JP6569841B1 - Hot-pressed steel sheet member and manufacturing method thereof - Google Patents

Abstract

A hot-pressed steel sheet member having both a tensile strength of 1780 MPa or more and excellent bending crushability is provided. It is a hot-pressed steel plate member, and in mass%, C: 0.30% or more and less than 0.50%, Si: 0.01% or more and 2.0% or less, Mn: 0.5% or more and 3.5% Hereinafter, Nb: 0.001% or more and 0.10% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% or more and 1.00% or less, and N: 0.01 %, The balance is Fe and inevitable impurities, and the ratio C / Nb of the C content (mass%) to the Nb content (mass%) is 22 to 100. Austenite grains have an average crystal grain size of 8 μm or less, a martensite volume ratio of 90% or more, and a microstructure in which the amount of solute C is 25% or less of the total C amount, and tensile strength Is a hot-pressed steel sheet member having 1780 MPa or more.

Description

  The present invention relates to a hot-pressed steel sheet member, and more particularly to a hot-pressed steel sheet member having both a tensile strength of 1780 MPa or more and excellent bending crushability. Moreover, this invention relates to the manufacturing method of the said hot press steel plate member.

In recent years, CO ₂ emission regulations have become stricter due to increasing environmental problems, and in the automobile field, it has become a challenge to reduce the weight of the vehicle body in order to improve fuel efficiency. Therefore, the use of high-strength steel sheets has been promoted to reduce the thickness of automobile parts. Specifically, the application of steel sheets having a tensile strength (TS) of 1780 MPa or more is being studied.

  However, high-strength steel sheets used for automobile structural members and reinforcing members are also required to have excellent formability and high dimensional accuracy after forming. Since the steel sheet having a tensile strength of 1780 MPa or more has low ductility, there is a problem that cracks are likely to occur during cold press forming. Further, since the steel sheet having a tensile strength of 1780 MPa or more has high yield strength, the spring back after cold press forming is large. Therefore, in the method of cold pressing a steel plate having a tensile strength of 1780 MPa or more, high dimensional accuracy cannot be obtained after forming.

  Therefore, in recent years, as a method for achieving high strength and high dimensional accuracy at the same time, attention has been paid to press molding by hot pressing (also called hot stamping, die quenching, press quenching, etc.). Hot pressing is a forming method in which a steel sheet is heated to an austenite single-phase temperature range, then formed at a high temperature and rapidly cooled (quenched) by contact with a mold. Since molding is performed in a heated and softened state, and then the strength is increased by quenching, a member having high strength and high dimensional accuracy can be obtained by hot pressing. Because of these characteristics, the hot press is used for manufacturing a member that is supplied with strength and accuracy, such as a member for an automobile.

  For example, Patent Document 1 proposes a method for manufacturing a hot press member used as an automobile member. In the method described in Patent Document 1, the toughness of the hot pressed member is improved by performing a quenching process and a heat treatment after the hot pressing.

JP2013-040390A

  By the way, in the member for motor vehicles, especially the member for frame | skeleton, it is calculated | required that it is excellent in bending crushability in addition to being excellent in intensity | strength. Here, the bending crushability refers to the property of being plastically deformed and crushed without causing cracks when bending deformation is applied to the member. In order to ensure the collision safety of the automobile, the automobile member is required to have excellent bending crushability.

  However, as described above, when the tensile strength of the member after pressing is set to 1780 MPa or more by rapid cooling at the time of hot pressing, the martensite as quenched is a factor, and plastic deformation does not occur when bending deformation is applied. It has been found that there is a problem that cracking is likely to occur.

  Moreover, according to the method described in Patent Document 1, a certain improvement in toughness is observed by performing heat treatment. However, even in the hot press member described in Patent Document 1, it has been found that sufficient bending crushability cannot be obtained when the tensile strength of the member after pressing is 1780 MPa or more.

  In order to reduce the weight required for automobile frame parts and to ensure collision safety, it is necessary that the member after hot pressing has a high tensile strength of 1780 MPa or more and an excellent bending crushability. However, it is difficult to improve the bending crushability of a hot-pressed steel sheet member having a TS of 1780 MPa or more, and the actual situation is that a method for obtaining a hot-pressed steel sheet member having these characteristics has not been developed.

  This invention is made | formed in view of the said actual condition, and it aims at providing the hot press steel plate member which combines the tensile strength of 1780 Mpa or more, and the outstanding bending crushability.

  As a result of studies to solve the above problems, the present inventors have obtained the following knowledge.

(1) In order to improve the bending crushability of the hot press member, it is important to control the amount of C dissolved in the hot press member. When the components of the steel sheet are adjusted to ensure a tensile strength of 1780 MPa or more, the amount of solute C in martensite increases as it is quenched. Therefore, although the hardness of a member becomes high, it becomes weak. Therefore, by precipitating the solid solution C by performing heat treatment, it is possible to improve the bending crushability while ensuring a certain degree of hardness. Adjustment of the amount of solute C can be performed by heat-treating under predetermined conditions after the steel sheet structure is made martensite by hot pressing.

(2) In order to improve the bending crushability of the hot press member, in addition to the control of the solid solution C amount, the ratio C / Nb of the C content to the Nb content may be controlled within a predetermined range. is important. Nb precipitates as NbC, and this NbC has an effect of refining austenite grains, and is therefore effective in improving the bending crushability. Therefore, it is considered desirable to add a sufficient amount of Nb from the viewpoint of generating NbC. However, generation of NbC consumes C necessary for strength improvement. Therefore, if the amount of C is too small compared to the amount of Nb, the required strength cannot be obtained. On the other hand, if the amount of C is too much compared to the amount of Nb, the bending crushability is lowered due to an increase in the amount of dissolved C or a decrease in the amount of NbC. Therefore, in order to achieve both strength and bending crushability, it is not sufficient to simply control the C content and the Nb content individually, and the ratio C / Nb of the C content to the Nb content is within a predetermined range. Need to control.

  The present invention has been completed based on the above findings, and the gist of the present invention is as follows.

1. A hot pressed steel plate member,
% By mass
C: 0.30% or more and less than 0.50%,
Si: 0.01% or more and 2.0% or less,
Mn: 0.5% or more and 3.5% or less,
Nb: 0.001% or more and 0.10% or less,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.01% or more and 1.00% or less, and N: 0.01% or less,
The balance is composed of Fe and unavoidable impurities, and the ratio C / Nb of C content (mass%) to Nb content (mass%) is 22-100,
Having a microstructure in which the average crystal grain size of the prior austenite grains is 8 μm or less, the volume fraction of martensite is 90% or more, and the solid solution C content is 25% or less of the total C content;
A hot-pressed steel sheet member having a tensile strength of 1780 MPa or more.

2. The component composition is mass%, and Mo: 0.35% or less,
Cr: 0.35% or less,
Ti: 0.15% or less,
B: 0.0050% or less,
Ca: 0.005% or less,
V: 0.05% or less,
Cu: 0.50% or less,
The hot-pressed steel sheet member according to 1 above, containing 1 or 2 or more selected from the group consisting of Ni: 0.50% or less, and Sn: 0.50% or less.

3. 3. The hot-pressed steel sheet member according to 1 or 2 above, further comprising an Al-based plating layer or a Zn-based plating layer on the surface.

4). % By mass
C: 0.30% or more and less than 0.50%,
Si: 0.01% or more and 2.0% or less,
Mn: 0.5% or more and 3.5% or less,
Nb: 0.001% or more and 0.10% or less,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.01% or more and 1.00% or less, and N: 0.01% or less,
A cold-rolled steel sheet having a component composition in which the balance is Fe and inevitable impurities, and the ratio C / Nb of the C content (mass%) to the Nb content (mass%) is 22 to 100 is greater than or equal to the Ac3 transformation point. Heated to a heating temperature of 1000 ° C. or less,
Hot-pressing the heated cold-rolled steel sheet to form a hot-pressed steel sheet,
Cooling the hot pressed steel sheet to the Mf point or less,
The cooled hot-pressed steel sheet is heat-treated under the conditions of heating temperature: 50 to 300 ° C. and holding time: 5 to 3600 seconds.
Manufacturing method of hot pressed steel sheet member.

5. The component composition is mass%, and Mo: 0.35% or less,
Cr: 0.35% or less,
Ti: 0.15% or less,
B: 0.0050% or less,
Ca: 0.005% or less,
V: 0.05% or less,
Cu: 0.50% or less,
The manufacturing method of the hot press steel plate member of said 4 containing 1 or 2 or more selected from the group which consists of Ni: 0.50% or less and Sn: 0.50% or less.

  ADVANTAGE OF THE INVENTION According to this invention, the hot press steel plate member which has the tensile strength of 1780 Mpa or more and the outstanding bending crushability can be provided.

  Hereinafter, a method for carrying out the present invention will be specifically described.

[Ingredient composition]
As for the hot-pressed steel plate member of the present invention, the steel plate portion has the above component composition. Hereinafter, the reason for limitation will be described. In the following description, “%” regarding the component composition means “% by mass”.

C: 0.30% or more and less than 0.50% C is an element effective for increasing the strength of a steel sheet, and is an important element for strengthening martensite after hot pressing to increase the strength of the steel. However, if the C content is less than 0.30%, the hardness of martensite after hot pressing becomes insufficient, and the desired tensile strength cannot be obtained. Therefore, the C content is 0.30% or more. On the other hand, when the C content is 0.50% or more, it becomes difficult to sufficiently reduce the solid solution C amount by heat treatment after cooling, and the bending crush resistance is lowered. Therefore, the C content is less than 0.50%, preferably less than 0.45%, more preferably less than 0.40%.

Si: 0.01% or more and 2.0% or less Si has an effect of strengthening the solid solution of ferrite and is an element effective for increasing the strength. However, excessive addition of Si reduces chemical conversion treatment properties, so the Si content is 2.0% or less, preferably 1.3% or less. On the other hand, since extremely low Si causes an increase in cost, the Si content is set to 0.01% or more.

Mn: 0.5% or more and 3.5% or less Mn is an element having an effect of improving hardenability, and contributes to martensite formation in cooling after hot pressing, that is, high strength. In order to acquire the said effect, Mn content shall be 0.5% or more, Preferably it is 1.0% or more. On the other hand, when Mn exceeding 3.5% is contained, Mn bands are generated excessively, so that the amount of dissolved C cannot be sufficiently reduced by heat treatment, and as a result, bending crushability is lowered. Therefore, the Mn content is 3.5% or less, preferably 2.5% or less.

Nb: 0.001% or more and 0.10% or less Nb is an element that contributes to an increase in strength by forming fine carbonitrides. Further, Nb is an element that contributes to the improvement of the bending crushability because it refines the austenite grain size during hot pressing. In order to acquire the said effect, Nb content shall be 0.001% or more, Preferably it is 0.003% or more. On the other hand, even if Nb is added in a large amount, the above effect is saturated, and the cost is increased. Therefore, the Nb content is 0.10% or less, preferably 0.07% or less, more preferably 0.03% or less, and further preferably 0.02% or less.

P: 0.05% or less P is an element that contributes to high strength by solid solution strengthening. However, when excessively added, segregation to the grain boundary becomes remarkable and the grain boundary becomes brittle, so that the bending crush resistance is lowered. Therefore, the P content is 0.05% or less, preferably 0.04% or less. On the other hand, the lower limit of the P content is not particularly limited, but since extremely low P causes an increase in steelmaking cost, the P content is preferably set to 0.0005% or more.

S: 0.01% or less If the S content is too high, a large amount of sulfide inclusions such as MnS are generated, and cracks are generated starting from the inclusions, so that the bending crushability is lowered. Therefore, the S content is 0.01% or less, preferably 0.005% or less. On the other hand, the lower limit of the Si content is not particularly limited, but since extremely low S causes an increase in steelmaking cost, the S content is preferably 0.0002% or more.

Al: 0.01% or more and 1.00% or less Al is an element necessary for deoxidation. In order to obtain the effect, the Al content is set to 0.01% or more. On the other hand, since the effect is saturated when the Al content exceeds 1.00%, the Al content is set to 1.00% or less, preferably 0.50% or less.

N: 0.01% or less N forms coarse nitrides and reduces bending crushability. If the N content is more than 0.01%, the influence on the bending crushability becomes significant. Therefore, the N content is 0.01% or less, preferably 0.008% or less.

  The steel plate in one embodiment of the present invention may contain the above components, and the balance may be composed of Fe and inevitable impurities.

  In another embodiment of the present invention, the component composition may further optionally contain one or more of the following elements.

Mo: 0.35% or less Mo is an element having an effect of improving hardenability, and contributes to martensite formation in cooling after hot pressing, that is, high strength. However, even if Mo is added excessively, the effect is saturated and the cost is increased. Moreover, excessive Mo addition reduces chemical conversion property. Therefore, when adding Mo, Mo content is made into 0.35% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of enhancing the effect of addition of Mo, the Mo content is preferably 0.005% or more, and more preferably 0.01% or more. .

Cr: 0.35% or less Cr is also an element having an effect of improving hardenability like Mo, and contributes to martensite formation in cooling after hot pressing, that is, high strength. However, even if Cr is added excessively, the effect is saturated and the cost is increased. Moreover, Cr forms a surface oxide and lowers the plating property. Therefore, when adding Cr, Cr content shall be 0.35% or less. On the other hand, the lower limit of the Cr content is not particularly limited, but from the viewpoint of enhancing the effect of adding Cr, the Cr content is preferably 0.005% or more, more preferably 0.01% or more. .

Ti: 0.10% or less Ti is an element that contributes to an increase in strength by forming fine carbonitrides. Ti is an element that contributes to the improvement of bending crushability by refining the austenite grain size during hot pressing. However, when Ti is added in a large amount, the elongation after hot pressing is significantly reduced. Therefore, when Ti is added, the Ti content is 0.10% or less, preferably 0.08% or less. On the other hand, the lower limit of the Ti content is not particularly limited, but from the viewpoint of enhancing the effect of adding Ti, the Ti content is preferably set to 0.005% or more.

B: 0.0050% or less B is an element having an effect of improving hardenability, and contributes to martensite formation in cooling after hot pressing, that is, high strength. In addition, since B segregates at the grain boundaries and improves the grain boundary strength, it is effective in improving the bending crushability. However, when B is added excessively, C and coarse precipitates are generated, and the bending crushability is lowered. Therefore, when B is added, the B content is set to 0.0050% or less, preferably 0.0035% or less. On the other hand, the lower limit of the B content is not particularly limited, but from the viewpoint of enhancing the effect of adding B, the B content is preferably set to 0.0002% or more.

Ca: 0.005% or less Ca is an element that controls the shape of sulfides and oxides and suppresses the formation of coarse MnS, and improves bending crushability. However, excessive addition deteriorates workability. Therefore, when Ca is added, the Ca content is set to 0.005% or less. On the other hand, the lower limit of the Ca content is not particularly limited, but from the viewpoint of enhancing the Ca addition effect, the Ca content is preferably 0.0005% or more.

V: 0.05% or less V is an element that contributes to an increase in strength by forming fine carbonitrides. However, excessive addition deteriorates the bending crushability. Therefore, when V is added, the V content is set to 0.05% or less. On the other hand, the lower limit of the V content is not particularly limited, but from the viewpoint of enhancing the effect of adding V, the V content is preferably set to 0.01% or more.

Cu: 0.50% or less Cu is an element contributing to high strength by solid solution strengthening. Cu also contributes to the improvement of the delayed fracture resistance by improving the corrosion resistance. However, even if added excessively, the effect is saturated, and surface defects caused by Cu tend to occur. Therefore, when adding Cu, the Cu content is 0.50% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but from the viewpoint of enhancing the effect of adding Cu, the Cu content is preferably 0.05% or more.

Ni: 0.50% or less Ni, like Cu, is an element that contributes to the improvement of delayed fracture resistance by improving the corrosion resistance. Further, Ni has an effect of suppressing surface defects caused by Cu by being added simultaneously with Cu. Therefore, the addition of Ni is particularly effective when Cu is added. However, when a large amount of Ni is added, the bending crushability is lowered and the tensile shear stress is lowered. Therefore, when Ni is added, the Ni content is 0.50% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of enhancing the effect of adding Ni, the Ni content is preferably 0.05% or more.

Sn: 0.50% or less Sn, like Cu, is an element that contributes to the improvement of delayed fracture resistance by improving the corrosion resistance. However, when a large amount of Sn is added, the bending crushability is lowered. Therefore, when adding Sn, Sn content shall be 0.50% or less. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of enhancing the effect of adding Sn, the Sn content is preferably 0.05% or more.

Zn: 0.10% or less Zn is an element that contributes to the formation of martensite after hot pressing, that is, to increase the strength, in order to enhance the hardenability during hot pressing. However, when a large amount of Zn is added, the bending crushability is lowered. Therefore, when Zn is added, the Zn content is set to 0.10% or less. On the other hand, the lower limit of the Zn content is not particularly limited, but from the viewpoint of enhancing the effect of adding Zn, the Zn content is preferably 0.005% or more.

Co: 0.10% or less Co, like Cu and Ni, is an element that has the effect of improving the hydrogen overvoltage and improving the corrosion resistance. Therefore, the addition of Co can improve the delayed fracture resistance. However, since a large amount of Co decreases the bending crushability, when Co is added, the Co content is 0.10% or less. On the other hand, the lower limit of the Co content is not particularly limited, but from the viewpoint of enhancing the Co addition effect, the Co content is preferably 0.005% or more.

Zr: 0.10% or less Zr, like Cu and Ni, is an element that contributes to improving delayed fracture resistance by improving corrosion resistance. However, a large amount of Zr addition reduces the bending crushability. Therefore, when Zr is added, the Zr content is set to 0.10% or less. On the other hand, the lower limit of the Zr content is not particularly limited, but from the viewpoint of enhancing the effect of adding Zr, the Zr content is preferably set to 0.005% or more.

Ta: 0.10% or less Ta, like Ti, is an element that generates carbides and nitrides and contributes to high strength. However, even if Ta is added excessively, the effect of addition is saturated and the alloy cost also increases. Therefore, when Ta is added, the amount of Ta added is 0.10% or less. On the other hand, the lower limit of the Ta content is not particularly limited, but from the viewpoint of enhancing the effect of adding Ta, the Ta content is preferably set to 0.005% or more.

W: 0.10% or less W, like Cu and Ni, is an element that contributes to the improvement of delayed fracture resistance by improving the corrosion resistance. However, the addition of a large amount of W decreases the bending crushability. Therefore, when adding W, W content shall be 0.10% or less. On the other hand, the lower limit of the W content is not particularly limited, but from the viewpoint of enhancing the effect of adding W, the W content is preferably set to 0.005% or more.

Moreover, the hot-pressed steel sheet member in another embodiment of the present invention is
C: 0.30% or more and less than 0.50%,
Si: 0.01% or more and 2.0% or less,
Mn: 0.5% or more and 3.5% or less,
Nb: 0.001% or more and 0.10% or less,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.01% or more and 1.00% or less,
N: 0.01% or less, and
Optionally
Mo: 0.35% or less,
Cr: 0.35% or less,
Ti: 0.15% or less,
B: 0.0050% or less,
Ca: 0.005% or less,
V: 0.05% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Sn: 0.50% or less,
Zn: 0.10% or less,
Co: 0.10% or less,
Zr: 0.10% or less,
Ta: 0.10% or less, and W: 1 or 2 or more selected from the group consisting of 0.10% or less,
The balance can have a component composition consisting of Fe and inevitable impurities.

C / Nb: 22-100
In this invention, it is important that ratio C / Nb of C content (mass%) with respect to Nb content (mass%) in a steel plate is 22-100. If C / Nb exceeds 100, the amount of Nb-based carbide produced decreases and the amount of solute C increases. Since Nb carbide has the effect of suppressing the growth of austenite grains due to the pinning effect, the decrease in Nb carbide causes coarsening of the crystal grains after hot pressing, resulting in lower bending crushability. To do. Moreover, when the amount of solute C increases, the toughness decreases, and as a result, the bending crushability also decreases. Therefore, C / Nb is set to 100 or less, preferably 80 or less, more preferably 70 or less. On the other hand, when C / Nb is less than 22, a large amount of Nb-based carbide that is the starting point of crack generation is generated, so that the bending crushability is lowered. C is an element having an effect of increasing the strength of steel. However, if C / Nb is less than 22, the proportion of C consumed by the formation of Nb-based carbides increases, and as a result, the tensile strength decreases. Therefore, C / Nb is 22 or more, preferably 25 or more, more preferably 30 or more.

[Microstructure]
Furthermore, it is important that the hot-pressed steel sheet member of the present invention has a microstructure that satisfies the following conditions.

Average crystal grain size of prior austenite grains: 8 μm or less When the average crystal grain size of prior austenite grains exceeds 8 μm, the toughness at the time of bending crushing is lowered, so that the bending crushability is lowered. Therefore, the average crystal grain size of the prior austenite grains is 8 μm or less, preferably 7 μm or less. On the other hand, the lower limit is not particularly limited, but is preferably 2 μm or more, more preferably 3 μm or more, and further preferably 5 μm or more.

Martensite volume ratio: 90% or more If the martensite volume ratio is less than 90%, it is difficult to obtain a tensile strength of 1780 MPa or more. Therefore, the volume ratio of martensite is 90% or more, preferably 95% or more. On the other hand, the upper limit of the volume ratio of martensite is not particularly limited, and may be 100%. In addition, about structures | tissues other than a martensite, it is not specifically limited, Arbitrary structures | tissues can be included. For example, the balance other than martensite may be one or more selected from the group consisting of ferrite, bainite, and pearlite.

Solid solution C amount: 25% or less of the total C amount When the solid solution C amount exceeds 25% of the total C amount, the toughness during bending crushing is lowered. Therefore, the solid solution C amount is 25% or less, preferably 20% or less, more preferably 15% or less of the total C amount. On the other hand, the lower limit of the solute C amount is not particularly limited, but is preferably 5% or more of the total C amount, and preferably 6% or more.

[Tensile strength]
TS: 1780 MPa or more The hot-pressed steel sheet member of the present invention has a tensile strength (TS) of 1780 MPa or more. TS is preferably 1800 MPa or more, more preferably 1850 MPa or more, and further preferably 1900 MPa or more. On the other hand, the upper limit of TS is not particularly limited, but may be usually 2500 MPa or less and 2450 MPa or less.

[Yield ratio]
When the yield ratio (YR) of a hot-pressed steel sheet member is high, collision safety performance can be further improved when the hot-pressed steel sheet member is used as an automobile member. Therefore, the yield ratio is preferably 65% or more, and more preferably 70% or more. Here, the yield ratio (YR) is a value defined as the ratio of the yield strength YS to the tensile strength TS, and specifically, calculated as YR = YS / TS × 100 (%). Can do.

[Plating layer]
The hot-pressed steel sheet member of the present invention may have no plating layer. In that case, the hot-pressed steel plate member is made of a steel material having the above-described component composition, microstructure, and tensile strength. However, in order to prevent oxidation during hot pressing and improve the corrosion resistance of the hot-pressed steel plate member, the hot-pressed steel plate member preferably further has a plating layer on the surface of the steel plate. When the hot-pressed steel plate member has a plating layer on the surface, the steel plate portion (base metal plate) excluding the plating layer has the above-described component composition and microstructure.

  As the plating layer, an Al-based plating layer or a Zn-based plating layer is suitable. By applying these plating layers to the surface of the steel sheet, oxidation of the steel sheet surface by hot pressing can be prevented, and further, the corrosion resistance of the hot pressed steel sheet member can be improved.

  Here, the Zn-based plating layer refers to a plating layer containing 50% by mass or more of Zn. As the Zn-based plating layer, both a Zn plating layer and a Zn-based alloy plating layer can be used. Examples of the Zn-based alloy plating layer include Si, Mg, Ni, Fe, Co, Mn, Sn, Pb, Be, B, P, S, Ti, V, W, and Mo in addition to Zn as a main component. A plating layer made of an alloy containing one or more selected from the group consisting of Sb, Cd, Nb, Cr, and Sr can be used. An example of a Zn-based plating layer that can be suitably used is a Zn-Ni alloy plating layer.

  Here, the Al-based plating layer refers to a plating layer containing 50% by mass or more of Al. As the Al plating layer, both an Al plating layer and an Al alloy plating layer can be used. Examples of the Al-based alloy plating layer include Si, Mg, Ni, Fe, Co, Mn, Sn, Pb, Be, B, P, S, Ti, V, W, and Mo in addition to Al as a main component. A plating layer made of an alloy containing one or more selected from the group consisting of Sb, Cd, Nb, Cr, and Sr can be used. An example of an Al-based plating layer that can be suitably used is an Al-Si plating layer.

  The formation method of the said plating layer is not specifically limited, It can form by arbitrary methods. For example, a hot-dip plating layer that is a plating layer formed by hot-dip plating, an electroplating layer that is a plating layer formed by an electroplating method, a vapor-deposited plating layer that is a plating layer formed by a vapor-deposition plating method, etc. Is also applicable. The plating layer may be an alloying plating layer that is a plating layer formed by performing an alloying treatment after the plating step.

  Examples of the Al-based plating layer that can be suitably used include a molten Al-Si plating layer formed by a hot-dip plating method. Examples of the Zn-based plating layer that can be suitably used include a hot-dip Zn plating layer formed by a hot-dip plating method, an alloyed hot-dip Zn plating layer obtained by alloying the hot-dip Zn plating layer, and an electroplating method. Examples thereof include an electric Zn plating layer and an electric Zn—Ni alloy plating layer. In particular, from the viewpoint of further improving the corrosion resistance of the hot press member and preventing liquid metal brittle cracking caused by molten Zn during hot press forming, a Zn-Ni alloy plating layer may be used as the Zn-based plating layer. preferable.

  In addition, when hot pressing is performed on a steel sheet provided with a plating layer, some or all of the elements contained in the plating layer may diffuse into the base steel sheet, producing a solid solution phase or an intermetallic compound. . Similarly, Fe, which is a base steel plate component, may diffuse into the plating layer to generate a solid solution phase or an intermetallic compound. Furthermore, an oxide film may be formed on the surface of the plating layer.

  For example, when an Al—Si plating layer is heated, the plating layer changes to a plating layer mainly composed of an Fe—Al intermetallic compound containing Si. Moreover, when a hot-dip Zn plating layer, an alloyed hot-dip Zn plating layer, an electric Zn plating layer, or the like is heated, an FeZn solid solution phase in which Zn is solid-solved in Fe, a ZnFe intermetallic compound, a surface ZnO layer, or the like is formed. Furthermore, when the electric Zn—Ni alloy plating layer is heated, a solid solution layer containing Ni in which a plating layer component is dissolved in Fe, an intermetallic compound mainly composed of ZnNi, a surface ZnO layer, and the like are formed. The

  In the present invention, as described above, a plating layer containing Al formed by heating a cold-rolled steel sheet for hot pressing provided with an Al-based plating layer is called an Al-based plating layer. The plating layer containing Zn formed by heating the cold-rolled steel sheet for hot pressing provided with the plating layer is referred to as a Zn-based plating layer.

The adhesion amount of the plating layer is not particularly limited, and can be an arbitrary amount. However, if the adhesion amount per side is less than 5 g / m ² , it may be difficult to ensure the corrosion resistance. Therefore, the adhesion amount per side is preferably 5 g / m ² or more. On the other hand, if the adhesion amount per one side exceeds 150 g / m ² , the plating peel resistance may be deteriorated. Therefore, the adhesion amount per one side is preferably 150 g / m ² or less.

[Production method]
Next, the manufacturing method of the hot press steel plate member of this invention is demonstrated. Although the manufacturing method of the hot press steel plate member of this invention is not specifically limited, In one Embodiment, it can manufacture by the process of following (1)-(4). Hereinafter, each step will be described.
(1) Heating of cold-rolled steel sheet (2) Hot pressing (3) Cooling (quenching)
(4) Heat treatment

[Cold rolled steel sheet]
As a cold-rolled steel sheet used as a raw material, what has the component composition mentioned above is used. That is, the component composition of the steel plate portion of the hot-pressed steel plate member finally obtained is basically the same as the component composition of the cold-rolled steel plate used as the material.

  The manufacturing method of the said cold-rolled steel plate is not specifically limited, It can manufacture according to a conventional method. For example, it can manufacture by hot-rolling the steel raw material (steel slab) which has the said component composition, and then cold-rolling. Further, temper rolling may be further performed on the cold-rolled steel sheet. When performing temper rolling, a suitable elongation is 0.05 to 2.0%.

  For example, a steel material (slab) having the above component composition is hot-rolled into a hot-rolled steel sheet at a finish rolling end temperature of 860 to 950 ° C. Next, the hot-rolled steel sheet is wound at a winding temperature of 650 ° C. or lower. In that case, after completion | finish of hot rolling, it cools by the cooling rate: 5 degree-C / s or more to the said winding temperature. Then, the wound hot-rolled steel sheet is rewound, pickled, and cold-rolled. After the cold rolling is completed, heat treatment is performed by heating to a temperature range of 650 to 950 ° C. at an average temperature increase rate of 2 ° C./s or more, and soaking in the temperature range for 5 seconds or more. Next, cooling is performed to cool the steel sheet to a cooling stop temperature of 2 ° C./s or more and 600 ° C. or less to obtain a cold rolled steel sheet.

  In addition, the said manufacturing conditions are an example, Comprising: It is not limited to this. This is because in the production method of the present invention, the microstructure of the steel sheet can be controlled by heating before hot pressing and cooling after hot pressing.

[Plating treatment]
The cold-rolled steel sheet can be subjected to the next heat treatment as it is (without being subjected to a plating treatment), but can be optionally subjected to a plating treatment prior to heating. The method for performing the plating treatment is not particularly limited, and any method such as a hot dipping method, an electroplating method, and a vapor deposition plating method can be used. Further, an alloying treatment may be performed after the plating treatment.

[heating]
Next, the cold-rolled steel sheet is heated to a heating temperature not lower than the Ac3 transformation point and not higher than 1000 ° C. When the heating temperature is less than Ac3 point, the austenite ratio in the heated steel sheet is lowered, so the volume ratio of martensite is less than 90% after hot pressing, and the desired tensile strength cannot be ensured. On the other hand, if the heating temperature is higher than 1000 ° C., the crystal grain size becomes excessively large, so that the bending crushability decreases. Here, the Ac3 transformation point can be obtained by the following equation (1).
Ac3 transformation point (° C.) = 881-206C + 53Si-15Mn-20Ni-1Cr-27Cu + 41Mo (1)
However, the element symbol in the formula (1) represents the content (% by mass) of each element. The content of elements not contained is calculated as 0.

  Although the said heating can be performed by arbitrary methods, without being specifically limited, Generally, what is necessary is just to heat using a heating furnace. As the heating furnace, for example, an electric furnace, a gas furnace, an electric heating furnace, a far infrared heating furnace, or the like can be used.

  Although it is possible to perform hot pressing immediately after heating to the heating temperature, it is preferable to hold the heating temperature for 0 to 600 seconds. When the holding time exceeds 600 seconds, the crystal grain size becomes excessively coarse, so that the bending crushability is lowered. Therefore, when holding, the holding time is 600 seconds or less.

[Hot press]
Next, the heated cold-rolled steel sheet is conveyed to a press machine and subjected to hot pressing. The method of hot pressing is not particularly limited, and can be performed by any method. The temperature at the time of pressing is not particularly limited, but it is preferable to perform hot pressing in the range of 550 to 800 ° C.

[cooling]
The hot-pressed steel sheet is cooled to the Mf point or less. By cooling the steel sheet heated above the Ac3 transformation point by the heating described above to the Mf point or less by this cooling, the volume ratio of austenite can be made 90% or more. The Mf point, which is the temperature at which the martensitic transformation ends, can be obtained based on a continuous cooling transformation curve (CCT curve).

  The method for performing the cooling is not particularly limited, and can be performed by any method. Usually, cooling may be performed by contact with a mold, as is done in general hot pressing. The cooling can be started simultaneously with hot pressing.

  Although the cooling rate is not particularly limited, it is preferable that the average cooling rate from the start of cooling to 150 ° C. is 10 ° C./s or more from the viewpoint of structure control. For example, by contact with the mold, the average cooling rate is cooled to a temperature of 10 ° C./s or higher and 150 ° C. or lower, and after cooling to 150 ° C. or lower, the mold is released. Thereafter, it can be arbitrarily cooled to room temperature.

[Heat treatment]
Next, the cooled hot-pressed steel sheet is heat-treated under the conditions of heating temperature: 50 to 300 ° C. and holding time: 5 to 3600 seconds. By the heat treatment, the solid solution C amount of the steel sheet can be 25% or less of the total C amount. When the heating temperature is less than 50 ° C., the amount of solute C increases, and the bending crushability decreases. On the other hand, when the heating temperature exceeds 300 ° C., the precipitated carbide is coarsened and the tensile strength is lowered. In addition, when the holding time is less than 5 seconds, the amount of dissolved C increases, and the bending crushability decreases. On the other hand, if the holding time exceeds 3600 seconds, the precipitated carbide is coarsened and the tensile strength is lowered.

  Next, examples of the present invention will be described. It should be noted that the present invention is not limited by the examples described below, and can be implemented with appropriate modifications within a range that can be adapted to the spirit of the present invention. Included in the technical scope.

  First, the cold-rolled steel plate used as a raw material for manufacturing a hot press steel plate member was manufactured in the following procedures.

  Steel with the composition shown in Table 1 was melted and cast to produce a steel slab. The steel slab was hot rolled under conditions of hot rolling heating temperature: 1250 ° C. and finish rolling finishing temperature (FDT): 900 ° C. to obtain a hot rolled steel sheet. The obtained hot rolled steel sheet was wound at a winding temperature of 600 ° C.

  The obtained hot-rolled steel sheet was rewound and pickled, and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.4 mm. The obtained cold-rolled steel sheet is subjected to an annealing treatment in a continuous annealing line (CAL) or continuous hot-dip plating line (CGL), and the final material is cold-rolled steel sheet (CR) or hot-dip galvanized steel sheet (GI). Got.

  In addition, about some steel plates, after performing the hot dip galvanization process, the alloying process was performed and it was set as the galvannealed steel plate (GA). In addition, some of the other steel plates were subjected to hot dip aluminum plating treatment to obtain hot dip aluminum plated steel plates (GAS). In addition, some of the other steel plates were annealed by CAL and then plated in an electrogalvanizing line (EGL) to obtain an electrogalvanized steel plate (EG) or an electrogalvanized nickel plated steel plate (EZN).

  Next, the obtained cold-rolled steel sheet was heated to the heating temperature shown in Table 2, and held at the heating temperature for 60 seconds. The heating was performed in the air using an infrared heating furnace or an atmosphere heating furnace. The heating rate during heating was 5 ° C./s.

  Next, the heated steel plate was conveyed to a press machine and hot pressed to produce a hat-shaped hot pressed steel plate member. The steel plate temperature during hot pressing was 700 ° C. The mold used for the hot press had a punch width: 120 mm, a punch shoulder R: 6 mm, a die shoulder R: 6 mm, and a molding depth of 40 mm.

  The cooling was performed by combining contact cooling by sandwiching a steel plate between punches and dies and air cooling on the die after releasing from the sandwiching. The average cooling rate from the press start temperature to 150 ° C. was 100 ° C./s. The average cooling rate was adjusted by changing the time for holding the punch at the bottom dead center in the range of 1 to 60 seconds.

  After air cooling to room temperature, heat treatment was performed at the heating temperature and holding time shown in Table 2. Specifically, it was heated to the heating temperature in an atmospheric furnace, held for the holding time, and then air-cooled.

  Next, for each of the obtained hot-pressed steel plate members, the microstructure, tensile properties, and bending crushability of the steel plate portion were evaluated by the following methods.

(Micro structure)
-Volume ratio of martensite The volume ratio of martensite in the steel plate portion of the hot-pressed steel plate member was evaluated by the following method. First, a plate thickness cross section parallel to the rolling direction of the steel plate is polished, then corroded with 3 vol% nital, and observed with a SEM (scanning electron microscope) at a magnification of 2000 times and 5000 times, and the point counting method (ASTM E562-83 (1988)), the area ratio of martensite was measured, and the area ratio was defined as the volume ratio.

-Average crystal grain size of prior austenite grains The average crystal grain size of prior austenite grains was determined by image analysis of a structural photograph obtained by SEM observation in the measurement of the martensite volume fraction. Specifically, the prior austenite crystal grains in the structure photograph were identified, the equivalent circle diameter was calculated, and the average value was taken as the average crystal grain size of the prior austenite grains. For the image analysis, Image-Pro of Media Cybernetics was used.

- amount of solute C solid solution C amount was determined by subtracting the total C content in steel from (C _total), the amount of C that is precipitated as carbide (C _p).

・ C _total
As the total C amount (C _total ) in the steel, the C content (mass%) of the steel sheet shown in Table 1 was used.

・ C _p
Examples of the carbide include cementite (M ₃ C). Further, when Nb, Ti, and V are contained, their carbides (NbC, TiC, VC) are precipitated. Therefore, the amount of C precipitated as carbide (C _p ) is obtained as the sum of the amount of C precipitated as cementite (C _p1 ) and the amount of C precipitated as NbC, TiC, and VC (C _p2 ). be able to.

・ C _p1
The amount of C precipitated as cementite was determined by using both TEM-EDX (transmission electron microscope-energy dispersive X-ray spectroscopy) analysis and ICP emission analysis of the extraction residue obtained by electrolytic extraction. . First, a measurement TEM observation sample was prepared from each of the obtained hot-pressed steel sheet members, and the concentration of the metal element constituting cementite was measured using EDX analysis. Here, the metal elements constituting cementite are Fe, Cr, and Mn. The said density | concentration was made into the average value of 10 places of measurements. From the obtained concentration, the atomic ratio of Fe, Mn, and Cr, F _Fe , F _Cr , and F _Mn were determined. Here, F _Fe + F _Cr + F _Mn = 1.

Next, extraction residues were obtained from each of the hot-pressed steel plate members by electrolytic extraction. As the electrolytic solution, a 10% acetylacetone-based electrolytic solution was used. The obtained extraction residue was analyzed by ICP (High Frequency Inductively Coupled Plasma) emission spectrometry, and the amount of Fe deposited in the steel as cementite: C _Fe (mass%) was determined.

Using the value obtained by the above measurement, the amount of C precipitated as cementite (C _p1 ) was calculated by the following formula.
C _p1 (mass%) = 12 / (M × 3) × C _Fe × 1 / (F _Fe )
Here, M = (56 × F _Fe + 52 × F _Cr + 54 × F _Mn )

・ C _p2
The amount of C deposited as NbC, TiC, and VC (C _p2 ) was determined by the following method. First, an extraction residue was obtained from each hot-pressed steel sheet member by electrolytic extraction. As the electrolytic solution, a 10% acetylacetone-based electrolytic solution was used. The obtained extraction residue was analyzed by ICP (High Frequency Inductively Coupled Plasma) emission spectroscopy, and Nb, Ti, and V amounts were measured. The measured amounts of Nb, Ti, and V are amounts of each metal element precipitated as NbC, TiC, and VC. Therefore, the amount of C (C _p2 ) precipitated as NbC, TiC, and VC was calculated from the measured values.

From the C _p1 and C _p2 obtained as described above, the solid solution C amount was calculated by the following formula.
[Solubility C amount (% by mass)] = C _total − (C _p1 + C _p2 )

(Tensile properties)
From the position of the hat bottom of the obtained hot-pressed steel sheet member, a JIS No. 5 tensile test piece was sampled and subjected to a tensile test according to JIS Z 2241. Yield strength (YS) and tensile strength (TS) Was measured.

(Bending crushability)
Three-point bending deformation was performed on the obtained hot-pressed steel sheet member, and the stroke and load were measured. In the three-point bending deformation, a die having a span of 280 mm and a punch 100R was used, and the test speed was set to 0.1 m / s. Bending crushability was evaluated as 0 when reaching the bottom dead center without cracking even when exceeding the maximum load, and x when cracking occurred.

Table 3 shows the measured steel structure, tensile properties, and bending crushability. In addition, solid solution C amount was shown as a ratio (%) with respect to the _total C amount ( _Ctotal ). As can be seen from this result, the hot-pressed steel sheet member satisfying the conditions of the present invention had a tensile strength of 1780 MPa or more and an excellent bending crushability.

Claims (5)

A hot pressed steel plate member,
% By mass
C: 0.30% or more and less than 0.50%,
Si: 0.01% or more and 1.3% or less,
Mn: 0.5% or more and 3.5% or less,
Nb: 0.005% or more and 0.019% or less ,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.01% or more and 1.00% or less, and N: 0.01% or less,
The balance is composed of Fe and unavoidable impurities, and the ratio C / Nb of C content (mass%) to Nb content (mass%) is 22-100,
Having a microstructure in which the average crystal grain size of the prior austenite grains is 8 μm or less, the volume fraction of martensite is 90% or more, and the solid solution C content is 25% or less of the total C content;
A hot-pressed steel sheet member having a tensile strength of 1780 MPa or more. The component composition is mass%, and Mo: 0.35% or less,
Cr: 0.35% or less,
Ti: 0.15% or less,
B: 0.0050% or less,
Ca: 0.005% or less,
V: 0.05% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Sn: 0.50% or less,
Zn: 0.10% or less,
Co: 0.10% or less,
Zr: 0.10% or less,
The hot-pressed steel sheet member according to claim 1, containing 1 or 2 or more selected from the group consisting of Ta: 0.10% or less and W: 0.10% or less.   The hot-pressed steel sheet member according to claim 1 or 2, further comprising an Al-based plating layer or a Zn-based plating layer on the surface. % By mass
C: 0.30% or more and less than 0.50%,
Si: 0.01% or more and 1.3% or less,
Mn: 0.5% or more and 3.5% or less,
Nb: 0.005% or more and 0.019% or less ,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.01% or more and 1.00% or less, and N: 0.01% or less,
A cold-rolled steel sheet having a component composition in which the balance is Fe and inevitable impurities, and the ratio C / Nb of the C content (mass%) to the Nb content (mass%) is 22 to 100 is greater than or equal to the Ac3 transformation point. Heated to a heating temperature of 1000 ° C. or less,
Hot-pressing the heated cold-rolled steel sheet to form a hot-pressed steel sheet,
Cooling the hot pressed steel sheet to the Mf point or less,
The cooled hot-pressed steel sheet is heat-treated under the conditions of heating temperature: 50 to 300 ° C. and holding time: 5 to 3600 seconds.
It has a microstructure in which the average crystal grain size of the prior austenite grains is 8 μm or less, the volume fraction of martensite is 90% or more, and the solid solution C content is 25% or less of the total C content, and the tensile strength The manufacturing method of the hot press steel plate member whose length is 1780 MPa or more . The component composition is mass%, and Mo: 0.35% or less,
Cr: 0.35% or less,
Ti: 0.15% or less,
B: 0.0050% or less,
Ca: 0.005% or less,
V: 0.05% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Sn: 0.50% or less,
Zn: 0.10% or less,
Co: 0.10% or less,
Zr: 0.10% or less,
The manufacturing method of the hot press steel plate member of Claim 4 containing 1 or 2 or more selected from the group which consists of Ta: 0.10% or less and W: 0.10% or less.