JP5071125B2 - High-strength cold-rolled steel sheet excellent in square tube drawing formability and shape freezing property, manufacturing method thereof, and automotive parts excellent in product shape - Google Patents

Description

The present invention mainly provides a high-strength cold-rolled steel sheet excellent in rectangular tube drawing formability and shape freezing property, which is suitable for use as an inner plate of an automobile, an underbody member, a structural member, and a reinforcing part of the structural member, and its manufacture It is about the method.
Further, the present invention relates to an automotive part having an excellent product shape obtained by subjecting the above-described high strength cold-rolled steel sheet excellent in square tube drawing formability and shape freezing property to square tube drawing.

  Automotive parts, including so-called box parts such as inner plates, underbody members, structural members and structural member reinforcement parts, are subjected to press working with deep drawing, bending, overhanging, etc. Therefore, deep drawing steel plates are often used as the material. In addition, since non-aging is an important required characteristic of materials for automobile parts, an ultra-low carbon Ti-added aluminum killed cold-rolled steel sheet having complete non-aging properties is a typical material for automobile parts.

Among the above-described automotive members, particularly, reinforcing parts and box parts are required to have square tube formability. For that purpose, conventionally, it is important that not only the Rankford value (r value), which is an index of deep drawability, is high, but also that the in-plane anisotropy (Δr value) of the r value is small. Such a steel sheet having a high r value and a controlled Δr value is disclosed. However, all of the above steel plates are mild steel plates having a TS of 340 MPa or less.
JP 2006-70339 A

Similarly, with increasing r value (average r value ≧ 2.7), regarding the Δr value, it is rather advantageous that the larger Δr value is advantageous for forming the rectangular tube, and the D direction (the direction that forms 45 ° with the rolling direction). Is disclosed in Patent Document 2 by improving the formability with Δr value ≧ 0.67 by lowering the r value of L) in the L direction (rolling direction) and the C direction (direction forming 90 ° with the rolling direction). Has been. However, although this steel sheet is not specifically disclosed, TS is an extremely low carbon IF steel in terms of components, and is estimated to be a mild steel sheet from the amount of alloy elements added. Moreover, in this technique, in order to obtain a high r value, hot rolling is performed by lubrication rolling in a temperature range from the Ar ₃ transformation point to 500 ° C. This is a technique peculiar to IF steel, and this method is usually used. When applied to the low carbon steel sheet, there is a problem that the rolling load becomes high and troubles during rolling are easily induced.
Japanese Patent No. 3460525

By the way, in recent years, high strength steel sheets having a tensile strength (TS) of 440 MPa or more have been frequently used for press parts as the strength of automobile bodies increases. Therefore, the demand for steel sheets having high strength and excellent square tube formability is increasing.
Moreover, since many of the automotive parts made of steel plates are manufactured by press forming as described above, the steel plates need excellent press formability, especially when high strength steel plates are applied, Excellent formability (dimensional accuracy) is also required.

Conventionally, among the formability and formability of high-strength cold-rolled steel sheets, particularly with respect to formability, it has been advantageous to lower the yield ratio YS / TS (YS is the yield strength) of the steel sheet. And multi-structure (Dual-Phase, DP) steel sheets containing martensite phase are considered effective.
Although patent document 3 is related with the manufacturing method of DP steel plate, in this method, there exists a disadvantage which requires water cooling equipment for cooling at the time of annealing.
Patent Document 4 is a DP steel containing martensite with a volume fraction of 25% or less, and (A) the average value of the X-ray random intensity ratio of {100} <011> to {223} <110> orientation groups. 4.0 or more, (B) the average value of the X-ray random intensity ratio of three crystal orientations of {554} <225>, {111} <112> and {111} <110> is 3.0 or more, (A) / ( B) Satisfies ≦ 4.0, at least one of the r values in the rolling direction or the direction perpendicular thereto is 0.7 or more, and the average value of r values is 0.8 or more. An attempt is made to obtain a cold-rolled steel sheet. However, if the above (A) is set to 4.0 or more, the average r value is low, and thus the problem remains in that the deep drawability is lowered.
JP 2006-233318 A JP 2004-124123 A

In addition, Patent Document 5 discloses a method for producing a high-strength steel sheet in which the average r value is 1.2 or more and the absolute value of Δr is as small as 0.3 or less.
However, with this method, it has been difficult to obtain a steel sheet with good shape.
JP 2005-120471 A

  The present invention advantageously solves the above problems, and provides a high-strength cold-rolled steel sheet having excellent rectangular tube drawability and shape (shape freezing property) even when TS has a high strength of 440 MPa or more. The object is to propose it together with its advantageous manufacturing method.

As a result of intensive studies to solve the above problems, the inventors have obtained the following knowledge.
That is, in order to improve the rectangular tube drawing formability and the shape freezing property, not only simply increasing the average r value, but also reducing the r value in the D direction of the steel sheet that hits the corner portion of the square tube in the L direction and the C direction. It is effective to make it higher than that, and especially for high-strength steel sheets of 500 MPa or more, unlike the conventional knowledge, by making the steel sheet with a large in-plane anisotropy on the negative side, New findings have been obtained that the moldability is advantageously improved.
Moreover, in order to improve the shape freezing property, it was found that it is advantageous to use a high yield ratio even for a composite structure steel plate, contrary to the conventional low yield ratio type.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.010 to 0.050%
Si: 1.0% or less,
Mn: 1.5-3.0%
P: 0.1% or less,
S: 0.01% or less,
Al: 0.005-0.5%
Ti: 0.005-0.05%
Nb: 0.01 to 0.3% and N: 0.01% or less, and
Mo: 0.01-0.5%
Cr: 0.05-0.8% and V: 0.01-0.2%
One or more selected from
M = [% Mn] +3 [% Mo] +1.3 [% Cr] +0.5 [% V] ≧ 2
The balance is the composition of Fe and inevitable impurities, the ferrite phase is the main phase, and has a steel structure consisting of a mixed structure containing a martensite phase with a volume fraction of 15% or less, The r value (rD) in the direction forming 45 ° with the rolling direction is 1.2 or more, and the in-plane anisotropy (Δr value) represented by the following formula satisfies the range of −1.0 ≦ Δr value ≦ −0.3. A high-strength cold-rolled steel sheet with excellent square tube drawability and shape freezing characteristics.
Record
Δr value = (r _L −2 × r _D + r _C ) / 2
Where r _L : r value in the rolling direction
r _D : r value in the direction of 45 ° with the rolling direction
r _C : r value in the direction of 90 ° with the rolling direction

2. 2. A high strength cold-rolled steel sheet excellent in square tube drawing formability and shape freezing property as described in 1 above, wherein the yield ratio exceeds 0.6.

3. 3. The high-strength cold-rolled steel sheet having excellent square-drawing formability and shape freezing property as described in 1 or 2 above, wherein the steel sheet surface has a plating layer.

4). % By mass
C: 0.010 to 0.050%
Si: 1.0% or less,
Mn: 1.5-3.0%
P: 0.1% or less,
S: 0.01% or less,
Al: 0.005-0.5%
Ti: 0.005-0.05%
Nb: 0.01 to 0.3% and N: 0.01% or less, and
Mo: 0.01-0.5%
Cr: 0.05-0.8% and V: 0.01-0.2%
One or more selected from
M = [% Mn] +3 [% Mo] +1.3 [% Cr] +0.5 [% V] ≧ 2
The steel slab that contains Fe and the inevitable impurities composition is hot rolled at a finish rolling temperature of 910 to 800 ° C, and then wound into a coil at a temperature of 550 to 700 ° C. Reduction ratio: After cold rolling at 55-80%, continuous annealing in a heat cycle in which at least 300-700 ° C temperature range is heated at a rate of 10 ° C / s or more and stays in the 800-900 ° C temperature range for 120s or more. A method for producing a high-strength cold-rolled steel sheet excellent in square tube drawing formability and shape freezeability.

5). An automotive part having an excellent product shape, which is obtained by subjecting the high-strength cold-rolled steel sheet according to any one of 1 to 3 above to square tube drawing.

According to the present invention, in a high-strength cold-rolled steel sheet with TS ≧ 440 MPa, excellent rectangular tube drawing formability and shape freezing property can be obtained.
Moreover, according to this invention, the automotive component excellent in the product shape can be obtained by carrying out square tube drawing of the above-mentioned high-strength cold-rolled steel sheet.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the cold-rolled steel sheet is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.010 to 0.050%
In order to make the final structure a steel sheet having a composite structure, at least 0.010% of C is required. However, since a large amount of C is not preferable for the r value, the upper limit value of C is set to 0.050%.

Si: 1.0% or less
Since Si contributes to increasing the strength of the steel material by the solid solution strengthening action, the necessary amount is added according to the desired strength. In order to obtain the effect of solid solution strengthening, it is preferable to contain 0.01% or more. However, if the content exceeds 1.0%, the r value tends to be in the relationship of r _D ≦ r _C , and not only does rD decrease and the in-plane anisotropy Δr deviates from the appropriate range, Since red scale is generated during hot rolling and the appearance of the steel sheet surface is likely to be impaired, Si is contained at 1.0% or less.

Mn: 1.5-3.0%
Mn also has the effect of preventing cracking during continuous casting (hot cracking due to S). However, in the present invention, in order to increase the strength of the steel material, it is annealed to obtain structural strengthening as well as solid solution strengthening. The effect of promoting the formation of the martensite phase during cooling (delaying the critical cooling rate at which the martensate phase is obtained) is utilized. In addition, since Mn delays the ferrite transformation during cooling, it contributes to the expansion of the rolling temperature range in the austenite phase region during hot rolling, and in addition to the recrystallization delay effect due to the addition of Nb, unrecrystallized austenite It is presumed that the in-plane anisotropy of the steel sheet is affected by increasing the amount of strain accumulated in the phase. From the viewpoint of both steel plate strength and in-plane anisotropy, Mn is contained in an amount of 1.5% or more. On the other hand, excessive addition of Mn causes deterioration of deep drawability and weldability, so the upper limit was made 3.0%.

P: 0.1% or less P contributes to increasing the strength of the steel material by the solid solution strengthening action, so the necessary amount is added according to the desired strength. In order to obtain the effect of solid solution strengthening, it is preferable to contain 0.005% or more. Further, from the viewpoint of dephosphorization cost in the steel making process, P is preferably 0.005% or more, preferably 0.01% or more. However, P is easily segregated at the grain boundary, not only deteriorates the secondary work brittleness resistance and weldability, but excessive addition tends to cause the r value to be r _D ≦ r _C , so P is 0.1%. Limited to:

S: 0.01% or less S is an impurity element that not only causes hot cracking during continuous casting, but also forms inclusions in the steel and degrades the formability of the steel sheet. Is preferred. Therefore, S is regulated to 0.01% or less.

Al: 0.005-0.5%
Al is an element useful as a deoxidizer for steel in the steel making process. In order to obtain a sufficient deoxidation effect, it is necessary to contain 0.005% or more as sol.Al. Further, Al is a ferrite transformation promoting element, and is also useful for adjusting the transformation point mainly from the viewpoint of recrystallization texture development in the ferrite region during annealing. However, excessive addition of Al not only increases the alloy cost but also increases Al inclusions in the steel and degrades the formability, so the upper limit was made 0.5%.

Ti: 0.005-0.05%
Ti, like Nb described later, has the effect of delaying recrystallization, and contributes to the improvement of the in-plane anisotropy of the steel sheet through this action. In addition to preferentially bonding with N in steel, there is an action of precipitating and fixing C, which contributes to aging resistance and contributes to improvement of formability. Moreover, since it also has an effect as a deoxidizing agent like Al, it can also be added as a deoxidizing agent. From such a viewpoint, Ti needs to be contained in an amount of 0.005% or more. However, since excessive addition tends to have an r value of r _D ≦ r _C , the upper limit was made 0.05%.

Nb: 0.01-0.3%
Nb, like Ti described above, has a recrystallization delay effect, promotes ferrite transformation after sufficiently accumulating strain in the non-recrystallized austenite state during finish rolling of hot rolling, and has a fine hot rolled sheet structure. It is a useful element when trying to make it easier. Nb affects the amount of accumulated strain and increases the amount of accumulated strain by delaying recrystallization between the rolling stands. Although details on this point are not yet clear, experimental experience suggests that the amount of strain accumulated in the unrecrystallized austenite has some influence on the in-plane anisotropy of the r value of the final characteristics. Is done. Refinement of the hot-rolled sheet structure leads to further refinement of the structure after annealing, and Nb tends to segregate along the grain boundary, in order to suppress the precipitation of coarse cementite at the grain boundary, These effects have the effect of strengthening the grain boundaries and thus improving the secondary work brittleness resistance. From these viewpoints, Nb is contained in an amount of 0.01% or more. However, excessive addition not only tends to degrade the ductility, but if IF steel is used, the in-plane anisotropy tends to be positive, so the upper limit of Nb was set to 0.3%.

N: 0.01% or less If N is too much, the normal temperature aging resistance is deteriorated, a large amount of inclusions and carbonitride formation are promoted, and various moldability is adversely affected. Therefore, it is desirable to reduce it as much as possible within the range allowed by the manufacturing cost, and the upper limit is regulated to 0.01% from the balance between moldability and cost.

One or more selected from Mo: 0.01-0.5%, Cr: 0.05-0.8% and V: 0.01-0.2%
Mo and Cr are useful elements that have the effect of delaying the critical cooling rate at which a martensite phase can be obtained, similar to Mn, when utilizing structural strengthening by a low-temperature transformation phase to increase the strength of steel materials. V is not only useful for precipitation fixing of N, but V-based carbides effectively contribute to precipitation strengthening. There is no difference in effect even if these elements are added alone or in combination. Furthermore, Mo and V, like Nb, tend to segregate along the grain boundary, and suppress coarse grainite precipitation at the hot-rolled grain boundary that hinders the increase of the r value, and are resistant to secondary work brittleness. There is an effect to improve. However, in any case, when the addition amount is less than the lower limit, the effect of addition is poor. On the other hand, excessive addition leads to an increase in the steel material cost and also tends to make the Δr value of the steel material positive. It was supposed to be included.

M = [% Mn] +3 [% Mo] +1.3 [% Cr] +0.5 [% V] ≧ 2
Among the above-described constituent components, Mn, Mo, Cr and V need to be contained in a range where the value of M defined by the above formula satisfies 2 or more.
This is because if the M value is less than 2, martensite is not formed in the cooling process after annealing, and sufficient strength cannot be obtained. The upper limit of the M value is not particularly limited, but if it is too large, not only will the alloy addition cost increase, but also the rolling load will increase, so the M value is preferably about 3.5 or less.

Next, the reason why the steel structure is limited as described above in the present invention will be described.
In the present invention, in order to increase the strength of TS: 440 MPa or more, preferably 500 MPa or more, the structure strengthening is effectively used without relying on the solid solution strengthening. For that purpose, it is important that the steel structure is a mixed structure containing a martensite phase having a ferrite phase as a main phase and a volume fraction of 15% or less with respect to the entire structure.
As phases other than the martensite phase, bainite, retained austenite, pearlite, and the like can be considered, but even if these phases are present, the ratio of the phases does not exceed the volume fraction of the martensite phase. There is no problem.

  From the viewpoint of increasing the strength, it is desirable that the martensite phase is 1% or more, preferably 3% or more, as a fraction of the entire structure. However, if this fraction exceeds 15%, a low yield ratio type (YR ≦ 0.6) is obtained, and the soft ferrite phase and the hard second phase (here, martensite, bainite, retained austenite, pearlite). In order to adversely affect plastic deformation at the interface and reduce the r value, the upper limit was made 15%.

  When trying to increase the strength of TS: 500 MPa or more by solid solution strengthening as before, the amount of alloying elements inevitably increases, the surface appearance is impaired, the plating property is deteriorated, and above all, the r value is to degrade. In this respect, if the structure strengthening is used as in the present invention, the amount of alloy addition can be suppressed, and the surface properties of the steel sheet can be kept good.

rD ≧ 1.2 and −1.0 ≦ Δr value ≦ −0.3
Here, Δr value = (r _L −2 × r _D + r _C ) / 2
Where r _L : r value in the rolling direction
r _D : r value in the direction of 45 ° with the rolling direction
r _C : r value in the direction perpendicular to the rolling direction According to the study by the inventors, as a result of studying the rectangular tube drawing formability of a high strength steel plate of TS ≧ 500 MPa, the average r value is lower than that of a conventional mild steel plate. However, it has been found that if the r value (r _D ) in the D direction of the steel sheet, which hits the corner portion of the square tube, is 1.2 or more, cracking of the corner portion of the square tube can be avoided.
As for the in-plane anisotropy, when the Δr value calculated by the above equation is less than −1, the inflow balance in each direction is reversed and the formability is deteriorated. Therefore, the lower limit of the Δr value is It was set to -1. On the other hand, when the Δr value is larger than −0.3, the inflow of the material at the corner portion of the rectangular tube is relatively deteriorated and press cracking is likely to occur. Therefore, the upper limit of the Δr value is set to −0.3.

Next, the manufacturing method of this invention is demonstrated.
The high-strength cold-rolled steel sheet of the present invention is a hot-rolled steel sheet obtained by hot rolling a steel slab adjusted to a predetermined component composition so that the finish rolling exit temperature is 910 to 800 ° C. A step of winding the rolled sheet at a temperature of 550 to 700 ° C, a step of cold rolling the rolled hot-rolled steel sheet at a rolling reduction of 55 to 80% to obtain a cold-rolled steel sheet, and at least 300 cold-rolled steel sheets In the temperature range of ˜700 ° C., the average temperature increase rate is 10 ° C./s or higher, and the step of recrystallization annealing is performed in a heat cycle in which the temperature is kept at 800 ° C. to 900 ° C. for 120 s or more.
Hereinafter, it demonstrates in order of each manufacturing process.

  In the present invention, first, a steel slab adjusted to the above-described preferred component composition is produced. As a method for producing a slab, a continuous casting method is desirable for preventing macro segregation of components, but it can also be produced by an ingot-making method or a thin slab casting method.

Next, in order to hot-roll steel slabs, in addition to the conventional method of cooling steel slabs to room temperature and then reheating and rolling them, hot rolling is performed immediately after continuous casting, or the temperature is not cooled to room temperature. An energy saving process such as a method of rolling a piece as it is after charging it into a heating furnace can be applied without any problem.
Here, the slab heating temperature is preferably set to 1000 ° C. or higher in order to avoid an increase in rolling load during hot rolling and the risk of troubles associated therewith, while scale loss accompanying an increase in oxidized weight. In order to prevent this increase, it is preferable that the temperature be 1300 ° C. or lower.
The slab after heating is made into a sheet bar by rough rolling. The conditions for rough rolling are not particularly specified, and may be performed according to a conventional method. When the heating temperature of the slab is lowered, it is preferable to heat the sheet bar using a sheet bar heater from the viewpoint of preventing troubles during rolling.

  The sheet bar is a hot-rolled sheet by finish rolling. At this time, the finishing temperature (FT) needs to be set to 800 ° C. or more and 910 ° C. or less in order not to increase the load during rolling and to accumulate strain due to rolling in the non-recrystallized austenite state. Further, in order to reduce the rolling load and make the shape and characteristics of the steel plate uniform, a part or all of the finishing rolling can be lubricated. The friction coefficient during lubrication rolling is preferably about 0.10 to 0.25. Furthermore, from the viewpoint of operational stability of hot rolling, it is advantageous to apply a continuous rolling process in which sheet bars are joined and continuously rolled.

  The coiling temperature (CT) after hot rolling needs to be 550 to 700 ° C. in order to precipitate an appropriately sized NbC (which may form a composite carbide with Mo or V). When CT exceeds 700 ° C, precipitates tend to become coarse, and the crystal grains of the hot-rolled sheet become coarse. As a result, the structure after cold-rolling annealing also tends to become coarse, resulting in reduced strength and deterioration of surface properties. May be incurred. On the other hand, if the CT is less than 550 ° C., precipitation of NbC hardly occurs and it is difficult to ensure deep drawability, and since relatively fine carbides precipitate at low temperature, the hot-rolled sheet strength is increased and cold rolling is performed. There is a disadvantage that the rolling load at the time increases. The preferred CT is in the range of 580-680 ° C.

  The hot-rolled sheet after winding is made into a cold-rolled sheet by cold rolling after removing the scale by pickling. The rolling reduction during cold rolling needs to be at least 55% from the viewpoint of improving deep drawability. From the viewpoint of the anisotropy of the r value in the annealing plate characteristics, it is preferably 60% or more. On the other hand, when the rolling reduction exceeds 80%, there is no appreciable increase in the r value in the D direction, and there is a concern of increasing the rolling load in the steel of the present invention to which Nb, which tends to increase the deformation resistance, is added. For this reason, the rolling reduction in cold rolling is limited to the range of 55 to 80%. In the present invention, the upper limit of the rolling reduction is set to 80%. However, depending on the equipment capacity, there is no particular problem even when rolling at a rolling reduction higher than this.

The cold-rolled plate is heated at an average temperature increase rate of 10 ° C./s or more in a temperature range of at least 300 to 700 ° C., and is subjected to recrystallization annealing at 800 to 900 ° C. The temperature increase rate in the temperature range from 300 ° C. to 700 ° C. is important from the viewpoint of the anisotropy of the r value, which is one of the points of the present invention, and the average temperature increase rate in this temperature range is 10 ° C./s. If it is less than 1, the desired anisotropy of the r value cannot be obtained. Therefore, in the present invention, box annealing (batch annealing) is not preferable, and it is essential to use a continuous annealing line.
Here, the rate of temperature increase in the temperature range from 300 ° C to 700 ° C is thought to affect the progress of recrystallization and transformation, and as a result, it is finer and smaller in amount than conventional DP steel. It is considered that the martensite phase can be formed uniformly and the shape is improved. In particular, it is speculated that the recrystallization delay due to the addition of Nb had some effect on the α-γ transformation in the subsequent two-phase annealing, but the details are unknown.
The average rate of temperature rise is preferably 30 ° C./s or less because the load on the equipment increases when it exceeds 30 ° C./s. More preferably, it is 25 ° C./s or less.

The annealing temperature is set to 800 ° C. or higher in order to set the annealing temperature to the recrystallization temperature or higher and to the (α + γ) two-phase region temperature or higher at which a structure including a ferrite phase and a martensite phase is obtained after cooling. However, when the temperature exceeds 900 ° C., the recrystallized grains are remarkably coarsened, and not only the texture is randomized and the r value is deteriorated, but also mechanical properties and surface properties tend to be remarkably deteriorated.
Although not particularly limited, in order to sufficiently develop the recrystallized grains and improve the deep drawability and hole expansibility, a temperature increase rate is preferable in the temperature range from 700 ° C. to the annealing temperature. Is preferably a slow heating of 5 ° C./s or less.
Furthermore, in order to sufficiently recrystallize and to sufficiently promote phase separation and concentration of solute C to austenite phase in (α + γ) two-phase castle, it stays in the temperature range of 800-900 ° C for 120 seconds or more. It is important to let However, if the residence time is too long, the crystal grains become coarse and various properties such as strength and surface properties tend to deteriorate, and from the viewpoint of productivity, the upper limit is preferably about 300 seconds.

  As for the cooling rate after annealing, it is desirable that the temperature range of 800 to 550 ° C. is cooled at an average cooling rate of 5 ° C./s or more from the viewpoint of martensite phase formation, but in the present invention, the M value is regulated. Since the critical cooling rate at which the martensite phase is obtained is lowered, the DP structure can be sufficiently obtained if the cooling rate is that of a general continuous annealing line facility. In addition, there is no problem to let the overaging band attached to the existing equipment pass through, but it is desirable that the overaging band should be 400 ° C or less as the temperature at which the martensite phase is not tempered. .

  Furthermore, a plated layer can be formed on the cooled steel sheet by electroplating or hot dipping. In addition, when performing hot dipping or alloying hot dipping online, the temperature range from 800 ° C. to just before dipping in the plating bath is average cooling rate: 5 ° C./s or more as described above from the viewpoint of martensite phase formation. It is desirable to cool with. At this time, the steel plate temperature immediately before immersion in the plating bath is approximately 480 to 520 ° C., the plating bath temperature is approximately 440 to 480 ° C., and the alloying temperature is approximately 500 to 600 ° C.

  In addition, the cold-rolled steel sheet or the plated steel sheet thus manufactured can be subjected to temper rolling or leveler processing for the purposes of shape correction and surface roughness adjustment. The total elongation of temper rolling or leveler processing is preferably about 0.2 to 15%. This is because if the elongation is less than 0.2%, the purpose of shape correction and surface roughness adjustment may not be achieved, while if it exceeds 15%, the ductility may be significantly reduced.

A steel slab having the component composition shown in Table 1 was heated to 1250 ° C. and then finish-rolled at the temperature shown in Table 2 and wound around a coil to obtain a hot rolled sheet having a thickness of 4 mm. Next, after pickling, after cold rolling at the rolling reduction shown in Table 2, the temperature range of 300-700 ° C. is heated at various heating rates and subjected to continuous annealing at the annealing temperature and residence time shown in Table 2. A cold-rolled annealed plate was used. The cold-rolled annealed sheet was then subjected to temper rolling with an elongation of 0.5%.
Table 3 shows the results of examining the mechanical properties, the second phase fraction, the r value, and the rectangular tube drawing formability of each cold-rolled annealed sheet thus obtained by a tensile test.

Each characteristic was investigated as follows.
(1) Mechanical properties (tensile test)
The mechanical properties by the tensile test were investigated as follows. That is, a No. 5 test piece defined in JIS Z 2201 was cut out from each cold-rolled annealed plate in a 90 ° direction (C direction) with respect to the rolling direction, and a tensile test was performed at room temperature. The tensile speed is constant at the crosshead speed: 10 mm / min. In accordance with JIS Z 2241, tensile strength TS, yield strength YS (0.2% flow stress), yield ratio YR (YS / TS) and elongation U.El (Uniform elongation) was measured.

(2) Second phase fraction A specimen is taken from each cold-rolled annealed plate, and a microscopic structure is imaged using an optical microscope or a scanning electron microscope with respect to a plate thickness section (L section) parallel to the rolling direction. Then, this was analyzed with an image analyzer, and the area ratio of the ferrite phase as the main phase, the type and area ratio of the second phase were obtained.

(3) r value Specified in JIS Z 2201 from each cold-rolled annealed sheet from the L direction (rolling direction), D direction (direction that forms 45 ° with the rolling direction) and C direction (direction that forms 90 ° with the rolling direction). No. 5 test piece was cut out, and the respective r values (r _L , r _D , r _C ) were obtained in accordance with the provisions of JIS Z 2254, and the average r value and in-plane anisotropy (Δr value) according to the following formula ) Was calculated. The applied plastic strain was 10% within the range of uniform elongation as specified.
Average r value = (r _L + 2 × r _D + r _C ) / 4
Δr value = (r _L −2 × r _D + r _C ) / 2

(4) Square tube drawability Each cold-rolled annealed sheet is blanked at a blank size of 200 x 200 mm, wrinkle holding force: 45 tons, punch size: 100 x 100 mm, punch (edge) Rp: 5 mm, punch (corner) ) Rc: 15 mm, dice (edge) Rd: 10 mm, dice (corner) Rc: 16.5 mm. What was shape | molded by this test was evaluated by (circle), and what was not able to be evaluated by x.

  As shown in Table 3, all the inventive examples that satisfy the requirements of the present invention have excellent square tube drawability in a high-strength cold-rolled steel sheet having a TS of 440 MPa or more.

In addition, the shape freezing property of some cold-rolled annealed plates was also investigated.
That is, a hat forming test was conducted in order to investigate the shape freezing property using various cold rolled steel sheets of No. 6, 8, 18 and 20 in Table 3. As shown in FIG. 1, the hat forming test was performed under the conditions of blank size: 80 × 340 mm, crease pressing force: 10 ton, die shoulder R: 2 mm, punch shoulder: 5 mm, stroke: 100 mm.
The obtained results are shown in FIG.
As shown in the figure, it can be seen that both the opening amount and the warpage amount are improved in the invention material having the yield ratio YR exceeding 0.6 as compared with the conventional composite structure steel plate (No. 20) having the same strength level.

It is the figure which showed the molding conditions of a hat molding test. It is the figure which showed the relationship between YR after a hat forming test, the amount of opening, and the amount of board curvature.

Claims (5)

% By mass
C: 0.010 to 0.050%
Si: 1.0% or less,
Mn: 1.5-3.0%
P: 0.1% or less,
S: 0.01% or less,
Al: 0.005-0.5%
Ti: 0.005-0.05%
Nb: 0.01 to 0.3% and N: 0.01% or less, and
Mo: 0.01-0.5%
Cr: 0.05-0.8% and V: 0.01-0.2%
One or more selected from
M = [% Mn] +3 [% Mo] +1.3 [% Cr] +0.5 [% V] ≧ 2
The balance is the composition of Fe and inevitable impurities, the ferrite phase is the main phase, and has a steel structure consisting of a mixed structure containing a martensite phase with a volume fraction of 15% or less, The r value (r _D ) in the direction forming 45 ° with the rolling direction is 1.2 or more, and the in-plane anisotropy (Δr value) represented by the following formula satisfies the range of −1.0 ≦ Δr value ≦ −0.3. A high-strength cold-rolled steel sheet with excellent square tube formability and shape freezing characteristics.
Record
Δr value = (r _L −2 × r _D + r _C ) / 2
Where r _L : r value in the rolling direction
r _D : r value in the direction of 45 ° with the rolling direction
r _C : r value in the direction of 90 ° with the rolling direction   The high strength cold-rolled steel sheet excellent in square tube drawing formability and shape freezing property according to claim 1, wherein the yield ratio exceeds 0.6.   The high-strength cold-rolled steel sheet excellent in square tube drawing formability and shape freezing property according to claim 1 or 2, wherein the steel sheet surface has a plating layer. % By mass
C: 0.010 to 0.050%
Si: 1.0% or less,
Mn: 1.5-3.0%
P: 0.1% or less,
S: 0.01% or less,
Al: 0.005-0.5%
Ti: 0.005-0.05%
Nb: 0.01 to 0.3% and N: 0.01% or less, and
Mo: 0.01-0.5%
Cr: 0.05-0.8% and V: 0.01-0.2%
One or more selected from
M = [% Mn] +3 [% Mo] +1.3 [% Cr] +0.5 [% V] ≧ 2
The steel slab that contains Fe and the inevitable impurities composition is hot rolled at a finish rolling temperature of 910 to 800 ° C, and then wound into a coil at a temperature of 550 to 700 ° C. Reduction ratio: After cold rolling at 55-80%, continuous annealing in a heat cycle in which at least 300-700 ° C temperature range is heated at a rate of 10 ° C / s or more and stays in the 800-900 ° C temperature range for 120s or more. A method for producing a high-strength cold-rolled steel sheet excellent in square tube drawing formability and shape freezeability.   An automotive part having an excellent product shape obtained by subjecting the high-strength cold-rolled steel sheet according to any one of claims 1 to 3 to square tube drawing.

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