JP4848958B2 - High-strength steel sheet excellent in deep drawability and secondary work brittleness resistance and method for producing the same - Google Patents

Description

  The present invention is useful for use in automobile steel sheets and the like, and has a high tensile strength (TS) of 440 MPa or more and a high r value (r value ≧ 1.3), deep drawability and secondary resistance. The present invention relates to a high-strength steel sheet excellent in work brittleness and a method for producing the same.

In recent years, in order to regulate CO ₂ emissions from the viewpoint of global environmental conservation, improvement in fuel efficiency of automobiles has been demanded. In addition, in order to ensure the safety of passengers in the event of a collision, improvement in safety centered on the collision characteristics of the automobile body is also required. As described above, the weight reduction of the automobile body and the reinforcement of the automobile body are being actively promoted.

In order to satisfy the weight reduction and strengthening of automobile bodies at the same time, it is said that it is effective to reduce the thickness by increasing the strength of the component materials and reducing the plate thickness within the range where there is no problem with rigidity. High-strength steel plates (also called high-tensile steel plates) are actively used in automobile parts.
This weight reduction effect increases as the strength of the steel sheet used increases. For example, in the automotive industry, the trend of using steel sheets with a tensile strength (TS) of 440 MPa or more as the panel material for inner and outer panels, for example. It is in.

  On the other hand, since many automotive parts made of steel plates are formed by press working, the steel plates for automobiles are required to have excellent press formability. However, high-strength steel sheets are significantly deteriorated in formability, especially deep drawability, compared to ordinary mild steel sheets. Therefore, TS 440 MPa is a good deep draw as a challenge in reducing the weight of automobiles. There is an increasing demand for steel sheets having formability. In this case, since it is required to maintain the formability at the conventional strength, a high-strength steel sheet having an average r value ≧ 1.3 is required with a Rankford value (hereinafter referred to as r value) which is an evaluation index of deep drawability. ing.

As a means to increase strength while maintaining a high r value, use ultra-low carbon steel, add Ti and Nb in amounts to fix carbon and nitrogen dissolved in the steel, IF (Interstitial atom free) There is a technique of adding a solid solution strengthening element such as Si, Mn, P or the like to the base of the converted steel (for example, Patent Document 1).
JP-A-56-139654

This patent document 1 has a composition of C: 0.002 to 0.015%, Nb: C% x 3 to C% x 8 + 0.020%, Si: 1.2% or less, Mn: 0.04 to 0.8%, P: 0.03 to 0.10%. This is a technology relating to a high-tensile cold-rolled steel sheet having excellent formability and having non-aging properties of 35 to 45 kgf / mm ² class (340 to 440 MPa class).

  However, with such ultra-low carbon steel technology, if an attempt is made to produce a steel sheet with a tensile strength of 440 MPa or higher, the amount of alloying elements added will increase, resulting in surface appearance problems and poor plating properties. It has been found that problems such as the manifestation of secondary processing brittleness arise. Further, when a solid solution strengthening component is added in a large amount, the r value deteriorates, so that there is a problem that the level of the r value decreases as the strength increases.

Further, Patent Document 2 discloses a steel in which C, Si, Mn, P, Ti, Nb and the like are specified, which are hot-rolled, cold-rolled, annealed and cooled under predetermined conditions, and from low-temperature transformation products and ferrite. A technique for improving press formability and bake hardenability by forming a mixed structure is disclosed.
Specifically, by weight, C: 0.0005 to 0.007%, Si: 0.001 to 0.8%, Mn: 0.8 to 4.0%, P: 0.003 to 0.15%, S: 0.0010 to 0.015%, Al: 0.005 to A slab having a composition comprising at least one of 0.1%, N: 0.0003 to 0.0060%, Ti: 0.003 to 0.1%, and Nb: 0.003 to 0.1%, the balance Fe and inevitable impurities (Ar ₃ -100 ) performs finishing hot rolling at ° C. or higher, coiling at a temperature of 750 ° C. from room temperature and the cold rolling at 60% of rolling reduction, the annealing temperature in the continuous annealing Ac ₁ transformation point or more Ae ₃ The temperature range from the annealing temperature to (Ar ₁ −50 ° C.) to (Ar ₁ + 50 ° C.) is cooled at an average cooling rate of 1 ° C./s to less than 30 ° C./s, and the total volume exceeds 5%. This is a mixed structure composed of a low-temperature transformation product and ferrite. This steel sheet is characterized by excellent secondary work brittleness resistance because the hard second phase suppresses the extension of cracks.
However, this technique has problems that it is difficult to control the low temperature transformation phase and that the annealing temperature has to be increased.
JP-A-6-116651

  The present invention has been developed in view of the above-described present situation, and has both high r value and secondary work brittleness resistance without high-temperature annealing in increasing the strength of a cold-rolled steel sheet excellent in deep drawability. The object is to propose a high-strength steel sheet having good formability together with its advantageous production method.

As a result of intensive studies to solve the above problems, the inventors controlled the annealing temperature in the final annealing to a temperature lower than the recrystallization completion temperature in a state where the hot rolled sheet structure was refined. It has been found that when the processed structure is left, the processed structure effectively suppresses the extension of cracks at low temperature brittleness, the secondary work brittleness resistance is improved, and further, high strength and high r value are obtained.
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: 0.0005 to 0.04%,
Si: 0.01 to 1.0%
Mn: 0.8-3%,
P: 0.003-0.15%
Al: 0.005-0.5%
Including S: 0.015% or less and N: 0.006% or less, and
Nb: 0.04-0.1% and
Ti: 0.003-0.1%
The balance is composed of Fe and inevitable impurities, the steel structure is composed of recrystallized ferrite and processed ferrite, the volume ratio of the recrystallized ferrite is 8% or more, and the volume ratio of the processed ferrite is 5% or more, the test load in the Vickers hardness test (JIS Z 2244): the Vickers hardness ratio of the processed ferrite to the recrystallized ferrite when measured at 0.2942 N is 1.2 or more, and the processed ferrite A high-strength steel sheet excellent in deep drawability and secondary work brittleness resistance, characterized by having an average colony diameter of 15 μm or less.

2. In 1 above, the steel sheet is
A high-strength steel sheet excellent in deep drawability and secondary work brittleness resistance, characterized by containing one or more selected from Mo, Cu and Ni: 0.5% or less.

3. 3. A high-strength steel sheet excellent in deep drawability and secondary work brittleness resistance, characterized in that the steel sheet further contains B: 0.01% or less by mass%.

4). % By mass
C: 0.0005 to 0.04%,
Si: 0.01 to 1.0%
Mn: 0.8-3%,
P: 0.003-0.15%
Al: 0.005-0.5%
Including S: 0.015% or less and N: 0.006% or less, and
Nb: 0.04-0.1% and
Ti: 0.003-0.1%
After the steel slab with a balance of Fe and inevitable impurities is heated to a temperature of 1000 ° C or higher and 1300 ° C or lower, hot rolling is performed at a temperature of (Ar ₃ -50) ° C or higher and 950 ° C or lower. After cooling, start cooling within 0.5 s after the end of hot rolling, cool to 750 ° C or less with an average cooling to 750 ° C of 30 ° C / s or more, and wind up at a temperature of 550 ° C to 720 ° C , Reduction ratio: Deep drawing and secondary resistance characterized by annealing at a temperature of (recrystallization completion temperature -50 ° C) or more (recrystallization completion temperature -10 ° C) after cold rolling at 50% or more A method for producing a high-strength steel sheet having excellent work brittleness.

5). In 4 above, the steel slab is further in mass%.
A method for producing a high-strength steel sheet excellent in deep drawability and secondary work brittleness resistance, characterized by containing one or more selected from Mo, Cu and Ni: 0.5% or less.

6). 4. A method for producing a high-strength steel sheet excellent in deep drawability and secondary work brittleness resistance, characterized in that the steel slab further contains B: 0.01% or less by mass% in the above 4 or 5.

According to the present invention, it is possible to stably obtain a high-strength steel sheet having good formability and having high r value and secondary work brittleness resistance without high-temperature annealing.
In addition, the high-strength steel sheet of the present invention is useful as an original sheet of a plated steel sheet. Therefore, according to the present invention, it is possible to obtain a plated steel sheet having a good formability having both a high r value and secondary work brittleness resistance. it can.

Hereinafter, the present invention will be specifically described.
The unit of element content is “mass%”, but hereinafter, it is simply indicated by “%” unless otherwise specified.
First, the reason why the component composition of the steel slab used in the present invention, that is, the high-strength steel sheet to be obtained in the present invention is limited to the above range will be described.

C: 0.0005-0.04%
Since C is effective for increasing the strength, 0.0005% or more is contained. However, if the amount becomes too large, the deep drawability deteriorates, so the upper limit was made 0.04%. More preferably, it is 0.03% or less.

Si: 0.01-1.0%
Si promotes ferrite transformation and increases the amount of C in untransformed austenite to facilitate the formation of a composite structure of ferrite and martensite, and also has an effect of solid solution strengthening. In order to obtain these effects, Si must be contained in an amount of 0.01% or more, and more preferably 0.05% or more. On the other hand, if it contains more than 1.0%, defects caused by the scale called red scale occur during hot rolling, which not only deteriorates the surface appearance of the product plate but also hot dip galvanizing. When applying (including alloying hot dip galvanizing), the wettability of the plating is deteriorated to cause uneven plating and the plating quality is deteriorated. Therefore, the Si amount needs to be 1.0% or less. More preferably, it is 0.7% or less.

Mn: 0.8-3%
Mn contributes to increasing the r value by reducing the hot-rolled sheet structure. In addition, the annealing plate is strengthened by solid solution strengthening and fine grain strengthening, which effectively contributes to high strength. Further, Mn is an element effective in preventing hot cracking due to S. From such a viewpoint, Mn needs to be contained by 0.8% or more. More preferably, it is 1.2% or more. However, on the other hand, excessive addition degrades the r value and weldability, so 3% is made the upper limit.

P: 0.003-0.15%
P is an element useful for solid solution strengthening. However, if the content is less than 0.003%, not only the effectiveness is poor, but also the dephosphorization cost is increased in the steelmaking process. Therefore, P is contained in an amount of 0.003% or more. More preferably, it is 0.01% or more. However, if it is added excessively exceeding 0.15%, P is segregated at the grain boundaries, and the secondary work embrittlement resistance and weldability are deteriorated. Further, when the hot dip galvanized steel sheet is used, the diffusion of Fe from the steel sheet to the plated layer at the interface between the plated layer and the steel sheet is suppressed during the alloying process after the hot dip galvanizing, and the alloying processability is deteriorated. Therefore, although an alloying treatment at a high temperature is required, the obtained plating layer is not preferable because plating peeling such as powdering and chipping easily occurs. Therefore, the upper limit of the P content is set to 0.15%.

Al: 0.005-0.5%
In addition to being useful as a deoxidizing element for steel, Al has an effect of fixing solid solution N to improve normal temperature aging resistance, so 0.005% or more is contained. However, addition exceeding 0.5% not only increases the alloy cost but also induces surface defects, so 0.5% was made the upper limit. More preferably, it is 0.1% or less.

S: 0.015% or less S is an impurity and not only causes hot cracking but also exists as an inclusion in the steel and degrades various properties of the steel sheet, so it is preferable to reduce it as much as possible, but 0.015% Up to 0.015%.

N: 0.006% or less If N is too much, the normal temperature aging resistance is deteriorated and a large amount of Al or Ti is required, so it is preferable to reduce it as much as possible, but 0.006% is acceptable, so the upper limit is 0.006 %.

Nb: 0.04-0.1%
Nb has an effect of refining the hot-rolled sheet structure and precipitating and fixing C as NbC in the hot-rolled sheet, and is an element that contributes to increasing the r value. It is also an element useful for suppressing recrystallization due to the presence of NbC and solute Nb, and increasing the r value in the D direction (a direction that forms 45 ° with the rolling direction) before completion of recrystallization. From this point of view, Nb is contained in an amount of 0.04% or more. On the other hand, excessive Nb addition causes a decrease in ductility, so the upper limit was made 0.1%.
Further, in order to achieve the effect of Nb addition, the ratio of Nb content (mass%) to C content (mass%) is (Nb content / 93) / (C content / 12) is 0.2 or more. It is useful to satisfy the range. If (Nb / 93) / (C / 12) is less than 0.2, the amount of dissolved C is large, and the formation of {111} recrystallized texture effective for increasing the r value is inhibited.

Ti: 0.003-0.1%
Ti, like Nb, is an element that contributes to increasing the r value by refining the hot rolled sheet structure and precipitating and fixing C as a carbide in the hot rolled sheet. However, since the effect of refinement of the hot-rolled sheet is large in Nb, it is preferable to add Ti as appropriate to the Nb-added steel. Furthermore, Ti brings about high moldability, such as high r value, by forming S and N and a precipitate in the high temperature range of hot rolling. From such a viewpoint, Ti is contained in an amount of 0.003% or more. On the other hand, in the present invention, excessive addition of Ti reduces ductility like Nb, so the upper limit was made 0.1%.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Total of one or more selected from Mo, Cu and Ni: 0.5% or less
As with Mn, Si, and P, all of Mo, Cu, and Ni are elements that increase the strength of the annealed plate by solid solution strengthening, while having little effect on ductility, r value, and the like. In particular, Mo is also an element that has an action of precipitating and fixing C and contributes to an increase in r value. However, excessive addition not only saturates these effects, but also raises the alloy cost. Therefore, it is preferable to contain these elements in a total amount of 0.5% or less when used alone or in combination.

B: 0.01% or less B is an element having an effect of strengthening grain boundaries, and is contained as necessary. However, if the amount of B exceeds 0.01%, the above effect is saturated.

In the present invention, the balance other than the above components is Fe and inevitable impurities.
In addition, if it is in the normal steel composition range, there is no problem even if Ca, REM and the like are further contained.
Ca and REM have the effect of controlling the morphology of sulfide inclusions, thereby preventing the deterioration of various properties of the steel sheet. Such an effect is saturated when the content of one or two selected from Ca and REM exceeds 0.01% in total, and is therefore preferably made less than this.

  Other unavoidable impurities include, for example, Sb, Sn, Zn, Co, etc. The allowable ranges of these contents are Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01%, respectively. Hereinafter, Co: 0.1% or less.

Next, the manufacturing method of the high strength steel plate according to this invention is demonstrated.
In the present invention, a steel slab adjusted to the above-mentioned preferred component composition is used as a raw material, heated, hot-rolled, and immediately after completion of hot rolling, cooled to a predetermined temperature at a predetermined speed, wound up, and then cooled. A high-strength steel sheet can be manufactured by finish annealing at a relatively low temperature after the intermediate rolling.
Hereinafter, each processing condition will be described.

  In producing the steel slab, it is desirable to produce it by a continuous casting method in order to prevent macro segregation of components, but it may be produced by an ingot-making method or a thin slab casting method. In addition to the conventional method in which the steel slab is manufactured and then cooled to room temperature and then heated again, it is directly fed into a heating furnace without being cooled and placed in a heating furnace for hot rolling, or a little heat retention Energy saving processes such as direct feed rolling and direct rolling that are immediately subjected to hot rolling after performing the above can be applied without any problem.

Slab heating temperature: 1000 ° C. or higher and 1300 ° C. or lower The slab heating temperature needs to be 1000 ° C. or higher in order to secure the finishing temperature described later. However, when the temperature exceeds 1300 ° C, not only the precipitation of NbC, TiC, TiN, Mo ₂ C, etc. during the heating is insufficient, but also austenite (γ) grain growth occurs, and the cost due to an increase in heating temperature. Increases and scale loss. Therefore, the slab heating temperature was limited to a range of 1000 ° C or higher and 1300 ° C or lower.

  The slab heated under the above conditions is subjected to hot rolling consisting of rough rolling and finish rolling. Here, the slab is made into a sheet bar by rough rolling. The conditions for rough rolling need not be specified, and may be performed according to a conventional method. In addition, it goes without saying that using a so-called sheet bar heater for heating the sheet bar is an effective method from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling.

Hot rolling end temperature: (Ar ₃ −50) ° C. or higher and 950 ° C. or lower Next, the sheet bar is subjected to finish rolling to obtain a hot rolled sheet. At this time, the hot rolling finish temperature, that is, the finish rolling outlet temperature (FT), needs to be (Ar ₃ −50) ° C. or higher. If the temperature is lower than that, the hot-rolled structure becomes coarse due to rolling in the high temperature region of the ferrite region, and good deep drawability cannot be obtained after cold rolling annealing. Even at temperatures exceeding 950 ° C., the γ grains become coarse, and good deep drawability cannot be obtained after cold rolling annealing, and problems of surface properties such as scale defects occur. Therefore, the hot rolling finish temperature is limited to the range of (Ar ₃ -50) ℃ above 950 ° C. or less.

  In addition, in order to reduce the rolling load at the time of hot rolling, a part or all pass of finish rolling can also be lubricated rolling. This lubrication rolling is effective from the viewpoint of uniforming the shape of the steel sheet and homogenizing the material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.10-0.25. Furthermore, it is also preferable to use a continuous rolling process in which the adjacent sheet bars are joined to each other and continuously subjected to finish rolling. It is desirable to apply the continuous rolling process from the viewpoint of the operational stability of hot rolling.

Cooling is started within 0.5 s after the end of hot rolling, and the average cooling rate up to 750 ° C. is reduced to 750 ° C. or less with an average cooling rate of 30 ° C./s or more. The average colony diameter of the tissue must be 15 μm or less. Effective for this is cooling after rolling. That is, when the time from the end of hot rolling to the start of cooling is long, the structure recovers and the colony diameter after cold rolling annealing becomes large. Therefore, in the present invention, cooling is started within 0.5 s after the end of hot rolling.
In addition, after the hot rolling, the heat history in the temperature range of 750 ° C. or higher greatly affects the colony diameter after cold rolling annealing. If the average cooling rate in this temperature range is less than 30 ° C./s, it is not in the γ range. Since the structure rolled in the recrystallized state is recovered and transformed at a higher temperature, the colony diameter after cold rolling annealing becomes large. Therefore, in the present invention, the average cooling rate up to 750 ° C. after the start of cold rolling is set to a rate of 30 ° C./s or more, and cooling is performed to 750 ° C. or less. In addition, it is not necessary to specifically limit about the cooling rate of 750 degrees C or less, You may continue cooling at the same rate, and you may stop forced cooling.

Winding temperature: 550 ° C or higher and 720 ° C or lower The coil winding temperature (CT) is 550 ° C or higher and 720 ° C or lower. This is because this temperature range is not only suitable for precipitating Nb and Ti carbides in the hot-rolled sheet, but especially when CT exceeds the upper limit temperature, the crystal grains become coarse and the strength decreases. This is because, at the same time, an increase in r value after cold rolling annealing is hindered. Preferably it is the temperature range of 550-680 degreeC.

Cold rolling reduction ratio: 50% or more Next, after pickling the hot-rolled sheet, it is cold-rolled to obtain a cold-rolled sheet. Pickling may be performed under normal conditions.
In general, cold rolling at a high pressure ratio is effective for increasing the r value. If the reduction ratio is less than 50%, the {111} recrystallization texture does not develop, and it is difficult to obtain excellent deep drawability. Become. Therefore, in the present invention, the rolling reduction during cold rolling is limited to 50% or more. More preferably, it is 60% or more. The r value increases as the cold rolling reduction is increased in the range of up to 90%, but when it exceeds 90%, not only the effect is saturated, but also the load on the roll during rolling increases, so the upper limit is 90% is preferable.

Finish annealing temperature: (recrystallization completion temperature −50 ° C.) or more (recrystallization completion temperature −10 ° C.) or less In the present invention, in order to improve the secondary work embrittlement resistance by leaving the work structure, the recrystallization temperature or less It needs to be annealed. In order to improve the secondary work brittleness resistance, a processed structure (processed ferrite) having a hardness ratio to recrystallized ferrite of 1.2 or more is required in order to suppress the extension of cracks. In this sense, the finish annealing temperature must be (recrystallization completion temperature −10 ° C.) or lower. However, if the annealing temperature is too low, the ductility is remarkably lowered, so the lower limit of the annealing temperature is (recrystallization completion temperature −50 ° C.).

  The cold-rolled annealed plate obtained as described above, and further, the plated steel plate subjected to the plating treatment can be subjected to temper rolling and leveler processing for the purpose of shape correction, surface roughness adjustment and the like. The temper rolling and leveler processing are preferably performed in the range of 0.2 to 15% in terms of elongation. If it is less than 0.2%, the intended purpose of shape correction and roughness adjustment cannot be achieved. On the other hand, if it exceeds 15%, the ductility is significantly reduced. In addition, although the processing form differs between temper rolling and leveler processing, it has been confirmed that there is no significant difference between the two. These temper rolling and leveler processing are effective even after the plating treatment.

The cold-rolled annealed plate obtained as described above needs to satisfy the following requirements in terms of its structure.
When the volume ratio of recrystallized ferrite is 8% or higher, the volume ratio of processed ferrite is 5% or higher, and the test load in the Vickers hardness test (JIS Z 2244) is 0.2942N. The Vickers hardness ratio of the processed ferrite to the recrystallized ferrite is 1.2 or more In order to suppress crack extension, it is necessary to leave a processed structure (processed ferrite) having a hardness ratio of 1.2 or more to the recrystallized ferrite. If this hardness ratio is less than 1.2, the effect of suppressing crack extension cannot be obtained. This hardness is measured by a micro Vickers hardness test with a test load of 0.2942 N (30 gf), and the hardness of the recrystallized ferrite and the processed ferrite is measured and determined from the ratio.
In order to suppress crack extension, a certain degree of processed structure must be dispersed. In that sense, the processed structure (processed ferrite) needs to have a volume ratio of 5% or more.

  Further, the ratio of recrystallized ferrite in the steel structure needs to be 8% or more by volume ratio. This is because if the ratio of the recrystallized ferrite is less than 8%, it is difficult not only to secure the r value ≧ 1.3, but also when the ferrite diameter is large, the extension of cracks cannot be suppressed, and the brittle transition temperature. This is because there is a problem of rising.

Average colony diameter of processed ferrite: 15 μm or less Even if the volume ratio of the above processed structure (processed ferrite) is 5% or more, if the structure is coarse, crack extension cannot be effectively suppressed. Therefore, such a processed structure needs to be made fine to some extent.
Here, since the processed ferrite forms a colony having a somewhat similar orientation, the size of the colony is defined in the present invention, and the average value is limited to 15 μm or less.

Here, the recrystallized ferrite, the processed ferrite, and the colony diameter of the processed ferrite were obtained as follows.
That is, the cross-sectional structure is measured by EBSD (Electron Back-Scatter Diffraction Pattern), and the recrystallized ferrite has a crystal orientation dispersion within 0.1 ° in the region surrounded by grain boundaries with an orientation difference of 15 ° or more. In the region surrounded by the grain boundaries having an orientation difference of 15 ° or more, the one having a crystal orientation dispersion of more than 0.1 ° was defined as processed ferrite. In this case, the region of the processed ferrite surrounded by the grain boundary having a misorientation of 15 ° was defined as a processed ferrite colony.
Moreover, the ratio (area ratio) of the processed ferrite in the entire steel structure was determined, and the volume ratio of the processed ferrite was determined, and the balance was determined as the volume ratio of the recrystallized ferrite. Furthermore, the colony diameter of the processed ferrite was evaluated based on the average value by calculating the equivalent circle diameter based on the area of each colony obtained as described above and setting it as the colony diameter.

The present invention has a steel structure composed of recrystallized ferrite and processed ferrite as described above. In addition to the recrystallized ferrite and processed ferrite described above, the presence of a structure such as martensite or bainite reduces ductility, so it is desirable to reduce these structures as much as possible. It is preferable to suppress to 2% or less.
In the steel of the present invention, carbides such as carbides of Nb and Ti or cementite are present and may be observed in the steel structure observation, but the abundance is small in the composition range of the present invention. Therefore, in determining the steel structure in the present invention, when these carbides are unavoidably observed, they are excluded from the steel structure, and the steel structure is determined.

In accordance with the present invention, the component composition is adjusted to the above range, and the steel structure is controlled to the above-described form, so that both the high r value and the secondary work brittleness resistance expected in the present invention are excellent. High strength steel sheet can be obtained. The reason is not necessarily clear, but the inventors consider as follows.
Usually, when the amount of C decreases, the grain boundary becomes clean, and P and the like segregate to lower the grain boundary strength. At a temperature lower than the recrystallization temperature, the processed structure is dispersed and moderately suppresses cracking when brittle. However, for that purpose, the processed structure needs to be high strength and fine. In this regard, by controlling the cooling conditions after hot rolling, it is possible to reduce the size of the processed structure colony (a group of grains having similar crystal orientations) in the pre-recrystallization stage.
Further, in a normal solid solution strengthened high-strength steel, the r value before recrystallization is usually low, but even before recrystallization is completed by refining the hot-rolled sheet and adjusting the recrystallization behavior. The value, particularly the r value in the D direction, which forms 45 ° with the rolling direction, was successfully increased. For this, the amount of Nb added is important. Nb addition is due to the refinement of hot-rolled sheet by strengthening rolling in the non-recrystallized γ region at the time of hot rolling and the effect of suppressing recrystallization by NbC and solute Nb. This is thought to affect the recrystallization behavior.

Next, examples of the present invention will be described.
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. After heating these steel slabs to 1250 ° C., they were made into sheet bars by rough rolling, and then subjected to finish rolling under the conditions shown in Table 2 to obtain hot rolled sheets. These hot-rolled sheets were pickled and then cold-rolled with a reduction ratio of 65% to obtain cold-rolled sheets with a sheet thickness of 1.4 mm. Subsequently, these cold-rolled sheets were subjected to continuous annealing in the continuous annealing line under the conditions shown in Table 2. Moreover, about some cold rolled sheets, the continuous annealing and the hot dip galvanization process were performed in the continuous hot dip galvanizing line. Subsequently, the obtained cold-rolled annealed sheet and hot-dip galvanized steel sheet were subjected to temper rolling with an elongation of 0.5%.
In the hot dip galvanizing treatment, the steel plate was dipped in a hot dip galvanizing bath, the steel plate was pulled up, the basis weight was adjusted by gas wiping, and an alloying treatment was performed. The conditions for the hot dip galvanizing treatment are as follows. The alloying treatment conditions were such that the Fe content in the alloyed hot-dip galvanized layer was 10.5%.
・ Plate temperature: 475 ℃
・ Plating bath: 0.13% Al-Zn
・ Bath temperature: 475 ℃
・ Immersion time: 3s
・ Weight per unit: 45 g / m ² (per side)

Further, the Ar ₃ transformation point and the recrystallization completion temperature were determined as follows.
Ar ₃ transformation point Measured with a machining formaster. That is, each steel having the component composition shown in Table 1 was heated to 1200 ° C., then processed 30% at 950 ° C., and measured for the transformation point when cooled at 15 ° C./s.
Recrystallization completion temperature The heating temperature was changed in a fluidized bed heat treatment furnace, holding time: after 10 seconds, cooling was performed, the processed ferrite ratio was measured, and the temperature at which the processed ferrite disappeared was defined as the recrystallization completion temperature. The heating temperature was changed from 600 ° C. at 20 ° C. intervals.
The microstructure, tensile properties and r value of the cold-rolled annealed sheet thus obtained were measured, and the secondary work brittleness resistance was investigated. In addition, about the hot dip galvanized material (No. 2 of Table 2), the plating processability was also evaluated.
The obtained results are also shown in Table 2.

In addition, the investigation method of the microscopic structure and each characteristic is as follows.
(1) Microstructure of cold-rolled annealed plates Specimens were collected from each cold-rolled annealed plate, and the thickness cross section (L cross section) parallel to the rolling direction was measured using an optical microscope or scanning electron microscope. The microscopic structure was imaged at a magnification of 2, and the type of phase was observed. As described above, the area ratio of hard ferrite (processed ferrite) and the average colony diameter of hard ferrite were obtained from an image of 400 times by EBSD.

(2) Hardness ratio of processed ferrite to recrystallized ferrite Using the Vickers hardness test method (JIS Z 2244), the hardness of recrystallized ferrite and processed ferrite was measured and obtained from the ratio.
The load in the Vickers hardness test was 0.2942N. Except when there was an indentation at the grain boundary, 30 or more of each of the recrystallized ferrite and the processed ferrite were measured and evaluated by the ratio of the average values.

(3) Tensile properties JIS No. 5 tensile test specimens were taken from each of the obtained cold-rolled annealed plates in the 90 ° direction (C direction) with respect to the rolling direction, and the crosshead speed was 10 mm in accordance with the provisions of JIS Z 2241. A tensile test was conducted at / min to determine tensile strength (TS) and elongation (El).

(4) r value JIS No. 5 tensile specimen from the rolling direction (L direction), 45 ° direction (D direction) with respect to the rolling direction, and 90 ° direction (C direction) with respect to the rolling direction. Were collected. Measure the width strain and plate thickness strain of each specimen when 10% uniaxial tensile strain was applied to these specimens, and use these measurements to determine the average r value in accordance with JIS Z 2254 regulations. (Average plastic strain ratio) was calculated from the following formula, and this was used as the average r value.
Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4
R ₀ , r _45, and r ₉₀ are plastic strain ratios obtained by measuring test pieces in the 0 °, 45 °, and 90 ° directions, respectively, with respect to the rolling direction of the plate surface.

(5) Secondary processing brittleness resistance
After blanking to 65Φ, a conical cup was made using 33Φ steel balls. Ear height (cup height from the bottom of the cup): A test cup was prepared by cutting the ear at a position of 27 mm. After cooling to a predetermined temperature, the cup was placed sideways, a 5 kg weight was dropped from the height of 80 cm in the vicinity of the ear cut part on the cup, and the transition temperature was determined by the presence or absence of cracks. As a criterion for judgment, three tests were conducted, and the lowest temperature at which none of the three cracks was determined as the brittle transition temperature.

(6) Plating property The surface of the obtained hot-dip galvanized steel sheet was visually observed to determine the presence or absence of non-plating defects, and the plating property was evaluated. In addition, the evaluation is ◯ when there is no non-plating defect (good plating property), △ when some non-plating defect occurs (slightly good plating property), and many non-plating defects occur (bad plating property) ) Was marked with x.

As is apparent from Table 2, all of the inventive examples obtained according to the present invention have a TS of 440 MPa or more, an r value of 1.3 or more and excellent deep drawability, and a brittle transition temperature of −45 ° C. or less. And the secondary work brittleness resistance was also excellent.
On the other hand, the comparative example manufactured on the conditions which remove | deviate from the range of this invention was a steel plate in which either r value or ductility fell with respect to intensity | strength.

According to the present invention, it is possible to stably and inexpensively produce a high-strength steel sheet having TS: 440 MPa or more, excellent secondary work brittleness resistance, and an r value of 1.3 or more and excellent deep drawability. There are remarkable effects in the industry.
For example, when the high-strength steel sheet of the present invention is applied to automobile parts, it is possible to increase the strength of parts that have been difficult to press-form so far, and it is possible to sufficiently contribute to collision safety and weight reduction of an automobile body. is there. Moreover, it is applicable not only to automobile parts but also to household appliance parts and pipe materials.

Claims (6)

% By mass
C: 0.0005 to 0.04%,
Si: 0.01 to 1.0%
Mn: 0.8-3%,
P: 0.003-0.15%
Al: 0.005-0.5%
Including S: 0.015% or less and N: 0.006% or less, and
Nb: 0.04-0.1% and
Ti: 0.003-0.1%
The balance is composed of Fe and inevitable impurities, the steel structure is composed of recrystallized ferrite and processed ferrite, the volume ratio of the recrystallized ferrite is 8% or more, and the volume ratio of the processed ferrite is 5% or more, the test load in the Vickers hardness test (JIS Z 2244): the Vickers hardness ratio of the processed ferrite to the recrystallized ferrite when measured at 0.2942 N is 1.2 or more, and the processed ferrite A high-strength steel sheet excellent in deep drawability and secondary work brittleness resistance, characterized by having an average colony diameter of 15 μm or less. The steel sheet according to claim 1, wherein the steel sheet is further in mass%.
A high-strength steel sheet excellent in deep drawability and secondary work brittleness resistance, characterized by containing one or more selected from Mo, Cu and Ni: 0.5% or less. The high-strength steel sheet having excellent deep drawability and secondary work brittleness resistance according to claim 1 or 2, wherein the steel sheet further contains B: 0.01% or less by mass%. % By mass
C: 0.0005 to 0.04%,
Si: 0.01 to 1.0%
Mn: 0.8-3%,
P: 0.003-0.15%
Al: 0.005-0.5%
Including S: 0.015% or less and N: 0.006% or less, and
Nb: 0.04-0.1% and
Ti: 0.003-0.1%
After the steel slab with a balance of Fe and inevitable impurities is heated to a temperature of 1000 ° C or higher and 1300 ° C or lower, hot rolling is performed at a temperature of (Ar ₃ -50) ° C or higher and 950 ° C or lower. After cooling, start cooling within 0.5 s after the end of hot rolling, cool to 750 ° C or less with an average cooling to 750 ° C of 30 ° C / s or more, and wind up at a temperature of 550 ° C to 720 ° C , Reduction ratio: Deep drawing and secondary resistance characterized by annealing at a temperature of (recrystallization completion temperature -50 ° C) or more (recrystallization completion temperature -10 ° C) after cold rolling at 50% or more A method for producing a high-strength steel sheet having excellent work brittleness. The steel slab according to claim 4, wherein the steel slab is further in mass%.
A method for producing a high-strength steel sheet excellent in deep drawability and secondary work brittleness resistance, characterized by containing one or more selected from Mo, Cu and Ni: 0.5% or less. 6. The method for producing a high-strength steel sheet excellent in deep drawability and secondary work brittleness resistance according to claim 4, wherein the steel slab further contains B: 0.01% or less by mass%.