TWI475114B - High strength cold rolled steel sheet excellent in workability and impact resistance and a method of manufacturing the same - Google Patents

Description

High-strength cold-rolled steel sheet excellent in workability and impact resistance and method for producing same

The present invention relates to a high-strength cold-rolled steel sheet excellent in formability for a frame member for a vehicle industry and a chassis member, and a method for producing the same.

In recent years, improving the fuel consumption rate of automobiles has become an important issue based on the viewpoint of global environmental protection. Therefore, the vehicle body material is increased in strength, and the thickness of the vehicle body is reduced, and the development of the weight of the vehicle body itself is made active. However, the high strength of the steel sheet causes deterioration in ductility (that is, the formability is deteriorated), and therefore it is desired to develop a material having both high strength and high workability.

In response to such a demand, various composite structure cold-rolled steel sheets such as ferrite iron, 麻田散铁 duplex steel (hereinafter referred to as DP steel), and TRIP steel using metamorphism-induced plasticity of residual Worth iron have been developed.

For example, Patent Document 1 discloses a method for producing a high-strength steel sheet excellent in workability, in which a large amount of Si is added to secure residual Worstian iron to achieve high ductility.

However, these DP steels and TRIP steels have excellent elongation properties but have a problem of poor hole expandability. The hole expandability is an index of workability when the machined hole portion is enlarged to perform flange forming, and the elongation characteristics are important characteristics required for a high-strength steel sheet.

As a method for producing a cold-rolled steel sheet having excellent flange formability, the technique disclosed in Patent Document 2 is a composite structure in which quenching-tempering is performed after annealing soaking to form ferrite iron and tempered granita iron. Improve hole expandability. However, although this technique can obtain high hole expandability, there is a problem that elongation is deteriorated.

As such, according to the conventional technique, a cold-rolled steel sheet having excellent elongation characteristics and flange formability cannot be obtained.

[Patent Document 1] Japanese Patent Laid-Open No. 2-101117

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2004-256872

The present invention has been developed in view of the above problems, and an object thereof is to provide a high-strength cold-rolled steel sheet having excellent elongation characteristics and flange formability and a method for producing the same.

In order to achieve the above-described problems, the present inventors have produced a high-strength cold-rolled steel sheet having excellent elongation properties and flange formability, and has been intensively studied from the viewpoint of steel sheet composition and microstructure. As a result, it has been found that by appropriately adjusting the alloying elements, it is forcibly cooled to a temperature range of 150 to 350 ° C during cooling from the soaking temperature during the annealing, and then reheated, thereby obtaining the ferrite iron 20 in terms of area ratio. More than 100% of the tempered granulated iron is 10~60%, and it contains 3~15% of the residual Worthite iron in volume ratio, and it has high ductility and flange formability.

In general, if there is residual Worthite iron, the TRIP effect of the residual Worthite iron can improve the ductility. However, it is known that the additional strain causes the residual Worth iron to be metamorphosed to produce a very hard granulated iron, and as a result, the difference in hardness from the ferrite of the main phase becomes large, resulting in deterioration of the flange formability.

However, under the composition and texture of the present invention, both high ductility and high flange formability can be achieved. Although the detailed reasons for the high flange formability of the residual Worthite iron are not well understood, the residual Worthite iron flange can be reduced by coexisting the residual Worthite iron and the tempered granulated iron. Adverse effects of formability.

Furthermore, it has been found that a steel sheet structure having an average crystal grain size of 3 μm or less in a low-temperature metamorphic phase composed of a granulated iron, a tempered granulated iron, and a residual volcanic iron can be improved in addition to high workability. Impact characteristics.

The present invention has been developed based on the above findings, and the gist thereof is explained below.

The high-strength cold-rolled steel sheet excellent in workability and impact resistance according to the first invention contains, by mass%, C: 0.05 to 0.3%, Si: 0.3 to 2.5%, Mn: 0.5 to 3.5%, and P: 0.003. 0.100%, S: 0.02% or less, Al: 0.010 to 0.5%, the remainder is composed of iron and unavoidable impurities, and has: 20% or more of ferrite-containing iron in area ratio, and tempered granulated iron 10~ 60%, Ma Tian loose iron 0~10%, containing 3~15% of residual Worthite iron in volume ratio.

According to a second aspect of the present invention, in the high-strength cold-rolled steel sheet having excellent workability and impact resistance as described in the first aspect of the invention, the low-temperature metamorphic phase composed of the above-mentioned 麻田散铁, tempered 麻田散铁, and residual Worth iron The average crystal grain size is 3 μm or less.

According to a third aspect of the invention, the high-strength cold-rolled steel sheet having excellent workability and impact resistance as described in the first invention or the second invention is further contained in a mass % selected from the group consisting of Cr: 0.005 to 2.00%, and Mo: 0.005. 1 or 2 or more elements of ~2.00%, V: 0.005 to 2.00%, Ni: 0.005 to 2.00%, and Cu: 0.005 to 2.00%.

According to a fourth aspect of the present invention, in the high-strength cold-rolled steel sheet having excellent workability and impact resistance as described in the first to third inventions, further comprising, by mass%, is selected from the group consisting of Ti: 0.01 to 0.20%, and Nb: 0.01~ One or two elements of 0.20%.

According to a fifth aspect of the invention, the high-strength cold-rolled steel sheet having excellent workability and impact resistance as described in the first to fourth inventions further contains, by mass%, B: 0.0002 to 0.005%.

According to a sixth aspect of the invention, in the high-strength cold-rolled steel sheet having excellent workability and impact resistance as described in the first to fifth inventions, further comprising, in mass%, Ca: 0.001 to 0.005%, REM: 0.001 One or two of the elements of 0.005%.

A method for producing a high-strength cold-rolled steel sheet having excellent workability and impact resistance according to a seventh aspect of the invention, characterized in that the steel slab having the components described in the first to sixth inventions is subjected to hot rolling and cold rolling to produce cold When the cold-rolled steel sheet is subjected to continuous annealing, the steel sheet is maintained at a temperature of 750 ° C or higher for 10 seconds or more, and then cooled from a temperature of 750 ° C at an average cooling rate of 10 ° C / s or more to a temperature range of 150 to 350 ° C. Heat to 350~600 °C for 10~600 seconds, then cool to room temperature.

According to a seventh aspect of the invention, in the method for producing a high-strength cold-rolled steel sheet having excellent workability and impact resistance according to the seventh aspect of the invention, the temperature is raised at an average heating rate of 10° C./s or more at a deformation point of 500° C. to Ac ₁ .

According to the present invention, a high-strength cold-rolled steel sheet excellent in workability can be obtained, and at the same time, the weight reduction of the automobile and the safety of impact can be improved, and the performance of the automobile body can be greatly improved.

The present invention will be specifically described below.

1. About the composition of ingredients

First, the reason why the steel component composition of the present invention is limited to the above range will be described. The expression % of the component means the mass % unless otherwise specified.

C: 0.05~0.3%

C is an element that stabilizes the Worthite iron. It is an essential element in order to increase the strength of the steel sheet in order to easily form a phase other than the ferrite iron, and to improve the TS-EL balance in order to composite the structure. When the amount of C is less than 0.05%, it is difficult to ensure a phase other than the ferrite iron even if the production conditions are optimized, and TS×EL is lowered. On the other hand, when the amount of C exceeds 0.3%, the degree of hardening of the welded portion and the heat-affected portion is increased, and the mechanical properties of the welded portion are deteriorated. Based on this point of view, the amount of C is set in the range of 0.05 to 0.3%. It is preferably in the range of 0.08 to 0.15%.

Si: 0.3~2.5%

Si is an element that contributes to the strengthening of steel. In addition, it is a ferrite-iron-forming element, which promotes the concentration of C in the Worthite iron and inhibits the formation of carbides, and promotes the formation of residual Worth iron. When the amount of Si is less than 0.3%, the effect of addition is poor, so the lower limit is made 0.3%. However, excessive addition causes surface characteristics and weldability to deteriorate, so the Si content is set to 2.5% or less. It is preferably in the range of 0.7 to 2.0%.

Mn: 0.5~3.5%

Mn is an element that contributes to the strengthening of steel, and promotes the formation of a low-temperature metamorphic phase such as tempered granules. This effect occurs when the Mn content is 0.5% or more. However, if the addition of Mn is excessively more than 3.5%, the excessive increase in the second phase fraction and solid solution strengthening result in deterioration of the ductility of the ferrite iron and deterioration of formability. Therefore, the amount of Mn is set in the range of 0.5 to 3.5%. It is preferably in the range of 1.5 to 3.0%.

P: 0.003~0.100%

P is an element which contributes to the strengthening of steel, and this effect is obtained at a content of 0.003% or more. However, if it is added too much and exceeds 0.100%, segregation due to grain boundary may cause embrittlement, resulting in deterioration of impact resistance. Therefore, the amount of P is set in the range of 0.003% to 0.100%.

S: 0.02% or less

S becomes an inclusion such as MnS, and causes deterioration of impact resistance and cracking of the metal flow along the welded portion. The lower the content, the better, and it is set to 0.02% or less from the viewpoint of production cost.

Al: 0.010~0.5%

Al has a function as a deoxidizer and is an element which contributes to the cleanliness of steel, and is preferably added in the deoxidation step. Here, if the amount of Al is less than 0.01%, the effect of addition is poor, so the lower limit is made 0.01%. However, if it is added in a large amount, the risk of occurrence of cracking of the steel sheet during continuous casting is increased, and the manufacturability is deteriorated. Therefore, the upper amount of Al added is limited to 0.5%.

The high-strength cold-rolled steel sheet according to the present invention comprises the above-mentioned component composition as a basic component, and the remainder is composed of iron and unavoidable impurities, and the components described below can be appropriately contained in accordance with desired characteristics.

One or more selected from the group consisting of Cr: 0.005 to 2.00%, Mo: 0.005 to 2.00%, V: 0.005 to 2.00%, Ni: 0.005 to 2.00%, and Cu: 0.005 to 2.00%.

When Cr, Mo, V, Ni, and Cu are cooled from the annealing temperature, the formation of a pearl-like structure can be suppressed, and the formation of a low-temperature metamorphic phase can be promoted to contribute to the strengthening of steel. This effect can be obtained by making the content of at least one of Cr, Mo, V, Ni, and Cu 0.005% or more. However, if the components of Cr, Mo, V, Ni, and Cu exceed 2.00%, the effect is saturated and the cost increases. Therefore, the contents of Cr, Mo, V, Ni, and Cu are set to be in the range of 0.005 to 2.00%, respectively.

One or two selected from the group consisting of Ti: 0.01 to 0.20% and Nb: 0.01 to 0.20%

Ti and Nb form carbonitrides, and the precipitation strengthening enhances the strength of the steel. This effect occurs when the content is 0.01% or more. On the other hand, even if the content of each of Ti and Nb exceeds 0.20%, the ductility is lowered due to excessively high strength. Therefore, the contents of Ti and Nb are set in the range of 0.01 to 0.20%, respectively.

B: 0.0002~0.005%

B suppresses the formation of ferrite iron from the Worthfield iron grain boundary and has the effect of increasing the strength. This effect is obtained at a content of 0.0002% or more. However, if the amount of B exceeds 0.005%, the effect is saturated and the cost increases. Therefore, the B content is set in the range of 0.0002 to 0.005%.

One or two selected from the group consisting of Ca: 0.001 to 0.005% and REM: 0.001 to 0.005%

Both Ca and REM can control the form of the sulfide and have an effect of improving the workability, and if necessary, one or two types of Ca and REM can be contained in an amount of 0.001% or more. However, too much addition will adversely affect the cleanliness, so it is set at 0.005% or less.

2. About the organization

Next, the structure of the steel will be described.

Area ratio of ferrite iron: 20% or more

If the area ratio of the ferrite iron is less than 20%, TS × EL will decrease, so it is set to 20% or more. It is preferably 50% or more.

Area ratio of tempered granulated iron: 10~60%

The tempered granulated iron is a high-difference-density ferrite iron and cemet carbon iron obtained by heating the granulated iron below the Ac ₁ metamorphic point (preferably at a lower temperature than the Ac ₁ metamorphic point). The composite organization is helpful for strengthening steel. Further, the structure obtained by heating the granulated iron at a temperature exceeding the metamorphic point of Ac ₁ is a ferritic carbon-iron structure in the ferrite iron, which is basically different from the tempered granulated iron of the present invention.

In addition, the adverse effect of the tempered granulated iron on the reaming property is smaller than that of the granulated iron, which is a phase that does not significantly reduce the reaming property and contributes to the strength. When the area ratio of the tempered granulated iron is less than 10%, it is necessary to ensure that the strength becomes difficult. If the TS×EL is decreased when it exceeds 60%, the area ratio of the tempered granulated iron is set to 10 to 60%.

Area ratio of Ma Tian loose iron: 0~10%

The granulated iron contributes to the high strength of the steel. If the area ratio exceeds 10%, the flange formability is significantly lowered. Therefore, the area ratio of the granulated iron in the field is set at 0 to 10%.

Volume ratio of residual Worth iron: 3~15%

The residual Worthite iron not only contributes to the strengthening of the steel, but also contributes to the improvement of the TS×EL of steel. This effect is obtained at a volume fraction of 3% or more. In addition, if the remaining Worth iron exceeds 15%, the hole expandability deteriorates. Therefore, the volume fraction of the remaining Worthite iron is set at 3 to 15%.

Average crystal grain size of low temperature metamorphic phase composed of 麻田散铁, tempered 麻田散铁, residual Worthite iron: 3μm or less

The low-temperature metamorphic phase of the Ma Tian loose iron, the tempered Ma Tian loose iron and the residual Worth iron contribute to the improvement of the impact resistance. In particular, the impact resistance property can be improved by finely dispersing the low-temperature metamorphic phase, and the effect is remarkable when the average crystal grain size of the low-temperature metamorphic phase is 3 μm or less. Therefore, the average crystal grain size of the low temperature metamorphic phase is set to 3 μm or less.

In addition, although the phases other than the granulated iron, the tempered granulated iron, and the residual Worth iron may contain a pearl-like structure and a toughened iron, there is no problem as long as the phase structure described above is satisfied. However, the pearly structure content is preferably 3% or less based on the viewpoint of ensuring ductility and hole expandability.

3. About manufacturing conditions

The steel adjusted to have the above composition is melted using a converter or the like, and a steel blank is obtained by a continuous casting method or the like. After the steel billet is subjected to hot rolling and cold rolling, continuous annealing is performed. The production method of casting, hot rolling, and cold rolling is not particularly limited, and the following is a description of a preferred production method.

Casting conditions

The steel blank to be used is preferably produced by a continuous casting method in order to prevent microsegregation of components, but it may be produced by agglomeration method or thin steel blank casting method. In addition, after the steel embryo is produced, once it is cooled to room temperature and then heated again, in addition to the conventional method, it can be used without any problem: it is inserted into the heating furnace in a warm sheet state without cooling to room temperature, or Immediately after the slight heat preservation, the energy-saving process such as direct rolling and direct rolling of the rolling is performed.

Hot rolling condition <Steel embryo heating temperature: 1100 ° C or more>

The temperature at which the steel embryo is heated is preferably low-temperature heating from the viewpoint of energy. When the heating temperature is less than 1,100 ° C, the carbide may not be sufficiently solid-solved, and the increase in the rolling load may increase the possibility of hindering during hot rolling. The problem. Further, the steel blast heating temperature is preferably 1300 ° C or lower, based on an increase in the oxidized scale loss associated with an increase in the oxidized weight.

Further, it is also possible to use a so-called sheet heater that heats a sheet bar, even if the steel billet heating temperature is lowered to prevent the hindrance during hot rolling.

<finishing finish temperature: above Ar ₃ metamorphic point>

If the finish rolling temperature does not reach the Ar ₃ metamorphic point, ferrite iron and Vostian iron are formed in the rolling, and the strip structure is easily formed in the steel sheet, and the strip structure is still after cold rolling or annealing. It may remain, which may cause anisotropy in material properties or cause deterioration in processability. Therefore, the finishing rolling temperature is preferably equal to or higher than the Ar ₃ metamorphic point.

<Winding temperature: 450~700°C>

If the coiling temperature is less than 450 ° C, the control of the coiling temperature becomes difficult and temperature unevenness is likely to occur, and as a result, problems such as deterioration of cold rolling property may occur. When the coiling temperature exceeds 700 ° C, problems such as decarburization may occur in the iron surface layer. Therefore, the coiling temperature should be set in the range of 450 to 700 °C.

Further, in the hot rolling step of the present invention, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be subjected to lubrication rolling. It is effective to perform lubrication rolling based on the viewpoint of uniformity of the shape of the steel sheet and uniformity of the material. Further, the friction coefficient at the time of lubrication rolling is preferably in the range of 0.25 to 0.10. Further preferably, the continuous fine rolling process in which the successively fed sheets are joined to each other and the finish rolling is continuously performed is employed. From the viewpoint of the stability of hot rolling operation, a continuous finish rolling pass is preferred.

Next, it is preferable to carry out cold rolling to obtain a cold-rolled steel sheet having a predetermined thickness after the oxidized scale on the surface of the hot-rolled steel sheet is removed by pickling. The pickling conditions and the cold rolling conditions are not particularly limited, and may be according to a usual method. The cold rolling reduction ratio is preferably 40% or more.

<500 ° C ~ Ac ₁ average heating rate of the metamorphic point: 10 ° C / s or more >

In the steel of the present invention, the average heating rate in the recrystallization temperature range (500 ° C to Ac ₁ transformation point) is set to 10 ° C / s or more, and recrystallization at the time of heating and heating can be suppressed, contributing to the above Ac ₁ transformation point. The refinement of the produced Worthite iron contributes to the refinement of the microstructure after annealing and cooling, and the average particle diameter of the low-temperature metamorphic phase can be made 3 μm or less.

When the average heating rate is less than 10 ° C / s, recrystallization of α occurs when the temperature is raised by heating, and the strain introduced into the ferrite iron is released, and sufficient refinement cannot be achieved. Therefore, the average heating rate of the 500 ° C to Ac ₁ transformation point is set to 10 ° C / s or more. The average heating rate is preferably in the range of 20 ° C / s or more.

<Keep at a temperature of 750 ° C or more for 10 seconds or more>

When the heating temperature is less than 750 ° C or the holding time is less than 10 seconds, the formation of Worstian iron during annealing is insufficient, and a sufficient amount of low-temperature metamorphic phase cannot be ensured after annealing cooling. Although the upper limit of the temperature and the holding time is not particularly specified, when the temperature is maintained at 900 ° C or higher and the holding time is 600 seconds or longer, the effect is saturated and the cost is increased. Therefore, it is preferable to keep the temperature below 900 ° C and the holding time is less than 600. second.

<Cooling from 750 ° C to an average cooling rate of 10 ° C / s or more to a temperature range of 150 to 350 ° C >

From a cooling rate of 750 ° C, a pearl-like structure is formed when the temperature is less than 10 ° C / s, and TS × EL and hole expandability are deteriorated. Therefore, the cooling rate from 750 ° C is set to 10 ° C / s or more. Cooling to temperature conditions is one of the most important conditions of the technology. When the cooling stops, a part of the Worth Iron will be transformed into a granulated iron, and the rest will become the untransformed Worth Iron. After reheating, electroplating, and alloying treatment, the mixture is cooled to room temperature, and the granulated iron becomes tempered granulated iron, and the untransformed Worth iron becomes residual volcanic iron or 麻田散铁. The lower the temperature from the annealing to the lower temperature, the more the amount of granulated iron produced in the cooling, the less the amount of iron in the untransformed Vostian, so by controlling the cooling to reach the temperature, the final granulated iron and the residual Worthite iron and back are determined. The area ratio of scattered iron in Hematian.

When the cooling reaches a temperature higher than 350 °C, when the cooling stops, the numbness of the granulated iron is insufficient and the amount of iron in the untransformed Vostian is excessive. Finally, an excessive amount of granulated iron or residual Worth iron is generated, which deteriorates the hole expandability. . In addition, if the cooling reaching temperature is lower than 150 ° C, most of the Worth iron in the cooling is transformed into the granulated iron in the field, so that the amount of unconformed Worth iron is reduced, and the residual Worth iron of 3% or more cannot be obtained. Therefore, the cooling arrival temperature is set in the range of 150 to 350 °C. As for the cooling method, any cooling method such as gas jet cooling, spray cooling, water cooling, or metal quenching may be employed as long as the target cooling rate and the cooling stop temperature can be achieved.

<heated to 350~600 °C and kept for 10~600 seconds>

After cooling in a temperature range of 150 to 350 ° C, the temperature is maintained at a temperature of 350 to 600 ° C for 10 seconds or more, thereby tempering the granulated iron which is generated during the cooling, and tempering the granulated iron to improve the hole expandability. Further, the undeformed Worth iron which has not been transformed into the granulated iron in the aforementioned cooling is stabilized, and finally 3% or more of the remaining Worth iron is obtained to improve the ductility.

Although the details of the mechanism for stabilizing the undeformed Worthite iron by reheating are not well understood, it should be from the solid solution of supersaturated C in the field of loose iron to allow C to diffuse into the untransformed Worthite iron. The C in the untransformed Worthfield iron is concentrated, thereby stabilizing the Worthite iron. At this time, if the precipitation of stellite in the granulated iron is earlier than the diffusion of C, the concentration of C in the untransformed Worth iron will become insufficient, so it is important to delay the precipitation of ferritic carbon, so More than 0.3% of Si must be added.

When the reheating temperature is less than 350 ° C, the tempering of the granulation of the granulated iron and the stabilization of the Worthite iron are insufficient, and the hole expandability and ductility are deteriorated. If the reheating temperature exceeds 600 ° C, the unconformed Worth iron will be transformed into a pearl-like structure when the cooling is stopped, and finally no more than 3% of the remaining Worthite iron can be obtained. Therefore, the heating temperature is set to 350 to 600 °C.

When the holding time is less than 10 seconds, the stability of the Worthite iron is insufficient. If it is more than 600 seconds, the untransformed Worthite iron will be transformed into a toughened iron when the cooling stops, and finally it is impossible to obtain more than 3% of the residual iron. Sita Iron. Therefore, the reheating temperature is set to a range of 350 to 600 ° C, and the holding time of the temperature range is set to 10 to 600 seconds.

Further, in order to correct the shape, adjust the surface roughness, and the like, the steel sheet after annealing can be subjected to temper rolling. Further, there is no problem even if the treatment of resin, grease coating, various coatings, and the like is performed.

[Example 1]

A steel having the composition shown in Table 1 and having the remainder in the form of Fe and unavoidable impurities was melted in a converter and a cast piece was obtained by a continuous casting method. The obtained cast piece was hot rolled into a sheet thickness of 3.0 mm. The hot rolling conditions were carried out at a finishing temperature of 900 ° C, a cooling rate after rolling, 10 ° C / s, and a coiling temperature of 600 ° C. Next, the hot-rolled steel sheet was pickled, and then cold-rolled to a thickness of 1.2 mm to obtain a cold-rolled steel sheet.

Next, for these cold-rolled steel sheets, annealing treatment was performed on the continuous annealing line under the conditions shown in Table 2.

The cross-sectional microstructure, tensile properties, and hole expandability of the obtained steel sheets were investigated. The results are shown in Table 3.

The cross-section microstructure of the steel plate was revealed by using a 3% Nital solution (3% nitric acid + ethanol), and a scanning electron microscope was used to observe the depth direction plate thickness of 1/4, and the photographed tissue photograph was used. The image analysis process is performed to quantify the fraction of the ferrite iron phase (a commercially available image processing software can be used for the image analysis process). The area ratio of the granulated iron in the field and the area ratio of the tempered granulated iron in the field are taken as SEM photographs of 1000 to 3000 times the appropriate magnification according to the fineness of the organization, and quantified using image processing software. The average particle diameter of the low-temperature metamorphic phase is obtained by dividing the area of the low-temperature metamorphic phase of the observed visual field by the number of low-temperature metamorphic phases to obtain an average area, and opening the root number to obtain an average particle diameter.

The volume fraction of the residual Worthite iron is obtained by grinding the steel sheet to a quarter surface in the thickness direction, and is obtained from the diffraction X-ray intensity of the 1/4 surface of the plate thickness. The incident X-rays are made of MoKα lines, {110}, {200}, {211} faces of the {111}, {200}, {220}, {311} faces of the residual Worthfield iron phase and the ferrite phase. The integrated intensity of the peaks is determined by the combination of all the strength ratios, and the average value of the peaks is the volume fraction of the remaining Worthite iron.

In addition, the tensile property was measured using a JIS No. 5 test piece (the sample was drawn so that the stretching direction was perpendicular to the rolling direction of the steel sheet), and the tensile test was performed in accordance with JIS Z2241, and TS (tensile strength) and EL (measured) were measured. Elongation), the balance value of the strength-elongation expressed by the product of the strength and the elongation (TS × EL).

Further, as an index for evaluating the formability of the flange, the hole expansion ratio λ was measured. The hole expansion ratio λ is a hole expansion test according to the Japanese Iron and Steel Federation specification JFST1001, and is obtained from the initial hole diameter (10 mm Φ) of the hole at the time of punching and the ratio of the hole diameter at the time of the crack penetration of the hole edge after the hole expanding process. .

The impact absorption characteristic was obtained by using a test piece having a parallel portion width of 5 mm and a length of 7 mm sampled in a direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test at a strain rate of 2000/s, and the stress-true strain curve taken was The amount of the variable is 0 to 10%, and the absorbed energy is calculated and evaluated (see: Steel and Iron 83 (1997) p. 748).

The steel sheet of the present invention has an excellent strength, ductility, and flange formability in which TS × EL is 22,000 MPa‧% or more and λ is 70% or more.

On the other hand, in the steel sheet of the comparative example which deviated from the range of the present invention, TS × EL was less than 22000 MPa ‧ % and (or) λ was less than 70%, and excellent strength, ductility and flange as in the steel sheet of the present invention could not be obtained. Formability. In addition, by setting the average particle diameter of the low-temperature metamorphic phase to 3 μm or less, it is possible to obtain excellent impact resistance characteristics in which the ratio (AE/TS) of the absorbed energy to TS is 0.063 or more.

According to the present invention, the high-strength cold-rolled steel sheet excellent in workability and impact resistance contributes to weight reduction of the automobile and reduction in fuel consumption rate.

Claims (5)

A high-strength cold-rolled steel sheet excellent in workability and impact resistance, in terms of mass%, contains C: 0.05 to 0.3%, Si: 0.3 to 2.5%, Mn: 0.5 to 3.5%, and P: 0.003 to 0.100%. S: 0.02% or less, Al: 0.010 to 0.5%, and the remainder is composed of iron and unavoidable impurities, and has 20% or more of ferrite-containing iron in an area ratio, and 10 to 60% of tempered granulated iron. Ma Tian loose iron 0~10%, containing 3~15% of the residual Worthite iron in volume ratio. The high-strength cold-rolled steel sheet having excellent workability and impact resistance as described in the first paragraph of the patent application, wherein the average of the low-temperature metamorphic phase composed of the above-mentioned 麻田散铁, tempered 麻田散铁, and residual Worth iron The crystal grain size is 3 μm or less. The high-strength cold-rolled steel sheet excellent in workability and impact resistance according to the first or second aspect of the patent application, further comprising at least one element selected from the group A to the group D, and the group A: in mass%, One or more elements selected from the group consisting of Cr: 0.005 to 2.00%, Mo: 0.005 to 2.00%, V: 0.005 to 2.00%, Ni: 0.005 to 2.00%, and Cu: 0.005 to 2.00%; Group B: In terms of mass%, one or two elements selected from the group consisting of Ti: 0.01 to 0.20% and Nb: 0.01 to 0.20%; Group C: in mass %, B: 0.0002 to 0.005%; Group D: mass %, one or two elements selected from the group consisting of Ca: 0.001 to 0.005% and REM: 0.001 to 0.005%. High-strength cold-rolled steel sheet with excellent workability and impact resistance A method of producing a cold-rolled steel sheet by subjecting a steel preform having the component according to any one of claims 1 to 3 to hot rolling and cold rolling, and performing continuous annealing on the cold-rolled steel sheet. After maintaining at a temperature of 750 ° C or higher for 10 seconds or more, it is cooled from 750 ° C at a cooling rate of 10 ° C / s or more to a temperature range of 150 to 350 ° C, and then heated to 350 to 600 ° C for 10 to 600 seconds. , cooled to room temperature. A method for producing a high-strength cold-rolled steel sheet having excellent workability and impact resistance as described in the fourth aspect of the invention, wherein the temperature is raised at an average heating rate of 10 ° C/s or more at a 500 ° C to Ac ₁ transformation point.