JP2014019928A - High strength cold rolled steel sheet and method for producing high strength cold rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet or the like having high ductility and excellent in stretch flange properties.SOLUTION: The high strength cold rolled steel sheet as a composition comprising, by mass%, 0.06% or higher to 0.12% or lower of C, 0.4% or higher to 0.8% or lower of Si, 1.6% or higher to 2.0% or lower of Mn, 0.01% or higher to 1.0% or lower of Cr, 0.001% or higher to 0.1% or lower of V, 0.05% or lower of P, 0.01% or lower of S, 0.01% or higher to 0.5% or lower of Sol.Al and 0.005% or lower of N, and the balance iron with inevitable impurities, and further, the volume ratio of equiaxial ferrite in a metallic structure is 50% or higher, the volume ratio of martensite is 5% or higher to 15% or lower, the volume ratio of a retained austenite phase is 1% or higher to 5% or lower, the average grain size of the retained austenite phase is 10 μm or lower, the aspect ratio of the retained austenite phase is 5% or lower, and the balance structure is made of bainite and/or pearlite.

Description

  The present invention relates to a high-strength cold-rolled steel sheet and a method for producing a high-strength cold-rolled steel sheet.

In recent years, in the automobile manufacturing field, from the viewpoint of global environmental conservation, there has been a strong demand for weight reduction of an automobile body for improving fuel efficiency for reducing CO ₂ emissions. On the other hand, from the viewpoint of ensuring the safety of passengers, it is naturally required to strengthen the automobile body against impacts. In order to achieve both weight reduction and high strength of the automobile body, conventionally, the steel sheet, which is the body material, has been increased in strength, and the plate thickness has been reduced in a range where rigidity is not a problem. It has been.

  By the way, in general, a high-strength steel plate is inferior in ductility as compared with a soft steel plate, and thus forming processing such as press forming is difficult. For this reason, in addition to increasing the strength and thickness of the steel sheet as described above, there is an increasing demand for higher ductility. In order to meet these requirements, the γ phase (austenite phase) has remained stable at room temperature so far, and this residual austenite phase (also referred to as residual γ phase) is transformed into martensite during plastic working. Therefore, so-called TRIP steel using transformation-induced plasticity (TRIP phenomenon) exhibiting high ductility has been proposed (see, for example, Patent Documents 1 and 2).

Japanese Patent Laid-Open No. 1-2230715 JP-A-1-272720

  However, the above-mentioned TRIP steel has a problem that stretch flangeability is inferior due to excessive hardening of the retained austenite transformed during molding.

  This invention is made | formed in view of the above, Comprising: It aims at providing the manufacturing method of the high strength cold-rolled steel plate which has high ductility and was excellent in stretch flangeability, and a high-strength cold-rolled steel plate.

  The inventors of the present invention have made extensive studies to solve the above problems, and as a result, adjusted the composition of the steel, the volume ratio of equiaxed ferrite, martensite, retained austenite phase and the form of retained austenite phase. By controlling the above, it was found that a high-strength cold-rolled steel sheet having high ductility and excellent stretch flangeability was obtained, and the following was found. Here, in the present invention, the high-strength cold-rolled steel sheet refers to a steel sheet having a tensile strength TS of 590 MPa or more, a total elongation El of 30% or more, and a hole expansion ratio λ of 60% or more.

  That is, in order to solve the above-described problems and achieve the object, the high-strength cold-rolled steel sheet according to the present invention has a component composition of mass%, C: 0.06% or more and 0.12% or less, Si: 0. 0.4% to 0.8%, Mn: 1.6% to 2.0%, Cr: 0.01% to 1.0%, V: 0.001% to 0.1%, P : 0.05% or less, S: 0.01% or less, Sol. Al: 0.01% or more and 0.5% or less, N: 0.005% or less, the balance is composed of iron and inevitable impurities, and the volume ratio of equiaxed ferrite in the metal structure is 50% The martensite volume fraction is 5% or more and 15% or less, the volume ratio of the retained austenite phase is 1% or more and 5% or less, the average particle size of the retained austenite phase is 10 μm or less, and the aspect ratio of the retained austenite phase Is 5 or less, and the remaining structure consists of bainite and / or pearlite.

  The high-strength cold-rolled steel sheet according to the present invention, in the above-described invention, further includes, as a component composition, mass%, Ti: 0.001% to 0.1%, Nb: 0.001% to 0.1%. % Or less, and Zr: 0.001% or more and 0.1% or less.

  The high-strength cold-rolled steel sheet according to the present invention, in the above-described invention, further has a component composition of Mo: 0.01% to 0.5% and / or B: 0.0001% to 0. It is characterized by containing 0020% or less.

  The method for producing a high-strength cold-rolled steel sheet according to the present invention comprises hot-rolling and cold-rolling a steel material having the component composition of the above-described invention, and then heating the steel material to a temperature range of 750 ° C. to 870 ° C. After holding in the temperature range for 10 seconds or more, the temperature is cooled to a temperature range of 600 ° C. to 700 ° C. at an average cooling rate of 20 ° C./sec. Cooling is performed as described above, and the temperature is maintained in the temperature range for 10 seconds or longer, and then cooled to room temperature.

  According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet having high ductility and excellent stretch flangeability and a method for producing a high-strength cold-rolled steel sheet.

  Hereinafter, the manufacturing method of the high-strength cold-rolled steel sheet and the high-strength cold-rolled steel sheet according to the present invention will be described in detail by dividing it into its component composition, metal structure, and manufacturing method.

  First, the component composition will be described. In the following description, “%” representing the content of the constituent elements of steel means “mass%” unless otherwise specified.

(C content)
C (carbon) has the effect of hardening the martensite phase to increase the strength of the steel sheet, and the effect of stabilizing the γ phase at room temperature to develop the TRIP phenomenon. However, if the C content is less than 0.06%, the effect cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.12%, stretch flangeability deteriorates. This is because if the amount of C in the steel is high, the hardness of the martensite phase and the retained austenite phase after transformation becomes high, which becomes a starting point of cracking during forming. Therefore, the C content is in the range of 0.06% to 0.12%. In order to further stabilize the strength, the C content is preferably 0.08% or more. In order to suppress casting cracks, the C content is desirably 0.10% or less.

(Si content)
Si (silicon) has an effect of suppressing the formation of iron carbide in the annealing step and increasing the C concentration in the γ phase. However, if the Si content is less than 0.4%, the effect cannot be sufficiently obtained. On the other hand, if the Si content exceeds 0.8%, not only the above-described effect is saturated, but also the chemical conversion property of the steel sheet is deteriorated. Therefore, the Si content is in the range of 0.4% to 0.8%. Furthermore, in order to improve the elongation stably, the Si content is desirably 0.5% or more. In order to stabilize the chemical conversion property, the Si content is desirably 0.6% or less.

(Mn content)
Mn (manganese) has the effect of optimizing the volume fraction of the γ phase and the amount of C in the γ phase in the annealing process, and enhancing the strength of the steel sheet and promoting the occurrence of the TRIP phenomenon. However, when the Mn content is less than 1.6%, the γ phase becomes too small and the strength decreases, and a desired tensile strength, specifically, a tensile strength TS of 590 MPa or more can be achieved. Can not. On the other hand, if the Mn content exceeds 2.0%, the volume ratio of martensite increases, so that the tensile strength rises more than necessary and the elongation deteriorates. Therefore, the content of Mn is in the range of 1.6% or more and 2.0% or less.

(Cr content)
Cr (chromium) has an effect of reducing the generation rate of the α phase (ferrite phase) from the γ phase in the annealing process. In particular, when it is added together with V (vanadium), there is an effect of stabilizing the amount of martensite and retained austenite generated regardless of the cooling rate after soaking in the annealing process. However, if the Cr content is less than 0.01%, the effect cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 1.0%, the amount of martensite produced becomes too large, and the expression of the TRIP phenomenon is suppressed. Therefore, the Cr content is in the range of 0.01% to 1.0%. In order to further enhance the above effect, the Cr content is desirably 0.5% or more. Moreover, from the viewpoint of chemical conversion treatment, the Cr content is preferably 0.7% or less.

(V content)
V is a quenching strengthening element, and forms carbides in the hot rolling process to increase the strength of the steel. In particular, when it is added together with the above Cr, the effect of increasing the strength is high. However, if the V content is less than 0.001%, the effect is not sufficiently obtained. On the other hand, if it exceeds 0.1%, the generated carbide becomes coarse and the elongation deteriorates. Therefore, the V content is within the range of 0.001% to 0.1%. In order to further enhance the above effect, the V content is preferably 0.04% or more. On the other hand, in order to further increase the elongation, the V content is preferably 0.06% or less.

(P content)
P (phosphorus) is an impurity mixed from the raw material and causes the spot welding strength to deteriorate. When the P content exceeds 0.05%, the spot weld strength is significantly deteriorated. Therefore, the content of P is set within a range of 0.05% or less. Furthermore, from the viewpoint of refining costs, the P content is preferably 0.005% or more. In order to improve spot weldability more effectively, the P content is desirably 0.02% or less. More preferably, it is 0.01% or less.

(S content)
S (sulfur) is an impurity mixed from the raw material and causes the spot welding strength to deteriorate as in the case of P. When the S content exceeds 0.01%, the spot weld strength is significantly deteriorated. Therefore, the content of S is set within a range of 0.01% or less. Furthermore, from the viewpoint of refining costs, the content of S is preferably 0.001% or more. In order to improve spot weldability more effectively, the S content is preferably 0.005% or less.

(Sol.Al content)
Al (aluminum) is an element added for the purpose of deoxidation in the steelmaking process. If the content of Al (solid aluminum) is less than 0.01%, the effect cannot be obtained sufficiently. On the other hand, Sol. When the Al content exceeds 0.5%, not only the above-described effect is saturated but also the production cost is increased. Therefore, Sol. The Al content is in the range of 0.01% to 0.5%. Furthermore, for detoxification of impurities N, Sol. The Al content is preferably 0.03% or more. Sol. If the Al content is 0.1% or more, there is a concern that the continuous castability deteriorates. The Al content is preferably less than 0.1%.

(N content)
N (nitrogen) is an impurity and degrades elongation and aging resistance. When the N content exceeds 0.005%, the above-described characteristic deterioration becomes remarkable. Therefore, the N content is within a range of 0.005% or less. Furthermore, from the viewpoint of refining costs, the N content is preferably 0.001% or more. In order to stabilize the elongation, the N content is preferably 0.004% or less.

(Contents of Ti, Nb, and Zr)
Ti (titanium), Nb (niobium), and Zr (zirconium) are precipitation strengthening elements, and generate carbides in the hot rolling process, and have an effect of increasing strength. For any element of Ti, Nb, and Zr, when the content is less than 0.001%, the effect cannot be sufficiently obtained. On the other hand, when it exceeds 0.1% and it adds excessively, the carbide | carbonized_material produced | generated will coarsen and cause deterioration of elongation. Therefore, when adding Ti, Nb, and Zr, the content is within the range of 0.001% to 0.1%, respectively. Furthermore, in order to enhance the effect of increasing the strength, the contents of Ti, Nb, and Zr are each preferably 0.01% or more. In order to suppress the decrease in elongation, the contents of Ti, Nb, and Zr are each preferably 0.06% or less.

(Mo and B contents)
Mo (molybdenum) and B (boron) have the effect of reducing the formation rate of α phase from the γ phase in the annealing process, and martensite and retained austenite regardless of the cooling rate after soaking in the annealing process. This has the effect of stabilizing the amount of the produced. However, when the content of Mo is less than 0.01%, the effect cannot be sufficiently obtained. On the other hand, if the Mo content exceeds 0.5%, the amount of martensite produced becomes too large, and the expression of the TRIP phenomenon is suppressed. In the case of B, if the content is less than 0.0001%, the above-described effects cannot be obtained sufficiently. On the other hand, if the content of B exceeds 0.0020%, the amount of martensite generated becomes too large, and the expression of the TRIP phenomenon is suppressed. Therefore, when Mo is added, the content is within a range of 0.01% to 0.5%, and when B is added, the content is 0.0001% to 0.00%. Within the range of 0020% or less.

  The balance other than the components having the above contents is composed of iron (Fe) and inevitable impurities. In addition, if it is a range which does not impair the effect of this invention, it does not refuse inclusion of components other than the above.

  Next, the metal structure will be described. In the present invention, the equiaxed ferrite, martensite, retained austenite, bainite, and pearlite may be referred to as the equiaxed ferrite phase, martensite phase, retained austenite phase, bainite phase, and pearlite phase, respectively.

(Volume ratio of equiaxed ferrite)
The equiaxed ferrite is rich in ductility and has an effect of improving elongation by forming a composite structure. However, if the volume ratio is less than 50%, the effect cannot be sufficiently obtained. Therefore, the volume ratio of equiaxed ferrite is set to 50% or more. The volume ratio of the equiaxed ferrite can be quantified by mirror-polishing the cross section of the steel sheet and corroding it, and then image-processing and analyzing an observation image obtained using, for example, an SEM (scanning electron microscope).

(Volume ratio of martensite)
Martensite is an important structure for improving the strength, and if the volume ratio is less than 5%, sufficient strength cannot be obtained. On the other hand, when the volume ratio of martensite exceeds 15%, elongation and stretch flangeability deteriorate. Therefore, the volume ratio of martensite is 5% or more and 15% or less. The martensite here includes tempered martensite.

(Volume ratio of residual austenite phase)
The residual austenite phase is essential for developing the TRIP phenomenon. However, if the volume fraction of the retained austenite phase is less than 1%, the effect of developing the TRIP phenomenon cannot be obtained sufficiently. On the other hand, if the volume fraction of the retained austenite phase exceeds 5%, the retained austenite transformed at the time of molding becomes a starting point of cracking, and the stretch flangeability is deteriorated. Therefore, the volume ratio of the retained austenite phase is 1% or more and 5% or less. The volume ratio of the retained austenite phase can be obtained using the SEM-EBSD method after mirror-polishing the cross section of the steel sheet.

(Average particle size of retained austenite phase)
When the crystal grain size of the retained austenite becomes coarse, the large transformed portion of retained austenite becomes the starting point of cracking at the time of molding, and the stretch flangeability is deteriorated. Accordingly, the average particle size of the retained austenite phase is 10 μm or less. The average grain size of the retained austenite phase can be determined using the SEM-EBSD method after mirror-polishing the cross section of the steel sheet.

(Aspect ratio of residual austenite phase)
When the aspect ratio of the retained austenite phase is increased, stress concentration is likely to occur around the retained austenite transformed at the time of molding, and it may become a starting point of cracks, so that the stretch flangeability is deteriorated. Therefore, the aspect ratio of the retained austenite phase is set to 5 or less. The aspect ratio of the retained austenite phase can be determined using the SEM-EBSD method after mirror-polishing the cross section of the steel sheet.

  The remaining structure other than the metal structure shown above consists of bainite and / or pearlite. Bainite is a structure containing bainitic ferrite and fine carbides. Pearlite is a layered structure of ferrite and cementite.

  In the present invention, by controlling the steel having the above component composition to the above metal structure, a high-strength cold-rolled steel sheet having high ductility and excellent stretch flangeability is obtained. Next, an embodiment of a manufacturing method for obtaining such a high-strength cold-rolled steel sheet will be described.

  In this production method, the steel slab (steel material) having the above-described composition that was melted by continuous casting or ingot forming was hot-rolled (hot-rolling step), and obtained through a winding step and a cold-rolling step. The steel strip is heat treated (annealing process). That is, as an annealing process, first, it is heated to a temperature range of 750 ° C. or more and 870 ° C. or less (heating process), held for 10 seconds or more in this temperature range (soaking process), and then to a temperature range of 600 ° C. or more and 700 ° C. or less. Cooling at an average cooling rate of 20 ° C./sec or less (primary cooling step), followed by cooling to a temperature range of 350 ° C. or more and 500 ° C. or less at an average cooling rate of 10 ° C./sec or more (secondary cooling step), Hold for 10 sec or longer in the zone (holding step after secondary cooling). After the above heat treatment, it is cooled to room temperature. By this production method, a high-strength cold-rolled steel sheet having high ductility and excellent stretch flangeability can be obtained.

(Hot rolling process)
In the hot rolling step, the steel slab having the above component composition is hot rolled as it is, or once cooled and then heated again to perform hot rolling. The final rolling temperature in the hot rolling is preferably from Ar ₃ transformation point to 890 ° C. This is because by refining the structure after hot rolling, the dissolution rate of carbides in the annealing stage is improved and the γ phase is stabilized.

(Winding process)
In the subsequent winding process, the obtained hot-rolled sheet is cooled and then wound. The coiling temperature is preferably 610 ° C. or lower. This is because by making the structure finer, the dissolution rate of carbides in the annealing stage is improved and the γ phase is stabilized.

(Cold rolling process)
In the subsequent cold rolling step, cold rolling is performed to obtain a desired thickness. The cold rolling rate is preferably 40% or more. This is because by making the structure finer, the dissolution rate of carbides in the annealing stage is improved and the γ phase is stabilized. In addition, after the winding step and before the cold rolling step, the steel plate may be subjected to pickling treatment (pickling step) in order to remove scale (oxide film) on the surface of the steel plate.

(Heating process / Soaking process)
When the heating temperature and the soaking temperature in the heating step / soaking step are less than 750 ° C., a sufficient amount of γ phase is not generated, resulting in a decrease in strength. On the other hand, when the heating temperature and the soaking temperature exceed 870 ° C., the γ phase becomes a single phase and the structure becomes coarse, so that the elongation deteriorates. Accordingly, the heating temperature and the soaking temperature are set to a temperature range of 750 ° C. or higher and 870 ° C. or lower. From the viewpoint of production stability, it is desirable to set the temperature range from 800 ° C. to 830 ° C. Moreover, the time (soaking time) for holding at the soaking temperature in the soaking step is 10 sec or more. This is because when the soaking time is less than 10 seconds, a sufficient amount of the γ phase is not generated and the strength is lowered.

(Primary cooling process)
By the cooling in the primary cooling step, the generation of the α phase is promoted, the amount of C in the γ phase is increased, and the TRIP effect is promoted. If the cooling rate (primary cooling rate) at the time of cooling in the primary cooling step exceeds 20 ° C./sec, the effect cannot be sufficiently obtained and ductility is lowered. Therefore, the cooling in the primary cooling step is performed at an average cooling rate of 20 ° C./sec or less up to a temperature range of 600 ° C. to 700 ° C.

(Secondary cooling process)
The formation of a pearlite phase is suppressed by cooling in the secondary cooling step. When the pearlite phase is generated, not only the strength is decreased, but also the amount of γ phase generated is decreased and the elongation is deteriorated. Therefore, the cooling in the secondary cooling step is performed at an average cooling rate of 10 ° C./sec or higher up to a temperature range of 350 ° C. or higher and 500 ° C. or lower.

(Holding process after secondary cooling)
In the subsequent holding step after the secondary cooling, a bainite phase is generated and the residual γ phase is stabilized. Thereby, the expression of the TRIP phenomenon can be promoted and the ductility of the steel sheet can be increased. In the holding step after the secondary cooling, the time (holding time) for holding in the temperature range of 350 ° C. or more and 500 ° C. or less, which is the temperature range in the secondary cooling step, is 10 seconds or more. This is because if the holding time is less than 10 seconds, the above-described effects cannot be obtained sufficiently and the elongation deteriorates.

  In addition, after the holding process after secondary cooling, after cooling to room temperature, it is desirable to perform temper rolling in order to eliminate yield point elongation. This temper rolling is preferably performed in a range of elongation rate of 0.1% to 1.0%. Moreover, you may apply | coat electroplating, hot dip galvanization, or a solid lubricant etc. to the surface of the obtained steel plate as needed.

Examples of the present invention will be described below. Table 1 shows the composition (mass%) of the chemical components of the steels of the present invention and comparative examples used as test materials in this example. In Table 1, values outside the range of the present invention are underlined. Further, “tr.” In Table 1 means a trace element (trace element).

[Example 1]
In Example 1, a steel ingot having a chemical component composition shown in Table 1 was melted and cast, and first heated to 1250 ° C. and hot-rolled to a thickness of 2.8 mm. The final pass outlet temperature in hot rolling was 860 ° C. Subsequently, after cooling at an average cooling rate of 20 ° C./sec, winding was simulated at 600 ° C., held for 1 hour, and then cooled in the furnace. Subsequently, pickling and cold rolling were performed to obtain a plate thickness of 1.2 mm, and then a heat treatment simulating continuous annealing was performed. In this heat treatment, the sample was heated to 810 ° C. at an average heating rate of 20 ° C./sec and held for 300 sec. Subsequently, it was cooled to 700 ° C. at an average cooling rate of 10 ° C./sec, subsequently cooled to 400 ° C. at an average cooling rate of 15 ° C./sec, and maintained for 480 seconds. And after cooling to room temperature, temper rolling with an elongation of 0.3% was performed.

And each steel plate obtained as described above was used as a test material, the volume ratio of equiaxed ferrite, the volume ratio of martensite, the volume ratio of the retained austenite phase, the average grain size and the aspect ratio, and the remaining structure. And the properties of tensile properties, hole expansion ratio, and chemical conversion properties were evaluated. The results are shown in Table 2. In Table 2, “B” represents bainite and “P” represents pearlite in the “remaining structure” item. In Table 2, values outside the range of the present invention and values that are not excellent in characteristics are shown with an underline.

  Here, the metal structure was mirror-polished on a cross section cut out in parallel with the rolling direction, corroded with 3% nital, and then imaged at a magnification of 1000 using JEOL Ltd./SEM (JSM-840F). From the point calculation method (Journal of the Japan Institute of Metals, 10 (1971), 279. Quantitative Metallography / Kento Sakuma, Taiji Nishizawa) The volume ratio of martensite was measured. At the same time, bainite and pearlite were identified. The volume ratio, average particle diameter, and aspect ratio of the retained austenite phase were obtained by photographing the above sample with JEOL Ltd./SEM (JSM-7001FA) and TSL Solutions / EBSD apparatus (VE1000SIT) at a magnification of 10,000 times. From 10 fields of view, attached software OIM Ver. 5 was measured. The volume fraction of the retained austenite phase was obtained by measuring the fraction of austenite from the Phase Map function of the same software. The average particle size of the retained austenite phase was determined using the Grain Chart function of the same software. The aspect ratio of the retained austenite phase was determined as an average value of the major axis / minor axis by measuring the major axis and minor axis of all austenite crystal grains from the same image.

  Tensile properties were determined by conducting a tensile test in accordance with JISZ2241 (1998) using a JIS5 test piece (JISZ2201) collected so that the tensile direction was perpendicular to the rolling direction of the steel sheet, yield strength YP, The strength TS and total elongation El were measured and evaluated. The hole expansion rate λ (%) is an evaluation index of stretch flangeability, and here, a hole expansion test was performed and evaluated in accordance with the Japan Iron and Steel Federation standard JFST1001-1996. The chemical conversion treatment is performed by degreasing with a commercially available alkaline degreasing solution (Nihon Parkerizing Co., Ltd./Fine Cleaner FC-E2001) and then immersing in a surface conditioning solution (Nihon Parkerizing Co., Ltd./PL-ZTH). Salt treatment (Nippon Parkerizing Co., Ltd./Palbond PB-L3080) was immersed and subjected to chemical conversion treatment under conditions of bath temperature: 43 ° C. and treatment time: 120 seconds, according to JEOL Ltd./SEM (JEM-840F). The density of the chemical conversion grains was evaluated by visually checking the SEM image. Here, the case where the coverage of the conversion crystal is not 100% is “X”, the coverage is 100%, and the conversion crystal grain is non-uniform and the maximum value of the conversion crystal grain size exceeds 4 μm is “Δ”, A case where the coverage was 100%, the chemical crystal grains were uniform and the chemical crystal grain size was 4 μm or less was evaluated as “◯” in three stages, and “Δ” or “◯” was judged as good.

  As shown in Table 2, in the example of the present invention, good evaluation was obtained for both the tensile properties and the hole expansion ratio. Specifically, in all of the examples of the present invention, the tensile strength TS was 590 MPa or more, the total elongation El was 30% or more, and the hole expansion ratio λ was 60% or more. Moreover, favorable evaluation was obtained also about the chemical conversion treatment property.

  On the other hand, in the comparative example, favorable evaluation was not obtained about one or more characteristics among the tensile characteristics, the hole expansion ratio, and the chemical conversion treatment. For example, the steel sheet “1” has a low tensile strength TS and a low strength. This is probably because the amount of C in the steel is low and the volume ratio of martensite is low. Further, the steel sheet “5” has a low total elongation El and a low hole expansion ratio λ. This is presumably because the amount of C in the steel is high, the volume ratio of equiaxed ferrite is low, and the volume ratio of martensite is high. Further, the steel sheet “6” has a low total elongation El. This is presumably because the amount of Si in the steel is low and the volume fraction of the retained austenite phase is low. Further, the steel sheet “9” has a low hole expansion ratio λ and is inferior in chemical conversion treatment. This is probably because the amount of Si in the steel is high and the volume ratio of the retained austenite phase is high. Further, the steel sheet “10” has a low tensile strength TS. This is probably because the amount of Mn and Cr in the steel is low and the volume ratio of martensite is low. Further, the steel sheet “13” has a low total elongation El. This is considered because the amount of Mn in steel is high and the volume fraction of equiaxed ferrite is low. Further, the steel sheet “15” has a low tensile strength TS. This is probably because the amount of V in the steel is low and the volume ratio of martensite is low.

[Example 2]
Table 3 shows the production conditions of the steel sheet of the present invention and the comparative example in Example 2. In Table 3, values outside the range of the present invention are underlined.

  In Example 2, a steel ingot having the composition of chemical components shown in Table 1 was melted and cast, and first heated to 1250 ° C. and hot-rolled. The final pass exit temperature in hot rolling was 870 ° C. (hot rolled sheet thickness: 2.8 mm). Subsequently, after cooling at an average cooling rate of 20 ° C./sec, winding was simulated according to the hot rolling conditions shown in Table 3, held for 1 hour, and then cooled in the furnace. Subsequently, cold rolling was performed to obtain a plate thickness of 1.2 mm, and then a heat treatment simulating continuous annealing was performed according to the annealing conditions shown in Table 3. And after cooling to room temperature, temper rolling with an elongation of 0.3% was performed.

And each steel plate obtained as described above was used as a test material, the volume ratio of equiaxed ferrite, the volume ratio of martensite, the volume ratio of the retained austenite phase, the average grain size and the aspect ratio, and the remaining structure. The tensile properties, the hole expansion ratio, and the chemical conversion property were evaluated in the same manner as in Example 1. The results are shown in Table 4. In Table 4, “B” represents bainite and “P” represents pearlite in the “remaining structure” item. Further, in Table 4, values outside the range of the present invention and values that are not excellent in the characteristics are shown with an underline.

  As shown in Table 4, in the examples of the present invention, good evaluation was obtained for both the tensile properties and the hole expansion ratio. Specifically, in all of the examples of the present invention, the tensile strength TS was 590 MPa or more, the total elongation El was 30% or more, and the hole expansion ratio λ was 60% or more. Moreover, favorable evaluation was obtained also about the chemical conversion treatment property.

  On the other hand, in the comparative example, favorable evaluation was not obtained about one or more characteristics among the tensile characteristics, the hole expansion ratio, and the chemical conversion treatment. For example, the steel sheet “B” has a remarkably low tensile strength TS. This is presumably because the soaking temperature in the soaking process is too low. Further, the steel sheet “F” has a low hole expansion ratio λ and is inferior in chemical conversion treatment. This is presumably because the soaking temperature in the soaking process was too high, the grain size of the retained austenite phase was coarsened, and the aspect ratio was also increased. Further, the steel sheet “G” has a low martensite volume fraction and a low tensile strength TS. This is probably because the soaking time in the soaking process is too short. Further, the steel sheet “H” has a low volume ratio of equiaxed ferrite, a high volume ratio of retained austenite phase, and a low total elongation El and a hole expansion ratio λ. This is considered because the primary cooling rate in the primary cooling step is too high. Further, the steel sheet “I” has a low martensite volume fraction and a low tensile strength TS. This is considered because the cooling rate (secondary cooling rate) in the secondary cooling step is too low. Further, the steel sheet “J” has a low total elongation El. This is probably because the lower limit temperature (secondary cooling stop temperature) in the temperature range during cooling in the secondary cooling step is too low and the amount of residual austenite phase produced is small. Further, the steel sheet “K” has a low total elongation El. This is presumably because the secondary cooling stop temperature in the secondary cooling step is too high and the amount of residual austenite phase produced is small. Further, the steel sheet “L” has a low total elongation El. This is probably because the holding time in the holding step after the secondary cooling is too short and the amount of residual austenite phase produced is small.

  As described above, according to the present invention, the component composition of steel is appropriately adjusted, the volume ratio of equiaxed ferrite, the volume ratio of martensite, the volume ratio of the retained austenite phase, the average particle diameter of the retained austenite phase, By controlling the aspect ratio and the like of the retained austenite phase, it is possible to realize a high-strength cold-rolled steel sheet that has excellent stretch flangeability and high formability and also has good ductility. More specifically, it is possible to obtain a high-strength cold-rolled steel sheet having good chemical conversion property while satisfying the tensile strength TS ≧ 590 MPa, the total elongation El ≧ 30%, and the hole expansion ratio λ ≧ 60%. Moreover, the manufacturing method of a high-strength cold-rolled steel sheet as described above can be stably provided by appropriately adjusting the manufacturing conditions. Therefore, according to the present invention, it is possible to provide a high-strength cold-rolled steel sheet having high ductility and excellent stretch flangeability and a method for producing a high-strength cold-rolled steel sheet.

  In particular, the high-strength cold-rolled steel sheet of the present invention is suitable for use as an automotive steel sheet used for an inner plate or an outer plate of an automobile body, and by applying to an automotive steel plate, an automobile structural member or a reinforcing member, Other mechanical structural parts can be reduced in weight and strength, contributing to global environmental conservation and passenger safety by improving fuel efficiency. However, the use of the high-strength cold-rolled steel sheet of the present invention is not limited to automobile steel sheets.

  As mentioned above, although embodiment of this invention was described, this invention is not limited by the description which makes a part of indication of this invention by this embodiment. That is, other embodiments, examples, operational techniques, and the like made by those skilled in the art based on the present embodiment are all included in the scope of the present invention. For example, in the series of heat treatment (annealing step) in the above-described manufacturing method, as long as the heat treatment conditions are satisfied, the equipment for performing the heat treatment on the steel sheet is not particularly limited.

Claims (4)

  As component composition, in mass%, C: 0.06% to 0.12%, Si: 0.4% to 0.8%, Mn: 1.6% to 2.0%, Cr: 0 0.01% or more and 1.0% or less, V: 0.001% or more and 0.1% or less, P: 0.05% or less, S: 0.01% or less, Sol. Al: 0.01% or more and 0.5% or less, N: 0.005% or less, the balance is composed of iron and inevitable impurities, and the volume ratio of equiaxed ferrite in the metal structure is 50% The martensite volume fraction is 5% or more and 15% or less, the volume ratio of the retained austenite phase is 1% or more and 5% or less, the average particle size of the retained austenite phase is 10 μm or less, and the aspect ratio of the retained austenite phase Is a high-strength cold-rolled steel sheet, wherein the remaining structure is bainite and / or pearlite.   As a component composition, Ti: 0.001% to 0.1%, Nb: 0.001% to 0.1%, and Zr: 0.001% to 0.1% by mass% The high-strength cold-rolled steel sheet according to claim 1, comprising at least one of them.   The component composition further comprises, in mass%, Mo: 0.01% to 0.5% and / or B: 0.0001% to 0.0020%. The high-strength cold-rolled steel sheet as described in 1.   After hot-rolling and cold-rolling a steel material having the component composition according to any one of claims 1 to 3, the steel material is heated to a temperature range of 750 ° C or higher and 870 ° C or lower, and maintained in the temperature range for 10 seconds or longer. Then, it is cooled at an average cooling rate of 20 ° C./sec or less to a temperature range of 600 ° C. or more and 700 ° C. or less, and subsequently is cooled at an average cooling rate of 10 ° C./sec or more to a temperature range of 350 ° C. or more and 500 ° C. or less, A method for producing a high-strength cold-rolled steel sheet, wherein the steel sheet is kept at a temperature range for 10 seconds or more and then cooled to room temperature.