JP5003785B2 - High tensile steel plate with excellent ductility and method for producing the same - Google Patents

Description

  The present invention relates to a high-tensile steel plate used in the industrial field such as transportation machinery represented by automobiles, and more particularly, to a high-tensile steel plate excellent in ductility with a tensile strength (TS) of 700 to 900 MPa and a method for producing the same.

  In recent years, from the viewpoint of suppressing global warming, reduction of carbon dioxide emissions has become an urgent issue, and there has been a strong demand for improving fuel efficiency of automobiles. For this reason, measures to reduce the weight of the vehicle body by reducing the thickness of the component parts by increasing the tension of the steel plate, which is the vehicle body material, are being actively studied. However, increasing the tensile strength of steel sheets inevitably causes a decrease in press formability, so the development of steel sheets having both high tension and good press formability has been promoted, and so far a two-phase structure consisting of ferrite and martensite. Various composite steel sheets such as steel sheets and retained austenitic steel sheets having transformation-induced plasticity have been applied to automobile parts, and have achieved certain effects.

  Recently, it has been decided that regulations on carbon dioxide emissions will be tightened in the near future, and the goal of reducing the weight of vehicle bodies has become very sophisticated. For this reason, it is necessary to reduce the thickness of parts with high forming difficulty, where steel sheets with a TS of 540 MPa or less were used in the past, and high strength steel sheets with a TS of 700 to 900 MPa that have the same press formability as conventional steel sheets. Is strongly desired.

  Under these circumstances, application of high-Mn austenitic steel to high-tensile steel sheets, which has been rarely applied to sheet metal materials, has been studied. High-Mn austenitic steel has austenite as the main phase even at room temperature and has been used as a non-magnetic steel or a low-temperature steel in the past, but it exhibits remarkable work hardening and extremely high ductility due to twin-induced plasticity of austenite. A new type of high-ductility high-tensile steel sheet utilizing this effect has been proposed. For example, in Patent Document 1, by weight, C: 1.0% or less, Si: 0.01-2.50%, Mn: 10-30%, sol.Al: 0.001-0.10%, P: 0.05% or less, S: 0.05 A steel slab having a steel composition consisting of iron and inevitable impurities with the balance being less than or equal to 1,100 ° C. or higher, after heating to a total rolling reduction of 90% or more of rough rolling and finish rolling, and a finishing temperature of 800 ° C. Finishing continuous hot finish rolling so that the final sheet thickness is 1.1 to 5.0 mm, and then cooling to 650 ° C. or less at a cooling rate of 10 to 100 ° C./s. A method for producing a high-strength hot-rolled steel sheet is disclosed. Patent Document 2 includes mass%, C: 1.00% or less, Mn: 7.00 to 30.00%, Al: 1.00 to 10.0%, Si: 2.50% to 8.00% or less, Al + Si: 3.50% to 12.00% Below, B: more than 0.00% and less than 0.01%, and as optional components, Ni: less than 8.00%, Cu: less than 3.00%, N: less than 0.60%, Nb: less than 0.30%, Ti: less than 0.30%, V: 0.30% A high-strength lightweight steel strip or steel plate excellent in cold formability having less than P and less than 0.01% is disclosed. Furthermore, Patent Document 3 includes, by weight, C: 0.5 to 0.7%, Mn: 17 to 24%, Si: 3% or less, Al: 0.050% or less, S: 0.030% or less, P: 0.08% or less, N: 0.1% or less, and as an option, Cr: 1% or less, Mo: 0.40% or less, Ni: 1% or less, Cu: 5% or less, Ti: 0.50% or less, Nb: 0.50% or less, V: It contains one or more of elements such as 0.50% or less, the balance is composed of Fe and inevitable impurities, the recrystallization rate exceeds 75%, the area ratio of carbide is less than 1.5%, average austenite Fe-C-Mn austenitic hot-rolled steel sheets with a grain size of less than 18μm exceeding 900MPa, TS x El (El: elongation at break) exceeding 45000MPa ・%, TS exceeding 950MPa, TS x El 45000MPa ・% An excess of Fe—C—Mn austenitic cold rolled steel sheet is disclosed. Furthermore, Patent Document 4 contains C: 0.15 to 0.70 wt%, Si: 0.10 to 3.00 wt%, Mn: 12 to 30 wt%, Ti: 0.01 to 0.10 wt%, the balance being iron and inevitable impurities A steel ingot or slab satisfying 60 × Cwt% + Mnwt% ≧ 36 wt% with respect to the content of C and Mn and having a cleanness of 0.03% or less with respect to the amount of non-metallic inclusions is 1050 to 1250 ° C. Discloses a method for producing a high-Mn non-magnetic steel having excellent local deformability, in which hot rolling is performed at a finishing temperature of 900 ° C. after heating.

Japanese Patent Laid-Open No. 4-259325 Special Table 2004-521192 Special Table 2006-528278 Japanese Patent Laid-Open No. 5-171273

  However, in the high-Mn austenitic steel sheets described in Patent Documents 1 to 4, the so-called plastic instability phenomenon that the work hardening behavior becomes unstable in a high strain region is likely to occur, so that it does not cause necking during press forming. There is a problem that it easily breaks.

  An object of the present invention is to provide a high-tensile steel sheet having a high ductility having a TS of 700 to 900 MPa and a method for producing the same, which can avoid a sudden break during press forming.

  As a result of intensive studies to achieve the above object, the present inventors have found the following.

  i) In order to avoid sudden breakage during press forming, the local elongation (l-El) measured using a No. 13B test piece specified in JIS Z2201 needs to be 5% or more.

  ii) In order to increase l-El to 5% or more, it is effective to form a microstructure composed of Ni addition of 3% by mass or more and recrystallized austenite grains having an average particle diameter of 5 μm or more.

  The present invention has been made based on such knowledge, and in mass%, C: 0.5 to 1.5%, Si: 0.1% or less, Mn: 10 to 25%, P: 0.1% or less, S: 0.05 %, Al: 0.1% or less, Ni: 3.0 to 8.0%, Mo: 0.1% or less, N: 0.01% or less, the balance is composed of Fe and inevitable impurities, the average particle size is 5 Provided is a high-tensile steel sheet having excellent ductility, characterized by having a microstructure composed of recrystallized austenite grains of ˜30 μm or other structures having an area ratio of 1% or less.

  The high-tensile steel sheet of the present invention is, for example, a steel slab having the above-described composition composition, reheated to a heating temperature of 1100 to 1300 ° C, hot-rolled at a finishing temperature of 800 ° C or higher, and a temperature range of 800 ° C or lower Is cooled to at least 600 ° C. at a cooling rate of 20 ° C./s or higher, and wound at a winding temperature of 600 ° C. or lower.

  After winding, the scale is further removed and annealed at an annealing temperature of 750 to 1050 ° C. The temperature range from the annealing temperature to at least 450 ° C is cooled at a cooling rate of 10 ° C / s or more, and the scale is removed. After cold rolling, annealing is performed at an annealing temperature of 750 to 1050 ° C., and a temperature range from the annealing temperature to at least 450 ° C. can be cooled at a cooling rate of 10 ° C./s or more.

  That is, the high-tensile steel sheet of the present invention is a hot-rolled steel sheet (hereinafter referred to as hot-rolled steel sheet), a hot-rolled steel sheet, a hot-rolled steel sheet, a hot-rolled steel sheet that has been cold-rolled and then annealed. Any steel plate.

  According to the present invention, sudden breakage during press forming can be avoided, and a high-tensile steel sheet with excellent ductility having a TS of 700 to 900 MPa can be produced. Since the high-tensile steel sheet of the present invention has an excellent strength-ductility balance, it can be applied to parts with a high degree of forming difficulty, and is extremely suitable for reducing the weight of an automobile body.

  The high-strength steel sheet excellent in ductility of the present invention and the manufacturing method thereof will be described in detail below. Note that “%” representing the amount of a component means “% by mass” unless otherwise specified.

1) Component composition
C: 0.5-1.5%
C is an essential element for stabilizing the austenite phase and plays a major role in increasing the tensile strength of steel. However, if the C content is less than 0.5%, the austenite phase is not sufficiently stabilized, and excellent ductility cannot be obtained. On the other hand, when the amount of C exceeds 1.5%, ductility decreases due to precipitation of carbides. Therefore, the C content is 0.5 to 1.5%, preferably 0.5 to 1.0%.

Si: 0.1% or less
Si is an element that can be added for deoxidation of steel. However, addition exceeding 0.1% as the Si content in the steel causes saturation of the deoxidation effect and increases in internal defects and surface defects due to an increase in inclusions. Therefore, the Si content is 0.1% or less. In order to obtain a sufficient deoxidation effect, the Si content is preferably 0.01 to 0.1%.

Mn: 10-25%
Like C, Mn is an essential element for stabilizing the austenite phase. However, if the Mn content is less than 10%, the austenite phase is not sufficiently stabilized, and excellent ductility cannot be obtained. On the other hand, if the amount of Mn exceeds 25%, the hot workability of the steel decreases and the manufacturability of the steel sheet is impaired. Therefore, the Mn content is 10 to 25%, preferably 15 to 25%. Furthermore, in order to stably realize the effect of improving ductility by twinning induced plasticity, it is preferable to control the amounts of C and Mn so as to satisfy the following formula (1).
32 ≦ 20 × [C] + [Mn] ≦ 36 (1)
However, [C] and [Mn] represent the contents of C and Mn, respectively.

  C and Mn affect the stabilization of the austenite phase as described above. The inventors examined the relationship between the stabilization of the austenite phase and the material properties, particularly the TS × El balance, and the C amount and Mn amount are within the scope of the present application, and the C amount and Mn amount are expressed by the formula (1). When satisfying the above, it was found that the TS × El balance was particularly good. This is because when 20 × [C] + [Mn] is less than the formula (1), that is, less than 32, the austenite phase is unstable and is likely to undergo martensitic transformation, while 20 × [C] + [Mn ] Exceeds (1), that is, exceeds 36, it is considered that the stacking fault energy becomes too high, and twin-induced plasticity hardly occurs.

P: 0.1% or less
When the P content exceeds 0.1%, the toughness of the steel decreases. Therefore, the P content is 0.1% or less, preferably 0.05% or less.

S: 0.05% or less
When the amount of S exceeds 0.05%, the hot workability of the steel decreases. Therefore, the S content is 0.05% or less, preferably 0.02% or less. More preferably, it is 0.01% or less.

Al: 0.1% or less
Al is an element that can be added for deoxidation of steel. However, the addition of more than 0.1% as the Al content in the steel causes saturation of the deoxidation effect and increases in internal defects and surface defects due to an increase in inclusions. Therefore, the Al content is 0.1% or less. In order to obtain a sufficient deoxidation effect, the Al content is preferably 0.01 to 0.1%.

Ni: 3.0-8.0%
Ni is the most important element in the present invention, and has the effects of increasing the stacking fault energy of steel, stabilizing the twin induced plasticity, and increasing the ductility. In particular, it is effective for suppressing plastic instability in a high strain region, and effective for improving l-El of a high Mn austenitic steel sheet. In order to obtain such effects sufficiently, the Ni content needs to be 3.0% or more. However, if the Ni content exceeds 8.0%, the effect is saturated and the manufacturing cost is increased. Therefore, the amount of Ni is set to 3.0 to 8.0%, preferably 3.0 to 6.0%.

Mo: 0.1% or less
Mo is an element that delays the recrystallization of steel and contributes to increasing the tensile strength of steel through refinement of austenite grains. In order to obtain such an effect, the Mo content is preferably 0.01% or more. However, when the Mo content exceeds 0.1%, TS exceeds 900 MPa, the tension becomes excessively high, and the ductility is significantly reduced. Therefore, the Mo content is 0.1% or less, preferably 0.05% or less.

N: 0.01% or less
If the N content exceeds 0.01%, the ductility of the steel decreases. Therefore, the N content is 0.01% or less, preferably 0.005% or less.

  The balance is Fe and inevitable impurities.

2) Microstructure The high-tensile steel sheet of the present invention has a microstructure composed of recrystallized austenite grains having an average grain size of 5 to 30 μm or other structures having an area ratio of 1% or less. In order to achieve high ductility by utilizing twin-induced plasticity of the austenite phase, the microstructure needs to be an austenite single phase. Further, in order to stably develop twin-induced plasticity up to a high strain region, it is necessary that the austenite grains are recrystallized grains whose internal strain energy is sufficiently released. Further, if the average grain size of the austenite grains is less than 5 μm, deformation twins are hardly generated in a high strain region, and a plastic instability phenomenon occurs. Therefore, in the high-tensile steel sheet of the present invention, the average grain size of the recrystallized austenite grains is 5 μm or more, preferably 10 μm or more. On the other hand, when the average particle size exceeds 30 μm, it is difficult to obtain a desired TS. For this reason, the average grain size of the recrystallized austenite grains is set to 30 μm or less.

  In addition, in a high Mn austenitic steel sheet like the present invention, other structures other than recrystallized austenite grains such as iron carbide and martensite phase may be generated depending on the cooling rate after hot rolling and the cooling rate after annealing. . In order to stably obtain high tension and excellent ductility, it is preferable to suppress these generations as much as possible. However, if the area ratio of the entire structure is about 1% or less, the object of the present invention is impaired. There is nothing. That is, the high-tensile steel sheet of the present invention has a microstructure composed of recrystallized austenite grains having an average grain size of 5 to 30 μm, or another structure such as an iron carbide or martensite phase having an area ratio of 1% or less. It is. The high-tensile steel sheet of the present invention has a microstructure in which the average grain size of recrystallized austenite grains is 5 to 30 μm and the area ratio of the recrystallized austenite grains in the entire steel sheet structure is 99% or more.

  Here, the average grain size of the recrystallized austenite grains is obtained by observing the structure of the thickness 1/4 position of the steel plate in the rolling direction parallel section at several magnifications of 1000 to 5000 times and using phase identification by EBSD analysis. And obtained by image analysis. Whether or not it is a recrystallized grain was judged by whether the aspect ratio was less than 2 depending on the crystal grain shape, or further confirmed by using the estimation of intra-grain strain by EBSD analysis.

3) Production conditions The following are preferred production conditions for the steel sheet of the present invention. In addition, the manufacturing method of the high strength steel plate of this invention is not limited to the following.

Steel slab heating temperature: 1100-1300 ℃
When the heating temperature of the steel slab exceeds 1300 ° C., hot workability deteriorates and energy required for heating increases. On the other hand, when the heating temperature is less than 1100 ° C., the load during hot rolling is increased. Therefore, the heating temperature of the steel slab is set to 1100 to 1300 ° C, preferably 1150 to 1250 ° C. In the heating of the steel slab, the steel slab cooled to room temperature may be reheated, or the steel slab having a high temperature during cooling after casting may be supplementarily heated or heat-retained.

Finishing temperature during hot rolling: 800 ° C or higher If the finishing temperature during hot rolling is less than 800 ° C, recrystallization and grain growth do not proceed sufficiently, and it is easy to become a hot rolled steel sheet with unrecrystallized grains remaining. Thereafter, when cold rolling is performed, the rolling load is increased. Therefore, the finishing temperature during hot rolling is 800 ° C. or higher, preferably 850 ° C. or higher. On the other hand, when the finishing temperature exceeds 1050 ° C., the crystal grains tend to be excessively coarsened, and the strength and ductility may be reduced. Therefore, the finishing temperature is desirably 1050 ° C. or lower. In order to secure the finishing temperature, the steel sheet being rolled can be supplementarily heated using a heating device such as an edge heater or a bar heater.

Cooling rate after hot rolling: 20 ° C / s or more in the temperature range of 800 ° C or less After cooling in the temperature range of 800 ° C or less at a cooling rate of less than 20 ° C / s, iron carbide is formed during cooling. Precipitates and ductility decreases. Therefore, after hot rolling, it is necessary to cool a temperature range of 800 ° C. or lower to at least 600 ° C. at a cooling rate of 20 ° C./s or higher. Note that if the cooling rate after hot rolling exceeds 100 ° C./s, recrystallization may not be completed, and therefore the cooling rate after hot rolling is preferably 100 ° C./s or less.

  When the finishing temperature exceeds 800 ° C., the temperature range up to 800 ° C. can be allowed to cool for 1 to 10 seconds (air cooling) in order to promote recrystallization. In this case as well, in the temperature range of 800 ° C. or lower, cooling is performed at a cooling rate of 20 ° C./s or higher to at least 600 ° C.

Winding temperature: 600 ° C. or less When the winding temperature exceeds 600 ° C., iron carbide is generated in the slow cooling process after winding, resulting in a decrease in ductility. Therefore, the coiling temperature is 600 ° C. or less, preferably 550 ° C. or less.

  The hot-rolled steel sheet produced in this way can be the high-tensile steel sheet of the present invention as it is, but after removing the scale of the hot-rolled steel sheet or after cold-rolling after removing the scale of the hot-rolled steel sheet, Annealing can also be performed under the following annealing conditions. The scale removal may be performed according to a conventional method such as pickling.

Annealing conditions: Annealing temperature: 750 to 1050 ° C, Cooling rate in temperature range from annealing temperature to at least 450 ° C: 10 ° C / s or more Annealing at 750 to 1050 ° C to promote grain growth of hot rolled steel sheet Annealing can be performed at a temperature. It is more preferable to perform annealing at an annealing temperature of 800 to 1000 ° C.

  Moreover, when annealing the steel plate which cold-rolled to the hot-rolled steel plate in order to set it as desired thickness, it is necessary to anneal at the annealing temperature of 750-1050 degreeC. This is because the microstructure of the steel sheet is a microstructure composed of recrystallized austenite grains having an average grain size of 5 to 30 μm. When the annealing temperature is less than 750 ° C., recrystallization is not completed and sufficient ductility cannot be obtained. On the other hand, when the annealing temperature exceeds 1050 ° C., the crystal grains are excessively coarsened, and the strength and ductility may be reduced. It is more preferable to perform annealing at an annealing temperature of 800 to 1000 ° C. The rolling reduction in cold rolling may be a rolling reduction that can be set to a desired thickness, and is not particularly limited, but is preferably about 50 to 70% from the viewpoint of production efficiency.

  Regardless of whether or not cold rolling is performed, if the cooling rate from the annealing temperature to at least 450 ° C. is less than 10 ° C./s, iron carbide is generated and ductility is reduced. Therefore, the temperature range from the annealing temperature to at least 450 ° C. needs to be cooled at a cooling rate of 10 ° C./s or more.

  To melt the steel of the present invention, both a converter and an electric furnace can be used. The steel thus melted is made into a slab by ingot-bundling rolling or continuous casting. If necessary, it is preferable to perform various pretreatments, secondary refining, surface treatment of slabs, and the like. Moreover, it is preferable from a viewpoint of productivity to implement annealing with a continuous annealing facility. The effect of the present invention is not impaired even if various types of plating are applied to a hot-rolled steel plate or a steel plate after annealing. The hot-rolled steel sheet, the steel sheet after annealing, or the steel sheet after plating treatment can be subjected to temper rolling for shape correction and surface roughness adjustment. Furthermore, the steel plate of the present invention can be subjected to various surface treatments such as painting and coating.

  Steel slabs of steels A to K having the composition shown in Table 1 were hot-rolled under the hot rolling conditions shown in Table 2 to obtain a hot-rolled steel plate with a thickness of 3 mm, and after removing the scale by pickling, For steel sheets, further annealed under the annealing conditions shown in Table 2, or annealed after cold rolling under the cold rolling reduction and annealing conditions shown in Table 2, hot rolled, hot rolled + annealed, cold rolled + Annealed steel sheets 1 to 20 were produced.

  About the produced steel plate, the microstructure was investigated by the above-described method, and the phase composition and the average grain size of recrystallized austenite grains were obtained. In Table 3, when the structure other than the recrystallized austenite grains is observed in an area ratio of more than 1%, the structure of the phase indicates the type of other structure, and when the other structure is an area ratio of 1% or less, Recrystallized austenite. In addition, sample No. 13B specified in JIS Z 2201 was sampled along the rolling direction, and a tensile test was performed in accordance with the method specified in JIS Z 2241. TS, El, l-El, TS I asked for El. In addition, when TS × El was 60 GPa ·% or more, it was determined that the steel sheet was excellent in ductility.

  The results are shown in Table 3. Each of the steel sheets of the present invention has a microstructure composed of recrystallized austenite grains having an average grain size of 5 μm or more, TS is 700 to 900 MPa, l-El is 5% or more, TS × El is 60 GPa It can be seen that it is a high-tensile steel sheet with excellent ductility that can avoid sudden breaks during press forming. In addition, when the formula (1) is satisfied, it is found that TS × El is particularly excellent.

Claims (4)

  In mass%, C: 0.5 to 1.5%, Si: 0.1% or less, Mn: 10 to 25%, P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, Ni: 3.0 to 8.0%, Mo : 0.1% or less, N: 0.01% or less, the remainder is composed of Fe and inevitable impurities, the average grain size is 5-30μm recrystallized austenite grains or other area ratio of 1% or less A high-tensile steel sheet with excellent ductility, characterized by having a microstructure composed of the following structure.   The steel slab having the component composition according to claim 1 is reheated to a heating temperature of 1100 to 1300 ° C, hot-rolled at a finishing temperature of 800 ° C or higher, and a temperature range of 800 ° C or lower is 20 ° C / s or higher. A method for producing a high-tensile steel sheet having excellent ductility, wherein the steel sheet is cooled to at least 600 ° C. at a cooling rate of 5 ° C. and wound at a winding temperature of 600 ° C. or lower.   The scale is further removed after winding, and annealing is performed at an annealing temperature of 750 to 1050 ° C, and the temperature range from the annealing temperature to at least 450 ° C is cooled at a cooling rate of 10 ° C / s or more. 2. A method for producing a high-tensile steel sheet having excellent ductility as described in 2.   After winding, further descaling, cold rolling, annealing at an annealing temperature of 750 to 1050 ° C, and cooling the temperature range from the annealing temperature to at least 450 ° C at a cooling rate of 10 ° C / s or more 3. The method for producing a high-tensile steel sheet having excellent ductility according to claim 2.