WO2011122237A1 - Steel sheet with high tensile strength and superior ductility and method for producing same - Google Patents

Abstract

Disclosed is a steel sheet with high tensile strength and superior ductility having tensile strength of 700 - 900 MPa and capable of avoiding abrupt failure during press molding and method for producing the same. The steel sheet with high tensile strength and superior ductility is characterized by having a composition containing 0.5 - 1.5% C, 0.1% or less Si, 10 - 25% Mn, 0.1% or less P, 0.05% or less S, 0.1% or less Al, 3.0 - 8.0% Ni, 0.1% or less Mo, and 0.01% or less N (% by mass), with the remainder being Fe and unavoidable impurities and by having recrystallized austenite particles with an average particle diameter of 5 - 30 µm or, further, a microstructure formed from another structure with an areal proportion of 1% or less.

Description

High tensile steel plate with excellent ductility and method for producing the same

The present invention relates to a high-tensile steel plate used in the industrial field such as a transportation machine represented by an automobile, particularly 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 structure 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 weight reduction of vehicle bodies has become very advanced. Therefore, it is necessary to reduce the thickness even for parts with high forming difficulty in which steel sheets having a TS of 540 MPa or less have been used in the past, and high tensile steel sheets with a TS of 700 to 900 MPa having 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 conventionally used as non-magnetic steel or low temperature steel, 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, Patent Document 1 discloses that, by weight, C: 1.0% or less, Si: 0.01-2.50%, Mn: 10-30%, sol. A steel slab having a steel composition containing Al: 0.001 to 0.10%, P: 0.05% or less, S: 0.05% or less, with the balance being iron and inevitable impurities, at 1100 ° C. or more After heating, the continuous hot finish rolling is finished so that the total rolling reduction of rough rolling and finish rolling is 90% or more, the finishing temperature is 800 ° C. or more, and the final sheet thickness is 1.1 to 5.0 mm. A method for producing a high-strength hot-rolled steel sheet for automobile parts, which has excellent workability for winding after cooling to 650 ° C. or less at a cooling rate of ˜100 ° C./s, is disclosed.

Further, in Patent Document 2, by mass%, C: 1.00% or less, Mn: 7.00 to 30.00%, Al: 1.00 to 10.00%, Si: more than 2.50% 8 0.000% or less, Al + Si: more than 3.50% and less than 12.00%, B: more than 0.00% and less than 0.01%, and as optional components, Ni: less than 8.00%, Cu: 3.00% Excellent cold formability having N: less than 0.60%, Nb: less than 0.30%, Ti: less than 0.30%, V: less than 0.30%, P: less than 0.01% A high strength lightweight steel strip or steel plate is disclosed.

Furthermore, in Patent Document 3, by weight, C: 0.5 to 0.7%, Mn: 17 to 24%, Si: 3% or less, Al: 0.050% or less, S: 0.030% Hereinafter, 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: 0.50% or less, including one or more of the elements, the balance being composed of Fe and inevitable impurities, Fe having a crystallinity of over 75%, a carbide area ratio of less than 1.5%, an average austenite grain size of less than 18 μm of TS exceeding 900 MPa, and TS × El (El: elongation at break) exceeding 45000 MPa ·% -C-Mn austenitic hot-rolled steel sheet or TS exceeds 950 MPa, T × El is Fe-C-Mn austenitic cold-rolled steel sheet exceeds 45000MPa ·% 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% And the balance is made of iron and inevitable impurities, satisfies 60 × Cwt% + Mnwt% ≧ 36 wt% with respect to the contents of C and Mn, and has a cleanness of 0.03% or less with respect to the amount of nonmetallic inclusions A method for producing a high-Mn nonmagnetic steel excellent in local deformability is disclosed in which a lump or steel slab is heated to 1050 to 1250 ° C. and then hot rolled at a finishing temperature of 900 ° C.

JP-A-4-259325 JP-T-2004-521192 JP-T-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. 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 breaks during press forming, the local elongation (l-El: local elongation) measured using a No. 13B test piece defined 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 comprising 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 on the basis of such knowledge. In mass%, C: 0.5 to 1.5%, Si: 0.1% or less, Mn: 10 to 25%, P: 0 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 has a component composition composed of Fe and inevitable impurities, and has a recrystallized austenite grain having an average particle diameter of 5 to 30 μm or a microstructure composed of other structures having an area ratio of 1% or less. A high-tensile steel sheet with excellent ductility is provided.

The high-tensile steel sheet of the present invention is, for example, a steel slab having the above component composition is re-heated to a heating temperature of 1100 to 1300 ° C. and then 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, 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 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 annealed steel sheet, a hot-rolled steel sheet annealed after cold rolling. Any steel plate.

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

The high-strength steel sheet having excellent ductility and a method for producing the same according to the present invention will be described in detail below. “%” Representing the amount of the component means “mass%” unless otherwise specified.

1) Component composition C: 0.5 to 1.5%
C is an essential element for stabilizing the austenite phase and plays a large role in increasing the tensile strength of steel. However, if the amount of C is less than 0.5%, the austenite phase is not sufficiently stabilized, and excellent ductility cannot be obtained. On the other hand, if the amount of C exceeds 1.5%, the 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, the addition of more than 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 amount is set to 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%
Mn, like C, is an essential element for stabilizing the austenite phase. However, if the amount of Mn 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 is lowered and the manufacturability of the steel sheet is impaired. Therefore, the amount of Mn 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 have studied the relationship between the stabilization of the austenite phase and the material properties, particularly the TS × El balance, and the amounts of C and Mn are within the scope of the present application, and the amounts of C and Mn are expressed by the formula (1). It has been found that the TS × El balance is particularly good when the above is satisfied. This is because when 20 × [C] + [Mn] is less than the formula (1), that is, less than 32, the austenite phase is unstable and easily martensitic, whereas 20 × [C] + [Mn ] Exceeds (1), that is, exceeds 36, the stacking fault energy becomes too high, and twin-induced plasticity is unlikely to occur.

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

S: 0.05% or less If 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 Al exceeding 0.1% 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 effect of increasing the stacking fault energy of steel, stabilizing the expression of twin-induced plasticity, and increasing ductility. In particular, it is effective for suppressing plastic instability in a high strain region, and is 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 amount of Ni 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 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 amount is preferably 0.01% or more. However, if the Mo content exceeds 0.1%, TS exceeds 900 MPa, excessive tension is increased, and ductility is significantly reduced. Therefore, the Mo amount is 0.1% or less, preferably 0.05% or less.

N: 0.01% or less When the N amount exceeds 0.01%, the ductility of the steel is lowered. 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. Furthermore, if the average particle 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 diameter exceeds 30 μm, it is difficult to obtain a desired TS. For this reason, the average grain size of the recrystallized austenite grains is 30 μm or less.

In the high Mn austenitic steel sheet as in 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 in the entire structure is about 1% or less, the object of the present invention is impaired. There is nothing. That is, the high-strength 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 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 at the 1/4 thickness position in the rolling direction parallel cross section of the steel sheet by several field SEM at a magnification of 1000 to 5000 times, and using phase identification by EBSD analysis in combination. And obtained by image analysis. Whether or not it is a recrystallized grain was determined by determining whether the aspect ratio was less than 2 depending on the crystal grain shape, or by further confirming the amount of strain in the grain by EBSD analysis.

3) Manufacturing conditions Below, the preferable manufacturing conditions of this invention steel plate are shown. In addition, the manufacturing method of the high strength steel plate of this invention is not limited to the following.

Heating temperature of steel slab: 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 kept warm.

Finishing temperature at the time of hot rolling: 800 ° C. or more If the finishing temperature at the time of hot rolling is less than 800 ° C., recrystallization and grain growth do not proceed sufficiently, and it becomes easy to become a hot rolled steel sheet in which unrecrystallized grains remain. 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 a temperature range of 800 ° C. or less When hot cooling and cooling a 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. In addition, since recrystallization may not be completed when the cooling rate after hot rolling exceeds 100 ° C./s, 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 lower, preferably 550 ° C. or lower.

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 the steel sheet as hot rolled Annealing can be performed at a temperature. It is more preferable to perform annealing at an annealing temperature of 800 to 1000 ° C.

In addition, when annealing a steel sheet that has been cold-rolled to a hot-rolled steel sheet in order to obtain a desired thickness, it is necessary to perform annealing at an annealing temperature of 750 to 1050 ° C. This is because the steel sheet has a microstructure made of recrystallized austenite grains having an average grain size of 5 to 30 μm. If 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 anneal at an annealing temperature of 800 to 1000 ° C. Note that the rolling reduction of the 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 the presence or absence of cold rolling, when 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.

Both converters and electric furnaces can be used for melting the steel of the present invention. 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 having a thickness of 3 mm, and after removing the scale by pickling, The steel sheet is further annealed under the annealing conditions shown in Table 2, or is annealed after cold rolling under the cold rolling reduction and annealing conditions shown in Table 2, and as hot rolled, hot rolled + annealed, cold rolled + Annealed steel sheets 1 to 20 were produced.

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

The results are shown in Table 3. Each of the steel plates 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, 1-El is 5% or more, and TS × El is 60 GPa · It can be seen that this is a high-tensile steel plate with excellent ductility that can avoid sudden breakage during press forming. Moreover, when satisfying said (1) Formula, it turns out that it is excellent in especially TSxEl.

Claims (4)

1. 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, with the balance being composed of Fe and unavoidable impurities, average grains A high-tensile steel sheet having excellent ductility, characterized by having a microstructure composed of recrystallized austenite grains having a diameter of 5 to 30 µm or other structures having an area ratio of 1% or less.
2. The steel slab having the component composition according to claim 1 is reheated to a heating temperature of 1100 to 1300 ° C, and then 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 less.
3. The scale is removed after winding, and 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 is cooled at a cooling rate of 10 ° C / s or more. The manufacturing method of the high-tensile steel plate excellent in the ductility as described in 2.
4. After winding, further descaling and 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. The manufacturing method of the high-tensile steel plate excellent in ductility of Claim 2 characterized by these.