JP2017039973A - Thin steel sheet for hot molding excellent in moldability and strength increase and hot molding method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet for hot molding capable of enhancing press moldability by hot molding of a high strength steel sheet with TS of 780 MPa or more while avoiding problems such as weldability, low temperature toughness and chemical conversion treatment by mold engineering and capable of increasing strength in use environment thereof (ordinary temperature) for a member after hot molding.SOLUTION: There is provided a steel having a component composition containing, by mass%, C:0.04 to 0.1%, Si:0.5% to 1.2%, Mn:2.5 to 3.5%, P:0.001 to 0.05%, S:0.0001 to 0.01%, Al:0.001 to 0.1%, N:0.0005 to 0.01% and Nb:0.01 to 0.1% and the balance Fe with inevitable impurities and a steel structure containing, by area percentage of polygonal ferrite:20% or more, bainite:10% or more and residual austenite:3% or more and having average crystal particle diameter of the residual austenite of 5 μm or less and further C concentration in the residual austenite after molding with strain amount of 0.10 at 300°C, (Cγ) of 0.2% or more.SELECTED DRAWING: Figure 2

Description

The present invention relates to a high-strength thin steel sheet for warm forming having a tensile strength (TS) of 780 MPa or more, which is suitable mainly for structural members of automobiles, and a warm forming method thereof, and particularly has high ductility during warm forming. In addition to obtaining excellent press formability by showing, it is intended to achieve high member strength by increasing the strength significantly after molding.
Examples of the high-strength thin steel sheet targeted in the present invention include high-strength hot-rolled steel sheets, high-strength cold-rolled steel sheets, and high-strength hot-dip galvanized steel sheets. In the present invention, the thin steel sheet means a sheet having a thickness of 10 mm or less in the case of a hot-rolled steel sheet, and a sheet thickness of 3 mm or less in the case of a cold-rolled steel sheet and a plated steel sheet.

In recent years, for the purpose of ensuring passenger safety in the event of a collision and improving fuel efficiency by reducing the weight of the vehicle body, the application of high-strength steel sheets having a TS of 780 MPa or more and a thin thickness to automobile structural members has been actively promoted. However, in general, increasing the strength of a steel sheet leads to a decrease in press formability of the steel sheet, so a steel sheet having both high strength and excellent formability is desired.
On the other hand, various techniques corresponding to said request | requirement are examined by performing warm forming which performs a plastic working in the state which heated the steel plate.

  As a technique for improving formability by utilizing deformation resistance reduction by performing warm forming, for example, Patent Document 1 discloses mass%, C: 0.010 to 0.10%, Si: 0.05 to 2.0%, and Mn: 0.50 to 3.00%, P: 0.003 to 0.15%, S: 0.01% or less, with the balance having a component composition composed of Fe and inevitable impurities, and the structure is composed of ferrite as the main phase and the second phase. A thin steel sheet excellent in warm press formability, characterized in that it is composed mainly of a certain martensite and (YS at 100 ° C.) / (YS at 20 ° C.) is 0.50 or less, has been proposed.

  In Patent Document 2, in mass%, C: 0.03 to 0.2%, Si: 0.5% or less, Mn: 1 to 3%, P: 0.1% or less, S: 0.1% or less, Cr: 0.01 to 1%, Al: 0.01 to 0.1%, N: 0.02% or less, with the balance being composed of Fe and impurities, characterized in that the ratio of the tensile strength at 450 ° C to the tensile strength at room temperature is 0.7 or less High strength thin steel sheets have been proposed.

  In Patent Document 3, C: 0.01 to 0.12%, Si: 2.0% or less, Mn: 0.01 to 2.0%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.0040 to 0.0200% by mass% And further comprising Ti: 0.001 to 0.1%, B: 0.01% or less, the composition consisting of the balance Fe and inevitable impurities, and a structure whose main phase is ferrite having an average crystal grain size of 8 μm or less, A high-tensile hot-rolled steel sheet excellent in warm press formability, characterized by having a solid solution N amount of 0.0040 to 0.0080% by mass%, has been proposed.

  Patent Document 4 discloses a high-strength hot-rolled steel sheet having a tensile property with a tensile strength TS at room temperature of 780 MPa or more and a yield ratio of 0.85 or more. The yield stress is 80% or less of the yield stress at room temperature and the total elongation is the total elongation at room temperature when heated to a temperature in the warm molding temperature range and subjected to a tensile test at the temperature in the warm molding temperature range. The yield stress when a tensile test is performed at room temperature after performing warm working to give a strain of 15% or less at a temperature in the warm forming temperature range and cooling to room temperature. A high-strength hot-rolled steel sheet for warm forming is proposed, which has a yield stress of 80% or more at room temperature and a total elongation of 80% or more of the total elongation at room temperature and is excellent in warm formability. ing.

  In Patent Document 5, the tensile strength at room temperature is 780 MPa or more, the yield stress in the heating temperature range from 400 ° C. to 700 ° C. is 80% or less of the yield stress at room temperature, and the total elongation in the heating temperature range is It is 1.1 times or more of the total elongation at room temperature, and the yield stress after cooling from the heating temperature to room temperature after applying a strain of 20% or less to the heating temperature range is the yield stress at room temperature before the heating. 70% or more, the total elongation after heating from the heating temperature to room temperature after applying 20% or less strain to the heating temperature range is 70% or more of the total elongation at room temperature before heating A high-strength steel sheet for warm forming has been proposed.

Patent Document 6 includes mass%, C: 0.05 to 0.3%, Si: 1 to 3%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.01% or less, Al: 0.001 to 0.1%, N: 0.002 to 0.03%, with the balance being the composition of iron and impurities, the area ratio to the whole structure, bainitic ferrite: 40 to 85%, residual austenite: 5 to 20%, It has a structure containing martensite + the above retained austenite: 10 to 50%, ferrite: 5 to 40%, and the above retained austenite has a C concentration (Cγ _R ) of 0.5 to 1.0% by mass. The presence of 1% or more of the ferrite grains in the area ratio relative to the entire structure reduces the strength sufficiently during warm molding at 150 to 250 ° C, while 980 MPa or more when used at room temperature after molding. A high-strength steel sheet excellent in warm formability that can secure a high strength is proposed.

Patent Document 7 includes mass%, C: 0.02 to 0.3%, Si: 1.0 to 3.0%, Mn: 1.8 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.001 to 0.1%, N: 0.01 to 0.03%, the balance is composed of iron and impurities, the area ratio to the whole structure, bainitic ferrite: 50-85%, residual austenite: 3% or more, martens Site + said retained austenite: 10 to 45%, ferrite: 5 to 40% of the structure containing each phase, C concentration (Cγ _R ) in the said retained austenite is 0.3 to 1.2% by mass A high-strength steel sheet having excellent formability at room temperature and warm, wherein a part or all of N in the component composition is solute N, and the amount of solute N is 30 to 100 ppm. Proposed.

In Patent Document 8, in mass%, C: 0.02 to 0.3%, Si: 1.0 to 3.0%, Mn: 1.8 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.001 to 0.1%, N: 0.002 to 0.008%, the balance is composed of iron and impurities, the area ratio to the whole structure, bainitic ferrite: 50 to 85%, residual austenite: 3% or more, martens Site + said retained austenite: 10 to 45%, ferrite: 5 to 40% of the structure containing each phase, C concentration (Cγ _R ) in the said retained austenite is 0.3 to 1.2% by mass A high-strength steel sheet excellent in formability at room temperature and warm is proposed, wherein a part of N in the component composition is solute N, and the amount of solute N is 12 ppm or less. Yes.
In Patent Documents 7 and 8, when processing in a temperature range of 100 to 250 ° C., the TRIP phenomenon used to obtain strength at room temperature is suppressed by increasing the amount of dissolved N, The strength at the warm is lowered, and the strength at room temperature is increased and the effect of reducing the molding load at the warm is improved at the same time.

On the other hand, as a technique for stabilizing the retained austenite in warm forming and improving formability, for example, Patent Document 9 discloses, in mass%, C: 0.05 to 0.6%, Si + Al: 0.5 to 3%, Mn : 0.5 to 3%, P: 0.15% or less, S: 0.02% or less, and the parent phase structure has bainitic ferrite and / or granular with an average hardness of 240 Hv or more in Vickers hardness. -Bainitic ferrite is contained in 70% or more of the entire structure, and the second phase structure contains residual austenite in the entire structure of 5-30%, The C concentration (Cγ _R ) in the retained austenite is 1.0% or more, and may further contain bainite / martensite. High strength steel sheets have been proposed.

In Patent Document 10, in mass%, C: 0.05 to 0.25%, Si: more than 1.00%, 2.5% or less, Al: 1.0% or less, Si + Al: 3% or less in total, Mn: 0.5 to 3%, P: 0.15 %, S: 0.02% or less, with the balance being a composition comprising iron and impurities, and the parent phase is an area of bainitic ferrite having an average Vickers hardness of 250 Hv or more with respect to the entire structure. The second phase contains the remaining austenite in an area ratio of 5 to 15% with respect to the whole structure, and the C concentration (Cγ _R ) in the residual austenite is 0.6% by mass or more and 1.0%. By making the structure less than% by mass and further containing bainite and / or martensite, it is possible to maximize the TRIP effect in warm conditions, and warm workability that can reliably increase the ductility. A high-strength steel sheet excellent in resistance is proposed.

Patent Document 11 includes, in mass%, C: 0.05 to 0.4%, Si + Al: 0.5 to 3%, Mn: 0.5 to 3%, P: 0.15% or less, S: 0.02% or less, with the balance being iron and impurities The total amount of martensite and / or bainitic ferrite in a total area of 45 to 80%, and polygonal ferrite in an area ratio of 5 to 5 Including 40%, including residual austenite in an area ratio of 5 to 20% with respect to the entire structure, and the C concentration (Cγ _R ) in the residual austenite being 0.6% by mass or more and less than 1.0% by mass; By adopting a structure that may contain bainite, it is possible to maximize the TRIP effect in the warm condition, while sacrificing the stretch flangeability to some extent, and with excellent warm workability that can further increase ductility. Strength steel plates have been proposed.

In Patent Document 12, in mass%, C: 0.05 to 0.3%, Si: 1 to 3%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.01% or less, Al: 0.001 to 0.1%, N: 0.002 to 0.03%, with the balance being the composition of iron and impurities, the area ratio to the whole structure, bainitic ferrite: 50 to 90%, residual austenite: 5 to 20%, Martensite + the above retained austenite: 10 to 50%, ferrite: 40% or less of the structure, the above retained austenite has a C concentration (Cγ _R ) of 0.5 to 1.2% by mass, its average circle By satisfying an equivalent diameter of 0.2 to 2 μm and an average aspect ratio (maximum diameter / minimum diameter) of less than 3.0, the amount of strain applied to the periphery of γ _R during processing-induced martensitic transformation is increased, thereby reducing flanges. Suppresses the progress of transformation to processing-induced martensite when compression such as A high strength steel sheet with excellent formability that can enhance deep drawability by increasing ductility of the vertical wall and promoting inflow of material from the flange has been proposed. .

In Patent Document 13, in mass%, C: 0.05 to 0.3%, Si: 1 to 3%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.01% or less, Al: 0.001 to 0.1%, N: 0.002 to 0.03%, with the balance being composed of iron and impurities, the area ratio to the whole structure, bainitic ferrite: 50 to 90%, residual austenite: 3% or more, martens Site + The above-mentioned residual austenite: 10 to 50%, ferrite: 40% or less of the structure, and the residual austenite has a C concentration (Cγ _R ) of 0.5 to 1.2% by mass. The presence of 0.3% or more of austenite surrounded by martensite makes it difficult for γ _R to be strained during plastic deformation, and suppresses work-induced transformation at the early stage of deformation, while at the later stage of deformation. However, by making it easier to cause deformation-induced transformation, work hardening is A high-strength steel sheet excellent in ductility that can be realized by surroundings has been proposed.

In Patent Document 14, in mass%, C: 0.02 to 0.3%, Si: 1.0 to 3.0%, Mn: 1.8 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.001 to 0.1%, N: 0.002 to 0.03%, the balance is composed of iron and impurities, the area ratio to the whole structure, bainitic ferrite: 45 to 85%, residual austenite: 3% or more, martens Site + said retained austenite: 10 to 50%, ferrite: 5 to 45% of the structure containing each phase, C concentration (Cγ _R ) of said retained austenite is 0.6 to 1.2% by mass, In the frequency distribution curve of the KAM (Kernel Average Misorientation) value, the relationship between the ratio X _{KAM ≦ 0.4 °} of the frequency where the KAM value is 0.4 ° or less to the total frequency and the area ratio Vα of the ferrite is X _{KAM ≦ 0.4 °} /Vα≧0.8 and exists at the interface between the ferrite and each phase other than the ferrite (hard second phase) Circle equivalent diameter 0.1μm or more cementite particles, by said hard second phase 1 [mu] m ² 3 per below, strength is increased in the low strain rate range, such as strength in the high strain rate range is lower, strength Using a material with high strain rate dependence, the TRIP phenomenon in the low strain rate region is promoted to promote work hardening and improve strength, thereby giving a strength difference between the punch shoulder and the reduced flange. By doing so, a high-strength steel sheet has been proposed that can improve the deep drawability and is excellent in warm deep drawability.

Patent Document 15 includes mass%, C: 0.02 to 0.3%, Si: 1.0 to 3.0%, Mn: 1.8 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.001 to 0.1%, N: 0.002 to 0.03%, the balance is composed of iron and impurities, the area ratio to the whole structure, bainitic ferrite: 50 to 85%, residual austenite: 3% or more, martens Site + the retained austenite: 10 to 45%, ferrite: 5 to 40% of the structure containing each phase, C concentration (Cγ _R ) in the retained austenite is 0.6 to 1.2% by mass Based on the Mn concentration distribution obtained by EPMA line analysis, the ratio Mnγ _R / Mn _av between the Mn concentration Mnγ _R in the retained austenite and the average Mn concentration Mn _av in the whole tissue is 1.2 or more. by, to achieve both improvement of uniform elongation by maximizing TRIP effect by improvement of ductility and gamma _R of the matrix (mother phase) Te, a high strength steel sheet excellent in deep drawability between room temperature and temperature may coexist at room temperature strength and deep drawability is proposed.

  In Patent Document 16, C: 0.10 to 0.30%, Si: more than 1.0% to 3.0% or less, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.010% or less, N: 0.0020 to 0.0300% or less, Al: 0.0010 to 0.1% is satisfied, the balance is composed of iron and inevitable impurities, and the microstructure is the area ratio of the whole structure with the sum of bainitic ferrite and bainite: 65% or more , Residual austenite: 5% or more, total of martensite and residual austenite: 35% or less, polygonal ferrite: 10% or less, and the rest of the structure: 5% or less, and the residual austenite The concentration of carbon in the steel is 1.3% or less and the interfacial spacing of bainitic ferrite and / or bainite is 1.4 μm or more, thereby further promoting the TRIP effect of residual austenite, and particularly ductility. And warm High strength with excellent ductility in warm and deep drawability that can improve ductility remarkably and increase the dependence of the material strength in the warm on the processing speed and also improve the deep drawability in the warm Steel plates have been proposed.

  In addition, as a technique for increasing the strength by processing-heat treatment during pressing, for example, Patent Document 17 describes, in wt%, C: 0.01 to 0.20%, Si: 0.01 to 3.0%, Mn: 0.01 to 3.0%, P: 0.002 to 0.2%, S: 0.001 to 0.020%, Al: 0.005 to 2.0%, N: 0.0002 to 0.01%, Mo: 0.01 to 1.5%, and further in wt%, Cr: 0.01 to 1.5%, Nb : 0.005 to 0.10%, Ti: 0.005 to 0.10%, V: 0.005 to 0.10%, B: 0.0003 to 0.005%, one or more of them are included, and the range of the specific component composition formula (A) A thin steel sheet excellent in heat-treating hardenability characterized by satisfaction has been proposed.

  In Patent Document 18, in mass%, C: 0.02 to 0.20%, Si: 0.5 to 2.0%, Mn: 0.5 to 2.5%, sol.Al: 0.15 to 1.2%, N: 0.020% or less, and Si ( %) + Sol.Al (%) ≥ 1.2 (%), the balance is Fe and inevitable impurities, contains 10% or more bainite by volume%, and the total of pearlite and martensite is volume% A high-tensile steel sheet for warm forming having a crystal structure of 10% or less has been proposed.

  Furthermore, as a technology that combines ductility improvement at the time of warm forming and strength increase at room temperature after warm forming, Patent Document 19 includes mass%, C: 0.04 to 0.2%, Si: 0.5 to 2.5%, A component composition containing Mn: 1.5 to 3.5%, P: 0.001 to 0.05%, S: 0.0001 to 0.01%, Al: 0.001 to 0.1%, N: 0.0005 to 0.01%, the balance being Fe and inevitable impurities. It has excellent formability and strength increasing capability, characterized in that the steel structure contains 30% or more of polygonal ferrite, 20% or more of martensite and less than 3% of retained austenite in terms of area ratio. A thin steel sheet for warm forming has been proposed.

Further, in Patent Document 20, in mass%, C: 0.1 to 0.3%, Si: 0.5 to 2.5%, Mn: 1.5 to 3.5%, P: 0.001 to 0.05%, S: 0.0001 to 0.01%, Al: 0.001 -0.1%, N: 0.0005-0.01%, with the balance being composed of Fe and inevitable impurities, steel structure is 40% or more of polygonal ferrite and 5% or more of bainite in area ratio And the thin steel plate for warm forming excellent in the formability and the strength raising ability characterized by containing 3% or more of retained austenite is proposed.
In Patent Documents 19 and 20, improvement of warm formability by stabilization of retained austenite by warm working or improvement of ductility by dynamic strain aging, and warm forming by strain aging hardening during warm forming It is possible to realize a subsequent increase in strength at room temperature.

JP 2000-87183 A Japanese Patent Laid-Open No. 2003-113442 JP 2001-234282 A JP 2013-133519 JP JP 2013-23721 JP 2013-181184 A Japanese Patent No. 5636347 JP 2013-40383 Japanese Patent Laid-Open No. 2004-190050 Japanese Patent No. 5537394 JP 2011-219859 JP 2012-122129 A JP 2012-122130 A JP 2012-180569 A JP 2012-180570 A JP 2012-188738 A JP 2000-234153 A Japanese Patent Laid-Open No. 2002-256388 JP 2012-107319 A JP 2012-92358 A

However, the techniques described in Patent Documents 1 and 2 are intended to improve dimensional accuracy using a decrease in deformation resistance in warm forming, and improve the formability (ductility) during warm forming. It is not a thing. In addition, the technique described in Patent Document 3 is intended to improve drawability using a change in deformation resistance in warm forming, and is applicable to molding without considering ductility during warm forming. There is a problem that the style is limited.
Moreover, none of Patent Documents 1 to 3 considers the strength at normal temperature (the environment in which the member is used) after warm working.

  The series of techniques described in Patent Documents 4 to 8 are mainly intended to reduce the forming load of super high tensile strength by reducing the strength at the time of warm forming at a relatively low temperature in residual austenitic steel. The improvement of ductility by warm forming is not intended, the strength at normal temperature (the environment in which the member is used) after the warm working remains as the original plate, and the increase due to the working-heat treatment is not considered.

  The technique described in Patent Document 9 is to improve the ductility and stretch flangeability by controlling the average hardness of the matrix structure, the C concentration in the retained austenite and the volume ratio thereof, but after warm working The strength at room temperature (the environment in which the material is used) is not considered.

A series of techniques described in Patent Documents 10 to 16 include the composition ratio of the constituent phases including residual austenite in the steel structure, the C concentration in the residual austenite, the form of each constituent phase, the internal structure, and the like. By controlling precisely, the stability of residual austenite, the local distribution of strain, the strain rate dependence of strength, and the like are controlled to improve the ductility and deep drawability of the retained austenitic steel. However, these patent documents mainly focus on improving the formability of ultra-high tensile strength, and are not intended to improve the strength by warm forming, and the strength at normal temperature (component usage environment) after warm working is It is only what maintains the same level as the original plate, and the rise due to the processing-heat treatment is not considered.
Furthermore, these series of patents are directed to residual austenitic steel, and it is essential to contain C and Si in a high concentration, which becomes a problem when they are added excessively. Practical properties such as processability, spot weldability, and low temperature toughness are not taken into consideration, and it is clear that these components are contained at a high concentration even in actually disclosed examples.

  The technique described in Patent Document 17 is intended to increase the strength by forming fine carbides by warm forming in a matrix structure having a high dislocation density, and the technique described in Patent Document 18 Is intended to increase the strength by strain-aging hardening of the matrix structure having a high dislocation density by warm forming, but none of them considers the formability during warm forming.

  The techniques described in Patent Documents 19 and 20 are intended to achieve both improvement in ductility during warm forming and improvement in strength at room temperature after warm forming. However, in Patent Document 19 in which residual austenite is not utilized, although the increase in strength due to strain aging is significant, it is difficult to say that the ductility in warm forming is sufficient. Patent Document 20 shows excellent ductility in warm forming due to the stability of retained austenite, but it is difficult to say that the increase in strength due to strain aging is sufficient. Furthermore, it is indispensable to contain C and Si at a high concentration, and consideration is given to practical properties such as chemical conversion, spot weldability, and low temperature toughness that become a problem when they are added excessively. It ’s hard to say.

  The present invention solves these problems, and in a high-strength steel sheet having a TS of 780 MPa or more, in consideration of practical properties such as weldability, low-temperature toughness, chemical conversion property, etc. The purpose is to provide a steel sheet for warm forming, together with its warm forming method, capable of significantly increasing the strength in the usage environment (room temperature) of the member after warm forming simultaneously with improvement of press formability. And

First, the experimental results on which the present invention is based will be described.
Steel X containing 0.095% C-0.78% Si-2.81% Mn-0.012% P-0.0011% S-0.040% Al-0.0033% N-0.055% Nb and 0.082% C-1.4 %% Si- Steel Y containing 2.63% Mn-0.011% P-0.0019% S-0.038% Al-0.0031% N-0.19% Cr-0.09% Mo, 0.121% C-0.15% Si-2.3% Mn-0.019% P- Steel Z containing 0.0010% S-0.032% Al-0.0024% N-0.23% Cr-0.19% Mo was melted in a laboratory in a vacuum melting furnace to obtain a slab. These were heated to 1250 ° C. and subjected to rough rolling, then finish rolling was performed at a temperature of 880 ° C., and a heat treatment equivalent to winding was performed at 620 ° C. to obtain a hot-rolled steel sheet. These hot-rolled steel sheets were acidified to remove the scale on the surface, and further subjected to cold rolling with a reduction rate of 50% to obtain cold-rolled steel sheets. Next, after soaking for 300 s at 750 to 900 ° C., cooling to 300 to 500 ° C., holding for 120 s and then cooling to room temperature, various steel sheets were produced.

About these steel plates, while identifying the steel structure, the area fraction of each structure was measured.
As for the microstructure, the steel microstructure was identified by magnifying the corrosion appearance structure due to the nitrile by 5000 times with a scanning electron microscope (SEM) on the thickness cross section parallel to the rolling direction of the steel sheet. This was analyzed by image analysis software (Image-Pro; manufactured by Cybernetics) to determine the area ratio of each phase. However, since martensite and retained austenite are difficult to distinguish, the total area ratio is obtained for these, and the area ratio of retained austenite is separately determined by the method described later, and this is subtracted to obtain martensite. The area ratio of the site was used.

In addition, after polishing the steel plate to the position of 1/4 thickness, the surface of which 0.1 mm was chemically polished was used as the measurement surface, and using Mo Kα rays with an X-ray diffractometer, the fcc iron (200), ( 220), (311) plane and bcc iron (200), (211), (220) plane of the integrated intensity of the peak is measured, fcc ratio is calculated for all the combinations, and the average value The volume ratio of the retained austenite was obtained and made three-dimensionally homogeneous, and this was used as the area ratio of the retained austenite. The average grain size of the retained austenite is determined by tempering in the portion corresponding to martensite by performing corrosion after performing heat treatment at 200 ° C. for 200 minutes in the above-mentioned SEM observation. Since it is easy to be released and shows a different form from retained austenite, the phase in which this substructure does not appear is regarded as retained austenite and obtained by performing the above-described image analysis. In the present invention, the lower limit value of the target retained austenite particle size was set to 0.01 μm, which is the measurement limit.
Further, the C concentration (Cγ) in austenite was calculated from the shift amount of the diffraction peak on the (220) plane of fcc iron.

  In addition, a JIS No. 5 tensile test specimen was taken in the direction perpendicular to the rolling direction, and a warm tensile test was performed in accordance with JIS Z 2241 at a crosshead speed of 20 mm / min, changing the test temperature from room temperature to 500 ° C. The mechanical properties were evaluated. Further, for some samples, the hot tensile test was stopped at a strain of 0.10 and cooled to room temperature, and then Cγ of retained austenite was measured. For some samples, the warm tensile test was stopped at a strain amount of 0.10, cooled to room temperature, and then the tensile test was performed again to evaluate the mechanical properties.

  In Steel X, the area ratio of polygonal ferrite is 52%, the average crystal grain size is 7.3 μm, the area ratio of bainite is 27%, the average crystal grain size is 5.4 μm, and the area ratio of residual austenite is In the sample 5% with an average grain size of 3.4 μm, Steel Y, the area ratio of polygonal ferrite is 56%, the average grain size is 15.8 μm, and the area ratio of bainite is 13%. In the sample, steel Z, in which the average crystal grain size is 11.0 μm, the retained austenite area ratio is 1%, and the average crystal grain diameter is 2.7 μm, the area ratio of polygonal ferrite is 57%, and the average crystal Evaluation was made on a sample with a grain size of 12.7μm, a bainite area ratio of 5%, an average crystal grain size of 9.0μm, a residual austenite area ratio of 2%, and an average crystal grain size of 2.5μm. Provided.

FIG. 1 shows the results of examining the relationship between the integrated values of TS and El during the warm tensile test and the test temperature for the samples X, Y, and Z described above.
As shown in the figure, Sample X shows that the integrated value of TS and El is remarkably high when the test temperature is in the range of 200 ° C. to 400 ° C., and it shows excellent moldability in the warm condition. On the other hand, in sample Y, an increase in the integrated value of TS and El was observed, but the temperature range was narrow and the increase in characteristics from room temperature was small. In Sample Z, no significant increase in the integrated value of TS and El was observed.

In addition, for the samples X, Y and Z described above, the tensile test in the warm was stopped at a strain amount of 0.10, and the C concentration (Cγ) in the retained austenite measured after cooling to room temperature was measured in the warm tensile test. FIG. 2 shows the results of examining the relationship between Cγ and the test temperature.
As shown in the figure, in sample X, Cγ increases when the test temperature is in the range of 200 ° C. to 400 ° C., and the residual γ is increased by increasing the C concentration in the retained austenite during the warm test process. It can be seen that it has been stabilized. In this respect, in samples Y and Z, no significant increase in Cγ was observed, and this effect was not manifested.

Further, for the above samples X, Y, and Z, the warm tensile test was stopped at a strain amount of 0.10, cooled to room temperature, and then the TS when the tensile test was performed at room temperature from the TS when the tensile test was performed again. The value obtained by subtracting the value, that is, the TS increase (ΔTS) was determined. The relationship between ΔTS and test temperature is shown in FIG.
As shown in the figure, in Sample X, ΔTS showed a remarkably high value in the test temperature range of 200 ° C. to 400 ° C., and showed a high strength increasing ability. On the other hand, in Samples Y and Z, no significant increase in ΔTS was observed.

Next, the present inventors, in a high-strength steel sheet having a TS of 780 MPa or more, based on the above-described experimental facts, improved ductility during warm forming, and increased strength at room temperature after warm forming As a result of intensive studies on how to achieve both, further gaining ingenuity, the following knowledge was obtained.
i) After optimizing the component composition so as to satisfy a specific relationship, the area ratio includes 20% or more of polygonal ferrite, 10% or more of bainite, and further 3% or more of retained austenite, and By making the average grain size of retained austenite 5 μm or less, in high-strength steel sheets with a TS of 780 MPa or more, improvement of ductility during warm forming, as well as improvement of formability, and a significant increase at room temperature after warm forming An increase in strength can be achieved at the same time.
ii) Improvement of such characteristics can be obtained by subjecting a thin steel plate having the above-described characteristics to a processing at a steel plate temperature of 200 to 400 ° C. and an equivalent plastic strain of 0.02 or more.

The present invention was developed after further research based on the above findings.
That is, the gist configuration of the present invention is as follows.
1. In mass%, C: 0.04-0.1%, Si: 0.5-1.2%, Mn: 2.5-3.5%, P: 0.001-0.05%, S: 0.0001-0.01%, Al: 0.001-0.1%, N: 0.0005- Contains 0.01% and Nb: 0.01-0.1%, the balance has a component composition consisting of Fe and inevitable impurities, the structure is area ratio, polygonal ferrite 20% or more, bainite 10% or more The austenite content is 3% or more, the average crystal grain size of the retained austenite is 5 μm or less, and the C concentration (Cγ ₃₀₀ ) in the retained austenite after molding with a strain amount of 0.10 at 300 ° C. is 0.2% or more. A thin steel sheet for warm forming which is excellent in formability and strength increasing ability, characterized by being.

2. 2. The thin steel sheet for warm forming as described in 1 above, wherein the average crystal grain sizes of polygonal ferrite and bainite are each 10 μm or less.

3. 3. The thin steel sheet for warm forming as described in 1 or 2 above, which contains martensite in an area ratio of 20% or more.

4). The volume fraction of retained austenite (Vγ ₀ ) at a strain amount of 0.10 at room temperature and the volume fraction of retained austenite (Vγ ₃₀₀ ) at a strain amount of 0.10 at 300 ° C. satisfy the following formula (1): The thin steel sheet for warm forming according to any one of 1 to 3.
Record
Vγ ₃₀₀ / Vγ ₀ ≧ 2.0 --- (1)

5). The work hardening rate (WHR ₀ ) at a strain amount of 0.05 at room temperature and the work hardening rate (WHR ₃₀₀ ) at a strain amount of 0.05 at 300 ° C. satisfy the following formula (2), and the tensile strength (TS at room temperature) ₀ ) and the tensile strength (TS ₃₀₀ ) at room temperature after applying pre-processing with a strain amount of 0.10 at 300 ° C. satisfies the following formula (3): Steel sheet for warm forming.
Record
WHR ₃₀₀ / WHR ₀ ≧ 1.2 --- (2)
TS ₃₀₀ − TS ₀ ≧ 150 MPa --- (3)

6). The decrease in tensile strength in the temperature range of 200 to 400 ° C with respect to the tensile strength at room temperature is 150 MPa or less, and the increase in yield strength in the temperature range of 200 to 400 ° C with respect to the yield strength at room temperature is 50 MPa or less. The thin steel sheet for warm forming as described in any one of 1 to 5 above.

7). The warm according to any one of 1 to 6 above, further comprising at least one element selected from Ti: 0.005 to 0.1% and V: 0.005 to 0.1% as a component composition by mass% Thin steel sheet for forming.

8). The thin steel sheet for warm forming as described in any one of 1 to 7 above, wherein the composition further contains B: 0.0003 to 0.0050% by mass%.

9. The composition further comprises at least one element selected from Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Ni: 0.01 to 1.0% and Cu: 0.01 to 1.0% in terms of mass%. The thin steel sheet for warm forming as described in any one of 1 to 8 above.

Ten. The thin steel sheet for warm forming according to any one of 1 to 9 above is excellent in formability and strength increasing capability, characterized by adding a processing of a steel sheet temperature of 200 to 400 ° C. and an equivalent plastic strain of 0.02 or more. Warm forming method.

  According to the present invention, in a high-strength steel sheet having a TS of 780 MPa or more, by applying warm forming, the press formability is improved and the degree of freedom of part shape is increased, and the high degree of difficulty in forming parts is increased. Strengthening is possible. Furthermore, by applying the structural member manufactured according to the present invention to the automobile body, it is possible to further improve the safety of the occupant and improve the fuel consumption by significantly reducing the weight of the vehicle body.

It is the figure which showed the relationship between the test temperature at the time of a warm tension test, and TSxEl value about steel X, Y, and Z. FIG. A graph showing the C concentration (Cγ) in retained austenite after stopping the tensile test at a strain of 0.10 for steel X, Y and Z and cooling to room temperature in relation to the test temperature. It is. For steel X, Y and Z, the tensile test in warm was stopped at a strain amount of 0.10, and after cooling to room temperature, the increase in TS (ΔTS) when the tensile test was performed again in relation to the test temperature. FIG. It is the figure which showed the press molding point. It is the figure which showed the opening amount of the molded article (hat member).

Hereinafter, the present invention will be specifically described. Note that “%” representing the content of component elements means “% by mass” unless otherwise specified.
1) Component composition C: 0.04 to 0.1%
C is an important element for strengthening steel, and has high solid solution strengthening ability, and is indispensable for stabilizing austenite and improving ductility by utilizing retained austenite during warm forming. Element. In addition, during warm forming, C in the steel exhibits a strong interaction at the forming temperature with the movable dislocations introduced in the process, and the movable dislocations remain fixed with C even after cooling to room temperature. The strength when deformed at room temperature after warm working is significantly increased. In order to obtain such effects, it is necessary to add at least 0.04% of C. On the other hand, excessive addition of C exceeding 0.1% deteriorates spot weldability, which is indispensable for the application of thin steel sheets, and martensite significantly hardens to lower ductility, or adversely affects low temperature toughness. In the present invention, it is possible to achieve stabilization of retained austenite while suppressing the addition of C by the structure control by the effect of Nb addition described later. Therefore, the upper limit of the C amount is 0.1%.

Si: 0.5-1.2%
Si increases the fraction of polygonal ferrite and suppresses the formation of carbides while unevenly distributing C in austenite, thereby increasing the concentration of solid solution C in austenite and stabilizing residual austenite. Further, it has an action of limiting the slip system during warm forming, promotes the growth of movable dislocations, increases the dislocation density, and contributes to an increase in strength by interaction with solute C. In order to fully and fully exhibit these actions, it is necessary to add 0.5% or more of Si. More preferably, it is 0.6% or more. However, the addition of Si exceeding 1.2% not only saturates the above-described effects, but also causes serious problems in the surface properties, and significantly deteriorates the corrosion resistance after coating by inhibiting chemical conversion treatment. Furthermore, cleavage fracture is promoted and low temperature toughness is reduced. In the present invention, stabilization of retained austenite is achieved while suppressing the addition of Si by controlling the structure by the effect of Nb addition described later. Therefore, the upper limit of Si content is 1.2%. More preferably, it is 0.8% or less.

Mn: 2.5-3.5%
Mn is not only effective for preventing hot embrittlement of steel and ensuring strength, but also stabilizes austenite and effectively acts to ensure the area ratio of residual austenite. Moreover, there exists an effect | action which refines | miniaturizes a steel structure by lowering transformation temperature. For this reason, in order to obtain retained austenite at a predetermined fraction and particle size, the Mn content needs to be 2.5% or more. On the other hand, when the amount of Mn exceeds 3.5%, the formation of a segregation layer is remarkably deteriorated in formability. Therefore, the Mn content is in the range of 2.5 to 3.5%.

P: 0.001 to 0.05%
P is an element to be added according to a desired strength, and is an element effective for forming a composite structure because it promotes ferrite transformation. In order to obtain such an effect, the P amount needs to be 0.001% or more. On the other hand, if the amount of P exceeds 0.05%, the weldability and the plating property are reduced. Therefore, the P content is in the range of 0.001 to 0.05%.

S: 0.0001 to 0.01%
S not only segregates at the grain boundaries and embrittles the steel during warm working, but also exists as a sulfide and lowers the local deformability, so the amount is made 0.01% or less. Preferably it is 0.003% or less, More preferably, it is 0.001% or less. However, it is difficult to reduce the amount of S to less than 0.0001% due to restrictions on production technology. Therefore, the S content is in the range of 0.0001 to 0.01%. Preferably it is 0.0001 to 0.003%, more preferably 0.0001 to 0.001% of range.

Al: 0.001 to 0.1%
Al is an element effective in generating ferrite and improving the strength-ductility balance. In order to obtain such an effect, the Al amount needs to be 0.001% or more. On the other hand, when the Al content exceeds 0.1%, the surface properties are deteriorated. Therefore, the Al content is in the range of 0.001 to 0.1%.

N: 0.0005-0.01%
N is an element that degrades the aging resistance of steel. In particular, when the N content exceeds 0.01%, the deterioration of aging resistance becomes significant. Therefore, the smaller the amount of N, the better. However, the amount of N needs to be 0.0005% or more due to restrictions on production technology. Therefore, the N content is in the range of 0.0005 to 0.01%.

Nb: 0.01-0.1%
Nb has a very important role in the present invention. Nb has the effect of suppressing recrystallization and phase transformation in manufacturing processes such as hot rolling and continuous annealing, and as a result, exhibits the effect of miniaturizing the steel structure. Under such a structure, the retained austenite is stabilized under the influence of plastic restraint, so even if the C concentration in the retained austenite is relatively small, the ductility is high in the warm forming process. Can be achieved. At the same time, the refinement of crystal grains acts as an obstacle to dislocation movement during warm forming, and contributes to an increase in strain age hardening due to an increase in mobile dislocation density. Furthermore, such refinement of the structure effectively contributes to the improvement of low temperature toughness. It also has the effect of increasing the strength by forming precipitates with C and N. To make these effects effective. The amount of Nb needs to be 0.01% or more. On the other hand, if Nb exceeds 0.1%, precipitation strengthening works excessively, leading to a decrease in ductility and low temperature toughness. Therefore, the Nb content is in the range of 0.01 to 0.1%.

The balance is Fe and inevitable impurities.
However, for the following reasons, at least one element selected from Ti: 0.005-0.1%, V: 0.005-0.1%, B: 0.0003-0.0050%, Cr: 0.01-1.0%, Mo: 0.01-1.0 %, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, at least one element selected from them can be contained.

Ti: 0.005-0.1%, V: 0.005-0.1%
Both Ti and V form precipitates with C and N and contribute effectively to the improvement of strength and toughness. Moreover, since steel is strengthened by precipitation strengthening, it can be added according to a desired strength. Further, when Ti is contained at the same time as B, since N is precipitated as TiN, the precipitation of BN is suppressed, and the effect of adding B described later is effectively expressed. In order to obtain such effects, the Ti content and the V content must each be 0.005% or more. On the other hand, when the Ti content and the V content exceed 0.1%, precipitation strengthening works excessively, leading to a decrease in ductility. Therefore, it is preferable to add Ti in the range of 0.005 to 0.1% and V in the range of 0.005 to 0.1%.

B: 0.0003-0.0050%
B is added as necessary to obtain martensite at a predetermined fraction in order to improve hardenability and facilitate complex organization. In order to obtain such an effect, the B amount needs to be 0.0003% or more. On the other hand, if the amount of B exceeds 0.0050%, the effect is saturated and ductility is reduced. Therefore, the B content is preferably added in the range of 0.0003 to 0.0050%.

Cr: 0.01-1.0%, Mo: 0.01-1.0%, Ni: 0.01-1.0%, Cu: 0.01-1.0%
Cr, Mo, Ni, and Cu not only serve as solid solution strengthening elements, but also to stabilize retained austenite and facilitate complex formation, so that retained austenite is obtained at a predetermined fraction. Add as needed. In order to obtain such effects, the Cr content, the Mo content, the Ni content, and the Cu content must each be 0.01% or more. On the other hand, if the Cr content, the Mo content, the Ni content, and the Cu content each exceed 1.0%, the plateability, formability, and spot weldability deteriorate. Accordingly, the Cr amount is 0.01 to 1.0%, the Mo amount is 0.01 to 1.0%, the Ni amount is 0.01 to 1.0%, and the Cu amount is 0.01 to 1.0%.

2) Microstructure Polygonal ferrite: 20% or more Bainite: 10% or more Area ratio of retained austenite: 3% or more Average crystal grain size of retained austenite: 5 μm or less In the present invention, improvement in ductility during warm forming and warm In order to simultaneously achieve an increase in strength at room temperature after processing, it is necessary to contain a fixed amount of a composite structure of polygonal ferrite and bainite. In order to obtain the effect of improving ductility by warm forming, it is necessary to contain a predetermined amount of retained austenite. Residual austenite has the effect of increasing the strain propagation property and improving the plastic deformability by strain-induced transformation, but since the retained austenite stabilizes at high temperatures, it will act in a higher strain region, further improving ductility. Is possible.

In particular, as in the present invention, when the amount of addition of C and Si is suppressed and stabilization due to C concentration in residual austenite cannot be expected, the crystal grain size of residual austenite is reduced. By the action of plastic restraint, the stability to processing can be increased and ductility can be improved. In warm forming, since the diffusion of C is promoted by heating, the C concentration in the retained austenite is increased, and this effect is remarkably exhibited. For this purpose, a certain amount of retained austenite needs to be left at the room temperature stage before warm forming, and this can be realized by the above-described effect of miniaturization. In addition, movable dislocations introduced by warm forming are mainly distributed in polygonal ferrite and are responsible for plastic deformation. The interaction between this movable dislocation and C in the steel increases work hardening ability and improves ductility. In addition, the movable dislocations mainly introduced into polygonal ferrite during warm forming interact with C in the steel, so that the movable dislocations are fixed in C even after cooling to room temperature. The strength when deformed at room temperature after inter-working is significantly increased. Furthermore, martensite and polygonal ferrite transformed from retained austenite exhibit a large hardness difference, but mixing bainite having intermediate hardness suppresses local stress concentration and contributes to improvement of ductility.
In order to effectively exhibit the above effects, the area ratio is set to 20% or more of polygonal ferrite, 10% or more of bainite, 3% or more of retained austenite, and the average crystal grain size of retained austenite is 5 μm. It is important that:

The upper limit of polygonal ferrite is preferably 70% from the viewpoint of securing strength.
The upper limit of bainite is preferably 50% from the viewpoint of securing strength.
Furthermore, the upper limit of retained austenite is preferably set to 30% because excessive content may cause a decrease in toughness and local ductility.
On the other hand, the structure other than polygonal ferrite, bainite and retained austenite is mainly martensite. In addition, pearlite may be mixed, but there is no problem if the mixed amount is 20% or less.

C concentration in residual austenite after forming with strain of 0.10 at 300 ° C (Cγ ₃₀₀ ): 0.2% or more In the case of residual austenite in the steel sheet before warm forming when the amount of C or Si is small Since it is difficult to increase the C concentration of C, it is necessary to concentrate C in the retained austenite not only during the manufacturing process of the steel sheet but also during warm forming. In order to contribute to ductility, the concentration needs to be at least 0.2% during warm forming. The upper limit of the amount of Cγ ₃₀₀ is not particularly limited, but about 1.2% is practical.

Furthermore, in the present invention, it is effective to satisfy the requirements described below.
Average grain size of polygonal ferrite and bainite: 10 μm or less For stabilization by plastic restraint by refining residual austenite grains, they are uniformly and finely dispersed to avoid local stress concentration There is a need. On the other hand, in order to efficiently promote concentration by diffusion of C from polygonal ferrite and bainite to retained austenite, it is preferable that many crystal grain boundaries serving as diffusion paths are included. Therefore, it is preferable that the crystal grain sizes of polygonal ferrite and bainite are fine. In order to obtain such an effect, the average crystal grain sizes of polygonal ferrite and bainite are preferably 10 μm or less, respectively.

Martensite area ratio: 20% or more Martensite has the effect of generating movable dislocations in the vicinity of the grain boundary with the adjacent polygonal ferrite due to volume expansion during its formation. This movable dislocation acts as a dislocation source during warm forming, and proliferates the dislocation as the plastic deformation progresses. These dislocation groups interact with C in steel to develop strain age hardening and contribute to an increase in strength at room temperature after warm working. In addition, the interaction between movable dislocations during warm forming and C in steel causes a decrease in ductility, as seen in a general dynamic strain aging phenomenon, and stress deformation during deformation called serration. Stabilize. However, with regard to this point, by appropriately mixing polygonal ferrite and martensite, the dislocation source is dispersed and the propagation of strain is enhanced, effectively improving the ductility by promoting work hardening and stress. Can be eliminated. In order to effectively exhibit such an effect, the area ratio of martensite is preferably 20% or more.

Relationship between volume fraction of retained austenite (Vγ ₀ ) at a strain amount of 0.10 at room temperature and volume fraction of residual austenite (Vγ ₃₀₀ ) at a strain amount of 0.10 at 300 ° C .: Vγ ₃₀₀ / Vγ ₀ ≧ 2.0
In the steel plate according to the present invention, as described above, with the stabilization of the retained austenite in the warm state, the strain propagation property is improved (uniform elongation is increased) and the ductility is improved. In order for this effect to contribute effectively to the improvement of press formability, residual austenite must remain up to a relatively high strain region. Specifically, the ratio of the volume ratio of residual austenite (Vγ ₃₀₀ ) at a strain amount of 0.10 at 300 ° C. to the volume ratio of residual austenite (Vγ ₀ ) at a strain amount of 0.10 at room temperature is set to 2.0 or more. It is preferable to do.

Relationship between work hardening rate (WHR ₀ ) at a strain amount of 0.05 at room temperature and work hardening rate (WHR ₃₀₀ ) at a strain amount of 0.05 at 300 ° C .: WHR ₃₀₀ / WHR ₀ ≧ 1.2
In the steel sheet according to the present invention, as described above, with the acceleration of work hardening based on the interaction between the movable dislocation introduced by warm forming and C, strain propagation is improved (uniform elongation is increased) and ductility is improved. . In order for this effect to contribute effectively to the improvement of press formability, the work hardening rate must increase in a relatively high strain region. Specifically, the ratio of the work hardening rate at a strain amount of 0.05 at 300 ° C. to the work hardening rate at a strain amount of 0.05 at room temperature is preferably 1.2 or more.

Relationship between tensile strength at room temperature (TS ₀ ) and tensile strength at room temperature (TS ₃₀₀ ) after pre-processing with strain of 0.10 at 300 ° C: TS ₃₀₀ −TS ₀ ≧ 150 MPa
In the steel sheet according to the present invention, the work hardening ability is improved by the interaction between the movable dislocation introduced during warm forming and C, so even if the same strain amount is processed, the warm forming is more in comparison with the forming at room temperature. Mobile dislocations are accumulated with a high dislocation density. Furthermore, many of these movable dislocations are in a state of being fixed by C in the steel. For this reason, when a deformation | transformation is again performed at room temperature after adding a pre-processing (press process) by warm forming, intensity | strength becomes high compared with the case where these series are performed at room temperature. In order to make this effect effective in the performance of automobile parts, especially the collision characteristics, and to contribute to the weight reduction of the car body by reducing the plate thickness, the tensile strength at room temperature after pre-processing with a strain amount of 0.10 at 300 ° C is It is preferable to have an increase margin of at least 150 MPa with respect to the tensile strength at room temperature.

Amount of decrease in tensile strength in the temperature range of 200 to 400 ° C. with respect to the tensile strength at room temperature: 150 MPa or less The steel sheet according to the present invention is warm-formed in the temperature range of 200 to 400 ° C. In general, steel sheet strength tends to decrease due to strain recovery and tempering action by increasing the forming temperature. Cause. Therefore, the amount of decrease in the tensile strength at a temperature range of 200 to 400 ° C. with respect to the tensile strength at room temperature is desirably 150 MPa or less.

Increase in yield strength in the temperature range of 200 to 400 ° C. with respect to yield strength at room temperature: 50 MPa or less The steel sheet according to the present invention improves work hardening ability by the interaction of movable dislocations and C introduced during warm forming. . This interaction tends to increase the yield strength. If the increase is extremely large, the amount of spring back may increase and the dimensional accuracy of the press member by warm forming may be impaired. Therefore, it is desirable that the amount of increase in the yield strength at a temperature range of 200 to 400 ° C. with respect to the yield strength at room temperature is 50 MPa or less.

3) Warm forming method In order to improve ductility by warm forming in which a thin steel plate is processed at a temperature of 200 to 400 ° C and an equivalent plastic strain of 0.02 or more, residual austenite is used to increase strain propagation. It is necessary to perform press molding in an appropriate temperature range that stabilizes in a high strain region as compared to room temperature. At the same time, in order to increase the strength due to warm forming, in order to allow the strain to be introduced to interact with C in the steel, the C can be sufficiently diffused, and the press is performed in an appropriate temperature range in which dislocations recover and do not disappear. It is necessary to mold.
When the steel plate temperature is less than 200 ° C., the retained austenite does not show sufficient stabilization, and even if movable dislocations are introduced, C cannot diffuse freely and the interaction is not effectively expressed. On the other hand, when the temperature exceeds 400 ° C., the retained austenite is excessively stabilized and does not cause strain-induced transformation, and even if a movable dislocation is introduced, it recovers and disappears, so that it contains a sufficient amount of C. However, the interaction is not effectively expressed.
In addition, when the amount of plastic strain due to warm forming is less than 0.02, not only the strain-induced transformation does not fully develop and the improvement of strain propagation property (uniform elongation) is not observed, but the movable dislocation introduced The density is not sufficient, and it is not possible to improve ductility or increase strength by promoting work hardening. Therefore, in warm forming, it is necessary to perform forming under the condition that the amount of equivalent plastic strain is 0.02 or more. Preferably, the amount of equivalent plastic strain is 0.10 or more.

The high strength thin steel sheet used in the present invention does not particularly define its production method, but can be produced using a general steel sheet production process. There are cold-rolled steel sheets and hot-dip galvanized steel sheets.
For example, when manufactured as a hot-rolled steel sheet, the slab is preferably manufactured by a continuous casting method in order to prevent macro segregation, but can also be manufactured by an ingot-making method or a thin slab casting method. When the slab is hot-rolled, the slab is reheated, but in order to prevent an increase in rolling load, the heating temperature is preferably 1150 ° C. or higher. Further, the upper limit of the heating temperature is preferably 1300 ° C. in order to prevent an increase in scale loss and an increase in fuel consumption rate.
The hot rolling is performed by rough rolling and finish rolling, but the finish rolling is preferably performed at a finishing temperature equal to or higher than the Ar ₃ transformation point in order to prevent a decrease in formability after cold rolling / annealing. In order to prevent the occurrence of non-uniform structure and scale defects due to the coarsening of crystal grains, the finishing temperature is preferably 950 ° C. or lower. The steel sheet after hot rolling is cooled to 650 ° C. or less after cooling at a cooling rate of 20 ° C./s or more within 1 second after finishing rolling from the viewpoint of securing the necessary area ratio of retained austenite. Winding at a winding temperature of 500 ° C. is preferred.
On the other hand, when the next process such as pickling and cold rolling continues as in the case of manufacturing as a cold-rolled steel sheet described below, from the viewpoint of preventing scale defects and ensuring good shape, It is preferable to wind at a winding temperature of 500 to 700 ° C. after rolling.

When manufacturing as a cold-rolled steel sheet, after removing the scale from the hot-rolled steel sheet after winding as described above by pickling, etc., in order to efficiently generate polygonal ferrite and bainite, a rolling reduction of 40% It is preferable to cold-roll as described above. After heating the steel sheet after cold rolling is Ac in ₁ transformation point or more (Ac ₃ +50) ° C. below the temperature range, 30～500S soaked, 3 to 30 ° C. / average cooling stop of 600 ° C. or less at a cooling rate of s When performing primary cooling to a temperature, in particular, after cooling to a cooling rate of 15 ° C./s or more in the range of 750 ° C. to 600 ° C., austempering treatment is performed for 10 s or more in the temperature range of 350 to 500 ° C. Then, it can be manufactured by a secondary cooling method.

When producing as a hot dip galvanized steel sheet, after removing the scale by pickling, etc. from the hot-rolled steel sheet after winding, in order to efficiently generate polygonal ferrite and bainite, a reduction rate of 40 It is preferable to cold-roll at% or more. After heating the steel sheet after cold rolling is Ac in ₁ transformation point or more (Ac ₃ +50) ° C. below the temperature range, 30～500S soaked, 3 to 30 ° C. / average cooling stop of 600 ° C. or less at a cooling rate of s After primary cooling to the temperature, austempering for 10 s or more in the temperature range of 350 to 500 ° C., then dipping in a galvanizing bath containing 0.10 to 0.20 mass% of Al, and then performing hot dip galvanizing treatment In order to adjust the weight per unit area, it is possible to manufacture by performing secondary cooling after performing wiping as necessary. In addition, in order to adjust the Fe concentration during plating and improve the adhesion of plating and the corrosion resistance after coating, alloying treatment of galvanizing is also possible in the temperature range of 450-600 ° C prior to secondary cooling. it can.

Example 1
Steel types A to K having the composition shown in Table 1 were melted in a vacuum melting furnace, and a sheet bar slab was formed by split rolling. These sheet bar slabs were heated to 1250 ° C to simulate the manufacturing process of hot-rolled steel sheets, and after rough rolling, finish rolling was performed at 850 to 920 ° C, and subsequently within 1 second. After cooling by dipping in an aqueous quenching liquid at a cooling rate of 25 ° C. / s up to 600 ° C., subjected to winding corresponding heat treatment for later furnace cooling and held for 1 hour at 3 from 00 to 600 ° C., and adjusted the organizational structure A hot-rolled steel sheet was produced.
Moreover, for some, imitating the manufacturing process of the cold-rolled steel sheet, the above-described hot-rolled steel sheet was pickled to remove the scale on the surface, and further subjected to cold rolling with a reduction rate of 50%. . Subsequently, after performing a soaking process for 300 s at 750 to 900 ° C, gas cooling was performed at an average cooling rate of 10 ° C / s from 200 to 500 ° C and a cooling rate of 750 ° C to 600 ° C at 20 ° C / s. After cooling and holding in a temperature range of 200 to 500 ° C. for 30 to 1800 s, it was cooled to room temperature, and various steel sheets with adjusted structure were prepared.
Furthermore, after the above-mentioned soaking, cooling, and holding steps, imitating the hot-dip galvanized steel sheet manufacturing process, reheating to 525 ° C, holding for 15 s, cooling to room temperature, and adjusting the structure A steel plate was prepared.
These steel sheets were classified into hot-rolled steel sheets (HOT), cold-rolled steel sheets (COLD), and alloyed hot-dip galvanized steel sheets (GA) according to the respective sample preparation procedures.

About the obtained steel plate, while identifying the steel structure, the area fraction was measured.
As for the microstructure, the steel microstructure was identified by magnifying the corrosion appearance structure due to the nitrile by 5000 times with a scanning electron microscope (SEM) on the thickness cross section parallel to the rolling direction of the steel sheet. This was analyzed by image analysis software (Image-Pro; manufactured by Cybernetics) to determine the area ratio of each phase. However, since martensite and retained austenite are difficult to distinguish, the total area ratio is obtained for these, and the area ratio of retained austenite is separately determined by the method described later, and this is subtracted to obtain martensite. The area ratio of the site was used.
In addition, after polishing the steel plate to the position of the thickness 1/4, the surface that was further chemically polished by 0.1 mm was used as the measurement surface, and using the Kα ray of Mo with an X-ray diffractometer, (200), (220 ), (311) surface and bcc iron's (200), (211), (220) surface peak integral strength is measured, fcc ratio is calculated for all the combinations, and the average value is used as the residual The volume ratio of austenite was determined and made three-dimensionally homogeneous, and this was defined as the area ratio of residual austenite.
At the same time, only the grain boundaries of ferrite and bainite are drawn from the microstructure by SEM observation described above, and the diameter (equivalent circle diameter) when each crystal grain is regarded as a circle is derived by image processing software, and these are averaged. Thus, the average crystal grain sizes of ferrite and bainite were obtained.
In addition, the average grain size of retained austenite is the result of tempering in the portion corresponding to martensite by performing corrosion after performing heat treatment at 200 ° C. for 200 minutes in the above-mentioned SEM observation. Since it easily shows a different form from retained austenite, the phase in which this substructure does not appear is regarded as retained austenite and obtained by performing the above-described image analysis. In the present invention, the lower limit value of the grain size of the retained austenite to be symmetric is set to 0.01 μm which is the measurement limit.

Further, JIS No. 5 tensile test specimens were collected in a direction perpendicular to the rolling direction, and a tensile test was performed at room temperature at a crosshead speed of 20 mm / min in accordance with JIS Z 2241 to measure TS and YS.
From these steel sheets, steel sheets having the steel structure and base material characteristics shown in Table 2 were extracted and subjected to the following warm tensile test and warm press test.

In order to evaluate the properties in warm forming, JIS No. 5 tensile test specimens were taken in the direction perpendicular to the rolling direction, and the test temperature was 200 to 400 at a crosshead speed of 20 mm / min in accordance with JIS Z 2241. Tensile tests were performed in the range of ° C. to evaluate mechanical properties. At this time, the test was stopped at a strain amount of 0.10 in the test at 300 ° C. and the test at room temperature, and the area ratio of residual austenite at this time was measured by the method described above, and the residual amount at a strain amount of 0.10 at 300 ° C. The ratio of the volume ratio of austenite to the volume ratio of residual austenite at a strain amount of 0.10 at room temperature was determined.
Further, from the stress-strain relationship in the test at 300 ° C. and the test at room temperature, the ratio of the work hardening rate at a strain amount of 0.05 at 300 ° C. to the work hardening rate at a strain amount of 0.05 at room temperature was obtained. By subtracting the lowest TS at a test temperature of 200 to 400 ° C. from the TS at room temperature, the amount of TS decrease during warm working was determined. Further, the amount of YS increase during warm working was determined by subtracting YS at room temperature from the highest YS at a test temperature of 200 to 400 ° C.
Further, the warm tensile test at 300 ° C. was stopped when the strain amount was 0.10, and after cooling to room temperature, the tensile test was performed again to evaluate the mechanical properties. The amount of increase in TS after warm working was determined by subtracting the tensile strength at room temperature from the tensile strength at room temperature after pre-processing with a strain amount of 0.10 at 300 ° C.

  These steel plates were subjected to warm press forming, and the press formability and impact resistance characteristics using the press formed products were evaluated. The steel plate is a blank plate of 220 mm x 300 mm size, heated to 300 ° C, and then subjected to press forming of a 300 mm hat cross-sectional shape in the longitudinal direction by the method schematically shown in Fig. 4 so that cracking does not occur The height (limit molding height) was measured, and the ratio of the limit molding height at 300 ° C. to the limit molding height at room temperature was determined. Further, the same test was stopped at a molding height of 30 mm, and the equivalent strain was calculated from the plate thickness reduction amount assuming a plane strain deformation for the vertical wall portion of the molded product.

  Moreover, as shown in FIG. 5, the opening amount of the molded product was measured, and the ratio of the opening amount at 300 ° C. to the opening amount at room temperature was determined. Furthermore, the same steel plate is welded to the bottom of this hat member to produce a prismatic part with a hat cross-sectional shape, and a weight of 750 kg is dropped and collided from a position of a height of 10 m in the longitudinal direction. And the load was measured. The absorbed energy was calculated by integrating the load value at this time up to a displacement of 50 mm, and the ratio of the absorbed energy of the hat part molded at 300 ° C. to the absorbed energy of the hat part molded at room temperature was obtained.

  The chemical conversion treatment property was evaluated by a chemical conversion film produced by subjecting these steel plates to a phosphate treatment. Phosphate treatment was performed by washing the surface of the steel sheet with an alkaline degreasing agent, washing with water, immersing in a phosphate treatment bath for 120 seconds, washing with water and drying. For comparison, a standard mild steel plate was processed in the same manner. The treated surface was magnified 5000 times with a scanning electron microscope (SEM) to observe the chemical conversion film. Further, the coating was dissolved and removed using chromic acid, and the coating weight was measured from the difference in weight before and after. A case where the chemical conversion film completely covered the surface of the steel sheet was determined to be acceptable (◯), and a case where the base was partially exposed was determined to be unacceptable (x). Moreover, the thing whose coating weight is 15% or more small compared with the comparative mild steel plate was determined to be disqualified (x).

  Spot weldability was evaluated by making a welded joint of two steel grades and the joint strength. Spot welding was performed according to WES7301 and was performed under the welding conditions corresponding to Class A in the welding conditions of low carbon steel. At this time, the condition for forming the nugget diameter corresponding to the minimum value of class A described in JIS Z 3140 by changing the welding current value in various ways was used as the standard condition. Under these conditions, the joint strength test was conducted with two types of shear tensile strength according to JIS Z 3136 and cross tensile strength according to JIS Z 3137. When the value obtained by dividing the cross tensile strength by the shear tensile strength at this time was 0.5 or more, it was judged as acceptable (◯), and the value less than 0.5 was judged as unacceptable (x).

The low temperature toughness was evaluated by a low temperature impact test using a Charpy tester. A Charpy test with a V-notch specimen shape was carried out at various temperatures according to JIS Z 2242, and the ductile fracture surface ratio was measured from the fracture surface observation. At this time, the fracture surface transition temperature, which is the temperature at which the ductile fracture surface ratio becomes 50%, was determined. Those having a fracture surface transition temperature of −40 ° C. or lower were determined to be acceptable (◯), and those exceeding −40 ° C. were determined to be unacceptable (x).
The obtained results are shown in Table 3.

As shown in the table, the steel plate according to the example of the present invention, when formed at 300 ° C., shows an improvement in the limit forming height of 20% or more compared to the case of forming at room temperature. The increase in the amount is within 5% and no decrease in dimensional accuracy is observed, and the press formability is remarkably improved by applying warm forming. In addition, the absorbed energy in the weight drop test also shows that the molded part at 300 ° C shows an increase of more than 15% compared to the molded part at room temperature, indicating that it has excellent impact resistance. Furthermore, practical properties such as chemical conversion treatment, spot weldability, and low temperature toughness are also good.

Claims (10)

In mass%, C: 0.04-0.1%, Si: 0.5-1.2%, Mn: 2.5-3.5%, P: 0.001-0.05%, S: 0.0001-0.01%, Al: 0.001-0.1%, N: 0.0005- Contains 0.01% and Nb: 0.01-0.1%, the balance has a component composition consisting of Fe and inevitable impurities, the structure is area ratio, polygonal ferrite 20% or more, bainite 10% or more The austenite content is 3% or more, the average crystal grain size of the retained austenite is 5 μm or less, and the C concentration (Cγ ₃₀₀ ) in the retained austenite after molding with a strain amount of 0.10 at 300 ° C. is 0.2% or more. A thin steel sheet for warm forming which is excellent in formability and strength increasing ability, characterized by being.   The thin steel sheet for warm forming according to claim 1, wherein the average crystal grain sizes of polygonal ferrite and bainite are each 10 μm or less.   The thin steel sheet for warm forming according to claim 1 or 2, wherein martensite is contained in an area ratio of 20% or more. The volume fraction of residual austenite (Vγ ₀ ) at a strain amount of 0.10 at room temperature and the volume fraction of residual austenite (Vγ ₃₀₀ ) at a strain amount of 0.10 at 300 ° C. satisfy the following formula (1): Item 4. The thin steel sheet for warm forming according to any one of Items 1 to 3.
Record
Vγ ₃₀₀ / Vγ ₀ ≧ 2.0 --- (1) The work hardening rate (WHR ₀ ) at a strain amount of 0.05 at room temperature and the work hardening rate (WHR ₃₀₀ ) at a strain amount of 0.05 at 300 ° C. satisfy the following formula (2), and the tensile strength (TS at room temperature) ₀ ) and the tensile strength (TS ₃₀₀ ) at room temperature after pre-processing with a strain amount of 0.10 at 300 ° C. satisfies the following formula (3): The thin steel sheet for warm forming described.
Record
WHR ₃₀₀ / WHR ₀ ≧ 1.2 --- (2)
TS ₃₀₀ − TS ₀ ≧ 150 MPa --- (3)   The decrease in tensile strength in the temperature range of 200 to 400 ° C with respect to the tensile strength at room temperature is 150 MPa or less, and the increase in yield strength in the temperature range of 200 to 400 ° C with respect to the yield strength at room temperature is 50 MPa or less. The thin steel sheet for warm forming according to any one of claims 1 to 5, wherein the thin steel sheet is for warm forming.   The temperature according to any one of claims 1 to 6, further comprising at least one element selected from Ti: 0.005-0.1% and V: 0.005-0.1% in terms of mass% as a component composition. Thin steel sheet for inter-forming.   The thin steel sheet for warm forming according to any one of claims 1 to 7, further comprising B: 0.0003 to 0.0050% by mass% as a component composition.   The composition further comprises at least one element selected from Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Ni: 0.01 to 1.0% and Cu: 0.01 to 1.0% in terms of mass%. The thin steel sheet for warm forming according to any one of claims 1 to 8.   The formability and strength increasing ability of the thin steel sheet for warm forming according to any one of claims 1 to 9, characterized in that the steel sheet temperature is 200 to 400 ° C, and processing with an equivalent plastic strain of 0.02 or more is applied. Excellent warm forming method.

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