JP2012240095A - Warm forming method of high-strength steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a warm forming method of a high-strength steel sheet capable of demonstrating its deep draw forming characteristic while ensuring the strength of ≥980 MPa of the high-strength steel sheet.SOLUTION: The high-strength steel sheet has the composition consisting of, by mass, C:0.05-0.3%, Si:1-3%, Mn:0.5-3%, P:≤0.1% (including 0%), S:≤0.01% (including 0%), Al:0.001-0.1%, N:0.002-0.03%, and the balance consisting of iron and impurities, and has a structure containing bainitic ferrite: 50-90%, residual austenite: 5-20%, martensite + the residual austenite: 10-50%, ferrite: ≤40% (including 0%) in terms of the area ratio to the entire structure. The residual austenite has the C concentration of 0.5-1.1 mass%, the average circle equivalent diameter of 0.4-2 μm, and the average aspect ratio (maximum diameter/minimum diameter) of <3.0. The die temperature of at least a shoulder part of a punch of a press forming die is set to be 250-350°C, and the die temperature of at least a shoulder part of the die is set to be 100-200°C.

Description

  The present invention relates to a method for warm-forming a high-strength steel plate, and more particularly to a method for warm-forming a high-strength TRIP steel plate using a press molding die. In addition, as a high strength steel plate used for this invention method, a cold-rolled steel plate, a hot dip galvanized steel plate, and an alloyed hot dip galvanized steel plate are contained.

  Thin steel plates used for automobile frame parts are required to have high strength in order to realize collision safety and fuel efficiency improvement. Therefore, it is required to ensure press formability while increasing the strength of the steel sheet to 980 MPa class or higher. It is known that in a high-strength steel sheet of 980 MPa class or higher, it is effective to use steel utilizing the TRIP effect to achieve both high strength and formability (for example, see Patent Document 1).

Patent Document 1 discloses a high-strength steel plate containing bainite or bainitic ferrite as a main phase and containing retained austenite (γ _R ) in an area ratio of 3% or more. However, this high-strength steel sheet has a tensile strength at room temperature of 980 MPa or more and does not reach an elongation of 20%, and further improvement in mechanical properties (hereinafter also simply referred to as “characteristics”) is required.

  On the other hand, it is known that the TRIP steel sheet is particularly superior in deep drawability over ductility represented by uniaxial tensile elongation (see, for example, Non-Patent Document 1 and Patent Document 2). That is, it is generally considered that the r-value dominates the deep drawability of the steel sheet, but in the case of a TRIP steel sheet, the ductility of the deep wall of the deep draw is improved by the TRIP effect, while the reduced flange part is reversed. It is known that when the TRIP phenomenon is suppressed, it is difficult to be cured, the inflow of the material is facilitated, and the deep drawability is improved.

  However, although the above knowledge can be applied to a TRIP steel sheet of 780 MPa class or less, it cannot be applied as it is to a TRIP steel sheet of 980 MPa class or more.

  Therefore, improvement of deep drawability, which is one of the most important indexes of formability in automobile parts, is an important point when using an ultra-high strength steel sheet of 980 MPa class or higher.

  On the other hand, since there is a limit to the formability of TRIP steel sheet in cold forming, the TRIP effect is further effectively expressed by increasing the elongation by warm working at 100 to 400 ° C for further improvement of elongation. Techniques have been proposed (see Non-Patent Document 2 and Patent Document 3).

As shown in Table 2 of Patent Document 3 above, the presence of γ _R having a carbon concentration of 1% by mass or more in the structure mainly composed of bainitic ferrite improves the elongation at around 200 ° C. to 23% in the 1200 MPa class. is made of. However, since these steel sheets have overstabilized γ _R by setting the carbon concentration to 1% by mass or more, the TRIP effect does not appear under conditions where the stability of γ _R is further increased, such as around 200 ° C. It will be enough. Furthermore, as shown in the table, these steel sheets contain a lot of elongated γ _R with a large aspect ratio, so the deep-drawing properties have not been evaluated, but are considered insufficient. The

Therefore, in order to provide a high-strength steel sheet having a deep drawing property and a warm working method using the same while ensuring a strength of 980 MPa class or higher, the applicant of the present invention has the same dislocation density as in the above prior art. Focusing on TRIP steel sheets containing bainitic ferrite with a high substructure (matrix) and retained austenite (γ _R ), further investigations have been made to further improve deep drawing properties while ensuring strength I came. As a result, (1) after ensuring the strength by partially introducing martensite into the structure, (2) γ _R having a carbon concentration of 0.5 to 1.2% by mass is 5% or more by area ratio. Inclusion increases the elongation due to the TRIP effect. (3) Further, the form of γ _R is 0.2 to 2 μm in average circle equivalent diameter and the aspect ratio is less than 3.0, so that the processing-induced martensitic transformation is achieved. The ductility of the vertical wall is increased by increasing the amount of strain that is sometimes applied to the periphery of γ _R , thereby increasing the effect of suppressing the progression of transformation to work-induced martensite when compression is applied, such as compression flange molding. Finding that deep drawability can be enhanced by securing and promoting the inflow of material from the flange portion, and the invention completed based on this finding (hereinafter collectively referred to as “prior invention”) High strength steel plate Called a row invention steel ".) Portion of this resin was already filed a patent application (Japanese Patent Application No. 2010-258151).

The prior invention steel sheet is in mass%,
C: 0.05 to 0.3%
Si: 1-3%
Mn: 0.2-3%,
P: 0.1% or less (including 0%),
S: 0.01% or less (including 0%),
Al: 0.001 to 0.1%,
N: 0.002 to 0.03%
And the balance has a component composition consisting of iron and impurities,
The area ratio for all tissues
Bainitic ferrite: 50-90%
Retained austenite: 5-20%,
Martensite + the above retained austenite: 10 to 50%,
Ferrite: 40% or less (including 0%)
Having an organization including
The residual austenite has a C concentration (Cγ _R ) of 0.5 to 1.2% by mass, an average equivalent circle diameter of 0.2 to 2 μm, and an average aspect ratio (maximum diameter / minimum diameter) of 3.0. It is a high-strength steel sheet that satisfies the following requirements.

Furthermore, the prior invention steel sheet has been found to be able to improve elongation and deep drawing properties by being processed before decomposition of γ _R occurs under a temperature condition where the stability of γ _R is appropriate. Based on this finding, a warm forming method (hereinafter referred to as “prior invention method”) was proposed in which the steel plate of the prior invention was uniformly heated to an appropriate temperature of 200 to 400 ° C. and then processed.

  However, as a result of further studies by the present inventors after that, the mold temperature of at least the shoulder of the punch of the press mold is not simply warm-heated by simply uniformly heating the steel sheet as in the above-described prior invention method. Of deep die forming characteristics by controlling the mold temperature at 250-350 ° C. and the mold temperature of at least the shoulder of the die to 100-200 ° C. I found out that

  Here, in Patent Document 4, when press-molding a high-strength steel plate (TRIP steel plate) having residual austenite transformation-induced plasticity, the die temperature of at least the shoulder portion of the die of the press-molding die is set to 150 to 200 ° C., punch There has been proposed a method in which the mold temperature of at least the shoulder portion is controlled in a temperature range of −30 to 0 ° C. and the temperature condition is changed depending on the processing site to perform press molding.

  However, in this method, it is necessary to heat the die shoulder and cool the punch shoulder to 0 ° C. or lower to perform press molding, and it is industrially easy to cool to 0 ° C. or lower in this way. Not.

  On the other hand, the method of the present invention is industrially easy because both the shoulder portion of the die and the shoulder portion of the punch are in different temperature ranges, but both are heated and press-molded. The method described in Document 4 and the method of the present invention are completely different in technical idea.

JP 2003-19319 A WO95 / 29268 pamphlet JP 2004-190050 A JP 2007-1111765 A

Manabu Takahashi, “Development of high-strength steel sheets for automobiles”, Nippon Steel Engineering Reports, 2003, No. 378, p. 2-6 Koichi Sugimoto, Takeshi Hoshi, Takeya Sakaguchi, Akihiko Nagasaka, Takahiro Kashima, “Warm Formability of Ultra High Strength Low Alloy TRIP Type Bainitic Ferritic Steel”, Iron and Steel, 2005, Vol. 91, No. 2, p. 34-40

  The present invention has been made paying attention to the above circumstances, and the purpose of the present invention is to provide a high-strength steel sheet capable of maximizing its deep-drawing properties while ensuring a high-strength steel sheet having room temperature strength of 980 MPa or higher. It is to provide a warm forming method.

The invention described in claim 1
A method of warm-forming a high-strength steel sheet with a press mold,
The high-strength steel plate is in mass% (hereinafter the same for chemical components).
C: 0.05 to 0.3%
Si: 1-3%
Mn: 0.5-3%,
P: 0.1% or less (including 0%),
S: 0.01% or less (including 0%),
Al: 0.001 to 0.1%,
N: 0.002 to 0.03%
And the balance has a component composition consisting of iron and impurities,
The area ratio for all tissues (hereinafter the same for tissues)
Bainitic ferrite: 50-90%
Retained austenite: 5-20%,
Martensite + said retained austenite: 10-50%,
Ferrite: 40% or less (including 0%)
Having an organization including
The retained austenite has a C concentration (Cγ _R ) of 0.5 to 1.1 mass%, an average equivalent circle diameter of 0.4 to 2 μm, and an average aspect ratio (maximum diameter / minimum diameter) of 3.0. Less than, and
Warm forming method of high strength steel sheet, characterized in that mold temperature of at least shoulder portion of punch of said press mold is 250-350 ° C and mold temperature of at least shoulder portion of die is 100-200 ° C. It is.

The invention described in claim 2
The component composition of the high-strength steel plate is further
Cr: 0.01 to 3%
Mo: 0.01 to 1%,
Cu: 0.01-2%,
Ni: 0.01-2%,
B: The method of warm forming a high-strength steel sheet according to claim 1, comprising one or more of 0.00001 to 0.01%.

The invention according to claim 3
The component composition of the high-strength steel plate is further
Ti: 0.01 to 0.1%,
V: 0.01-0.1%
The method for warm forming a high-strength steel sheet according to claim 1 or 2, comprising one or more of Zr: 0.01 to 0.1%.

The invention according to claim 4
The component composition of the high-strength steel plate is further
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
The warm forming method for a high-strength steel sheet according to any one of claims 1 to 3, wherein the REM contains one or more of 0.0001 to 0.01%.

According to the present invention, as a high-strength steel sheet, bainitic ferrite: 50 to 90%, retained austenite: 5 to 20%, martensite + retained austenite: 10 to 50%, ferrite in terms of area ratio to the entire structure : Having a structure containing 40% or less (including 0%), the residual austenite has a C concentration (Cγ _R ) of 0.5 to 1.1% by mass and an average equivalent circle diameter of 0.4 to When using a steel sheet satisfying an average aspect ratio (maximum diameter / minimum diameter) of less than 3.0 using 2 μm and warm-forming this high-strength steel sheet with a press mold, at least one of the punches of the press mold Deep draw forming while ensuring a room temperature strength of 980 MPa or higher in a high-strength steel sheet by setting the temperature of the shoulder to 250 to 350 ° C. and the temperature of at least the shoulder of the die to 100 to 200 ° C. Sex and it becomes possible to provide the warm molding method of the high strength steel sheet that may be exhibited to the maximum.

It is a graph which shows the relationship between the tensile temperature (forming temperature) and tensile strength TS in a TRIP steel plate. It is a graph which shows the relationship between the tensile temperature (forming temperature) and total elongation EL in a TRIP steel plate.

As described above, the present applicant pays attention to the TRIP steel sheet containing bainitic ferrite having a substructure (matrix) with a high dislocation density and retained austenite (γ _R ), similar to the above-described prior art, Further studies have been made to further improve the elongation and deep drawability while ensuring the above.

As a result, (1) after securing strength by partially introducing martensite into the structure, (2) γ _R having a carbon concentration of 0.5 to 1.1% by mass is 5% or more by area ratio. By adding, the elongation is increased by the TRIP effect. (3) Furthermore, the shape of γ _R is 0.4-2 μm in average circle equivalent diameter and the aspect ratio is less than 3.0, so that the processing-induced martensitic transformation is achieved. The ductility of the vertical wall is increased by increasing the amount of strain that is sometimes applied to the periphery of γ _R , thereby increasing the effect of suppressing the progression of transformation to work-induced martensite when compression is applied, such as compression flange molding. It has been found that the deep drawability can be enhanced by securing and promoting the inflow of the material from the flange portion. Based on this knowledge, the high-strength steel plate used in the method of the present invention (hereinafter referred to as “the steel plate using the method of the present invention”). " The led to the specific.

  Hereinafter, the structure characterizing the steel sheet using the method of the present invention will be described first.

[Structure of steel sheet using the method of the present invention]
As described above, the steel sheet using the method of the present invention is based on the structure of TRIP steel as in the above-described prior art, and particularly contains a predetermined amount of martensite and a carbon concentration of 0.5 to 1.1 mass. % Γ _R is contained in an area ratio of 5% or more, and the shape of γ _R is controlled to an average equivalent circle diameter of 0.4 to 2 μm and an aspect ratio of less than 3.0. Is different.

<Bainitic ferrite: 50-90%>
“Bainitic ferrite” in the steel sheet used in the method of the present invention means that the bainite structure has a substructure having a lath-like structure with a high dislocation density, and has no carbide in the structure. It is clearly different from the structure, and it is also different from the polygonal ferrite structure with a substructure with little or very little dislocation density, or with a quasi-polygonal ferrite structure with a substructure such as fine subgrains. Issued by Basic Research Society “Steel Bainite Photobook-1”). This structure exhibits an acicular shape when observed with an optical microscope or SEM, and is difficult to distinguish. Therefore, in order to determine a clear difference from a bainite structure or a polygonal / ferrite structure, the structure of the lower structure by TEM observation is determined. Identification is necessary.

  As described above, the structure of the steel sheet using the method of the present invention can improve the balance between strength and formability by using bainitic ferrite, which is uniform and fine, rich in ductility, and has a high dislocation density and high strength as the parent phase. it can.

In the steel sheet using the method of the present invention, the amount of the bainitic ferrite structure is 50 to 90% (preferably 60 to 90%, more preferably 70 to 90%) in terms of area ratio with respect to the entire structure. is necessary. This is because the effect of the bainitic ferrite structure is effectively exhibited. Note that the amount of the bainitic ferrite structure is determined by the balance with γ _R, and it is recommended that the amount be controlled appropriately so that desired characteristics can be exhibited.

<Contains retained austenite (γ _R ) in an area ratio of 5 to 20% with respect to the entire structure>
γ _R is useful for improving the total elongation, and in order to effectively exhibit such action, the area ratio is 5% or more (preferably 10% or more, more preferably 15% or more) with respect to the entire structure. It is necessary to exist. On the other hand, the stretch flangeability deteriorates too much when present in large amounts, so the upper limit was set to 20%.

<Martensite + the retained austenite (γ _R ): 10 to 50%>
For securing strength, but introduces some martensite in the tissue, since the moldability amount of martensite is too large can not be secured, 10% total area fraction of martensite + gamma _R for all tissues It is limited to 50% or less (preferably 12% or more, more preferably 16% or more).

<Ferrite: 40% or less (including 0%)>
Since ferrite is a soft phase, it does not contribute to increase in strength, but is effective in increasing ductility. Therefore, it may be introduced in an area ratio of 40% or less that can guarantee strength.

<C concentration of residual austenite (γ _R ) (Cγ _R ): 0.5 to 1.1% by mass>
Cγ _R is, γ _R at the time of processing is an indicator that affects the stability of the transformation to martensite. When C gamma _R is too low, gamma for _R is unstable, after stressing, since the deformation-induced martensitic transformation occurs prior to plastic deformation, not bulging property can be obtained. On the other hand, when the C gamma _R is too high, gamma _R becomes too stable, since the addition of machining work-induced martensitic transformation does not occur, not too bulging property can be obtained. To obtain a sufficient stretch forming property, C gamma _R is required to be 0.5 to 1.1 mass%. Preferably it is 0.6-1.0 mass%.

In the prior invention steel (Japanese Patent Application No. 2010-258151), the upper limit of the C gamma _R was about to 1.2 wt%, considering that cause ensure deformation-induced martensitic transformation due during processing, The upper limit was reviewed, and the content of the steel sheet using the method of the present invention was 1.1% by mass.

<Average equivalent circular diameter of retained austenite (γ _R ): 0.4 to 2 μm, average aspect ratio (maximum diameter / minimum diameter): less than 3.0>
It coarsens the gamma _R, and by close its shape equiaxed, to increase the effect of suppressing martensitic transformation during contraction flange deformation, in order to enhance the deep drawability. In addition, by making γ _R coarse and equiaxed, and making γ _R slightly unstable at room temperature, the part subjected to processing at a temperature around 300 ° C. (the steel plate part in contact with the shoulder of the punch) by proper stability set larger the ratio of gamma _R obtained, it is possible to increase both elongation and deep drawability of the same sites.

In order to effectively exhibit the effect of coarsening of the gamma _R is, gamma average circle equivalent diameter of _R must be at least 0.4μm but too small number of particles coarsened excessively when gamma _R Since the effect of suppressing martensitic transformation at the time of deformation of the reduced flange cannot be obtained, the upper limit is set to 2 μm.

In the prior invention steel (Japanese Patent Application No. 2010-258151), the lower limit of the average circle equivalent diameter of gamma _R has had a 0.2 [mu] m, considering that to more reliably exhibit the effect of coarsening of the gamma _R Then, the lower limit was reviewed, and the steel sheet using the method of the present invention was 0.4 μm.

In order to effectively exhibit the effect obtained by the equiaxed the gamma _R, it is necessary to average aspect ratio of gamma _R (the maximum diameter / minimum diameter) less than 3.0.

<Others: Bainite (including 0%)>
The steel sheet used in the method of the present invention may consist only of the above structure (martensite and / or bainitic ferrite, polygonal ferrite and γ _R mixed structure), but in a range not impairing the action of the present invention, As another heterogeneous structure, bainite may be included. This structure can inevitably remain in the manufacturing process of the steel sheet using the method of the present invention, but the smaller the better, the better the area ratio to 5% or less, more preferably 3% or less with respect to the entire structure. Is recommended.

[Each phase area ratio, gamma C concentration of _R (C gamma _R), and each method of measuring the average circle equivalent diameter and the aspect ratio of gamma _R]
Here, each phase area ratio, gamma C concentration of _R (C gamma _R), and will be described the method of measuring the average circle equivalent diameter and the aspect ratio of gamma _R.

The area ratio of the microstructure in the steel sheet is determined by corroding the steel sheet and observing the structure by, for example, defining a white region as “martensite + residual austenite (γ _R )” by observation with a transmission electron microscope (TEM; magnification: 1500 times). After that, the area ratio of the tissue was measured by observation with an optical microscope (magnification 1000 times).

Incidentally, gamma C concentration area ratio and gamma _R of the _R (C gamma _R), after grinding to 1/4 the thickness of the steel sheet was measured by X-ray diffraction method from the chemical polishing (ISIJ Int.Vol.33 (1933), No. 7, p. 776). The area ratio of ferrite was determined by corroding the steel plate with nital and identifying the black area as ferrite by observation with a scanning electron microscope (SEM; magnification 2000 times).

The average equivalent circular diameter of γ _R is measured by measuring the distribution of γ phase in EBSP (Electron Back Scatter Diffraction Pattern) over 30 μm × 30 μm in 0.2 μm steps. The area measured as a grain was converted to an equivalent circle diameter, and the equivalent circle diameter of each gamma grain was determined by arithmetic averaging. The average aspect ratio of gamma _R, for each gamma particle, the maximum Feret diameter, was measured minimum Feret diameter, defining the ratio (maximum diameter / minimum diameter) and the aspect ratio, the aspect ratio of each gamma particle Obtained by arithmetic averaging.

  Next, the component composition constituting the steel sheet using the method of the present invention will be described. Hereinafter, all the units of chemical components are mass%.

[Component composition of steel sheet used in the method of the present invention]
C: 0.05-0.3%
C is an essential element for obtaining a desired main structure (bainitic ferrite + martensite + γ _R ) while ensuring high strength, and 0. It is necessary to add 05% or more (preferably 0.10% or more, more preferably 0.15% or more). However, if it exceeds 0.3%, it is not suitable for welding.

Si: 1-3%
Si is an element that effectively suppresses the generation of carbides by decomposition of γ _R. In particular, Si is useful as a solid solution strengthening element. In order to exhibit such an action effectively, it is necessary to add Si 1% or more. Preferably it is 1.1% or more, More preferably, it is 1.2% or more. However, if Si is added in excess of 3%, the formation of bainitic ferrite + martensite structure is hindered, the hot deformation resistance is increased, and the welds are easily embrittled. It also has an adverse effect on the surface properties of the film, so the upper limit is made 3%. Preferably it is 2.5% or less, More preferably, it is 2% or less.

Mn: 0.5 to 3%
In addition to effectively acting as a solid solution strengthening element, Mn also exerts an effect of promoting transformation and promoting the formation of bainitic ferrite + martensite structure. Furthermore, it is an element necessary for stabilizing γ and obtaining a desired γ _R. In order to exhibit such an action effectively, it is necessary to add 0.5% or more. Preferably it is 0.7% or more, More preferably, it is 1% or more. However, when it is added in excess of 3%, adverse effects such as slab cracking are observed. Preferably it is 2.5% or less, More preferably, it is 2% or less.

P: 0.1% or less (including 0%)
P is inevitably present as an impurity element, but is an element that may be added to ensure desired γ _R. However, when it exceeds 0.1%, secondary workability deteriorates. More preferably, it is 0.03% or less.

S: 0.01% or less (including 0%)
S is also an element unavoidably present as an impurity element, forms sulfide inclusions such as MnS, and becomes a starting point of cracking and deteriorates workability. Preferably it is 0.01% or less, More preferably, it is 0.005% or less.

Al: 0.001 to 0.1%
Al is an element which is added as a deoxidizer and effectively suppresses the generation of carbides by decomposition of γ _{R in} combination with Si. In order to exhibit such an action effectively, it is necessary to add 0.001% or more of Al. However, even if added excessively, the effect is saturated and is economically wasteful, so the upper limit is made 0.1%.

N: 0.002 to 0.03%
N is an unavoidable element, but forms a precipitate when combined with carbonitride-forming elements such as Al and Nb, and contributes to strength improvement and microstructure refinement. And austenite grain coarsening the N content is too low, as a result, the aspect ratio for gamma _R which elongated lath structure becomes mainly increases. On the other hand, if the N content is too high, casting becomes difficult with low carbon steel such as the material of the present invention, and therefore the production itself cannot be performed.

  The steel sheet used in the method of the present invention basically contains the above components, and the balance is substantially iron and inevitable impurities, but in addition, the following allowable components may be added within the range not impairing the function of the present invention. it can.

Cr: 0.01 to 3%
Mo: 0.01 to 1%,
Cu: 0.01-2%,
Ni: 0.01-2%,
B: One or more elements of 0.00001 to 0.01% These elements are useful elements for strengthening steel and are effective in stabilizing γ _R and securing a predetermined amount. In order to effectively exhibit such an action, Mo: 0.01% or more (more preferably 0.02% or more), Cu: 0.01% or more (more preferably 0.1% or more), Ni : 0.01% or more (more preferably 0.1% or more) and B: 0.00001% or more (more preferably 0.0002% or more) are recommended. However, even if Cr is added in an amount of 3%, Mo is added in an amount of 1%, Cu and Ni are added in an amount of more than 2%, and B is added in an amount exceeding 0.01%, the above effect is saturated, which is economically wasteful. More preferably, Cr is 2.0% or less, Mo is 0.8% or less, Cu is 1.0% or less, Ni is 1.0% or less, and B is 0.0030% or less.

Ti: 0.01 to 0.1%,
V: 0.01-0.1%
One or more of Zr: 0.01 to 0.1% These elements have an effect of precipitation strengthening and refinement of the structure, and are effective elements for increasing the strength. In order to effectively exhibit such an action, it is recommended that each element is added in an amount of 0.01% or more, more preferably 0.02% or more. However, even if any element is added in an amount exceeding 0.1%, the above effect is saturated, which is economically useless. More preferably, any element is 0.08% or less.

Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: One or more of 0.0001 to 0.01% These elements are effective elements for controlling the form of sulfide in steel and improving workability. Here, examples of the REM (rare earth element) used in the present invention include Sc, Y, and lanthanoid. In order to effectively exhibit the above-mentioned action, Ca and Mg are each added to 0.0005% or more (more preferably 0.0001% or more), and REM is added to 0.0001% or more (more preferably 0.0002% or more). It is recommended to do. However, even if Ca and Mg are added in an amount of 0.01% and REM is added in excess of 0.01%, the above effects are saturated, which is economically wasteful. More preferably, Ca and Mg are 0.003% or less, and REM is 0.006% or less.

  Next, the warm forming method characterizing the method of the present invention will be described.

(Warm forming method)
The steel sheet using the method of the present invention is excellent in strength and elongation at room temperature, but is warm-formed under appropriate temperature conditions in press forming of parts, particularly deep drawing with a high degree of workability. As a result, more excellent deep drawing characteristics are exhibited.

  As specific temperature conditions, the mold temperature of at least the shoulder portion of the punch of the press mold is set to 250 to 350 ° C., and the mold temperature of at least the shoulder portion of the die is set to 100 to 200 ° C. Hereinafter, the reason for setting such temperature conditions will be described.

  In order to further improve the deep drawing characteristics of the deep drawing of the TRIP steel sheet, the strength of the portion that contacts the shrink flange portion and the shoulder portion of the die is reduced as much as possible to reduce the forming load, and at the same time, the vertical wall portion and It is important to increase the deformation resistance of the portion that contacts the shoulder of the punch as much as possible to prevent breakage due to molding load.

  On the other hand, in order to investigate the influence of the forming temperature on the mechanical properties by using the TRIP steel sheet different from the steel sheet using the method of the present invention in the already filed Japanese Patent Application No. 2010-46721, the present inventors have investigated the forming temperature ( Tensile temperature was varied from room temperature (20 ° C.) to 350 ° C., and tensile strength (TS) and total elongation (EL) were measured by a tensile test, and the relationship between tensile temperature and TS and EL was clarified. . These relationships are shown in FIGS. 1 and 2 (corresponding to FIGS. 1 and 2 of Japanese Patent Application No. 2010-46721, respectively). In addition, the “reference invention steel plate” in these figures is indicated as “invention steel plate” in FIG. 1 and FIG. 2 of Japanese Patent Application No. 2010-46721. In order to avoid confusion, the display is changed to “reference invention steel plate”.

  Therefore, in order to further improve the deep drawing characteristics of the TRIP steel sheet, the present inventors show that the mold temperature of at least the shoulder portion (flat portion and shoulder portion of the die) of TS is the lowest in FIG. On the other hand, from FIG. 1 and FIG. 2, the mold temperature of at least the shoulder of the punch (vertical wall and shoulder of the punch) is 250 to 350 ° C. where TS and EL are the highest. I thought that it should be in the range.

  And since it is assumed that the steel sheet using the method of the present invention also shows a tendency as shown in FIGS. 1 and 2, the temperature condition was determined.

  By performing warm forming under such temperature conditions, it is possible to reduce the forming load by minimizing the deformation resistance (TS) of the portion that contacts the shrink flange portion and the shoulder portion of the die, and at the same time, the vertical wall portion and the punch By increasing the deformation resistance (TS) and the total elongation (EL) of the portion in contact with the shoulder of the sheet as much as possible, the molding depth can be increased while preventing breakage due to the molding load, and the deep drawing characteristics are further improved.

  In the press molding method described in Patent Document 4, the die temperature of at least the shoulder portion (flat portion and shoulder portion of the die) of the die is 150 to 200 ° C., and TS is maximized as in the method of the present invention. Is selected, but the mold temperature of at least the shoulder (the vertical wall and shoulder of the punch) of the punch is −30 to 0 ° C., and judging from FIG. 1 and FIG. Although it becomes as high as 250-300 degreeC of an invention method, it is assumed that EL does not raise and it is thought that the deep drawing characteristic as the method of this invention cannot be obtained.

  Next, the preferable manufacturing method for obtaining the said steel plate using the method of this invention is demonstrated below.

[Preferred production method of steel sheet using the method of the present invention]
The steel sheet using the method of the present invention is manufactured by hot rolling a steel material satisfying the above component composition, followed by cold rolling, followed by heat treatment, but in order to increase the size of γ _R , the initial structure is changed. It needs to be coarse.

[Hot rolling conditions]
Therefore, the hot rolling finish temperature (rolling end temperature, FDT) is set to 900 to 1000 ° C., and the coiling temperature is set to 600 to 700 ° C., which is higher than the conventional one, thereby making the structure of the hot rolled material coarser than before. By doing so, the structure formed in the subsequent heat treatment process becomes coarse, and as a result, the size of γ _R also increases.

[Cold rolling conditions]
In addition, by reducing the cold rolling ratio during cold rolling to 10 to 30% (more preferably 10 to 20%), the recrystallized structure during heating in the subsequent annealing step is roughened and further cooled. The reverse transformation structure in is roughened.

[Heat treatment conditions]
Regarding heat treatment conditions, soaking in austenitic (γ + α) two-phase region or γ single-phase region, soaking at a predetermined cooling rate and supercooling, followed by a predetermined time at the supercooling temperature A desired tissue can be obtained by holding and austempering. It should be noted that plating or further alloying treatment may be performed without significantly degrading the desired structure and within the range not impairing the action of the present invention.

  Therefore, as specific heat treatment conditions, (1) when a cold-rolled steel sheet is manufactured on a continuous annealing line, (2) when a hot-dip galvanized steel sheet (GI steel sheet) is manufactured on a hot-dip galvanizing line, and (3) When manufacturing a hot dip galvanized steel plate (GA steel plate) with a hot dip galvanizing line, it demonstrates for each.

(1) When manufacturing a cold-rolled steel sheet with a continuous annealing line In order to austenize the cold-rolled material after the cold rolling, 0.6Ac1 + 0.4Ac3 or more, which is a (γ + α) two-phase region or a γ single-phase region (Preferably 0.4 Ac1 + 0.6 Ac3 or more) After holding for 1800 s or less (preferably 900 s or less) in a temperature range of 950 ° C. or less (930 ° C. or less), 3 ° C./s or more (preferably 5 ° C./s or more, More preferably, it is rapidly cooled to a temperature range of 380 to 500 ° C. (preferably 390 to 460 ° C., more preferably 400 to 420 ° C.) at an average cooling rate of 10 ° C./s or more, particularly preferably 20 ° C./s or more. The mixture is supercooled, held at this rapid cooling stop temperature (supercooling temperature) for 100 to 1800 s (preferably 200 to 800 s) for austempering, and then cooled to room temperature.

In the above-mentioned prior invention steel plate (Japanese Patent Application No. 2010-258151), the lower limit of the austempering temperature was 350 ° C., but in consideration of increasing the size of γ _R more reliably, the lower limit was reviewed, In the method of the present invention, the temperature was 380 ° C.

(2) When manufacturing a hot-dip galvanized steel sheet (GI steel sheet) in a hot-dip galvanizing line, in order to austenitize the cold-rolled material after the cold rolling, it is a (γ + α) two-phase region or a γ single-phase region. 0.6Ac1 + 0.4Ac3 or more (preferably 0.4Ac1 + 0.6Ac3 or more) After holding at a temperature of 950 ° C. or less (930 ° C. or less) for 1800 s or less (preferably 900 s or less), 3 ° C./s or more (preferably 380 to 500 ° C. (preferably 390 to 460 ° C., more preferably 400 to 420 ° C.) at an average cooling rate of 5 ° C./s or more, more preferably 10 ° C./s or more, particularly preferably 20 ° C./s or more. Is rapidly cooled to a temperature range of 10 to 100 s (preferably 20 to the same heat treatment conditions as in the case of (1) above) at this quenching stop temperature (supercooling temperature). Hold for 60 s) and austemper, then cool to room temperature.

(3) When manufacturing a hot-dip galvanized steel sheet (GA steel sheet) in a hot-dip galvanizing line, in order to austenitize the cold-rolled material after the cold rolling, it is a (γ + α) two-phase region or a γ single-phase region. 0.6Ac1 + 0.4Ac3 or more (preferably 0.4Ac1 + 0.6Ac3 or more) After holding at a temperature of 950 ° C. or less (930 ° C. or less) for 1800 s or less (preferably 900 s or less), 3 ° C./s or more (preferably 380 to 500 ° C. (preferably 390 to 460 ° C., more preferably 400 to 420 ° C.) at an average cooling rate of 5 ° C./s or more, more preferably 10 ° C./s or more, particularly preferably 20 ° C./s or more. Is rapidly cooled to a temperature range of 10 to 100 s (preferably 20 to the same heat treatment conditions as in the case of (1) above) at this quenching stop temperature (supercooling temperature). ˜60 s), and after austempering (the same heat treatment conditions as in the above (2)), the temperature range of 480 to 600 ° C. (preferably 480 to 550 ° C.) is 1 to 100 s. After reheating for a time and alloying, it is cooled to room temperature.

  In addition to the coarsening of the structure by adjusting the conditions for hot rolling and cold rolling as described above, the conditions for hot rolling and cold rolling are the same as in the prior art, such as continuous annealing. Prior to the heat treatment, the same structure can be obtained by preheating the structure in a temperature range of 500 ° C. to A1 point or lower for 1 to 30 hours to coarsen the structure and then performing a heat treatment such as continuous annealing. be able to.

  First, test steel sheets were produced by changing the component composition and production conditions in various ways. That is, after vacuum-melting the test steels having the respective component compositions shown in Table 1 to form a slab having a thickness of 30 mm, the slab was hot-rolled and cold-rolled under the production conditions shown in Table 2. Thereafter, heat treatment was performed. Specifically, the slab is heated to 1200 ° C., hot-rolled to a sheet thickness tmm at a rolling finish temperature (FDT) T1 ° C., then placed in a holding furnace at a coiling temperature T2 ° C., and air-cooled. Simulated board winding. Then, it cold-rolled by the cold rolling rate r%, and was set as the cold-rolled board of plate thickness 12mm. The cold-rolled material was heated to a soaking temperature T3 ° C. at 10 ° C./s, held at that temperature for 90 s, then cooled at a cooling rate R4 ° C./s, and held at a supercooling temperature T5 ° C. for t5 seconds. Thereafter, it was cooled with air, or held at a supercooling temperature T5 ° C. for t5 seconds, and further held at a holding temperature T6 ° C. for t6 seconds, and then cooled with air.

  In addition, Ac1 and Ac3 in Table 1 were calculated | required using the following formula 1 and formula 2 (translated by Shigeyasu Koda, "Leslie Steel Materials Science", Maruzen Co., 1985, p. 273).

Formula 1: Ac1 (degreeC) = 723 + 29.1 [Si] -10.7 [Mn] +16.9 [Cr] -16.9 [Ni]
Formula 2: Ac3 (° C.) = 910−203√ [C] +44.7 [Si] −30 [Mn] +700 [P] +400 [Al] +400 [Ti] +104 [V] −11 [Cr] +31.5 [Mo] -20 [Cu] -15.2 [Ni]
However, [] shows content (mass%) of each element.

The thus-obtained sample steel plate, by a measuring method described in the section of [Description of the Invention, each phase area ratio, C concentration of γ _R (Cγ _R), as well as, gamma The average equivalent circle diameter and aspect ratio of _R were measured.

  In addition, in order to evaluate the mechanical properties of the test steel plate at room temperature, tensile strength (TS) and elongation [total elongation (EL)] were measured by a tensile test using a JIS No. 5 test piece. The strain rate in the tensile test was 1 mm / s.

  Next, in order to evaluate the influence on the deep drawing characteristics due to the difference in temperature conditions during the warm forming of the test steel sheet, a cylinder having a die diameter of 53.4 mm, a punch diameter of 50.0 mm, and a shoulder R of 8 mm. Under the various temperature conditions by using a die, incorporating a heater in each die and punch and attaching a thermocouple, and independently controlling the die temperature near each shoulder of the die and punch. A test piece having a diameter of 80 to 140 mm was deep-drawn at a wrinkle-reducing pressure of 9.8 kN, and the deep punching characteristics [limit drawing ratio (LDR)] and the maximum punch load when the drawing ratio was 2.1 were measured.

  These results are shown in Tables 3-5. Table 3 shows the influence of the composition of the test steel sheet, Table 4 shows the influence of the manufacturing conditions of the steel sheet, and Table 5 shows the influence of the temperature conditions during deep drawing.

  As shown in these tables, Test No. 100, 102 to 104, 106 to 113, 122, 123, and 129 to 132 are all manufactured using the steel types satisfying the range of the component composition of the steel sheet used in the method of the present invention under the recommended manufacturing conditions. It is an example of the present invention that satisfies the requirements of the structure definition of the steel sheet used, and also satisfies the warm forming conditions specified by the method of the present invention. Both the room temperature TS, the punch maximum load during deep drawing and the LDR are judgment criteria. Thus, a warm forming method of a high strength steel sheet capable of maximizing the deep drawing properties while maintaining a room temperature strength of 980 MPa or higher in the high strength steel sheet was obtained.

  In contrast, test no. 101, 105, 114-121, 124-128, 133-136 are at least of the requirements of the component composition and structure defined for the steel sheet used in the method of the present invention, and the warm forming conditions defined in the method of the present invention. This is a comparative example that does not satisfy any of the above, and at least one of the room temperature TS, the maximum punch load at the time of deep drawing, and the LDR does not satisfy the criterion.

For example, test no. In 101, 105, and 114, C, Mn, and Si are too small in the component composition of the test steel sheet, the area ratio of γ _R is insufficient, and at least one of the maximum punch load and LDR at the time of deep drawing is selected. The criteria are not met.

  In addition, Test No. In 116-120 and 125, although the component composition of the test steel sheet satisfies the component composition defined for the steel sheet used in the method of the present invention, the manufacturing conditions are out of the recommended range, and the resulting structure is Outside the specified range, at least one of room temperature TS, punch maximum load during deep drawing and LDR does not satisfy the criteria.

  In addition, Test No. In 126-128 and 133-136, the test steel plate satisfies the requirements of the component composition and the structure specified for the steel plate used in the method of the present invention, but the temperature condition during deep drawing is out of the specified range, As a result, at least one of the punch maximum load and the LDR at the time of deep drawing does not satisfy the criterion.

Claims (4)

A method of warm-forming a high-strength steel sheet with a press mold,
The high-strength steel plate is in mass% (hereinafter the same for chemical components).
C: 0.05 to 0.3%
Si: 1-3%
Mn: 0.5-3%,
P: 0.1% or less (including 0%),
S: 0.01% or less (including 0%),
Al: 0.001 to 0.1%,
N: 0.002 to 0.03%
And the balance has a component composition consisting of iron and impurities,
The area ratio for all tissues (hereinafter the same for tissues)
Bainitic ferrite: 50-90%
Retained austenite: 5-20%,
Martensite + said retained austenite: 10-50%,
Ferrite: 40% or less (including 0%)
Having an organization including
The retained austenite has a C concentration (Cγ _R ) of 0.5 to 1.1 mass%, an average equivalent circle diameter of 0.4 to 2 μm, and an average aspect ratio (maximum diameter / minimum diameter) of 3.0. Less than, and
Warm forming method of high strength steel sheet, characterized in that mold temperature of at least shoulder portion of punch of said press mold is 250-350 ° C and mold temperature of at least shoulder portion of die is 100-200 ° C. . The component composition of the high-strength steel plate is further
Cr: 0.01 to 3%
Mo: 0.01 to 1%,
Cu: 0.01-2%,
Ni: 0.01-2%,
B: The method of warm forming a high-strength steel sheet according to claim 1, comprising one or more of 0.00001 to 0.01%. The component composition of the high-strength steel plate is further
Ti: 0.01 to 0.1%,
V: 0.01-0.1%
The warm forming method for high-strength steel sheets according to claim 1 or 2, comprising one or more of Zr: 0.01 to 0.1%. The component composition of the high-strength steel plate is further
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
The method of warm forming a high-strength steel sheet according to any one of claims 1 to 3, wherein the REM contains one or more of 0.0001 to 0.01%.

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