JPWO2013132821A1 - Warm press molding method and automotive framework parts - Google Patents

Abstract

When forming a steel sheet with a tensile strength of 440 MPa or more into a press-molded product consisting of a flange part and other parts by press forming, the steel sheet is heated to a temperature range of 400 to 700 ° C. In addition, press molding is performed using draw molding, and at that time, the state is maintained for 1 second or more and 5 seconds or less at the bottom dead center of the molding, thereby suppressing the shape change of the springback and improving the dimensional accuracy of the panel. Furthermore, it is possible to easily obtain desired mechanical properties in the press-formed product.

Description

The present invention relates to a warm press forming method capable of suppressing a dimensional accuracy defect caused by a shape change such as a spring back, which occurs when a high strength steel plate is press formed.
The present invention also relates to an automobile frame part manufactured by the warm press molding method.

  High-strength steel sheets are being applied to vehicle parts in order to achieve both weight reduction of the vehicle body for the purpose of improving fuel efficiency and improvement of collision safety for occupant protection. However, high-strength steel sheets are generally inferior in press formability, have a large shape change (spring back) due to elastic recovery after mold release, and are prone to dimensional accuracy, so the parts to which press forming is applied are limited. This is the current situation.

  Therefore, for the purpose of improving press formability and shape freezing property (decreasing spring back), Patent Document 1 discloses that hot press forming is performed on a high-strength steel plate by press forming after heating the steel plate to a predetermined temperature. An applied example is disclosed.

JP 2005-205416 A

  The above hot press forming reduces the deformation resistance of the steel sheet during press forming by forming at a higher temperature than cold press forming, in other words, improves the deformability and improves the shape freezing property. This is a technique to achieve this together with prevention of press cracking.

The hot press forming disclosed in Patent Document 1 performs press forming by draw (drawing) forming. In this draw forming, the edge of a heated steel plate (hereinafter also referred to as a blank) is clamped by a die mold and a blank holder (wrinkle presser) during forming. There is a difference in contact time with In addition, since the blank temperature of the contacted portion falls during press molding, it is not effective in the press-molded product immediately after molding (hereinafter also referred to as a panel) due to the influence of the difference in contact time with the above-described mold or the like. A uniform temperature distribution occurs.
As a result, especially in automobile framework parts to which high-strength steel sheets are applied, the panel shape changed during air cooling after hot press forming, and there was a problem that a panel with sufficiently satisfactory dimensional accuracy could not be obtained. .

  Also, in general hot press forming, the steel sheet is heated to the austenite region and accompanied by quenching and phase transformation at the time of cooling. There was a problem of large variations in characteristics.

The present invention has been developed to solve the above-described problems, and suppresses a shape change such as a spring back to improve the dimensional accuracy of the panel and easily obtain desired mechanical characteristics in a press-formed product. It is an object of the present invention to provide a warm press molding method that can be used.
Another object of the present invention is to provide an automobile frame part manufactured by the warm press molding method.

Now, in order to solve the above problems, the inventors have determined the heating temperature of the steel sheet that had to be heated to the austenite region when applying a high-strength steel sheet in the conventional hot press forming, the austenite transformation temperature. Tried to lower than.
At the same time, in order to find a condition that can suppress the amount of change in shape due to the springback, various studies were made on various molding methods and molding conditions.

As a result, when forming a high-strength steel sheet into a press-formed product consisting of a flange part and other parts by press molding,
(1) Heat the steel plate to the so-called warm forming temperature range,
(2) Next, when the heated steel sheet is subjected to press molding by draw molding, the state is maintained for a predetermined time or longer at the bottom dead center of molding.
As a result, it was found that the intended purpose can be achieved advantageously.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. When forming a steel sheet with a tensile strength of 440 MPa or more into a press-molded product consisting of a flange part and other parts by press molding,
The steel sheet is heated to a temperature range of 400 to 700 ° C.
Next, the heated steel sheet is subjected to press forming by draw forming, and the state is maintained at 1 to 5 seconds at the bottom dead center.
A warm press molding method.

2. 2. The warm press molding method according to 1 above, wherein an average temperature difference between the flange portion of the press-molded product immediately after the draw molding and other portions is set to 150 ° C. or less.

3. 3. The warm press forming method according to 1 or 2 above, wherein the press-formed product has a tensile strength of 80% to 110% of the tensile strength of the steel plate.

4). The steel sheet is in mass%,
C: 0.015-0.16%,
Si: 0.2% or less,
Mn: 1.8% or less,
P: 0.035% or less,
S: 0.01% or less,
Al: 0.1% or less,
N: 0.01% or less and
Ti: 0.13-0.25%
In a range satisfying the relationship of the following formula (1), the balance has a component composition consisting of Fe and inevitable impurities,
A structure in which the proportion of the ferrite phase in the entire structure is 95% or more in area ratio, the average crystal grain size of ferrite is 1 μm or more, and carbides having an average grain size of 10 nm or less are dispersed and precipitated in the ferrite crystal grains. Having
The warm press molding method according to any one of the above items 1 to 3.
Record
2.00 ≧ ([% C] / 12) / ([% Ti] / 48) ≧ 1.05… (1)
Here, [% M] is the content of M element (mass%)

5. The steel sheet is further mass%,
V: 1.0% or less,
Mo: 0.5% or less,
W: 1.0% or less,
Nb: 0.1% or less,
Zr: 0.1% or less and
Hf: The warm press molding method according to 4 above, which contains one or two or more selected from 0.1% or less and satisfies the relationship of the following formula (1) ′.
Record
2.00 ≧ ([% C] / 12) / ([% Ti] / 48 + [% V] / 51 + [% W] / 184 + [% Mo] / 96 + [% Nb] / 93 + [% Zr] / 91 + [% Hf] / 179) ≧ 1.05… (1) '
Here, [% M] is the content of M element (mass%)

6). 6. The warm press forming method according to 4 or 5 above, wherein the steel sheet further contains B: 0.003% or less by mass%.

7). The steel sheet further contains one or more selected from the group consisting of Mg: 0.2% or less, Ca: 0.2% or less, Y: 0.2% or less, and REM: 0.2% or less in terms of mass%. The warm press molding method according to any one of 4 to 6.

8). Any of 4 to 7 above, wherein the steel sheet further contains, by mass%, one or more selected from Sb: 0.1% or less, Cu: 0.5% or less, and Sn: 0.1% or less. The warm press molding method according to 1.

9. The warm press forming as described in any one of 4 to 8 above, wherein the steel sheet further contains, by mass%, one or two selected from Ni: 0.5% or less and Cr: 0.5% or less. Method.

10. The steel sheet is further in mass%, O, Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir. The warm as described in any one of 4 to 9 above, containing a total of 2.0% or less of one or more selected from Ru, Os, Tc, Re, Ta, Be and Sr Press molding method.

11. 11. The warm press forming method according to any one of 1 to 10, wherein the steel sheet has a plating layer on the surface thereof.

12 An automobile frame part manufactured by the warm press molding method according to any one of 1 to 11 above.

According to the present invention, it is possible to suppress the shape change that occurs when the panel after press molding is air-cooled, and thereby it is possible to manufacture an automobile skeleton component with good dimensional accuracy. As a result, a high-strength steel sheet that could not be applied due to poor dimensional accuracy can be applied to automobile frame parts, which can greatly contribute to improvement of environmental problems through weight reduction of the vehicle body.
Further, according to the present invention in which the press forming is performed warmly, the mechanical properties of the steel sheet as a raw material can be utilized as it is without quenching or phase transformation before and after forming, so that a press-formed product having desired properties is obtained. Can be obtained stably.

It is a figure explaining the press molding by draw (drawing) shaping | molding, (a) represents the state in the time of a shaping | molding start, (b) in the middle of shaping | molding, (c) represents the state in shaping | molding bottom dead center (at the time of shaping | molding completion). . (A) It is a figure which shows an example of the automobile frame components manufactured from the panel obtained by press molding. (B) It is a figure explaining the flange part of the panel obtained by press molding using draw molding. (A) The average temperature difference between the flange portion of the panel warm-formed by draw molding and the other portions, and the amount of change in the shape of the panel immediately after press molding (when the panel is removed from the mold) and after air cooling It is a figure explaining the relationship. (B) It is a figure explaining the shape change amount of the panel immediately after press molding (at the time of removing a panel from a metal mold | die) and after air cooling. (A) It is a figure explaining the relationship between the average temperature difference of the flange part of the panel warm-press-molded by draw molding, and the other part, and the holding time in a shaping | molding bottom dead center. (B) The relationship between the amount of change in the shape of the panel immediately after press molding (when the panel is removed from the mold) and the air cooling after the warm press molding by draw molding and the holding time at the bottom dead center of molding will be described. FIG. (A) It is a figure which shows the outline of a center pillar upper press panel. (B) It is a figure explaining the shape change amount of the panel immediately after press molding (at the time of removing a panel from a metal mold | die) and after air cooling.

Hereinafter, the present invention will be specifically described.
First, the reason why the heating temperature of the steel sheet before press forming is set in the range of 400 to 700 ° C. will be described.

Steel plate heating temperature: 400 ~ 700 ℃
By heating the steel plate to 400 ° C. or higher, the strength decreases and the ductility increases. For this reason, it becomes easy to deform | transform a steel plate along a metal mold | die during press molding, a press crack can be prevented, and also generation | occurrence | production of a wrinkle can also be suppressed. However, when the heating temperature of the steel sheet exceeds 700 ° C., the material strength becomes too low, and there is a risk of cracking or breaking. Therefore, the heating temperature of the steel sheet is in the range of 400 to 700 ° C. In particular, when the heating temperature of the steel sheet is 400 ° C. or higher and lower than 650 ° C., it is possible to suppress the occurrence of oxidation and cracking on the surface of the steel sheet, and it is more advantageous because the press load does not increase excessively.

  Next, in the present invention, when press molding by draw molding is performed, the reason why the state is maintained at 1 to 5 seconds at the molding bottom dead center will be described.

In order to press-mold a panel that requires the height of the side wall, it is generally performed by draw (drawing) molding. When performing this draw forming, even if it is warm (or hot) press forming, a wrinkle presser is arranged as shown in FIG. In general, molding is performed while applying tension to the side wall while clamping the blank edge with a die.
In FIG. 1, reference numeral 1 is a die, 2 is a punch, 3 is a crease presser, 4 is a heated steel plate (blank), 5 is a press-formed product (panel) after forming, 6 is a flange portion, and 7 is a side wall portion. It is.

  For example, as shown in FIG. 2 (a), an automobile skeleton component often forms a closed cross section by joining substantially hat cross-sectional members by spot welding or the like. Here, the blank edge portion narrowed as shown in FIG. 2B becomes a flange portion of the panel after molding, and this flange portion becomes a portion for joining the panels to each other by spot welding or the like. Therefore, it is required to be flat. Therefore, as described above, molding is performed while applying a wrinkle holding force to the blank edge.

  In the case of the draw molding as described above, the blank edge portion is always pinched by the wrinkle presser and the upper die (die) from the beginning of molding to the completion of molding. For this reason, when press-molding a heated steel plate (blank), heat transfer from the blank edge to the mold occurs, the temperature of the blank edge tends to drop, and the flange part of the panel immediately after molding and the rest The temperature difference from this part will increase.

  If there is such a temperature difference in the panel, the amount of thermal shrinkage in the process of cooling to room temperature will differ depending on the part in the panel, so residual stress will be generated in the panel, and this stress will be reduced. The shape of the panel changes to open. The inventors considered that this is the main factor of shape change during cooling.

Therefore, the inventors firstly made the relationship between the average temperature difference between the flange portion of the panel and the other portions when performing press molding by draw molding, and the amount of change in the shape of the panel immediately after press molding and after air cooling. We focused on and investigated this.
In addition, the description of "difference in average temperature" here means the average temperature difference immediately after press molding, and is used in this sense unless otherwise specified. Here, “immediately after press molding” is after the holding treatment at the bottom dead center of molding, and corresponds to the time of starting air cooling after removing the panel from the mold. The “shape change amount” means a difference (change amount) between the shape at the time when the panel is removed from the mold immediately after the warm press is formed and the shape after the panel is air-cooled. .

  Fig. 3 (a) shows the average temperature difference between the flange portion of the panel having a substantially hat cross-sectional shape immediately after warm press molding by draw molding and the other portions, and the panel when removed from the mold immediately after press molding. And the change in shape of the panel after air cooling. Here, a 980 MPa grade steel plate was used, and the heating temperature of the steel plate was 600 ° C. Further, as shown in FIG. 3B, the above-described shape change amount was evaluated by an opening amount a at the flange end with respect to a reference panel (a panel at the time of removal from the mold immediately after press molding). In the figure, reference numeral 8 is a reference panel (broken line), 9 is a panel after air cooling (thick solid line), and 10 is a panel at the bottom dead center of molding (thin solid line).

  As shown in FIG. 3A, as the average temperature difference increases, the amount of change in the shape of the panel immediately after being removed from the mold immediately after press molding and the panel after air cooling increases. In particular, when the average temperature difference exceeds 150 ° C, the shape change amount exceeds 1.0 mm. Therefore, in order to reduce the shape change amount due to the temperature difference in the panel, this average temperature difference is within 150 ° C, preferably It can be said that it is important to suppress the temperature within 100 ° C.

  And by the above investigation, there is a strong correlation between the average temperature difference between the flange part of the panel and the other parts, and the amount of change in the shape of the panel after being removed from the mold immediately after press molding and the panel after air cooling. The inventors who have found that there is a repeated investigation on a method for suppressing the above-described average temperature difference when performing the draw molding. As a result, it has been conceived that the state is maintained for a certain time or longer at the bottom dead center of molding shown in FIG.

Here, the reason why the above average temperature difference can be suppressed by maintaining the state at the bottom dead center of molding is as follows.
That is, when a panel formed from a blank is held at the bottom dead center, not only the flange portion restrained by the die and the wrinkle presser, but also the portions other than the flange portion such as the side wall portion are in contact with the die and the punch mold and cooled. Is done. Thereby, soaking | uniform-heating in a panel advances and the average temperature difference of a flange part and other parts is suppressed.

  FIG. 4 (a) shows the relationship between the average temperature difference between the flange part of the panel having a substantially hat cross-sectional shape warm-formed by draw molding and the other part and the holding time at the bottom dead center. FIG. 4 (b) shows the relationship between the amount of change in the shape of the panel immediately after press forming and the panel after air cooling, and the holding time at the bottom dead center. Here, a 980 MPa grade steel plate was used, and the heating temperature of the steel plate was 600 ° C., 650 ° C., and 700 ° C.

As shown in FIGS. 4 (a) and 4 (b), when the heating temperature is 600 ° C., the holding time at the bottom dead center of the molding is set to 1 second or longer so that the flange portion of the panel and the others It can be seen that the average temperature difference from this part can be kept within 150 ° C., and the change in the shape of the panel can be suppressed within 1.0 mm.
Furthermore, by setting the holding time at the bottom dead center of molding to 3 seconds or more, even if the heating temperature is 650 ° C or 700 ° C, the average temperature difference between the flange portion of the panel and the other portions is 150 ° C. It can be seen that the temperature can be kept within the temperature range, and the shape change amount of the panel can be suppressed within 1.0 mm.
However, if the holding time at the molding bottom dead center exceeds 5 seconds, the amount of change in shape becomes almost constant regardless of the heating temperature, but it is disadvantageous in terms of production efficiency.
From the above, in the present invention, when press molding by draw molding is performed, the state is maintained at 1 to 5 seconds at the bottom dead center of molding. Preferably, it is 3 seconds or more and 5 seconds or less.

  As described above, in order to keep this average temperature difference within 150 ° C. for any strength steel sheet, the heating temperature of the steel sheet is set to 400 to 700 ° C., and the holding time at the bottom dead center is set to 3 seconds or more. That's fine. At this time, the draw molding conditions such as the press speed are not particularly limited, but the press speed is about 10 to 15 spm (Strokes per minute: number that can be processed in one minute. However, when holding at the bottom dead center of the molding. The retention time is further added).

  In addition, the draw molding has an advantage that wrinkles of the flange portion are hardly generated because the flange portion is continuously clamped during molding. Furthermore, in the present invention, since the holding at the molding bottom dead center is performed as described above, the generation of wrinkles in the flange portion can be more effectively suppressed.

  In addition, about the heating of a steel plate, the same effect is exhibited irrespective of the kind of heating method, such as heating by an electric furnace, rapid heating by energization heating and far infrared heating.

  In addition, as described above, the warm press forming method of the present invention targets a steel plate having a tensile strength of 440 MPa or more. Furthermore, the warm press forming method of the present invention can be suitably used for steel sheets having a tensile strength of 780 MPa or more, and further 980 MPa or more.

And, as described above, according to the warm press forming method of the present invention, the mechanical properties of the blank steel plate can be utilized as it is. Therefore, in the press-formed product after press forming, the tension of the steel plate before press forming is achieved. The difference with strength is small, and a tensile strength of 80% to 110% can be obtained.
Furthermore, depending on the forming conditions and the characteristics of the steel sheet, the tensile strength of the steel sheet before press forming is almost maintained after press forming (having a tensile strength of 95 to 100% of the tensile strength of the steel sheet before press forming). ) A press-molded product can be obtained.
Therefore, according to the required characteristics of the press-formed product, if a steel plate having the corresponding characteristics is used as a blank, a press-formed product having the desired characteristics can be stably obtained.

  Hereinafter, the component composition range of a steel plate suitable as a blank in the present invention will be described. In addition, unless otherwise indicated, the "%" display regarding a component shall mean "mass%".

C: 0.015-0.16%
C is an important element that combines with Ti, V, Mo, W, Nb, Zr, and Hf to form carbides and finely disperses in the matrix to increase the strength of the steel sheet. Here, in order to achieve a tensile strength of 440 MPa or more, the C content is preferably 0.015% or more. On the other hand, when the amount of C exceeds 0.16%, ductility and toughness are remarkably lowered, and good impact absorption capacity (for example, expressed by tensile strength TS × total elongation El) cannot be secured. For this reason, it is preferable to make C into the range of 0.015 to 0.16%. More preferably, it is 0.03-0.16%, More preferably, it is 0.04-0.14% of range.

Si: 0.2% or less
Si is a solid solution strengthening element and inhibits workability in the warm forming temperature range (warm formability) in order to suppress a decrease in strength in the high temperature range. For this reason, it is preferable to reduce as much as possible in the present invention, but up to 0.2% is acceptable. Therefore, Si is preferably 0.2% or less. More preferably, it is 0.1% or less, More preferably, it is 0.06% or less. Si may be reduced to the impurity level.

Mn: 1.8% or less
Mn, like Si, is a solid solution strengthening element and inhibits workability in the warm forming temperature range (warm formability) in order to suppress a decrease in strength in the high temperature range. For this reason, it is preferable to reduce as much as possible in the present invention, but up to 1.8% is acceptable. For these reasons, Mn is preferably 1.8% or less. More preferably, it is 1.3% or less, More preferably, it is 1.1% or less. Note that if the Mn content is extremely low, the austenite (γ) → ferrite (α) transformation temperature is excessively increased, and there is a concern that the carbides become coarse, so Mn may be 0.5% or more. preferable.

P: 0.035% or less P is an element that has a very high solid-solution strengthening ability and inhibits workability (warm formability) in the warm forming temperature range in order to suppress a decrease in strength in the high temperature range. Furthermore, since P segregates at the grain boundaries, it lowers the ductility during and after warm forming. For these reasons, it is preferable to reduce P as much as possible, but it is acceptable up to 0.035%. Therefore, P is preferably 0.035% or less. In addition, More preferably, it is 0.03% or less, More preferably, it is 0.02% or less.

S: 0.01% or less S is an element that exists as an inclusion in steel. It combines with Ti to reduce strength, or combines with Mn to form sulfides. Reduce ductility. For this reason, S is preferably reduced as much as possible, but is acceptable up to 0.01%. For this reason, S is preferably 0.01% or less. In addition, More preferably, it is 0.005% or less, More preferably, it is 0.004% or less.

Al: 0.1% or less
Al is an element that acts as a deoxidizer. In order to obtain such an effect, it is desirable to contain 0.02% or more. However, if Al exceeds 0.1%, oxide inclusions increase, and the ductility drop during warming becomes significant. For this reason, Al is preferably 0.1% or less. More preferably, it is 0.07% or less.

N: 0.01% or less N is combined with Ti, Nb, or the like at the steel making stage to form coarse nitrides. For this reason, when N is contained in a large amount, the steel sheet strength is remarkably lowered. For these reasons, it is preferable to reduce N as much as possible, but it is acceptable up to 0.01%. Therefore, N is preferably 0.01% or less. More preferably, it is 0.007% or less.

Ti: 0.13-0.25%
Ti is an element that combines with C to form carbides and contributes to strengthening of the steel sheet. In order to ensure the tensile strength at room temperature of the steel sheet to be used in the present invention: 440 MPa or more, it is preferable to contain 0.13% or more of Ti. On the other hand, when Ti exceeding 0.25% is contained, coarse TiC remains when the steel material is heated, and microvoids are generated. For this reason, it is preferable to make Ti amount into 0.25% or less. In addition, More preferably, it is 0.14-0.22%, More preferably, it is 0.15-0.22% of range.

The preferred range of each component has been described above. However, it is not sufficient that each component satisfies the above range, and it is important to satisfy the relationship of the following formula (1) for C and Ti.
2.00 ≧ ([% C] / 12) / ([% Ti] / 48) ≧ 1.05… (1)
Here, [% M] is the content of M element (mass%)

That is, the formula (1) is a requirement necessary for developing precipitation strengthening by carbides described later and ensuring a desired high strength after warm forming. By satisfying the relationship of the formula (1) with respect to the contents of C and Ti, a desired amount of carbide can be precipitated, and thereby a desired high strength can be ensured.
Further, when the value of [[% C] / 12) / ([% Ti] / 48) is less than 1.05, not only the grain boundary strength is lowered but also the thermal stability of the carbide with respect to heating is lowered. For this reason, it becomes easy to coarsen the carbide, and the desired increase in strength cannot be achieved. On the other hand, when the value of ([% C] / 12) / ([% Ti] / 48) exceeds 2.00, cementite is excessively precipitated. For this reason, micro void generation is generated during warm forming, which causes cracks during warm forming. A more preferable range of ([% C] / 12) / ([% Ti] / 48) is 1.05 or more and 1.85 or less.

  Although the basic components have been described above, the steel plate suitable for use in the warm press forming method of the present invention can appropriately contain the following elements in addition to the above-described components.

One or more selected from V: 1.0% or less, Mo: 0.5% or less, W: 1.0% or less, Nb: 0.1% or less, Zr: 0.1% or less, and Hf: 0.1% or less V, Mo, W, Nb, Zr and Hf are elements that contribute to the strengthening of the steel sheet by forming carbides like Ti. Therefore, when further strengthening of the steel sheet is required, it can be selected from V, Mo, W, Nb, Zr and Hf in addition to Ti, and can be contained in one or more kinds. . In order to obtain such an effect, V is 0.01% or more, Mo is 0.01% or more, W is 0.01% or more, Nb is 0.01% or more, Zr is 0.01% or more, and Hf is 0.01% or more. Is preferred.
On the other hand, when V exceeds 1.0%, the carbide tends to be coarsened, and the carbide is coarsened particularly in the warm forming temperature range. Therefore, the average particle size of the carbide after cooling to room temperature can be adjusted to 10 nm or less. It becomes difficult. Therefore, V is preferably 1.0% or less. In addition, More preferably, it is 0.5% or less, More preferably, it is 0.2% or less.
Further, when Mo and W exceed 0.5% and 1.0%, respectively, the γ → α transformation is extremely delayed. For this reason, a bainite phase and a martensite phase are mixed in the steel sheet structure, and it becomes difficult to obtain a ferrite single phase described later. For these reasons, Mo and W are preferably 0.5% or less and 1.0% or less, respectively.
Furthermore, when Nb, Zr and Hf are contained in amounts exceeding 0.1%, coarse carbides cannot be completely dissolved and remain when the slab is reheated. For this reason, it becomes easy to produce a micro void during warm forming. For these reasons, Nb, Zr and Hf are each preferably 0.1% or less.
In addition, when including each above-mentioned element, it replaces with said Formula (1), and it is necessary to satisfy the range of following Formula (1) '. The reason for this is the same as that described for (1).
2.00 ≧ ([% C] / 12) / ([% Ti] / 48 + [% V] / 51 + [% W] / 184 + [% Mo] / 96 + [% Nb] / 93 + [% Zr] / 91 + [% Hf] / 179) ≧ 1.05… (1) '
Here, [% M] is the content of M element (mass%)

  Furthermore, in the steel plate suitable for use in the warm press forming method of the present invention, the elements described below can be appropriately contained.

B: 0.003% or less B has an action of inhibiting the nucleation of the γ → α transformation and lowering the γ → α transformation point, and this action contributes to the refinement of carbides. In order to obtain such an effect, it is desirable to contain 0.0002% or more of B. However, even if it contains B exceeding 0.003%, the effect is saturated and it is economically disadvantageous. Therefore, B is preferably 0.003% or less. More preferably, it is 0.002% or less.

One or more selected from Mg: 0.2% or less, Ca: 0.2% or less, Y: 0.2% or less, and REM: 0.2% or less
Mg, Ca, Y, and REM all have the effect of refining inclusions, which suppresses stress concentration in the vicinity of the inclusions and the base material during warm forming and improves ductility. Has an effect. For this reason, these elements can be contained as needed. REM is an abbreviation for Rare Earth Metal and refers to a lanthanoid element.
However, if Mg, Ca, Y, and REM are excessively contained in excess of 0.2%, castability (property of molten steel flow when solidified by putting molten steel in a mold) is lowered, and ductility is rather reduced. Incurs a decline. For this reason, Mg: 0.2% or less, Ca: 0.2% or less, Y: 0.2% or less, REM: 0.2% or less are preferable. More preferably, Mg is 0.001 to 0.1%, Ca is 0.001 to 0.1%, Y is 0.001 to 0.1%, and REM is 0.001 to 0.1%.
Further, the total amount of these elements is desirably adjusted to be 0.2% or less, and more preferably 0.1% or less.

One or more selected from Sb: 0.1% or less, Cu: 0.5% or less, and Sn: 0.1% or less
Sb, Cu and Sn are concentrated near the surface of the steel sheet and have the effect of suppressing the softening of the steel sheet due to nitriding of the steel sheet surface during warm forming, and can contain one or more kinds as necessary. . Cu also has the effect of improving corrosion resistance. In order to obtain such an effect, it is desirable to contain 0.005% or more of Sb, Cu and Sn, respectively. However, if the Sb content exceeds 0.1%, Cu content exceeds 0.5%, and Sn content exceeds 0.1%, the surface properties of the steel sheet deteriorate. For this reason, it is preferable to set Sb: 0.1% or less, Cu: 0.5% or less, and Sn: 0.1% or less.

One or two selected from Ni: 0.5% or less and Cr: 0.5% or less
Both Ni and Cr are elements that contribute to high strength, and one or two selected from these can be included as necessary. Here, Ni is an austenite stabilizing element, which suppresses the formation of ferrite at high temperatures and contributes to increasing the strength of the steel sheet. Cr is a hardenability-enhancing element and, like Ni, suppresses the formation of ferrite at a high temperature and contributes to increasing the strength of the steel sheet.
In order to obtain such an effect, it is preferable to contain 0.01% or more of Ni and Cr. However, when Ni and Cr are excessively contained in excess of 0.5%, generation of low-temperature transformation phases such as martensite phase and bainite phase is induced. A low temperature transformation phase such as a martensite phase or a bainite phase recovers during heating, and thus reduces the strength after warm forming. For this reason, Ni and Cr are each preferably 0.5% or less. In addition, More preferably, it is 0.3% or less.

O, Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re, One or more selected from Ta, Be, and Sr is 2.0% or less in total If these elements are 2.0% or less in total, they do not affect the strength and warm formability of the steel sheet So acceptable. More preferably, it is 1.0% or less.
The balance other than the above components is Fe and inevitable impurities.

Next, a suitable structure of the above steel plate will be described.
Ratio of ferrite phase in entire structure: 95% or more in area ratio In the present invention, the metal structure of the steel sheet is a ferrite single phase. The term “ferrite single phase” here includes not only the case where the ferrite phase is 100% in area ratio but also the case where the ferrite phase is substantially a ferrite single phase of 95% or more.
By making the metal structure a ferrite single phase, excellent ductility can be maintained, and further, material change due to heat can be suppressed. When a bainite phase or a martensite phase, which are hard phases, is mixed, dislocations introduced into the hard phase are recovered and softened by heating, so that the steel sheet strength cannot be maintained after warm forming. For this reason, it is better not to contain a pearlite, bainite phase, or martensite phase, but such a hard phase and further a retained austenite phase are acceptable as long as the area ratio to the entire structure is 5% or less.

Here, when the metal structure is substantially a ferrite single phase, the metal structure of the steel sheet is substantially single ferrite even when heated to a temperature range of 400 ° C. to 700 ° C. (warm forming temperature range). It remains in phase. And since the above-mentioned steel plate is heated and ductility increases, it can ensure favorable total elongation in a warm forming temperature range.
Further, when the steel sheet is formed in the warm forming temperature range, the steel sheet is formed while recovering the dislocation, so that there is almost no decrease in ductility during the warm forming. And even if it cools to room temperature after warm forming, a structure change does not arise, Therefore The metal structure of a steel plate is maintained with the ferrite single phase substantially, and shows the excellent ductility.

Average grain size of ferrite: 1μm or more If the average grain size of ferrite is less than 1μm, crystal grains tend to grow during warm forming. The material stability is reduced. Therefore, the average crystal grain size of ferrite is preferably 1 μm or more.
On the other hand, if the average crystal grain size of ferrite exceeds 20 μm and becomes excessively large, strengthening due to the refinement of the structure cannot be obtained, and it becomes difficult to ensure the desired steel plate strength. For this reason, the average crystal grain size of ferrite is preferably 15 μm or less. More preferably, it is 12 μm or less.

  In order to obtain a structure in which the average crystal grain size of ferrite is 1 μm or more, it is effective to prevent the number of ferrite nucleation sites from becoming excessive. The number of nucleation sites is closely related to the strain energy accumulated in the steel sheet during rolling, and it is necessary to prevent the accumulation of excessive strain energy in order to prevent the refinement of ferrite grains. For this purpose, the finish rolling finish temperature is preferably 840 ° C. or higher.

Average particle diameter of carbides in ferrite crystal grains: 10 nm or less With the above ferrite single phase structure, it is difficult to obtain a steel sheet having a sufficiently high tensile strength and yield ratio. In this regard, if fine carbides having an average particle diameter of 10 nm or less are precipitated in the ferrite crystal grains, the strength of the steel sheet can be increased. Here, when the average particle diameter of the carbide exceeds 10 nm, it becomes difficult to obtain the above-described high tensile strength and yield ratio. A more preferable average particle diameter of the carbide is 7 nm or less.

  Examples of fine carbides include Ti carbide, and further V carbide, Mo carbide, W carbide, Nb carbide, Zr carbide, and Hf carbide. These carbides are not coarsened when the heating temperature of the steel sheet is 700 ° C. or less, and the average particle diameter is maintained at 10 nm or less. Therefore, even if the steel sheet is heated to a warm forming temperature range of 400 ° C or higher and 700 ° C or lower and subjected to warm forming, the coarsening of carbides is suppressed, so the steel sheet strength is reduced after cooling to room temperature after warm forming. There will be no significant decrease in. Therefore, if a steel sheet having a structure containing the above-described carbide having an average particle diameter of 10 nm or less in a matrix of ferrite single phase substantially, the steel sheet is heated to a warm forming temperature range of 400 ° C. or more and 700 ° C. or less, It is possible to effectively suppress a decrease in yield stress of a press-formed product obtained by performing warm forming.

  In addition, the above steel plate may have a plating layer such as a hot dip galvanized layer. Examples of such a plating layer include an electroplating layer, an electroless plating layer, and a hot dipping layer. Furthermore, an alloyed plating layer may be used.

Next, the manufacturing method of a steel plate suitable for use in the warm press forming method of the present invention will be described.
A steel plate suitable for use in the warm press forming method of the present invention is obtained by heating a steel material, then subjecting it to hot rolling consisting of rough rolling and finish rolling, and then rolling it into a coil to obtain a hot-rolled steel plate.
The method for producing the steel material is not particularly limited, but the molten steel having the above composition is melted by a known melting method such as a converter or an electric furnace, or further in a vacuum degassing furnace. After secondary refining, it is preferable to cast a steel material such as a slab by a known casting method such as a continuous casting method. In view of productivity and quality, the continuous casting method is preferable.

Hereinafter, preferable manufacturing conditions will be described.
Heating temperature of steel material: 1100-1350 ° C
When the heating temperature of the steel material is less than 1100 ° C., coarse carbides do not dissolve, so the amount of fine carbides dispersed and precipitated in the finally obtained steel sheet decreases, making it difficult to ensure the desired high strength. On the other hand, when the heating temperature of the steel material exceeds 1350 ° C., the oxidation becomes remarkable, and the oxide scale is caught during hot rolling, thereby deteriorating the surface properties of the steel sheet, thereby reducing the warm formability of the steel sheet. For this reason, it is preferable to make the heating temperature of a steel raw material into the range of 1100-1350 degreeC. In addition, More preferably, it is the range of 1150-1300 degreeC.

Finish rolling end temperature: 840 ° C. or higher If the finish rolling end temperature is less than 840 ° C., the structure becomes a structure in which ferrite grains are extended, and a mixed grain structure in which individual ferrite grain sizes differ greatly, and the steel sheet strength is significantly reduced. If the finish rolling finish temperature is less than 840 ° C., the strain energy accumulated in the steel sheet during rolling becomes excessive, and it becomes difficult to obtain a structure in which the average crystal grain size of ferrite is 1 μm or more. For this reason, it is preferable that finish rolling completion temperature shall be 840 degreeC or more. In addition, More preferably, it is 860 degreeC or more.

Time from the end of hot rolling to the start of forced cooling: within 3 seconds After the completion of the above hot rolling, the obtained hot rolled steel sheet is forcibly cooled. If the time from the end of this hot rolling to the start of forced cooling exceeds 3 seconds, a large amount of strain-induced precipitation of carbide occurs, making it difficult to ensure desired fine carbide precipitation. For this reason, it is preferable that the time from the end of hot rolling to the start of forced cooling be within 3 seconds. More preferably, it is within 2 seconds.

Average cooling rate from the start of cooling to cooling stop: 30 ° C / second or more If the average cooling rate from the start of cooling to cooling stop is less than 30 ° C / second, the time to maintain at high temperature is long, and the coarseness of carbide due to strain-induced precipitation It becomes easy to progress. For this reason, it is preferable that the forced cooling after hot rolling described above is rapidly cooled to a predetermined temperature at an average cooling rate of 30 ° C./second or more. More preferably, it is 50 ° C./second or more.
The cooling stop temperature is set so that the coiling temperature falls within the target temperature range in consideration of the temperature drop of the steel sheet between the cooling stop and winding. That is, after the cooling is stopped, the temperature of the steel sheet is lowered by air cooling. Therefore, the cooling stop temperature is usually set to a temperature of about 5 to 10 ° C.

Winding temperature: 500 ~ 700 ℃
When the coiling temperature is less than 500 ° C., carbides precipitated in the steel sheet are insufficient, and it becomes difficult to ensure a desired steel sheet strength. On the other hand, when the coiling temperature exceeds 700 ° C., the precipitated carbide is coarsened, so that it is difficult to secure a desired steel sheet strength. For this reason, it is preferable to make winding temperature into the range of 500-700 degreeC. In addition, More preferably, it is the range of 550-680 degreeC.

  Moreover, the obtained hot-rolled steel sheet can be plated by a known method to form a plated layer on the surface. As the plating layer, a hot dip galvanized layer, an alloyed hot dip galvanized layer, an electroplated layer and the like are preferable.

Next, the mechanical properties of the steel sheet suitable for use in the warm press forming method of the present invention obtained by the above manufacturing method will be described.
Here, the mechanical properties of the preferred steel sheet are as follows.
(a) Tensile strength at room temperature: 780 MPa or more, and yield ratio at room temperature: 0.85 or more
(b) Yield stress YS ₂ in the warm forming temperature range of 400 to 700 ° C .: 80% or less of the yield stress YS ₁ at room temperature
(c) Total elongation El ₂ in the warm forming temperature range of 400 to 700 ° C .: 1.1 times or more of total elongation El ₁ at room temperature Hereinafter, each of these characteristics will be described.

Tensile strength at room temperature: 780 MPa or more, and yield ratio at room temperature: 0.85 or more The warm press forming method of the present invention is intended for steel sheets having a tensile strength at room temperature of 440 MPa or more. Accordingly, it is possible to obtain a steel sheet having a TS ₁ of 780 MPa or more and a yield ratio at room temperature of 0.85 or more.
Here, TS ₁ means the tensile strength at room temperature, and room temperature means (22 ± 5) ° C.

Warm forming temperature range in which 400 to 700 ° C. Yield stress at YS _2: yield stress YS ₂ at 400 to 700 ° C. is 80% or less warm molding temperature range of the yield stress YS ₁ at room temperature, the yield at room temperature If the stress YS ₁ exceeds 80%, the deformation resistance of the steel sheet during warm forming is not sufficiently reduced, so that it is necessary to increase the load load (press load) during warm forming, and the die life is shortened. In addition, in order to apply a large load (press load), the processing machine (press machine) body inevitably becomes large. When the processing machine (pressing machine) body becomes large, it takes a long time to transport the steel plate heated to the warm forming temperature to the processing machine, which causes a decrease in the temperature of the blank and warm forming at the desired temperature. Becomes difficult. Furthermore, since the shape freezing property is not sufficiently improved, the effect of using warm forming is reduced.
Therefore, the yield stress YS ₂ in the warm forming temperature range of 400 to 700 ° C. is preferably 80% or less of the yield stress YS ₁ at room temperature. More preferably, it is 70% or less.

Total elongation at 400 to 700 ° C. is a warm molding temperature region El _2: total elongation El ₂ at 400 to 700 ° C. is a warm molding temperature region above 1.1 times the total elongation El ₁ at room temperature, all at room temperature If it is the elongation El ₁ 1.1 times or more, since the workability during warm forming is sufficiently improved, without defects such as cracks occur, easily molded steel plate member having a complicated shape.
Accordingly, the total elongation El ₂ in the warm molding temperature range of 400 to 700 ° C. is preferably 1.1 times or more of the total elongation El ₁ at room temperature. More preferably, it is 1.2 times or more.

  Furthermore, in addition to the mechanical properties described above, a steel plate that exhibits the following mechanical properties after being formed into a press-formed product is more suitably used for the warm press-forming method of the present invention.

Yield stress YS ₃ and total elongation El ₃ of the press-formed product at room temperature are 80% or more of the yield stress YS ₁ and total elongation El ₁ of the steel plate before press forming, respectively. Yield stress YS ₃ of the press-formed product at room temperature and If the total elongation El ₃ is less than 80% of the yield stress YS ₁ at room temperature and the total elongation El ₁ of the steel sheet before press forming, respectively, the strength and total elongation of the member after warm forming are insufficient. If such a steel plate is used to form an automobile member having a desired shape by warm press forming, the impact absorbing performance at the time of automobile collision is insufficient, so the reliability as an automobile member is impaired.
Therefore, it is preferable that the yield stress YS ₃ and the total elongation El ₃ of the press-formed product at room temperature are 80% or more of the yield stress YS ₁ and the total elongation El ₁ of the steel plate before press forming, respectively. More preferably, it is 90% or more.

Example 1
Thickness: 1.6mm, tensile strength: 440MPa ~ 1180MPa class steel plate is heated under the conditions shown in Table 1, and then formed into a center pillar that is one of the automotive framework parts shown in Fig. 5 (a). Molded into an upper press panel.

Here, an electric furnace was used for heating the steel sheet. The in-furnace time was set to 300 seconds, and the entire blank was heated so as to have a uniform temperature distribution. The heated blank was taken out from the furnace, and after 10 seconds of conveyance time, it was fed into the press machine, and the holding time at the bottom dead center of molding was changed as shown in Table 1 to perform molding.
Immediately thereafter, the temperature difference between the flange portion of the molded panel and the other portions was measured. That is, with a non-contact type thermometer, the panel flange portion (location indicated by point X in FIG. 5A) has 6 points, and the other portion (location indicated by Y point in FIG. 5A) has 5 points. The temperature at the point was measured, and the difference between the average temperature at the point X and the average temperature at the point Y was defined as the average temperature difference between the flange portion and the other portions.

Moreover, the press machine uses a servo press machine, and the press speed during press molding is 15 spm (Strokes per minute: the number that can be processed in 1 minute. However, if holding at the bottom dead center of molding, The holding time is further added.).
After the molded panel is air-cooled for a sufficient period of time, the cross-sectional shape of the center pillar upper press panel shown in FIG. 5B is air-cooled with respect to the reference panel shape (the shape when removed from the mold immediately after press molding). The shape change amount “a” at the end of the rear panel was measured with a laser displacement meter. These measurement results are also shown in Table 1.

  As shown in Table 1, all of Invention Examples Nos. 1, 2, and 5 to 9 having a holding time at the bottom dead center of molding of 1 second or more are formed between the flange portion of the press-formed product and the other portions. The average temperature difference was within 150 ° C., and the shape change amount a was within 1.0 mm, and good dimensional accuracy was obtained.

  On the other hand, in Comparative Examples No. 10 to 16, where the holding time at the molding bottom dead center was less than 1 second, the average temperature difference between the flange portion of the press-molded product and the other portions exceeded 150 ° C. Thus, the shape change amount a was 1.2 to 2.6 mm, and sufficient dimensional accuracy could not be obtained.

  From the above results, according to the warm press forming method of the present invention, it is possible to suppress the average temperature difference between the flange portion and the other portions, thereby changing the shape of the panel immediately after press forming and after air cooling. It is clear that a press-molded product with a reduced amount and greatly improved dimensional accuracy can be obtained.

(Example 2)
Molten steel having the composition shown in Table 2 was melted in a converter and cast by a continuous casting method to obtain a slab (steel material). These slabs (steel materials) are heated to the heating temperature shown in Table 3, held soaked, roughly rolled, then finish-rolled under the hot rolling conditions shown in Table 3, cooled, and wound in a coil shape. To obtain a hot-rolled steel sheet (thickness: 1.6 mm). The steel plates a, i, k, m were heated to 700 ° C. in a continuous hot dip galvanizing line and then immersed in a hot dip galvanizing bath at a liquid temperature of 460 ° C. to form a hot dip galvanized layer on the surface. The plating layer was alloyed at 530 ° C. to form an alloyed hot dip galvanized layer. In addition, the plating adhesion amount was 45 g / m ² .

Next, test pieces were collected from the obtained hot-rolled steel sheet and subjected to structure observation, precipitate observation, and tensile test. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation is collected from the obtained hot-rolled steel sheet, a cross section (L cross section) parallel to the rolling direction is polished, corroded (corrosive liquid: 5% nital liquid), and scanned. Using an electron microscope (magnification: 400 times), the central part of the plate thickness was observed, and 10 fields of view were imaged. The obtained structure photograph was subjected to image analysis, and the identification of the structure, the structure fraction of each phase, and the average crystal grain size of each phase were measured.

  That is, using the obtained structure photograph, first, the ferrite phase and the other phases were separated, the area of the ferrite phase was measured, the area ratio with respect to the entire observation field was obtained, and the area ratio of the ferrite phase was determined . The ferrite phase was observed as a smooth curve with no grain marks in the grains, but the grain boundaries observed as a linear form were counted as a part of the ferrite phase. Further, the average crystal grain size of ferrite was obtained by a cutting method based on ASTM E 112-10 using the obtained structure photograph.

(2) Precipitate observation In addition, a specimen for transmission electron microscope observation was collected from the central portion of the thickness of the obtained hot-rolled steel sheet, and a thin film for observation was obtained by mechanical polishing and chemical polishing. About the obtained thin film, the deposit (carbide) was observed using the transmission electron microscope (magnification: 120,000 times). With respect to 100 or more carbides, the particle diameter was measured, and the arithmetic average value thereof was defined as the average particle diameter of the carbide in each steel plate. In the measurement, coarse cementite and nitride larger than 1 μm were excluded.

(3) Tensile test From the obtained hot-rolled steel sheet, a JIS 13B tensile test piece was sampled according to JIS Z 2201 (1998) so that the direction perpendicular to the rolling direction was the tensile direction. Using these collected specimens, a tensile test was performed according to JIS G 0567 (1998), and mechanical properties at room temperature (22 ± 5 ° C) (yield stress YS ₁ , tensile strength TS ₁ , total elongation El ₁ ) and mechanical properties at high temperatures (yield stress YS ₂ , tensile strength TS ₂ , total elongation El ₂ ) at each temperature shown in Table 4 were measured. All tensile tests were performed at a crosshead speed of 10 mm / min. Also, in the test to measure the mechanical characteristics at high temperature, the test piece is heated using an electric furnace, and after the test piece temperature is stably obtained within ± 3 ° C of the test temperature, hold for 15 min. Then, a tensile test was performed.
Tables 3 and 4 show the test results of (1) to (3).

  Next, after heating the steel plate obtained as described above under the conditions shown in Table 5, the center pillar upper which is one of the automobile frame parts shown in FIG. Molded into a press panel. The heating conditions and draw molding conditions other than those shown in Table 5 are the same as in the case of Example 1.

And after the air cooling with respect to the panel shape (the shape at the time of removing from the mold immediately after the press molding) and the temperature difference between the flange portion of the panel immediately after the molding and other portions under the same conditions as in Example 1. The shape change amount “a” of the panel end was measured.
In addition, JIS 13 B tensile test specimens were collected from the molded panels, and these tensile test specimens were subjected to a tensile test at room temperature under the same conditions as described above to obtain mechanical properties (yield stress (YS ₃ ), Tensile strength (TS ₃ ), total elongation (El ₃ )).
The obtained results are also shown in Table 5.

As shown in Table 5, in Nos. 17 to 42, which are invention examples, the average temperature difference between the flange portion and the other portions is within 150 ° C., and the shape change amount a is within 1.0 mm. Accuracy was obtained.
In particular, Invention Examples Nos. 17-22, 29-36, 40, 41 using steel sheets with suitable composition and structure are all used after forming, despite using high-strength steel sheets of 780 MPa or more. Good dimensional accuracy is obtained in press-formed products, and the mechanical properties are also extremely good, such as the tensile strength TS ₃ of the press-formed product is 99 to 104% of the tensile strength TS ₁ of the steel plate before press forming. Met.

1 Die 2 Punch 3 Wrinkle presser 4 Heated steel plate (blank)
5 Press-molded products (panels)
6 Flange 7 Side wall 8 Reference panel (Panel when removed from the mold immediately after press molding)
9 Panel after air cooling 10 Panel at bottom dead center 11 Center pillar upper press panel

Claims (12)

When forming a steel sheet with a tensile strength of 440 MPa or more into a press-molded product consisting of a flange part and other parts by press molding,
The steel sheet is heated to a temperature range of 400 to 700 ° C.
Next, the heated steel sheet is subjected to press forming by draw forming, and the state is maintained at 1 to 5 seconds at the bottom dead center.
A warm press molding method.   The warm press molding method according to claim 1, wherein an average temperature difference between the flange portion of the press-molded product and the other portion immediately after the draw molding is set to 150 ° C or less.   The warm press forming method according to claim 1 or 2, wherein a tensile strength of the press-formed product is 80% to 110% of a tensile strength of the steel plate. The steel sheet is in mass%,
C: 0.015-0.16%,
Si: 0.2% or less,
Mn: 1.8% or less,
P: 0.035% or less,
S: 0.01% or less,
Al: 0.1% or less,
N: 0.01% or less and
Ti: 0.13-0.25%
In a range satisfying the relationship of the following formula (1), the balance has a component composition consisting of Fe and inevitable impurities,
A structure in which the proportion of the ferrite phase in the entire structure is 95% or more in area ratio, the average crystal grain size of ferrite is 1 μm or more, and carbides having an average grain size of 10 nm or less are dispersed and precipitated in the ferrite crystal grains. Having
The warm press molding method according to any one of claims 1 to 3.
Record
2.00 ≧ ([% C] / 12) / ([% Ti] / 48) ≧ 1.05… (1)
Here, [% M] is the content of M element (mass%) The steel sheet is further mass%,
V: 1.0% or less,
Mo: 0.5% or less,
W: 1.0% or less,
Nb: 0.1% or less,
Zr: 0.1% or less and
The warm press forming method according to claim 4, wherein the hot press forming method contains one or more selected from Hf: 0.1% or less and satisfies the relationship of the following formula (1) ′.
Record
2.00 ≧ ([% C] / 12) / ([% Ti] / 48 + [% V] / 51 + [% W] / 184 + [% Mo] / 96 + [% Nb] / 93 + [% Zr] / 91 + [% Hf] / 179) ≧ 1.05… (1) '
Here, [% M] is the content of M element (mass%)   The warm press forming method according to claim 4 or 5, wherein the steel sheet further contains B: 0.003% or less in terms of mass%.   The steel sheet further contains one or more kinds selected from Mg: 0.2% or less, Ca: 0.2% or less, Y: 0.2% or less, and REM: 0.2% or less in mass%. Item 7. The warm press molding method according to any one of Items 4 to 6.   The steel sheet according to any one of claims 4 to 7, further comprising one or more selected from Sb: 0.1% or less, Cu: 0.5% or less, and Sn: 0.1% or less in terms of mass%. The warm press molding method according to claim 1.   The warm press according to any one of claims 4 to 8, wherein the steel sheet further contains, by mass%, one or two selected from Ni: 0.5% or less and Cr: 0.5% or less. Molding method.   The steel sheet is further in mass%, O, Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir. The temperature according to any one of claims 4 to 9, comprising a total of 2.0% or less of one or more selected from Ru, Os, Tc, Re, Ta, Be and Sr. Inter-press forming method.   The warm press forming method according to any one of claims 1 to 10, wherein the steel sheet has a plating layer on a surface thereof.   An automobile skeleton part manufactured by the warm press molding method according to claim 1.

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