JP4427110B2 - Press forming method of thin steel sheet for processing - Google Patents

Description

【００３９】
【表２】

【図面の簡単な説明】
【図１】鉄炭化物、鉄窒化物、鉄炭窒化物の１種または２種以上の合計の割合が１０個／μｍ² 以上含まれる結晶粒、ベイナイト、マルテンサイトの合計の体積分率と鋼板強度の低下比（ΔＴＳ／ＴＳ）の関係を説明する概念図である。
【図２】鋼板強度、Ｋ値、Ｙ値、熱処理により可能な強度の低下量の関係を説明する概念図である。
【図３】Ｋ値、Ｙ値の比（Ｋ／Ｙ）と４００℃×３０sec の熱処理による鋼板の強度低下比（ΔＴＳ／ＴＳ）を示す図である。
【図４】部分軟化鋼板の軟質部硬質部のパターン例を示す図である。
【図５】円盤状鋼板を局部加熱する装置と局部加熱された円盤を示す図である。
【符号の説明】
１…部分軟化鋼板
２…ブランク（切り板）
３…プレス部品
４…側面から見た４５０℃に加熱したダイス
５…側面から見た円盤状の鋼板
６…上から見た円盤の未加熱部
７…上から見た円盤の加熱部
Ａ…軟質部
Ｂ…硬質部[0001]
BACKGROUND OF THE INVENTION
The present invention is a thin steel plate applied to a place where structural strength is required, such as a structural part such as a body of an automobile, and has a formability improved by partial short-time heat treatment. The present invention relates to a press forming method for a thin steel sheet.
[0002]
[Prior art]
In order to reduce the weight and the strength of structural parts, it is usually attempted to form a high-strength steel sheet. However, high-strength steel sheets have high yield stress and poor ductility, so that press formability is difficult, and studies are being made to improve formability. For example, there are a bake-hardened steel sheet that is soft before pressing and hardens in a baking process during subsequent electrodeposition coating, and a method of curing by irradiation with a beam of high energy density after completion of forming.
[0003]
The bake hardened steel sheet increases the yield stress after bake, but the deformation strength itself increases only at most by about 5%, and the absolute amount of strength is insufficient.
Further, as a method of irradiating and curing a high energy density beam after completion of molding, for example, a method of irradiating a laser after molding as disclosed in JP-A-61-99629 is disclosed. However, in this case, since the irradiation is performed three-dimensionally, the processing equipment becomes economically expensive, and the accuracy error due to thermal distortion due to the transformation of steel becomes very large.
[0004]
In view of this, a method has been devised in which only a portion requiring strength in the state of a steel plate is reinforced and softened where formability is required. For example, Japanese Patent Application Laid-Open No. 60-238424 discloses that a steel sheet is partially irradiated with a laser to form a hardened structure, both hard and soft parts are present, forming is performed in the soft part, and the hard part has strength. A method is disclosed. However, since the transformation of hardened steel is used, deformation of the steel plate due to transformation strain is inevitable. In addition, the hardness difference between the transformation hard part and the soft part is remarkably large, and there is an excessive difference in deformability. Therefore, breakage often occurs from the boundary, and it was not always possible to improve the moldability. Japanese Patent Application Laid-Open No. 9-87737 discloses a method in which a high-tensile steel sheet is partially irradiated with an arc or laser and melted to soften the steel sheet. However, since this method also transforms steel, the influence of transformation strain cannot be avoided. As a method not utilizing the transformation of steel, for example, Japanese Patent Laid-Open No. 9-143554 is disclosed. In this method, plastic strain is applied to the steel sheet, and then partially heat-treated at 800 ° C. to recover or recrystallize and soften. However, since this method is also heated to 800 ° C. or higher, there is a problem of thermal strain of the steel sheet, and since plastic strain is used, there is a defect that the ductility of the hard part is remarkably deteriorated. It is not a solution to the problem.
[0005]
In addition, as in tailored blanks (for example, Application of Laser-Beam-Welded Sheet Metal, SAE Technical Paper Series, 890853, 1989), instead of making a soft part and a hard part separately on a single plate, means such as laser welding Then, there is a method of joining and finishing to a single plate. However, even in this method, it is inevitable that the joint is hardened by welding, which becomes an obstacle in molding. Moreover, since it joins by welding, it is difficult to join materials finely.
[0006]
Thus, an optimal method has not yet been obtained in manufacturing a high-strength structural component. Therefore, there has been a strong demand for a press forming method for a high strength steel plate that can be easily formed by press forming or the like like a mild steel plate.
[0007]
[Problems to be solved by the invention]
In order to solve the problems as described above, the present inventors have conducted intensive studies such as various forming materials made of thin steel sheets, heat treatment for improving formability, and optimum forming methods.
[0008]
[Means for Solving the Problems]
The present inventors have found that, if limited to short-time heat treatment, for example, a thin steel plate of 0.1 to 7.0 mm suppresses heat conduction as much as possible, and finds that local heat treatment is possible, and that a specific structure and component are selected. It was found that the steel material can be remarkably changed within the range of the short-time heat treatment. Furthermore, it has been newly discovered that when local heat treatment is applied to a portion that requires deformation during molding, the moldability is surprisingly improved. The present invention has been achieved by pursuing an optimum material for short-time heat treatment for the purpose of improving the moldability. The gist is
(1) C: 0.0005% to 0.25% by mass ratio
Si: 0.01% to 3.0%
Mn: 0.01% to 3.0%
P: 0.002% to 0.20%
S: 0.001% to 0.03%
N: 0.0002% to 0.02%
And one or more of Mo, Al, Ni, Cu, Nb, Ti, V, Cr, B in mass%, K / Y ≧ 0.77,
K = {TS (MPa)-(280 + 390 * C + 98 * Si + 65 * Mn + 882 * P + 207 * Al + 980 * Ti + 2000 * Nb + 980 * V + 200 * Mo + 38 * Ni + 55 * Cu + 22 * Cr)}
Y = {30 * √Si + 40 * √Mn + 80 * √Mo + 4 * √Ni + 35 * √Nb + 40 * √Ti + 55 * √V + 40 * √Cr + 70 * √P + 500 * √B},
In the range of
The balance is composed of Fe and inevitable impurities , and the crystal grain contains a ratio of one or more of iron carbide, iron nitride, and iron carbonitride in a microstructure of 10 pieces / μm ² or more. A steel sheet in which the total proportion of one or more of bainite and martensite is 7% or more in space factor is subjected to a heat treatment within 30 sec at a temperature below the Ac1 transformation point before forming, A press forming method of a thin steel sheet for processing excellent in formability-improving heat treatment capability, wherein a soft part having a strength reduced by 5% or more is formed and then formed.
(2) The thin steel plate for processing according to (1), wherein the heat treatment is performed locally, and the soft portion is two-dimensionally patterned in a unit of 1 mm ² or more in the steel plate surface. Press forming method ,
It is.
[0009]
Here, the formability-enhancing heat treatment ability refers to the ability to change the tensile strength of a steel sheet by a short time heat treatment within 30 seconds at a heat treatment temperature below the Acl transformation point of the steel sheet. If limited to this heat treatment, thermal strain can be suppressed as much as possible with a thin steel plate of 0.1 to 7.0 mm, and transformation strain hardly occurs. Furthermore, since heat conduction is suppressed, heat treatment can be performed with a resolution of about 1 mm. Further, as the amount of change in tensile strength due to this heat treatment, it is desirable that the tensile strength change by 5% or more. When heat treatment is locally performed using this steel plate, the formability is remarkably improved without substantially changing the strength of the whole steel plate. For example, when a heat treatment is locally applied to a portion that needs to be deformed, the strength of the portion that needs to be deformed is lowered, and deformation easily occurs, thereby improving the moldability.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors conducted extensive research on the steel sheet structure and steel plate composition, whose strength changes greatly by local heat treatment within 30 seconds at temperatures below the Acl transformation point, which improves the formability of high-strength steel sheets. If the size of the iron nitride and iron carbonitride is controlled, the iron carbide, iron nitride, and iron carbonitride are immediately dissolved or coarsened by heat treatment within 30 seconds at a temperature below the Acl transformation point, and the steel plate reduced in strength, press formability of the steel sheet was found that you improved by implementing prior to shaping it.
[0011]
We also found that martensite has a similar effect. Furthermore, the amount of change in strength of the steel sheet is affected by the ratio of iron carbide, iron nitride, and iron carbonitride containing fine structure, bainite structure, and martensite structure, with a space factor of 7% or more. At one point, I found that the effect was great.
In addition, the amount of strength reduction of the steel sheet depends greatly on the chemical composition of the steel sheet,
K / Y ≧ 0.77,
K = {TS (MPa)-(280 + 390 * C + 98 * Si + 65 * Mn + 882 * P + 207 * Al + 980 * Ti + 2000 * Nb + 980 * V + 200 * Mo + 38 * Ni + 55 * Cu + 22 * Cr)}
Y = {30 * √Si + 40 * √Mn + 80 * √Mo + 4 * √Ni + 35 * √Nb + 40 * √Ti + 55 * √V + 40 * √Cr + 70 * √P + 500 * √B},
It was found that the amount of decrease in the strength of the steel sheet can be increased by limiting to the above range.
[0012]
Moreover, earnestly researched about the manufacturing method of this steel plate, and found the optimal manufacturing method.
The present invention is described in detail below.
First, the reason for limiting the steel structure will be described below.
As a heat treatment for improving the formability, a local heat treatment is performed within 30 seconds at a temperature below the Acl transformation point to partially soften the high-strength steel sheet. For this reason, since it is necessary to change the strength at a low temperature in a short time, it is most suitable to use the diffusion phenomenon of C and N, which has a high moving speed, in the steel, iron carbide, iron nitride, iron carbonitride and It is most effective to use bainite and martensite.
[0013]
If there are crystal grains containing a total ratio of one or more of iron carbide, iron nitride, and iron carbonitride of 10 / μm ² or more, the steel can be strengthened by precipitation strength. On the other hand, this iron carbide, iron nitride or iron carbonitride can be easily reduced in strength by melting and coarsening by a short heat treatment.
Bainite or martensite can increase the strength of steel by transformation strengthening. On the other hand, bainite or martensite can be easily tempered by short-time heat treatment to precipitate coarse carbides and reduce the strength. In order to locally reduce the strength of the steel sheet, it is necessary to contain a structure whose strength changes by short-time heat treatment at a certain rate or more in terms of space factor. That is, the crystal grain, bainite, martensite structure in which the total ratio of one or more of iron carbide, iron nitride, and iron carbonitride is 10 / μm ² or more is constant in the ratio of the whole steel. There must be more than a proportion. Iron carbide or iron nitride or Tetsusumi nitride 10 / [mu] m ² or more Included grain, bainite, martensite structure, other structure of the steel (e.g., ferrite, pearlite, etc. containing no fine iron nitride ), The contribution to the strength of the steel sheet is large, and if the total volume ratio is approximately 7% or more, the strength of the steel sheet can be changed by 5% or more when heat-treated for a short time. Iron carbide, iron nitride, one or the total proportion of the two or more ten Tetsusumi nitride / [mu] m ² or more Included grain, bainite, change in the intensity to the intensity and the rapid thermal annealing of the martensitic structure The amount of softening varies depending on the amount of carbon and nitrogen contained in these structures. When the amount of carbon and nitrogen is large, the strength of the steel sheet is 5% or more even when the volume ratio of these structures is 7% or less. It can be changed. However, in the range of carbon (wt% C ≦ 0.25) and nitrogen (wt% N ≦ 0.02) normally used as a thin steel plate, one of iron carbide, iron nitride, iron carbonitride or When the crystal grains, bainite, and martensite structures containing ^two or more total ratios of 10 pieces / μm ² or more are in a space factor of 7% or more in total, the strength of the steel sheet can be changed by 5% or more. 7% is the lower limit.
[0014]
In addition, as the volume ratio of crystal grains, bainite, and martensite structure including one or more of iron carbide, iron nitride, and iron carbonitride is 10 / μm ² or more increases, Since it becomes easy to change the strength of the steel sheet, an upper limit is not specified, but the processing method deteriorates, so it is desirable to adjust the volume ratio according to the processed part.
[0015]
The term “iron carbide” as used herein refers to iron-carbon compounds such as cementite, ε carbide, χ carbide, iron-carbon complex, and iron nitride refers to iron-nitrogen compounds such as Fe ₄ N, Fe ₁₆ N ₂ , iron-nitrogen complex, iron Carbonitride refers to a form in which iron and carbon compounds or iron nitrides are mixed, one in which some C in the iron and carbon compounds are replaced with N, one in which some N in iron nitrides are replaced with C, etc. .
[0016]
Iron carbide, iron nitride 1, one or the total proportion of the two or more are 10 / [mu] m ² or more Included grain, bainite, the total space factor of martensite structure steel sheet Tetsusumi nitride The relationship of strength reduction ratio is shown. The proportion of crystal grains containing 10 or more μm ² of the total of one or more of iron carbide, iron nitride, and iron carbonitride is iron carbide or iron nitride for each crystal grain in the microscope field of view. The space factor was calculated by counting the number of products or iron carbonitrides and measuring the number of crystal grains contained in 10 / μm ² or more and the average crystal grain size. The space factor of bainite and martensite was also calculated by measuring the number and average size in the microscope field. For the strength measurement, a JIS No. 5 tensile test piece was prepared, heat-treated at 400 ° C. for 30 seconds, cooled to room temperature, and then subjected to a tensile test at room temperature. A value obtained by dividing the amount of decrease in tensile test strength (ΔTS) at this time by TS before heat treatment was shown as the strength decrease ratio. The total space factor of the crystal grains, bainite and martensite structure including one or more of iron carbide, iron nitride and iron carbonitride is 10% / μm ² or more is 7% or more. It can be seen that the reduction ratio of the steel sheet strength is remarkable.
[0017]
Next, the reason for limiting the components of steel will be described.
C is for obtaining crystal grains, bainite, and martensite structure in which the total proportion of one or more of iron carbide, iron nitride, and iron carbonitride according to the present invention is 10 / μm ² or more. It is an essential element. When the content is increased, the structure becomes easy to obtain, but the weldability deteriorates. Therefore, it is 0.25% or less. Further, if it is less than 0.0005%, crystal grains, bainite, and martensite structures containing a total ratio of one or more of iron carbide, iron nitride, and iron carbonitride of 10 pieces / μm ² or more are obtained. Therefore, the alloy cost is increased, the manufacturing cost is drastically increased, and it is not economical, so 0.0005% is made the lower limit.
[0018]
If Si is less than 0.010%, the effect of increasing the strength during heat treatment is small, so 0.010% is made the lower limit. Preferably, it is 0.200% or more. If it exceeds 3.00%, workability deteriorates, so 3.00% is made the upper limit.
Mn is used for securing the strength. However, if it is less than 0.01%, the manufacturing cost is drastically increased and it is not economical. Deteriorates, so 3.00% is made the upper limit.
[0019]
If P is less than 0.002%, the effect of increasing the strength during heat treatment is small, so 0.002% is set as the lower limit. Preferably, it is 0.02% or more. If it exceeds 0.20%, the toughness is remarkably deteriorated and brittle, so 0.20% is made the upper limit.
If S is less than 0.001%, the production cost is drastically increased and it is not economical, so 0.001% is the lower limit, and if it exceeds 0.03%, red hot brittleness occurs during hot rolling, and the surface cracks. In order to cause so-called hot brittleness, the upper limit is made 0.03%.
[0020]
If N is less than 0.0002%, the manufacturing cost will increase dramatically and it will not be economical, so 0.0002% is the lower limit, and if it exceeds 0.02%, the workability deteriorates, so 0.02% Is the upper limit.
In the present invention, when C, Si, Mn, Mo, Ni, Al, Cu, Nb, Ti, V, Cr, P, and B are contained in the range of the following formula, the effect is remarkable.
K / Y ≧ 0.77,
K = {TS (MPa)-(280 + 390 * C + 98 * Si + 65 * Mn + 882 * P + 207 * Al + 980 * Ti + 2000 * Nb + 980 * V + 200 * Mo + 38 * Ni + 55 * Cu + 22 * Cr)}
Y = {30 * √Si + 40 * √Mn + 80 * √Mo + 4 * √Ni + 35 * √Nb + 40 * √Ti + 55 * √V + 40 * √Cr + 70 * √P + 500 * √B},
TS (MPa) is the tensile strength (MPa) of steel.
[0021]
In using the steel sheet of the present invention, as a heat treatment for improving the formability, a heat treatment is performed at a temperature of Acl or lower and within 30 seconds.
The K value in the above equation is mainly defined because heat treatment is performed at a temperature below Acl, and the Y value in the above equation is mainly defined because heat treatment is performed within 30 seconds. is there.
[0022]
First, the K value in the above equation will be described. C, Si, Mn, P, Al, Ti, Nb, V, Mo, Ni, Cu, and Cr are elements that strengthen steel. These elements act directly or indirectly by various hardening mechanisms such as solid solution strengthening, precipitation strengthening, transformation strengthening, and strengthen steel. In the present invention, the strength of the steel is softened by heat treatment at a temperature not higher than the Acl transformation point, and the strength is lowered. Although the amount of softening increases as the heat treatment time increases, it has been found that there is a strength that the steel does not soften no matter how long the heat treatment is. It can be seen that the strength that does not soften greatly depends on the steel composition and can be expressed in units of MPa (280 + 390 * C + 98 * Si + 65 * Mn + 882 * P + 207 * Al + 980 * Ti + 2000 * Nb + 980 * V + 200 * Mo + 38 * Ni + 55 * Cu + 22 * Cr). It was. The strength not to be softened varies depending on the component, and Nb is most effective per unit mass , and then becomes Ti and P in this order. The contribution of this effect is a coefficient given before the element of the first term of the formula shown in claim 2. That is, the coefficient is 390 for C, 98 for Si, 65 for Mn, 882 for P, 882 for Al, 980 for Ti, 980 for Ti, 2000 for Nb, 980 for V, 200 for Mo. It was found that Ni was 38, Cu was 55, and Cr was 22.
[0023]
Accordingly, the strength of the steel whose strength can be changed by heat treatment at a temperature below the Acl transformation point is {TS (MPa) − (280 + 390 * C + 98 * Si + 65 * Mn + 882 * P + 207 * Al + 980 * Ti + 2000 * Nb + 980 * V + 200 * Mo + 38 * Ni + 55 * Cu + 22 * Cr)}
This value is defined as K.
[0024]
Next, the Y value of the above equation will be described. Si, Mn, Mo, Ni, Nb, Ti, V, Cr, P, and B are elements that affect the softening rate of steel. Dissolution, coarsening, and bainite of iron carbide, iron nitride, and iron carbonitride It is an element that delays softening of the structure and martensite structure. Therefore, if these elements are contained in a certain amount or more, the steel cannot be softened by short-time heat treatment. This effect largely depends on the composition and content of steel,
{30 * √Si + 40 * √Mn + 80 * √Mo + 4 * √Ni + 35 * √Nb + 40 * √Ti + 55 * √V + 40 * √Cr + 70 * √P + 500 * √B}
It turns out that it is proportional to. This value is defined as Y.
[0025]
The effect cost varies depending on the component, and B is the most effective per unit mass , followed by Mo and P. The contribution of this effect is a coefficient given before the element of the second term of the formula shown in claim 2. That is, this coefficient is 30 for Si, 40 for Mn, 80 for Mo, 4 for Ni, 35 for Nb, 40 for Ti, 55 for V, 40 for Cr, 70 for P. , B was found to be 500.
[0026]
In addition, it has been clarified that this effect is proportional to the square root of the concentration of the contained element. Although this mechanism is not clear, the interaction of these elements with C, N, dislocations, and vacancies delays the dissolution and coarsening of iron carbide, iron nitride, and iron carbonitride, and also includes bainite and martensite. The present inventors consider that the precipitation of carbides and the recovery of dislocations are suppressed, and the softening of bainite and martensite is delayed.
[0027]
Further, an important point of the present invention is that the steel structure is excellent in formability-improving heat treatment by limiting the structure of the steel and well balancing the strength of the steel that is not softened and the amount of the element that delays the softening of the steel. Is achieved. That is, when the ratio between the K value and the Y value is 0.77 or more, the steel strength was successfully changed by 5% by short-time heat treatment.
[0028]
If this is expressed in an expression,
K / Y ≧ 0.77,
K = {TS (MPa)-(280 + 390 * C + 98 * Si + 65 * Mn + 882 * P + 207 * Al + 980 * Ti + 2000 * Nb + 980 * V + 200 * Mo + 38 * Ni + 55 * Cu + 22 * Cr)}
Y = {30 * √Si + 40 * √Mn + 80 * √Mo + 4 * √Ni + 35 * √Nb + 40 * √Ti + 55 * √V + 40 * √Cr + 70 * √P + 500 * √B},
When it is within the range, a steel sheet excellent in heat treatment for improving formability can be obtained. FIG. 2 shows a conceptual diagram of the above description.
[0029]
In addition, FIG. 3 shows the strength reduction ratio of the steel sheet when heat treatment is performed for a short time using various concentrations of steel.
For the strength measurement, a JIS No. 5 tensile test piece was prepared, heat-treated at 400 ° C. for 30 seconds, cooled to room temperature, and then subjected to a tensile test at room temperature. A value obtained by dividing the amount of decrease in tensile test strength (ΔTS) at this time by TS before heat treatment was shown as the strength decrease ratio.
[0030]
FIG. 3 shows that when K / Y is less than 0.77, almost no effect is observed, and a strength decrease of 5% or more is obtained at 0.77% or more.
The processing steel sheet excellent in formability improvement heat treatment ability of the present invention is a hot-rolled steel sheet, cold-rolled steel sheet, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, electrogalvanized so long as it satisfies the above structure and composition. Any of steel plates can be used, and the effects of the invention can be enjoyed.
[0031]
In the case of a galvanized steel sheet, it is preferable to perform heat treatment for improving formability at a temperature of 550 ° C. or lower in order to prevent deterioration of the galvanized layer, but it is sufficiently possible to use the steel sheet of the present invention.
Further, the plate thickness is not limited, but is particularly effective at 0.4 to 6 mm.
In press-forming the steel sheet of the present invention, blanking is performed in accordance with the shape of the steel sheet, and a heat treatment for improving formability is locally performed at places where processing is severe with a press before forming. Although the heat treatment is partial, the formability of the entire steel sheet is remarkably improved, and a high-strength press-formed body having a complicated shape can be obtained.
[0032]
Further, in the present invention, the steel sheet is partially softened by performing a heat treatment for improving the formability in a pattern matched to the parts in advance, not before forming, and is two-dimensionally patterned in units of 1 mm ² or more in the steel sheet surface. Soft part and hard part can be made. This is the third invention with a portion softened steel sheet press-forming method of the present invention. At this time, in order to improve formability, it is preferable that the soft portion has a lower tensile strength of steel by 5% or more than the hard portion. If the necessary parts are softened according to the part shape in advance, it is not necessary to perform heat treatment to improve the formability separately after cutting the steel sheet into the part shape, so the part heat treatment manufacturing process of the part can be reduced and special Therefore, a high-strength press-molded body having a complicated shape can be obtained with ordinary equipment.
[0033]
FIG. 4 shows an example pattern. The thin steel plate for partial softening according to the present invention is not limited to this pattern example, and can be patterned according to the part shape.
[0034]
【Example】
The present invention will be specifically described below based on examples.
Steels having the components shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. And it heated to 1200 degreeC in the heating furnace, it hot-rolled with the finishing temperature of 880 degreeC, wound up at the temperature of 500 degreeC, and then pickled and made the hot rolled steel sheet.
[0035]
Further, after cold rolling at a rolling reduction of 60%, recrystallization annealing is performed at 830 ° C. for 60 seconds, and the product is gradually cooled to 700 ° C., and then cooled to a temperature of 100 ° C./sec and a temperature of 250 ° C. Thus, a 1.2 mm cold-rolled steel sheet was obtained. The steel structure obtained at this time is also shown in Table 1. Moreover, a part was electrogalvanized and the zinc layer was provided to the surface layer of the steel plate.
[0036]
The obtained cold-rolled steel sheet was processed into a JIS No. 5 tensile test piece, and the mechanical property value (no heat treatment) was evaluated. Separately, a JIS No. 5 tensile test piece was prepared, subjected to heat treatment at 450 ° C. for 10 seconds, and evaluated for mechanical property values (with heat treatment).
Separately, the steel sheet was punched into a 90φ-120φ disk, and deep drawing was performed using a 50φ cylindrical punch maintained at 25 ° C. and a die having an inner diameter of 54φ. The limit drawing ratio capable of deep drawing was determined by changing the wrinkle pressure. Some discs are softened by sandwiching the periphery of the disc with a die with an inner diameter of 60φ heated to 450 ° C for the purpose of heat-treating the flange equivalent part for a short time, and the change in material strength locally as shown in Fig. 5 Created a disc. Thereafter, similarly to the disk without heat treatment, deep drawing was performed using a 50φ cylindrical punch maintained at 25 ° C. and a die having an inner diameter of 54φ.
[0037]
The above results are also shown in Table 1.
As is apparent from Table 1, when the steel sheet of the present invention is used, the material strength can be made soft by 5% or more by short-time heat treatment, and formability can be improved when heat treatment for improving formability is performed.
[0038]
[Table 1]

[0039]
[Table 2]

[Brief description of the drawings]
FIG. 1 shows the total volume fraction of steel grains, bainite, martensite, and steel sheets containing a total of 10 or more μm ² of one or more of iron carbide, iron nitride, and iron carbonitride. It is a conceptual diagram explaining the relationship of intensity | strength fall ratio ((DELTA) TS / TS).
FIG. 2 is a conceptual diagram illustrating the relationship between steel plate strength, K value, Y value, and the amount of strength reduction that can be achieved by heat treatment.
FIG. 3 is a graph showing a ratio of K value and Y value (K / Y) and a strength reduction ratio (ΔTS / TS) of a steel sheet by heat treatment at 400 ° C. × 30 sec.
FIG. 4 is a diagram showing a pattern example of a soft part hard part of a partially softened steel sheet.
FIG. 5 is a diagram showing an apparatus for locally heating a disk-shaped steel plate and a locally heated disk.
[Explanation of symbols]
1 ... Partially softened steel plate 2 ... Blank (cut plate)
3 ... Pressed part 4 ... Dies heated to 450.degree. C. seen from the side..Disk-like steel plate 6 seen from the side..Unheated part 7 of the disk seen from above..Heating part A of the disk seen from above .... soft Part B ... Hard part

Claims (2)

C: 0.0005% to 0.25% by mass ratio
Si: 0.01% to 3.0%
Mn: 0.01% to 3.0%
P: 0.002% to 0.20%
S: 0.001% to 0.03%
N: 0.0002% to 0.02%
And one or more of Mo, Al, Ni, Cu, Nb, Ti, V, Cr, B in mass%, K / Y ≧ 0.77,
K = {TS (MPa)-(280 + 390 * C + 98 * Si + 65 * Mn + 882 * P + 207 * Al + 980 * Ti + 2000 * Nb + 980 * V + 200 * Mo + 38 * Ni + 55 * Cu + 22 * Cr)}
Y = {30 * √Si + 40 * √Mn + 80 * √Mo + 4 * √Ni + 35 * √Nb + 40 * √Ti + 55 * √V + 40 * √Cr + 70 * √P + 500 * √B},
In the range of
The balance is composed of Fe and inevitable impurities , and the crystal grain contains a ratio of one or more of iron carbide, iron nitride, and iron carbonitride in a microstructure of 10 pieces / μm ² or more. A steel sheet in which the total proportion of one or more of bainite and martensite is 7% or more in space factor is subjected to a heat treatment within 30 sec at a temperature below the Ac1 transformation point before forming, A press forming method of a thin steel sheet for processing excellent in formability-improving heat treatment capability, wherein a soft part having a strength reduced by 5% or more is formed and then formed. ^2. The press forming of a thin steel sheet for processing according to claim 1, wherein the heat treatment is locally performed, and the soft portion is patterned in a unit of 1 mm ² or more two-dimensionally in a steel plate surface. Method.

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