JP2000178684A - Method for producing thin steel sheet and high-strength press-formed body excellent in heat treatment hardening ability - Google Patents

Abstract

(57) Abstract: The present invention provides a method for producing a steel sheet and a press-formed body capable of increasing the strength by 60 MPa or more in low-temperature heat treatment after press-forming. SOLUTION: C: 0.01 to 0.20%, Mn: 0.010 to 3.0
%, S: 0.001 to 0.020%, N: 0.0002 to 0.01%, Si:
0.01 to 3.0%, Al: 0.005 to 2.0%, P: 0.005 to 0.2
%, Or in addition, Mo: 0.01-2.0%,
Cr: 0.01 to 2.0%, Ti: 0.005 to 0.10%, Nb: 0.005 to 0.
10%, V: 0.005 to 0.10%, B: 0.0003 to 0.0050%
Or two or more species, and the average grain size of the steel is
After performing press forming in which the average particle diameter of the iron-carbon compound is 1 μm or less and the plastic equivalent strain is 2% or more, the temperature T (expressed by 17,000 <T (30 + lnt) <30,000 The present invention relates to a method for producing a thin steel sheet excellent in heat treatment hardening ability for performing low-temperature heat treatment maintained in the range of K) and time t seconds, and a high-strength pressed molded body.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a press or the like suitable for application to a place where structural strength, particularly strength and / or rigidity at the time of deformation is required, such as structural parts of an automobile. High-strength steel sheet with excellent heat-hardening ability (heat-strength heat-hardening ability after molding) used as a material for a molded body subjected to a tensile strength increase heat treatment in a predetermined temperature range after work forming by high temperature The present invention relates to a method for producing a strength press molded body. The heat treatment hardening ability of the present invention means an ability to increase both the tensile strength and the yield strength after molding.

[0002]

2. Description of the Related Art In manufacturing a press-formed body made of a thin steel sheet, there is a method of increasing the strength of parts by softening before press-forming and easy to press-forming, and hardening after press-forming.
There is a method of baking at 200 ° C. or lower. For example,
In an Nb-added steel as described in JP-A-55-141526 and JP-A-55-141555, Nb is added in accordance with the content of C, N, and AI in the steel, and Nb is added in at%. / (Solute C + Solute N)
Is controlled within a certain range, and the cooling rate after annealing is controlled to adjust the solute C and solute N in the steel sheet. Alternatively, as described in JP-B-61-45689, Ti and It is disclosed that the bake hardenability is improved by the complex addition of Nb.

However, since the above-mentioned steel sheet is made of a material having excellent deep drawability, the strength of the steel sheet is low and is not always sufficient as a structural material. Further, as disclosed in Japanese Patent Application Laid-Open No. 577-143464, a technique of increasing the solid solution C in steel by adding Si to improve the bake hardenability, or as described in Japanese Patent Application Laid-Open No. 5-25549. W on steel,
A technique for improving bake hardenability by adding Cr or Mo alone or in combination is disclosed. However, the increase in strength due to bake hardening is due to solid solution C contained in the steel sheet,
Since solid solution N is used, only the yield strength of the material is increased, but the tensile strength is not increased. For this reason, there is only an effect of increasing the starting stress of deformation of the part,
The effect of increasing the stress required for deformation (deformation strength characteristic) over the entire area during deformation from the start of deformation to the end of deformation of the component is not necessarily sufficient.

[0004] In addition, bake hardening type steel sheets using these solid solution C and solid solution N have a limited bake hardening ability because solid solution C and solid solution N are left as long as the aging property at normal temperature is not deteriorated. Was.
On the other hand, as a curing method other than the baking of the press-formed body, there is a method by soft nitriding after press-forming. For example, as described in JP-A-2-80539, a method in which a nitride-forming element such as Cr, Al, V, or the like is contained in steel so that the strength is increased by nitriding, or JP-A-3-122255. Discloses a method of increasing the hardness of a member by precipitating and hardening Cu using the heat of nitriding treatment. However, these methods have the drawback that the heating temperature is high and the heat treatment time is long, so that the accuracy of the parts after the heat treatment tends to be disordered. Japanese Patent Application Laid-Open No. 2-57634 discloses a technique of depositing Ti and V by heat treatment at 300 to 800 ° C. However, long-term heat treatment of at least 10 minutes is required in the low temperature range of 300-500 ° C to precipitate Ti and V. In the case of heat treatment at 300 ° C or less, the strength of steel (particularly tensile strength) ) Could not be sufficiently strengthened.

[0005] As described above, before forming, it is easy to perform forming such as press forming with relatively soft high-strength steel, and after forming such as press forming, heat treatment is performed at a low temperature for a short time to increase the strength. As a result, there has been a strong demand for a thin steel plate as a material capable of increasing the tensile strength or hardness and increasing the deformation strength of a member or component, or increasing the rigidity.

[0006]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present inventors have studied the workability in forming various molding materials and parts made of thin steel sheets, and the members and parts. We conducted intensive research on the heat treatment method of hardening by heat treatment, and the deformation strength characteristics of a press-formed body as a part made of the steel sheet.

[0007]

Means for Solving the Problems The present inventors have proposed a steel plate having an average crystal grain size of steel of 20 μm or less and an average particle size of iron carbon compound in the steel of 1 μm or less, exceeding 200 ° C. to 50 ° C.
The present inventors have newly found that a high strength increase (or an increase in hardness) can be obtained in a short time when heated to a temperature range of 0 ° C., thereby achieving the present invention.

The gist is as follows: (1) C: 0.01 to 0.20%, Mn: 0.010 to 3.0% by weight.
%, S: 0.001 to 0.020%, N: 0.0002 to 0.01%, Si:
0.01 to 3.0%, Al: 0.005 to 2.0%, P: 0.005 to 0.2
%, The balance being iron and unavoidable impurities, and the steel having an average crystal grain size of 20 μm or less and an iron carbon compound in the steel having an average particle size of 1 μm or less. Thin steel sheet with excellent heat treatment hardening ability after strength increase after forming. (2) By weight%, C: 0.01 to 0.20%, Mn: 0.010 to 3.0
%, S: 0.001 to 0.020%, N: 0.0002 to 0.01%, Si:
0.01 to 3.0%, Al: 0.005 to 2.0%, P: 0.005 to 0.2
%, And in terms of weight%, Si, Al, P is 0.2 ≦ S
i% + 1.4 Al% + 6.3 P% ≦ 3.0, the average grain size of the steel is 20 μm or less, and the average grain size of the iron-carbon compound in the steel is 1 μm or less. High strength after heat treatment.

Here, Si% + 1.4Al% + 6.
The expression represented by 3P% indicates the ability to increase the heat treatment strength after forming a thin steel sheet. (3) By weight%, C: 0.01 to 0.20%, Mn: 0.010 to 3.0
%, S: 0.001 to 0.020%, N: 0.0002 to 0.01%, Si:
0.01 to 3.0%, Al: 0.005 to 2.0%, P: 0.005 to 0.2
%, And by weight%, Mo: 0.01 to 2.0%, Cr:
0.01 to 2.0%, Ti: 0.005 to 0.10%, Nb: 0.005 to 0.10
%, V: 0.005 to 0.10%, B: 0.0003 to 0.0050%, 1
A steel having an average crystal grain size of not more than 20 μm and an iron carbon compound in the steel having an average particle size of not more than 1 μm. Excellent thin steel plate. (4) By weight%, C: 0.01 to 0.20%, Mn: 0.010 to 3.0
%, S: 0.001 to 0.020%, N: 0.0002 to 0.01%, Si:
0.01 to 3.0%, Al: 0.005 to 2.0%, P: 0.005 to 0.2
%, Si, Al, P in the range of 0.2 ≦ Si% + 1.4 Al% + 6.3 P% ≦ 3.0 by weight%, and further, by weight%, Mo: 0.01 to 2.0%, C
r: 0.01 to 2.0%, Ti: 0.005 to 0.10%, Nb: 0.005 to
0.10%, V: 0.005 to 0.10%, B: 0.0003 to 0.0050%,
Wherein the steel has an average crystal grain size of not more than 20 μm and an average particle size of the iron-carbon compound in the steel is not more than 1 μm. Thin steel sheet with excellent hardening ability. (5) The thin steel sheet according to any one of (1) to (4) is subjected to press forming in which plastic strain of 2% or more is applied to at least a portion where strength is required, and thereafter, 17000 <T (30 + 1n
t) A method for producing a high-strength press-formed body, characterized by performing a heat treatment for maintaining the temperature T (K) represented by <30,000 and a time period t seconds.

[0010] The post-forming strength increasing heat treatment refers to a temperature T (K) and a time t (t) expressed by 17000 <T (30 + lnt) <30,000 after a forming process in which a strain of 2% or more is applied as plastic equivalent strain. In the heat treatment held in the range of seconds, the tensile strength after the thermomechanical treatment is 60
This shows processing that can be improved by MPa or more (more preferably 90 Mp or more). Alternatively, a heat treatment capable of increasing the Vickers hardness (Hv) by 18 or more (more preferably 27 or more) after the thermomechanical treatment is shown. However, this heat treatment does not require the addition of a hardening inducing element from the outside to the molded body as in a nitriding treatment or the like.

In addition, the heat treatment hardening ability (ΔTS) for hardening at a tensile strength of 60 MPa or more refers to an increase in nominal stress as shown in FIG.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present researchers have proposed a method for imparting deformation strength characteristics to members and components while ensuring workability such as press-formability of members and components, by using a steel plate, a heat treatment method, a formability ( In particular, the present inventors have conducted intensive studies on press formability) and found that the steel contains an appropriate amount of C, Si, Al, and P, has an average crystal grain size of 20 μm or less, and has an average particle size of iron-carbon compound in steel. It has been found that if a steel sheet having a particle size of 1 μm or less is processed by a press forming method giving a strain of 2% or more and subjected to a heat treatment at a temperature of more than 200 ° C. to 500 ° C., the steel sheet is significantly hardened.
In addition, Mo, Cr, Ti, Nb, V, B
Was found to increase the amount of curing by the complex addition of.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the components of steel will be described. C is an element that affects the workability of steel, and when the content is large, the workability is deteriorated. Also, 0.
If it is less than 010%, the amount of carbide precipitated during heat treatment maintained within the range of temperature T (K) represented by 17000 <T (30 + lnt) <30,000 and time t seconds is small, and the effect of increasing strength during heat treatment , The lower limit is 0.010%.

If the Mn content is less than 0.010%, the production cost rises dramatically and becomes uneconomical, so the lower limit is 0.010%, and if it exceeds 3.00%, the workability deteriorates, so the upper limit is 3.00%. If S is less than 0.001%, the production cost increases dramatically and becomes uneconomical.
If it exceeds 020%, red hot embrittlement occurs at the time of hot rolling and cracks occur at the surface, that is, so-called hot embrittlement, so the upper limit is 0.020%.

N is preferably as small as possible in order to ensure workability, but if it is less than 0.0002%, the production cost will increase dramatically and it will not be economical.
%, The workability deteriorates, so the upper limit is 0.0100%. In order to exhibit the effect of increasing the tensile strength at the time of the predetermined low-temperature heat treatment after the working, an appropriate amount of Si, Al, or P is further contained in the steel as an essential element in addition to the above elements.

If the content of Si is less than 0.010%, the effect of increasing the strength (tensile strength) is small even if the heat treatment is performed after molding, so the lower limit is set to 0.010%. It is more preferably 0.200% or more for increasing the strength. If it exceeds 3.00%, the workability will deteriorate after heat treatment after molding, so the upper limit is 3.00%. If the content of Al is less than 0.005%, the effect of increasing the strength (tensile strength) is small even if the heat treatment is performed after molding, so the lower limit is 0.005%. It is more preferably 0.10% or more due to an increase in strength. If it exceeds 2.00%, the workability will deteriorate, so the upper limit is 2.00%.

If P is less than 0.005%, the effect of increasing the strength (tensile strength) is small even if the heat treatment is performed after molding, so the lower limit is 0.005%. The increase in strength is more preferably 0.2% or more. If it exceeds 0.2%, the toughness deteriorates remarkably and the material becomes brittle, so the upper limit is 0.2%. As described above, a steel sheet containing a predetermined amount of Si, Al, and P is subjected to working such as press forming to impart a strain of 2% or more, and a temperature T (17000 <T (30 + lnt) <30,000 When the heat treatment is performed in the range of K) and the time t seconds, the tensile strength of the steel sheet increases.

The steel contains predetermined amounts of Si, Al and P,
The reason for the increase in tensile strength during this heat treatment is not clear, but the application of a strain of 2% or more introduces a considerable amount of dislocation, and the diffusion of C in the steel is dramatically increased. Further, carbides are formed on the dislocations with the introduced dislocations as nuclei, and further suppress the coarsening of the carbides in which Si, Al, and P are precipitated, and increase the tensile strength of members and components because a large number of carbides are precipitated. The present inventors believe. Si relating to the increase in tensile strength in the post-processing heat treatment,
The degree of influence of Al and P differs for each element, and Al
1.4 times and P is 6.3 times that of Si. The effect on the increase in tensile strength when Si, Al and P are added in combination is as follows: Si +
It turned out that it can be shown by 1.4Al + 6.3P. Si +
The range of 1.4Al + 6.3P is preferably 0.2% or more in order to obtain the effect of the present invention, but if it exceeds 3%, the workability is deteriorated. Therefore, the addition of 3% or less is desirable. FIG. 2 shows the formula for increasing the heat treatment strength after forming the thin steel sheet: Si + 1.4Al.
The relationship between + 6.3P and the increase in the tensile stress (ΔTS) is shown. It can be seen that the strength rise is remarkable at 0.2% or more.

As will be described later, this effect is remarkable when the crystal grain size of the steel microstructure is 20 μm or less and the particle size distribution of the iron-carbon compound in the steel is mainly 1 μm or less. . In addition, in order to further increase the tensile strength during the heat treatment after forming, in addition to the above Si, Al, and P,
Mo, Cr, Ti, Nb, V, and B can be added as selective elements. It is not clear why the effect of increasing the tensile strength and hardness of the member or component containing Mo, Cr, Ti, Nb, V, and B increases, but when Mo, Cr, Ti, Nb, V, and B are added. , Synergistic with the action of the low-temperature heat treatment by the addition of Si, Al and P.

The present inventors consider that Mo has an effect of preventing dislocations imparted in a steel sheet by press forming from disappearing during low-temperature heat treatment and promoting the precipitation of iron carbide. . If it is less than 0.01%, the effect of increasing the strength during heat treatment is small, so the lower limit is 0.01%, and if it exceeds 2.0%, the workability is deteriorated, so the upper limit is 2.0%. Cr
Dissolves in the precipitated iron carbide during low-temperature heat treatment,
The present inventors consider that iron carbide has a function of preventing coarsening of iron carbide and precipitating fine iron carbide. If it is less than 0.01%,
At the time of heat treatment, the effect of increasing the strength is small, so the lower limit is 0.01%. Further, Cr is an element that increases the strength of steel, and if it exceeds 2.0%, the workability deteriorates, so the upper limit is 2.0%.

Ti, Nb and V form fine carbides in the steel sheet. The present inventors believe that the fine carbide effectively causes the dislocations to grow effectively with respect to the strain imparted during pressing, and exhibits the effect of increasing the amount of strain. T
i is an element that enhances the effect of increasing the strength during heat treatment. If it is less than 0.005%, the effect is small, so 0.005%
Is the lower limit. Further, Ti is an element that increases the strength of steel, and if it exceeds 0.10%, the workability deteriorates, so the upper limit is 0.10%.

Nb is an element that enhances the effect of increasing the strength during heat treatment. If the content is less than 0.005%, the effect is small, so the lower limit is 0.005%. Also, Nb is an element that increases the strength of steel, and if it exceeds 0.10%, the workability deteriorates, so the upper limit is 0.10%. V is an element that enhances the effect of increasing the strength during the heat treatment. If the content is less than 0.005%, the effect is small, so the lower limit is 0.005%. Further, V is an element that increases the strength of steel, and if it exceeds 0.10%, the workability deteriorates, so the upper limit is 0.10%.

The steel composition is adjusted as described above.
In order to enhance the effect of increasing the strength during the post-forming heat treatment, it is desirable that the C content in the steel sheet be in a solid solution state at the post-forming heat treatment temperature, so that the carbide forming elements Ti, N
b, V is the amount of Ti, ｛(48/12) × C [%] + (48 /
14) × N [%]} or less, or the amount of Nb is {(93
/ 12) × C [%] + (93/14) × N [%]} or less, or V amount is {(51 × 4/12/3) × C
[%] + (51/14) × N [%]｝ or less, or T
In the case where i, Nb, and V are combined, 複合 Ti [%] ×
12/48 + Nb [%] × 12/93 + V [%] × 12
× 3/51/4｝ <C [%] + N [%] × 12/14,
Is desirably added to satisfy the following.

In order to further increase the tensile strength during the heat treatment after forming, B can be added as a selective element. It is not clear why the effect of increasing the tensile strength or hardness of the member or component containing B increases.
The present inventors consider that B attracts C to the vicinity of the grain boundary where dislocation density is high, and effectively forms fine carbides and hardens the steel during heat treatment after pressing.

B is an element that enhances the effect of increasing the strength during the heat treatment. If the content is less than 0.0003%, the effect is small, so the lower limit is 0.0003%. If the content exceeds 0.0050%, the effect is saturated, so the upper limit is 0.0050%. Then
The reason why the microstructure is limited according to the composition of the steel will be described. The microstructure of steel has an average grain size of 20 μm or less,
In addition, the average iron carbide particle diameter in the steel is 1 μm or less (the average is calculated by the number fraction). The technical idea of the present invention is that, unlike a bake hardened steel sheet in which solid solution C and solid solution N are present before forming a steel sheet (for example, before pressing), heat treatment after forming (for example, after pressing) dissolves iron carbide to form a solid solution. C, to form solute N, and then to re-precipitate on dislocations introduced during forming (for example, during pressing). Unlike baking hardened steel sheets, both tensile strength and yield strength of thin steel sheets are significantly increased. be able to. It is not always necessary to leave solid solution C and solid solution N before forming (for example, before pressing) as in a conventional bake hardened steel sheet.

In order to dissolve the iron carbide by heat treatment after forming (for example, after pressing), the average iron carbide particle diameter in the steel is set to 1 μm or less, and the interface energy and strain energy at the ferrite-iron carbide boundary are increased to dissolve the iron carbide. It is necessary to control the iron carbide particle size to be promoted. The iron carbide may be a carbide containing a metal element other than iron as long as iron is present. In addition, the average crystal grain size of steel is 20μ.
m or less, the iron carbides at the grain boundaries become finer and dissolve faster, and furthermore, solid solution C and solid solution N resulting from the dissolution of the iron carbide are formed over the entire crystal grains within the heat treatment time after forming (for example, after pressing). Because it is widespread, the effect of hardening the steel sheet is enhanced.
The iron carbide mentioned here may be any chemical bond form of iron and carbon as long as it is a compound of iron and carbon such as cementite, ε carbide, χ carbide, iron-carbon complex, iron-carbon compound containing N or a third element. Further, the existence form of the iron carbide may be any of a ferrite grain boundary, ferrite inside, and a form in which ferrite and iron carbide are mixed (fine pearlite, bainite), and do not depart from the present invention.

FIG. 3 shows the relationship between the average iron carbide diameter, the average crystal grain diameter, and the increase in the tensile strength (ΔTS) after the heat treatment.
The average carbide particle diameter was calculated by counting the number distribution of each carbide particle diameter in the microscope visual field and calculating the average carbide particle diameter. The average crystal grain size was measured by the ferrite crystal grain size test method of JIS G 0552. For bainite, the packet size was defined as the crystal grain size. It can be seen that ΔTS of 60 MPa or more was obtained within the range of the present invention, and the amount of increase in tensile strength after heat treatment was remarkable.

The steel sheet for heat treatment for increasing the strength (tensile strength) after working according to the present invention may be any of a hot-rolled steel sheet, a cold-rolled steel sheet, a hot-dip galvanized steel sheet, a galvannealed steel sheet, an electro-galvanized steel sheet and the like. However, the effect of the present invention can be enjoyed, but when a layer containing 1 mg / m ² or more of zinc is provided on at least one surface of the thin steel sheet, oxidation and decarburization during heat treatment after forming (for example, after pressing) are prevented, The effects of the present invention can be more effectively enjoyed. The layer containing 1 mg / m ² or more zinc on at least one side may be applied by any method such as an electroplating method, a hot-dip plating method, a coating method, and a vapor deposition method, and the method is not limited. . Further, the layer containing 1 mg / m ² or more of zinc may contain elements other than zinc at all.

The steel sheet of the present invention is preferably a cold-rolled steel sheet from which a steel sheet having a small crystal grain size can be obtained relatively easily. Also, the thickness of the sheet is not limited.
6 mm is particularly effective. The production method of the steel of the present invention may be appropriately selected, and the molten steel adjusted to the above components is made into a slab or a slab by continuous casting, and is made into a slab by an ingot ingot method, and is heated at a high temperature. Hot rolling or hot rolling after heating. After hot rolling, a descaling treatment is performed to form a hot-rolled steel sheet, or hot-dip galvanizing is performed to form a hot-dip galvanized steel sheet. The hot-dip galvanized steel sheet may be subjected to a heat alloying treatment to form an alloyed hot-dip galvanized steel sheet. There are no particular restrictions on the hot rolling and winding conditions, but in order for the crystal grain size of the microstructure of the steel to be 20 μm or less and the iron carbon compound particle size distribution in the steel to be mainly 1 μm or less, After hot rolling, cool down, 55
It is desirable to perform winding at 0 ° C or less.

Alternatively, after hot rolling, a descaling treatment is performed and cold rolling is performed to obtain a cold-rolled steel sheet. Then, it is annealed to form a cold-rolled steel sheet, or is annealed and hot-dip galvanized to form a hot-dip galvanized steel sheet. The hot-dip galvanized steel sheet may be subjected to a heat alloying treatment to form an alloyed hot-dip galvanized steel sheet. The annealing temperature at this time is not particularly limited. However, in order that the crystal grain size of the steel microstructure is 20 μm or less and the particle size distribution of the iron carbon compound in the steel is 1 μm or less, the Ac1 transformation is required. When performing annealing at a temperature equal to or higher than the temperature, and then cooling, it is desirable to cool the Ar1 transformation point at a speed of 20 ° C./sec or more. Further, in order to avoid coarsening of iron carbide in the steel, it is desirable to cool at a cooling rate up to 250 ° C. at a rate of 3 ° C./sec or more. The heating method of the heat alloying treatment is not particularly limited, and direct heating by combustion gas, induction heating, direct current heating, or the like can be appropriately selected.

After forming a high-strength hot-rolled steel sheet, a cold-rolled steel sheet, a galvanized steel sheet, or an alloyed hot-dip galvanized steel sheet, a steel sheet (Dull) subjected to temper rolling to improve workability and appearance after processing. Finished steel sheet, bright-finished steel sheet, steel sheet with a specific shape pattern transferred on the surface, etc.), and steel sheet with an oil film layer such as rust-preventive oil and lubricating oil on the surface. In any of the steel sheets described above, the effects of the present invention can be sufficiently enjoyed as long as the steel sheet has the component range of the present invention.

Further, as described later, when a plastic strain of 2% or more is imparted to the steel sheet, the amount of increase in tensile strength after heat treatment is remarkable. Therefore, it is difficult to impart a strain of 2% or more, so that the effect of the present invention can be effectively enjoyed by imparting a strain of 2% or more in advance by temper rolling.

Then, using the steel sheet of the present invention, press forming such as drawing is performed. In carrying out the press working, a region where a strength (tensile strength) or hardness is required to give an appropriate amount of dislocation to the steel sheet is subjected to forming in which a plastic equivalent strain of 2% or more is applied.
If the amount of strain is small, the effect of increasing the strength of the present invention cannot be sufficiently exhibited even if heat treatment is performed after molding, so the amount of strain applied during pressing is set to 2% or more, preferably 5% or more. The press forming method is not particularly limited as long as it gives a strain of more than 2%, and there is no problem even if drawing, stretching, bending, ironing, punching or the like is added. FIG. 4 shows the relationship between the amount of strain during press forming and the amount of increase in tensile strength (ΔTS) after press forming and heat treatment. 2% or more, preferably 5%
%, The increase in tensile strength is remarkable.

After press molding, 17,000 <T (30 + lnt) <30,0
A heat treatment for maintaining the temperature T (K) represented by 00 and the time t seconds is performed. If the temperature is in the range of 17,000 <T (30 + lnt) <30,000, the heat treatment temperature and time are not particularly limited, but the heat treatment temperature is generally more than 200 ° C to 500 ° C, and the heat treatment time is 1 second to 1 hour. is there. At this time, T (30 ＋ lnt)
If ≦ 17,000, the effects of the present invention cannot be exhibited, so
When T <30 (30 + lnt) is the lower limit and T (30 + lnt) ≧ 30,000, not only does the component accuracy deteriorate due to thermal strain, but also the oxidation reaction of the surface of the hot-rolled steel sheet, cold-rolled steel sheet, and galvanized steel sheet proceeds. Since the corrosion resistance of the finished component may be impaired, the upper limit is T (30 + lnt) <30,000.

The heat treatment method for heating in the range of 17,000 <T (30 + lnt) <30,000 is not particularly specified.
Partial high frequency heating, energizing heating, hot bath heat treatment, infrared heating,
Any method, such as hot air heating, may be used as long as it heats at least the distortion imparting portion to a predetermined range. FIG. 5 shows the relationship between the heat treatment temperature and the amount of increase in the tensile strength (ΔTS) after the heat treatment. The heat treatment is performed at 50 ° C to 600 ° C for 10 minutes. 1
7,000 <T (30 + lnt) <30,000, preferably 18,000 <T (30
It can be seen that the amount of increase in tensile strength is remarkable in the heat treatment of (+ lnt) <23,000.

[0036]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. Steels having the components shown in Table 1 were melted and continuously cast into slabs according to a conventional method. Then, it is heated to 1200 ° C. in a heating furnace, and hot-rolled at a finishing temperature of 880 ° C.,
The film was wound at a temperature of 500 ° C. and then pickled to obtain a hot-rolled steel sheet.

A part of the hot-rolled steel sheet was further cold-rolled at a rolling reduction of 80%, and then recrystallized and annealing at a temperature of 830 ° C. for 60 seconds to obtain a cold-rolled steel sheet. In addition, a part was electrogalvanized to provide a zinc layer on the surface layer of the steel sheet.
The obtained hot-rolled steel sheet and cold-rolled steel sheet were processed into JIS No. 5 tensile test pieces, and mechanical property values (without heat treatment) were evaluated.

Further, separately, the steel sheet is formed by pressing,
This was a hat-shaped press-formed product shown in FIG. At this time, the wrinkle holding pressure was adjusted, and 5%
A 2% plastic equivalent strain was applied to the flat portion B. Table 1
And then air-cooled and heated. Tensile test pieces were cut out from the vertical wall A and the flat part B of the component, and the tensile strength was measured. In the tensile test after press working, the true stress-strain relationship is measured, so in order to see the rise in nominal stress, the thickness before press working is converted to the test piece thickness. The nominal stress was used. The above results are shown in the table.

As is evident from Tables 1 and 2, the steel sheet of the present invention is more excellent in the heat treatment hardenability.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

As described above, according to the present invention,
The strength and rigidity of the press-formed body can be improved by the heat treatment for increasing the strength after forming, and a thin steel sheet and a high-strength press-formed body excellent in heat treatment hardening ability can be provided.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a conceptual diagram illustrating a heat treatment hardening ability of the present invention for hardening at a tensile strength of 60 MPa or more.

FIG. 2 is a diagram showing a relationship between a formula representing a heat treatment strength increasing ability after forming a thin steel sheet and a heat treatment hardening amount.

FIG. 3 is a graph showing the relationship between the average iron carbide diameter and average crystal grain size of the steel sheet of the present invention and the amount of heat treatment hardening. The numbers in the figure indicate the heat treatment hardening amounts (ΔTS, MPa).

FIG. 4 is a diagram showing the relationship between the amount of strain imparted by press forming to the steel sheet of the present invention and the amount of heat treatment hardening.

FIG. 5 is a diagram showing a relationship between a heat treatment temperature and a heat treatment hardening amount.

FIG. 6 is a schematic view showing the shape of a hat-shaped press-formed product.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuichi Taniguchi 5-3 Tokaicho, Tokai City, Aichi Prefecture Inside Nippon Steel Corporation Nagoya Works (72) Inventor Kazumasa Yamazaki 5- Tokaicho, Tokai City, Aichi Prefecture 3 Nippon Steel Corporation Nagoya Works (72) Inventor Eiji Eiko 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Shinji Shibata 1 Toyota Town, Toyota City, Aichi Prefecture Toyota (72) Inventor Hiroshi Kawaguchi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation

Claims (5)

[Claims] C .: 0.01 to 0.20%, Mn: 0.010 to 3.0%, S: 0.001 to 0.020%, N: 0.0002 to 0.01%, Si: 0.01 to 3.0%, Al: 0.005 to 2.0% by weight%. , P: 0.005 to 0.2%, the balance being iron and unavoidable impurities, and the steel having an average crystal grain size of 20 μm or less;
A thin steel sheet excellent in heat-hardening ability, wherein the average particle size of the iron-carbon compound in the steel is 1 μm or less. 2. In% by weight, C: 0.01 to 0.20%, Mn: 0.010 to 3.0%, S: 0.001 to 0.020%, N: 0.0002 to 0.01%, Si: 0.01 to 3.0%, Al: 0.005 to 2.0% , P: 0.005 to 0.2%, and further contains Si, Al, P by weight in the range of 0.2 ≦ Si% + 1.4Al% + 6.3P% ≦ 3, and the average grain size of the steel. Is not more than 20 μm, and the average particle size of the iron-carbon compound in the steel is 1 μm
A thin steel sheet excellent in heat treatment hardening ability, characterized in that: 3. In% by weight, C: 0.01 to 0.20%, Mn: 0.010 to 3.0%, S: 0.001 to 0.020%, N: 0.0002 to 0.01%, Si: 0.01 to 3.0%, Al: 0.005 to 2.0% , P: 0.005 to 0.2%, and by weight%, Mo: 0.01 to 2.0%, Cr: 0.01 to 2.0%, Ti: 0.005 to 0.10%, Nb: 0.005 to 0.10%, V: 0.005 to 0.10 %, B: 0.0003 to 0.0050%, and the steel has an average crystal grain size of 20 μm or less, and the iron carbon compound in the steel has an average grain size of 1 μm or less. Thin steel sheet with excellent heat-hardening ability. 4. In% by weight, C: 0.01 to 0.20%, Mn: 0.010 to 3.0%, S: 0.001 to 0.020%, N: 0.0002 to 0.01%, Si: 0.01 to 3.0%, Al: 0.005 to 2.0% , P: 0.005 to 0.2%, and further containing Si, Al, P in the range of 0.2 ≦ Si% + 1.4Al% + 6.3P% ≦ 3 by weight%, and further, by weight%, Mo: 0.01% ~ 2.0%, Cr: 0.01 ~ 2.0%, Ti: 0.005 ~ 0.10%, Nb: 0.005 ~ 0.10%, V: 0.005 ~ 0.10%, B: 0.0003 ~ 0.0050%, one or more of the following: A thin steel sheet excellent in heat-hardening ability, wherein the steel has an average crystal grain size of 20 μm or less, and the iron-carbon compound in the steel has an average grain size of 1 μm or less. 5. The thin steel sheet according to any one of claims 1 to 4, which is subjected to press forming in which a plastic strain of 2% or more is applied to at least a portion where strength is required.
A method for producing a high-strength press-formed body, characterized by performing a heat treatment for maintaining a temperature T (K) expressed by <T (30 + 1nt) <30,000 and a time period t seconds.