JP2003266123A - Method of forming high tensile strength steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of forming a high tensile strength steel sheet by which the increase of the strength in the steel sheet after press forming-heat treatment is uniformized, and the strength can stably be exhibited. <P>SOLUTION: A high tensile strength steel sheet having a composition containing ≤0.10% C, 0.1 to 1.0% Si, 0.1 to 3.5% Mn, ≤0.05% P, ≤0.01% S, ≤0.02% Al and 0.0050 to 0.025% N, also satisfying N/Al of ≥0.3, and containing N as a solid solution state in ≥0.0010%, and the balance Fe with inevitable impurities, and having excellent strain age hardening properties is subjected to forming so that the bend radius R (mm) on the forming or the radius R (mm) of the shoulder of a die satisfies R/t:≤8 (t: sheet thickness (mm)). In addition to the above composition, one or more kinds of elements selected from Cu, Ni, Cr and Mo, one or more kinds of elements selected from Nb, Ti, V and B, or one or two kinds of elements selected from Ca and rare earth metals can be incorporated as well. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a highly workable high-strength thin steel sheet suitable for manufacturing automobile parts such as bumpers, center pillars and members. In addition,
The present invention is preferably applied to a high-strength steel sheet having a tensile strength of 440 MPa or more, and particularly an ultra-high-strength steel sheet having a tensile strength of 980 MPa or more. Steel plates and hot dip plated steel plates are also included.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of protecting the global environment, reducing the weight of a vehicle body of an automobile has become an extremely important issue in relation to the regulation of exhaust gas of the automobile. To reduce the weight of the vehicle body, increase the strength of the steel plate used, that is, apply a high-strength steel plate (also called a high-strength steel plate),
It is effective to reduce the thickness of the steel sheet used.

However, even a part using a thin high-strength steel plate is required to sufficiently and sufficiently exhibit the performance imposed according to its role. The performance as a part differs for each part, but one example is static strength against bending and torsional deformation. That is, the high-strength steel sheet used for automobile parts is required to have excellent properties after the forming process. Since these characteristics are related to the strength of the steel sheet after forming, the strength lower limit of the high-strength steel sheet to be used is usually set.

However, when a part is manufactured by press molding using a steel plate having high strength, (1) shape freezeability is deteriorated, and (2) ductility is low, which causes problems such as cracking and necking during molding. There is a problem such as (3) deterioration in dent resistance because the sheet thickness is reduced.

All of these problems have hindered the widespread application of high-strength steel sheets to automobile bodies. On the other hand, for example, as a cold-rolled steel sheet for an outer panel, a steel sheet is known in which ultra-low carbon steel is used as a raw material and the amount of C finally remaining in a solid solution state is controlled within an appropriate range. This type of steel sheet is soft during press forming and secures shape fixability and ductility, while increasing the yield stress by utilizing the strain age hardening phenomenon that occurs in the coating baking process at 20 ° C for about 20 min after press forming. Therefore, the dent resistance is to be secured. However, in this type of steel sheet, from the viewpoint of preventing the occurrence of stretcher strain, which is a surface defect, the amount of increase in strength due to strain age hardening is kept low. For this reason, there is a drawback that the part that actually contributes to the weight reduction of parts is small.

On the other hand, for applications in which the appearance does not matter so much, for example, in Japanese Examined Patent Publication No. 7-30408, solid solution N
The steel plate whose bake hardening amount is further increased by using
Japanese Examined Patent Publication (Kokoku) No. 8-23048 proposes a steel sheet having a further improved bake hardenability by having a composite structure of ferrite and martensite. However, although the steel plate manufactured by the technique described in Japanese Patent Publication No. 8-23048 is manufactured at an extremely low coiling temperature, the tensile strength increases due to strain age hardening, but the yield stress increases. There is a problem in that there is a large variation in the mechanical properties and the variation in other mechanical properties is also large.

[0007]

As described above, despite the extremely strong demand, a thin steel sheet which is soft and excellent in formability at the time of forming, and which can secure high strength after forming-painting baking is industrially inexpensive. And, there was no stable manufacturing technology. Particularly in recent years, the ultra-high strength cold rolled steel sheet with a tensile strength TS of 980 MPa or more, which is used for bumpers, center pillars and members, whose application amount has been expanding, has a high strength, and therefore has a higher strength after forming-paint baking. There was a problem that it was difficult to achieve a stable increase.

In order to solve the above problems, the present inventors manufactured steel sheets with various compositions and manufacturing methods and conducted many material evaluation experiments. As a result, by using nitrogen (N), which has not been used positively in the field where high workability is required, as a strengthening element, and by effectively utilizing the large strain age hardening effect of this strengthening element, It was found that it is possible to easily achieve both the improvement of the properties and the increase in the strength after molding. Then, the applicant of the present invention is disclosed in JP-A-2002-47536.
As described in the publication, in order to utilize the strain age hardening effect of N, it is necessary to favorably combine it with the paint baking condition of the automobile or more positively with the post-molding heat treatment. Therefore, the manufacturing condition is optimized. Therefore, it is effective to control the microstructure and the amount of solid solution N within a certain range. Further, in order to stably develop the strain age hardening effect of N, the content of Al is adjusted according to the N content. It was also clarified that it is extremely important to control it. In this way, the steel sheet with the optimized microstructure and the amount of solid solution N is the conventional solid solution strengthened C
-It is a steel sheet that has remarkably superior formability compared to Mn-based steel sheets and precipitation-strengthened steel sheets, and excellent strain-age hardening characteristics that conventional steel sheets do not have.

However, even if a steel sheet having the above-mentioned microstructure and the amount of solid solution N optimized is used, there is still a large variation in the strength increase of the parts that have undergone the press forming-painting baking process. was there. Such variations in the increase in strength after the press forming-painting baking step are particularly large when the high-strength steel sheet is applied to parts that require reliability, such as ultra-high-strength steel sheet with TS: 980 MPa or more. It was an obstacle.

The present invention solves the above-mentioned problems and provides a high-strength steel sheet having excellent strain age hardening characteristics for automobile bodies which require high workability and stable quality characteristics, after press forming and heat treatment. An object of the present invention is to propose a method for forming a high-strength steel sheet which can uniformly increase the strength (strain age hardening amount) without variation and stably exhibit a strength increase of a certain amount or more (50 MPa or more). The "high-strength steel sheet having excellent strain age hardening characteristics" referred to in the present invention has TS of 440 MPa or more, is soft during press molding, and does not cause problems such as defective shape during press molding and cracking. ,Tensile strength
It is a high-tensile steel plate having a strength of 440 MPa or more, which can secure a strength increase amount ΔTS of 50 MPa or more after being subjected to plastic strain and then subjected to a heat treatment of 170 ° C. × 20 min, for example.

[0011]

Means for Solving the Problems In order to achieve the above-mentioned objects, the inventors of the present invention have earnestly studied the factors of variation in strength increase after press molding and heat treatment. As a result, the inventors of the present invention found that the variation in the increase in strength after press molding-heat treatment was caused by the fact that sufficient consideration was not given to press molding conditions.

First, the experimental results which are the basis of the present invention will be described. % By mass, 0.085% C-0.55% Si-2.
55% Mn-0.005% P-0.0008% S-0.005% Ti-0.015
% Nb-0.012% Al-0.0135% N (N / Al = 1.13) composition steel slab (Steel No. HN) and 0.078% C-0.48% Si-3.
15% Mn-0.011% P-0.0009% S-0.007% Ti-0.019
% Nb-0.011% Al-0.0018% N (N / Al = 0.16) composition steel slab (steel No. LN) is used as the material, slab heating temperature: 1250 ° C, finish rolling outlet temperature: 880 ° C, Winding temperature: Hot rolled at 450 ℃ to make hot rolled sheet,
Then, these hot-rolled sheets are cold-rolled to obtain cold-rolled sheets, and then these cold-rolled sheets are heated to a temperature rising rate of 2.5 ° C / s and an annealing temperature of 810 ° C and a cooling rate of 20 ° C / s. s, Cooling stop temperature: Cooling to 250 ℃ and 150 at 250 ℃ after cooling stop
Continuous annealing, which consists of holding for s, was performed to obtain a cold rolled annealed plate (plate thickness 1.6 mm).

For these cold rolled annealed sheets (sheet thickness t), R
Press molding tests were carried out by changing / t variously. The press molding test simulates press molding on a press machine.
In this test, a cold rolled annealed plate is used as a test plate, and the test plate is pulled out by winding it around a tool (die shoulder radius R). By varying R, the die shoulder radius R in press forming can be changed. The effects of can be investigated. Figure 2
The test outline is shown in.

The cold-rolled and annealed sheet was subjected to the above-mentioned press forming test and then heat-treated at 170 ° C.-1200 s. After the heat treatment, a tensile test is carried out to obtain the tensile strength TS _HT after the heat treatment, and the increase amount of the tensile strength before and after the heat treatment ΔTS (= TS _HT −
TS, TS: tensile strength before heat treatment) was determined. The obtained results are shown in FIG. 1 as a relationship between the amount of increase in strength ΔTS after molding and heat treatment and R / t. The cold-rolled steel sheet manufactured from Steel No. HN has a solid solution N content of 0.0095% by mass,
Cold-rolled steel sheet manufactured using Steel No.LN as a solid solution N
Was 0.0004% by mass.

From FIG. 1, steel N containing a large amount of solid solution N is present.
In o.HN, by adjusting the die shoulder radius and performing R / t: 8 or less, it is possible to stably obtain a high strength increase ΔTS (strain age hardening characteristic) after forming-heat treatment. . When R / t exceeds 8, N in solid solution state
Even a steel sheet having a large content (a steel sheet having excellent strain age hardening characteristics) can obtain only a low value of ΔTS (strain age hardening characteristics). On the other hand, in steel No. LN containing a small amount of N in the solid solution state, ΔTS is low in all cases, and high strain age hardening characteristics are not exhibited.

As described above, the inventors of the present invention may not obtain the expected strain age-hardening property even when the steel plate having the excellent strain age-hardening property is out of the proper range of the forming conditions such as press forming. I found that there is. The present invention has been completed by further studies based on the above findings. That is, the gist of the present invention is as follows. (1) C: 0.10% or less, Si: 0.1 to 1.0% by mass%,
Mn: 0.1-3.5%, P: 0.05% or less, S: 0.01% or less,
Al: 0.02% or less, N: 0.0050 to 0.025% inclusive, and N
/ Al is 0.3 or more, 0.0010% or more of N in the form of solid solution is contained, and the balance of Fe and unavoidable impurities makes the high-tensile steel plate into a predetermined shape. R (mm) or die shoulder radius R (m
m) is expressed by the following equation (1) R / t ≦ 8 ... (1) (where, R: bending radius R (mm) or die shoulder radius R
(Mm), t: Steel plate thickness (mm)) is adjusted so as to satisfy the method for forming a high-tensile steel plate. (2) In (1), in addition to the above composition, further, in mass%, the following Group A, Group B and Group C Group A: one or more of Cu, Ni, Cr, Mo in total. 1.0% or less, B group: 0.1% or less of one or more of Nb, Ti, V, and B in total, and C group: 0.001 to 0. 1 or 2 of Ca, REM in total.
A method for forming a high-strength steel sheet, which comprises one or more groups selected from the group consisting of 010%. (3)% by mass, C: 0.10% or less, Si: 0.1 to 1.0%,
Mn: 0.1-3.5%, P: 0.05% or less, S: 0.01% or less,
Al: 0.02% or less, N: 0.0050 to 0.025% inclusive, and N
/ Al is 0.3 or more, 0.0010% or more of N is contained as a solid solution, and the balance is Fe and unavoidable impurities. In the method for manufacturing a steel plate part, the forming process is performed by bending radius R (mm) or die shoulder radius R
(Mm) is expressed by the following equation (1) R / t ≦ 8 ... (1) (where, R: bending radius R (mm) or die shoulder radius R
(Mm), t: Steel plate thickness (mm) adjusted to satisfy the above-mentioned heat treatment at 100 to 300 ° C for 30s or more and 20min or less. Manufacturing method of parts. (4) In (3), in addition to the above composition, further, in mass%, the following Group A, Group B and Group C Group A: one or more of Cu, Ni, Cr, Mo in total. 1.0% or less, B group: 0.1% or less of one or more of Nb, Ti, V, and B in total, and C group: 0.001 to 0. 1 or 2 of Ca, REM in total.
A method for producing a high-tensile steel plate part, characterized by containing one group or two or more groups selected from 010%.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the composition of the steel sheet used in the present invention will be explained. In the following, mass% is
Simply write in%. C: 0.10% or less C is an element that increases the strength of steel, and it is preferable to contain 0.03% or more to secure TS440MPa or more, and 0.05% or more to secure 980MPa or more. Preferably. On the other hand, when the content exceeds 0.10%, the weldability is significantly deteriorated, and the ductility of the steel sheet and further the formability are significantly reduced due to the increase in the amount of carbides in the steel. Therefore, in the present invention, C
Was limited to 0.10% or less. From the viewpoint of improving moldability, it is preferably 0.085% or less.

Si: 0.1 to 1.0% Si is a useful strengthening element capable of increasing the strength of a steel sheet without significantly reducing the ductility of the steel. Such a desired effect is exhibited at a content of 0.1% or more.
On the other hand, if Si is contained in a large amount, the transformation point is greatly increased, which makes it difficult to secure quality and shape during hot rolling, or it adversely affects the beauty of the steel sheet surface such as surface properties and chemical conversion treatability. Also, it is an element that adversely affects even the above, and is limited to 1.0% or less in the present invention. When the content is 1.0% or less, a remarkable increase in transformation point can be suppressed by adjusting the amount of Mn added at the same time, and good surface properties can be secured. For this reason, Si is limited to the range of 0.1 to 1.0%. Incidentally, when producing a steel sheet having high ductility at TS of 980 MPa or more, there is a trade-off with the amount of Mn, but Si is 0.
It is preferably 2% or more from the viewpoint of the balance between strength and ductility. It is more preferably 0.2 to 0.6%.

Mn: 0.1-3.5% Mn is an effective element that prevents hot cracking due to S and increases the strength of the steel sheet, and is preferably contained according to the amount of S contained, and is 0.1% or more. Requires inclusion. If a strength of TS980 MPa or higher is required,
It is desirable to contain about 2.5% or more. On the other hand, if it is contained excessively, the formation of ferrite is remarkably suppressed, the structure of the hot-rolled sheet becomes non-uniform, and the ductility decreases, so it was limited to 3.5% or less. In applications where both corrosion resistance and formability are required, it is preferably 3.3% or less. In addition,
It is preferably 2.7 to 3.2%.

P: 0.05% or less P is an element effective as a solid solution strengthening element for steel, and is preferably contained in an amount of 0.01% or more from the viewpoint of increasing strength. On the other hand, if it is contained excessively, the steel becomes brittle, and further the stretch flange formability of the steel sheet is deteriorated. Further, P has a strong tendency to segregate in the steel, so that it causes embrittlement of the welded portion. Therefore, P is limited to 0.05% or less. In addition, P is preferably 0.04% or less in applications where stretch flange formability and weld toughness are particularly important.

S: 0.01% or less S is an element that exists as an inclusion in the steel sheet, reduces ductility of the steel sheet, and further causes deterioration of corrosion resistance, and is preferably reduced as much as possible.
It was set to 0.01%. In addition, it is desirable that S is 0.0030% or less in an application where particularly good workability is required,
If particularly good stretch flangeability is required, it is preferably 0.0015% or less.

Al: 0.02% or less Al is an element that is added as a deoxidizing element for steel and is useful for improving the cleanliness of steel, and is also an element that is desirable to be contained for refining the structure. Is. In the present invention, solid solution N is used as a strengthening element, but an aluminum killed steel containing Al in an appropriate range is superior in mechanical properties to a conventional rimmed steel containing no Al. . On the other hand, excessive Al content deteriorates the surface properties of the steel sheet or significantly reduces N in a solid solution state, making it difficult to secure the minimum required tensile strength of 440 MPa. Therefore, Al is limited to 0.02% or less.
From the viewpoint of material stability, 0.001 to 0.020%
Is desirable.

N: 0.0050 to 0.0250% N is an element that increases the strength of the steel sheet by solid solution strengthening and strain age hardening, and is the most important element in the present invention. In the present invention, by containing N in an appropriate range, further adjusting the Al content to an appropriate value as described above, and further controlling the production conditions such as hot rolling conditions and annealing conditions, cold rolled products or To secure the necessary and sufficient solid solution N for plated products. As a result, the effect of increasing the strength (yield stress and tensile strength) by solid solution strengthening and strain age hardening is sufficiently exerted, and the amount of increase in strength ΔT due to the heat treatment after forming is ΔT.
S can be stably secured.

When N is less than 0.0050%, the above-described strength increasing effect is difficult to appear stably. On the other hand, when N exceeds 0.0250%, the internal defect occurrence rate of the steel sheet increases, and slab cracks and the like frequently occur during continuous casting. Therefore, N is limited to the range of 0.0050 to 0.0250%. From the viewpoint of improving the stability and yield of the material in consideration of the entire manufacturing process, N is preferably in the range of 0.0070 to 0.0170%. If the amount of N is within the range of the present invention, there is no adverse effect on weldability and the like.

Solid solution N: 0.0010% or more In order to secure sufficient strength in a cold rolled product and to effectively exert strain age hardening by N, N in a solid solution state in the steel sheet
(Also referred to as solid solution N) must be present at least 0.0010% or more. Here, the amount of solute N is calculated from the total amount of N in steel,
The value obtained by subtracting the amount of precipitated N is taken as the amount of dissolved N. As a method for analyzing the amount of precipitated N, it is effective that the inventors of the present invention compare various methods and, as a result, carry out electrolytic extraction analysis using a potentiostatic electrolysis method. In addition, as a method of dissolving the ground iron used for the extraction analysis, there are an acid decomposition method, a halogen method and an electrolytic method. Among them, the electrolytic method can stably dissolve only the base iron without decomposing extremely unstable precipitates such as carbides and nitrides. An acetyl-acetone system is used as the electrolytic solution, and electrolysis is performed at a constant potential. In the present invention, the result of measuring the amount of precipitated N using the potentiostatic electrolysis method showed good correspondence with the actual change of the material.

From the above, in the present invention, the residue extracted by the potentiostatic electrolysis method is chemically analyzed to determine the amount of N in the residue, and this is used as the amount of precipitated N. When higher strain age hardening is required, the amount of solute N is 0.0020% or more,
In order to obtain a higher value, it is preferably 0.0030% or more. The upper limit of the amount of solute N is not particularly limited, but the total N
Even if all the amount remains, the deterioration of mechanical properties is small.

N / Al (ratio of N content and Al content): 0.3
In order to stabilize the solid solution N by 0.0010% or more and leave it in the product state, it is necessary to limit the amount of Al, which is an element that strongly fixes N. N content in the composition range of the present invention (0.
(0050-0.0250%) and Al content (0.02% or less) was studied in a wide range of steel sheets, and as a result, N / Al, which is the ratio of N content (mass%) and Al content (mass%), was determined. 0.
It was found that by setting the content to 3 or more, the solid solution N in the cold rolled product and the plated product can be stably set to 0.0010% or more. Therefore, N / Al is limited to 0.3 or more. From the viewpoint of stable enhancement of strain age hardening characteristics, N / Al is preferably 0.5 or more. More preferably, it is 0.7 or more.

In the present invention, in addition to the above composition, the following groups A to C: group A: 1% or more of Cu, Ni, Cr and Mo in total of 1.0% or less, group B: One or more of Nb, Ti, V, and B is 0.1% or less in total, and Group C: one or two of Ca and REM is 0.001 to 0.
It is preferable to contain one or more groups selected from 010%.

Group A: Cu, Ni, Cr, and Mo are all elements that contribute to increasing the strength of the steel sheet, and can be contained alone or in combination as required. Such an effect is Cu:
0.1% or more, Ni: 0.1% or more, Cr: 0.05% or more, Mo: 0.0
It becomes noticeable when it contains 5% or more of each. On the other hand, if the content is too large, the hot deformation resistance increases, or the chemical conversion treatability and surface treatment properties in a broad sense deteriorate, and the weld zone hardens and the weld zone formability deteriorates.
It is preferable to limit the total amount of Cu, Ni, Cr, and Mo to 1.0% or less.

Group B: Nb, Ti, V, and B are all elements that contribute to the miniaturization and homogenization of crystal grains, and can be contained alone or in combination as required. Such effects are as follows: Nb: 0.002% or more, Ti: 0.002% or more, V: 0.002% or more.
%, And B: 0.0002% or more, respectively, it becomes noticeable. However, if the content is too high,
Hot deformation resistance increases, and chemical conversion treatability and surface treatment properties in a broad sense deteriorate. Further, there is also an adverse effect of reducing the solid solution N. Therefore, the total amount of Nb, Ti, V, and B is preferably 0.1% or less.

Group C: Ca and REM are elements which are useful for controlling the morphology of inclusions, and it is preferable to contain them individually or in combination if there is a demand for stretch flange formability. If the total amount of the elements of group C is less than 0.0010%, the effect of controlling the morphology of inclusions is insufficient, while if it exceeds 0.010%, the occurrence of surface defects becomes conspicuous. Therefore, it is preferable to limit the total amount of the elements of the C group to the range of 0.0010 to 0.010%, whereby the stretch-flange formability can be improved without causing surface defects.

It should be noted that even if the groups A, B, and C are contained alone or in combination, these desired effects are not offset. The balance other than the above components is Fe and inevitable impurities. Next, a preferable method for producing a steel sheet used in the present invention will be described by taking a case of producing a high-strength cold-rolled steel sheet as an example.

The cold-rolled steel sheet (steel sheet) used in the present invention is basically made of a steel slab having the above-mentioned composition, and the material is heated and then roughly rolled to form a sheet bar, and the sheet bar is finish-rolled. Hot rolling process of applying and finishing rolling to cool to form a rolled hot rolled sheet, cold rolling step of subjecting the hot rolled sheet to pickling and cold rolling to form a cold rolled sheet, and the cold rolled sheet It is manufactured by sequentially performing a cold-rolled sheet annealing step in which the continuous annealing is performed.

The steel slab used as a raw material in the present invention is
It is desirable to manufacture by a continuous casting method in order to prevent macrosegregation of the components, but it may be manufactured by an ingot making method or a thin slab casting method. In addition, after manufacturing the slab, in addition to the conventional method of once cooling to room temperature and then heating again, direct feeding rolling of charging and rolling in a heating furnace without cooling, or slight heat retention was performed. An energy-saving process such as direct rolling which is immediately rolled later can be applied without any problem. In particular, direct feed rolling is one of the useful techniques for effectively securing N in a solid solution state.

First, preferable conditions for the hot rolling step will be described. Slab heating temperature: 1000 ° C or higher The slab heating temperature should be 1000 ° C or higher in order to secure the necessary and sufficient amount of solute N in the initial state and to satisfy the target value of solute N in the product. Is preferred. In addition, it is desirable to set the temperature to 1280 ° C. or lower because the loss increases with the increase in the oxidized weight.

The slab heated under the above-mentioned conditions is roughly rolled into a sheet bar. The conditions for rough rolling need not be specified in particular, and may be performed according to a conventional method. However, from the viewpoint of securing the amount of solid-dissolved N, it is desirable to carry out in as short a time as possible. Then, the sheet bar is finish-rolled to obtain a hot rolled sheet. It should be noted that it is desirable to join the successive sheet bars between the rough rolling and the finish rolling, and continuously roll the sheets. Also, on the entry side of the finish rolling mill between rough rolling and finish rolling, one of a sheet bar edge heater that heats the widthwise end of the sheet bar and a sheet bar heater that heats the lengthwise end of the sheet bar. It is preferable to use both or both to uniformize the temperature distribution in the width direction and the longitudinal direction of the sheet bar.

Finishing rolling outlet temperature: 800 ° C. or higher The finishing rolling outlet temperature FDT is preferably 800 ° C. or higher in order to obtain a uniform and fine hot rolled mother sheet structure. FDT
When the temperature is lower than 800 ° C, the structure of the steel sheet becomes non-uniform, and the work structure remains in part, and even after the cold rolling annealing step, the non-uniformity of the structure does not disappear and remains. Therefore, the risk of causing various problems during press molding increases. Further, if a high coiling temperature is adopted in order to avoid remaining of the processed structure, coarse crystal grains are generated and the same problem occurs. Further, by raising the coiling temperature to a high temperature, the amount of solute N is remarkably reduced, so that it becomes difficult to obtain the target tensile strength. From the viewpoint of improving the mechanical properties, the FDT is more preferably 820 ° C or higher.

Further, although the upper limit of FDT is not specified, when the FDT is excessively high, scale defects and the like become remarkable. The FDT is preferably up to about 1000 ° C. Winding temperature: 300 to 550 ℃ When the winding temperature CT is less than 300 ℃, the shape of the steel sheet is significantly disturbed, the risk of defective shape during cold rolling increases, and the uniformity of the steel sheet material decreases. Therefore, the CT is preferably 300 ° C. or higher. On the other hand, C
When T exceeds 550 ° C, the hot rolled sheet structure becomes a layered structure and Mn
Segregation of elements such as, for example, adversely affect the material properties of the cold rolled sheet. Therefore, the winding temperature CT is preferably 300 to 550 ° C. If higher material uniformity is required, it is desirable to set the temperature to 350 ° C to 500 ° C.

In the present invention, in finish rolling, lubrication rolling may be performed in order to reduce the hot rolling load. By performing the lubrication rolling, there is an effect that the shape and material of the hot-rolled sheet are made more uniform. The coefficient of friction during lubrication rolling is preferably in the range of 0.25-0.10.
Further, by combining the lubrication rolling and the continuous rolling, the operation of hot rolling is further stabilized.

The hot rolled sheet that has been subjected to the above hot rolling step is then subjected to pickling and cold rolling in the cold rolling step to become a cold rolled sheet. The pickling conditions may be generally known conditions and are not particularly limited. When the scale of the hot rolled sheet is extremely thin, cold rolling may be performed immediately without performing pickling.

The cold rolling condition may be a known condition and is not particularly limited. The cold reduction is preferably 50% or more from the viewpoint of ensuring the uniformity of the structure. Next, preferable conditions for the cold rolled sheet annealing step will be described. The cold-rolled sheet is then subjected to a cold-rolled sheet annealing step consisting of continuous annealing and cooling. Continuous annealing temperature: 750 ℃ ~ 900 ℃ When the continuous annealing temperature is higher than 900 ℃, the austenite grain size during annealing becomes coarse, ferrite transformation in the subsequent cooling process is suppressed, and ductility tends to decrease significantly. . If the temperature is lower than 750 ℃, the effect of cold work structure cannot be completely removed, the ductility is decreased, and a two-phase structure in which expanded grains are present is formed, and the workability such as bending and stretch flange deformation is generally improved. descend. From the viewpoint of ensuring good moldability, the temperature is preferably in the range of 780 ° C to 860 ° C.

The temperature rising rate in continuous annealing is preferably 1.7 ° C./s or more in order to suppress the recovery of the worked structure, recrystallization, and coarsening of the metal structure associated with grain growth.
At a heating rate slower than 1.7 ° C / s, the crystal grain size of the final product tends to become coarse, which may cause deterioration of bendability due to rough skin during molding, and deterioration of bend flangeability due to nonuniform organization. To be done.

The holding time of the continuous annealing time is preferably as short as possible from the viewpoints of production efficiency, refinement of the structure, and securing of the amount of solid solution N. The holding time is preferably 20 s or more from the viewpoint of operation stability, and is preferably 130 s or less from the viewpoint of refining the structure and securing the amount of dissolved N. From the viewpoint of stabilizing the material, it is more preferably 40 s or more.

Cooling after continuous annealing: Cooling at a cooling rate of 100 ° C./s or less Cooling after soaking in continuous annealing is important from the viewpoint of refining the structure and securing the amount of solute N. In the present invention, 100 ° C
Continuous cooling at a cooling rate of / s or less. It is preferably 50 ° C./s or less in order to secure a better elongation. If the cooling rate exceeds 100 ° C / s, ferrite transformation does not occur easily, and the uniformity of the material in the width direction of the steel sheet is insufficient. If the cooling rate is less than 5 ° C / s, the cooling rate is slow, and a uniform and fine structure and a sufficient amount of solid solution N cannot be obtained. Therefore, the cooling rate should be 5 ° C / s or more. Is more preferable. The cooling is at least 200-350 ° C.
It is desirable to continuously cool to the temperature of. Also, especially T
In order to obtain high strength of S980 MPa or more, the cooling stop temperature in cooling after continuous annealing is set to 200 to 350 ° C,
It is preferable to carry out a holding treatment for holding for 300 s or less at a temperature in the range of 200 to 350 ° C. Thereby, the second phase is softened and the moldability is improved. Hold temperature after cooling is 350
When the temperature exceeds ℃, the strength of the second phase other than ferrite decreases, and it becomes particularly difficult to secure the strength of TS980 MPa or more. On the other hand, if the holding temperature is lower than 200 ° C, the second phase becomes too hard, and the workability of the entire steel sheet deteriorates. Further, when the holding time is longer than 300 s, the solid solution N is precipitated and fixed, so that the strain aging property is likely to be deteriorated. If the second phase is cooled to room temperature without holding it, the second phase easily hardens. Even if the holding time is longer than 300 s,
The decrease in hardness of the second phase is small.

Needless to say, the cold rolling annealing step may be followed by temper rolling or leveler processing with an elongation of 0.5 to 10% for the purpose of shape correction and roughness adjustment. The total elongation of temper rolling or leveler processing is 0.
If it is less than 5%, the intended purpose of shape correction and roughness adjustment cannot be achieved. On the other hand, if it exceeds 10%, the ductility is lowered. From the viewpoint of ensuring ductility, it is more preferably 5% or less. In addition, it has been confirmed that the tempering rolling and the leveler processing have different processing types, but their effects are not so different. Temper rolling and leveler processing are effective even after plating.

A high-strength steel sheet having the above composition and excellent strain age hardening characteristics is used to form into a predetermined shape. In the present invention, the forming process is an ordinary steel plate bending process (steel plate process) or press forming. In the present invention, in the forming process, the bending radius R at the time of forming a steel plate or the die shoulder radius R at the time of press forming is expressed by the following formula (1) R / t ≦ 8 ... (1) ( Where R: bending radius R (mm) or die shoulder radius R
(Mm), t: steel plate thickness (mm)).

In the bending or press forming of a steel sheet, the inside portion of the steel sheet which is in contact with the bending tool (press die shoulder) receives compression, the outside portion receives tension, and the introduction of strain is simple.
It is completely different from axial tensile deformation. In order to stably secure a desired strength increase amount of a certain amount or more, when performing bending processing and press forming of the steel plate, the bending radius R depends on the plate thickness of the steel plate.
(Mm) or the die shoulder radius R is adjusted so that a predetermined amount of strain or more can be uniformly introduced into the steel plate.

When R / t exceeds 8, as shown in FIG. 1, the amount of plastic strain given under bending deformation in consideration of the plate thickness becomes insufficient, and ΔTS decreases to less than 50 MPa. Therefore, in the present invention, the bending radius R (mm) or the die shoulder radius R
Satisfies R / t: 8 or less. In addition, more preferably, R / t: 6 or less. R / t: By setting the ratio to 6 or less, the amount of increase in strength after molding-heat treatment (ΔTS)
Has a tendency to be saturated, and the strength of the part after molding-heat treatment can be uniformly increased.

In the present invention, a steel sheet is formed into a predetermined shape and then heat-treated to obtain a product (part) having a predetermined strength. The heat treatment is preferably performed at 100 to 300 ° C. for 30 seconds or more and 20 minutes or less. The conventional paint baking conditions are
170 ° C. × 20 min is adopted as a standard. Generally, although strain aging has an upper limit, heat treatment at high temperature for a long time is advantageous for strain age hardening treatment. However, with the high-strength steel sheet used in the present invention, after forming, the temperature is generally 100 ° C.
Strain age hardening becomes remarkable by heating to. On the other hand, 300
When the temperature exceeds ℃, the effect is saturated and, on the contrary, it tends to be slightly softened. Therefore, in the present invention, the heat treatment temperature is 100 to 30
The temperature is preferably 0 ° C.

Within this temperature range, almost sufficient strain age hardening can be achieved if the temperature is maintained for about 30 seconds or more.
In order to achieve a larger and stable strain age hardening, it is preferable to hold for 60 s or more. However, heat treatment for a long time of 20 minutes or more is not preferable because it causes a remarkable decrease in production efficiency in practical use. Therefore, in the present invention, the heat treatment time is 30 s or more and 20 mi or more.
It is preferably n or less.

Further, the heating method in the heat treatment is not limited to the case of heating the atmosphere in a heating furnace as in the case of ordinary coating baking, but, for example, induction heating, non-oxidizing flame, laser, plasma, etc. are also effective. . It should be noted that the steel sheet used in the present invention can be left alone at room temperature after the forming process even if it is not heated and subjected to accelerated aging (artificial aging).
Strength can be expected to increase, and it is possible to expect a minimum of approximately 40% of complete aging. On the other hand, in the state where no plastic working is applied, so-called aging deterioration (here, YS increase and EL decrease) due to holding at room temperature does not occur.

According to the method of the present invention, the tensile strength increase amount (ΔTS) is generally determined by the following (2) by performing the above-mentioned heat treatment after bending the steel plate or passing through the die shoulder of press forming. Formula 150−2.5 × (R / t) + 1.1 × (R / t) ² −0.14 × (R / t) ³ ……… (2) Below, the following (3) Formula 40−2.5 × (R / t) ) +1.1 × (R / t) ² −0.14 × (R / t) ³ ……… (3) The above can be secured.

In accordance with the present invention, intensity levels generally range from 1 to
It is possible to raise 1.5 grades. For example TS980MPa
It is possible to achieve an increase in strength from the grade of 1030 to 1180MPa.
Reduce plate thickness by 1 grade (eg 2.0 → 1.6mm
You can The effect of the present invention is exhibited even when the plate thickness is thick, but in the case of a steel plate having a thickness of more than 2.0 mm, the effect of strain aging intended by the present invention is difficult to be exhibited in the use of automobile steel plate. Therefore, the thickness of the steel sheet used in the present invention is 2.
It is desirable to use a thin steel plate of 0 mm or less.

[0054]

EXAMPLE A molten steel containing the components shown in Table 1 and the balance being substantially Fe was melted in a converter and made into a steel slab by a continuous casting method. Using these steel slabs as raw materials, the slabs were heated under the conditions shown in Table 2, rough rolling, and then a hot rolling step of performing finish rolling under the conditions shown in Table 2 was used to obtain hot-rolled sheets.

These hot-rolled sheets were pickled and the rolling reduction was 50% to 70%.
A cold-rolled sheet was obtained by a cold rolling process consisting of cold rolling. Next, these cold rolled sheets were continuously annealed under the conditions shown in Table 2 to obtain cold rolled annealed sheets. About the obtained cold rolled annealed plate, the amount of solid solution N and the tensile property were investigated. Further, the hole expansion ratio λ and the critical bending radius were investigated and the workability was examined. The survey method is as follows. (1) Investigation of the amount of solute N The amount of solute N was determined by subtracting the amount of precipitated N from the total amount of N in the steel obtained by chemical analysis. Here, the amount of deposited N was determined by the analysis method using the above-mentioned potentiostatic electrolysis method. (2) Tensile properties JIS No. 5 test pieces were taken from each cold rolled annealed sheet in the rolling direction,
Cross head speed: 10mm / in accordance with JIS Z 2241
Conduct a tensile test at min and yield stress YS, tensile strength T
S and elongation El were determined. (3) Hole expansion ratio λ Test pieces were taken from each cold-rolled annealed sheet, and specified by Japan Iron and Steel Federation
It was measured by the hole expansion test specified in JFS T1001. The hole expansion rate λ is a value defined by the following equation.

Λ (%) = {(D _H −D ₀ ) / D ₀ } × 100 where D _H : hole diameter after fracture, D ₀ : initial hole diameter (10 mm) (3) critical bending radius The No. 3 test piece (bending test piece) specified in JIS Z 2204 is sampled from the cold rolled annealed plate so that the direction perpendicular to the rolling direction is the longitudinal direction of the test piece, and specified in JIS Z 2248. A bending test was carried out by the pressing and bending method. After the test, the presence or absence of cracks is visually observed to obtain the minimum bending radius _Rmin without cracks, and the minimum bending radius R with respect to the plate thickness t (mm).
_The critical bending radius was evaluated by the ratio R _min / t of _min .

The results obtained are shown in Table 2.

[0058]

[Table 1]

[0059]

[Table 2]

Next, the cold-rolled annealed sheet having the above-mentioned characteristics was subjected to a press forming test under the conditions (R / t) shown in Table 3 to simulate press forming with a press machine.
Then, heat treatment under the conditions shown in Table 3 was performed. In the press molding test, as shown in FIG. 2, the plate to be tested is wound around a tool (die shoulder radius R) and pulled upward.

After the above-mentioned press molding test-heat treatment, tensile test pieces (JIS No. 5 test pieces) were sampled and subjected to a tensile test at a crosshead speed of 10 mm / min to obtain tensile properties (deformation stress YS _HT , Tensile strength TS _HT ), the difference between the press forming test and the tensile properties (yield stress YS, tensile strength TS) before heat treatment, the amount of increase in deformation stress ΔYS = YS _HT −YS,
The amount of increase in tensile strength ΔTS = TS _HT -TS was obtained.

The results obtained are shown in Table 3. For reference, Table 3 shows ΔT calculated by the equations (2) and (3).
S is also shown.

[0063]

[Table 3]

In each of the examples of the present invention, a high deformation stress increase amount ΔYS and a high tensile strength increase amount ΔTS are shown. on the other hand,
In the comparative example outside the scope of the present invention, the amount of increase in deformation stress ΔY
Both S and tensile strength increase amount ΔTS are only low values of 30 MPa or less. Further, it was found that in the examples of the present invention, the hole expansion ratio λ was large, the critical bending radius was small, and the workability was good.

Further, as an example, the case of forming a cold-rolled steel sheet was described, but the same examination was conducted for a hot-rolled steel sheet having a plate thickness of 2 mm or less, and it was confirmed that the effect of the present invention was obtained.

[0066]

EFFECTS OF THE INVENTION According to the present invention, a sufficient amount of solid solution N, which is obtained by optimizing the composition, hot rolling and cold rolling conditions, is secured, has good press formability, and further strain aging. Steel sheets with excellent hardening characteristics can be formed so that sufficient strength can be uniformly and stably ensured as parts, and a marked effect in industry is achieved.

[Brief description of drawings]

[Fig. 1] Press molding test-increase in tensile strength after heat treatment Δ
It is a graph which shows the influence of R / t which affects TS.

FIG. 2 is an explanatory diagram showing an outline of a press molding test.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/06 C22C 38/06 38/58 38/58 (72) Inventor Takashi Sakata Chuo-ku, Chiba City, Chiba Prefecture Kawasaki-cho No. 1 Kawasaki Steel Co., Ltd. Technical Research Institute F-term (reference) 4E063 AA01 AA20 BA01 CA03 DA03 MA18 4K042 AA25 BA01 CA02 CA03 CA05 CA06 CA08 CA09 CA12 CA13 CA14 DA05 DA06 DC02 DC03 4K043 AA01 AB01 AB02 AB03 AB07 AB16 AB13 AB13 AB15 AB18 AB20 AB21 AB22 AB25 AB26 AB27 AB29 AB30 AB33 BA01 BA03 BA05 BB04 BB06 BB08 DA00

Claims (4)

[Claims] 1. In mass%, C: 0.10% or less, Si: 0.1 to 1.0%, Mn: 0.1 to 3.5%, P: 0.05% or less, S: 0.01% or less, Al: 0.02% or less, N: 0.0050. ~ 0.025%, N / Al is 0.3 or more, N in solid solution state
When forming a high-strength steel sheet containing 0.0010% or more and the balance being Fe and unavoidable impurities into a predetermined shape, the bending radius R (mm) or die shoulder radius R (mm) at the time of the forming process is set. A method for forming a high-strength steel sheet, which comprises adjusting to satisfy the following formula (1). Note R / t ≦ 8 (1) where R: bending radius R (mm) or die shoulder radius R (m
m) t: Steel plate thickness (mm) 2. The composition according to claim 1, further comprising, in mass%, one or more groups selected from the following groups A, B and C: Method for forming high-strength steel sheet of. Note Group A: 1% or more of Cu, Ni, Cr, Mo or more total 1.0% or less, Group B: 1% or more of Nb, Ti, V, B 0.1 or more total 0.1 % Or less, C group: one or two of Ca and REM in total of 0.001 to 0.
010% 3. In mass%, C: 0.10% or less, Si: 0.1 to 1.0%, Mn: 0.1 to 3.5%, P: 0.05% or less, S: 0.01% or less, Al: 0.02% or less, N: 0.0050. ~ 0.025%, N / Al is 0.3 or more, N in solid solution state
In the method for producing a high-tensile steel plate-made component containing 0.0010% or more, the balance being a high-tensile steel plate having a composition consisting of Fe and unavoidable impurities, which is subjected to heat treatment, and then subjected to heat treatment. The bending radius R (mm) or die shoulder radius R (mm) was adjusted to satisfy the following formula (1), and the heat treatment was performed at 100-300 ° C for 30 minutes.
A method for producing a high-tensile steel plate part, characterized by performing heat treatment for s or more and 20 min or less. Note R / t ≦ 8 (1) where R: bending radius R (mm) or die shoulder radius R (m
m) t: Steel plate thickness (mm) 4. In addition to the above composition, further comprising:
The method for producing a high-tensile steel plate component according to claim 3, further comprising one group or two or more groups selected from the group, the group B, and the group C. Note Group A: 1% or more of Cu, Ni, Cr, Mo or more total 1.0% or less, Group B: 1% or more of Nb, Ti, V, B 0.1 or more total 0.1 % Or less, C group: one or two of Ca and REM in total of 0.001 to 0.
010%