JP2003321734A - High formability, high tensile strength, hot rolled steel sheet having excellent material uniformity, production method therefor and working method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high formability, high tensile strength, hot rolled steel sheet having excellent material uniformity which has a reduced variation in material in a coil, particularly that of yield strength in the width direction and can be industrially put into practical use, to provide a production method therefor, and to provide a working method therefor. <P>SOLUTION: The steel containing, by weight, ≤0.1% C and 0.02 to 0.2% Ti, and containing one ore more kinds of metals selected from 0.05 to 0.6% Mo and 0.01 to 1.5% W is cast. The steel is thereafter hot-rolled, is coiled round a coil, and is heat-treated. By the process, carbides containing Ti and one or more kinds of metals selected from Mo and W are dispersively precipitated in the ranges satisfying the following (1) into a structure mainly made up of ferrite, and the difference in yield stress between the central part and edge parts in the width direction of the steel sheet is ≤30 MPa: 0.1≤Ti/[48ä(Mo/96)+(W/184)}]≤3.5 (1). <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high formability and high strength hot rolled steel sheet which is suitable for a member used in a transportation machine such as an automobile and has excellent material uniformity, and a manufacturing method and a processing method thereof.

[0002]

2. Description of the Related Art In recent years, in the field of transportation machines representing automobiles, weight reduction of vehicle bodies has been studied for the purpose of improving fuel efficiency. As one of the studies for reducing the weight of the vehicle body, the strength of the used steel sheet is being promoted. Further, since the members for automobiles are difficult to process, workability is also required.

Japanese Unexamined Patent Publication No. 6-200351 and Japanese Unexamined Patent Publication No. 6-287685 propose a technique relating to a precipitation-strengthened high-strength hot-rolled steel sheet having a structure mainly composed of ferrite. In the former, the majority of the structure is made of polygonal ferrite to secure the required workability, and high strength is realized by precipitation strengthening and solid solution strengthening centering on TiC. In the latter case, most of the structure is made ferrite to secure the required workability, and the addition amount of Ti and Cu is controlled to increase the strength by precipitation strengthening and solid solution strengthening centering on TiC and Cu. ing.

However, in these steel sheets, TiC
The precipitation temperature of TiC is in a narrow range, and TiC is thermally unstable and easy to coarsen. Therefore, there is a change in the material in the width direction due to the difference in the cooling history between the center and the end in the width direction from the hot-rolled runout table to the coiler. There has been a problem that the yield strength becomes non-uniform between the central portion and the end portion in the width direction of the steel sheet due to the change in the cooling rate of the runout table-shaped strip caused by the change in the rolling speed.

[0005]

As described above, in the prior art, although a high-strength hot-rolled steel sheet having relatively good workability has been obtained, since the fluctuation of the tensile property in the coil becomes large, the steel sheet is This causes problems in the shape freezeability and the strength characteristics of the final product during press molding.

The present invention has been made in view of the above circumstances, and has a small change in the material in the coil, particularly a small change in the yield strength in the width direction, and is industrially practically excellent in high formability and high formability. It is an object of the present invention to provide a tension hot-rolled steel sheet and a manufacturing method and a processing method thereof.

[0007]

The inventors of the present invention have conducted extensive studies in order to reduce the fluctuation of the yield strength in the coil, and as a result, have found that the ferrite single-phase structure is formed of fine precipitates satisfying a predetermined atomic ratio. It was found that the strength can be reduced to a fluctuation within a desired range.

The present invention has been made based on such findings, and provides the following (1) to (12).

(1) C ≦ 0.1% by weight, Ti:
0.02 to 0.2% and Mo: 0.05 to 0.
6%, W: containing one or more selected from 0.01 to 1.5%, after casting, after hot rolling and winding into a coil,
By going through the heat treatment step, the ferrite structure is substantially added with Ti and M in a range satisfying the following formula (1).
Carbide containing at least one of o and W is dispersed and precipitated, and the difference in yield stress between the central portion and the end portion in the steel sheet width direction is 30 M.
A high-formability, high-strength hot-rolled steel sheet having excellent material uniformity, which is Pa or less. 0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1) However, in the above formula (1), Ti, Mo, and W are% by weight of each component. Represents

(2) In the above (1), in% by weight, Nb: 0.005 to 0.1%, V: 0.01 to
A high formability, high strength hot rolled steel sheet excellent in material uniformity, characterized by containing at least one selected from 0.1%.

(3) In weight%, C ≦ 0.1%, Si ≦
0.5%, Mn ≦ 2%, P ≦ 0.06%, S ≦ 0.01
%, Al ≦ 0.1%, N ≦ 0.006%, Cr ≦ 0.5
%, Ti: 0.02 to 0.2%, and Mo:
0.05 to 0.6%, W: 0.01 to 1.5%, and at least one selected from the group consisting essentially of Fe. After casting, it was hot rolled and wound into a coil. Later, through a heat treatment step, carbides containing Ti and at least one of Mo and W are dispersed and precipitated substantially in the ferrite structure in a range satisfying the following formula (1), and the steel plate width A high formability, high strength hot rolled steel sheet excellent in material uniformity, characterized in that the difference in yield stress between the central portion and the end portion in the direction is 30 MPa or less. 0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1) However, in the above formula (1), Ti, Mo, and W are% by weight of each component. Represents

(4) In weight%, C ≦ 0.1%, Si ≦
0.5%, Mn ≦ 2%, P ≦ 0.06%, S ≦ 0.01
%, Al ≦ 0.1%, N ≦ 0.006%, Cr ≦ 0.5
%, Ti: 0.02 to 0.2%, and Mo:
0.05 to 0.6%, W: 0.01 to 1.5%, at least one selected from, and Nb: 0.005 to 0.1
%, V: 0.01 to 0.1%, and the balance substantially consists of Fe. After casting, hot rolling is performed and wound into a coil, followed by a heat treatment step. As a result, a carbide containing at least one of Mo and W and Ti is dispersed and precipitated substantially in the ferrite structure in the range that satisfies the following formula (1), and the yield of the central portion and the end portion in the width direction of the steel sheet is obtained. A high formability, high tensile hot-rolled steel sheet having excellent material uniformity, which is characterized by a stress difference of 30 MPa or less. 0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1) However, in the above formula (1), Ti, Mo, and W are% by weight of each component. Represents

(5) In the above (1) or (3),
A high formability, high strength hot rolled steel sheet having excellent material uniformity, which contains C, Ti, and at least one of Mo and W so as to satisfy the following formula (2). 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} ≦ 1.5 (2) However, in the above formula (2), C, Ti, Mo and W represent the weight% of each component.

(6) In the above (2) or (4),
C, Ti, one or more kinds of Mo and W, and one or more kinds of Nb and V are contained so as to satisfy the following expression (3). Formable high-strength hot-rolled steel sheet. 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184) + (Nb / 93) + (V / 51)} ≦ 1.5 (3) , In the above formula (3), C, Ti, Mo, W, Nb,
V represents the weight% of each component.

(7) In any of the above (1) to (6), a high formability, high strength hot rolled steel sheet having excellent material uniformity, characterized in that it has a hot dip galvanized coating film on its surface.

(8) Upon heating a cast slab having any of the above-mentioned composition (1) to (6) to a temperature in the austenite single-phase region, hot rolling is performed, and finish rolling is completed at 800 ° C. or higher. And after winding at 675 ° C or lower,
A method for producing a high-formability, high-strength hot-rolled steel sheet excellent in material uniformity, which comprises heat treatment at 0 ° C.

(9) In the above (8), the heat treatment after winding is performed so as to satisfy the following expression (4), and a high formability and high strength hot rolled steel sheet excellent in material uniformity is manufactured. Method. 16500 ≦ (T + 273) {log (t) +1
6} ≦ 19500 (4) where T: heat treatment temperature (° C.), t: heat treatment time (second)

(10) A first step of preparing a member made of the steel sheet according to any one of the above (1) to (7), and press-forming the member to form a press-formed product having a desired shape. A method for processing a high-formability, high-strength hot-rolled steel sheet, which comprises two steps.

(11) In the above (10), the press-formed product is a method for processing a high-formability, high-strength hot-rolled steel sheet which is an automobile part, particularly an automobile underbody member.

(12) An automobile part manufactured from the steel sheet according to any one of (1) to (7) above.

According to the present invention having such a configuration, (1)
There is no or minimal precipitation of coarse Fe carbides that form a ferrite structure and whose shape changes with the manufacturing heat history of cementite, and (2) carbides that are stable in a wide temperature range are used throughout the steel sheet. By uniformly and finely precipitating in the steel sheet, a steel sheet having excellent yield strength uniformity can be obtained. Further, since the above-mentioned fine carbides are dispersed and precipitated substantially in the ferrite structure, high formability and high strength are realized.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. The hot rolled steel sheet according to the present invention has a weight percentage of C ≦ 0.
1%, containing Ti: 0.02-0.2%, and Mo:
By including at least one selected from 0.05 to 0.6% and W: 0.01 to 1.5%, after casting, hot rolling, winding into a coil, and then heat treatment. In a substantially ferrite structure, a carbide containing Ti and at least one of Mo and W is dispersed and precipitated in a range satisfying the following (1), and a central portion and an end in the width direction of the steel sheet. The difference in yield stress between the parts is 30 MPa or less. Furthermore, Nb: 0.005 to 0.1%, V: 0.01 to
One or more kinds selected from 0.1% may be contained. 0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1) (In the above formula (1), Ti, Mo and W are the weights of the respective components. Represents%.)

After the hot rolling and winding into the coil, the heat treatment is performed in order to make the fine carbide precipitates and the structure more uniform in the central portion and the end portion in the as-hot-rolled state more uniform. Is.

The reason why the matrix has a substantially ferrite structure is that the composite structure has to control the formation of two or more kinds of structures and it is difficult to realize material uniformity, whereas the ferrite single phase This makes it possible to eliminate the difficulty of controlling multiple organizations at the same time. For example, the morphology of Fe carbide changes depending on the heat history of the strip or coil, and if a large amount of this is contained, it causes a material variation.

In the present invention, it suffices that the ferrite structure is substantially formed, and it is not necessary that the ferrite structure is 100% completely. It should be ferrite. It is preferably at least 98%. Further, if the coarse Fe carbides other than the fine precipitates are less than 1% by volume, the effect of the present invention is not impaired.

In the hot-rolled steel sheet of the present invention in which the matrix is substantially ferrite, the strength is secured by the fine precipitates. In a fine precipitate composed of a carbide containing Ti and at least one of Mo and W, if these elements are not in an appropriate ratio, the form of the precipitate is likely to change due to the thermal history of the strip or coil, Rapid progress of coarsening due to heating. Therefore, the strength fluctuations in the coil caused by the difference in cooling rate between the center and the end in the width direction and the difference in cooling rate between the outer circumference and the center of the coil, which are observed in the coil as hot-rolled, are Large even if heat-treated. This is because the progress of coarsening due to heating is fast, so where the fine carbide was precipitated during the winding after hot rolling, the carbide was coarsened by the heat treatment, and the fine carbide was removed by the heat treatment when it was not precipitated during the winding. This is because precipitation occurs and strength fluctuations still occur in the coil. On the other hand, in the case of a carbide containing Ti and at least one of Mo and W, and when these elements are in a proper ratio, the progress of coarsening due to heating is slow. Therefore, when the coil is heat-treated as it is hot-rolled so that the inside of the coil has a uniform ferrite structure, fine carbide is uniformly precipitated in the entire area of the coil, and the strength fluctuation in the coil is reduced. Among the strength fluctuations, the effect on the fluctuation of the yield strength is remarkable.

The above Ti / [48 {(Mo / 96) +] calculated by the weight% of Ti, Mo and W in the fine carbide.
(W / 184)}] has a value of 0.1 to 3.5,
This is because if it is less than 0.1 or more than 3.5, fine carbides are not uniformly precipitated and the material variation cannot be reduced. It is preferably 0.7 to 1.5.

Next, the composition will be described. C: C is fixed as a carbide containing Ti, Mo and W,
It is an essential element for bearing the strength of steel. But,
If the content exceeds 0.1%, the ductility deteriorates due to the formation of coarse Fe carbide and the formation of island martensite. Therefore, the upper limit of the amount of C is set to 0.1%. From the viewpoint of reducing the amount of Fe carbide produced, 0.08% or less is desirable. On the other hand, in order to maintain the strength of 540 MPa or more, 0.01% or more is preferably contained.

Ti: Ti forms fine carbides together with Mo and W, and bears the strength of the steel sheet. However, if the content is less than 0.02%, it is insufficient to secure the necessary strength, while if it exceeds 0.2%, the transformation point is markedly increased, and finish rolling is completed in the austenite region. It becomes difficult to do so, and the workability decreases. Therefore, the Ti content is set to 0.02 to 0.2%.

Mo and W: Mo and W are both effective in delaying the coarsening of fine carbide due to heat, and are effective in reducing the strength variation in the coil through the heat treatment step after hot rolling. At least one is added. Mo is 0.05%
If it is less than 0.6%, it cannot be sufficiently precipitated as carbides. On the other hand, if it exceeds 0.6%, the hot strength becomes high and hot rolling becomes difficult. Therefore, the Mo content is 0.05 to 0.
It was 6%. It is preferably 0.5% or less. Also, W
If it is less than 0.01%, it cannot be sufficiently precipitated as carbides, while if it exceeds 1.5%, the hot strength becomes high and hot rolling becomes difficult. Therefore, the W content is set to 0.01 to 1.5%.

Nb, V: Nb and V together form a carbide and are effective for bearing the strength of the steel sheet, and at least one of them can be added. However, when Nb is 0.
If it is less than 005%, the effect of Nb carbide precipitation cannot be obtained, and if it exceeds 0.1%, the hot strength becomes high and hot rolling becomes difficult. If V is less than 0.01%, the effect of V carbide precipitation cannot be obtained, and if V exceeds 0.1%, the effect is saturated. Therefore, when Nb is added, its content is set to 0.005 to 0.1%, and when V is added, its content is set to 0.01 to 0.1%.

A more preferable component composition in the present invention is
% By weight, C ≦ 0.1%, Si ≦ 0.5%, Mn ≦ 2
%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.1
%, N ≦ 0.006%, Cr ≦ 0.5%, Ti: 0.0
2 to 0.2%, and Mo: 0.05 to 0.6
%, W: one or more selected from 0.01 to 1.5%, and if necessary, Nb: 0.005 to 0.1%,
V: contains one or more selected from 0.01 to 0.1%,
The balance consists essentially of Fe. The components other than C, Ti, Mo and W will be described below.

Si: Si has often been used as a solid solution strengthening element. However, Si produces red scale and deteriorates the surface properties. Therefore, the amount of Si is preferably 0.5% or less. Furthermore, 0.2% or less is desirable.

Mn: Mn is used as a solid solution strengthening element. However, if it exceeds 2%, segregation during casting tends to occur. Therefore, the Mn content is preferably 2% or less.

P: P is a solid solution strengthening element, but 0.06
%, The segregation to the grain boundaries is caused and the ductility is deteriorated, so 0.06% or less is preferable.

S: S is fixed as MnS and TiS. Therefore, S is Mn, Ti that effectively acts on the material properties.
Since the amount is reduced and the ductility is also reduced, 0.0
It is preferably 1% or less. More preferably 0.005% or less

Al: Al in steel is used as a deoxidizing material. However, if the content exceeds 0.1%, the ductility of the steel is reduced, so 0.1% or less is preferable.

N: N is an impurity in steel. If its content exceeds 0.006%, it causes coarse nitride formation which reduces ductility, so 0.006% or less is preferable.

Cr: Cr has an effect of suppressing ferrite transformation. After finish rolling, if ferrite transformation proceeds at 750 ° C or higher on the runout table, coarse Ti, Mo, W
Since the carbide containing carbon is formed, and the coarse carbide once formed cannot be refined again due to the subsequent thermal history, it becomes impossible to effectively strengthen the steel. Can be suppressed. However, if Cr exceeds 0.5%, the surface properties are deteriorated, so 0.5% or less is preferable. In order to exert such effects effectively, 0.04% or more is preferable.

In the present invention, it is preferable that the following formula (2) is satisfied in addition to the above composition. 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} ≦ 1.5 (2) (However, in the above formula (2), C, Ti , Mo, W represent the weight% of each component.) This is the atomic number ratio of C and (Ti + Mo + W) in steel,
That is, (C / 12) / {(Ti / 48) + (Mo /
By adjusting the contents of C, Ti, Mo, and W such that the value of (96) + (W / 184)} is 0.5 to 1.5, Ti, which effectively secures strength, This is because the carbide containing Mo and W is likely to be homogeneously and finely dispersed and precipitated.
If the value of the above formula (2) is less than 0.5, the strength cannot be made uniform throughout the coil, and if it exceeds 1.5, carbides are coarsened and pearlite that impairs stretch flangeability is formed. It deteriorates workability.

In addition to C, Ti, Mo and W, Nb,
When one or more kinds of V are added, it is preferable that the following formula (3) is satisfied. 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184) + (Nb / 93) + (V / 51)} ≦ 1.5 (3) ( However, in the above formula (3), C, Ti, Mo, W, N
b and V represent the weight% of each component. ) In this case, the same as the above case, C in the steel and (Ti + M
o + W + Nb + V), the number ratio of atoms, that is, (C / 1
2) / {(Ti / 48) + (Mo / 96) + (W / 18
4) + (Nb / 93) + (V / 51)} is 0.5 to
This is because by adjusting the contents of C, Ti, Mo, W, Nb, and V so as to be 1.5, the carbides are likely to be homogeneously and finely dispersed and precipitated. The value of the above equation (3) is 0.
If it is less than 5, the strength of the entire coil cannot be made uniform,
If it exceeds 1.5, carbides are coarsened and pearlite is formed which impairs stretch flangeability, resulting in deterioration of strength and workability.

(C / 12) / {(Ti / 48) +
The value of (Mo / 96) + (W / 184)}, and (C /
12) / {(Ti / 48) + (Mo / 96) + (W / 1
The value of 84) + (Nb / 93) + (V / 51)} is 0.8
~ 1.3 is more desirable.

Elements other than the above may be added in trace amounts within the range where the effects of the present invention are maintained, and other unavoidable impurity elements may be contained.

Next, preferable manufacturing conditions of the hot rolled steel sheet of the present invention as described above will be described. Here, after heating the steel having the above-mentioned composition to the temperature of the austenite single phase region,
In hot rolling, finish rolling is completed at 800 ° C. or higher, wound at 675 ° C. or lower, and then heat treated at 575 to 750 ° C.

Bringing the slab to austenite single phase region temperature: Prior to hot rolling, the slab is brought to austenite single phase region temperature in order to form a solid solution of Ti, Nb, V, Mo and W carbides. . Desirably, the temperature is 1200 ° C. or higher.

Finishing rolling temperature of 800 ° C. or higher: The finishing rolling temperature is important for making the material uniform. If the temperature is lower than 800 ° C., strain-induced precipitation of carbide occurs during rolling, but since this precipitation occurs at high temperature, the carbide does not coarsen and does not become finer again in the subsequent heat history.
Carbides do not work effectively to increase strength. Therefore, the finish rolling temperature is set to 800 ° C. or higher.

Winding temperature: 675 ° C. or lower: In the steel of the present invention, it is necessary that the carbide is not precipitated or the carbide is finely precipitated after the hot rolling and winding. If the coiling temperature exceeds 675 ° C., the carbides after hot rolling and coiling will become coarse and will not be refined again due to the subsequent heat history, so that the steel cannot be effectively strengthened. Therefore, the winding temperature is set to 675 ° C or lower.

Heat treatment at 575 to 750 ° C .: In the present invention,
The coil that has been hot rolled and wound is further heat treated. This enhances the material uniformity over the entire coil. If the heat treatment temperature is lower than 575 ° C, homogenization of ferrite structure and carbide precipitation cannot be achieved in the entire area of the coil, and if it exceeds 750 ° C, the carbide is coarsened and the strength decreases.

More preferable heat treatment conditions after winding the hot rolled coil are as shown in the following equation (4). 16500 ≦ (T + 273) {log (t) +16} ≦ 19500 (4) (where T: heat treatment temperature (° C.), t: heat treatment time (sec)) Thus, (T + 273) {log (t) +16 } Value of 16500 or more further enhances the uniformity of yield strength. However, if the value exceeds 19,500, the coarsening of the carbide proceeds, making it difficult to effectively exert the strengthening effect of the carbide, which is a feature of the present invention.

The high-strength hot-rolled steel sheet of the present invention includes a hot-dip galvanized steel sheet having a hot-dip galvanized coating film formed on the surface thereof. Since the high-strength hot-rolled steel sheet of the present invention has good workability, good workability can be maintained even if a hot dip galvanized coating film is formed. Here, the hot-dip zinc-based plating is hot-dip plating mainly composed of zinc and zinc, and includes those containing alloy elements such as Al and Cr in addition to zinc. The high-strength hot-rolled steel sheet of the present invention which has been subjected to such hot dip galvanizing may be subjected to an alloying treatment as it is or after the plating. Regarding the annealing temperature before plating, if the temperature is lower than 450 ° C., plating does not occur, and if it exceeds 750 ° C., the strength tends to decrease. Therefore, the annealing temperature is 450
C. or higher and 750.degree. C. or lower are preferable.

The hot-rolled steel sheet of the present invention has no difference in its characteristics whether it is a black skin or a pickling material. The temper rolling is not particularly limited as long as it is usually performed. Also,
The hot-dip galvanizing does not cause any problem even after pickling or as black skin. Electroplating is also possible for zinc plating. There is no particular problem with the chemical conversion treatment. The effect of the present invention is not affected even if direct-feed rolling is performed immediately after casting or after performing heating for the purpose of supplementing heat and then performing hot rolling as it is.
Further, after the rough rolling and before the finish rolling, even if the rolled material is heated, or after the rough rolling, the continuous rolling is performed by joining the rolled materials, or even if the heating and the continuous rolling of the rolled material are performed at the same time. The effect of the present invention is not impaired.

The hot-rolled steel sheet of the present invention is excellent in formability and has little fluctuation in material inside the coil.
By taking advantage of its characteristics, it is possible to manufacture an automobile member, in particular, a member having a complicated cross-sectional shape at the time of pressing such as a suspension member such as a suspension arm with good quality.
It can contribute to the weight reduction of the press-molded product. Hereinafter, a method for processing a hot-rolled steel sheet according to the present invention, in other words, a method for manufacturing a press-formed product will be specifically described.

FIG. 1 is a flow chart showing an example of a work flow of a method for processing a hot rolled steel sheet according to the present invention. This work flow usually has a pre-process of manufacturing the steel sheet according to the present invention or transporting the manufactured steel sheet into a coil, for example, and delivering it to a target place. First, the hot-rolled steel sheet according to the present invention is prepared. Start by doing (S0, S
1). Before this steel sheet is pressed, the steel sheet may be pre-processed (S2) or may be processed into a predetermined size or shape by a cutting machine (S2).
3). In the former step S2, for example, a press-formed product or a press-formed product having a predetermined size and shape is formed by cutting or punching at a predetermined position in the width direction of the steel sheet, and at a stage after the subsequent press working or in the process of the press working. Be prepared so that it can be separated as a member. In the latter step S3, the size, shape, etc. of the final press-formed product are taken into consideration in advance so that a steel plate member having a predetermined size and shape is processed (and thus cut). After that, the member that has gone through the steps of S2 and S3 is subjected to press working, and finally a desired press-formed product having a desired size and shape is manufactured (S4). This press working is usually carried out in multiple stages, often in three stages or more and seven stages or less.

The step S4 may include a step of further cutting the member having undergone the steps S2 and S3 into a predetermined size and shape. The operation of "cutting" in this case is, for example, an operation of separating unnecessary portions from the final press-formed product such as the end portion of the member that has passed through the steps of S2 and S3 at least in the process of press working. Alternatively, it may be an operation of separating the member to be pressed along the widthwise cuts or perforations of the steel plate provided in the step S2.

In FIG. 1, N1 to N3 are steel plates,
In some cases, the member and the press-formed product may be transported mechanically or by an operator.

The press-formed product thus manufactured is sent to the next step as needed. As the next step, for example,
The process of further machining the press-formed product to adjust the dimensions and shape, the process of transporting the press-formed product to a predetermined place and storing it, the step of surface-treating the press-formed product, and the automobile molding using the press-formed product. There is an assembly process for assembling such an object.

FIG. 2 is a block diagram showing the relationship between the apparatus for actually performing the work shown in FIG. 1 and the flow of steel plates, members, and press-formed products. In this figure, the hot-rolled steel sheet according to the present invention is prepared in a coil shape, and a press-formed product is manufactured by a press machine. The press machine is a model that performs multi-stage pressing, but the present invention is not limited to this.

There are cases where a cutting machine and other pretreatment machines are installed in front of the press machine (FIG. 2 (a)), and cases where they are not installed (FIG. 2 (b)). When a cutting machine is installed, a member having a required size or shape is cut from a long steel plate according to the present invention supplied from a coil, and this member is pressed by a press working machine to obtain a predetermined press. It becomes a molded product. When a pretreatment machine for making notches and perforations in the width direction of the steel sheet is installed, cutting may be performed along the notches and perforations in the press machine. When the pretreatment machine is not installed, cutting is performed in the process of pressing the steel sheet by the press machine, and finally a press-formed product having a predetermined size and shape is manufactured. The meaning of “cutting” in FIG. 2 is the same as the cutting in FIG.

The press-formed product produced in this manner uses the steel sheet according to the present invention as a raw material thereof, which has excellent surface properties and ductility, and has little fluctuation in the material in the coil, so that it has good and uniform quality. The production yield of such press-molded products is high. Such a feature is particularly useful when the press-formed product is an automobile member, particularly a suspension member such as a suspension arm.

[0060]

(Example) (Example 1) Steel having the chemical composition shown in Table 1 was melted, the heating temperature was 1250 ° C, and the finishing rolling temperature was about 900.
Hot rolled at a coiling temperature of about 610 ° C and a sheet thickness of 2.
A 3 mm steel plate was produced. A sample for tensile test and a sample for hole expansion test were taken from the coil, and then the hot rolled coil was heat-treated in a continuous annealing facility. At this time, the conditions for heating in the continuous annealing equipment were 700 ° C. and 2 minutes. A sample for tensile test and a sample for hole expansion test were taken from the obtained final heat-treated steel sheet, and a thin film was taken, and observation of precipitates by a transmission electron microscope (TEM) and Ti, Mo, W in the precipitates, The composition of Nb and V was analyzed by an energy dispersive X-ray spectrometer (EDX) equipped in the TEM. The structure of the matrix was observed with a scanning electron microscope (SEM). The results are also shown in Table 1. In Table 1, the A value is Ti / [48 {(M
o / 96) + (W / 184)}], and the B value is
(C / 12) / {(Ti / 48) + (Mo / 96) +
(W / 184)} or (C / 12) / {(Ti / 4
8) + (Mo / 96) + (W / 184) + (Nb / 9
3) + (V / 51)}.

The tensile test was conducted by collecting JIS No. 5 test pieces in the coil longitudinal direction from the center and the end in the width direction. The yield stress in the width direction was compared by the absolute value of the strength difference.
The hole expanding test for evaluating the stretch flangeability (λ) of the steel sheet was performed according to the Japan Iron and Steel Federation Standard. These characteristics are shown in Table 2.

As shown in Table 1, No. 1 to 13 have a chemical composition within the range of the present invention, the matrix structure is ferrite, and the A value of the precipitate is 0.1 or more and 3.5.
The following are examples of the present invention, and as shown in Table 2, good material characteristics and material uniformity could be obtained.

On the other hand, in the case of No. 14
In addition, pearlite is generated, the stretch-flange formability is deteriorated, and the yield strength of the center portion and the end portion in the width direction after heat treatment is not uniform. Perlite is formed and A value is 0.1
No. less than In No. 15, the stretch flangeability is particularly low, and the yield strength of the center portion and the end portion in the width direction after heat treatment is also nonuniform. No. A having an A value of less than 0.1 and a B value of less than 0.5. Even in No. 16, the yield strength non-uniformity at the center portion and the end portion in the width direction is not eliminated after the heat treatment. A value is less than 0.1,
No. B value exceeding 1.5. In No. 17, since pearlite is generated, stretch flangeability is low, strength reduction in the widthwise central portion is large after heat treatment, and uneven yield strength in the widthwise central portion and the end portion is not eliminated. No. A value exceeding 3.5. In No. 18, the uneven yield strength in the width direction after heat treatment is not eliminated.

[0064]

[Table 1]

[0065]

[Table 2]

(Example 2) Steel Nos. Shown in Table 1 Using the cast piece of No. 6, hot rolling was carried out at a heating temperature of 1250 ° C., a finish rolling temperature of about 900 ° C., and a winding temperature of about 600 ° C. to produce a hot rolled coil having a plate thickness of 2.3 mm. From that coil,
Take a sample for tensile test from the center and end in the width direction,
Heat treatment was performed under various conditions. FIG. 3 shows the stress difference ΔYS of the yield strength between the central portion and the end portion in the width direction at the heat treatment temperature and the heat treatment time in that case. As shown in this figure, when the heat treatment temperature is 550 ° C., ΔYS does not decrease so much from the as-rolled state.
It became Pa or less. From this result, the heat treatment temperature was 575 ° C.
From the above, it is understood that a low ΔYS can be obtained. Also,
FIG. 4 shows the heat treatment temperature and heat treatment time at that time.
The tensile strength TS of the central portion in the width direction is shown. As shown in this figure, it is understood that when the heat treatment temperature exceeds 750 ° C., TS is significantly reduced and the strengthening effect by the carbide disappears.

The heat treatment temperature and time and Δ obtained in FIG.
From the value of YS, (T + 273) {log (t) +1
6} and ΔYS were determined. The result is shown in FIG. As shown in FIG. 5, (T + 273) {log (t)
It was confirmed that ΔYS is particularly reduced when +16} is 16500 or more. Further, from the heat treatment temperature and time obtained in FIG. 4 and the value of the tensile strength TS in the widthwise central portion, (T
+273) The relationship between {log (t) +16} and the tensile strength TS of the width direction center part was calculated | required. The result is shown in FIG.
As shown in FIG. 6, (T + 273) {log (t) +1
6} was 19500 or less, it was confirmed that the decrease in TS was particularly small.

[0068]

As described above, according to the present invention,
Appropriately controlling the composition of the steel, after casting, after hot rolling and winding into a coil, through a heat treatment step, substantially in the ferrite structure, Ti of a specific composition, and
Since the carbide containing at least one of Mo and W is dispersed and precipitated, the precipitation of coarse Fe carbide whose shape changes due to the manufacturing heat history of cementite and the like can be suppressed to a minimum and a wide temperature range can be obtained. By uniformly and finely precipitating carbides that are stable throughout the steel sheet, a steel sheet having excellent yield strength uniformity can be obtained. Further, since the above-mentioned fine carbides are dispersed and precipitated substantially in the ferrite structure, high formability and high strength are realized.

[Brief description of drawings]

FIG. 1 is a flowchart showing an example of a work flow of a hot-rolled steel sheet processing method according to the present invention.

FIG. 2 is a block diagram showing the relationship between the apparatus for actually performing the work shown in FIG. 1 and the flow of steel plates, members, and press-formed products.

FIG. 3 shows various heat treatment temperatures and heat treatment times.
The graph which shows the stress difference (DELTA) YS of the yield strength of a width direction center part and an edge part.

FIG. 4 shows various heat treatment temperatures and heat treatment times.
The graph which shows the tensile strength TS of the width direction center part.

FIG. 5: Heat treatment temperature and time and ΔYS obtained in FIG.
And (T + 273) {log (t) +1
6} and a graph showing the relationship between ΔYS.

6 is obtained from the heat treatment temperature and time obtained in FIG. 4 and the value of the tensile strength TS at the center portion in the width direction, (T + 27).
3) A graph showing the relationship between {log (t) +16} and the tensile strength TS at the widthwise central portion.

Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/58 C22C 38/58 (72) Inventor Kunikazu Tomita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Stocks In-company F-term (reference) 4K037 EA01 EA04 EA05 EA11 EA15 EA17 EA18 EA19 EA23 EA25 EA27 EA31 EA32 EA33 EB06 EB08 FA00 FA01 FA02 FA03 FB00 FC03 FC04 FC05 FE01 FE02 FE03 FF01 FF02 GA05 HA03 JA03 HA03

Claims (12)

[Claims] 1. By weight%, C ≦ 0.1%, Ti: 0.02 to 0.2% is included, and Mo: 0.05 to 0.6%, W: 0.01 to 1.5. % Of at least one selected from the group consisting of%, and after being cast, hot-rolled, wound into a coil, and then subjected to a heat treatment, so that the ferrite structure is substantially satisfied within a range satisfying the following formula (1). , Ti and at least one of Mo and W are dispersed and precipitated, and the difference in yield stress between the central portion and the end portion in the steel sheet width direction is 30 MPa.
A high formability, high tensile hot-rolled steel sheet excellent in material uniformity, characterized in that: 0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1) However, in the above formula (1), Ti, Mo, and W are% by weight of each component. Represents 2. The composition further comprises, by weight, at least one selected from Nb: 0.005 to 0.1% and V: 0.01 to 0.1%.
High-formability, high-strength hot-rolled steel sheet with excellent material uniformity as described in. 3. By weight%, C ≦ 0.1%, Si ≦ 0.5%, Mn ≦ 2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.1%, N ≤ 0.006%, Cr ≤ 0.5%, Ti: 0.02 to 0.2% are included, and Mo: 0.05 to 0.6% and W: 0.01 to 1.5% are selected. And the balance is substantially composed of Fe. After casting, hot rolling, winding into a coil, and heat treatment are carried out, so that the ferrite structure is substantially converted into the following (1 ), The carbide containing Ti and at least one of Mo and W is dispersed and precipitated, and the difference in the yield stress between the central portion and the end portion in the steel sheet width direction is 30 MPa.
A high formability, high tensile hot-rolled steel sheet excellent in material uniformity, characterized in that: 0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1) However, in the above formula (1), Ti, Mo, and W are% by weight of each component. Represents 4. By weight%, C ≦ 0.1%, Si ≦ 0.5%, Mn ≦ 2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.1%, N ≦ 0.006%, Cr ≦ 0.5%, Ti: 0.02 to 0.2%, Mo: 0.05 to 0.6%, W: 0.01 to 1.5% After casting, containing at least one selected from Nb: 0.005 to 0.1%, and at least one selected from V: 0.01 to 0.1%, the balance being substantially Fe. After hot rolling and winding into a coil, a heat treatment step is performed, whereby substantially one or more of Mo and W and Ti are added to the ferrite structure in a range satisfying the following formula (1). Carbide containing is dispersed and precipitated, and the difference in yield stress between the central portion and the end portion in the steel sheet width direction is 30 MPa.
A high formability, high tensile hot-rolled steel sheet excellent in material uniformity, characterized in that: 0.1 ≦ Ti / [48 {(Mo / 96) + (W / 184)}] ≦ 3.5 (1) However, in the above formula (1), Ti, Mo, and W are% by weight of each component. Represents 5. The material according to claim 1, wherein C, Ti, and one or more kinds of Mo and W are contained so as to satisfy the following expression (2). High formability and high strength hot rolled steel sheet with excellent uniformity. 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} ≦ 1.5 (2) However, in the above formula (2), C, Ti, Mo and W represent the weight% of each component. 6. C, Ti, one or more kinds of Mo and W, and one or more kinds of Nb and V are contained so as to satisfy the following expression (3). The high-formability, high-strength hot-rolled steel sheet having excellent material uniformity according to claim 2 or claim 4. 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184) + (Nb / 93) + (V / 51)} ≦ 1.5 (3) , In the above formula (3), C, Ti, Mo, W, Nb,
V represents the weight% of each component. 7. The high formability, high tensile hot-rolled steel sheet excellent in material uniformity according to any one of claims 1 to 6, which has a hot dip galvanized coating film on its surface. 8. A slab having the composition of any one of claims 1 to 6 is heated to a temperature in the austenite single-phase region and then hot-rolled, and finish rolling is completed at 800 ° C. or higher. After winding at 675 ° C or lower, 575 to 750
A method for producing a high-formability, high-strength hot-rolled steel sheet having excellent material uniformity, which comprises heat treatment at ℃. 9. The method for producing a high-formability, high-strength hot-rolled steel sheet having excellent material uniformity according to claim 8, wherein the heat treatment after winding is performed so as to satisfy the following expression (4). . 16500 ≦ (T + 273) {log (t) +16} ≦ 19500 (4) where T: heat treatment temperature (° C.), t: heat treatment time (second) 10. A first step of preparing a member made of the steel sheet according to any one of claims 1 to 7, and a step of press-forming the member to form a press-formed product having a desired shape. A method for processing a high-formability, high-strength hot-rolled steel sheet, which comprises two steps. 11. The method for processing a high-formability, high-strength hot-rolled steel sheet according to claim 10, wherein the press-formed product is an automobile part. 12. An automobile part manufactured from the steel sheet according to any one of claims 1 to 7.