JP2003321739A - High tensile strength hot rolled steel sheet having excellent workability, production method therefor and working method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength hot rolled steel sheet which has excellent elongation and stretch-flanging properties, and is suitable for an automobile member, to provide a production method therefor, and to provide a working method therefor. <P>SOLUTION: The high tensile strength hot rolled steel sheet has a composite structure containing, by a volume ratio, 5 to 70% bainite, and the balance substantially ferrite. Precipitates containing Ti and Mo in the ranges satisfying the following relationship (1) are dispersively precipitated into the ferrite: (Mo/96)/ä(Ti/48)+(Mo/96)}≥0.25 (1); wherein, Ti and Mo denote the weight % of each component. <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-strength hot-rolled steel sheet having a tensile strength of 690 MPa or more and excellent workability, which is suitable for a member used in a transportation machine such as an automobile, and a manufacturing method and a processing method thereof. .

[0002]

2. Description of the Related Art Ferrite is used as a steel sheet with excellent workability.
Bainite composite structure steel sheets have been proposed. As such a technique, for example, the technique disclosed in JP-A-6-240356 can be mentioned. This technique aims to improve elongation and stretch flangeability by devising manufacturing conditions and producing a large amount of TiN. However, in this technique, since the hardness difference between ferrite and bainite is large, the stretch flangeability is insufficient for press-forming a complicated shape such as a member for automobiles.

Further, the technique disclosed in Japanese Patent Application Laid-Open No. 8-199298 aims at improving the balance between elongation and stretch-flangeability in ferrite-bainite composite structure steel sheet by fine-graining ferrite and strengthening precipitation by TiC. It is a thing. However, since TiC has a very high tendency to form Ti carbide, it tends to be coarsened, and if it is attempted to strengthen ferrite, a large amount of TiC is required and the ductility of ferrite itself is impaired.

[0004]

As described above, the ferrite-bainite composite structure steel sheet disclosed as the prior art has a problem that it cannot cope with severe working conditions.

The present invention has been made in view of the above circumstances, and is a high-strength hot-rolled steel sheet having excellent workability represented by elongation and stretch-flangeability, which is suitable for automobile members, and a manufacturing method and a processing method thereof. The purpose is to provide.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted extensive research to obtain a high-strength hot-rolled steel sheet having excellent elongation and stretch-flangeability. It has been found that by strengthening with a fine precipitate satisfying a specific element ratio, high tensile strength and excellent workability are exhibited. That is, by using fine precipitates, the ferrite is strengthened with a minimum amount of precipitates, so that the reduction in ductility can be avoided as much as possible.

The present invention has been completed based on these findings and provides the following (1) to (12).

(1) A composite structure containing bainite in an amount of 5 to 70% by volume and the balance being substantially ferrite.
A high-strength hot-rolled steel sheet excellent in workability having a tensile strength of 690 MPa or more, characterized in that precipitates containing Mo and Mo are dispersed and precipitated. (Mo / 96) / {(Ti / 48) + (Mo / 96)} ≧ 0.25 (1) However, in the above formula (1), Ti and Mo are% by weight of each component in the precipitate. Indicates.

(2) It has a composite structure containing bainite in an amount of 5 to 70% by volume, and the balance substantially consisting of ferrite, and T within a range satisfying the following expression (2) in ferrite.
A high-tensile hot-rolled steel sheet having a tensile strength of 690 MPa or more and excellent workability, characterized in that precipitates containing i, Mo, and W are dispersed and precipitated. {(Mo / 96) + (W / 184)} / {(Ti / 48) + (Mo / 96) + (W / 184)} ≧ 0.25 (2) However, in the above formula (2) , Ti, Mo, and W represent the weight% of each component in the precipitate.

(3) In the above item (1), in% by weight,
C: 0.03 to 0.1%, Si ≦ 2.0%, Mn: 0.
5 to 1.7%, P ≦ 0.06%, S ≦ 0.01%, Al
≤0.1%, N≤0.006%, Mo: 0.1-0.5
%, Ti: 0.07 to 0.2%, the balance being substantially Fe, which is a high-strength hot-rolled steel sheet excellent in workability.

(4) In the above item (1), in% by weight,
C: 0.03 to 0.1%, Si ≦ 2.0%, Mn: 0.
5 to 1.7%, P ≦ 0.06%, S ≦ 0.01%, Al
≤0.1%, N≤0.006%, Mo: 0.1-0.5
%, Ti: 0.07 to 0.2%, and Nb: 0.
005 to 0.02%, and V: 0.01 to 0.15%
A high-strength hot-rolled steel sheet having excellent workability, comprising at least one of the above, and the balance being substantially Fe.

(5) In the above (3) or (4),
A high-strength hot-rolled steel sheet having excellent workability, which contains C, Ti, and Mo so as to satisfy the following formula (3). 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (3) However, in the formula (3), C, Ti, and Mo are each in steel. Represents the weight percent of the component.

(6) In the above item (2), in% by weight,
C: 0.03 to 0.1%, Si ≦ 2.0%, Mn: 0.
5 to 1.7%, P ≦ 0.06%, S ≦ 0.01%, Al
≤0.1%, N≤0.006%, Mo: 0.1-0.5
%, W: 0.01 to 1.0%, Ti: 0.07 to 0.2
%, And the balance is substantially Fe, which is a high-tensile hot-rolled steel sheet excellent in workability.

(7) In the above item (2), in% by weight,
C: 0.03 to 0.1%, Si ≦ 2.0%, Mn: 0.
5 to 1.7%, P ≦ 0.06%, S ≦ 0.01%, Al
≤0.1%, N≤0.006%, Mo: 0.1-0.5
%, W: 0.01 to 1.0%, Ti: 0.07 to 0.2
%, And at least one of Nb: 0.005 to 0.02% and V: 0.01 to 0.15%, the balance being substantially Fe. Excellent high-strength hot-rolled steel sheet.

(8) In the above (6) or (7),
A high-strength hot-rolled steel sheet having excellent workability, which contains C, Ti, Mo, and W so as to satisfy the following formula (4). 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} ≦ 1.5 (4) However, in the formula (4), C, Ti, Mo and W represent the weight% of each component in steel.

(9) Immediately or at 1200 ° C. after slabs made of steel having the composition of any of (3) to (8) above.
When heating to the above temperature and performing hot rolling, 8
Finished at 00 ℃ or higher, and then from 600 to 750 ℃ 2
After cooling at a cooling rate of 0 ° C / sec or more, after air cooling for 3 seconds or more, it is cooled to 575 to 350 ° C at a cooling rate of 10 ° C / sec or more and wound up. An excellent method for producing a high-strength hot-rolled steel sheet.

(10) The first step of preparing a member made of the high-strength hot-rolled steel sheet according to any one of (1) to (8) above, and press-molding the member with a desired shape. A method for processing a high-strength hot-rolled steel sheet, comprising:

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

(12) An automobile part manufactured from the high-strength hot-rolled steel sheet according to any one of (1) to (8).

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. The high-strength hot-rolled steel sheet according to the present invention has a composite structure containing bainite in a volume ratio of 5 to 70% and the balance substantially consisting of ferrite. The composite structure containing bainite in such a ratio is that in the present invention, ductility is ensured by soft ferrite and strengthened by hard bainite,
For that purpose, it is important to control the volume ratio of bainite, and if the volume ratio of bainite is less than 5%, sufficient strength cannot be obtained, and if it exceeds 70%, the ductility of the entire material is impaired.

In the present invention, the following (1) is added to ferrite.
Precipitates containing Ti and Mo are dispersed and deposited within a range that satisfies the formula. (Mo / 96) / {(Ti / 48) + (Mo / 96)} ≧ 0.25 (1) (In the above formula (1), Ti and Mo are the weight of each component in the precipitate. % Is shown.)

In this way, it is possible to disperse and deposit the precipitates containing Ti and Mo within the range satisfying the above formula (1).
It is important to the present invention. Precipitates containing Ti and Mo containing Mo in a specific ratio or more suppress growth or coarsening after precipitation, which is different from Ti carbides that are well known in the related art. This is because strengthening can be realized. This leads to avoiding a decrease in ductility of ferrite as much as possible. However, (Mo / 96) / {(Ti / 48) + (Mo / 9
6)} is less than 0.25, the effect of suppressing coarsening of precipitates is insufficient, so this value was made 0.25% or more.

In the present invention, the following (2) is added to ferrite.
Precipitates containing Ti, Mo and W may be dispersed and deposited within a range satisfying the formula. {(Mo / 96) + (W / 184)} / {(Ti / 48) + (Mo / 96) + (W / 184)} ≧ 0.25 (2) (However, the above formula (2) In the above, Ti, Mo and W represent% by weight of each component in the precipitate.) As described above, by dispersing and depositing a precipitate containing Ti, Mo and W in the range satisfying the above formula (2), The same effect as that of the precipitate containing Ti and Mo can be obtained. This is because W has the same effect as Mo and can replace Mo. In the case of this value as well, if it is less than 0.25, the effect of suppressing coarsening of precipitates is insufficient, so this value was made 0.25% or more. This Ti, Mo
One or more of Nb and V may be added to the precipitate containing and.

By forming the metal structure as described above, a high-strength hot-rolled steel sheet excellent in elongation and stretch-flangeability and having a tensile strength of 690 MPa or more can be realized.

In the present invention, as long as the above metal structure is satisfied, the desired elongation and stretch flangeability and 690MP are obtained.
A strength of a or more is obtained, and the chemical composition is not particularly limited, but in the case where the precipitate contains Ti and Mo, the weight% is
C: 0.03 to 0.1%, Si ≦ 2.0%, Mn:
0.5-1.7%, P ≦ 0.06%, S ≦ 0.01%,
Al ≦ 0.1%, N ≦ 0.006%, Mo: 0.1
0.5%, Ti: 0.07 to 0.2%, the balance is preferably substantially Fe, and the precipitate is Ti or Mo.
And when W is included, W: 0.01 to 1.0
% Is preferable. Further, in any of these cases, it is preferable that at least one of Nb: 0.005 to 0.02% and V: 0.01 to 0.15% is further contained. Hereinafter, each of these components will be described.

C: C is fixed in the ferrite phase as a carbide containing Ti and Mo or a carbide containing Ti, Mo and W, and contributes to an increase in strength. However, 0.
If it is less than 03%, it is difficult to maintain and secure the tensile strength of 690 MPa or more, and if it exceeds 0.1%, coarse Fe carbides are generated, and the stretch flangeability is deteriorated.
Therefore, the C content is preferably 0.03 to 0.1%.

Si: Si promotes the formation of ferrite. However, if it exceeds 2.0%, the steel becomes brittle, so the Si content is preferably 2.0% or less.

Mn: Mn suppresses the formation of cementite and promotes the formation of bainite. However, if it is less than 0.5%, the effect of suppressing the generation of cementite is low, and if it exceeds 1.7%, segregation during casting of steel becomes remarkable and workability is deteriorated. Therefore, the Mn content is preferably 0.5 to 1.7%.

P: P is a solid solution strengthening element, but 0.06
If added in excess of 0.1%, remarkable segregation to the grain boundaries is caused and ductility is reduced, so 0.06% or less is preferable.

Since S: S is fixed as MnS and TiS, it reduces the amounts of Mn and Ti that effectively act on the strength, and also reduces the ductility.
Therefore, the S content is preferably 0.01% or less,
It is more preferably 005% or less.

Al: Al is added as a deoxidizer. 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 the steel, and if its content exceeds 0.006%, it causes the formation of coarse nitrides that reduce the ductility, so 0.006% or less is preferable.

Mo: Mo is an important element in the present invention and forms fine carbides together with Ti or Ti and W and suppresses coarsening of precipitates. But 0.1
If it is less than 0.5%, the effect is small, and if it exceeds 0.5%, the formation of ferrite is hindered. Therefore, the Mo content is 0.
1 to 0.5% is preferable.

Ti: Ti forms fine precipitates together with Mo or Mo and W and strengthens ferrite. However, if it is less than 0.07%, it is difficult to secure the necessary strength, and if it exceeds 0.2%, the transformation point is remarkably increased, and it becomes difficult to finish the finish rolling in the austenite region, resulting in workability. Is reduced. Therefore, Ti
The content is preferably 0.07 to 0.2%.

W: W has an effect of suppressing coarsening of fine precipitates like Mo, and it is possible to replace a part of Mo. If W is less than 0.01%, its effect is small, and if it exceeds 1.0%, the hot strength becomes high and hot rolling becomes difficult. Therefore, when W is added, its content is 0. 01-1.0% is preferable.

Nb, V: Nb and V are both effective for making fine ferrite grains, and one or more of them can be added if necessary. If Nb is less than 0.005%, its effect is small, and if it exceeds 0.02%, the formation of ferrite is hindered. Therefore, when Nb is added, its content is 0.
005-0.02% is preferable. Also, V is 0.01%
If it is less than 0.15%, the above effect cannot be obtained, and if it exceeds 0.15%, the effect is saturated. Therefore, when V is added, its content is preferably 0.01 to 0.15%.

Elements other than the above may be added in trace amounts, for example Cr, Ni, Cu may be added, but these elements deteriorate the surface properties of the steel sheet, and
The tendency becomes remarkable when the content is 5% or more, and therefore, when these are added, the content is set to 0.15% or less.

In the present invention, in addition to satisfying the above component composition,
It is desirable to contain C, Ti, and Mo so as to satisfy the following expression (3). 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (3) (However, in the formula (3), C, Ti, and Mo are in the steel. Represents the weight% of each component.)

(C / 12) / {(Ti / 48) + (Mo
/ 96)} represents the atomic ratio of C and (Ti + Mo) in the steel, and the contents of C, Ti, and Mo are adjusted so that this value is 0.5 to 1.5. As a result, the precipitates containing C, Ti, and Mo can easily and effectively be dispersed and precipitated uniformly in the ferrite. When the value is less than 0.5, the amount of solid solution Ti and solid solution Mo is increased without forming carbides and the ductility is decreased, and when it exceeds 1.5, C which does not form carbides increases the hardness of bainite. Therefore, the difference in hardness between ferrite and bainite becomes too large, and the stretch-flange formability deteriorates.

When W is added, C, Ti,
It is desirable to contain Mo and W so as to satisfy the following formula (4). 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} ≦ 1.5 (4) (However, in the above formula (4), C and Ti , Mo, W represent the weight% of each component in the steel.)

As described above, W has the same action as that of Mo. Therefore, when W is added, by satisfying the equation (4) corresponding to the above equation (3), C can be effectively contained in the ferrite. Precipitates containing Ti, Mo, and W tend to be homogeneously and finely dispersed and precipitated. (C / 12) / {(Ti / 48)
If the value of + (Mo / 96) + (W / 184)} is less than 0.5, the amounts of solid solution Ti, solid solution Mo, and solid solution W increase and the ductility decreases, without forming carbides. When it exceeds 5, C which does not form a carbide raises the hardness of bainite, so that the difference in hardness between ferrite and bainite becomes too large and the stretch flangeability is deteriorated.

Next, the manufacturing method will be described. In the present invention, after the steel having the above-mentioned composition is made into a slab, it is heated immediately or at 1200 ° C or higher, and when hot rolling is performed, finish rolling is finished at 800 ° C or higher, and then 600
After cooling to 750 ° C. to 750 ° C. at a cooling rate of 20 ° C./second or more, air cooling is performed for 3 seconds or more, and then cooling to 575 to 350 ° C. at a cooling rate of 10 ° C./second or more and winding.
Hereinafter, these conditions will be described.

Immediately after making steel into a slab or 1
Hot-rolling by heating to 200 ° C or higher: Immediately after heating the steel into a slab or heating at 1200 ° C or higher, hot-rolling is performed in a state where Ti, Mo, Nb, and V are solid-solved. This is for rolling.

Finishing rolling end temperature 800 ° C. or higher: The reason why the finishing rolling end temperature is 800 ° C. or higher is that if the temperature is lower than 800 ° C., the ductility of the steel sheet remarkably deteriorates.

After the rolling is finished, 20 to 600 to 750 ° C.
Cooling at a cooling rate of ° C / sec or more: When the cooling rate from 600 to 750 ° C is less than 20 ° C / sec, the structure after rolling is
Due to excessive recovery and recrystallization, coarse austenite grains are formed, which hinders subsequent ferrite formation. For this reason, after rolling, it was decided to cool to 600 to 750 ° C. at a cooling rate of 20 ° C./sec or more.

After the above cooling, air cooling for 3 seconds or more: After cooling at 600 to 750 ° C. at a cooling rate of 20 ° C./second or more, air cooling for 3 seconds or more is from less than 600 ° C. With air cooling, the precipitates in the ferrite phase are insufficient,
This is because if air cooling is performed from a temperature exceeding 750 ° C., the formation of ferrite is hindered. Also, this air cooling time is 3
If it is less than a second, the generation of ferrite and precipitates will be insufficient.

After air cooling, the film was cooled to 575 to 350 ° C. and cooled at a cooling rate of 10 ° C./sec or more and wound up: If the cooling rate at this time was less than 10 ° C./sec, pearlite was precipitated and stretch flangeability was increased. Deteriorate. Further, when the coiling temperature exceeds 575 ° C, a large amount of pearlite is precipitated, and when the coiling temperature is lower than 350 ° C, martensite is formed, and both deteriorate stretch flangeability. For this reason, after air cooling, it was decided to cool to 575 to 350 ° C. at a cooling rate of 10 ° C./sec or more and to wind.

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. Electroplating is 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.

Since the high-strength steel sheet of the present invention is excellent in workability, when it is press-formed, its characteristics are utilized, and a press-formed member such as an automobile member, particularly a suspension member such as a suspension member is formed. It can be manufactured with good quality. Hereinafter, a method for processing a high-strength 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 the work flow of the method for processing a high-strength steel sheet according to the present invention. This work flow generally 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 transporting it to a target place. First, the high-tensile 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 passed through 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 manufactured in this manner is sent to the next step, if necessary. 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 high-tensile steel plate 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 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 high-strength steel sheet according to the present invention, which is excellent in workability as a raw material, and particularly excellent in stretch flangeability, so that the cross-sectional shape during pressing is complicated. Even if it exists, it can be manufactured with good quality and is lightweight. Such a feature is particularly useful when the press-formed product is an automobile member, particularly a suspension member such as a suspension arm.

[0057]

Example A steel slab having the chemical composition shown in Table 1 was made into a plate having a thickness of 3.2 mm under the hot rolling conditions shown in the table, cooled, and wound to obtain a hot rolled steel plate. After pickling the obtained steel sheet, the thin film produced from the steel sheet was observed for precipitates by a transmission electron microscope (TEM). The composition of Ti, Mo, etc. was grasped from the analysis by the energy dispersive X-ray spectrometer (EDX) equipped in the TEM. The structure of the matrix was observed with a scanning electron microscope (SEM). In Table 1, the value of the above formula (2) regarding the precipitate, the value of the above formula (4) regarding the steel composition, and the value of the bainite volume ratio are also shown. If W is not included, (2)
The expression matches the expression (1), and the expression (4) matches the expression (3).

Further, the tensile properties and stretch flangeability of the obtained hot rolled steel sheets were investigated. The tensile test is JI
It carried out about the L direction (rolling direction) using the No. S5 tensile test piece. Moreover, the hole expansion ratio showing stretch flangeability was measured according to the Japan Iron and Steel Federation standard. The test results are summarized in Table 1.

As shown in Table 1, no.
Each of 1 to 5 had a tensile strength of 690 MPa or more, and showed good elongation and stretch flangeability. On the other hand, No. In Nos. 6 and 7, both elongation and stretch flangeability were low. Specifically, No. 6 is Nb
Since the quantity is high, No. 7 has a high Mn content and no Mo added, and thus a high Nb content. Therefore, the values of the above formula (1) (formula (2) in Table 1), which are composition ratios of carbides, are low, and the bainite volume ratio is 7 The higher the value, the lower the elongation and stretch flangeability.

[0060]

[Table 1]

[0061]

As described above, according to the present invention,
It is possible to obtain a hot-rolled steel sheet having excellent elongation and stretch-flangeability while having a high tensile strength of 690 MPa or more, and is suitable for a severely machined member used for transportation machines such as automobiles.

[Brief description of drawings]

FIG. 1 is a flowchart showing an example of a work flow of a method for processing a high-strength steel sheet 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.

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Claims (12)

[Claims] 1. A bainite content of 5 to 70% by volume,
The tensile strength is 690 MPa, which has a composite structure in which the balance substantially consists of ferrite, and precipitates containing Ti and Mo are dispersed in the ferrite in a range satisfying the following formula (1). A high-strength hot-rolled steel sheet with excellent workability. (Mo / 96) / {(Ti / 48) + (Mo / 96)} ≧ 0.25 (1) However, in the above formula (1), Ti and Mo are% by weight of each component in the precipitate. Indicates. 2. A bainite content of 5 to 70% by volume,
The balance has a composite structure consisting essentially of ferrite, and within the range of the following formula (2) in the ferrite, Ti, Mo
A high-strength hot-rolled steel sheet having a tensile strength of 690 MPa or more and excellent workability, characterized in that precipitates containing and W are dispersed and precipitated. {(Mo / 96) + (W / 184)} / {(Ti / 48) + (Mo / 96) + (W / 184)} ≧ 0.25 (2) However, in the above formula (2) , Ti, Mo, and W represent the weight% of each component in the precipitate. 3. C: 0.03 to 0.1% by weight, S
i ≦ 2.0%, Mn: 0.5 to 1.7%, P ≦ 0.06
%, S ≦ 0.01%, Al ≦ 0.1%, N ≦ 0.006
%, Mo: 0.1 to 0.5%, Ti: 0.07 to 0.2
%, And the balance is substantially Fe, and the high-strength hot-rolled steel sheet with excellent workability according to claim 1. 4. C: 0.03 to 0.1% by weight, S
i ≦ 2.0%, Mn: 0.5 to 1.7%, P ≦ 0.06
%, S ≦ 0.01%, Al ≦ 0.1%, N ≦ 0.006
%, Mo: 0.1 to 0.5%, Ti: 0.07 to 0.2
%, And at least one of Nb: 0.005 to 0.02% and V: 0.01 to 0.15%, with the balance being essentially Fe. A high-strength hot-rolled steel sheet having excellent workability according to 1. 5. The high-strength hot-rolled steel sheet having excellent workability according to claim 3 or 4, which contains C, Ti, and Mo so as to satisfy the following expression (3). 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (3) However, in the formula (3), C, Ti, and Mo are each in steel. Represents the weight percent of the component. 6. C .: 0.03 to 0.1% by weight, S:
i ≦ 2.0%, Mn: 0.5 to 1.7%, P ≦ 0.06
%, S ≦ 0.01%, Al ≦ 0.1%, N ≦ 0.006
%, Mo: 0.1 to 0.5%, W: 0.01 to 1.0
%, Ti: 0.07 to 0.2%, the balance being substantially Fe. The high-strength hot-rolled steel sheet with excellent workability according to claim 2. 7. C: 0.03 to 0.1% by weight, S
i ≦ 2.0%, Mn: 0.5 to 1.7%, P ≦ 0.06
%, S ≦ 0.01%, Al ≦ 0.1%, N ≦ 0.006
%, Mo: 0.1 to 0.5%, W: 0.01 to 1.0
%, Ti: 0.07 to 0.2%, and Nb: 0.
005 to 0.02%, and V: 0.01 to 0.15%
The high-strength hot-rolled steel sheet having excellent workability according to claim 2, comprising at least one of the above, and the balance being substantially Fe. 8. A high-tensile hot-rolled wire having excellent workability according to claim 6 or 7, wherein C, Ti, Mo and W are contained so as to satisfy the following formula (4). steel sheet. 0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (W / 184)} ≦ 1.5 (4) However, in the formula (4), C, Ti, Mo and W represent the weight% of each component in steel. 9. A steel sheet having a composition according to any one of claims 3 to 8 is formed into a slab, and immediately or after heating to 1200 ° C. or higher and hot rolling is performed, finish rolling is performed at 800
Finished above ℃, then 20 ℃ up to 600-750 ℃
After being cooled at a cooling rate of not less than 1 sec / sec, it is subjected to air cooling for not less than 3 sec and then cooled at a cooling rate of not less than 10 ° C./sec and wound up to 575 to 350 ° C., which is excellent in workability. Method for manufacturing high-strength hot-rolled steel sheet. 10. A first step of preparing a member made of the high-strength hot-rolled steel sheet according to claim 1, and a press-formed product having a desired shape by press-forming the member. A method for processing a high-strength hot-rolled steel sheet, comprising: 11. The method for processing a 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 high-strength hot-rolled steel sheet according to any one of claims 1 to 8.