JP5610089B2 - High-tensile hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength hot rolled steel sheet and method for producing the same, which are suitable for materials for transportation equipment and structural materials such as automobile parts.

In order to reduce CO ₂ emissions from the viewpoint of global environmental conservation, maintaining the strength of the car body while reducing its weight and improving the fuel efficiency of the car has always been an important issue in the automobile industry. . In order to reduce the weight of the vehicle body while maintaining the strength of the automobile body, it is effective to reduce the thickness of the steel sheet by increasing the strength of the steel sheet used as a material for automobile parts. For example, the reduction in the strength and thickness of steel plates for undercar parts of automobiles is a very effective means for improving automobile fuel consumption because it leads to a significant reduction in weight of the automobile body. Therefore, there is a strong demand for increasing the strength of these component materials.

On the other hand, since many automotive parts made of steel plates are formed by pressing or burring, etc., steel plates for automobile parts are required to have excellent elongation and stretch-flange formability. The For example, undercarriage parts have a complicated shape, steel sheets as materials for undercarriage parts are regarded as important in workability as well as strength, and high-tensile steel sheets with excellent workability such as elongation and stretch flangeability are used. It has been demanded.

However, in general, the workability of steel materials decreases with increasing strength. Therefore, in order to apply high-tensile hot-rolled steel sheets to undercarriage parts, etc., it is essential to develop high-tensile hot-rolled steel sheets that have both strength and workability. Proposed.

For example, Patent Document 1 discloses that by weight%, C: 0.03-0.25%, Si: 2.0% or less, Mn: 2.0% or less, P: 0.1% or less, S: 0.007% or less, Al: 0.07% or less, and Cr : A composition containing 1.0% or less, a composite structure composed of ferrite and the second phase, and by specifying the hardness, volume ratio, and particle size of the second phase, the tensile strength (TS) is 490 N / mm A technique for improving fatigue properties and stretch flangeability of high-strength hot-rolled steel sheets exceeding ² (490 MPa) has been proposed. The second phase is one or more of pearlite, bainite, martensite, and retained austenite.

  In Patent Document 2, wt%, C: 0.01 to 0.10%, Si: 1.5% or less, Mn: more than 1.0% to 2.5%, P: 0.15% or less, S: 0.008% or less, Al: 0.01 to 0.08%, total of one or two of Ti and Nb: It is a chemical component containing 0.10 to 0.60%, the ferrite content is 95% or more by area ratio, and the average crystal grain size of ferrite is 2.0 to 10.0 μm There has been proposed a technique for improving the fatigue strength, particularly the stretch flangeability of a high-strength hot-rolled steel sheet having a tensile strength (TS) of 490 MPa or more by making the structure free from martensite and retained austenite. And by the technique proposed by patent document 2, it is supposed that the strength of a steel plate will improve and a fine ferrite grain will be obtained by making Mn content more than 1.0%-2.5%.

Further, in Patent Document 3, in mass%, C: 0.01 to 0.1%, S ≦ 0.03%, N ≦ 0.005%, Ti: 0.05 to 0.5%, Si: 0.01 to 2%, Mn: 0.05 to 2% , P ≦ 0.1%, Al: 0.005 to 1.0%, and a composition containing Ti in a range satisfying Ti−48 / 12C−48 / 14N−48 / 32S ≧ 0%, and particles in steel are 5 nm or more of by a minimum interval between 10 ¹ nm ultra 10 ⁴ nm or less in 10 ¹ to 10 ³ nm the average size of the precipitates containing Ti, tensile strength (TS) of the high-strength hot-rolled steel sheet is not less than 640MPa Techniques for improving Burring formability and fatigue properties have been proposed.

JP-A-4-329848 JP 2000-328186 A JP 2002-161340 A

  However, in the technique proposed in Patent Document 1, when a steel sheet is pressed to form a desired part shape, the interface between the soft ferrite and the hard second phase is the starting point for cracking during processing. It has the problem that it is easy and processability is not stable. Further, the technique proposed in Patent Document 1 also has a problem that when the tensile strength (TS) of the steel sheet is increased to 590 MPa class, the workability, particularly the stretch flangeability, is insufficient for the current requirements. (See the example of Patent Document 1).

Further, in the technique proposed in Patent Document 2, since the Mn content of the steel sheet is high, Mn segregates in the central part of the thickness of the steel sheet, and when the steel sheet is press-formed, cracks are induced during processing, which is excellent. It is difficult to stably secure stretch flangeability, and sufficient stretch flangeability cannot always be obtained. In the technique proposed in Patent Document 2, Ti carbide is formed with Ti as a predetermined content to reduce solid solution C that adversely affects stretch flangeability. When Ti is contained, Ti carbide tends to be coarsened, and there is a problem that a desired strength cannot be stably obtained.

Moreover, in the technique proposed by patent document 3, the size distribution of the precipitate contained in a steel plate is large, and the problem that desired intensity | strength cannot be ensured stably is seen. Moreover, with the technique proposed by patent document 3, the stretch flangeability of a steel plate is inadequate (refer the Example of patent document 3).

For automobile parts that are mass-produced, it is necessary to industrially mass-produce hot-rolled steel sheets in order to stably supply the materials, but the above-mentioned conventional technology has a tensile strength (TS) of 590 MPa or more. In addition, it is difficult to stably supply a high-tensile hot-rolled steel sheet having excellent workability (stretch flangeability). The present invention advantageously solves the above-mentioned problems of the prior art, and is suitable as a material for automobile parts. Tensile strength (TS): 590 MPa or more and excellent workability (stretch flangeability), specifically An object of the present invention is to provide a high-tensile hot-rolled steel sheet having a hole expansion ratio λ: 100% or more and a method for producing the same.

In order to solve the above-mentioned problems, the present inventors diligently studied various factors affecting the high strength and workability (stretch flangeability) of a hot-rolled steel sheet. As a result, the following findings were obtained.
1) When the steel sheet structure is a ferrite single-phase structure with low dislocation density and excellent workability, and fine carbides are dispersed and precipitated to strengthen the precipitation, the strength is improved while maintaining the stretch flangeability of the hot-rolled steel sheet. .
2) To obtain hot-rolled steel sheets with excellent workability and high tensile strength (TS): 590 MPa or more, fine carbides with an average particle diameter of less than 10 nm effective for precipitation strengthening are sufficiently dispersed and precipitated. That you need to do.
3) As fine carbides contributing to precipitation strengthening, carbides containing Ti are effective from the viewpoint of ensuring strength and the like.

4) In order to disperse and precipitate carbide containing Ti with an average particle size of less than 10 nm and sufficient tensile strength of 590 MPa or more, it is necessary to secure the amount of Ti that forms Ti carbide as a precipitation nucleus. It is necessary to contain Ti (Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48)) more than a predetermined amount with respect to the N and S content in the steel material.
5) To precipitate carbide containing Ti finely (average particle diameter: less than 10 nm) in the ferrite phase, the desired ratio of B content and Mn content in the material steel (B ≥ 0.0003- It is effective to control to 0.00025Mn).
6) When the Ti content of the carbide containing Ti exceeds the C content by atomic ratio, the carbide is liable to be coarsened and adversely affect hot-rolled steel sheet characteristics.
7) In order to suppress the coarsening of carbides by setting the Ti content of carbides containing Ti to less than the C content by atomic ratio, the Ti, N, and S contents with respect to the C content in the steel used as the material are within a predetermined range. It is effective to control to (C / 12> Ti / 48-N / 14-S / 32).

The present invention has been completed based on the above findings, and the gist thereof is as follows.
・ By mass%
C: 0.010% or more and 0.050% or less, Si: 0.2% or less,
Mn: 0.1% to 0.8%, P: 0.025% or less,
S: 0.01% or less, N: 0.01% or less,
Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, containing S, N, and Ti so that the following formula (1) is satisfied, with the balance consisting of Fe and inevitable impurities, and the ferrite phase High tensile hot rolling with a matrix having an area ratio of 95% or more with respect to the entire structure and a structure in which fine carbides containing Ti and having an average particle diameter of less than 10 nm are dispersed and having a tensile strength of 590 MPa or more. steel sheet.
Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48) (1)
(S, N, Ti: content of each element (mass%))

[2] A high-tensile-strength hot-rolled steel sheet that contains B: 0.0035% or less in mass% so as to satisfy the following formula (2) in [1].
B ≧ 0.0003−0.00025Mn (2)
(Mn, B: content of each element (mass%))
[3] The high-tensile hot-rolled steel sheet according to [2], wherein B is 0.0003% or more and 0.0020% or less.

[4] A high-tensile hot-rolled steel sheet according to [1] or [2], wherein the composition satisfies the following formula (3).
C / 12> Ti / 48−N / 14−S / 32 (3)
(C, S, N, Ti: content of each element (mass%))

[5] The high-tensile hot-rolled steel sheet according to [1] or [2], wherein a volume ratio of the fine carbide to the entire structure is 0.0005 or more.
[6] The high-tensile hot-rolled steel sheet according to [5], wherein the volume ratio is 0.0005 or more and 0.003 or less.

[7] In the above [1] or [2], in addition to the composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, A high-strength hot-rolled steel sheet containing at least 0.1% of any one of V.

[8] The high-tensile hot-rolled steel sheet according to [1] or [2], which has a plating film on the steel sheet surface.

[9] In the method for producing a high-strength hot-rolled steel sheet, the steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling is cooled, wound, and hot-rolled steel sheet.
The steel material in mass%,
C: 0.010% or more and 0.050% or less, Si: 0.2% or less,
Mn: 0.1% to 0.8%, P: 0.025% or less,
S: 0.01% or less, N: 0.01% or less,
Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, S, N, and Ti are contained so as to satisfy the following formula (1), and the balance is composed of Fe and inevitable impurities,
The final rolling temperature of the finish rolling is 880 ° C or higher, the average cooling rate of the cooling is 10 ° C / s or higher, the winding temperature is 550 ° C or higher and lower than 800 ° C, and the tensile strength is 590 MPa or higher. A method for producing a hot-rolled steel sheet.
Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48) (1)
(S, N, Ti: content of each element (mass%))

[10] The method for producing a high-tensile hot-rolled steel sheet according to [9], further including B: 0.0035% or less by mass% in addition to the composition so as to satisfy the following formula (2)
B ≧ 0.0003−0.00025Mn (2)
(Mn, B: content of each element (mass%))
[11] The method for producing a high-tensile hot-rolled steel sheet according to [10], wherein B is 0.0003% or more and 0.0020% or less.

[12] A method for producing a high-tensile hot-rolled steel sheet, wherein the composition in [9] or [10] satisfies the following formula (3):
C / 12> Ti / 48−N / 14−S / 32 (3)
(C, S, N, Ti: content of each element (mass%))

[13] In the above [9] or [10], in addition to the composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, A method for producing a high-tensile hot-rolled steel sheet containing at least 0.1% of any one of V in total.

  According to the present invention, excellent processing that can be applied satisfactorily as a material for undercarriage parts, etc. suitable for automobile steel sheets and the like, having a tensile strength (TS) of 590 MPa or more and a complicated sectional shape at the time of pressing. It is possible to provide a high-tensile hot-rolled steel sheet having the properties (stretch flangeability), and there is a remarkable industrial effect.

Hereinafter, the present invention will be described in detail.
First, the structure of the steel sheet of the present invention and the reasons for limiting the carbide will be described.
The hot-rolled steel sheet of the present invention has a matrix in which the ferrite phase is 95% or more in area ratio with respect to the entire structure, and a structure in which fine carbides containing Ti and having an average particle diameter of less than 10 nm are dispersed and precipitated in the matrix.

Ferrite phase: 95% or more in area ratio with respect to the entire structure In the present invention, formation of a ferrite phase is essential to ensure the workability (stretch flangeability) of a hot-rolled steel sheet. In order to improve the elongation and stretch flangeability of the hot-rolled steel sheet, it is effective to make the structure of the hot-rolled steel sheet a ferrite phase having a low dislocation density and excellent ductility. In particular, to improve stretch flangeability, the structure of the hot-rolled steel sheet is preferably a ferrite single-phase structure, but even if it is not a complete ferrite single-phase structure, it is substantially a ferrite single-phase structure, that is, If the area ratio with respect to the entire structure is 95% or more of the ferrite phase, the above effect is sufficiently exhibited. Therefore, the area ratio of the ferrite phase to the entire structure is 95% or more. Preferably, it is 97% or more.

In the hot-rolled steel sheet of the present invention, examples of the structure other than the ferrite phase include cementite, pearlite, bainite phase, martensite phase, retained austenite phase, and the like. However, it is acceptable if it is preferably about 3% or less.

Fine carbide containing Ti
A carbide containing Ti has a strong tendency to become a fine carbide having an extremely small average particle diameter. For this reason, in the present invention for increasing the strength of a hot-rolled steel sheet by dispersing and precipitating fine carbides in the hot-rolled steel sheet, fine carbides containing Ti are used as the fine carbides to be dispersed and precipitated.

Average particle diameter of fine carbide: Less than 10 nm The average particle diameter of fine carbide is extremely important for imparting desired strength (tensile strength: 590 MPa or more) to a hot-rolled steel sheet. In the present invention, fine carbide containing Ti is used. The average particle size of is less than 10 nm. When fine carbide precipitates in the matrix, the fine carbide acts as a resistance to dislocation movement that occurs when deformation is applied to the steel sheet, strengthening the hot-rolled steel sheet, but the average particle diameter of the fine carbide is 10 nm. If it is less than the above, the above action becomes even more remarkable. Therefore, the average particle diameter of the fine carbide containing Ti is set to less than 10 nm. More preferably, it is 5 nm or less.

In order to stably obtain hot-rolled steel sheet strength, it is effective to control the dispersion and precipitation state of fine carbides containing Ti.In the present invention, fine carbides containing Ti and having an average particle size of less than 10 nm are effective. It is preferable to disperse and precipitate so that the volume ratio with respect to the entire structure is 0.0005 or more. However, if the volume ratio exceeds 0.003, the strength becomes too high and the stretch flangeability may be deteriorated. Therefore, the volume ratio is preferably 0.0005 or more and 0.003 or less.

In the present invention, as the precipitation form of fine carbide containing Ti, in addition to the row precipitation that is the main precipitation form, even if there is a mixture of randomly precipitated fine carbide, without affecting the characteristics at all, Regardless of the form of precipitation, various precipitation forms are collectively referred to as dispersion precipitation.

Next, the reason for limiting the component composition of the hot-rolled steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.
C: 0.010% to 0.050%
C is an essential element for forming fine carbides and strengthening the hot-rolled steel sheet. If the C content is less than 0.010%, a tensile strength of 590 MPa or more cannot be obtained. On the other hand, when the C content exceeds 0.050%, the strength increases and pearlite is easily formed in the steel sheet, and it is difficult to obtain excellent stretch flangeability. Therefore, the C content is 0.010% or more and 0.050% or less. Preferably they are 0.020% or more and 0.035% or less. More preferably, it is 0.020% or more and 0.030% or less.

Si: 0.2% or less
Si is a solid solution strengthening element and is an element effective for increasing the strength of steel. However, when the Si content exceeds 0.2%, C precipitation from the ferrite phase is promoted, coarse Fe carbides are likely to precipitate at the grain boundaries, and stretch flangeability is deteriorated. In addition, excessive Si adversely affects the plating properties. Therefore, the Si content is 0.2% or less. Preferably it is 0.05% or less. Further, it is preferably 0.005% or more for strengthening the solid solution.

Mn: 0.1% or more and 0.8% or less
Since Mn is a solid solution strengthening element and is an element effective for increasing the strength of steel, it is desirable to increase the Mn content from the viewpoint of strengthening the hot-rolled steel sheet. If the Mn content is less than 0.1%, solid solution strengthening cannot be obtained. On the other hand, if the Mn content is less than 0.1%, the Ar ₃ transformation point becomes too high, and the fine carbide containing Ti tends to become coarse as will be described later. On the other hand, when the Mn content exceeds 0.8%, segregation is likely to occur, and a phase other than the ferrite phase, that is, a hard phase is formed, and stretch flangeability is deteriorated. Therefore, the Mn content is 0.1% or more and 0.8% or less. Preferably they are 0.1% or more and 0.5% or less. More preferably, it is 0.1% or more and 0.45% or less.

P: 0.025% or less
P is a solid solution strengthening element and is an element effective for increasing the strength of steel. However, when the P content exceeds 0.025%, segregation becomes significant and stretch flangeability deteriorates. Therefore, the P content is 0.025% or less. Preferably it is 0.02% or less. Further, it is preferably 0.005% or more for strengthening the solid solution.

S: 0.01% or less
S is an element that decreases the hot workability (hot rollability), and increases the hot cracking susceptibility of the slab, and also exists as MnS in the steel and degrades the stretch flangeability of the hot rolled steel sheet. Therefore, in the present invention, it is preferable to reduce S as much as possible, and set it to 0.01% or less. Preferably it is 0.005% or less.

N: 0.01% or less
N is a harmful element in the present invention and is preferably reduced as much as possible. In particular, when the N content exceeds 0.01%, the stretch flangeability deteriorates due to the formation of coarse nitrides in the steel. Therefore, the N content is 0.01% or less. Preferably it is 0.006% or less.

Al: 0.06% or less
Al is an element that acts as a deoxidizer. In order to acquire such an effect, it is desirable to contain 0.001% or more, but inclusion exceeding 0.06% reduces elongation and stretch flangeability. For this reason, Al content shall be Al: 0.06% or less.

Ti: 0.05% or more and 0.10% or less
Ti is the most important element in the present invention. Ti is an element that contributes to increasing the strength of the steel sheet while maintaining excellent stretch flangeability by forming carbides. If the Ti content is less than 0.05%, the desired hot-rolled steel sheet strength (tensile strength: 590 MPa or more) cannot be ensured. On the other hand, when the Ti content exceeds 0.10%, the stretch flangeability tends to decrease. Therefore, the Ti content is 0.05% or more and 0.10% or less. Preferably they are 0.065% or more and 0.095% or less.

The hot-rolled steel sheet of the present invention contains S, N, and Ti in the above-described range and satisfying the expression (1).
Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48) (1)
(S, N, Ti: content of each element (mass%))
The above formula (1) is a requirement to be satisfied in order to bring the fine carbide containing Ti into the above-described desired precipitation state, and is an extremely important index in the present invention.

Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48) (1)
As described above, in the present invention, fine carbide containing Ti is dispersed and precipitated in the hot-rolled steel sheet, but this fine carbide dissolves the carbide in the steel material by heating before hot rolling, mainly after hot rolling. Precipitated during winding. Here, in order to stably disperse and precipitate the fine carbide with an average particle diameter of less than 10 nm, it is necessary to secure a sufficient amount of Ti as a precipitation nucleus of the fine carbide. However, at a high temperature range, Ti is easier to form nitrides and sulfides than carbides. Therefore, if the Ti content is insufficient with respect to the N and S contents of the steel material, the amount of Ti that becomes the precipitation nuclei of fine carbides decreases with the precipitation of the nitrides and sulfides, and the fine content containing Ti is reduced. It becomes difficult to sufficiently precipitate the carbide.

  Therefore, in the present invention, the Ti, N, and S contents are controlled so as to satisfy the formula (1) Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48). As a result, a sufficient amount of Ti serving as a nucleus for precipitation of fine carbide can be secured, and the fine carbide can be stably precipitated with an average particle size of less than 10 nm.

In the present invention, the steel material is heated to the austenite region before hot rolling to dissolve carbides in the steel material, and carbides containing Ti are precipitated simultaneously with the subsequent austenite → ferrite transformation. However, when the austenite → ferrite transformation temperature is high, the precipitated carbide containing Ti becomes coarse. Therefore, in the present invention, it is preferable to precipitate the carbide containing Ti at the time of winding by adjusting the temperature of the austenite → ferrite transformation (Ar ₃ transformation point) to the winding temperature range. Thereby, the coarsening can be suppressed, and a carbide having an average particle diameter of less than 10 nm can be obtained.

In order to adjust the temperature of the austenite → ferrite transformation (Ar ₃ transformation point) to the coiling temperature range, in addition to the above-mentioned composition, B: 0.0035% or less is contained so as to satisfy the following formula (2) It is preferable to do.
B ≧ 0.0003−0.00025Mn (2)

B: 0.0035% or less
B is an element that lowers the Ar ₃ transformation point of steel. In the present invention, by adding B to lower the Ar ₃ transformation point of steel, it is possible to refine the carbide containing Ti. In order to obtain such an effect, the B content is preferably 0.0003% or more. On the other hand, the above effect is saturated even if the content exceeds 0.0035%. Therefore, the B content is preferably 0.0035% or less. More preferably, it is 0.0003% or more and 0.0020% or less.

B ≧ 0.0003−0.00025Mn (2)
In the present invention, when B is contained, it is also important to control the ratio of the B content and the Mn content in the steel within an appropriate range. The present inventors studied a means for finely dispersing (including an average particle diameter of less than 10 nm) a carbide containing Ti in a matrix having an area ratio of 95% or more with respect to the entire structure of the ferrite phase. As a result, adjusting the temperature of the austenite → ferrite transformation (Ar ₃ transformation point) in the hot rolling process to the coiling temperature range described later refines the carbide containing Ti to an average particle size of less than 10 nm. It was newly discovered that it is a very effective means.

Further, the present inventors have found that further studying, in the steel composition of the present invention, by which the B content and Mn content of the steel material is controlled to satisfy the desired relationship, steel Ar ₃ It was clarified that the transformation point can be adjusted to the target range. Here, in the above formula, when the value on the right side (0.0003−0.00025Mn) is equal to or less than zero, the value on the right side is regarded as zero.

In the present invention, if the content of Mn, which is a solid solution strengthening element, exceeds 0.35%, a desired steel plate strength (tensile strength: 590 MPa or more) can be ensured without using the effect of B described above. it can. However, if the Mn content is 0.35% or less, it may be difficult to secure a desired steel sheet strength without using the above-described effect of B. Therefore, when the Mn content is 0.35% or less, it is preferable to contain B for the purpose of making the carbide containing Ti finer.

In the present invention, it is preferable to adjust the contents of C, S, N, and Ti so as to satisfy the expression (3) within the above range.
C / 12> Ti / 48−N / 14−S / 32 (3)
(C, S, N, Ti: content of each element (mass%))
As described above, the carbide containing Ti has a strong tendency to become a fine carbide having an extremely small average particle diameter. However, when the Ti bonded to C becomes C or more in atomic ratio, the carbide is easily coarsened. And it becomes difficult to ensure desired hot-rolled steel sheet strength (tensile strength: 590 MPa or more) with the coarsening of carbides.

Therefore, in the present invention, it is preferable to define the contents of C, Ti, N, and S as shown in formula (3). That is, in the present invention, for C and Ti contained in the steel material, the atomic% of C (C / 12) is more than the atomic% of Ti that can contribute to carbide formation (Ti / 48-N / 14-S / 32). It is preferable to increase the amount. Thereby, the coarsening of the fine carbide containing Ti can be suppressed.

  In the steel sheet of the present invention, one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, and V are combined in a total of 0.1% or less, preferably 0.03. % Or less. Components other than the above are Fe and inevitable impurities.

  The steel sheet of the present invention may have a plating film on the surface. By forming a plating film on the surface of the steel sheet, the corrosion resistance of the hot-rolled steel sheet is improved, and a hot-rolled steel sheet suitable for a part exposed to a severe corrosive environment, for example, a material for an undercarriage part of an automobile is obtained. Examples of the plating film include a hot dip galvanizing film and an alloyed hot dip galvanizing film.

Next, the manufacturing method of the hot rolled steel sheet of the present invention will be described.
In the present invention, hot rolling consisting of rough rolling and finish rolling is performed on a steel material, and after finishing rolling, the steel material is cooled and wound to obtain a hot rolled steel sheet. At this time, the finish rolling temperature of the finish rolling is 880 ° C. or higher, the average cooling rate of the cooling is 10 ° C./s or higher, and the winding temperature of the winding is 550 ° C. or higher and lower than 800 ° C. To do.

  In the present invention, the method for melting the steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. In addition, after melting, it is preferable to use a slab (steel material) by a continuous casting method because of problems such as segregation, but it may also be used as a slab by a known casting method such as an ingot-bundling rolling method or a thin slab continuous casting method. good. In addition, when hot-rolling the slab after casting, the slab may be rolled after being reheated in a heating furnace, and when the temperature is maintained at a predetermined temperature or higher, direct rolling without heating the slab You may do it.

The steel material obtained as described above is subjected to rough rolling and finish rolling. In the present invention, it is necessary to dissolve carbides in the steel material before rough rolling. In the present invention containing Ti which is a carbide forming element, the heating temperature of the steel material is preferably set to 1150 ° C. or higher. However, if the heating temperature of the steel material is excessively high, the surface is excessively oxidized and TiO ₂ is generated and Ti is consumed. Is preferably 1300 ° C. or lower. In addition, as described above, when the steel material before rough rolling maintains a temperature equal to or higher than a predetermined temperature, and the carbide in the steel material is dissolved, the step of heating the steel material before rough rolling is It can be omitted. The rough rolling conditions are not particularly limited.

Finishing rolling temperature: 880 ° C. or more Optimization of the finishing rolling temperature is important for securing elongation and stretch flangeability of the hot-rolled steel sheet and reducing the rolling load of finishing rolling. When the finish rolling temperature is less than 880 ° C., the crystal grains of the hot rolled steel sheet surface layer are coarse and the stretch flangeability is impaired. In addition, since rolling is performed in the non-recrystallization temperature range, coarse Ti carbide precipitates at the prior austenite grain boundaries, and stretch flangeability deteriorates. Accordingly, the finish rolling temperature is 880 ° C. or higher. Preferably it is 900 degreeC or more. If the finish rolling temperature becomes excessively high, the crystal grains become coarse and adversely affect the securing of the desired steel sheet strength (tensile strength: 590 MPa or more). Therefore, the finish rolling temperature is preferably 1000 ° C. or less.

Average cooling rate: 10 ° C / s or higher If the average cooling rate from 880 ° C or higher to the coiling temperature is less than 10 ° C / s after finishing rolling, the Ar ₃ transformation point increases and carbides containing Ti Is not sufficiently refined. Therefore, the average cooling rate is 10 ° C./s or more. Preferably it is 30 ° C./s or more. Further, it is preferably less than 200 ° C./s in order to obtain a ferrite structure.

Winding temperature: 550 ° C or higher and less than 800 ° C This is extremely important in forming a structure in which fine matrixes containing Ti and an average particle diameter of less than 10 nm are dispersed and precipitated.

  When the coiling temperature is less than 550 ° C, precipitation of fine carbides becomes insufficient at the end in the width direction of the rolled material, which tends to be supercooled, and the desired steel sheet strength (tensile strength: 590 MPa or more) can be imparted. It becomes difficult. Moreover, the problem that it becomes difficult to ensure the running stability on a run-out table arises. On the other hand, when the coiling temperature is 800 ° C. or higher, pearlite is generated, and it becomes difficult to obtain a matrix in which the ferrite phase is 95% or more in terms of the area ratio with respect to the entire structure. Therefore, the coiling temperature is set to 550 ° C. or higher and lower than 800 ° C. Preferably they are 550 degreeC or more and less than 700 degreeC, More preferably, they are 580 degreeC or more and less than 700 degreeC.

  As described above, high-tensile hot-rolled steel sheet with excellent workability (stretch flangeability) that can be applied as a material for undercarriage parts with a tensile strength (TS) of 590 MPa or more and a complicated cross-sectional shape. In order to manufacture, it is necessary to disperse and precipitate fine carbides having an average particle diameter of less than 10 nm over the entire width direction of the steel sheet.

  However, in the present invention, a predetermined amount or more of Ti (Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48)) is included with respect to the N and S content in the steel used as the material of the hot-rolled steel sheet. Further, fine carbides having an average particle diameter of less than 10 nm by containing so that the content of B and Mn in the steel that is the raw material of the hot-rolled steel sheet satisfies a predetermined relationship (B ≧ 0.0003−0.00025Mn) Is controlled so as to sufficiently disperse and precipitate. Therefore, according to the present invention, it is possible to disperse and precipitate fine carbides having an average particle diameter of less than 10 nm over the entire width direction without strictly specifying the production conditions of the hot-rolled steel sheet. Uniform and good characteristics (tensile strength, stretch flangeability) are imparted over the entire direction.

In the present invention, the hot-rolled steel sheet manufactured as described above may be plated to form a plating film on the surface of the steel sheet. For example, a hot dip galvanizing process may be performed as a plating process to form a hot dip galvanized film, or an alloyed hot dip galvanized film may be formed on the surface of the steel sheet by further alloying after the hot dip galvanizing process. .

Molten steel having the composition shown in Table 1 was melted and continuously cast by a generally known technique to obtain a slab (steel material) having a thickness of 250 mm. These slabs are heated to 1250 ° C, roughly rolled, and subjected to finish rolling at the finish rolling temperature shown in Table 2. After finishing rolling, the temperature range from 880 ° C to the coiling temperature is shown in Table 2. The steel sheet was cooled at the average cooling rate shown and wound at the winding temperature shown in Table 2 to obtain a hot rolled steel sheet having a plate thickness of 2.3 mm. Some hot-rolled steel sheets (hot-rolling numbers a2, b2, and c2) are immersed in a 480 ° C zinc plating bath (0.1% Al-Zn) and molten zinc with an adhesion amount of 45 g / m ² per side. After forming the plating film, alloying treatment was performed at 520 ° C. to obtain an alloyed hot-dip galvanized steel sheet.

  Samples are collected from the hot-rolled steel sheet obtained as described above, microstructure observation, tensile test, hole expansion test, ferrite phase area ratio, average particle diameter and volume ratio of fine carbide containing Ti, tensile strength, The hole expansion rate (stretch flangeability) was determined. The test method was as follows.

(I) Microstructure observation A test piece was collected from the obtained hot-rolled steel sheet, a cross section parallel to the rolling direction of the test piece was mechanically polished, corroded with nital, and then magnified with a scanning electron microscope (SEM): Using a structure photograph (SEM photograph) taken at a magnification of 3000 times, the type of the structure other than the ferrite phase and the ferrite phase and the area ratio thereof were determined by an image analyzer.
Moreover, the thin film produced from the hot-rolled steel sheet was observed with a transmission electron microscope (TEM) at a magnification of 260,000 times, and the particle diameter of fine carbide containing Ti was determined.
The particle size of fine carbide containing Ti is calculated based on the observation results of 30 fields of view at 260,000 times, and the individual particle size is obtained by image processing using circular approximation, and the obtained particle size is arithmetically averaged to obtain the average particle size. The diameter.
The volume ratio of fine carbides containing Ti is determined by the weight of Ti carbide by extraction residue analysis of the residue collected by filtering and collecting the residue using 10% acetylacetone-1% tetramethylammonium chloride-methanol solution (AA solution). The volume was determined by dividing this by the density of Ti carbide (TiC), and this volume was divided by the volume of the dissolved iron.

(Ii) Tensile test From the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece (JIS Z 2201) with the direction perpendicular to the rolling direction as the tensile direction was sampled and a tensile test in accordance with the provisions of JIS Z 2241. The tensile strength (TS) was measured.

(Iii) Hole expansion test From the obtained hot-rolled steel sheet, a test piece (size: 130 mm x 130 mm) was collected, and a hole having an initial diameter d ₀ : 10 mmφ was formed in the test piece by punching. Using these test pieces, a hole expansion test was performed. That is, a conical punch with an apex angle of 60 ° is inserted into the hole, the hole is expanded, and the diameter d of the hole when the crack penetrates the hot-rolled steel sheet (test piece) is measured. The rate λ (%) was calculated.
Hole expansion ratio λ (%) = {(d−d ₀ ) / d ₀ } × 100
The obtained results are shown in Table 3.

Each of the examples of the present invention is a hot-rolled steel sheet having both a high strength of tensile strength TS: 590 MPa or more and an excellent stretch flangeability of a hole expansion ratio λ: 100% or more. On the other hand, in a comparative example outside the scope of the present invention, a predetermined high strength cannot be ensured or the hole expansion rate λ cannot be ensured.

Claims (9)

% By mass
C: 0.010% to 0.050%, Si: 0.2% or less, Mn: 0.1% to 0.8%, P: 0.025% or less, S: 0.01% or less, N: 0.01% or less, Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less , B: 0.0003% or more and 0.0020% or less , S, N, Ti, B And a matrix containing Mn so as to satisfy the following formulas (1) and (2) , the balance being Fe and unavoidable impurities, and a matrix having an area ratio of 95% or more of the entire structure of the ferrite phase: And a high-strength hot-rolled steel sheet having a structure in which fine carbides containing Ti and having an average particle diameter of less than 10 nm are dispersed and precipitated and have a tensile strength of 590 MPa or more.
Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48) (1)
(S, N, Ti: content of each element (mass%))
B ≧ 0.0003-0.00025Mn (2)
(Mn, B: content of each element (mass%)) The high-tensile hot-rolled steel sheet according to claim 1, wherein the composition satisfies the following formula (3).
C / 12> Ti / 48-N / 14-S / 32 (3)
(C, S, N, Ti: content of each element (mass%)) The high-tensile hot-rolled steel sheet according to claim 1, wherein a volume ratio of the fine carbide to the whole structure is 0.0005 or more. The high-tensile hot-rolled steel sheet according to claim 3 , wherein the volume ratio is 0.0005 or more and 0.003 or less. In addition to the above-mentioned composition, in addition, by mass%, any one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, and V is 0.1 in total. The high-tensile hot-rolled steel sheet according to claim 1, which is contained in an amount of not more than%. The high-tensile hot-rolled steel sheet according to claim 1, which has a plating film on the steel sheet surface. The steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and after the finish rolling is finished, the steel material is cooled, wound, and made into a hot-rolled steel plate. C: 0.010% or more and 0.050% or less, Si: 0.2% or less, Mn: 0.1% or more and 0.8% or less, P: 0.025% or less, S: 0.01% Hereinafter, N: 0.01% or less, Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, B: 0.0003% or more and 0.0020% or less , S, N, Ti , B, and Mn are contained so as to satisfy the following formula (1) and the following formula (2) , the balance is composed of Fe and unavoidable impurities, and the finish rolling temperature of the finish rolling is 880 ° C. or more. The average cooling rate of the cooling is 10 ° C./s or more, and the winding temperature is 55 ° C. or higher and less than 800 ° C., with a tissue and the matrix area ratio is 95% or more to the whole structure of ferrite phase, fine carbide average grain size of less than 10nm include Ti are dispersed precipitated, tensile strength A method for producing a high-tensile hot-rolled steel sheet of 590 MPa or more.
Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48) (1)
(S, N, Ti: content of each element (mass%))
B ≧ 0.0003-0.00025Mn (2)
(Mn, B: content of each element (mass%)) The manufacturing method of the high-tensile-strength hot-rolled steel sheet according to claim 7 , wherein the composition satisfies the following formula (3): C / 12> Ti / 48-N / 14-S / 32 ... (3)
(C, S, N, Ti: content of each element (mass%)) In addition to the above-mentioned composition, in addition, by mass%, any one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, and V is 0.1 in total. The manufacturing method of the high-tensile-strength hot-rolled steel plate according to claim 7, which is contained in an amount of% or less.