TW201317367A - High strength hot rolled steel sheet and method for producing the same - Google Patents

Abstract

The high-strength hot-rolled steel sheets of the present invention includes a constitution including 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% to 0.10% by mass%, satisfying Ti ≥ 0.04 + (N/14*48 + S/32*48), and the rest including Fe and unavoidable impurities; a matrix that an area ratio of a ferrite phase compared to the whole structure is 95% or more; and a structure that is formed by dispersed precipitation of fine carbides including Ti, wherein the average particle size of the fine carbides is less than 10 nm.

Description

High-tension hot-rolled steel sheet and manufacturing method thereof

The present invention relates to a high strength hot rolled steel sheets and method for producing the same, which is suitable for a conveying device for an automobile part or the like and a material for building materials.

From the viewpoint of protecting the environmental environment, it is often important in the automotive industry to reduce the amount of CO ₂ emissions, to maintain the strength of the vehicle body, to reduce the weight of the vehicle body, and to improve the fuel consumption of the automobile. Question. In order to reduce the weight of the vehicle body while maintaining the strength of the vehicle body, it is effective to reduce the thickness of the steel sheet by increasing the strength of the steel sheet for the material for automobile parts. For example, the high strength and thinning of the steel plate for the chassis parts of automobiles makes the automobile body significantly lighter, and thus is an extremely effective means for improving the fuel consumption of the automobile. Therefore, the expectation of high strength of the materials for these parts is very strong.

On the other hand, most of the automobile parts using steel sheets as materials are formed by press working or burring processing, and therefore, it is required to have excellent elongation and stretchability for steel sheets for automobile parts (stretch-flange) Formability). For example, in the steel plate which is a material for the chassis component, the strength and workability of the steel plate are taken into consideration, and a high-tensile steel plate excellent in workability such as elongation and stretchability is sought.

However, in general, steel materials are accompanied by high strength and processability. reduce. Therefore, when a high-tensile hot-rolled steel sheet is used for a chassis part or the like, development of a high-tensile hot-rolled steel sheet having both strength and workability is required, and various studies have been conducted so far, and various techniques have been proposed.

For example, Patent Document 1 proposes a technique in which carbon (C): 0.03% to 0.25%, cerium (Si): 2.0% or less, and manganese (Mn): 2.0% are contained in terms of % by weight. Hereinafter, phosphorus (P): 0.1% or less, sulfur (S): 0.007% or less, aluminum (Al): 0.07% or less, and chromium (Cr): 1.0% or less; and ferrite (ferrite) and Two-phase composite structure; by adjusting the hardness, volume ratio, and particle size of the second phase, the fatigue of the high-strength hot-rolled steel sheet having a tensile strength (TS) exceeding 490 N/mm ² (490 MPa) is improved. Fatigue property and extension. The second phase is one or more of pearlite, bainite, martensite, and retained austenite.

Further, Patent Document 2 proposes a technique in which the following chemical components are contained, in terms of wt%, including C: 0.01% to 0.10%, Si: 1.5% or less, Mn: more than 1.0% to 2.5%, and P: 0.15. % or less, S: 0.008% or less, Al: 0.01% to 0.08%, and a total of one or two kinds of titanium (Ti) and niobium (Nb): 0.10% to 0.60%; and the following structure, fat The iron content is 95% or more in area ratio, and the average crystal grain size of the ferrite iron is 2.0 μm to 10.0 μm, and does not contain the granulated iron and retained austenite; thereby increasing the tensile strength (TS) to 490 MPa or more. The fatigue strength and especially the edge of the high-strength hot-rolled steel sheet. Further, in the technique proposed in Patent Document 2, by setting the Mn content to more than 1.0% to 2.5%, the strength of the steel sheet can be improved and fine fine iron particles can be obtained.

Further, Patent Document 3 proposes a technique of including C: 0.01% to 0.1%, S≦ 0.03%, N≦0.005%, Ti: 0.05% to 0.5%, Si in terms of mass%. : 0.01%~2%, Mn: 0.05%~2%, P≦0.1%, Al: 0.005%~1.0%, and further satisfy the range of Ti-48/12C-48/14N-48/32S≧0% Ti is contained; the average size of precipitates containing 5 nm or more of Ti in the steel is set to 10 ¹ nm to 10 ³ nm, and the minimum interval is set to be more than 10 ¹ nm and 10 ⁴ nm or less. The high-strength hot-rolled steel sheet having a tensile strength (TS) of 640 MPa or more has a Burkin formability and fatigue characteristics.

Prior technical literature Patent literature

Patent Document 1: Japanese Patent Laid-Open No. Hei 4-329848

Patent Document 2: Japanese Patent Laid-Open Publication No. 2000-328186

Patent Document 3: Japanese Patent Laid-Open Publication No. 2002-161340

However, in the technique proposed in Patent Document 1, when the steel sheet is subjected to press working or the like and formed into a desired part shape, the interface between the soft ferrite iron and the hard second phase is likely to be a starting point of cracking during processing. Therefore, there is a problem of unstable workability. Further, in the technique proposed in Patent Document 1, when the tensile strength (TS) of the steel sheet is increased to the level of 590 MPa, the problem that the workability, particularly the stretchability, is not sufficient with respect to the current situation is also found ( Refer to 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 is segregated toward the center portion of the thickness of the steel sheet, and thus the steel sheet is At the time of press forming, cracks are generated during processing, so that it is difficult to stably ensure excellent stretchability, and it is not always possible to obtain sufficient stretchability. Further, in the technique proposed in Patent Document 2, Ti is formed to have a predetermined content to form Ti carbide, whereby the solid solution C which adversely affects the edge resistance can be reduced, but if it is excessive with respect to C When Ti is contained in the ground, the Ti carbide is easily coarsened, and it has been found that there is a problem that the desired strength cannot be stably obtained.

Further, in the technique proposed in Patent Document 3, the distribution of the size of the precipitate contained in the steel sheet is large, and it has been found that there is a problem that the desired strength cannot be stably ensured. Further, in the technique proposed in Patent Document 3, the stretchability of the steel sheet is insufficient (refer to the example of Patent Document 3).

In the case of a mass-produced automobile part, it is necessary to industrially mass-produce a hot-rolled steel sheet for stably supplying the material of the automobile part. However, in the above prior art, it is difficult to stably supply a tensile strength (TS) of 590 MPa or more. A high-tension hot-rolled steel sheet having excellent workability (extension). The present invention advantageously solves the problems faced by the prior art described above, and an object thereof is to provide a high-tensile hot-rolled steel sheet suitable for use as an automobile part, tensile strength (TS): 590, and a method of manufacturing the same. MPa or more and excellent workability (stretching property), specifically, hole expansion ratio λ: 100% or more.

In order to solve the above problems, the inventors of the present invention have actively studied various factors that affect the high strength and workability (extension) of hot-rolled steel sheets. As a result, the following findings were obtained.

1) The steel sheet structure is a single-phase structure of the ferrite-grained iron having excellent dislocation density and excellent workability, and further, when the fine carbide is dispersed and precipitated to enhance precipitation, the strength of the hot-rolled steel sheet is maintained and the strength is improved. improve.

2) In order to obtain a high-strength hot-rolled steel sheet having excellent tensile properties (TS): 590 MPa or more, it is necessary to sufficiently disperse and precipitate fine carbides having an average particle diameter of less than 10 nm which is effective for precipitation strengthening.

3) From the viewpoint of ensuring strength and the like, as a fine carbide which contributes to precipitation strengthening, it is effective as a carbide containing Ti.

4) In order to sufficiently disperse and precipitate the carbide containing Ti so that the average particle diameter is less than 10 nm and the tensile strength of 590 MPa or more can be obtained, it is necessary to ensure the amount of Ti which forms the Ti carbide as the precipitation nucleus. Further, it is necessary to contain Ti (Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48)) in a predetermined amount or more with respect to the N content and the S content in the steel as the material.

5) In the ferrite-rich iron phase, in order to precipitate fine Ti-containing carbides (average particle diameter: less than 10 nm), it is effective to control the B content and the Mn content in the steel as the material to be desired. Ratio (B≧0.0003-0.00025Mn).

6) If the Ti content of the carbide containing Ti exceeds the C content by atomic ratio, the carbide tends to be coarsened, which adversely affects the characteristics of the hot rolled steel sheet.

7) In the case where the Ti content of the carbide containing Ti is set to be smaller than the C content by atomic ratio and the coarsening of the carbide is suppressed, it is effective to use the Ti content and the N content relative to the C content in the steel as the material. The S content is controlled within the specified range (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.

[1] A high-tensile hot-rolled steel sheet comprising C: 0.010% or more and 0.050% or less in mass%, and Si: 0.2% or less in such a manner that S, N, and Ti satisfy the following formula (1). Mn: 0.1% or more and 0.8% or less, 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, 0.10% or less, and the balance containing Fe And an unavoidable composition of impurities; a matrix having an area ratio of the ferrite grain iron phase relative to the entire structure of the substrate of 95% or more; and a structure obtained by dispersing and depositing fine carbides containing Ti and having an average particle diameter of less than 10 nm; Tensile strength is above 590 MPa, Ti≧0.04+(N/14×48+S/32×48)......(1)

(S, N, Ti: content (% by mass) of each element).

[2] The high-tensile hot-rolled steel sheet according to the above [1], containing boron (B): 0.0035% or less, in a mass %, in a manner satisfying the following formula (2). B≧0.0003-0.00025Mn.........(2)

(Mn, B: content (% by mass) of each element).

[3] The high-tensile hot-rolled steel sheet according to [2], wherein the B is 0.0003% or more and 0.0020% or less.

[4] The high-tensile hot-rolled steel sheet according to the above [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 (% by mass) of each element).

[5] The high-tensile hot-rolled steel sheet according to the above [1] or [2], wherein the 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] The high-tensile hot-rolled steel sheet according to the above [1] or [2], in addition to the above composition, further contains, in mass%, copper (Cu), tin (Sn), and nickel (Ni) in a total amount of 0.1% or less. ), calcium (Ca), magnesium (Mg), cobalt (Co), arsenic (As), chromium (Cr), tungsten (W), niobium (Nb), lead (Pb), tantalum (Ta), molybdenum (Mo Any one or more of vanadium (V).

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

[9] A method for manufacturing a high-tension hot-rolled steel sheet, which is applied to a steel material Hot rolling including rough rolling and finish rolling, after completion of finish rolling, cooling and winding to form a hot-rolled steel sheet, the steel material including: in mass %, so that S, N, and Ti satisfy the following The formula (1) contains C: 0.010% or more and 0.050% or less, Si: 0.2% or less, Mn: 0.1% or more, 0.8% or less, P: 0.025% or less, S: 0.01% or less, and N: 0.01%. Hereinafter, Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, and the remainder contains a composition of Fe and unavoidable impurities; and the finish rolling temperature of the finish rolling is set to 880 ° C or higher, and the average cooling rate of the above cooling When it is 10 ° C / s or more, the winding temperature is 550 ° C or more and less than 800 ° C, and the tensile strength is 590 MPa or more, Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48)... (1)

(S, N, Ti: content (% by mass) of each element).

[10] The method for producing a high-tensile hot-rolled steel sheet according to the above [9], in addition to the above-described composition, further contains, in mass%, a formula of the following formula (2), and contains B: 0.0035% or less. B≧0.0003-0.00025Mn.........(2)

(Mn, B: content (% by mass) of each element).

[11] The method for producing a high-tensile hot-rolled steel sheet according to [10], wherein the B is 0.0003% or more and 0.0020% or less.

[12] The method for producing a high-tensile hot-rolled steel sheet according to the above [9] or [10], wherein the composition satisfies the following formula (3): C/12>Ti/48-N/14-S/32... (3)

(C, S, N, Ti: content (% by mass) of each element).

[13] The method for producing a high-tensile hot-rolled steel sheet according to the above [9] or [10], further comprising Cu, Sn, Ni, Ca, Mg in a total amount of 0.1% or less, in addition to the above-mentioned composition, in total by mass% or less Any one or more of Co, As, Cr, W, Nb, Pb, Ta, Mo, and V.

According to the present invention, it is possible to provide an industrially high-efficiency effect by providing a high-tensile hot-rolled steel sheet which is suitable for an automobile steel sheet or the like, has a tensile strength (TS) of 590 MPa or more, and has a pressing property. The materials such as the chassis parts with complicated cross-sectional shapes are very suitable for excellent workability (extension).

Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the structure and carbide of the steel sheet of the present invention are carried out. Description.

The hot-rolled steel sheet according to the present invention has a matrix in which the ferrite-grained iron phase is 95% or more with respect to the area ratio of the entire structure, and the fine carbide containing Ti and having an average particle diameter of less than 10 nm is dispersed and precipitated in the matrix. Into the organization.

Fertilizer iron phase: 95% or more based on the area ratio of the whole tissue

In the present invention, it is necessary to form a ferrite-grain iron phase in order to ensure the workability (stretching property) of the hot-rolled steel sheet. In order to improve the elongation and the edge of the hot-rolled steel sheet, it is effective to set the structure of the hot-rolled steel sheet to a ferrite-rich iron phase having a low dislocation density and excellent ductility. In particular, in order to improve the edge-stretching property, it is preferable to set the structure of the hot-rolled steel sheet into a single-phase structure of the ferrite-grained iron, but even in the case of a single-phase structure of the ferrite-free iron, it is substantially The single-phase structure of the ferrite-grained iron, that is, the effect of the above-mentioned effect can be fully exerted as long as 95% or more of the area ratio with respect to the entire structure is the ferrite-grain iron phase. Therefore, the area ratio of the ferrogranular iron phase to the entire structure is set to 95% or more. It is preferably 97% or more.

Further, in the hot-rolled steel sheet according to the present invention, examples of the structure other than the ferrite-grain iron phase include celite, bundite, toughened iron phase, mashed iron phase, and retained austenite, as long as The total of these is 5% or less, preferably about 3% or less, with respect to the area ratio of the entire structure, and is acceptable.

Fine carbide containing Ti

The carbide containing Ti tends to be a fine carbide having an extremely small average particle diameter. Therefore, by dispersing fine carbides into hot rolled steel In the present invention in which the strength of the hot-rolled steel sheet is increased in the sheet, the fine carbide which is deposited and precipitated is a fine carbide containing Ti.

Average particle size of fine carbide: less than 10 nm

The average particle diameter of the fine carbide is extremely important in imparting a desired strength (tensile strength: 590 MPa or more) to the hot-rolled steel sheet. In the present invention, the average particle diameter of the fine carbide containing Ti is set to be less than 10 nm. When the fine carbide is precipitated in the matrix, the fine carbide acts as a resistance against the movement of dislocations generated when the deformation is applied to the steel sheet, whereby the hot-rolled steel sheet is strengthened, but if the fine carbide is averaged When the diameter is set to be less than 10 nm, the above effect is more remarkable. Therefore, the average particle diameter of the fine carbide containing Ti is set to be less than 10 nm. More preferably 5 nm or less.

In order to stably obtain the strength of the hot-rolled steel sheet, it is effective to control the state of dispersion and precipitation of the fine carbide containing Ti. In the present invention, it is preferred that the fine carbide is dispersed and precipitated so that the volume ratio of the fine carbide containing Ti and having an average particle diameter of less than 10 nm to the entire structure is 0.0005 or more. However, when the volume ratio exceeds 0.003, the strength is too high and the edge-strengthening property is lowered. Therefore, the volume is preferably 0.0005 or more and 0.003 or less.

In addition, in the present invention, in the form of precipitation of the fine carbide containing Ti, in addition to the columnar precipitation as the main precipitation form, even if finely precipitated fine carbides are mixed, the properties are hardly affected. Therefore, regardless of the form of precipitation, it can be referred to as dispersion precipitation in accordance with various precipitation forms.

Next, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention are determined. Description. In addition, the % which shows the following component composition means the mass % unless it demonstrates especially.

C: 0.010% or more and 0.050% or less

C is an element necessary for strengthening a hot rolled steel sheet to form fine carbides. When the C content is less than 0.010%, tensile strength of 590 MPa or more cannot be obtained. On the other hand, when the C content is more than 0.050%, the strength is increased, and the ferrite is easily formed in the steel sheet, so that it is difficult to obtain excellent stretchability. Therefore, the C content is set to be 0.010% or more and 0.050% or less. It is preferably 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 is more than 0.2%, the precipitation of C from the ferrite-rich iron phase is promoted, and coarse Fe carbide is easily precipitated at the grain boundary, and the elongation property is lowered. Moreover, excessive Si adversely affects plating properties. Therefore, the Si content is set to 0.2% or less. It is preferably 0.05% or less. Further, in order to solid solution strengthening, it is preferably 0.005% or more.

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. When the Mn content is less than 0.1%, solid solution strengthening cannot be obtained. In addition, when the Mn content is less than 0.1%, the Ar ₃ transformation point becomes too high, and the fine carbide containing Ti as described later tends to be coarsened. On the other hand, when the Mn content exceeds 0.8%, segregation is likely to occur, and a phase other than the ferrite-grained iron phase, that is, a hard phase is formed, and the edge-hanging property is lowered. Therefore, the Mn content is set to be 0.1% or more and 0.8% or less. It is preferably 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. When the P content exceeds 0.025%, segregation is remarkable, and the elongation is lowered. Therefore, the P content is set to 0.025% or less. It is preferably 0.02% or less. Further, in order to solid solution strengthening, it is preferably 0.005% or more.

S: 0.01% or less

S is an element which lowers hot workability (hot rolling property), and in addition to improving the hot-tearing sensitivity of a slab, it is present as manganese sulfide (MnS) in steel, thereby making the hot rolled steel sheet The elongation of the edge deteriorates. Therefore, in the present invention, it is preferable to reduce S as much as possible and to set it to 0.01% or less. It is preferably 0.005% or less.

N: 0.01% or less

N is a harmful element in the present invention, and it is preferred to reduce it as much as possible. In particular, if the N content exceeds 0.01%, the coarse nitride is formed in the steel, so that the stretchability is lowered. Therefore, the N content is set to 0.01% or less. It is preferably 0.006% or less.

Al: 0.06% or less

Al is an element that functions as an oxygen scavenger. In order to obtain this effect, it is desirable to contain 0.001% or more, and the inclusion of more than 0.06% lowers elongation and stretchability. Therefore, the Al content is set to Al: 0.06% or less.

Ti: 0.05% or more and 0.10% or less

Ti is the most important element in the invention. Further, Ti is an element which maintains excellent elongation by forming carbides and contributes to high strength of the steel sheet. When the Ti content is less than 0.05%, the desired strength of the hot-rolled steel sheet (tensile strength: 590 MPa or more) cannot be ensured. On the other hand, when the Ti content exceeds 0.10%, the stretchability tends to be lowered. Therefore, the Ti content is set to 0.05% or more and 0.10% or less, preferably 0.065% or more and 0.095% or less.

The hot-rolled steel sheet of the present invention contains S, N, and Ti in such a range as to satisfy the formula (1).

Ti≧0.04+(N/14×48+S/32×48)......(1)

(S, N, Ti: content of each element (% by mass))

The above formula (1) is a requirement to satisfy the above-described desired precipitation state of the fine carbide containing Ti, 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 a hot-rolled steel sheet which melts carbides in the steel material during heating before hot rolling, and mainly after hot rolling Precipitated during winding. Here, in order to stably disperse and precipitate the fine carbides in order to set the size of the fine carbides to have an average particle diameter of less than 10 nm, it is necessary to sufficiently ensure the production. It is the amount of Ti of the precipitated core of the fine carbide. However, in the high temperature range, Ti is more likely to form nitrides or sulfides than carbides. Therefore, when the Ti content is insufficient with respect to the N content and the S content of the steel material, the amount of Ti which is a precipitation nucleus of the fine carbide is reduced as the nitride or sulfide is precipitated, and it is difficult to make the Ti-containing fine. The carbide is sufficiently precipitated.

Therefore, in the present invention, the Ti content, the N content, and the S content are controlled so as to satisfy the formula (1) of Ti≧0.04+(N/14×48+S/32×48). Thereby, the amount of Ti which is a precipitated nucleus of the fine carbide is sufficiently ensured, and the size of the fine carbide can be set such that the average particle diameter is less than 10 nm, and the fine carbide can be stably precipitated.

Further, in the present invention, the steel material is heated to austenite range before hot rolling to melt the carbide in the steel material, and the Ti-containing carbide is precipitated simultaneously with the subsequent austenite→fertilizer iron transformation. . However, if the austenite→fertilizer iron metamorphic temperature is high, the precipitated Ti-containing carbide becomes coarse. Therefore, in the present invention, it is preferred to adjust the temperature (Ar ₃ metamorphic point) of the austenite→fertilizer iron deformation state in the winding temperature range, thereby precipitating the Ti-containing carbide at the time of winding. Thereby, the coarsening of the above-mentioned Ti-containing carbide can be suppressed, and a carbide having an average particle diameter of less than 10 nm can be obtained.

In the case of adjusting the temperature (Ar ₃ metamorphic point) of the austenite → ferrite iron in the winding temperature range, it is preferable to contain B: 0.0035% in addition to the above-described composition and further satisfy the following formula (2). the following.

B≧0.0003-0.00025Mn.........(2)

B: 0.0035% or less

B is that the Ar ₃ transformation point of steel reducing element of the present invention, reduced by the addition of B Ar ₃ transformation point of steel, can be miniaturized carbides containing Ti. In order to obtain the above effects, it is preferred to set the B content to 0.0003% or more. On the other hand, even if it exceeds 0.0035%, the effect of containing B is saturated. Therefore, the B content is preferably set to 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, in the case where B is contained, it is also important to control the ratio of the B content to the Mn content in the steel within an appropriate range. The inventors of the present invention have studied a method in which a carbide containing Ti is fine (having an average particle diameter of less than 10 nm) in a matrix having an area ratio of ferrite-grain iron phase to the entire tissue of 95% or more. The ground is dispersed and precipitated. As a result, it was found that the temperature of the austenite→fertilizer iron metamorphosis (Ar ₃ metamorphic point) during the hot rolling process is adjusted in the winding temperature range described later, and the carbide containing Ti is refined to an average particle diameter: less than 10 An extremely efficient method of nm.

Further, the present inventors have further studied and found that the steel composition of the present invention can be controlled in such a manner that the B content and the Mn content of the steel material satisfy the desired relationship, and the steel can be adjusted within the target range. Ar ₃ metamorphosis point. Here, in the above formula, when the value of the right side (0.0003-0.00025Mn) is zero or less, the value on the right side is regarded as zero.

In addition, in the present invention, when the content of Mn as a solid solution strengthening element exceeds 0.35%, the desired steel sheet strength (tensile strength: 590 MPa or more) can be secured without using the effect of the above B. However, when the content of Mn is 0.35% or less, it is difficult to ensure the desired strength of the steel sheet without using the effect of the above B. Therefore, when the content of Mn is 0.35% or less, it is preferable to contain B in order to make the carbide containing Ti finer.

Further, in the present invention, it is preferred to adjust the contents of C, S, N, and Ti within the above range and satisfying the formula (3).

C/12>Ti/48-N/14-S/32.........(3)

(C, S, N, Ti: content of each element (% by mass))

As described above, the carbide containing Ti tends to be a fine carbide having an extremely small average particle diameter. However, when the Ti bonded to C is C or more in atomic ratio, the carbide is easily coarsened. Further, with the coarsening of the carbide, it is difficult to secure the desired strength of the hot-rolled steel sheet (tensile strength: 590 MPa or more).

Therefore, in the present invention, it is preferred to set the C, Ti, N, and S contents as in the formula (3). That is, in the present invention, with respect to C and Ti contained in the steel material, it is preferred that the atomic % (C/12) of C is more than the atomic % of Ti which contributes to the formation of carbides (Ti/48- N/14-S/32). Thereby, the coarsening of the fine carbide containing Ti can be suppressed.

The steel sheet of the present invention may further contain 0.1% or less in total, preferably 0.03% or less of any one of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, and V. Further, the components other than the above are Fe and unavoidable impurities.

Further, the steel sheet of the present invention may be provided with a coating 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, such as a material of a chassis part of an automobile, can be obtained. In addition, examples of the plating film include a hot-dip galvanized film, an alloyed hot-dip galvanized film, and the like.

Next, a method of producing the hot-rolled steel sheet of the present invention will be described.

In the present invention, hot rolling is performed on the steel material including rough rolling and finish rolling, and after completion of the finish rolling, cooling and winding are performed to form a hot rolled steel sheet. The present invention is characterized in that 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 more, and the winding temperature of the winding is 550 ° C or higher. Less than 800 ° C.

In the present invention, the method of melting the steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, after melting, it is preferred to form a slab (steel material) by a continuous casting method due to problems such as segregation, but a known casting method such as agglomerate-block rolling method or thin slab continuous casting method may be used. To form a billet. Further, when the slab is hot-rolled after casting, the slab may be reheated by a heating furnace and then rolled to maintain the temperature at a predetermined temperature or higher, or the slab may be heated without being heated. .

The steel material obtained as described above is subjected to rough rolling and finish rolling, and in the present invention, it is necessary to melt the carbide in the steel material before rough rolling. In the invention containing Ti as a carbide forming element, it is preferred to set the heating temperature of the steel material to 1150 ° C or higher. However, if the heating temperature of the steel material becomes too high, the surface is excessively oxidized to generate TiO ₂ to consume Ti, and in the case of forming a steel sheet, a decrease in hardness near the surface is likely to occur, so that the above heating temperature is preferably set. It is below 1300 °C. Further, as described above, the steel material before the rough rolling is maintained at a temperature equal to or higher than the predetermined temperature, and when the carbide in the steel material is melted, the step of heating the steel material before the rough rolling can be omitted. In addition, the rough rolling conditions are not particularly limited.

Finishing temperature: above 880 °C

The appropriateness of the finish rolling temperature is important for ensuring the elongation and the edge of the hot-rolled steel sheet and reducing the rolling load of the finish rolling. When the finish rolling temperature is less than 880 ° C, the crystal grains of the surface layer of the hot-rolled steel sheet become coarse and the edge-stretching property is broken. Further, since the rolling is performed in the non-recrystallization temperature region, coarse Ti carbides are precipitated in the old austenite grain boundary, and the elongation is lowered. Therefore, the finish rolling temperature is set to 880 ° C or higher. It is preferably set to 900 ° C or higher. In addition, when the finish rolling temperature is too high, the crystal grains are coarsened and the desired steel sheet strength (tensile strength: 590 MPa or more) is adversely affected. Therefore, the finish rolling temperature is preferably 1000 ° C or lower.

Average cooling rate: 10 ° C / s or more

After the completion of the finish rolling, if the average cooling rate from the temperature of 880 ° C or higher to the winding temperature is less than 10 ° C / s, the Ar ₃ transformation point becomes high, and the Ti-containing carbide is not sufficiently refined. Therefore, the above average cooling rate is set to 10 ° C / s or more. It is preferably 30 ° C / s or more. Moreover, in order to obtain the ferrite iron structure, it is preferably less than 200 ° C / s.

Winding temperature: 550 ° C or more and less than 800 ° C

The appropriateness of the winding temperature is extremely important in that the structure of the hot-rolled steel sheet is a desired structure for the entire region in the width direction of the hot-rolled steel sheet, that is, the ferrite-grain phase is relative to the entire structure. The area ratio is 95% or more of a matrix, and a structure obtained by dispersing and depositing fine carbides containing Ti and having an average particle diameter of less than 10 nm.

When the winding temperature is less than 550 ° C, the super-cooling state is likely to occur, and precipitation of fine carbides is insufficient at the end portions in the width direction of the rolled material, and it is difficult to impart desired steel sheet strength (tensile strength: 590 MPa or more). . Moreover, it is difficult to ensure the stability of the transition on the Run-Out-Table. On the other hand, when the winding temperature is 800 ° C or higher, the ferrite is generated, and it is difficult to form a ferrite-grained iron phase having a matrix ratio of 95% or more with respect to the entire area ratio of the structure. Therefore, the winding temperature is set to 550 ° C or more and less than 800 ° C. It is preferably 550 ° C or more and less than 700 ° C, more preferably 580 ° C or more and less than 700 ° C.

As described above, in the case of producing a high-tensile hot-rolled steel sheet having excellent workability (stretching property), which is applicable to a material such as a chassis member having a tensile strength (TS) of 590 MPa or more and a complicated cross-sectional shape, It is necessary to disperse and precipitate fine carbides having an average particle diameter of less than 10 nm over the entire region in the width direction of the steel sheet.

However, in the present invention, Ti (Ti≧0.04+(N/14×48+S/32×48)) is contained in a predetermined amount or more with respect to the N content and the S content in the steel which is the material of the hot-rolled steel sheet. Or, in the steel which is the material of the hot-rolled steel sheet, the B content and the Mn content satisfy the predetermined relationship. It is contained in the form of (B≧0.0003-0.00025Mn), whereby it is possible to control a composition in which fine carbides having an average particle diameter of less than 10 nm are sufficiently dispersed and precipitated. Therefore, according to the present invention, even if the production conditions of the hot-rolled steel sheet are not so strict, fine carbides having an average particle diameter of less than 10 nm can be dispersed and precipitated over the entire width direction, so as to extend throughout the width direction of the hot-rolled steel sheet. The zone imparts uniform and good properties (tensile strength, elongation).

Further, in the present invention, the hot-rolled steel sheet produced as described above is subjected to a plating treatment, whereby a plating film may be formed on the surface of the steel sheet. For example, a hot-dip galvanizing treatment may be performed on the surface of the steel sheet by performing a hot-dip galvanizing treatment as a plating treatment to form a hot-dip galvanized film, or after performing a alloying treatment after the hot-dip galvanizing treatment.

Instance

A steel slab (steel material) having a wall thickness of 250 mm was formed by melting and continuously casting the molten steel of the composition shown in Table 1 by a generally known method. After heating the slabs to 1,250 ° C, the rough rolling was performed, and finish rolling was performed at the finishing rolling temperature shown in Table 2, and after the completion of the finish rolling, the temperature range from 880 ° C to the winding temperature was The average cooling rate shown in Table 2 was cooled, and wound at the winding temperature shown in Table 2 to form a hot-rolled steel sheet having a thickness of 2.3 mm. Further, some of the hot-rolled steel sheets (hot-rolled steel sheets a2, b2, and c2) were immersed in a galvanizing bath (0.1% Al-Zn) at 480 ° C to form a hot-dip coating having a single-sided adhesion amount of 45 g/m ² . After the zinc film, it was alloyed at 520 ° C to form an alloyed hot-dip galvanized steel sheet.

A test piece was taken from the hot-rolled steel sheet obtained as described above, and subjected to a structure observation, a tensile test, and a hole expansion test to determine the area ratio of the ferrite grain iron phase, the average particle diameter and volume ratio of the fine carbide containing Ti, and the tensile strength. Reaming rate Marginality). The test method is as follows.

(i) Organizational observation

A test piece was taken from the obtained hot-rolled steel sheet, and a cross section parallel to the rolling direction of the test piece was mechanically polished, and after being etched in a nital etching solution (Nital), a scanning electron microscope (SEM) was used. The tissue photograph (SEM photograph) photographed at a magnification of 3000 times was used to determine the type of tissue other than the ferrite grain iron phase and the fertiliser iron phase and the area ratios thereof by the image analysis device.

Furthermore, the film produced from the hot-rolled steel sheet was observed by a transmission electron microscope (TEM) at a magnification of 260,000 times, and the particle diameter of the fine carbide containing Ti was determined.

With respect to the particle diameter of the fine carbide containing Ti, each particle diameter is obtained by image processing using an approximate circle based on the observation result of 30 fields of view at 260,000 times, and the obtained particle diameter is arithmetically calculated. On average, it is set to the average particle diameter.

The α-iron was electrolyzed using 10% acetamidine-1% tetramethylammonium chloride-methanol solution (AA solution), and the weight of the Ti carbide was determined by analyzing the residue of the residue captured by filtration. The volume is determined by the density of Ti carbide (TiC), and the volume is divided by the volume of the melted α-iron to determine the volume ratio of the fine carbide containing Ti.

(ii) Tensile test

From the obtained hot-rolled steel sheet, JIS No. 5 tensile test piece (JIS Z 2201) in which the direction perpendicular to the rolling direction is a perpendicular direction is taken, and a tensile test according to JIS Z 2241 is performed. Determination of stretch Strength (TS).

(iii) Hole expansion test

A test piece (size: 130 mm × 130 mm) was taken from the obtained hot-rolled steel sheet, and a hole having an initial diameter d ₀ : 10 mm Φ was formed on the test piece by press working. A hole expansion test was carried out using these test pieces. In other words, a taper punch having a vertex angle of 60° was inserted into the hole, and the hole was expanded, and the diameter d of the hole when the crack penetrated the hot-rolled steel sheet (test piece) was measured, and the following formula was used to calculate The hole expansion ratio λ (%).

The hole expansion ratio λ (%) = {(dd ₀ ) / d ₀ } × 100 The results obtained are shown in Table 3.

In the example of the present invention, a hot-rolled steel sheet having a high tensile strength TS of 590 MPa or more and an excellent extensibility of λ: 100% or more is formed. On the other hand, the comparative example beyond the scope of the present invention cannot ensure the regulation High strength, or the hole expansion rate λ cannot be ensured.

Claims (13)

A high-tensile hot-rolled steel sheet comprising, by mass%, S, N, and Ti satisfying the following formula (1), and contains C: 0.010% or more and 0.050% or less, and Si: 0.2% or less, Mn: 0.1% or more and 0.8% or less, 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, and the balance containing Fe and inevitable a composition of impurities; a matrix having an area ratio of ferrite-grain iron phase relative to the entire structure of the substrate of 95% or more; and a structure obtained by dispersing and depositing fine carbides containing Ti and having an average particle diameter of less than 10 nm; and the tensile strength is 590 MPa or more, Ti≧0.04+(N/14×48+S/32×48) (1) (S, N, Ti: content (% by mass) of each element). The high-tensile hot-rolled steel sheet according to claim 1, wherein, in addition to the above composition, further in mass%, the following formula (2) is satisfied. In the case, B: 0.0035% or less, B≧0.0003-0.00025Mn (2) (Mn, B: content (% by mass) of each element). The high-tensile hot-rolled steel sheet according to claim 2, wherein the B is 0.0003% or more and 0.0020% or less. The high-tensile hot-rolled steel sheet according to claim 1 or 2, wherein the above composition satisfies the following formula (3): C/12>Ti/48-N/14-S/32...( 3) (C, S, N, Ti: content (% by mass) of each element). The high-tensile hot-rolled steel sheet according to the first or second aspect of the invention, wherein the volume ratio of the fine carbide to the entire structure is 0.0005 or more. The high-tensile hot-rolled steel sheet according to claim 5, wherein the volume ratio is 0.0005 or more and 0.003 or less. The high-tensile hot-rolled steel sheet according to the first or second aspect of the invention, in addition to the above composition, further contains, by mass%, Cu, Sn, Ni, Ca, Mg, Co in a total amount of 0.1% or less. Any one or more of As, Cr, W, Nb, Pb, Ta, Mo, and V. High-tension hot rolling as described in item 1 or 2 of the patent application scope A steel sheet in which a surface of the steel sheet is coated. A method for producing a high-tensile hot-rolled steel sheet, comprising: performing hot rolling including rough rolling and finish rolling on a steel material, and cooling and winding after the finishing rolling is completed to form a hot-rolled steel sheet; In a mass%, S, N, and Ti satisfy the following formula (1), and contain C: 0.010% or more and 0.050% or less, Si: 0.2% or less, and Mn: 0.1% or more and 0.8% or less. 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, 0.10% or less, and the remainder contains Fe and unavoidable impurities; The finishing rolling temperature is set to 880 ° C or higher; the average cooling rate of the cooling is set to 10 ° C / s or more; and the winding temperature is set to 550 ° C or more and less than 800 ° C, and the tensile strength is 590 MPa or more, Ti ≧ 0.04+(N/14×48+S/32×48) (1) (S, N, Ti: content (% by mass) of each element). The method for producing a high-tensile hot-rolled steel sheet according to claim 9, wherein in addition to the above composition, the mass % is further satisfied to satisfy In the following formula (2), B: 0.0035% or less, B≧0.0003-0.00025Mn (2) (Mn, B: content (% by mass) of each element) is contained. The method for producing a high-tensile hot-rolled steel sheet according to claim 10, wherein the B is 0.0003% or more and 0.0020% or less. The method for producing a high-tensile hot-rolled steel sheet according to claim 9 or 10, wherein the above composition satisfies the following formula (3): C/12>Ti/48-N/14-S/32... (3) (C, S, N, Ti: content (% by mass) of each element). The method for producing a high-tensile hot-rolled steel sheet according to the ninth or tenth aspect of the present invention, further comprising, in addition to the above composition, Cu, Sn, Ni, Ca in a total amount of 0.1% or less. Any one or more of Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, and V.