KR20030076430A - High tensile hot rolled steel sheet excellent in elongation property and elongation flanging property, and method for producing the same - Google Patents

Abstract

The present invention provides a high tensile strength hot rolled steel sheet having a TS of 780 MPa or more, or more than 980 MPa, which satisfies TS × EL ≧ 20,000 MPa% and also satisfies TS × λ ≧ 82000 MPa% and a method of manufacturing the same. Specific solutions are as follows. 0.04% by mass or more and 0.25% by mass or less of C, 0.4% by mass or more and 2.0% by mass or less of Si, 3.0% by mass or less of Mn, 0.2% by mass or less of Al, 0.007% by mass or less of S, and 0.08% by mass or more of Ti. It is contained in mass% or less, and remainder consists of Fe and an unavoidable impurity, and content of said C, said Si, and said Ti is ([% C] / 12-[% Ti] / 48) / ([% Si] / 28) A composition satisfying ≤ 0.4, containing ferrite, bainite, and retained austenite, wherein the fraction of the ferrite is 40% or more with respect to the entire structure, and the average particle diameter of the ferrite is 5 µm or less, and the bainite The high tensile strength hot-rolled steel sheet is characterized in that the fraction of 20% to 48% of the total tissue, and the residual austenite fraction is 2% to 7% of the total tissue.

Description

High tensile hot rolled steel sheet with excellent elongation characteristics and elongation flange characteristics and its manufacturing method {HIGH TENSILE HOT ROLLED STEEL SHEET EXCELLENT IN ELONGATION PROPERTY AND ELONGATION FLANGING PROPERTY, AND METHOD FOR PRODUCING THE SAME}

The present invention relates to a high tensile strength hot rolled steel sheet having excellent elongation characteristics and excellent elongation flange characteristics and a method of manufacturing the same.

Among the hot rolled steel sheets for automobiles, structural members, suspension members (eg, wheels, rims, chassis, etc.) and strength members (eg, bumpers, door guard bars, etc.) of the vehicle body have high tensile strength hot rolled steel sheets having a tensile strength of 780 MPa to 980 MPa. This is used. Among them, hot rolled steel sheets used for vehicle bodies are required to satisfy high strength and high workability in order to achieve low fuel efficiency and improved crash safety of automobiles.

Hot-rolled steel sheets developed from this point of view include composite steels (so-called DP steels) having a structure mainly composed of ferrite and martensite, or residual austenite steels having a structure composed of ferrite, bainite and residual austenite. Known.

In recent years, there is a tendency to increase the weight of the vehicle body by installing equipment considering safety and environment. Therefore, attempts have been made to reduce the weight of the vehicle body by actively using a high tensile hot rolled steel sheet having a thin tensile strength of 780 MPa or more.

For example, Patent Document 1 discloses that steel having C, Si, and Mn as a basic component is hot-rolled at a rolling reduction ratio of 80% or more and a rolling temperature of 780 ° C to 900 ° C, and less than 40 ° C / sec after rolling is completed. Cooling is started at a cooling rate of and the cooling is terminated at a predetermined temperature, and then cooled at a cooling rate of 40 ° C / sec or more and wound up at 350 ° C to 500 ° C, whereby the droplet rate of polygonal ferrite is 61% or less, poly Manufacture of hot-rolled steel sheet having a ratio of the droplet ratio and the particle size of gonnal ferrite to 18 or more, a second phase consisting of bainite and residual austenite, and a structure in which the residual austenite is 5% or more in the second phase A method is disclosed.

In this technique, the TS x EL value calculated by the tensile strength TS (MPa) and the elongation EL (%) can achieve 20000 MPa%, thereby obtaining a hot rolled steel sheet having excellent elongation characteristics. However, extension flange characteristics, which are important characteristics required for automotive high strength steel sheets, are not considered at all. Elongation flange characteristic is an index which evaluates the workability of the steel plate which shows generally using the hole expansion rate obtained by a hole expansion test. There is no relationship between stretch flange properties and stretch properties. Therefore, even if the technique disclosed in Patent Literature 1 is used, it is difficult to produce a high tensile strength hot rolled steel sheet having excellent stretch flange characteristics and excellent stretch characteristics.

In addition, Patent Document 2 discloses a high strength steel sheet having excellent stretch flange characteristics. This is characterized by having C, Si, Mn, and B as basic components, limiting the S content to 0.02% or less, and having a structure composed of three phases of polygonal ferrite, bainite, and martensite.

In this technique, a hot-rolled steel sheet having a tensile strength of 66 kgf / mm 2 (= 647 MPa) achieves a hole expansion rate of? 150% (ie, TS x? = 97050 MPa%). However, the elongation characteristic is only 24% (that is, TS x EL = 15528 MPa%), and there is a problem that the application to the suspension member which requires a lot of elongation characteristics is limited. In addition, Patent Document 2 does not describe a high tensile strength hot rolled steel sheet having a tensile strength of 780 MPa or more (so-called TS 780 MPa grade hot rolled steel sheet), and it is difficult to apply it to a high tensile strength hot rolled steel sheet having a tensile strength of 780 MPa.

In addition, Patent Document 3 discloses a high strength hot rolled steel sheet excellent in stretch flange characteristics. This is characterized in that C, Si, Mn, Ti, and Nb are the basic components, and the ferrite having an average particle diameter of 25 µm or less has an area ratio of 70% to 95%, and the balance is made of martensite or a structure containing residual austenite. Doing.

Since this technique contains martensite in the structure, the tensile strength achieves 99 kgf / mm 2 (= 970 MPa). However, in this technique, even at TS 80 kgf / mm 2 (= 784 MPa), the hole expansion ratio λ is 48%, and the extension flange characteristics are not sufficient.

In addition, Patent Document 4 discloses a high tensile steel sheet having excellent burring characteristics. This is characterized by having a structure composed of C, Si, Mn, and Ti as a basic component and comprising a main phase (i.e., ferrite) having an average particle diameter of 5 µm or less and a second phase having an average particle diameter of 3.5 µm or less.

This technique is intended to produce a high tensile strength steel sheet having a good TS-EL balance and a TS-λ balance, particularly excellent burring characteristics (i.e., hole expansion workability). However, since the pearlite is contained in the second phase, the tensile strength disclosed is 740 MPa at maximum, but does not reach 780 MPa.

Patent Document 1: Japanese Patent Application Laid-Open No. 3-10049

Patent Document 2: Japanese Patent Application Laid-open No. 58-167750

Patent Document 3: Japanese Patent Application Laid-Open No. 9-125194

Patent Document 4: Japanese Unexamined Patent Publication No. 2000-192191

In order to achieve a lighter weight of the vehicle body, a high tensile hot rolled steel sheet having a tensile strength of TS of 780 MPa or more, or even of 980 MPa or more, having an elongation characteristic of achieving TS × EL ≥ 20000 MPa%, and attaining TS × λ 82000 MPa%. There was a need for a steel sheet that also had stretch flange characteristics to achieve. That is, in the case of TS 780 MPa, for example, a high tensile strength hot rolled steel sheet having characteristics of EL ≧ 25.5% and λ ≧ 105% is required. However, as mentioned above, there is no conventional technique that can achieve this.

The present invention solves the above problems, the TS is 780 MPa or more, or even 980 MPa or more, the elongation characteristics are good, that is, TS × EL ≥ 20000 MPa%, and the elongation flange characteristics are good, that is TS An object of the present invention is to provide a high tensile strength hot rolled steel sheet that satisfies λ ≧ 82000 MPa% and a method of manufacturing the same.

The present inventors earnestly researched in order to achieve the above object, and as a result, refine the ferrite produced after hot rolling with Ti as an essential component, and at the same time, determine the fraction of bainite and residual austenite produced from unmodified austenite. By adjusting to the range of, it has been found that the elongation characteristics and the elongation flange characteristics of a high tensile strength hot rolled steel sheet having a tensile strength of 780 MPa or more or even 980 MPa or more can be significantly improved.

In addition, it was found that such a high tensile strength hot rolled steel sheet can be stably produced by keeping the addition amount of C and Si within a predetermined range.

In the present invention, C is 0.04% by mass or more, 0.25% by mass or less, Si is 0.4% by mass or more, 2.0% by mass or less, Mn is 3.0% by mass or less, Al is 0.2% by mass or less, S is 0.007% by mass or less, Ti is 0.08% It contains in mass% or more and 0.3 mass% or less, the balance consists of Fe and an unavoidable impurity, and content of said C, said Si, and said Ti satisfy | fills following formula (1), ferrite, bainite, and remainder Containing austenite, the fraction of ferrite is 40% or more with respect to the whole tissue, the average particle diameter of the ferrite is 5 µm or less, the fraction of bainite is 20% to 48% with respect to the whole tissue, and the residue It is a high tensile strength hot rolled steel sheet characterized by having a tissue having a fraction of austenite of 2% to 7% of the whole tissue.

In the present invention, C is 0.04% by mass or more and 0.25% by mass or less, Si is 0.4% by mass or more and 2.0% by mass or less, Mn is 3.0% by mass or less, Al is 0.2% by mass or less, S is 0.007% by mass or less, Ti Is contained in an amount of 0.08% by mass or more and 0.3% by mass or less, the balance consists of Fe and unavoidable impurities, and the content of C, Si and Ti is represented by the following formula (1):

([% C] / 12-[% Ti] / 48) / ([% Si] / 28)? (One)

(% C): C content (mass%)

(% Ti): Ti content (mass%)

[% Si]: Si content (mass%)

The composition is heated to have the steel slab to below 1150 ℃ satisfying then, (Ar ₃ transformation point + 20 ℃) over yet (Ar ₃ transformation point + 100 ℃) hot rolled in a finish rolling temperature not higher than, and the resulting hot rolled steel sheet 30 ℃ Cooling at a cooling rate of at least 2 seconds / second and staying for 2 seconds to 20 seconds in a temperature range of 600 ° C. to 750 ° C., followed by cooling at a cooling rate of 15 ° C./sec or more to the hot rolled steel at a temperature range of 380 ° C. to 520 ° C. It is a method for producing a high tensile strength hot rolled steel sheet, wherein the steel sheet is wound up.

Best Mode for Carrying Out the Invention

First, the composition of the high tensile strength hot rolled steel sheet of the present invention will be described.

C: 0.04 mass% or more and 0.25 mass% or less

C is an element necessary to improve the strength of the hot rolled steel sheet, combine with Ti to be described later to form TiC to refine the structure of the hot rolled steel sheet, and to produce bainite and residual austenite within a range of a fraction to be described later. In order to obtain a tensile strength of 780 MPa or more, it is necessary to contain C 0.04 mass% or more. Moreover, when it exceeds 0.25 mass%, the weldability of a hot rolled steel sheet will remarkably deteriorate. Therefore, C needs to satisfy the range of 0.04 mass% or more and 0.25 mass% or less. In addition, in order to further prevent deterioration of weldability, it is preferable to make C amount into 0.20 mass% or less. More preferably, they are 0.08 mass% or more and 0.16 mass% or less.

Si: 0.4 mass% or more and 2.0 mass% or less

Si is an element that acts as a deoxidation element in the steelmaking process. In addition, Si contained in the hot-rolled steel sheet improves the strength of the hot-rolled steel sheet without impairing the yield ratio or strength-elongation balance (extension characteristics) by solid solution strengthening, and activates the transformation from austenite to ferrite by unmodified austenite. It is an element that promotes C concentration on the nit phase. In addition, an essential element for forming a tissue composed of ferrite, bainite and residual austenite by suppressing the formation of carbides, such as Fe ₃ C. In order to acquire these effects, it is necessary to contain Si 0.4 mass% or more. Moreover, when it exceeds 2.0 mass%, these effects will be saturated, and also the scale hard to peel on the hot rolled steel sheet will generate | occur | produce, and scale defect will generate | occur | produce, and it becomes difficult to apply it to the use which requires an external appearance. Therefore, Si needs to satisfy the range of 0.4 mass% or more and 2.0 mass% or less. Moreover, Preferably they are 0.7 mass% or more and 1.5 mass% or less.

Mn: 3.0 mass% or less

Mn is an element which improves the strength of the hot rolled steel sheet and improves hardenability. Moreover, when S mentioned later is precipitated as MnS, it also has an effect which suppresses deterioration of the various characteristic which originates in S. When the content of Mn exceeds 3.0% by mass, bainite transformation after winding of the hot rolled steel sheet is suppressed, and residual austenite is significantly reduced. Therefore, Mn was made into 3.0 mass% or less. Moreover, in order to acquire said effect, it is preferable to contain Mn 0.5 mass% or more. Moreover, it is further more preferable to set it as 1.0 mass% or more and 2.5 mass% or less.

Al: 0.2 mass% or less

Al acts as a deoxidizer in the steelmaking process. When the content of Al exceeds 0.2% by mass, the deoxidation effect is saturated, and the toughness and elongation flange properties of the hot rolled steel sheet are deteriorated. Therefore, Al was made into 0.2 mass% or less. In addition, in order to acquire the said effect, it is preferable to contain Al 0.01 mass% or more. Moreover, it is further more preferable to set it as 0.02 mass% or more and 0.05 mass% or less.

S: 0.007 mass% or less

Since S is an element that degrades the toughness and elongation flangeability of a hot rolled steel sheet, it is necessary to reduce S as much as possible. When the content of S exceeds 0.007% by mass, the toughness and elongation flange properties of the hot rolled steel sheet are significantly degraded. Therefore, S was made into 0.007 mass% or less. More preferably, it is 0.005 mass% or less, More preferably, you may be 0.0025 mass% or less. Moreover, in the refinement | refining technique of image development, in order to reduce S to less than 0.001 mass%, since enormous refining time and various additives are required, cost rises. For this reason, the lower limit of the amount of S in the manufacturing technique of image development is about 0.001 mass%.

Ti: 0.08 mass% or more and 0.3 mass% or less

Ti combines with C by heat treatment of the steel slab prior to hot rolling to form TiC. As a result, the particle size of the austenite in the heat treatment is approximately 50 µm or less, and coarsening of the ferrite particles of the hot rolled steel sheet is prevented. That is, by hot rolling a steel slab having austenite particles having a particle diameter of approximately 50 µm or less, recrystallization of the austenite particles proceeds to produce finer austenite particles. In addition, during cooling of the hot rolled steel sheet, ferrite transformation is promoted to produce fine ferrite particles, and at the same time, untransformed austenite is refined. In the subsequent cooling process, bainite and austenite produced in the low temperature region are also refined to obtain a hot rolled steel sheet having a uniform and fine structure.

The hot rolled steel sheet thus obtained has excellent elongation characteristics and elongation flange properties. In order to acquire such an effect, it is necessary to contain Ti by 0.08 mass% or more. If the content exceeds 0.3% by mass, the recrystallization of austenite is significantly inhibited, not only the structure of the hot rolled steel sheet is coarsened but also the deterioration characteristics and the elongation flange characteristics are deteriorated. Therefore, Ti needs to satisfy the range of 0.08 mass% or more and 0.3 mass% or less. Moreover, Preferably they are 0.12 mass% or more and 0.25 mass% or less.

In addition, C content, Ti content, and Si content need to satisfy following formula (1) in order to form the mixed structure containing ferrite, bainite, and residual austenite which are mentioned later.

([% C] / 12-[% Ti] / 48) / ([% Si] / 28)? (One)

(% C): C content (mass%)

(% Ti): Ti content (mass%)

[% Si]: Si content (mass%)

Bainite and residual austenite are produced from unmodified austenite in the cooling process after hot rolling. When the hot rolled steel sheet is cooled, diffusion of C is promoted in the high temperature region, and diffusion of C is suppressed in the low temperature region. As the diffusion of C is accelerated, the ferrite increases, and the fraction of bainite and residual austenite decreases. In other words, the diffusion behavior of C has a great influence on the production of ferrite, bainite and residual austenite.

In addition, Si suppresses the production of cementite in the hot-rolled steel sheet, and promotes the diffusion of C from ferrite into unmodified austenite. As a result, the C content of ferrite, bainite, and retained austenite reaches saturation in a short time, so that the effect on the formation of ferrite, bainite, and retained austenite even if the cooling conditions (e.g., cooling rate, etc.) change. Is suppressed. That is, Si has an enormous influence on the diffusion behavior of C.

In addition, since Ti fixes C as TiC, it greatly affects the diffusion behavior of C.

Therefore, the diffusion behavior of C is changed by the interaction of C, Si and Ti. The interaction of these elements can be evaluated by an index calculated by the number of atoms of each. That is, within the range satisfying the formula (1), diffusion of C is promoted, and a hot-rolled steel sheet having a mixed structure containing ferrite, bainite and residual austenite as described later is obtained stably. In addition, a hot rolled steel sheet made of ferrite, bainite, and retained austenite is obtained without being affected by variations in cooling conditions after hot rolling.

Next, the structure of the high tensile strength hot rolled steel sheet of this invention is demonstrated.

In the high tensile strength hot rolled steel sheet of the present invention, the fraction of ferrite is made 40% or more of the whole structure. This is because when the fraction of ferrite is 40% or more, the elongation characteristics are improved. In addition, in the case where the tensile strength is 780 MPa, and the elongation characteristics are improved, it is preferable that the ferrite is a main phase (that is, the fraction of the ferrite is 50% or more with respect to the entire structure).

In addition, the average particle diameter of ferrite particle needs to be 5 micrometers or less. When the average particle diameter exceeds 5 mu m, the elongation flange properties deteriorate remarkably. Since ferrite particles having an average particle diameter of 5 µm or less can be produced, the addition amount of the alloying element can be reduced, so that the mechanical properties such as the elongation characteristics and the elongation flange characteristics of the hot-rolled steel sheet are not deteriorated, and thus 780 MPa or even 980 MPa. Class tensile strength is obtained. Moreover, preferably, it is 4 micrometers or less in average particle diameter.

A phase other than ferrite is a mixed phase containing bainite and residual austenite. Since bainite is softer than residual austenite and martensite, the hardness difference with ferrite is small. In general, cracks in elongation flange processing occur at an interface of a phase with a large hardness difference (for example, an interface between ferrite and martensite). Therefore, the more the soft bainite, the longer the extension flange property is.

This effect is obtained when the fraction of bainite is 20% or more with respect to the whole tissue. In addition, if the fraction of bainite exceeds 48%, the fraction of ferrite decreases and the extension characteristics deteriorate. In addition, the C content in the unmodified austenite is remarkably lowered and the residual austenite is also reduced, which causes deterioration in extension characteristics. Therefore, the fraction of bainite needs to be 20% to 48% of the whole tissue. In addition, in the case where the tensile strength is 780 MPa class to improve the elongation characteristics, the fraction of bainite is preferably 40% or less, and more preferably 25% to 35%.

Residual austenite exhibits uniform and high elongation characteristics by generation of processed organic martensite. This effect is obtained when the fraction of retained austenite is 2% or more with respect to the whole tissue. In addition, when the fraction of the retained austenite exceeds 7%, the retained austenite is hardened by stretching flange processing, and the hardness difference with the ferrite becomes large. As a result, cracks tend to occur at the interface between the ferrite and the retained austenite by extension flange processing. Therefore, the fraction of retained austenite needs to be 2% to 7% of the whole tissue. Also, it is preferably 4% to 6%.

In addition, in the manufacturing process of a hot rolled steel sheet, martensite may be produced in addition to ferrite, bainite, and retained austenite. Martensite is the hardest phase of the hot rolled steel. For this reason, cracks tend to occur at the interface between ferrite and martensite by extension flange processing. Therefore, the smaller the fraction of martensite is, the better, and 5% or less is preferable for the whole tissue.

In this way, ferrite and retained austenite which improves the elongation characteristics, and bainite which improves the elongation flange characteristics are produced at appropriate fractions, so that a high tensile strength hot rolled steel sheet having excellent elongation characteristics and excellent elongation flange characteristics is produced. Obtained.

Next, the manufacturing method of the high tensile strength hot rolled steel sheet of this invention is demonstrated.

The molten steel of the composition mentioned above is melted, and a steel slab is manufactured by the conventionally known method, such as a continuous casting method or an ingot method. Then, the steel slab is charged into a heating furnace and heated to 1150 ° C. or less. When the heating temperature of the steel slab exceeds 1150 ° C., TiC is dissolved and micronization of austenite particles cannot be achieved. As a result, the ferrite is coarsened, and the extension characteristics and the extension flange characteristics are deteriorated.

As for the lower limit of the heating temperature of a steel slab, in order to ensure the finishing rolling temperature mentioned later, 1050 degreeC or more is preferable. Moreover, the more preferable range of the heating temperature of a steel slab is 1050 degreeC-l100 degreeC.

In this way, hot rolling is performed on the heated steel slab. Finish rolling temperature of the hot rolling, to be within the above, yet to exceed the Ar ₃ transformation point (Ar ₃ transformation point + 20 ℃) range of not more than (Ar ₃ transformation point + 100 ℃). By performing hot rolling at the finishing rolling temperature in this range, the fraction of bainite can be maintained in the range of 20% to 48% with respect to the whole structure. If the finish rolling temperature is lower than (Ar ₃ transformation point + 20 ° C), the fraction of bainite does not reach 20%, so that the fraction of ferrite and the fraction of retained austenite are increased. In addition, (Ar ₃ transformation point + 100 ℃) and if it exceeds, the dialog austenite grain growth and tighten the tissue, the elongation properties and stretch-flange properties is deteriorated.

The hot rolled steel sheet obtained by hot rolling is subjected to first stage cooling to 600 ° C to 750 ° C at a cooling rate of 30 ° C / sec or more. By making cooling rate 30 degreeC / sec or more, coarsening of a structure can be suppressed. Moreover, if the temperature which stops 1st stage cooling is out of the range of 600 degreeC-750 degreeC, ferrite transformation will be delayed by 2nd stage cooling mentioned later. As a result, the fractions of ferrite, bainite, and retained austenite cannot be properly maintained. Further, the stop temperature of the first stage cooling is preferably 650 ° C to 700 ° C.

In this way, the hot-rolled steel sheet which stopped the first stage cooling at 600 ° C to 750 ° C is kept in the temperature range of 600 ° C to 750 ° C for 2 seconds to 20 seconds. By maintaining the hot rolled steel sheet at 600 ° C to 750 ° C, the concentration of C to bainite and residual austenite can be promoted. If the residence time is less than 2 seconds, the concentration of C to austenite is insufficient, and the fractions of ferrite, bainite, and retained austenite cannot be properly maintained. Further, if it exceeds 20 seconds, ferrite transformation proceeds excessively to produce pearlite, and deterioration of stretching characteristics and stretching flange characteristics. In addition, the residence time is preferably for 4 seconds to 10 seconds. In addition, in order to hold | maintain 2 second-20 second in the said temperature range, 1st stage cooling may be stopped and air-cooled (cooling) may be carried out, and you may heat-retain using a heating apparatus.

Then, after performing the second stage cooling of the hot-rolled steel sheet to 380-520 degreeC with the cooling rate of 15 degrees-C / sec or more, the hot-rolled steel sheet is wound up. By making cooling rate 15 degreeC / sec or more, coarsening of a structure can be suppressed. In addition, by winding the hot rolled steel sheet by stopping the second stage cooling at 380 ° C to 520 ° C, generation of martensite can be suppressed to produce bainite, and residual austenite can be produced by bainite transformation. If the stop temperature (ie, the coiling temperature) of the second stage cooling is less than 380 ° C, the hot rolled steel sheet will be flared due to the decrease in the coiling temperature. In addition, martensite is produced in excess, causing deterioration of the extension flange properties. In addition, when it exceeds 520 degreeC, a pearlite is produced | generated and generation | occurrence | production of bainite and a retained austenite is suppressed, and extension property and extension flange property deteriorate. In addition, the stop temperature (ie, coiling temperature) of the second stage cooling is preferably 400 ° C to 500 ° C.

Example

Preparing a steel slab of a composition shown in Table 1, and the Ar ₃ transformation point (℃) was measured by sampling test pieces from each of the steel slabs. That is, the test piece was heated and maintained at 1250 ° C. for 30 minutes, cooled at a cooling rate of 1 ° C./sec, and the Ar ₃ transformation point was measured by a differential thermal expansion system. Table 1 shows all the measured values of the Ar ₃ transformation point.

The steel slabs (A) to (D) are examples of satisfying the component range of the present invention. In addition, the steel slab (E) is an example in which the S content is outside the scope of the present invention, and the steel slab (F) is (1) Example in which the content of Si and Ti is outside the scope of the present invention without satisfying the formula, and example in which the content of C and Mn is outside the scope of the present invention, as in the steel slab G, the steel slab is Example in which the content is outside the range of the present invention, the steel slab (I) does not satisfy the formula (1), and Example in which the C content is outside the range of the present invention, the steel slab J does not satisfy the formula (1). Yes.

These steel slabs were hot rolled under various conditions to prepare hot rolled steel sheets having a thickness of 2.9 mm. The conditions of hot rolling are as Table 2 and Table 3 showing.

The test piece was extract | collected from the hot-rolled steel sheet obtained in this way, and the particle diameter and fraction of ferrite were measured. The particle size was measured in accordance with the cutting method in the ferrite grain size test method specified in JIS Standard G0552 after photographing with an electron microscope about the rolling direction cross section. The fraction was an image analysis of the photograph taken with the electron microscope, the area ratio was calculated | required, and this was made into the fraction. The results are shown in Tables 2 and 3.

In addition, using the test piece collected from the hot-rolled steel sheet, the type of structure other than ferrite, the fraction of bainite, the fraction of retained austenite, and the fraction of martensite were examined. In addition, the tissue of the second phase was irradiated with an electron microscope. The fraction of bainite was examined by image analysis of electron micrographs. The fraction of retained austenite was calculated from the integral strengths of the (200) and (220) planes of the austenite phase and the (200) and (211) planes of the austenitic phase using an X-ray diffractometer using Co Kα rays. The fraction of was investigated by image analysis of electron micrographs. The results are shown in Tables 2 and 3.

Next, the JIS No. 5 tensile test piece was extract | collected from the rolling width direction (namely, the direction orthogonal to a rolling direction) of a hot rolled steel sheet, and the tensile test was done. The results are shown in Tables 2 and 3.

The hole expansion test was conducted in accordance with the Japanese Iron and Steel Federation Standard JFS-Tl001-1996. That is, an initial hole is formed by punching a hole diameter (do = 10 mm) with a clearance of 12.5% in the hot rolled steel sheet, and a cone punch (right angle 60 °) with the bur of the initial hole as the die side (i.e., opposite to the cone punch). Was inserted into the initial hole to expand the hole, and the hole diameter (d) at the time when the crack penetrated the hot-rolled steel sheet was obtained. The hole expansion rate (lambda) (%) was computed from following formula (2) using these d _o and d values. The results are shown in Tables 2 and 3.

lambda = 100 x (d-d _o ) / d _o . (2)

In addition, the surface of the hot-rolled steel sheet was visually observed to investigate the presence of scale defects and cracks. And the case where a scale defect or a crack is not observed was evaluated as good (0), and the case where a scale defect or a crack was observed as defect (x). The results are shown in Tables 2 and 3.

As is apparent from Tables 2 and 3, the hot-rolled steel sheets of the invention examples all satisfied the tensile strength of 780 MPa or more, and also satisfied TS x EL ≥ 20000 MPa% and TS x λ 8200 OMPa%. Moreover, evaluation of the external appearance was also favorable.

According to the present invention, a hot rolled steel sheet having tensile strength TS of 780 MPa class or even 980 MPa class and satisfying TS × EL ≥ 20000 MPa% and TS × λ ≥ 82000 MPa%, namely elongation characteristics and elongation flange High tensile hot rolled steel sheet with excellent characteristics is obtained.

Claims (2)

0.04% by mass or more and 0.25% by mass or less of C, 0.4% by mass or more and 2.0% by mass or less of Si, 3.0% by mass or less of Mn, 0.2% by mass or less of Al, 0.007% by mass or less of S, and 0.08% by mass or more of Ti. It is contained in mass% or less, remainder consists of Fe and an unavoidable impurity, and content of said C, said Si, and said Ti is following (1) Formula: ([% C] / 12-[% Ti] / 48) / ([% Si] / 28)? (One) (% C): C content (mass%) (% Ti): Ti content (mass%) [% Si]: Si content (mass%) Containing ferrite, bainite, and retained austenite, and the fraction of the ferrite is 40% or more with respect to the entire structure, the average particle diameter of the ferrite is 5 µm or less, and the fraction of the bainite is A high tensile strength hot rolled steel sheet, characterized in that it has 20% to 48% of the total, and the fraction of the retained austenite is 2% to 7% of the total tissue. 0.04% by mass or more and 0.25% by mass or less of C, 0.4% by mass or more and 2.0% by mass or less of Si, 3.0% by mass or less of Mn, 0.2% by mass or less of Al, 0.007% by mass or less of S, and 0.08% by mass or more of Ti. It is contained in mass% or less, remainder consists of Fe and an unavoidable impurity, and content of said C, said Si, and said Ti is following formula (1): ([% C] / 12-[% Ti] / 48) / ([% Si] / 28)? (One) (% C): C content (mass%) (% Ti): Ti content (mass%) [% Si]: Si content (mass%) The composition is heated to have the steel slab to below 1150 ℃ satisfying then, (Ar ₃ transformation point + 20 ℃) over yet (Ar ₃ transformation point + l00 ℃) hot rolled in a finish rolling temperature not higher than, and the resulting hot rolled steel sheet 30 ℃ Cooling at a cooling rate of at least 2 seconds / second and staying for 2 seconds to 20 seconds in a temperature range of 600 ° C. to 750 ° C., and then cooling at a cooling rate of at least 15 ° C./sec to give the hot rolled steel at a temperature range of 380 ° C. to 520 ° C. A method for producing a high tensile strength hot rolled steel sheet, the steel sheet being wound up.