JP2013133499A - High-tensile hot-rolled steel sheet excellent in bendability and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-tensile hot-rolled steel sheet with high tensile strength of 590-750 MPa excellent in bendability, and a manufacturing method therefor.SOLUTION: A steel raw material with a composition containing, by mass%, 0.010-0.055% C, 0.2% or less Si, 0.7% or less Mn, 0.02% or less S, 0.01% or less N, 0.1% or less Al, and 0.06-0.095% Ti, or further containing 0.0035% or less B is subjected to rough rolling and finish rolling, the finish rolling being performed by a tandem system rolling in which finish rolling machines are arranged in 5 or more stages successively, an entry side temperature of the finish rolling machine being set to 1,000°C or higher and an exit side temperature to 900°C or higher. After the finish rolling is finished, the hot rolled steel sheet is cooled and wound at a winding temperature of 700°C or lower. Thereby, the hot-rolled steel sheet has a structure in which the matrix consisting of ferrite crystal grain occupies 95% or more in the area rate, Ti carbide with a mean diameter of less than 6 nm is dispersed and precipitated in the ferrite crystal grain, and TiS with a mean diameter of 0.5 μm or less is dispersed and precipitated in the matrix.

Description

  The present invention is a high-tensile hot-rolled steel sheet having a high tensile strength TS of 590 MPa or more and 750 MPa or less, which is suitable as a structural member for transportation machinery parts such as automobiles, switchboards and buildings, and the production thereof. It is related to the method and relates to the improvement of workability, especially bending workability.

In recent years, improving the fuel efficiency of automobiles has always been an important issue in the automobile industry in order to reduce carbon dioxide CO ₂ emissions from the viewpoint of protecting the global environment. It is effective to reduce the weight of an automobile body to improve the fuel efficiency of the automobile, but it is necessary to reduce the weight of the vehicle body while maintaining the strength of the automobile body. If the strength of the steel sheet used for automobile parts is increased and the material is made thinner, the weight of the car body can be reduced without reducing the strength of the car body. For example, increasing the strength of steel sheets for automobile undercarriage parts leads to a significant reduction in weight of the automobile body, and is an extremely effective means for improving the fuel efficiency of automobiles. For these reasons, recently, there is a strong demand for increasing the strength of these component materials. In addition, with respect to other structural members, there is a growing demand for higher strength for the purpose of weight reduction.

  On the other hand, since many automotive parts made of steel sheets are formed by press working, the steel sheet for automobile parts has a large elongation and has excellent workability, in particular, excellent bending workability. Required. For example, in an automobile undercarriage part, since it is press-formed using a steel plate having a relatively thick plate thickness, bending processing is locally applied to the surface of the steel plate. Therefore, workability, especially bending workability is regarded as important as well as strength in a steel plate as a material for automobile undercarriage parts. For this reason, as a material for automobile undercarriage parts, a high-tensile steel plate having high elongation and excellent bending workability is required.

However, the workability of steel materials generally decreases with increasing strength. Therefore, in order to apply a high strength steel plate to an undercarriage component, a high strength steel plate having both high strength and excellent workability is required. For these reasons, a high-tensile hot-rolled steel sheet having both high strength and excellent workability is desired as a material for undercarriage parts.
In response to such a request, for example, in Patent Document 1, C: 0.03 to 0.25%, Si: 2.0% or less, Mn: 2.0% or less, P: 0.1% or less, S: 0.007% or less, A composition containing Al: 0.07% or less, Cr: 1.0% or less, and satisfying (Si + 20 × P) / (Mn + Cr) of 0.6 to 1.5, ferrite and second phase (pearlite, bainite, martensite, retained austenite 1) or more), the hardness of the second phase, the volume fraction of the second phase, and the particle size of the second phase satisfy a predetermined relationship, and have excellent fatigue characteristics and stretch flangeability A high-strength hot-rolled steel sheet having a tensile strength of 490 MPa or more has been proposed.

  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 %, Ti, Nb total of 1 type or 2 types: composition containing 0.10 ~ 0.60%, ferrite content is 95% or more in area ratio, and the average grain size of ferrite is 2.0 ~ 0.0μm, martensite In addition, a high-strength hot-rolled steel sheet with an ultrafine ferrite structure having a structure not containing residual austenite and a tensile strength of 490 MPa or more and excellent stretch flangeability has been proposed. In the technique described in Patent Document 2, fatigue strength is also improved.

Further, in Patent Document 3, in mass%, C: 0.01 to 0.1%, S: 0.03% or less, N: 0.005% or less, Ti: 0.05 to 0.5%, Si: 0.01 to 2%, Mn: 0.05 to 2 %, P: 0.1% or less, Al: 0.005 to 1.0%, and (Ti-48 / 12C-48 / 14N-48 / 32S) containing Ti in a range satisfying 0% or more, and in steel with an average size of 10 ¹ to 10 ³ nm of precipitates containing 5nm or more Ti particle, the minimum interval is less than 10 ¹ nm ultra 10 ⁴ nm, the tensile strength of more than 640MPa at the burring workability fatigue properties An excellent hot-rolled steel sheet has been proposed.

Patent Document 4 contains, in mass%, C: 0.02 to 0.08%, Si: 0.01 to 1.5%, Mn: 0.1 to 1.5%, Ti: 0.03 to 0.06%, P: 0.1% or less, S : 0.005% or less, Al: 0.5% or less, N: 0.009% or less, further, the total content of Nb, Mo, V is limited to 0.01% or less, Ti / C: 0.375 to 1.6, An alloy-saving high-strength hot-rolled steel sheet with an average diameter of TiC precipitates in the grains of 0.8 to 3 nm, an average number density of 1 × 10 ¹⁷ pieces / cm ³ and a tensile strength of 540 to 650 MPa has been proposed. ing.

Japanese Unexamined Patent Publication No. 04-32984 JP 2000-328186 A JP 2002-161340 A JP 2011-26690 A

  However, according to the study by the present inventors, in the technique described in Patent Document 1, when a steel sheet is subjected to press working or the like to form a desired part shape, soft ferrite and hard second phase It is difficult to say that this interface is a hot-rolled steel sheet that easily becomes a starting point for cracking during processing and has excellent bending workability. In addition, the technique described in Patent Document 1 relates to a hot-rolled steel sheet having a tensile strength of about 490 MPa. The technique described in Patent Document 1 has both higher strength and excellent bending workability. I was left with problems.

  Moreover, in the technique described in Patent Document 2, the Mn content is high, the portions where Mn segregates are scattered in the steel sheet, and when the steel sheet is press-formed, the Mn segregated portion is an excellent starting point for cracking. There was a problem that it was difficult to ensure stable bending workability. Moreover, in the technique described in Patent Document 2, the total amount of one or two of Ti and Nb is limited to a predetermined range to form strong carbides and reduce the amount of dissolved C to almost zero. However, when excessive Ti and Nb are contained with respect to C, there is a problem that the carbide tends to be coarsened and a desired high strength cannot be secured stably.

Moreover, in the technique described in Patent Document 3, since the average size of precipitates containing Ti is in a wide range of 10 ¹ to 10 ³ nm, there is a problem that a desired high strength cannot be secured stably. .
In the technique described in Patent Document 4, as shown in the examples, the tensile strength of the obtained steel sheet is in the range of 540 to 650 MPa, the elongation is about 30%, and the tensile strength is less than 590 MPa. In many cases, there has been a problem that high strength until a tensile strength of 590 MPa or more can be stably secured has not yet been secured while maintaining excellent bending workability.

  The present invention provides a high-tensile hot-rolled steel sheet having a high tensile strength of not less than 590 MPa and not more than 750 MPa, excellent workability, particularly excellent bending workability, and a method for producing the same. The purpose is to do. The “excellent bending workability” mentioned here is a bending test in which a steel sheet having a thickness t is bent using a punch having a tip radius R on a V-shaped block having a vertex angle of 90 degrees, and a crack is formed on the outer side of the bending portion. This means that the limit bend radius R / t, which is the lowest bend radius that does not occur, is small.

  In order to achieve the above-described object, the present inventors diligently studied various factors affecting the high strength and bending workability of hot-rolled steel sheets. As a result, in a composite structure in which phases with different hardnesses are combined, the bending workability is lowered. Therefore, in order to ensure excellent bending workability, it was first necessary to have a single-phase structure. It was. In a single-phase structure, a ferrite phase having a low dislocation density is most suitable from the viewpoint of improving workability.

In order to increase the strength with a ferrite single structure having a low dislocation density, it is conceivable that fine carbides are precipitated in the ferrite crystal grains constituting the matrix. However, it has been found that bending workability may not be improved in a ferrite single structure in which fine carbides containing, for example, Ti as a main carbide constituent element are precipitated in the matrix.
Therefore, the reason why the bending workability is not improved was studied. As a result, it was found that the coarsened sulfide deteriorates the bending workability. In steel containing Ti, two types of sulfides, TiS and composite sulfides of TiS and MnS, are precipitated. Sulfides in steel are likely to be coarser than carbides, and among these, composite sulfides of TiS and MnS are particularly likely to be coarse.

In order to prevent coarsening of this composite sulfide, the present inventors have conceived that hot rolling should be performed in a temperature range in which recrystallization occurs instantaneously. It has been found that fine sulfides can be precipitated by rolling in such a specific temperature range.
This is because austenite is recrystallized by rolling, so that concentration of S due to diffusion to the austenite grain boundary is suppressed, and when rolling is performed in a temperature range where recrystallization occurs instantaneously, the accumulated energy due to rolling is instantaneously recrystallized. Therefore, sulfide coarsening is unlikely to occur. In addition, if tandem rolling is used, the effect becomes remarkable because S can be rolled before diffusing into the grain boundaries of the recrystallized austenite grains. When rolling is performed in the non-recrystallization temperature range, the sulfide is coarsened by using the energy accumulated by rolling as a driving force.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.010 to 0.055%, Si: 0.2% or less, Mn: 0.7% or less, P: 0.025% or less, S: 0.02% or less, N: 0.01% or less, Al: 0.1% or less, Ti: a composition containing 0.06 to 0.095%, the balance consisting of Fe and inevitable impurities, and a matrix consisting of 95% or more area ratio ferrite ferrite grains, and an average diameter of 6 nm in the ferrite crystal grains constituting the matrix High tensile strength TS: 590MPa or more and 750MPa or less, characterized by excellent bending workability with carbides containing less than Ti dispersed and a structure in which TiS with an average diameter of 0.5μm or less is dispersed and precipitated in the matrix Tensile hot-rolled steel sheet.
(2) A high-tensile hot-rolled steel sheet according to (1), further containing, in addition to the above composition, B: 0.0035% or less by mass%.
(3) In (1) or (2), in addition to the above-mentioned composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta are also contained in mass%. , REM, V, Cs, Zr, Hf, 1% or more in total, 1% or less in total.
(4) The high-tensile hot-rolled steel sheet according to any one of (1) to (3), having a plating layer on the surface.
(5) A method for producing a hot-rolled steel sheet, in which a steel material is heated and subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling is cooled and wound. C: 0.010 to 0.055%, Si: 0.2% or less, Mn: 0.7% or less, P: 0.025% or less, S: 0.02% or less, N: 0.01% or less, Al: 0.1% or less, Ti: 0.06 to 0.095 %, And the finish rolling is a tandem rolling machine in which the finishing mill is continuously installed in five or more stages, and the entry side of the finishing mill Tensile strength TS, characterized in that the temperature is 1000 ° C or higher, the exit side temperature of the finishing mill is 900 ° C or higher, and the coiling temperature after cooling is 500 ° C or higher and 700 ° C or lower. : 590MPa or more and 750MPa or less, a manufacturing method of high-tensile hot-rolled steel sheet with excellent bending workability.
(6) The method for producing a high-tensile hot-rolled steel sheet according to (5), further comprising B: 0.0035% or less by mass% in addition to the above composition.
(7) In (5) or (6), in addition to the above-mentioned composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta are further contained in mass%. , REM, V, Cs, Zr, Hf, 1% or more in total, containing 1% or less, A method for producing a high-tensile hot-rolled steel sheet,

  According to the present invention, tensile strength TS: high strength of 590 MPa to 750 MPa and bending workability, which is suitable as a structural member for parts of transportation machinery including automobiles, switchboards and buildings, etc. It is possible to easily produce a high-tensile hot-rolled steel sheet that is superior to the above, and has a remarkable industrial effect. The high-tensile hot-rolled steel sheet according to the present invention is particularly suitable as a material for automobile undercarriage parts and the like that have a complicated cross-sectional shape during press forming and undergo a bending process on the surface.

The hot-rolled steel sheet of the present invention is in mass%, C: 0.010 to 0.055%, Si: 0.2% or less, Mn: 0.7% or less, P: 0.025% or less, S: 0.02% or less, N: 0.01% or less, Al: It contains 0.1% or less, Ti: 0.06 to 0.095%, and has a composition composed of the balance Fe and inevitable impurities.
First, the reasons for limiting the composition of the high-tensile hot-rolled steel sheet of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.

C: 0.010-0.055%
C has the effect of forming fine carbides and increasing the strength of the steel sheet. In order to secure a desired tensile strength of 590 MPa or higher, it is necessary to contain 0.010% or more. On the other hand, if it exceeds 0.055%, the strength increases excessively and pearlite is easily formed. Since pearlite is a starting point for cracking during bending, the formation of pearlite is a factor that reduces bending workability. For this reason, C was limited to the range of 0.010 to 0.055%. In addition, Preferably it is 0.015 to 0.030%.

Si: 0.2% or less
If Si is contained in excess of 0.2%, a low melting point oxide containing Si is formed on the surface during rolling, the surface properties are lowered, cracks are easily generated from the surface during bending, and bending workability is reduced. . For this reason, Si was limited to 0.2% or less. In addition, Preferably it is 0.05% or less.
Mn: 0.7% or less
When Mn is contained exceeding 0.7%, segregation of Mn tends to occur. In the Mn segregation part, a crack is likely to occur during bending, and therefore bending workability is lowered. For these reasons, Mn is limited to 0.7% or less. In addition, Preferably it is 0.5% or less.

P: 0.025% or less When P is contained in a large amount exceeding 0.025%, segregation becomes prominent and bending workability deteriorates. In this invention, it is preferable to reduce as much as possible from a viewpoint of a bending workability improvement. Therefore, P is limited to 0.025% or less. In addition, Preferably it is 0.02% or less.
S: 0.02% or less In the present invention containing Mn and Ti, S combines with Ti to form TiS and Mn to form MnS. These sulfides are easily coarsened, and may be coarsened to about several μm. Such a coarse sulfide is likely to be a starting point of crack generation during bending, and causes a decrease in bending workability. If S is contained in excess of 0.02%, generation of coarse sulfides cannot be suppressed, and bending workability is deteriorated. For this reason, S was limited to 0.02% or less. In addition, Preferably it is 0.01% or less, More preferably, it is 0.004% or less.

N: 0.01% or less N is a harmful element that lowers the bending workability in the present invention, and is desirably reduced as much as possible. In particular, when the content exceeds 0.01%, coarse nitrides are formed, and bending workability is lowered. For this reason, N was limited to 0.01% or less. In addition, Preferably it is 0.006% or less.
Al: 0.1% 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. On the other hand, if the content exceeds 0.1%, the deoxidation product aggregates and coarsens, so that the bending workability decreases. For this reason, Al was limited to 0.1% or less.

Ti: 0.06-0.095%
Ti is the most important element in the present invention. Ti contributes to increasing the strength of the steel sheet by forming fine carbides. In order to obtain such an effect, the content of 0.06% or more is required. However, if the content exceeds 0.095%, coarse sulfides are likely to be generated, and bending workability tends to be lowered. For this reason, Ti was limited to the range of 0.06 to 0.095%. In addition, Preferably it is 0.07 to 0.09%.

The above-mentioned components are basic components, but as a selective element in addition to the basic composition, B: 0.0035% or less, and / or Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, One or two or more of W, Nb, Pb, Ta, REM, V, Cs, Zr, and Hf in total can be selected and contained as necessary.
B: 0.0035% or less B is an element that segregates at austenite grain boundaries and refines sulfides. In order to obtain such an effect, B is preferably contained in an amount of 0.0010% or more. On the other hand, when the content exceeds _{0.0035%, Fe 23 (CB)} 6 precipitates, bending workability is deteriorated. For this reason, when it contains, it is preferable to limit B to 0.0035% or less.

In addition to the above components, the present invention includes Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta, REM, V, Cs, Zr, and Hf. 1 type or 2 types or more may be contained, but when it is contained, the total content is preferably 1% or less. More preferably, the total content is 0.5% or less.
The balance other than the components described above consists of Fe and inevitable impurities.

Next, the reason for limiting the structure of the hot-rolled steel sheet of the present invention will be described.
The hot-rolled steel sheet of the present invention is a matrix composed of ferrite crystal grains with an area ratio of 95% or more, and carbide containing Ti having an average diameter of less than 6 nm is dispersed in the ferrite crystal grains constituting the matrix, and further in the matrix. It has a structure in which TiS having an average diameter of 0.5 μm or less is dispersed and precipitated.
Matrix: ferrite crystal grains having an area ratio of 95% or more In the present invention, in order to ensure excellent bending workability, it is effective that the matrix is a ferrite phase having a low dislocation density and a high ductility. In order to ensure particularly excellent stretch flangeability, a single structure is formed by ferrite crystal grains. Note that “single” here does not need to be 100% in terms of area ratio, and may be substantially single. Here, “substantially single” means that the ferrite grains are 95% or more, preferably 97% or more in terms of the area ratio with respect to the entire structure.

Examples of the second phase other than the ferrite phase include cementite, pearlite, bainite phase, martensite phase, and retained austenite phase. The total of these second phases is preferably about 5% or less in terms of area ratio. More preferably, it is about 3% or less.
Carbide containing Ti: average diameter less than 6nm
A carbide containing Ti has a strong tendency to precipitate as a fine carbide having an extremely small average diameter. In the present invention, carbides containing fine Ti are dispersed and precipitated in ferrite crystal grains that substantially constitute the matrix, and a desired high strength is ensured. The carbide containing Ti finely precipitated in the ferrite crystal grains acts as a resistance to dislocation movement generated when the steel sheet is deformed, and contributes to strengthening of the hot-rolled steel sheet. For this reason, the average diameter of the carbide containing Ti is set to less than 6 nm. If the average diameter is 3 nm or less, the effect becomes more remarkable. For this reason, the average diameter of the carbide containing Ti is set to less than 6 nm. In addition, Preferably it is 3 nm or less.

TiS: Average diameter 0.5μm or less
In the hot-rolled steel sheet of the present invention containing Ti, sulfide containing Ti is precipitated in the matrix in addition to precipitation of carbide containing fine Ti. Examples of the sulfide containing Ti that precipitates include single TiS and a composite sulfide of TiS and MnS. Ti-containing sulfides are more likely to be coarser than carbides, and TiS and MnS composite sulfides are particularly likely to be coarser. When sulfides containing Ti are coarsely deposited, bending workability is adversely affected. In the present invention, single TiS that is not combined with MnS and fine TiS are deposited. The average diameter of the deposited TiS is 0.5 μm or less. Thereby, the bad influence which acts on bending workability can be excluded. When the average diameter exceeds 0.5 μm and the TiS becomes coarse, it becomes a starting point of cracking during bending, which lowers the bending workability. For this reason, the average diameter of TiS was limited to 0.5 μm or less.

In the present invention, a plating film may be formed on the steel sheet surface. By forming the plating film, the corrosion resistance of the hot-rolled steel sheet is improved, and the hot-rolled steel sheet is suitable for use in parts exposed to severe corrosive environments, for example, undercarriage parts of automobiles. Examples of the plating film include a hot dip galvanized film, an alloyed hot dip zinc film, and an electroplated film.
Below, the preferable manufacturing method of this invention high tension hot-rolled steel plate is demonstrated.

In the present invention, the steel material having the above composition is heated and subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling, the steel material is cooled and wound to obtain a hot-rolled steel sheet (steel strip).
The method for producing a steel material such as a slab having the above composition is not particularly limited. As the melting method, any conventional melting method such as a converter or an electric furnace can be applied. The molten steel is preferably made into a steel material such as a slab by a continuous casting method because of problems such as segregation, but a known casting method such as an ingot-bundling rolling method or a thin slab continuous casting method is used. There is no problem using it.

The obtained steel material is then hot rolled to form a hot rolled sheet. In addition, when performing hot rolling on a steel material, even if it is charged in a heating furnace and reheated to a predetermined temperature, or if the steel material retains heat above a predetermined temperature, reheat it. There is no problem even with direct or direct rolling, in which hot rolling is performed only by holding for a short time.
The reheating temperature of the steel material is preferably 1150 ° C or higher and 1300 ° C or lower. In the present invention using a steel material containing Ti which is a carbide forming element, it is necessary to completely dissolve carbides and sulfides in the steel material before rough rolling, so TiS and MnS and It is preferable to heat to 1150 ° C. or higher so that the composite sulfide can be completely dissolved. On the other hand, if the heating temperature exceeds 1300 ° C and the temperature becomes excessively high, the austenite grain boundaries on the surface of the steel material (slab) are abnormally oxidized, and the surface properties of the resulting hot-rolled steel sheet are lowered and bending workability is lowered. To do. For this reason, the heating temperature for hot rolling of the steel material is preferably limited to a range of 1150 to 1300 ° C.

The heated steel material is then subjected to rough rolling and finish rolling to form a hot rolled sheet.
The conditions for rough rolling are not particularly limited as long as the rough rolling bar having a predetermined size and shape can be obtained.
Finish rolling is a tandem rolling method in which five or more finishing mills are continuously installed and continuously rolled in one direction. By using a tandem rolling method with five or more stages, it is possible to significantly prevent the coarsening of sulfides. If the finish rolling mill is installed in less than five stages, the number of recrystallizations of austenite decreases, TiS diffuses into the austenite grain boundaries and becomes easy to concentrate, and TiS coarsely precipitates, making it impossible to expect the desired effect.

  Furthermore, in finish rolling, the entrance side temperature of the finish rolling mill is set to 1000 ° C. or more, and the exit side temperature is set to 900 ° C. or more. In the present invention, finish rolling is performed in a temperature range where recrystallization occurs instantaneously to prevent the coarsening of sulfides. If rolling is performed in a temperature range in which recrystallization occurs instantaneously, the accumulated energy in the rolling is released instantaneously, the driving force for coarsening the sulfide disappears, and coarsening can be prevented. When the inlet side temperature is less than 1000 ° C., rolling in the non-recrystallization temperature region becomes long, and the risk of the sulfide becoming coarse increases. On the other hand, when the outlet temperature is less than 900 ° C., the residence time in the non-recrystallization temperature range is long, the amount of reduction in the non-recrystallization temperature range tends to increase, and the coarsening of the sulfide becomes remarkable. For this reason, the inlet side temperature of the finishing mill was limited to 1000 ° C. or higher, and the outlet side temperature was limited to 900 ° C. or higher. In addition, the temperature range of finish rolling is preferably 1050 to 920 ° C.

  After finishing rolling, it is cooled and wound at a predetermined winding temperature. The cooling after finishing rolling is not particularly limited, but is preferably 50 ° C./s or more at an average cooling rate up to 700 ° C. from the viewpoint of strength. If the cooling rate is much lower than 50 ° C./s on average, it is impossible to substantially secure a single ferrite structure. For this reason, the average cooling rate is preferably 50 ° C./s or more.

The coiling temperature is 500 ° C or higher and 700 ° C or lower.
Optimization of the coiling temperature is important in order to secure a desired steel sheet structure or to suppress the coarsening of sulfides. When the coiling temperature is higher than 700 ° C., the sulfide becomes coarse after winding, and the bending workability is deteriorated. The winding temperature is 500 ° C. or more from the viewpoint of forming a ferrite single structure. When the winding temperature is less than 500 ° C., bainite is generated and a ferrite single structure cannot be obtained. In addition, Preferably it is 550 degreeC or more and 670 degrees C or less.

  The hot-rolled steel sheet manufactured in the above process may be subjected to a plating process, for example, a hot dip galvanizing process to form a plating film on the surface. An alloying treatment may be performed after the hot dip galvanizing treatment to form an alloyed hot dip galvanized film. Alternatively, electroplating may be formed by electroplating.

Molten steel having the composition shown in Table 1 was melted in a converter and continuously cast into a slab (steel material) having a thickness of 250 mm. These slabs were heated to the heating temperature shown in Table 2 and subjected to rough rolling, and then, as finish rolling, tandem continuous rolling in which seven finishing rolling mills were continuously installed was performed under the conditions shown in Table 2. Then, it cooled on the conditions shown in Table 2, and wound up by the winding temperature shown in Table 2, and obtained the hot rolled sheet steel of board thickness: 3.6mm. For steel plate No. 41, rolling was completed in 4 stages of a 7-high mill, and the remaining 3 stages were unused. Some hot-rolled steel plates (steel plates No. 9, No. 10, No. 11, No. 29, No. 30, No. 31) were pickled to remove the surface scale and then melted. Apply galvanizing treatment (immersion in 480 ° C zinc plating bath (0.1% Al-Zn bath)) to form a hot dip galvanized film with an adhesion amount of 45 g / m ² , and then apply alloying treatment at 520 ° C. An alloyed hot-dip galvanized steel sheet was obtained.

Test specimens were collected from the obtained hot-rolled steel sheet and subjected to structure observation, tensile test, and bending test. The test method is as follows.
(1) Microstructure observation A specimen for microstructural observation is collected from the obtained hot-rolled steel sheet, a cross section (L cross section) parallel to the rolling direction is mechanically polished and corroded with a nital solution, and then a scanning electron microscope (magnification: The tissue was observed and imaged at 3000x. Using the obtained structure photograph, the type of structure other than the ferrite crystal grains and the ferrite phase, and the structure fraction (area ratio) thereof were determined by an image analysis apparatus.

  Moreover, the thin film for transmission electron microscope observation was produced from the obtained hot-rolled steel plate, and it observed with the transmission electron microscope (magnification: 120,000-260,000 times), and calculated | required the particle diameter of the fine carbide | carbonized_material and TiS containing Ti. In addition, about fine carbide | carbonized_material containing Ti, it imaged by observing 30 or more visual fields at 340000 times. And about the obtained structure | tissue photograph, the diameter was calculated | required by the circular approximation about the fine carbide | carbonized_material containing a total of 300 or more Ti using the image-analysis apparatus. The obtained values were arithmetically averaged to obtain the average particle diameter of fine carbide containing Ti in the steel sheet (test piece).

In addition, TiS was observed and imaged at 30 times or more at 10,000 magnifications. And about the obtained structure | tissue photograph, the diameter was calculated by circular approximation about 20 or more TiS in total using the image-analysis apparatus. The obtained values were arithmetically averaged to obtain the average diameter of TiS in the steel sheet (test piece).
(2) Tensile test From the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece (GL: 50mm) with the direction perpendicular to the rolling direction as the tensile direction was sampled, and the tensile test was performed in accordance with the provisions of JIS Z 2241. And tensile strength TS was determined.
(3) Bending test A bending test piece (size: 30 mm x 150 mm) was collected from the obtained hot-rolled steel sheet. The bending specimen is placed on the V-shaped block so that the longitudinal center of the specimen coincides with the apex of the V-shaped block (vertical angle: 90 °), and the center position of the bending specimen is punched (tip radius R). The bending test was performed by changing the punch tip radius until the crack was generated on the outside of the bent portion of the bending test piece. After the test, the outside of the bent part of the test piece was visually observed to investigate the presence or absence of cracks. The number of repetitions was three. In all three bending tests, the case where no crack was generated on the outside of the bent portion was accepted, and the case where even one crack occurred was rejected. Then, R / t obtained by dividing the smallest punch tip radius R that passes by the plate thickness t is defined as a limit bending radius, which is used as an evaluation standard for bending workability. It is evaluated that R / t is 2 or less and excellent in bending workability.
The obtained results are shown in Table 3.

  Each of the examples of the present invention is a hot-rolled steel sheet having both high strength of tensile strength TS: 590 MPa and good bending characteristics. On the other hand, the comparative example outside the scope of the present invention does not ensure the desired high strength, or has a large limit bending radius and does not ensure the desired bending workability.

Claims (7)

% By mass
C: 0.010 to 0.055%, Si: 0.2% or less,
Mn: 0.7% or less, P: 0.025% or less,
S: 0.02% or less, N: 0.01% or less,
Al: 0.1% or less, Ti: 0.06-0.095%
A composition comprising the balance Fe and inevitable impurities,
Furthermore, 95% or more of the area ratio is a matrix made of ferrite crystal grains, and carbide containing Ti having an average diameter of less than 6 nm diffuses into the ferrite crystal grains constituting the matrix, and the average diameter is 0.5 μm or less in the matrix. High tensile hot-rolled steel sheet with tensile strength TS: 590MPa or more and 750MPa or less characterized by excellent bending workability with a structure in which TiS is dispersed and precipitated.   The high-tensile hot-rolled steel sheet according to claim 1, further comprising, in addition to the composition, B: 0.0035% or less by mass%.   In addition to the above-mentioned composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta, REM, V, Cs, Zr, Hf The high-tensile hot-rolled steel sheet according to claim 1 or 2, wherein one or more of the above are contained in total of 1% or less.   The high-tensile hot-rolled steel sheet according to any one of claims 1 to 3, wherein the surface has a plating layer. A steel material is heated and subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling, cooled and wound, a method for producing a hot rolled steel sheet,
The steel material in mass%,
C: 0.010 to 0.055%, Si: 0.2% or less,
Mn: 0.7% or less, P: 0.025% or less,
S: 0.02% or less, N: 0.01% or less,
Al: 0.1% or less, Ti: 0.06-0.095%
A steel material having a composition comprising the balance Fe and inevitable impurities,
The finish rolling is a tandem rolling machine in which the finish rolling mill is continuously installed in 5 stages or more, and the finish rolling mill has an inlet temperature of 1000 ° C. or more and an exit temperature of the finish mill of 900 ° C. or more. And rolling
A method for producing a high-tensile hot-rolled steel sheet having a tensile strength of 590 MPa to 750 MPa and excellent in bending workability, wherein the coiling temperature after cooling is 500 ° C to 700 ° C.   6. The method for producing a high-tensile hot-rolled steel sheet according to claim 5, further comprising, in addition to the composition, B: 0.0035% or less by mass%.   In addition to the above-mentioned composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta, REM, V, Cs, Zr, Hf 1 or 2 types or more in total are contained 1% or less, The manufacturing method of the high-tensile-strength hot-rolled steel plate of Claim 5 or 6 characterized by the above-mentioned.