JP5700139B2 - High-tensile hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-tensile hot-rolled steel sheet having a high strength with a tensile strength TS of 780 MPa or more and 900 MPa or less, which is suitable as a structural member for automobiles and other transportation machinery and construction, and a method for producing the same. In particular, it relates to improvement of workability, particularly 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 chassis 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.
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, in particular, as a material for automobile undercarriage parts, a high-tensile steel plate having a large elongation and excellent bending workability is required.
However, the workability of steel materials generally decreases with increasing strength. Therefore, when applying a high-tensile steel plate to an undercarriage part, it is necessary to make it a high-tensile steel plate having both high strength and excellent workability. 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 Fatigue properties and stretch flangeability (stretch flange) in which 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 A high-strength hot-rolled steel sheet having an excellent formability and a tensile strength of 490 MPa or more has been proposed.

  Further, in Patent Document 2, in terms of% by weight, 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 in average size 10 ¹ to 10 ³ nm particle precipitates containing 5nm or more Ti, the minimum interval is less than 10 ¹ nm ultra 10 ⁴ nm, the tensile strength of burring workability than 640MPa (burring formability ) And hot-rolled steel sheets with excellent fatigue properties have 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. 04-329848 Japanese Unexamined Patent Publication No. 2000-328186 Japanese Unexamined Patent Publication No. 2002-161340 Japanese Unexamined Patent Publication No. 2011-26690

  However, in the technique described in Patent Document 1, when the steel sheet is pressed into a desired part shape by pressing or the like, the interface between the soft ferrite and the hard second phase is the starting point of cracking during processing. It is difficult to say that it is a hot-rolled steel sheet having excellent bending workability. And the technique described in patent document 1 is about the hot-rolled steel plate whose tensile strength is 490 MPa or more, and in the technique described in patent document 1, the further increase in strength and the outstanding bending workability are provided. There was a problem with having to combine.

  Moreover, in the technique described in Patent Document 2, the Mn content is high, the portions where Mn segregates are scattered in the steel plate, and when the steel plate is press-formed, the bending workability is stably secured. There was a problem that was difficult. 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.

In addition, in the technique described in Patent Document 3, the average size of precipitates containing Ti of 5 nm or more is in a wide range of 10 ¹ to 10 ³ nm, so that a desired high strength cannot be secured stably. There's a problem.

  In the technique described in Patent Document 4, the tensile strength of the obtained steel sheet is at most 650 MPa, and there remains a problem in combining further high strength and excellent workability.

  The present invention provides a high-strength hot-rolled steel sheet having a high tensile strength of 780 MPa to 900 MPa and excellent workability, particularly excellent bending workability, and a method for producing the same, which solves the problems of the prior art. 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. R is the lowest bend radius that does not cause R, and it means that the limit bend radius ratio defined by R / t 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-phase structure having a low dislocation density, it is conceivable to precipitate fine carbides in the ferrite crystal grains. However, it has been found that bending workability may not be improved in a ferrite structure in which fine carbides containing, for example, Ti as a main carbide constituent element are precipitated in ferrite crystal grains.
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 steels containing Ti, two types of sulfides are precipitated: a single sulfide TiS and a composite sulfide of TiS and MnS. Sulfides in steel are likely to be coarser than carbides, and among these, complex 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 when rolling in a temperature range where recrystallization occurs instantaneously, the accumulated distortion energy accumulated during rolling is released instantly by recrystallization, so the driving force for coarsening of sulfides is released instantly. As a result, the coarsening of the sulfide hardly occurs. Moreover, if tandem rolling is used, the effect becomes remarkable. 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: 0.035 to 0.065%, 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 0.1 to 0.2%, the balance consisting of Fe and inevitable impurities, and a metal structure (microstructure) consisting of ferrite grains with an area ratio of 95% or more, and the average grain size in the ferrite grains A high-strength hot-rolled steel sheet having a tensile strength TS of 780 MPa to 900 MPa, which has a structure in which Ti carbide of less than 6 nm and a structure in which TiS having an average particle diameter of 0.5 μm or less is dispersed and precipitated in the metal structure are dispersed.
(2) The high-tensile hot-rolled steel sheet according to (1), further containing B: 0.0035% or less by mass% in addition to the above composition.
(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, Zn, a high-tensile hot-rolled steel sheet containing 1% or less in total of one or more of them.
(4) The high-tensile hot-rolled steel sheet according to any one of (1) to (3), having a plating layer on the surface.
(5) The high-tensile hot-rolled steel sheet according to any one of (1) to (4), wherein a limit bending radius ratio is 2 or less.
(6) A method for producing a hot-rolled steel sheet, in which a steel material is heated and subjected to hot rolling comprising rough rolling and finish rolling, and after finishing rolling is cooled and wound.
The steel material in mass%,
C: 0.035 to 0.065%, 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: 0.1-0.2%
A steel material having a composition comprising the balance Fe and inevitable impurities,
The finish rolling is tandem rolling in which five or more finishing mills are continuously installed. The finishing entry temperature of the finishing mill is 1000 ° C. or more, and the exit temperature of the finishing mill. (Rolling delivery temperature) is set to 900 ° C or higher,
The coiling temperature after cooling is 500 ° C. or more and 700 ° C. or less,
A method for producing high-tensile hot-rolled steel sheets with a tensile strength of 780 MPa to 900 MPa.
(7) The method for producing a high-tensile hot-rolled steel sheet according to (6), further containing B: 0.0035% or less in mass% in addition to the composition.
(8) In addition to the above composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta, REM, V, Cs, Zr, The method for producing a high-tensile hot-rolled steel sheet according to (6) or (7), containing one or more of Hf and Zn in total of 1% or less.

  According to the present invention, tensile strength TS: high strength of 780 MPa to 900 MPa, which is suitable as a structural member for automobiles and other transportation machinery and construction, and excellent bending workability. The high-tensile hot-rolled steel sheet can be easily manufactured, 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 mass%, C: 0.035 to 0.065%, 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: 0.1-0.2%, and has a composition consisting 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: 0.035-0.065%
C has the effect of forming fine carbides and increasing the strength of the steel sheet. In order to ensure a high strength of 780 MPa or more which is a desired tensile strength, a content of 0.035% or more is required. On the other hand, if the content exceeds 0.065%, 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 0.035 to 0.065%. In addition, Preferably it is 0.040 to 0.055%.

Si: 0.2% or less
When 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 lowered. . For this reason, Si was limited to 0.2% or less. In addition, Preferably it is 0.05% or less. There is no problem even if the Si content is zero.

Mn: 0.7% or less
If Mn exceeds 0.7%, segregation of Mn is likely to occur. In the Mn segregation part, a crack is likely to occur during bending, and therefore bending workability is lowered. Moreover, since TiS precipitates faster, it tends to be coarser and bending workability tends to decrease. For these reasons, Mn is limited to 0.7% or less. In addition, Preferably it is 0.5% or less. There is no problem even if the Mn content is zero.

P: 0.025% or less When P is contained in a large amount exceeding 0.025%, segregation becomes prominent and bending workability deteriorates. For this reason, P was limited to 0.025% or less. In addition, Preferably it is 0.02% or less. There is no problem even if the P content is zero.

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. There is no problem even if the S content is zero.

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. There is no problem even if the N content is zero.

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%, deoxidation products aggregate and coarsen, so that the bending workability is lowered. For this reason, Al was limited to 0.1% or less.

Ti: 0.1-0.2%
Ti is the most important element in the present invention. Ti forms fine carbides and contributes to increasing the strength of steel sheets while maintaining excellent stretch flangeability. In order to obtain such an effect, the content of 0.1% or more is required. However, when the content exceeds 0.2%, coarse sulfides are likely to be generated, and bending workability tends to be lowered. For this reason, Ti was limited to the range of 0.1 to 0.2%. In addition, Preferably it is 0.14-0.18%.

  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, Hf, and Zn in total can be selected and contained as necessary. For example, elements (Cu and the like) mixed from ore and scrap do not need to be reduced as long as they are below the total content.

B: 0.0035% or less B is an element that segregates at austenite grain boundaries and refines sulfides. To obtain such an effect, B is preferably contained in an amount of 0.0005% 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 components described above, in the present invention, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta, REM, V, Cs, Zr, Hf, Zn Of these, one or more of them may be contained, but when they are 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 comprises a metal structure in which 95% or more of the area ratio is occupied by ferrite crystal grains, a Ti carbide having an average particle diameter of less than 6 nm in the ferrite crystal grains constituting the matrix, and further in the metal structure. It has a structure in which TiS having an average particle size of 0.5 μm or less is dispersed and precipitated.

In the present invention, in order to ensure excellent bending workability, it is effective to occupy the metal structure with ferrite crystals having a low dislocation density and high ductility. is there. In order to ensure particularly excellent stretch flangeability, a ferrite single phase structure is adopted. Here, the “single phase” does not need to be 100% in terms of area ratio, and may be substantially a single phase. Here, “substantially single phase” means that the ferrite phase composed of ferrite crystal grains is 95% or more, preferably 97% or more in terms of the area ratio with respect to the entire structure. The “metal structure” herein refers to a metal structure observed at a magnification of 500 to 5000 times using an optical microscope or a scanning electron microscope.
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 acceptable if the area ratio is about 5% or less, preferably about 3% or less.

Ti carbide: average particle size less than 6nm
Ti carbide has a strong tendency to precipitate as fine carbide having an extremely small average particle size. In the present invention, fine Ti carbides are dispersed and precipitated in ferrite crystal grains constituting a metal structure that is substantially a ferrite single phase to ensure a desired high strength. Ti carbide finely precipitated in ferrite crystal grains acts as a resistance to dislocation movement that occurs when deformation is applied to the steel sheet, and contributes to strengthening of the hot-rolled steel sheet. For this reason, the average particle diameter of Ti carbide precipitated in the ferrite crystal grains is set to less than 6 nm. If the average particle size is 3 nm or less, the effect becomes more remarkable. For this reason, the average particle size of Ti carbide was set to less than 6 nm. In addition, Preferably it is 4 nm or less. In addition, Ti carbide may have solid solution of elements contained in steel in addition to Ti as a carbide constituent element, but Ti carbide in the present invention is a Ti carbide in which such other elements are dissolved. In addition to Ti, both Ti carbides that do not contain carbide constituent elements are included. Note that Ti is an element that can be added at a relatively low cost, and it is advantageous to use Ti carbide that does not substantially contain carbide constituent elements other than Ti. In this case, among the selective elements, Mo, W, Nb, and V, which have a strong tendency to form carbides, are preferably not added (content of about impurities).

TiS: Average particle size of 0.5μm or less
In the hot-rolled steel sheet of the present invention containing Ti, sulfide containing Ti is precipitated in the metal structure 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 particle size 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 particle size exceeds 0.5 μm and TiS becomes coarse, it becomes a starting point of cracking during bending, and bending workability is lowered. For this reason, the average particle size of TiS was limited to 0.5 μm or less.

  In the present invention, a plating film may be formed on the steel plate 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 may be used. There is no problem using it.

  The obtained steel material is then hot rolled into 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, when the heating temperature exceeds 1300 ° C. and becomes excessively high, the austenite grain boundaries are abnormally oxidized, the surface properties of the resulting hot-rolled steel sheet are lowered, and the bending workability is lowered. 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 adopting tandem rolling, the position of the austenite grain boundary is changed in a short time, whereby the refinement of sulfides precipitated at the austenite grain boundary is achieved. That is, before the sulfide is precipitated at the austenite grain boundary, the coarsening of the sulfide can be prevented by changing the position of the austenite grain boundary by rolling and subsequent recrystallization. By combining this effect with the effect of promoting austenite recrystallization, the sulfide can be efficiently refined. If the finish rolling mill is installed in less than 5 stages, the number of recrystallizations of austenite is reduced, the refinement of sulfide is not promoted, and the desired effect cannot be expected. The rolling reduction per stage of the tandem finish rolling mill is not particularly limited, but a rolling reduction of 5% or more is preferable.
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. If the inlet temperature is less than 1000 ° C., rolling in the non-recrystallization temperature range 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 increases, 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 securing the tensile strength. If the cooling rate is smaller than 50 ° C./s on average, it becomes difficult to substantially secure the structure of the ferrite single phase. 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 coiling temperature is 500 ° C. or more from the viewpoint of securing a ferrite single phase structure. When the coiling temperature is less than 500 ° C., a large amount of bainite is generated and bending workability is deteriorated. In addition, Preferably it is 580 degreeC or more and 680 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. Moreover, electroplating may be performed to form an electroplating film.

Molten steel having the composition shown in Table 1 was melted in a converter and continuously cast into a slab (steel material) having a wall thickness of 280 mm. These slabs were heated to the heating temperature shown in Table 2 and subjected to rough rolling, followed by tandem continuous rolling in which seven finish rolling mills were installed continuously as finish rolling 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 steel plate of board thickness: 2.9mm. In addition, some hot-rolled steel sheets are pickled to remove the scale of the surface layer, and then subjected to hot dip galvanizing treatment (immersion in a 490 ° C zinc plating bath (0.1% Al-Zn bath)) for adhesion An amount: 48 g / m ² of a hot dip galvanized film was formed, and further alloyed at 550 ° C. to obtain an alloyed hot dip galvanized steel sheet.

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, corroded with nital liquid (nital), and then a scanning electron microscope The tissue was observed at (magnification: 3000 times) and imaged. Using the obtained structure photograph, the type of structure other than the ferrite phase 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 Ti carbide. In addition, the particle diameter of the fine Ti carbide was observed at 260,000 times and 30 or more fields of view, and the particle diameter of each particle was determined for a total of 100 or more Ti carbides. Using the obtained structural photograph, the particle diameter of each particle was obtained by circular approximation by image analysis, and the value was arithmetically averaged to obtain the average particle diameter of Ti carbide in the steel sheet (test piece).
In addition, TiS was observed and imaged at 30 times or more at 10,000 magnifications. The obtained tissue was subjected to image analysis, and the particle diameter of each particle was calculated by circular approximation for a total of 20 or more TiS. The obtained values were arithmetically averaged to obtain the average grain size 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: 50 mm) with the direction perpendicular to the rolling direction as the tensile direction was collected, and a 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 of pressing against the V-shaped block with the punch tip radius changed. After the test, the outside of the bent part of the test piece was visually observed to investigate the presence of cracks. The number of repetitions was three. In all three bending tests, the case where no cracks were visually observed on the outer side of the bent part was acceptable, and the case where even one crack was generated was rejected. And R / t which divided the smallest tip radius R to pass by board thickness t was defined as a limit bending radius ratio, and it was set as an evaluation standard of bending workability. When R / t is 2 or less, it is evaluated that the bending workability is excellent.

  The obtained results are shown in Table 3.

Each of the examples of the present invention is a hot-rolled steel sheet having a high tensile strength TS: 780 MPa or more and a good bending property. On the other hand, the comparative example which is out of the scope of the present invention does not ensure the desired high strength, or has a large limit bending radius ratio and cannot ensure the desired bending workability.

Claims (7)

% By mass
C: 0.035 to 0.065%, 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: 0.1 to 0.2% Including the composition of the balance Fe and inevitable impurities,
Further, the metal structure is composed of ferrite crystal grains with an area ratio of 95% or more, and Ti carbide having an average particle size of less than 6 nm is dispersed in the ferrite crystal particles, and TiS having an average particle size of 0.5 μm or less is dispersed in the metal structure. A high-tensile hot-rolled steel sheet having a precipitated structure , a limit bending radius ratio of 2 or less, and a tensile strength TS: 780 MPa to 900 MPa.   The high-strength hot-rolled steel sheet according to claim 1, further comprising B: 0.0035% or less by mass% in addition to the composition.   In addition to the above composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta, REM, V, Cs, Zr, Hf, Zn The high-tensile-strength hot-rolled steel sheet according to claim 1 or 2, containing one or more of them 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: 0.035 to 0.065%, 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: 0.1 to 0.2% Including a steel material having a composition comprising the balance Fe and inevitable impurities,
The finish rolling is a tandem rolling method in which the finish rolling mill is continuously installed in five or more stages, 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. Rolling, the coiling temperature after cooling is 500 ° C. or more and 700 ° C. or less, and the metal structure is composed of ferrite crystal grains with an area ratio of 95% or more, and the average grain size is less than 6 nm in the ferrite crystal grains Ti carbide and a structure in which TiS with an average particle size of 0.5 μm or less is dispersed and precipitated in the metal structure, the critical bending radius ratio is 2 or less, and the tensile strength is 780 MPa or more and 900 MPa or less. A method for producing a hot-rolled steel sheet. The method for producing a high-tensile hot-rolled steel sheet according to claim 5 , further comprising B: 0.0035% or less by mass% in addition to the composition. In addition to the above composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta, REM, V, Cs, Zr, Hf, Zn The manufacturing method of the high-tensile-strength hot-rolled steel sheet of Claim 5 or 6 which contains 1 type or 2 types or more in total in 1% or less.