JP6052503B2 - High-strength hot-rolled steel sheet and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet that is suitable for use mainly in automobile members, home appliances, structures, heavy machinery, and the like, and that is excellent in bending workability, and a method for producing the same.

  In recent years, from the viewpoint of protecting the global environment and ensuring the safety of passengers, in order to reduce the weight and strength of automobile bodies, steel materials used in automobile bodies have been increased in strength and thickness. It is being advanced. An automobile body is composed of a member or a structural material obtained by processing a steel plate, and the strength thereof generally increases as the strength of the material steel plate increases.

  In addition, high-strength hot-rolled steel sheets are often used as skeletal members and strength members for structures that are not directly visible from the outside. The strength of the member is often required to be high.

  Conventional methods for strengthening steel include a solid solution strengthening method in which alloy elements are added and solid solution strengthened, a structure strengthening method in which a hard transformation phase having a high dislocation density is generated and strengthened, and fine precipitation. A precipitation strengthening method for increasing strength by dispersing substances, or a method for increasing strength by combining the above methods is known.

  Among the high-strength steel sheets produced by these methods, as a hot-rolled steel sheet in which the matrix of the steel structure is composed of a ferrite single phase, for example, the majority of the structure proposed in Patent Document 1 is polygonal ferrite, There is a precipitation-strengthening-type high-strength hot-rolled steel sheet that has been strengthened by combining precipitation strengthening and solid solution strengthening with precipitates centered on TiC. However, since the precipitation strengthening method disclosed in Patent Document 1 requires the addition of a large amount of Ti, coarse precipitates are likely to be generated, and the resulting strength and workability are likely to be unstable. Further, the strength obtained is only about 780 MPa class at the maximum with the tensile strength TS.

  Patent Documents 2 and 3 disclose techniques for stably increasing the strength of steel sheets by precipitating fine carbides of Ti and Mo. The techniques described in these documents are intended to ensure high strength by precipitation strengthening of fine carbides while ensuring workability by making the matrix of the steel structure a ferrite single phase. However, the tensile strength obtained is only up to 980 MPa class.

  Patent Document 4 discloses that a tensile strength of 980 MPa or more can be obtained by dispersing and precipitating a composite carbide of Ti, Mo, and V in steel that is substantially a ferrite single-phase structure. Discloses that a steel sheet having a tensile strength of 1180 MPa or more can be obtained. However, in the technique disclosed in Patent Document 4, it is difficult to control the temperature so that both the composition and size of the composite carbide are matched to preferable conditions, and the desired strength and workability are stably imparted to the obtained steel sheet. There is a problem that can not be.

  Patent Document 5 discloses a method for producing a hot-rolled steel sheet having excellent bendability in which equiaxed ferrite having a predetermined hardness has a steel structure of 70 vol% or more. However, the technique disclosed in Patent Document 5 has no technical idea of controlling coarse precipitates precipitated in the slab, and the tensile strength obtained is less than 1180 MPa.

Japanese Patent Laid-Open No. 06-200351 Japanese Patent No. 3637885 Japanese Patent No. 3882577 JP 2007-063668 A JP 2006-161111 A

  By the way, conventionally, when fine carbide precipitation is used to increase the strength of hot-rolled steel sheets, the size of the precipitate is mainly controlled by cooling control and coil winding temperature control in the run-out table after hot rolling. I have been.

  However, in the conventional method, the steel slab manufactured by continuous casting is once cooled to near room temperature, and then reheated to a predetermined temperature and hot-rolled again, and the steel slab is cooled to near room temperature. At the stage, coarse precipitates (mainly carbides) are precipitated in the slab. It is difficult to dissolve the entire coarse precipitate even if the component design and the heating conditions are set so that the entire precipitate is dissolved during reheating of the slab. For this reason, in conventional precipitation-strengthened hot-rolled steel sheets, which are hot-rolled after reheating the cooled steel slab, the carbides are refined using hot-rolling cooling control technology and coiling temperature control technology after hot rolling. Even if it is attempted to precipitate, the amount of precipitation is insufficient, and there is a limit to the degree of strengthening, it is difficult to stably obtain a high-strength steel sheet having a tensile strength TS of 1180 MPa class, and good bending workability is achieved. It is actually difficult to obtain.

  The present invention has been made in view of the above-mentioned problems of the prior art, and its purpose is to provide a high-strength hot-rolled steel sheet having a tensile strength TS of 1180 MPa or more and excellent bending workability. It is to propose an advantageous manufacturing method.

  In order to solve the above-mentioned problem of achieving both high strength and bending workability, the inventors have earnestly devised measures for improving bending workability in high-strength hot-rolled steel sheets utilizing precipitation strengthening of carbides precipitated in the slab. Repeated research. As a result, cracks in the bending process occur starting from coarse precipitates scattered in the steel, and the coarse precipitates are 1200 before the slab (slab) once cooled to near room temperature is hot rolled. In order to increase the strength by precipitating fine carbides after hot rolling and improve bending workability, it is necessary to reduce the cooling rate of the slab after solidification. The present inventors have found that it is effective to control the holding time in slab reheating before hot rolling within an appropriate range, and completed the present invention.

The present invention based on the above knowledge is C: 0.1 to 0.4 mass%, Si: 0.2 mass% or less, Mn: 1.5 mass% or less, P: 0.03 mass% or less, S: 0.03 mass% or less Al: 0.1 mass% or less, N: 0.005 mass% or less, and Zr: 0.05 to 3.5 mass%, Ti: 0.04 to 2.0 mass%, and Nb: 0.05 to One or two selected from 3.0 mass%, and Mo: 0.01 to 0.5 mass%, V: 0.01 to 1.0 mass%, and W: 0.01 to 1.0 mass% One or more selected from among the following (1) formula;
0.7 ≦ (Ti / 48 + Nb / 93 + Zr / 91 + Mo / 96 + V / 51 + W / 184) / (C / 12) ≦ 1.2 (1)
(The element symbol in the above formula (1) represents the content (mass%) of each element.)
A strip test piece with a thickness of 1 mm and a cut-out strip test piece having a tensile strength of 1180 MPa or more , having a component composition consisting of Fe and inevitable impurities, the balance being Fe and inevitable impurities. When a bending test is performed in which the punch is bent in the C direction using a punch and a die having a V-shaped cross section (angle 90 °) with a radius of curvature of 0.5 mm, the test piece is free from cracks. It is a strength hot-rolled steel sheet.

  The high-strength hot-rolled steel sheet of the present invention is characterized in that a plating layer is formed on the surface of the steel sheet obtained by hot rolling.

  The plating layer in the high-strength hot-rolled steel sheet of the present invention is a zinc-based plating layer.

The zinc-based plating layer in the high-strength hot-rolled steel sheet of the present invention is a hot- dip galvanized layer or an alloyed hot- dip galvanized layer.

The invention also Steels having the chemical composition described above SL, a steel slab obtained by continuous casting, from the freezing point to 1300 ° C. and cooled at 10 to 300 ° C. / min, then 900 It is charged into a heating furnace without cooling to less than ℃, after reheating at 1150 to 1300 ℃ × 40 min or less, hot rolled to finish finish rolling at a temperature of 820 ℃ or more, and coiled at a temperature of 700 to 500 ℃ Is wound into a V-shaped cross section (angle 90 °) with a 1 mm thick strip test piece having a tensile strength of 1180 MPa or more and a radius of curvature of 0.5 mm at the tip. A method for producing a high-strength hot-rolled steel sheet, characterized in that a hot-rolled steel sheet having no cracks in the test piece is obtained when a bending test in which it is bent in the C direction using a punch and a die having .

  In the method for producing a high-strength hot-rolled steel sheet of the present invention, the ratio (R / t) of the curvature radius R (m) at the curved portion to the thickness t (m) of the steel slab is 25 or more in the production of the steel slab. It is characterized by using the continuous casting machine which is.

  Moreover, the manufacturing method of the high intensity | strength hot-rolled steel plate of this invention forms a plating layer on the surface of the steel plate obtained by the said hot rolling.

  According to the present invention, coarse precipitates that have been scattered in the slab before hot rolling can be refined, and nitrides and sulfides that have not been effectively used in the prior art can be refined. Therefore, it is possible to stably provide a high-strength hot-rolled steel sheet having a tensile strength TS of 1180 MPa or more and excellent bending workability.

First, the component composition of the high-strength hot-rolled steel sheet of the present invention will be described.
C: 0.1 to 0.4 mass%
C is an element essential for the formation of carbides necessary for precipitation strengthening of steel. If C is less than 0.1 mass%, the amount of precipitates is small, and it is difficult to stably obtain a tensile strength of 1180 MPa. On the other hand, the addition exceeding 0.4 mass% causes a decrease in weldability or TiC crystallizes in the molten steel, resulting in a decrease in strength of the obtained steel sheet. Therefore, C is set to a range of 0.1 to 0.4 mass%. Preferably it is the range of 0.12-0.4 mass%.

Si: 0.2 mass% or less Si is an element added as a deoxidizing agent, and has been actively used as an element for solid solution strengthening of steel. However, since Si has the effect of expanding the temperature range of δ ferrite in which carbide precipitates, it is desirable to reduce it as much as possible. Therefore, in the present invention, Si is set to 0.2 mass% or less. Preferably it is 0.1 mass% or less.

Mn: 1.5 mass% or less Mn is a useful element for solid solution strengthening of steel and an element for improving hardenability. However, if the hardenability becomes too high, bainite and martensite are generated, and it becomes difficult to make the steel structure a ferrite single phase and to finely precipitate carbide in the ferrite grains. Therefore, in this invention, Mn shall be 1.5 mass% or less. Preferably it is 1.0 mass% or less, More preferably, it is 0.7 mass% or less.

P: 0.03 mass% or less P, like Si, has the effect of expanding the temperature range of δ ferrite in which carbides precipitate, and also causes embrittlement during welding, so it is desirable to reduce it as much as possible. Therefore, P is set to 0.03 mass% or less. Preferably it is 0.01 mass% or less.

S: 0.03 mass% or less S is conventionally combined with Ti and Zr at a high temperature to form coarse TiS and ZrS, and consumes Ti and Zr which are carbide forming elements contributing to precipitation strengthening. It was thought to be a harmful element that impairs bending workability. However, in the manufacturing method of the present invention, since coarsening of TiS and ZrS is suppressed and finely precipitated, these precipitates can be used for increasing the strength of steel, and are also harmless to bendability. To some extent, it is acceptable. However, the content exceeding 0.03 mass% produces MnS and inhibits hot workability. Therefore, in the present invention, S is set to 0.03 mass% or less. Preferably it is 0.01 mass% or less.

Al: 0.1 mass% or less Al is an element added as a deoxidizer for steel. However, excessive addition increases the amount of non-metallic inclusions such as alumina, which adversely affects internal quality and surface quality. Moreover, since the alumina _(Al 2 _{O 3)} such as coarse TiN as nuclei tend to precipitate, the less it preferred. Therefore, the upper limit of Al is 0.1 mass%.

N: 0.005 mass% or less N has been conventionally considered to be a harmful element that combines with Ti, Nb, and Zr to form nitrides at high temperatures and consumes carbide-forming elements that contribute to precipitation strengthening. However, in the manufacturing method of the present invention, the coarsening of TiN, NbN, and ZrN is suppressed, and the precipitate is finely precipitated. Therefore, these nitrides can also be effectively used for precipitation strengthening, and bending workability is not impaired. However, since addition exceeding 0.005 mass% promotes formation of coarse nitrides, the upper limit of N is set to 0.005 mass%.

The high-strength hot-rolled steel sheet of the present invention is one or more selected from Ti, Nb and Zr which are carbide forming elements, in addition to the above components, from the viewpoint of ensuring high strength stably. Must be contained within the following range.
Ti: 0.04 to 2.0 mass%
Ti is a useful element that forms fine charcoal / nitrides and precipitates to contribute to the strengthening of steel. However, if the Ti content is less than 0.04 mass%, it is difficult to stably obtain a tensile strength of 1180 MPa or more. On the other hand, addition exceeding 2.0 mass% produces coarse Ti oxides, and coal / nitrides aggregate to reduce the precipitation strengthening ability. Therefore, the tensile strength of 1180 MPa or more is also stabilized. It can no longer be obtained. Therefore, when adding Ti, it is set as the range of 0.04-2.0 mass%.

Nb: 0.05 to 3.0 mass%
Nb, like Ti, is a useful element that forms charcoal / nitrides and precipitates to contribute to increasing the strength of steel. However, if the Nb content is less than 0.05 mass%, it is difficult to stably obtain a tensile strength of 1180 MPa or more. On the other hand, when the addition exceeds 3.0 mass%, coarse Nb charcoal / nitride is generated and aggregated, and a tensile strength of 1180 MPa or more cannot be stably obtained. Therefore, when adding Nb, it is set as the range of 0.05-3.0 mass%.

Zr: 0.05 to 3.5 mass%
Zr, like Ti and Nb, is a useful element that forms and precipitates carbon / nitride and contributes to increasing the strength of steel. However, if the Zr content is less than 0.05 mass%, it is difficult to stably obtain a tensile strength of 1180 MPa or more. On the other hand, when the addition exceeds 3.5 mass%, coarse Zr oxide is formed, and carbon / nitride aggregates thereon, so that a tensile strength of 1180 MPa or more cannot be stably obtained. Therefore, when adding Zr, it is set as the range of 0.05-3.5 mass%.

  Moreover, the high-strength hot-rolled steel sheet of the present invention is further added with one or more selected from Mo, V, and W in addition to the above components from the viewpoint of securing high strength more stably. can do. Mo, V, and W have a weak carbide forming ability as compared with Ti, Nb, and Zr. When these are added, they are added in combination with other carbide forming elements, thereby stabilizing fine carbides. Can be formed. Therefore, after adding one or more selected from Ti, Nb and Zr, it is necessary to contain any one or more of Mo, V and W within the following range. is there.

Mo: 0.01-0.5 mass%
Mo is a useful element that forms fine carbides and precipitates and contributes to increasing the strength of steel. However, if the Mo content is less than 0.01 mass%, it is difficult to stably obtain a tensile strength of 1180 MPa or more. On the other hand, addition exceeding 0.5 mass% produces coarse Mo oxides, and carbides agglomerate to this to lower the precipitation strengthening ability, so that a tensile strength of 1180 MPa or more cannot be stably obtained. . Therefore, when adding Mo, it is set as the range of 0.01-0.5 mass%.

V: 0.01-1.0 mass%
V is a useful element that forms and precipitates fine carbonitrides and contributes to increasing the strength of steel. However, if the V content is less than 0.01 mass%, it is difficult to stably obtain a tensile strength of 1180 MPa or more. On the other hand, addition exceeding 1.0 mass% produces coarse V oxide, and carbonitride aggregates to lower the precipitation strengthening ability, so that a tensile strength of 1180 MPa or more can be stably obtained. It becomes impossible. Therefore, when adding V, it is set as the range of 0.01-1.0 mass%.

W: 0.01-1.0 mass%
W is a useful element that forms and precipitates fine carbides and contributes to increasing the strength of steel. However, if the W content is less than 0.01 mass%, it is difficult to stably obtain a tensile strength of 1180 MPa or more. On the other hand, addition exceeding 1.0 mass% produces a coarse W oxide, and carbides agglomerate to this to lower the precipitation strengthening ability, so that a tensile strength of 1180 MPa or more cannot be obtained stably. . Therefore, when adding W, it is set as the range of 0.01-1.0 mass%.

Further, in addition to one or more selected from Ti, Nb and Zr, when adding one or more selected from Mo, V and W, the following formula (1):
0.7 ≦ (Ti / 48 + Nb / 93 + Zr / 91 + Mo / 96 + V / 51 + W / 184) / (C / 12) ≦ 1.2 (1)
(The element symbol in the above formula (1) represents the content (mass%) of each element.)
It is necessary to satisfy and contain.
Here, the above formula (1) is the ratio of the total number of atoms of Ti, Nb, Zr, Mo, V, and W that form fine precipitates with respect to the number of C atoms, that is, C and the carbide-forming element. It represents the atomic ratio. If this ratio is less than 0.7, the amount of increase in strength due to carbide precipitation is insufficient. On the other hand, if it exceeds 1.2, the precipitate becomes coarse and the strengthening ability becomes insufficient, so that the above range is set. Preferably, the value on the left side of the equation (1) is 0.8 and the value on the right side is 1.1.

One or more selected from Cr, Hf, Ta, Be, B, Cu, Ni, Au, Ag, Co, Pt, Sb, Sn, Zn, Ca, Mg, and REM: 2.0 mass in total % Or less Cr, Hf and Ta are elements that contribute to high strength by actively forming carbides in steel, like Ti, Nb and Zr described above, and Be and B are solid solution strengthening. Since it is an element that contributes to grain boundary strengthening, it can be added appropriately as a strengthening element when higher strength is desired.

  Cu is an impurity element usually mixed from scrap or the like, but is an element effective for increasing the strength of steel. Therefore, in the present invention, it is possible to allow Cu contamination to some extent, and to actively utilize scrap which is a recycling resource, and to reduce the raw material cost. In the steel sheet of the present invention, the influence on the material of Cu is small, but if excessively mixed, it may cause surface defects due to cracking due to hot brittleness during hot rolling, so the upper limit of Cu content Is preferably limited to about 0.3 mass%.

  Of the above elements, Cr, B and Cu are elements that enhance the hardenability like Mn. If the hardenability becomes too high, bainite and martensite are generated, and a ferrite single phase structure is formed. It becomes difficult to obtain and inhibits fine precipitation in ferrite grains. Therefore, these elements are preferably each or a total of 1 mass% or less.

  Ni has a small effect on the material of the steel sheet, but is an element effective in preventing hot brittleness due to the addition of Cu and improving the surface quality. Since this effect is obtained by addition of ½ or more of the Cu content, when Cu is contained, it is preferable to add Ni of ½ or more of the Cu content. However, excessive addition of Ni causes surface defects due to non-uniformity of scale, so the upper limit is preferably about 0.3 mass%.

  In addition, Au, Ag, Co, Pt, Sb, Sn, and Zn suppress the surface oxidation and nitridation, or the decarburization of the steel sheet surface layer of several tens of microns, resulting in improved fatigue characteristics and aging resistance. Therefore, it can be added as appropriate. However, Sn is preferably added in an amount of 0.005 mass% or more in order to obtain the above effect, but excessive addition causes a reduction in the toughness of the steel, so the upper limit is preferably about 0.2 mass%. .

  Further, Ca, Mg, and REM are all effective elements for improving workability through the form control of inclusions, and therefore can be added as appropriate.

  The elements to be selectively added are added from various viewpoints as described above, but the total addition amount of these elements is preferably limited to 2.0 mass% or less. This is because if it exceeds 2 mass%, the formability is lowered and the alloy cost is increased.

One or more selected from As, Cs, Pb, Se, and Sr: 2.0 mass% or less in total As, Cs, Pb, Se, and Sr are elements that are positioned as inevitable impurities in the present invention It is. However, these elements tend to increase in recent years when the amount of scrap used is increasing, and a large amount of refining costs are required to remove them. However, these elements do not particularly impair the effects of the present invention as long as the total content is 2.0 mass% or less. Therefore, these elements are allowed within a range of 2.0 mass% or less in total.

Next, the steel structure of the high strength hot rolled steel sheet of the present invention will be described.
The high-strength hot-rolled steel sheet of the present invention requires that the matrix of the steel structure is substantially a ferrite single phase in order to increase the strength and obtain good bending workability by precipitating carbides finely. is necessary. In a bainite or martensite structure, it becomes difficult to precipitate fine carbides in the ferrite phase as a matrix, and the tensile strength is insufficient. Further, when pearlite appears, C is consumed by the formation of cementite, and the precipitation of fine carbides is suppressed, so that the tensile strength is also insufficient. However, the phases other than ferrite can be allowed if the total area ratio is about 5% or less.

  The high-strength hot-rolled steel sheet of the present invention has excellent bending workability when the tensile strength TS is 1180 MPa or more only when all the conditions of the above component composition and steel structure are satisfied.

Next, the manufacturing method of the high intensity | strength hot-rolled steel plate of this invention is demonstrated.
The high-strength hot-rolled steel sheet of the present invention is obtained by melting steel adjusted to the above-described component composition by a conventional refining process using a converter, an electric furnace, a vacuum degassing apparatus, etc., and continuously casting to a slab (steel) Then, the steel slab was cooled from the freezing point to 1300 ° C. at 10 to 300 ° C./min, and then the steel slab was charged into a heating furnace without cooling to less than 900 ° C. After reheating at 1300 ° C. × 40 min or less, it is hot rolled to finish the finish rolling at a temperature of 820 ° C. or more, and is wound around a coil at a temperature of 700 to 500 ° C.
The steel slab may be a normal steel slab having a thickness of 100 mm or more, or a so-called thin slab having a thickness of 100 mm or less.

  As described above, in order to produce a high-strength hot-rolled steel sheet having a tensile strength TS of 1180 MPa or more and good bending workability, the size of carbides precipitated in the steel sheet after hot rolling is refined. It is necessary to do it. For that purpose, it is important not to deposit coarse carbides in the slab after continuous casting and before hot rolling.

  Therefore, in the present invention, the temperature range from at least 1300 ° C. after the continuous cast slab (slab) solidifies, that is, the δ ferrite region where the solid solubility limit of carbide Heisei elements such as Ti, Nb, Zr, etc. is small. By passing through and moving to the austenite region where the solid solubility limit is large at an early stage, the precipitation of carbides in the δ ferrite region is suppressed, and the diffusion of alloy elements in the steel is promoted to reduce segregation. In order to precipitate fine composite carbide, cooling is performed at a cooling rate of 10 ° C./min to 300 ° C./min. When the cooling rate exceeds 300 ° C./min, the drop in the transformation point due to supercooling becomes large, and it stays in the δ region for a long time. It becomes difficult to precipitate fine carbide stably. In addition, at a cooling rate of less than 10 ° C / min, productivity is hindered, and by keeping it at a high temperature for a long time, coarse precipitates (mainly nitrides) precipitate and high strength cannot be obtained. Cool at 10 ° C / min or more. In addition, the said freezing point and the temperature of 1300 degreeC in this invention are the temperature of slab thickness center part obtained by heat transfer calculation from slab surface temperature.

  In addition, according to the research by the inventors, it has been clarified that the strain applied to the continuously cast steel slab promotes the precipitation of carbides. At present, a continuous casting machine used for manufacturing a steel slab is mainly a curved mold composed of a curved portion and a straightening portion or a vertical bending die composed of a vertical portion, a curved portion and a straightening portion. In the correction part, distortion is introduced by correcting the curved slab, but this distortion promotes the precipitation of carbides, and thus greatly inhibits the precipitation strengthening ability of carbides. Therefore, in order to enjoy the effects of the present invention, the curvature radius R (m) of the curved portion and the slab thickness t (m) and the ratio (R / t) are 25 or more in order to reduce the correction distortion in the continuous casting machine. Is preferable. More preferably, it is 28 or more.

The steel slab obtained as described above is then reheated to a predetermined temperature and hot-rolled to obtain a hot-rolled sheet (high-strength hot-rolled steel sheet) having a predetermined thickness.
Usually, in hot rolling, the slab is once cooled to near room temperature to form a cold piece, and then charged into a heating furnace and reheated to a predetermined temperature. The slab after continuous casting is reheated at a temperature of 1150 to 1300 ° C. for 40 minutes or less without being cooled to less than 900 ° C., and then hot-rolled. Here, the reason why the steel slab after continuous casting is not cooled to less than 900 ° C. is that when it is cooled to less than 900 ° C., an α-ferrite phase having a small solid solubility limit of carbide forming elements begins to appear again, and precipitation of carbide is promoted. This is because that.

The reason why the reheating condition of the steel slab is set to 1150 to 1300 ° C. × 40 min or less is that, in addition to ensuring high strength stably, the effect of improving bending workability is obtained.
The reason why such an effect can be obtained is as follows. By heating the slab to the reheating temperature, it is possible to redissolve the carbide precipitation nuclei that have already precipitated, so that the refinement of the carbide precipitated after hot rolling can be further promoted. However, if the temperature is lower than 1150 ° C., the above-described effect cannot be obtained, and the coarsening of the precipitate is promoted by holding the film at a low temperature with low solubility for a long time. On the other hand, if the reheating temperature exceeds 1300 ° C. or the heating time exceeds 40 min, the precipitation nuclei of carbides that have already precipitated grow and become coarse, or re-dissolved precipitation nuclei reprecipitate. Therefore, it is not preferable.

  The reheated steel slab is then subjected to hot rolling with a finish rolling finish temperature of 820 ° C. or higher. The reason why the finish rolling finish temperature is set to 820 ° C. or more is that if it is less than 820 ° C., carbides precipitated in the processed austenite are likely to be coarsened, and a high strength of 1180 MPa or more cannot be obtained. A preferable finish rolling end temperature is 850 ° C. or higher.

  In addition, in order to ensure the said finish rolling completion temperature, it is preferable that the temperature which starts hot rolling shall be 1000 degreeC or more. However, when the temperature at which the hot rolling is started is reduced to about 1000 ° C., the rolling load increases, and it becomes difficult to perform hot rolling including normal rough rolling and finish rolling. In such a case, a thin slab having a thickness of 100 mm or less may be employed. Further, in this case, rough rolling may be omitted. This is because when a thin slab is employed and rough rolling is omitted, the slab residence time at a high temperature can be shortened, which is preferable in suppressing the coarsening of carbides.

  The hot-rolled steel sheet (steel strip) that has been hot-rolled to a predetermined thickness is then cooled and wound on a coil. The coiling temperature at this time needs to be in the range of 500 to 700 ° C. in order to increase the strength by depositing carbonitrides such as Ti, Nb and Zr uniformly and finely. If the coiling temperature is less than 500 ° C, it becomes a bainite-based structure and cannot be made into a ferrite single-phase structure, and fine carbides do not sufficiently precipitate in the ferrite grains, ensuring the desired tensile strength and bending workability. Can not do it. On the other hand, if the CT exceeds 700 ° C., the precipitated carbide is coarsened and the precipitation strengthening ability is lowered, so that the desired tensile strength and bending workability cannot be ensured.

  The hot-rolled steel sheet obtained as described above has a steel structure composed of a ferrite single phase, carbides are finely precipitated in the ferrite, a tensile strength of 1180 MPa or more, and excellent bending workability. It becomes. Moreover, the above-mentioned excellent characteristics of the high-strength hot-rolled steel sheet of the present invention can be maintained as it is not only by electrogalvanization but also by hot dip galvanization with heat treatment and alloying treatment after hot dip galvanization. it can. Therefore, the high-strength hot-rolled steel sheet of the present invention can be suitably used as a material for a surface-treated steel sheet that forms a plated layer such as electrogalvanized, hot-dip galvanized, and alloyed hot-dip galvanized on the steel sheet surface. The plating layer is not limited to a zinc-based plating layer, and may be an Al-based or other metal plating layer.

C: 0.18 mass%, Si: 0.02 mass%, Mn: 1.30 mass%, P: 0.020 mass%, S: 0.001 mass%, Al: 0.04 mass%, N: 0.003 mass%, Ti : 0.19 mass%, Mo: 0.3 mass% and V: 0.41 mass% ((Ti / 48 + Nb / 93 + Zr / 91 + Mo / 96 + V / 51 + W / 184) / (C / 12): 1.02) ((Ti / 48 + Nb / 93 + Zr / 91 + Mo / 96 + V / 51 + W / 184) / (C / 12) = 1.01), a steel having a component composition consisting of Fe and unavoidable impurities in the remainder is melted and vertically bent continuously A steel slab was manufactured using a casting machine with a radius of curvature R (m) of the bent portion and a slab thickness t (m) of R / t in the range of 26.7 to 36.4, and then solidified. To 1300 ° C. at the cooling rate shown in Table 1, and then cooled to the lowest temperature shown in Table 1, and then reheated under the reheating conditions shown in Table 1, and then also shown in Table 1. Hot rolling was performed under the conditions to obtain hot rolled steel sheets having various thicknesses.
Some slabs were cooled to room temperature RT, then charged in a heating furnace and reheated. Further, some slabs were hot-rolled without rough rolling.

Moreover, about a part of hot-rolled steel plate obtained as mentioned above, after pickling and removing a scale, it passed through the continuous hot dip galvanizing line CGL, and annealed at 650-850 degreeC. Then, it was immersed in a hot dip galvanizing tank to obtain a hot dip galvanized steel sheet GI, or was further alloyed at 500 to 550 ° C. to obtain an alloyed hot dip galvanized steel sheet GA.
Furthermore, about a part of hot-rolled steel plate obtained as mentioned above, after pickling and removing a scale, it passed through the electrogalvanization line EGL, and the Zn-Ni type electrogalvanized steel plate EG It was.

  A sample was taken from the hot-rolled steel sheet thus obtained, and the steel structure of the steel sheet was confirmed using an optical microscope and a scanning electronic distance mirror (SEM). Further, a JIS No. 5 tensile test piece having the L direction as the tensile direction was taken from the above sample, and a tensile test was performed in accordance with JIS Z2241, to determine the tensile strength TS. The bendability was evaluated by mechanically grinding the hot-rolled steel sheet to a thickness of 1 mm, cutting out a strip test piece, and using a punch and a die having a V-shaped cross section (angle 90 °) with a radius of curvature of 0.5 mm at the tip. Using this, a bending test was performed in which the strip test piece was bent in the C direction, and the presence or absence of cracks was examined with a 10-fold loupe. In order to prevent cracking from the end face of the test piece, the end face of the test piece was ground, and 3 mm of both end portions of the width of the bent part of the test piece were excluded from the evaluation as unsteady parts. In addition, the above bending test was carried out in n number of 3 under each condition, and if all three cracks were not cracked, the bending workability was good (◯), and if even one crack was found, the bending workability was poor (×). . The results are also shown in Table 1.

As shown in Table 1, No. of manufacturing conditions satisfying all the conditions of the present invention. It can be seen that the steel sheets 1 to 4 (invention examples) all have a ferrite single phase metal structure, a tensile strength TS of 1180 MPa or more, and are excellent in bending workability.
On the other hand, no. 5-10 steel plates (comparative examples) are inferior in any one or more of tensile strength and bending workability.
Further, No. 1 satisfying all the conditions of the present invention. The steel sheets 11 to 13 (invention examples) can maintain the above-mentioned excellent characteristics as they are after electrogalvanizing, after hot dip galvanizing with heat treatment, and after alloying treatment. I understand that.

  A to P steels having the composition shown in Table 2 were melted, and the R / t at the bending radius of curvature R (m) and slab thickness t (m) was 26. After producing a steel slab in the range of 7 to 36.4, the steel slab was cooled from the freezing point to 1300 ° C. at the cooling rate shown in Table 3, and then cooled to the lowest temperature shown in Table 3. Similarly, after reheating to the reheating temperature shown in Table 3, hot rolling was performed under the same conditions as shown in Table 3 to obtain hot rolled steel sheets having various plate thicknesses. Some slabs were cooled to room temperature, charged into a heating furnace, and reheated. Also, rough rolling was omitted for some slabs.

  A sample was taken from the hot-rolled steel sheet thus obtained, and the steel sheet structure was confirmed using an optical microscope and a scanning electronic distance microscope (SEM). Further, a JIS No. 5 tensile test piece having the L direction as the tensile direction was taken from the above sample, a tensile test was performed in accordance with JIS Z2241, the tensile strength TS was obtained, and the bending workability was the same as in Example 1. And evaluated. The results are also shown in Table 3.

As shown in Table 3, all of the steel component composition and production conditions satisfy the conditions of the present invention. Steel sheets 1 to 5 and 7 to 10 (invention examples , where 2, 3, 5, 6, and 9 are reference examples ) all have a single-phase ferrite structure, a tensile strength of 1180 MPa or more, and Excellent bending workability.
On the other hand, any of the steel composition and production conditions deviates from the present invention. It can be seen that the steel plates 6 and 11 to 18 (comparative examples) are inferior to the steel plate of the invention example in any one or more of the above characteristics.

Claims (8)

C: 0.1 to 0.4 mass%, Si: 0.2 mass% or less, Mn: 1.5 mass% or less, P: 0.03 mass% or less, S: 0.03 mass% or less, Al: 0.1 mass% or less N: 0.005 mass% or less and Zr: 0.05 to 3.5 mass%, and further selected from Ti: 0.04 to 2.0 mass% and Nb: 0.05 to 3.0 mass% 1 or 2 selected from the group consisting of Mo: 0.01 to 0.5 mass%, V: 0.01 to 1.0 mass%, and W: 0.01 to 1.0 mass%. The seed or more is contained so as to satisfy the following formula (1), and the remainder has a component composition consisting of Fe and inevitable impurities,
A punch and die having a V-shaped cross section (angle 90 °) having a steel structure made of a ferrite single phase, a tensile strength of 1180 MPa or more , a cut 1 mm thick strip test piece with a radius of curvature of 0.5 mm at the tip. A high-strength hot-rolled steel sheet characterized by having no cracks in the test piece when subjected to a bending test in which the specimen is bent in the C direction .
0.7 ≦ (Ti / 48 + Nb / 93 + Zr / 91 + Mo / 96 + V / 51 + W / 184) / (C / 12) ≦ 1.2 (1)
(The element symbol in the above formula (1) represents the content (mass%) of each element.) The high-strength hot-rolled steel sheet according to claim 1, wherein a plated layer is formed on the surface of the steel sheet obtained by hot rolling. The high-strength hot-rolled steel sheet according to claim 2, wherein the plating layer is a zinc-based plating layer. The high-strength hot-rolled steel sheet according to claim 3 , wherein the zinc-based plated layer is a hot- dip galvanized layer. The high-strength hot-rolled steel sheet according to claim 3 , wherein the zinc-based plated layer is an alloyed hot- dip galvanized layer. A steel slab obtained by melting and continuously casting steel having the component composition according to claim 1 is cooled from a freezing point to 1300 ° C. at 10 to 300 ° C./min, and then cooled to less than 900 ° C. Without being charged into a heating furnace and reheating at 1150 to 1300 ° C. for 40 minutes or less, and then hot-rolling to finish the finish rolling at a temperature of 820 ° C. or more and winding it on a coil at a temperature of 700 to 500 ° C. ,
A punch and die having a V-shaped cross section (angle 90 °) having a steel structure made of a ferrite single phase, a tensile strength of 1180 MPa or more , a cut 1 mm thick strip test piece with a radius of curvature of 0.5 mm at the tip. A method for producing a high-strength hot-rolled steel sheet, comprising obtaining a hot-rolled steel sheet having no cracks in the test piece when a bending test is performed in which the specimen is bent in the C direction . The steel slab is manufactured using a continuous casting machine having a ratio (R / t) of a radius of curvature R (m) at a curved portion to a thickness t (m) of the steel slab of 25 or more. Item 7. A method for producing a high-strength hot-rolled steel sheet according to Item 6. The method for producing a high-strength hot-rolled steel sheet according to claim 6 or 7, wherein a plating layer is formed on a surface of the steel sheet obtained by hot rolling.