JPWO2015118864A1 - Manufacturing method of high strength hot-rolled steel sheet - Google Patents

Abstract

The present invention provides a high workability high strength hot-rolled steel sheet having a tensile strength of 590 MPa or more, excellent workability, and low tensile strength anisotropy, and a method for producing the same. The steel sheet according to the present invention is mass%, C: more than 0.010% and 0.070% or less, Si: 0.30% or less, Mn: more than 0.30% and 1.00% or less, P: 0.030. %: S: 0.030% or less, Al: 0.10% or less, N: 0.0100% or less, Ti: 0.050% or more and 0.120% or less, and Nb and B which are impurity elements Nb: 0.005% or less, B: 0.0005% or less, the balance is composed of Fe and inevitable impurities, the ferrite phase area ratio is 95% or more, the ferrite phase crystal grains The average aspect ratio is 3.0 or less, carbides containing Ti are finely precipitated in the ferrite phase crystal grains, and the carbide has an average particle diameter of less than 10 nm.

Description

  The present invention is a high-strength hot-rolled steel sheet suitable for structural steel materials such as parts for transportation machinery including automobiles and building steel, and has a tensile strength (TS) of 590 MPa or more. The present invention relates to a high-strength hot-rolled steel sheet having excellent stretch flangeability and small anisotropy in tensile strength and a method for producing the same.

From the viewpoint of global environmental conservation, for the purpose of reducing CO ₂ emissions, maintaining the strength of the car body while reducing the weight and improving the fuel efficiency of the car has always been an important issue in the automobile industry. . In order to reduce the weight of the vehicle body while maintaining the strength of the automobile body, it is effective to reduce the thickness of the steel sheet by increasing the strength of the steel sheet used as a material for automobile parts. For example, by reducing the thickness and strength of steel plates for undercarriage parts of automobiles, which often use thick steel plates, it is possible to expect a significant reduction in the weight of automobile bodies.

  On the other hand, since many automobile parts made of steel plates are formed by press working or burring, etc., excellent formability is required for steel sheets for automobile parts. For example, parts for automobile undercarriages are often subjected to burring due to the shape of the parts, and steel plates having excellent stretch flangeability are required.

  In addition, when pressing or burring a steel sheet with a large tensile strength anisotropy, fluctuations in the amount of springback due to this anisotropy will increase. The direction is limited. In contrast, steel sheets with low anisotropy in tensile strength have little variation in the amount of springback due to anisotropy, and the direction in which parts are punched from the steel sheet is not limited, so a significant improvement in yield is expected. it can. Therefore, it is extremely important in the industry to reduce the anisotropy of the tensile strength of the steel sheet. In particular, when punching out parts having complicated shapes such as parts for automobile undercarriage, the yield is significantly improved.

  For the reasons described above, a high-strength steel sheet that is excellent in stretch flangeability and has a small tensile strength anisotropy is particularly desired as a steel sheet applied to automobile undercarriage parts. However, in general, steel materials tend to have lower ductility and stretch flangeability with increasing strength, and further increase the anisotropy of mechanical properties. For this reason, when a high-strength steel plate is applied to an automobile undercarriage part, a steel plate that is excellent in stretch flangeability and has low mechanical property anisotropy is required in addition to high strength.

  Many researches and developments have been made on high-strength hot-rolled steel sheets that have both strength and workability, and various technologies have been proposed. Above all, the technology to increase the strength of the steel sheet by making the metal structure substantially ferrite single phase and precipitating fine carbides in the ferrite phase grains is to achieve both excellent strength and excellent flange flangeability. It is known to be a very useful technique.

  For example, Patent Document 1 relates to a hot-rolled steel sheet, in which the steel sheet composition is 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%, total of one or two of Ti and Nb: 0.10 to 0.60% The composition of which the main component is Fe, the steel sheet structure has a ferrite content of 95% or more by area ratio, and the average crystal grain size of ferrite is 2.0 to 10.0 μm, and substantially contains martensite and retained austenite. A technology that does not include the organization has been proposed. And in patent document 1, recrystallization in a hot rolling process is suppressed by making 1 type or 2 types of total content of Ti and Nb into 0.10 to 0.60%, and also Mn content is made into As a result of suppressing the coarsening of the ferrite grains by setting it to more than 1.0% to 2.5%, fine ferrite grains are obtained, and a hot rolled steel sheet having a tensile strength of 490 MPa or more is obtained without impairing stretch flangeability. It is stated that

In addition, Patent Document 2 relates to a hot-rolled steel sheet, and the composition of the steel sheet in mass% is C: 0.01 to 0.1%, S: 0.03% or less, N: 0.005% or less, Ti: 0.00. Steel containing 0.5 to 0.5%, further containing Ti in a range satisfying (Ti-48 / 12C-48 / 14N-48 / 32S) ≧ 0%, with the balance being Fe and inevitable impurities, A technology has been proposed in which the average size of precipitates containing Ti of 5 nm or more among the particles therein is 10 ¹ to 10 ³ nm and the minimum interval is more than 10 ¹ nm to 10 ⁴ nm. Patent Document 2 provides a hot-rolled steel sheet having a tensile strength of 640 MPa or more excellent in burring workability and fatigue characteristics by defining the average size and minimum interval of precipitates containing Ti of 5 nm or more as described above. It is stated that

  On the other hand, many researches and developments have been made on techniques for producing high-strength hot-rolled steel sheets with small anisotropy. For example, Patent Document 3 relates to a hot-rolled steel sheet, with ferrite or bainite being the phase with the largest volume fraction, optionally containing martensite or residual austenite with a volume fraction of 1% to 25%, and at least 1/2. The average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation groups on the plate thickness in the plate thickness is 2.5 or more, and {554} <225>, {111} < The average value of the X-ray random intensity ratios of the three crystal orientations 112> and {111} <110> is 3.5 or less, and at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction. A technique has been proposed in which one is 0.7 or less, the uniform elongation anisotropy ΔuE1 is 4% or less, and the local elongation anisotropy ΔLE1 is less than or equal to.

  In Patent Document 3, a specific texture as described above, and by defining ΔuEl and ΔLEl, a highly workable and high strength hot rolled steel sheet having low shape anisotropy and low anisotropy can be obtained. Have been described.

JP 2000-328186 A JP 2002-161340 A JP 2004-250743 A

  However, in the technique proposed in Patent Document 1, it is necessary to add more than 1.0% of Mn to the steel sheet, so Mn segregation inevitably occurs in the steel sheet. Thus, in the steel sheet in which Mn segregation occurs, the strength of the Mn segregation part is significantly higher than the strength of the non-segregation part. There is a problem with stability. Further, in the technique proposed in Patent Document 1, ferrite grains are contained by containing 0.10 to 0.60% of one or two of Ti and Nb in total to suppress recrystallization in the hot rolling process. The strength of the steel sheet and stretch flangeability are ensured. In this way, if recrystallization in the hot rolling process is suppressed, recrystallization after finish rolling is suppressed, resulting in a problem that the ferrite grain shape after transformation becomes flat and the anisotropy of tensile strength increases. . Therefore, the technique proposed in Patent Document 1 cannot secure the strength and stretch flangeability of the steel sheet while suppressing the anisotropy of the tensile strength.

  Further, in the technique proposed in Patent Document 2, carbide is formed by containing Ti in a range satisfying (Ti-48 / 12C-48 / 14N-48 / 32S) ≧ 0%, and solid solution C is reduced. We are trying to make it. Thus, when excess Ti is contained with respect to C, the amount of solid solution Ti increases remarkably in the vicinity of the coiling temperature in the hot rolled steel sheet manufacturing process, and the coarsening of Ti carbide after coiling is promoted. Therefore, the technique proposed in Patent Document 2 has a problem that the stability of the steel sheet strength with respect to fluctuations in manufacturing conditions (mainly winding temperature) is significantly reduced.

  With the technique proposed in Patent Document 3, it is difficult to obtain a specific texture stably in the longitudinal direction and the width direction of the steel sheet. Moreover, in the technique proposed in Patent Document 3, since martensite and retained austenite are positively contained as a steel sheet structure, there is a problem that strength stability against hot rolling conditions is remarkably lowered. Furthermore, since the hot-rolled steel sheet obtained by the technique proposed in Patent Document 3 has a composite structure in which martensite and retained austenite are positively contained in addition to ferrite or bainite, a hard phase such as martensite and ferrite Cracks due to hardness differences are likely to occur at the interface with the phase, and it is difficult to say a steel plate with excellent stretch flangeability.

  As described above, according to the prior art, it is extremely difficult to obtain a high-strength hot-rolled steel sheet having small tensile strength anisotropy while ensuring strength and stretch flangeability.

  The present invention advantageously solves the problems of the prior art and is suitable as a material for automobile parts. Tensile strength (TS): High strength of 590 MPa or more, excellent stretch flangeability, and tensile strength An object of the present invention is to provide a high-strength hot-rolled steel sheet having a small anisotropy and a method for producing the same. In addition, when it only describes with tensile strength in this invention, the tensile strength of C direction (rolling perpendicular direction) is represented.

  In order to solve the above-mentioned problems, the present inventors focused on hot-rolled steel sheets having a ferrite single-phase structure with good workability, and increased the strength of the hot-rolled steel sheets while maintaining excellent workability. The method for reducing the anisotropy of the thickness was studied earnestly.

  As a method for increasing the strength of a hot-rolled steel sheet having a ferrite single phase structure, solid solution strengthening by Si or Mn and precipitation strengthening by Ti or Nb can be mentioned. However, Si and Mn, which have been extremely useful for increasing the strength of steel sheets and have been positively added to high-strength steel sheets as solid solution strengthening elements, have improved stretch flangeability when segregated at the center of the plate thickness. It causes a decrease. Also, in steel sheets with high strength due to precipitation of Ti and Nb, in the manufacturing process of hot-rolled steel sheets, the ferrite grains become flat because Ti and Nb inhibit recrystallization after finish rolling, and the mechanical properties are anisotropic. Tend to be large.

  In order to solve such a problem, the present inventors have observed the structure of hot rolled steel sheets to which Mn and Si are added, and have conducted a thorough investigation on the relationship between the contents of Mn and Si and stretch flangeability. As a result, when Mn and Si are included in excess of 1.5% in total, segregation inevitably exists in the central portion of the plate thickness, and changes in the hardness and shape of the structure due to segregation cause elongation of the steel sheet. The knowledge that it has a bad influence on flangeability was obtained. And, it is said that the influence of the segregated structure can be suppressed by setting the contents of Mn and Si to a predetermined amount or less, specifically, the Si content to 0.30% or less and the Mn content to 1.00% or less. Obtained knowledge.

  Based on these findings, the present inventors have developed an anisotropy of the mechanical properties of the steel sheet while combining excellent stretch flangeability and high strength by means of increasing the strength of the steel sheet by solid solution strengthening with Si and Mn. It was judged that it was extremely difficult to reduce the size. As a technique for increasing the strength of a hot rolled steel sheet having a ferrite single phase structure, an attempt was made to utilize precipitation strengthening by carbides of Ti and Nb while reducing the amount of Si and Mn added.

  By precipitating fine carbides of Ti and Nb on the steel plate, significant improvement in steel plate strength can be expected. However, the addition of Ti and Nb, which are carbide forming elements, is a factor that delays the recrystallization of austenite after finish rolling in the manufacturing process of hot-rolled steel sheets. When the recrystallization of austenite is not promoted, the austenite before the γ → α transformation has a structure elongated in the rolling direction. As a result, the ferrite after transformation also has a structure elongated in the rolling direction, and there is a marked difference in mechanical properties between the rolling direction and the direction perpendicular to the rolling direction.

  Accordingly, the present inventors have further studied and sought a means for making the ferrite grains after hot rolling equiaxed even when a sufficient amount of carbide forming element is added to obtain a desired steel plate strength. As a result, when Nb is used as the carbide forming element, it is difficult to make the ferrite grains equiaxed, but while using Ti as the carbide forming element, the addition amount of Si and Mn is reduced to a predetermined amount or less, In addition, by controlling the reduction ratio of finish rolling in hot rolling, the rolling temperature and the holding time after rolling, the ferrite grains become equiaxed or nearly equiaxed, while maintaining desired strength and excellent stretch flangeability. In addition, the inventors have found that the anisotropy of tensile strength can be suppressed.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: more than 0.010% and 0.070% or less, Si: 0.30% or less, Mn: more than 0.30% and 1.00% or less, P: 0.030% or less, S : 0.030% or less, Al: 0.10% or less, N: 0.0100% or less, Ti: 0.050% or more and 0.120% or less, and Nb and B which are impurity elements are contained in mass%. Nb: 0.005% or less, B: 0.0005% or less, the balance is composed of Fe and inevitable impurities, the area ratio of the ferrite phase is 95% or more, the ferrite phase The crystal grains have an average aspect ratio of 3.0 or less, carbides containing Ti are finely precipitated in the ferrite phase grains, and the carbide has an average particle diameter of less than 10 nm. High-strength hot-rolled steel characterized by having a thickness of 590 MPa or more .
[2] A high-strength hot-rolled steel sheet according to [1], wherein the composition satisfies the following formula (1):
((Ti− (48/14) × N− (48/32) × S) / 48) / (C / 12) <1.0 (1)
(C, S, N, Ti in formula (1): content of each element (mass%))
[3] In the above [1] or [2], in addition to the composition, in addition to REM, Zr, V, As, Cu, Ni, Sn, Pb, Ta, W, Mo, Cr, Sb, A high-strength hot-rolled steel sheet containing 1.0% or less in total of one or more selected from Mg, Ca, Co, Se, Zn, and Cs.
[4] The high-strength hot-rolled steel sheet according to any one of [1] to [3], wherein the steel sheet surface has a plating layer.
[5] A steel material having the composition according to any one of [1] to [3] is heated to an austenite single-phase region, hot-rolled, cooled, wound, hot-rolled steel sheet and In doing so, in the finish rolling of the hot rolling, rolling at a rolling reduction of 15% or more in a temperature range of 900 ° C. or more and 1100 ° C. or less is performed three or more times, the finish rolling temperature is set to 860 ° C. or more, and after the finish rolling is finished, The cooling is started after being held for 0.3 s or more in a temperature range of 850 ° C. or more, the average cooling rate from 850 ° C. to 750 ° C. of the cooling is 30 ° C./s or more, and the winding temperature of the winding is 580 A method for producing a high-strength hot-rolled steel sheet, characterized by having a temperature of from ℃ to 750 ℃.

  According to the present invention, the tensile strength (TS): suitable for structural steel materials such as parts for transportation machinery including automobiles and construction steel materials, has a high strength of 590 MPa or more and excellent workability, and A high-strength hot-rolled steel sheet with a small tensile strength anisotropy is obtained. Moreover, since the outstanding steel plate characteristic is acquired as mentioned above, this invention enables the further use expansion | deployment of a high intensity | strength hot-rolled steel plate, and has an industrial remarkable effect.

FIG. 1 is a diagram showing the relationship between the average aspect ratio of the ferrite phase and the anisotropy of tensile strength (ΔTS).

  Hereinafter, the present invention will be specifically described.

  First, the reasons for limiting the component composition of the hot-rolled steel sheet of the present invention will be described. In addition,% which represents the following component composition shall mean the mass% (mass%) unless there is particular notice.

C: more than 0.010% and 0.070% or less C is an essential element for forming carbide containing Ti in the steel sheet and increasing the strength of the hot-rolled steel sheet. When the C content is 0.010% or less, a carbide precipitation amount sufficient to make the tensile strength of the steel sheet 590 MPa or more cannot be obtained, and a tensile strength of 590 MPa or more cannot be obtained. On the other hand, when the C content exceeds 0.070%, pearlite is easily generated, and the stretch flangeability of the steel sheet is deteriorated. Therefore, the C content is more than 0.010% and 0.070% or less. Preferably, it is 0.020% or more and 0.060% or less.

Si: 0.30% or less Si is normally positively contained in a high-strength steel sheet as an effective element for improving the steel sheet strength without reducing ductility (elongation). However, Si is an element that promotes Mn segregation at the center of the plate thickness to be avoided in the hot-rolled steel sheet of the present invention and also segregates Si itself. Therefore, in the present invention, the Si content is limited to 0.30% or less for the purpose of suppressing the Mn segregation and suppressing the segregation of Si. Preferably it is 0.10% or less, More preferably, it is 0.05% or less.

Mn: more than 0.30% and 1.00% or less Mn is a solid solution strengthening element, and is actively contained in a normal high-strength steel sheet like Si. However, if Mn is positively contained in the steel sheet, Mn segregation at the center of the plate thickness is unavoidable, which causes the stretch flangeability of the steel sheet to deteriorate. Therefore, in the present invention, the Mn content is limited to 1.00% or less for the purpose of suppressing the Mn segregation. Preferably it is 0.90% or less.

  On the other hand, when the Mn content is 0.30% or less, the γ → α transformation point increases, and it is difficult to refine the carbide containing Ti. The carbide containing Ti is aged at the same time as the γ → α transformation or in the ferrite during the cooling or winding process after finish rolling in the hot rolled steel sheet manufacturing process. Here, when the [gamma]-> [alpha] transformation point becomes high temperature, the carbide containing Ti is precipitated in a high temperature region, so that the carbide becomes coarse. And as a result of the carbide | carbonized_material containing Ti coarsening, desired steel plate intensity | strength will no longer be obtained. Therefore, the Mn content is more than 0.30%. Preferably it is more than 0.35%.

P: 0.030% or less P is a harmful element that segregates at grain boundaries to reduce the elongation of the steel sheet and induces cracks during processing. Therefore, the P content is 0.03% or less.

S: 0.030% or less S is present in the steel as MnS or TiS. MnS and TiS promote the generation of voids during the punching process of hot-rolled steel sheets, and further serve as a starting point for the generation of voids during the processing, thereby reducing the stretch flangeability of the steel sheet. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and set it to 0.030% or less. Preferably it is 0.010% or less.

Al: 0.10% or less Al is an element that acts as a deoxidizer. In order to obtain such an effect, it is desirable to contain 0.01% or more of Al. However, when the Al content exceeds 0.10%, Al remains as an Al oxide in the steel sheet, and the Al oxide tends to agglomerate and become coarse, which causes the stretch flangeability of the steel sheet to deteriorate. . Therefore, the Al content is set to 0.10% or less. Preferably it is 0.05% or less.

N: 0.0100% or less Since N is present as TiN in the steel, if the N content increases, the amount of Ti that contributes to the formation of carbides decreases due to the presence of N, and the desired steel plate strength cannot be obtained. . In addition, TiN promotes the generation of voids during the punching process of hot-rolled steel sheets, and further, since it becomes a starting point for the generation of voids during processing, it reduces the stretch flangeability of the steel sheet. For the above reasons, in the present invention, it is preferable to reduce the N content as much as possible, and it is set to 0.0100% or less. Preferably it is 0.0060% or less.

Ti: 0.050% or more and 0.120% or less Ti is an indispensable element for forming a carbide containing Ti and increasing the strength of the steel sheet. When the Ti content is less than 0.050%, it is difficult to obtain a desired hot-rolled steel sheet strength (tensile strength: 590 MPa or more). On the other hand, when the Ti content is excessive, the carbide containing Ti tends to be coarsened, and it becomes difficult to obtain a desired hot-rolled steel sheet strength (tensile strength: 590 MPa or more). Ti is an element that inhibits recrystallization of austenite. Therefore, when Ti is contained in a large amount exceeding 0.120%, recrystallization after finish rolling is delayed, the ferrite structure after transformation is elongated in the rolling direction, and the anisotropy of the mechanical properties of the steel sheet Becomes larger. Therefore, the Ti content is set to 0.050% or more and 0.120% or less. Preferably they are 0.060% or more and 0.110% or less, More preferably, they are 0.060% or more and 0.100% or less.

Nb: 0.005% or less In the present invention, Nb is an impurity (unavoidable impurity). Nb, like Ti, is an element that forms carbides and contributes to steel plate strength. However, Nb inhibits austenite recrystallization more strongly than Ti. Therefore, when the Nb content exceeds 0.005%, the recrystallization of austenite after finish rolling is delayed, and the ferrite structure after transformation becomes a structure elongated in the rolling direction. Increases sex. Therefore, in the present invention, the strength of the steel sheet is not increased by Nb carbide, and the Nb content is reduced as much as possible, and is limited to 0.005% or less. Preferably it is 0.003% or less.

B: 0.0005% or less In the present invention, B is an impurity (inevitable impurity). B, like Nb, strongly inhibits the recrystallization of austenite and delays the recrystallization of austenite after finish rolling. Therefore, the ferrite structure after transformation extends in the rolling direction, and the anisotropy of the mechanical properties of the steel sheet increases. Therefore, in the present invention, the B content is reduced as much as possible, and is limited to 0.0005% or less. Preferably it is 0.0003% or less.

  Moreover, in this invention, in order to suppress the coarsening of the carbide | carbonized_material containing Ti, it is preferable to adjust content of C, S, N, and Ti so that following (1) Formula may be satisfied.

((Ti− (48/14) × N− (48/32) × S) / 48) / (C / 12) <1.0 (1)
(C, S, N, Ti in formula (1): content of each element (mass%))
In the present invention, desired steel plate strength is obtained by finely precipitating a carbide containing Ti in the ferrite phase. A carbide containing Ti has a strong tendency to become a fine carbide having an extremely small average particle diameter. However, when the atomic concentration of Ti contained in the steel is equal to or higher than the atomic concentration of C, the amount of dissolved Ti at the coiling temperature is rapidly increased, so that the carbide containing Ti is easily coarsened. As a result of coarsening of the carbide containing Ti, it becomes difficult to obtain a desired steel plate strength (tensile strength: 590 MPa or more). For the above reasons, it is desirable that the atomic percent of C contained in the steel be larger than the atomic percent of Ti that contributes to carbide formation.

  As will be described later, in the present invention, a predetermined amount of Ti is added to the steel material, the carbide in the steel material is dissolved by heating before hot rolling, and the carbide contains Ti mainly during winding after hot rolling. To precipitate. However, the total amount of Ti added to the steel material does not contribute to carbide formation, and a part of Ti added to the steel material is used for the formation of nitrides and sulfides. This is because Ti forms nitrides and sulfides more easily than carbides in a temperature range higher than the coiling temperature, and Ti forms nitrides and sulfides before the winding process when manufacturing a hot-rolled steel sheet. . Therefore, among the Ti contained in the steel material, the amount of Ti that can contribute to carbide generation can be represented by “Ti− (48/14) N− (48/32) S”.

  For the above reasons, in the present invention, the atomic percent of C (C / 12) is converted to atomic percent of Ti that can contribute to carbide formation ((Ti− (48/14) × N− (48/32) × S) / For the purpose of increasing more than 48), it is preferable to contain each element of C, N, S, and Ti so as to satisfy the above formula (1). More preferably, ((Ti− (48/14) × N− (48/32) × S) / 48) / (C / 12) <0.9.

  In addition to the above elements, the hot-rolled steel sheet of the present invention further includes REM, Zr, V, As, Cu, Ni, Sn, Pb, Ta, W, Mo, Cr, Sb, Mg, Ca, Co, One or more selected from Se, Zn, and Cs may be contained in a total of 1.0% or less. If the total content of these elements is 1.0% or less, the various properties of the hot-rolled steel sheet will not be adversely affected. Components other than the above are Fe and inevitable impurities (inevitable impurities excluding Nb and B).

  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 has the above-described composition, and the area ratio of the ferrite phase is 95% or more, the average aspect ratio of the crystal grains of the ferrite phase is 3.0 or less, and the ferrite phase The carbide containing Ti is finely precipitated in the crystal grains, and the carbide has a metal structure having an average particle diameter of less than 10 nm.

In the present invention, it is essential to form a ferrite phase in the hot-rolled steel sheet in order to obtain a desired stretch-flange property of the hot-rolled steel sheet. In order to improve stretch flangeability, the metal structure of the hot-rolled steel sheet is preferably a ferrite single phase. However, even if it is not a complete ferrite single phase, the above effect is sufficiently exhibited if the ferrite single phase is substantially the ferrite phase, that is, if the area ratio with respect to the entire metal structure is 95% or more. Therefore, in the present invention, the metal structure of the hot-rolled steel sheet is a structure containing a ferrite phase having an area ratio of 95% or more. Preferably it is 97% or more.

  In the hot-rolled steel sheet of the present invention, examples of the structure other than the ferrite phase that can be contained in the metal structure include cementite, pearlite, bainite phase, martensite phase, and retained austenite phase. When these structures are present in the metal structure, the stretch flangeability of the steel sheet is lowered. Therefore, it is preferable to reduce the area ratio of these tissues. However, it is permissible if the total area ratio with respect to the entire metal structure is 5% or less. Preferably it is 3% or less.

Average aspect ratio of ferrite phase crystal grains: 3.0 or less The aspect ratio of ferrite grains in the present invention is the length in the rolling direction relative to the length in the thickness direction of the ferrite grains in a cross section parallel to the rolling direction of the hot rolled steel sheet. Ratio. A method for obtaining the average aspect ratio will be described later. In the present invention, the aspect ratio is reduced to make the ferrite grain shape as close to the equiaxed axis as possible. If the aspect ratio is small, the difference between the length of the ferrite grains in the rolling direction and the length in the sheet width direction is also small. By setting the average aspect ratio to 3.0 or less, the anisotropy of the tensile strength of the hot-rolled steel sheet, that is, the difference in tensile strength between the rolling direction and the sheet width direction can be reduced. When the average aspect ratio of the ferrite grains increases beyond 3.0, the tensile strength anisotropy due to the ferrite grain shape becomes significant. Therefore, the average aspect ratio of ferrite grains is limited to 3.0 or less. Preferably it is 2.0 or less, More preferably, it is 1.5 or less. The average aspect ratio of the ferrite grains of the hot-rolled steel sheet obtained in the present invention was 1.1 at the minimum.

Carbide containing Ti In the present invention, the carbide finely precipitated on the hot-rolled steel sheet is a carbide containing Ti. When the hot-rolled steel sheet contains only Ti as a carbide constituent element, the carbide containing Ti is Ti carbide. Further, when the hot-rolled steel sheet also contains carbide constituent elements (V, Mo, etc.) other than Ti, in addition to Ti carbide, composite carbide of Ti and V, or further, carbide constituent elements such as Mo, etc. in the carbide. Including.

Average particle diameter of carbides containing Ti: less than 10 nm The average particle diameter of carbides containing Ti is extremely important for making a hot-rolled steel sheet have a desired strength (tensile strength: 590 MPa or more). Therefore, in this invention, the average particle diameter of the carbide | carbonized_material containing Ti shall be less than 10 nm. When carbide containing Ti is finely precipitated in the ferrite phase crystal grains, the carbide acts as a resistance against dislocation movement that occurs when the steel sheet is deformed, thereby increasing the strength of the hot-rolled steel sheet. However, when the carbide containing Ti is coarsened, the carbide is sparsely precipitated, and the interval for stopping the dislocation is widened, so that the precipitation strengthening ability is lowered. And when the average particle diameter of the carbide containing Ti is 10 nm or more, sufficient steel plate strengthening ability to compensate for the decrease in steel plate strength caused by reducing the content of Mn and Si as solid solution strengthening elements is obtained. Absent. Therefore, the average particle diameter of the carbide containing Ti is less than 10 nm. More preferably, it is 6 nm or less.

  In addition, although the effect of invention is not specifically limited, the fine precipitate (carbide containing Ti) in this invention may be observed so that it may be located in a line depending on the angle to observe. However, even in this case, the precipitates are actually randomly distributed in the plane where the rows of precipitates are observed, and when observed with a transmission electron microscope, the precipitates are often not observed in rows.

  By defining the composition and structure as described above, a high-strength hot-rolled steel sheet having a desired strength (tensile strength: 590 MPa or more), excellent stretch flangeability, and small tensile strength anisotropy. Is obtained. Moreover, even if a plated layer is provided on the surface of the hot-rolled steel sheet of the present invention for the purpose of imparting corrosion resistance to the steel sheet, the above-described effects of the present invention are not impaired. In the present invention, the type of the plating layer provided on the surface of the steel sheet is not particularly limited, and any of an electroplating layer and a hot dipping layer can be applied. The alloy component of the plating layer is not particularly limited, and preferred examples include a galvanized layer and an alloyed galvanized layer. Of course, the present invention is not limited to these examples. By forming a plating layer on the surface, the corrosion resistance of the hot-rolled steel sheet is improved, and it becomes possible to apply it to automobile parts used in severe corrosive environments.

  Next, the manufacturing method of the hot rolled steel sheet of the present invention will be described.

  In the present invention, a steel material having the above composition is heated to an austenite single-phase region, subjected to hot rolling, then cooled, wound, and formed into a hot-rolled steel sheet. At this time, in the finish rolling of the hot rolling, rolling at a reduction rate of 15% or more is performed three times or more in a temperature range of 900 ° C. or more and 1100 ° C. or less, the finish rolling temperature is 860 ° C. or more, and after the finish rolling is finished, The cooling is started after being held for 0.3 s or more in a temperature range of 850 ° C. or more, the average cooling rate from 850 ° C. to 750 ° C. of the cooling is 30 ° C./s or more, and the winding temperature of the winding is 580 It is characterized by being not less than 750 ° C. and not more than 750 ° C.

  In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, a slab is cast from the molten steel. From the viewpoint of productivity and the like, the slab (steel material) is preferably formed by a continuous casting method. However, the slab may be formed by a known casting method such as ingot-bundling rolling or continuous slab casting. In the present invention, the content of Mn and Si causing segregation is suppressed for the purpose of improving the workability (stretch flangeability, etc.) of the steel sheet. Therefore, when the continuous casting method advantageous for suppressing segregation is employed, the effect of the present invention becomes more remarkable.

  The steel material obtained as described above is hot-rolled. In the present invention, the steel material is heated to an austenite single-phase region prior to hot rolling. If the steel material before hot rolling is not heated to the austenite single-phase region, remelting of carbides (carbides containing Ti) in the steel materials will not proceed, and fine precipitation of carbides containing Ti after rolling will not occur. Cannot be realized.

For the above reasons, in the present invention, prior to hot rolling, the steel material is heated to an austenite single phase region, preferably 1200 ° C. or higher. However, if the heating temperature of the steel material becomes too high, the surface of the steel material is excessively oxidized, and TiO ₂ is generated in the vicinity of the surface. As a result of the generation of TiO ₂ , Ti for forming carbide is reduced in the vicinity of the surface, and as a result, the hardness in the vicinity of the surface of the finally obtained steel sheet is likely to decrease. Therefore, the heating temperature is preferably 1350 ° C. or lower. In addition, when hot rolling the steel material, if the steel material (slab) after casting has a temperature in the austenite single-phase region, the steel material is not heated or directly heated after being heated for a short time. You may roll.

  Hot rolling usually consists of rough rolling and finish rolling. In the present invention, the conditions for rough rolling are not particularly limited. In particular, when a thin slab casting method is employed, rough rolling may be omitted. Finish rolling is performed under the following conditions.

Rolling in a temperature range of 900 ° C. or higher and 1100 ° C. or lower: rolling at a rolling reduction of 15% or more 3 times or more Generally, ferrite transformed from austenite stretched by hot rolling tends to form a structure elongated in the rolling direction. Therefore, the anisotropy of the tensile strength of the steel sheet becomes a factor. On the other hand, since austenite recrystallized after hot rolling becomes equiaxed, the ferrite transformed from the austenite after recrystallization tends to have an equiaxed structure, and the anisotropy of the tensile strength of the steel sheet becomes small. For this reason, in the present invention, it is necessary to promote recrystallization after finish rolling and to transform ferrite from austenite after recrystallization.

  Here, austenite can be recrystallized by introducing processing strain by rolling in an appropriate temperature range. However, as long as the austenite structure is coarse, a region that does not partially recrystallize remains in the steel. This non-recrystallized portion has an austenite structure that extends greatly in the rolling direction, and also induces anisotropy because it is a region in which recrystallization hardly occurs during holding after finish rolling described later. This non-recrystallized austenite part disappears gradually by repeating rolling and recrystallization, and thereby a uniform and fine-grained recrystallized austenite structure can be obtained. Austenite recrystallization is achieved by rolling at a rolling reduction of 15% or more in a temperature range of 900 ° C. or higher and 1100 ° C. or lower. In order to obtain a recrystallized austenite structure that is uniform and fine throughout the entire steel plate, it is necessary to perform rolling at a reduction rate of 15% or more at least three times in a temperature range of 900 ° C. or higher and 1100 ° C. or lower. Preferably it is 4 times or more. Note that the upper limit is substantially about seven due to the restriction on the number of finish rolling stands.

Finish rolling temperature: 860 ° C. or higher As described above, in order to obtain a steel sheet with low tensile strength anisotropy, it is necessary to promote recrystallization after finish rolling and to transform ferrite from austenite after recrystallization. . Here, when the finish rolling temperature is less than 860 ° C., it becomes difficult for the austenite grains to be equiaxed by recrystallization before the start of accelerated cooling after finish rolling. Therefore, if the finish rolling temperature is less than 860 ° C., the shape of the ferrite grains after transformation becomes a shape elongated in the rolling direction, and the anisotropy of the tensile strength of the steel sheet tends to increase. For the above reasons, the finish rolling temperature is set to 860 ° C. or higher. Moreover, it is preferable to set it as 880 degreeC or more, and it is more preferable to set it as 900 degreeC or more. However, if the finish rolling temperature is higher than necessary, wrinkles and roughness due to the secondary scale of the surface are likely to occur, so the substantial upper limit of the finish rolling temperature is about 1050 ° C.

After completion of finish rolling, holding time in a temperature range of 850 ° C. or higher: 0.3 s or more In the present invention, accelerated cooling is not started immediately after finish rolling, but is held in a high temperature range for a predetermined time, thereby extending by finish rolling. Austenite recrystallization proceeds to make the transformed ferrite grains equiaxed or close to equiaxed. In order to sufficiently advance the recrystallization of austenite, it is necessary to hold the steel sheet after finish rolling for 0.3 seconds or more in a temperature range of 850 ° C. or more. Preferably it is 0.5 second or more, More preferably, it is 0.8 second or more.

  However, if the holding time in the above temperature range becomes too long, it becomes difficult to cool to the target winding temperature due to the restriction of the length of the cooling equipment. Therefore, the substantial upper limit of the holding time in the temperature range is about 10 seconds. After completion of finish rolling, accelerated cooling is started after holding at a temperature range of 850 ° C. or higher for 0.3 seconds or longer. Accelerated cooling is performed under the following conditions.

Average cooling rate from 850 ° C. to 750 ° C .: 30 ° C./s or more In order to precipitate fine carbides (carbides containing Ti) having an average particle diameter of less than 10 nm, accelerated cooling and γ at the lowest possible temperature → It is necessary to make α transformation occur. When the average cooling rate in the temperature range from 850 ° C. to 750 ° C. is less than 30 ° C./s, the γ → α transformation occurs at a high temperature, and the carbides precipitated in the ferrite are easily coarsened. Therefore, the average cooling rate in the temperature range from 850 ° C. to 750 ° C. is set to 30 ° C./s or more. Preferably it is 50 degrees C / s or more. However, if the average cooling rate in the temperature range becomes excessively large, it is difficult to control the coiling temperature and it may be difficult to obtain a stable strength. Therefore, the average cooling rate in the temperature range is 300 ° C./s or less. It is preferable to do.

Winding temperature: 580 ° C. or higher and 750 ° C. or lower The optimization of the winding temperature is performed by precipitating fine carbides (carbides containing Ti) in the ferrite and forming a desired metal structure (area ratio of ferrite phase) in the hot rolled steel sheet. : 95% or more, average particle diameter of carbide containing Ti: less than 10 nm). When the coiling temperature is less than 580 ° C., martensite and bainite are likely to be generated, and it becomes difficult to make the metal structure substantially a ferrite single phase structure. On the other hand, when the coiling temperature exceeds 750 ° C., the coarsening of the carbide containing Ti is promoted during cooling of the coil after winding, and therefore it is difficult to make the average particle diameter of the carbide containing Ti less than 10 nm. It becomes. Therefore, the coiling temperature is set to 580 ° C. or higher and 750 ° C. or lower. Preferably they are 600 degreeC or more and 680 degrees C or less.

  Furthermore, holding the coil after winding in a temperature range of 580 ° C. or higher and 750 ° C. or lower for 60 seconds or more is more preferable because a uniform structure can be easily obtained. The thickness of the hot-rolled steel sheet of the present invention is not particularly limited, but is generally 1.0 mm or more and 8.0 mm or less.

  As described above, a high-strength hot-rolled steel sheet having high tensile strength (TS): 590 MPa or more, excellent workability (particularly stretch flangeability) and small tensile strength anisotropy can be obtained. In the present invention, the desired effect is exhibited regardless of whether the material is a black skin material (as hot rolled material) or a white skin material (hot rolled pickling material).

  In the present invention, the hot-rolled steel sheet produced as described above may be plated to form a plating layer on the steel sheet surface. Even if the plating layer is formed, the effect of the present invention is not impaired. As the plating treatment, either electroplating or hot dipping can be applied. Further, the alloy component of the plating layer is not particularly limited, and preferred examples include a hot dip galvanized layer and an alloyed hot dip galvanized layer. Of course, the present invention is not limited to these examples. Aluminum or aluminum alloy can also be plated.

  The hot-rolled steel sheet obtained by the present invention is suitable for a material for press molding performed at room temperature, and also suitable for warm forming in which a steel sheet before pressing is heated from 400 ° C. to 750 ° C. and then press-formed immediately. .

  Molten steel was melted and continuously cast by a generally known method to obtain a slab (steel material) having a thickness of 300 mm having the composition shown in Table 1. These slabs are heated under the conditions shown in Table 2, and after rough rolling, finish rolling is performed under the conditions shown in Table 2. After finishing rolling, the slab is kept in a temperature range of 850 ° C. or higher for a predetermined time, accelerated cooling, Winding, plate thickness: 2.3 mm hot rolled steel sheet.

Subsequently, after pickling the hot-rolled steel sheet obtained above to remove the surface scale, some hot-rolled steel sheets (steel plates No. S40, S41, S42) are hot-dip galvanized lines with an annealing temperature of 720 ° C. And immersed in a 480 ° C. zinc plating bath (plating composition: 0.1 mass% Al—Zn) to form a hot dip galvanized layer with an adhesion amount of 45 g / m ² per side on the surface of the steel plate and melt A galvanized steel sheet (GI material) was used. Further, for some hot-rolled steel sheets (No. S43, S44), after forming a hot-dip galvanized layer as described above, alloying treatment is performed at 520 ° C. to form an alloyed hot-dip galvanized steel sheet (GA material). It was.

Samples are taken from the hot-rolled steel sheet (hot-rolled steel sheet, GI material, GA material) obtained as described above, and subjected to structure observation, tensile test, and hole expansion test. The type and area ratio, the average aspect ratio of the crystal grains of the ferrite phase, the average particle diameter of the carbide containing Ti, the tensile strength, the elongation, and the hole expansion ratio (stretch flangeability) were determined. The observation method and test method were as follows.
(I) Microstructure observation A test piece is taken from the obtained hot-rolled steel sheet (hot-rolled steel sheet, GI material, GA material), and a cross-section (L cross-section) at a 1/4 thickness position parallel to the rolling direction of the test piece is obtained. After polishing and corroding with nital, a structure photograph was taken with an optical microscope (magnification: 400 times) and a scanning electron microscope (magnification: 2000 times). Subsequently, using the photographed structure | tissue photograph, the kind of structure | tissue other than a ferrite phase and a ferrite phase and those area ratios were calculated | required with the image-analysis apparatus.

  In addition, a test piece is taken from the obtained hot-rolled steel sheet (hot-rolled steel sheet, GI material, GA material), and a cross section (L cross section) at a 1/4 thickness position parallel to the rolling direction of the test piece is polished. After corroding with nital, a structure photograph was taken with a scanning electron microscope (magnification: 800 times). Next, the average aspect ratio of the ferrite phase crystal grains was determined by image analysis using the photographed structure photograph. Here, the average aspect ratio of the ferrite phase crystal grains is the length of the vertical lines cutting the ferrite crystal grains when three horizontal lines and three vertical lines are drawn for each photograph taken at a magnification of 800 times (average crystal grain section length). )) To the average grain section length of the horizontal line. In the photograph, a horizontal line is drawn in a direction parallel to the rolling direction.

Moreover, the thin film produced from the plate | board thickness 1/4 position of the obtained hot-rolled steel plate (hot-rolled steel plate, GI material, GA material) is observed with a transmission electron microscope (TEM), and the average particle diameter of the carbide containing Ti Asked. The average particle diameter of the carbide containing Ti is measured using a photograph taken with a transmission electron microscope (magnification: 340000 times) for a minimum of 100 carbides (carbide containing Ti) in a total of five fields of view, The average value was defined as the average particle size. The carbide particle diameter is determined by measuring the maximum diameter d of the carbide (the diameter of the largest portion) and the diameter (thickness) t in the direction orthogonal thereto, and the arithmetic average value d _def = (d + t) / 2. As sought. Moreover, the carbide | carbonized_material containing Ti was analyzed by EDX attached to TEM.
(Ii) Tensile test From the obtained hot-rolled steel sheet (hot-rolled steel sheet, GI material, GA material), the direction perpendicular to the rolling direction (C direction), the rolling direction and parallel direction (L direction), the rolling direction and 45 A JIS No. 5 tensile test piece (JIS Z 2241) with the tensile direction (D direction) as the tensile direction is sampled and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to determine the tensile strength (TS) and elongation (El). It was measured. Next, the difference in tensile strength ΔTS between the direction with the highest tensile strength and the direction with the lowest tensile strength among the C direction, L direction, and D direction is obtained, and the anisotropic tensile strength that causes fluctuations in the springback amount. It was used as an index of sex. In addition, the case where the tensile strength of a C direction was 590 Mpa or more was evaluated as having the desired steel plate strength. Moreover, when ΔTS was less than 50 MPa, it was evaluated that the anisotropy of tensile strength was small.
(Iii) Hole expansion test From the obtained hot-rolled steel sheet (hot-rolled steel sheet, GI material, GA material), a test piece (size: 130 mm × 130 mm) was sampled, and an initial diameter d _{0 of} 10 mmφ was obtained on the test piece. Holes were formed by punching (clearance: 12.5% of the test piece plate thickness). Using these test pieces, a hole expansion test was performed. That is, a conical punch having an apex angle of 60 ° is inserted from the punch side at the time of punching into the hole, the hole is expanded, and the diameter d of the hole when the crack penetrates in the thickness direction of the steel plate (test piece) is set. The hole expansion ratio λ (%) was calculated by the following formula. Here, when the hole expansion ratio λ is 100% or more, it is determined that the workability is excellent.

Hole expansion rate λ (%) = {(d−d ₀ ) / d ₀ } × 100
The results obtained as described above are shown in Table 3. Moreover, the relationship between the average aspect ratio of the crystal grains of the ferrite phase obtained by the above (i) and ΔTS obtained by the above (ii) is shown in FIG.

As shown in Table 3, all the inventive examples have high tensile strength TS: 590 MPa or more, elongation El: 22% or more, and excellent workability of hole expansion ratio λ: 100% or more. A hot-rolled steel sheet is obtained. Moreover, as for the hot-rolled steel plate of the example of an invention, the average aspect-ratio of a ferrite grain is 3.0 or less, (DELTA) TS is less than 50 Mpa, and TS anisotropy is suppressed. Furthermore, as shown in FIG. 1, it can be understood that by setting the average aspect ratio of ferrite grains to 3.0 or less, ΔTS is less than 50 MPa, and a hot-rolled steel sheet having a small TS anisotropy can be obtained. On the other hand, the hot-rolled steel sheet of the comparative example outside the scope of the present invention does not ensure a predetermined high strength, does not secure a sufficient hole expansion rate, or has an average aspect ratio of ferrite grains of 3.0. And ΔTS is 50 MPa or more.

Claims (5)

In mass%, C: more than 0.010% and 0.070% or less, Si: 0.30% or less, Mn: more than 0.30% and 1.00% or less, P: 0.030% or less, S: 0.00. Nb and B containing 030% or less, Al: 0.10% or less, N: 0.0100% or less, Ti: 0.050% or more and 0.120% or less, and Nb and B as impurity elements in Nb : 0.005% or less, B: 0.0005% or less, the balance is composed of Fe and inevitable impurities,
The area ratio of the ferrite phase is 95% or more, the average aspect ratio of the ferrite phase crystal grains is 3.0 or less, and a carbide containing Ti is finely precipitated in the ferrite phase crystal grains. Having a structure with an average particle size of less than 10 nm,
A high-strength hot-rolled steel sheet having a tensile strength of 590 MPa or more. The high-strength hot-rolled steel sheet according to claim 1, wherein the composition satisfies the following formula (1).
((Ti− (48/14) × N− (48/32) × S) / 48) / (C / 12) <1.0 (1)
(C, S, N, Ti in formula (1): content of each element (mass%))   In addition to the above-mentioned composition, REM, Zr, V, As, Cu, Ni, Sn, Pb, Ta, W, Mo, Cr, Sb, Mg, Ca, Co, Se, Zn, Cs The high-strength hot-rolled steel sheet according to claim 1, wherein the high-strength hot-rolled steel sheet according to claim 1, containing at least one selected from the group consisting of 1.0% or less.   The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, further comprising a plating layer on the steel sheet surface.   When the steel material having the composition according to any one of claims 1 to 3 is heated to an austenite single-phase region and hot-rolled, and then cooled, wound, and hot-rolled steel sheet, In the finish rolling of rolling, rolling at a reduction rate of 15% or more is performed three times or more in a temperature range of 900 ° C. or more and 1100 ° C. or less, and the finish rolling temperature is set to 860 ° C. or more. The cooling is started after being held for 0.3 s or more, and the average cooling rate from 850 ° C. to 750 ° C. of the cooling is set to 30 ° C./s or more. A method for producing a high-strength hot-rolled steel sheet.

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