TW201544607A - Hot rolled steel sheet and manufacturing method thereof - Google Patents

Abstract

A hot rolled steel sheet is disclosed, wherein the hot rolled steel sheet has a predetermined chemical composition, wherein a total of Si content and Al content of the hot rolled steel sheet is more than 0.20% and less than 0.81%, wherein a microstructure of the hot rolled steel sheet has, in area ratio, a ferrite of 90 to 99 %, a martensite of 1 to 10%, and a bainite limited to 5 %, wherein a grain size of the martensite is 1 to 10[mu]m, and wherein X ray random intensity ratio in {211} (011), which is parallel to a rolled surface and a rolling direction, is 3.0 or less.

Description

Hot rolled steel sheet and method of manufacturing same Field of invention

The present invention relates to a high-strength hot-rolled steel sheet having excellent appearance and a balance between elongation and hole expandability and having a tensile strength of 590 MPa or more and a method for producing the same.

Background of the invention

In recent years, in order to improve the fuel economy of automobiles and improve the safety of collisions, a lot of efforts have been placed on the weight reduction of the body of high-strength steel sheets. In the case where a high-strength steel sheet is applied to a vehicle body or the like of an automobile, it is important to ensure press formability. Moreover, in order to improve surface design, for example, in a wheel disk for an automobile, it is required to remove the Si scale pattern as much as possible. Moreover, since the elongation processing and the burring processing are applied, a steel sheet as a raw material is required to have a good appearance, high elongation, and hole expandability.

Patent Document 1 proposes a method of setting the microstructure of the granulated iron to a hot-rolled steel sheet of 3% or more and less than 10%. Patent Document 1 discloses that the ferrite iron is precipitated and strengthened by using Ti and Nb to increase the strength thereof. In the case of the method, a hot-rolled steel sheet excellent in balance between elongation and hole expandability can be obtained.

Patent Document 2 discloses that in order to prevent the generation of Si scale which is a cause of deterioration in chemical conversion treatability, Al is added to make the ratio of the ferrite iron in the microstructure 40% or more with fertilized iron and 麻田散铁. Composite steel.

Advanced technical literature Patent literature

Patent Document 1: Japanese Patent Laid-Open No. 2011-184788

Patent Document 2: Japanese Patent Laid-Open Publication No. 2005-120438

Summary of invention

The technique described in Patent Document 1 is to add Ti and Nb for the precipitation strength of the ferrite iron. Therefore, the hot rolling delay makes the texture developed and the plastic anisotropy of the ferrite iron becomes stronger. As a result, sufficient hole expandability cannot be obtained.

Further, in the technique described in Patent Document 1, 0.5% or more of Si is added. Therefore, the scale generated by the hot rolling delay forms a stripe pattern (hereinafter referred to as a scale pattern) on the steel sheet, so that a good appearance cannot be obtained.

In the technique described in Patent Document 2, by adding Al as a substitute for Si in a steel sheet, the appearance and chemical conversion property are improved. However, since the addition of Al causes the temperature at which the ferrite-grain metamorphism starts to be elevated, coarse ferrite iron and 麻田散铁 are formed. As a result, the steel plate described in Patent Document 2 is easily broken at the interface between the ferrite iron and the granulated iron. Cracking, resulting in insufficient elongation and hole expansion.

In view of the above, it is an object of the present invention to provide a high-strength hot-rolled steel sheet having a good external appearance and excellent tensile strength and hole expandability at a tensile strength of 590 MPa or more and a method for producing the same.

In the present invention, the term "good appearance" means that the surface scale pattern is less generated, and the so-called balance between extensibility and hole expandability is excellent, and has an elongation of 20% or more and 100% or more. The case of the hole expansion ratio.

The inventors of the present invention conducted various reviews on the means for solving the above problems.

When the microstructure contains the granulated iron, although the strength will increase, there will be doubts that the hole expandability is lowered. Therefore, in order to increase the strength, it is considered to use the precipitation strengthening of Ti and Nb instead of the strength enhancement (metamorphic strengthening) formed by the loose iron in the field. However, when it is made to contain Ti and Nb, a texture is formed in the hot rolling.

Further, in order to improve the appearance, when Al is contained in place of Si which is a cause of the scale pattern, coarse granulated iron is formed, and the hole expandability is deteriorated. The inventors of this case, in order to solve these two problems, rediscovered that it is important to control the Oswego organization that is about to metamorphosis.

Specifically, it has been found that by setting the final pass rolling reduction ratio of the finished rolling to 20% or more, and setting the finishing rolling temperature to 880 ° C or higher and 1000 ° C or lower, it is possible to promote the re-entry of Osbane Iron. Crystallization, whereby the texture can be improved. In addition, it was discovered that by the end of the rolling delay, The water cooling of the steel sheet is started between 0.01 seconds and 1.0 second, and the recrystallization can be completed in a short time, whereby the fine recrystallized Aosta iron can be purified. The metamorphosis from the fine recrystallized Austin iron is mostly carried out at the nucleation site of the ferrite iron, and it will metamorphose very quickly. Therefore, by performing air cooling after completion of the above cooling, fine ferrite iron can be formed, and the Osbane iron remaining in the air cooling is also finely retained. As a result, it is feasible to refine the metamorphic granulated iron in the field.

The present invention has been made based on the above knowledge. The gist of the present invention is as follows.

(1) That is, the hot-rolled steel sheet according to one aspect of the present invention has the following chemical composition: C: 0.02 to 0.10%, Si: 0.005 to 0.1%, Mn: 0.5 to 2.0%, in terms of mass%, P: 0.1% or less, S: 0.01% or less, Al: 0.2 to 0.8%, N: 0.01% or less, Ti: 0.01 to 0.11%, Nb: 0 to 0.10%, Ca: 0 to 0.0030%, Mo: 0.02~ 0.5%, Cr: 0.02~1.0%, and the remainder consists of Fe and impurities. The total content of Si and Al is more than 0.20% and less than 0.81%; the microstructure has 90~99% of fertilizer by area ratio. Granular iron and 1~10% of granulated iron, and the toughened iron is limited to 5% or less; the aforementioned granulated iron has a particle size of 1~10μm; parallel to the rolling surface of the steel plate, and parallel to the rolling direction The X-ray random intensity ratio of the {211}<011> orientation is 3.0 or less; the tensile strength is 590 MPa or more.

(2) In the hot-rolled steel sheet according to the above (1), the chemical component may further contain one or more of the following: Nb: 0.01% to 0.10%, Ca: 0.0005 to 0.0030%, Mo: 0.02. ~0.5%, Cr: 0.02~1.0%.

(3) A method for producing a hot-rolled steel sheet according to another aspect of the present invention There is a casting step of continuously casting a steel material having the chemical composition described in the above (1) or (2) to obtain a flat steel slab; and a heating step of heating the flat steel embryo to 1200 ° C or higher. The rough rolling step is a rough rolling process of the heated flat steel preform; the finishing rolling step is performed after the rough rolling step, and is arranged in a straight row with a plurality of rolling mills. Finishing rolling mill series, in order to make the final pass rolling reduction rate of 20% or more, and the completion rolling temperature is 880~1000 ° C, the above-mentioned flat steel blank is continuously rolled to obtain a steel plate; The step is started from the end of the completion rolling step to 0.01 second to 1.0 second, and the steel sheet is water-cooled to a temperature range of 600 to 750 ° C at a cooling rate of 30 ° C /sec or more; the air cooling step is as described above. After the primary cooling step, the steel sheet is air-cooled for 3 to 10 seconds; and the secondary cooling step is performed after the air-cooling step, and the steel sheet is water-cooled to a temperature lower than 200 ° C at a cooling rate of 30 ° C /sec or more; Taking the step after the aforementioned secondary cooling step The aforementioned steel sheet is taken up.

According to the above aspect of the present invention, it is possible to obtain a predetermined chemical composition, and the microstructure fraction of the ferrite iron in the microstructure is 90% or more and 99% or less, and the particle size of the granulated iron is 1 μm or more and 10 μm or less. The tissue fraction of the granulated iron is 1% or more and 10% or less, and the X-ray random intensity ratio parallel to the rolling surface and parallel to the {211}<011> direction of the rolling direction is 3.0 or less, and the tensile strength is 590 MPa. The above hot rolled steel sheet. This hot rolled steel sheet is excellent in appearance and balance between extensibility and hole expandability.

Moreover, the flat steel preform having a predetermined chemical composition is subjected to a hot rolling delay, and the finished rolling degree is set to be 880 ° C or more and 1000 ° C or less. In order to promote the recrystallization of Osbane Iron and seek for the improvement of texture. In addition, by setting the completion rolling reduction ratio (the final rolling reduction ratio) to 20% or more, and starting water cooling within 0.01 second or more and 1.0 second after the end of rolling, recrystallization can be completed in a short time. And refined fine recrystallized Aosta iron. The metamorphosis from the fine recrystallized Austin iron is mostly at the nucleation site of the ferrite iron, and it will metamorphose very quickly. Therefore, fine ferrite iron can be formed by performing air cooling afterwards. In addition, since the Osbane iron remaining in the air cooling also remains fine, it is feasible to refine the metamorphic granulated iron. That is, according to the above aspect of the present invention, it is possible to produce a high tensile strength of 590 MPa or more with a predetermined microstructure and an X-ray random intensity ratio, and having a good appearance and excellent balance of elongation and hole expandability. Hot rolled steel sheet.

Fig. 1 is a graph showing the relationship between the X-ray random intensity ratio and the hole expansion ratio.

Fig. 2 is a flow chart showing an example of a method of producing a hot-rolled steel sheet according to the embodiment.

Form for implementing the invention

Hereinafter, a hot-rolled steel sheet according to an embodiment of the present invention (hereinafter sometimes referred to as a hot-rolled steel sheet according to the present embodiment) will be described.

The hot-rolled steel sheet according to the embodiment is a high-strength hot-rolled steel sheet having a tensile strength of 590 MPa or more. In such a high-strength hot-rolled steel sheet, 90% is formed in the microstructure (metal structure) in order to achieve the improvement of the hole expandability. The above-mentioned ferrite iron fraction (area ratio) and 10% or less of the granita iron fraction (area ratio) are effective. The tissue fraction and the particle diameter of each of the tissues can be obtained by, for example, performing image analysis on a photograph of the tissue obtained by optical micrograph (field of view: 500 × 500 μm field of view) of the steel sheet which has been excessively corroded. In the method of obtaining such a structure, for example, as shown in Patent Document 1, a steel sheet containing 0.5% or more of Si is subjected to a run-out-table in a hot rolling step (hereinafter, It is called ROT), which is air-cooled (intermediate air-cooling), and promotes the deformation of ferrite and iron. However, Si is the cause of the scale pattern of Si scale. Therefore, if Si is contained, the appearance of the steel sheet during use is a problem.

On the other hand, when Si is not added, in order to promote the deformation of the ferrite and iron, there is a need to lower the temperature of the finished rolling. However, lowering the temperature of the finished rolling temperature leads to the development of the texture of the steel sheet. Specifically, {211}<110> parallel to the rolling surface and parallel to the rolling direction is developed. When such a texture is developed, the anisotropy of plastic deformation becomes strong, and the hole expandability is deteriorated.

That is, it is impossible to achieve the balance between the elongation and the hole expandability in the steel sheet to which Si is not added.

In the hot-rolled steel sheet according to the present embodiment, Al is used as a substitute for Si to promote the deformation of the ferrite. By making it contain a predetermined amount of Al, the ferrite iron can be metamorphosed from the fine Austenite iron, and it becomes feasible to avoid the coarsening of the ferrite iron.

In addition, in the completion rolling, the finishing temperature is set to 880~1000 °C, the final rolling reduction rate is set to 20% or more, and after the completion rolling, the cooling starts between 0.01 and 1.0 seconds. . In this one cooling, yes Cool to 600~750 °C at a cooling rate of 30 ° C / sec or more. After cooling once, it is air-cooled for 3 to 10 seconds, and after air cooling, it is cooled to a cooling rate of 200 ° C or lower at a cooling rate of 30 ° C /sec or more, and then coiled. Through the above-mentioned manufacturing method, the tissue fraction of ferrite iron can be obtained from 90 to 99%, the particle size of Ma Tian loose iron is 1 to 10 μm, and the tissue fraction of Ma Tian iron is 1 to 10%, and the texture of the steel sheet ( Texture) A hot-rolled steel sheet having a random X-ray intensity ratio of 3.0 or less and a tensile strength of 590 MPa or more parallel to the rolling surface and parallel to the {211}<011> direction of the rolling direction. Moreover, this hot-rolled steel sheet is excellent in appearance and balance between extensibility and hole expandability.

Hereinafter, the hot-rolled steel sheet according to the embodiment will be described in detail.

First, the reasons for limiting the chemical composition will be explained.

C: 0.02~0.10%

C is an important element for improving the strength of the steel sheet. In order to obtain this effect, the lower limit of the C content is set to 0.02%. A more desirable lower limit of the C content is 0.04%. On the other hand, when the C content exceeds 0.10%, the toughness is deteriorated, and the basic characteristics of the steel sheet cannot be ensured. Therefore, it is preferable to set the upper limit of the C content to 0.10%.

Si: 0.005~0.1%

Si is an essential element for preliminary deoxidation. Therefore, it is preferable to set the lower limit of the Si content to 0.005%. On the other hand, since Si is also an element causing a defect in appearance, the upper limit of the Si content is set to 0.1%. The Si content is preferably less than 0.1%, preferably 0.07% or less, more preferably 0.05% or less.

Mn: 0.5~2.0%

Mn is permeable to hardenability and solid solution strengthening to the strength of the steel sheet Enhance the elements that contribute. In order to obtain the target strength, the lower limit of the Mn content is set to 0.5%. However, when the Mn content is excessive, MnS which is isotropic to toughness is generated. Therefore, the upper limit of the Mn content is set to 2.0%.

P: 0.1% or less

P is an impurity and has an adverse effect on workability and weldability, and is also an element which lowers fatigue characteristics. Therefore, the lower the P content, the better, but from the viewpoint of the dephosphorization cost, the lower limit can also be set to 0.0005%. When the P content exceeds 0.1%, the adverse effect becomes remarkable, so the P content is limited to 0.1% or less.

S: 0.01% or less

S causes inclusions such as MnS which is isotropic and is harmful to toughness. Therefore, the lower the S content, the better, but from the viewpoint of the desulfurization cost, the lower limit can also be set to 0.0005%. When the S content exceeds 0.01%, the adverse effect is remarkable, so the S content is limited to 0.01% or less. In the case where the low temperature toughness is particularly required, the S content is preferably limited to 0.006% or less.

Al: 0.2~0.8%

Al is an important element in the hot-rolled steel sheet of the embodiment. In order to promote the deformation of the ferrite and iron in the cooling of the ROT after the completion of the rolling, the lower limit of the Al content is set to 0.2%. However, when the Al content becomes excessive, alumina which precipitates into agglomerates is formed, and the toughness is deteriorated. Therefore, it is preferable to set the upper limit of the Al content to 0.8%.

N: 0.01% or less

N is an element which can form precipitates in Ti in a higher temperature range than S. When the N content is excessive, not only will the Ti effective on the fixed S be reduced, but also The formation of coarse Ti nitride deteriorates the toughness of the steel sheet. Therefore, the N content is limited to 0.01% or less.

Ti: 0.01~0.11%

Ti is an element that enhances the strength of the steel sheet by precipitation strengthening. In order to precipitate the ferrite-rich iron and obtain a balance between excellent elongation and hole expandability, the lower limit of the Ti content is set to 0.01%. However, when the Ti content exceeds 0.11%, inclusions due to TiN are formed, which deteriorates hole expandability. Therefore, the upper limit of the Ti content is set to 0.11%.

0.20%<Si+Al<0.81%

Both Si and Al are elements that promote the metamorphosis of ferrite. When the Si+Al content of the Si content and the Al content is 0.20% or less, the ferrite iron metamorphism cannot be performed in the intermediate air cooling, and the target ferrite iron composition fraction cannot be obtained in the ROT cooling. On the other hand, when Si+Al is 0.81% or more, the fermented iron iron metamorphic temperature becomes too high, and the ferrite iron metamorphosis occurs during rolling, so that the anisotropy of the texture becomes strong. It is desirable to make Si+Al more than 0.20% and less than 0.60%.

The hot-rolled steel sheet according to the present embodiment is basically composed of the above chemical components and the remainder of which is composed of Fe and impurities. However, in order to reduce the manufacturing variability and to increase the strength, it may be one or more selected from the group consisting of Nb, Ca, Mo, and Cr in the following range. Furthermore, since these chemical elements are not necessarily added to the steel sheet, the lower limit is 0%.

Nb: 0.01~0.10%

Nb can make the crystal grain size of hot-rolled steel sheets smaller and pass through NbC The precipitation strengthening enhances the strength of the steel sheet. In order to obtain these effects, it is preferred to set the Nb content to 0.01% or more. On the other hand, when the Nb content exceeds 0.10%, the effect is saturated. Therefore, it is preferable to set the upper limit of the Nb content to 0.10%.

Ca: 0.0005~0.0030%

Ca has an effect of dispersing a plurality of fine oxides in molten steel to refine the structure. In addition, Ca is an element which fixes the spherical inclusions of S in the molten steel and suppresses the generation of the extended inclusions such as MnS, thereby improving the hole expandability of the steel sheet. In order to obtain these effects, it is preferred to set the Ca content to 0.0005% or more. On the other hand, since the effect is saturated even if the Ca content exceeds 0.0030%, the upper limit of the Ca content is set to 0.0030%.

Mo: 0.02~0.5%

Mo is an effective element for precipitating and strengthening iron. In order to obtain this effect, it is desirable to set the Mo content to 0.02% or more. However, when the Mo content becomes excessive, the cracking susceptibility of the flat steel embryo becomes high and the treatment of the flat steel embryo becomes difficult. Therefore, the upper limit of the Mo content is set to 0.5%.

Cr: 0.02~1.0%

Cr is an effective element for increasing the strength of the steel sheet. To obtain this effect, the Cr content should be set to 0.02% or more. However, when the Cr content becomes excessive, the elongation is lowered. Therefore, the upper limit of the Cr content is set to 1.0%.

Next, the microstructure of the hot-rolled steel sheet according to the embodiment and The X-ray random intensity ratio is explained.

A steel sheet having a high strength and a high elongation is a composite structural steel of a steel sheet in which a hard structure such as a granulated iron is dispersed in a soft and extensible ferrite. This composite structural steel has high strength and high elongation. However, in the case of composite structural steel, since the high strain is concentrated in the vicinity of the hard structure, the crack propagation speed is increased, so that the hole expandability is low.

In order to suppress the deterioration of the hole expandability caused by the presence of the loose iron in the field, the particle size of the granulated iron is 10 μm or less, and the tissue fraction (area ratio) of the granulated iron in the microstructure is made. Less than 10% is effective. On the other hand, in order to ensure the balance of fatigue characteristics and elongation and strength, it is necessary to set the area ratio of the granulated iron to 1% or more. In addition, when the area ratio of the granulated iron is reduced to 10% or less in order to suppress the deterioration of the hole expandability, there is a fear that sufficient strength cannot be obtained. Therefore, in the hot-rolled steel sheet according to the present embodiment, as a means for ensuring the elongation and improving the strength, 90% or more of the ferrite iron which can be precipitated and strengthened by Ti is used as an area ratio. necessary. However, for the purpose of precipitation strengthening, when Ti is contained in the steel sheet, since recrystallization of the Osbane iron in the completion rolling is suppressed, an enhanced deformation texture is formed by the completion rolling. organization. Since the deformed structure also persists after the metamorphosis, the texture in the deformed steel sheet exhibits a strong degree of integration and deteriorates the hole expandability. Therefore, in the hot-pored steel sheet according to the present embodiment, in addition to the optimization of the area ratio of the ferrite iron and the granulated iron, the texture of the steel sheet is used as an index, and will be parallel to the rolling surface and parallel to The X-ray random intensity ratio of the {211}<011> orientation in the rolling direction is set to 3.0 or less. It is feasible to have both high elongation and hole expandability by setting the tissue fraction and texture to an optimum range as described above.

Moreover, the toughening iron has a poor elongation and hole expandability with respect to the ferrite iron, and the strength rise is also lower than that of the granulated iron. Therefore, it is preferable to control the area ratio of the toughened iron to 5% or less for the reason that the balance between the elongation and the hole expandability becomes difficult. In the hot-rolled steel sheet according to the present embodiment, it is not necessary to specify the area ratio of the structure other than the ferrite iron, the granulated iron, and the toughened iron.

Next, a method of manufacturing the hot-rolled steel sheet according to the embodiment will be described.

First, a steel material having the above chemical composition is continuously cast to obtain a continuous cast flat steel blank (hereinafter referred to as a flat steel blank) (casting step). Before the hot rolling, the flat steel is heated to above 1200 ° C (heating step). When the flat steel is heated at less than 1200 ° C, TiC cannot be fully melted in the flat steel, which leads to the precipitation of the ferrite iron. On the other hand, when the heating temperature is 1300 ° C or more, the amount of scale generated and the maintenance cost of the heating furnace are increased, which is not preferable.

The rough flat steel is subjected to rough rolling (rough rolling step), and further continuous rolling is performed on the tandem finishing mill in which the plurality of rolling mills are arranged in a straight row (finish rolling step) . At this time, the final rolling reduction rate of the finished rolling (the final rolling reduction ratio of the finished rolling) is set to be 20% or more, and the finished finishing temperature FT (the temperature at the end of the final pass) is finally completed. Set to 880~1000 °C. In order to produce recrystallization of Osbane iron at a high temperature, it is necessary to use a rolling reduction ratio of 20% or more as the final rolling reduction ratio. Final road When the second reduction ratio is less than 20%, the driving power necessary for recrystallization is insufficient, and grain growth occurs between the end of the final pass of the completion rolling and the start of cooling. As a result, the coarsening of the granulated iron is deteriorated, and the hole expandability is deteriorated. When the finishing rolling temperature is lower than 880 °C, the texture of the steel sheet is developed due to the inability to recrystallize the Austenite iron, so as to be parallel to the rolling surface and parallel to the rolling direction {211}< The 011>azimuth X-ray random intensity ratio exceeds 3.0, thus making the hole expandability worse. When the finishing rolling temperature exceeds 1000 ° C, the crystal grain size of the Osbane iron is coarsened, and the difference in the discharge density is drastically lowered, which causes a large delay in the deformation of the ferrite. As a result, the tissue fraction of the ferrite iron of 90% or more could not be obtained.

Furthermore, in order to recrystallize Osbane Iron more reliably, it is advisable to set the finishing rolling temperature above 900 °C.

The completion of the rolling is performed once (cooling step). This primary cooling is started between 0.01 and 1.0 seconds after the completion of the rolling. Although the primary cooling is performed by water cooling, in order to complete the recrystallization of the Osbane iron after the rolling, it is necessary to perform air cooling (natural cooling) for 0.01 second or more from the completion of the completion rolling to the start of the cooling. In order to surely complete the recrystallization, it is preferred to set the time from the completion of the rolling to the start of the cooling to 0.02 second or longer, preferably 0.05 seconds or longer. However, when the air cooling time becomes long, the crystal grains of the recrystallized Austenite iron are coarsened, and the fat body and the metamorphosis state are largely delayed to form a coarse granulated iron. In order to suppress the voids generated at the interface between the ferrite iron and the granulated iron, and to obtain excellent hole expandability, it is important to set the particle size of the granulated iron to 10 μm or less. Since it is necessary to suppress the coarsening of the crystal grains of the Osbane iron, it is necessary to One cooling is started within 1.0 second after the completion of the rolling.

The primary cooling after finishing rolling is cold at 30 ° C / sec or more. However, the speed is set such that the cooling stop temperature becomes a temperature range of 600 to 750 °C. Further, after the completion of the primary cooling, intermediate gas cooling (air cooling step) of 3 to 10 seconds is performed in this temperature range. Since the fine Osbane iron has a high growth rate of crystal grains, when the cooling rate is lower than 30 ° C / sec, grain growth is carried out during cooling, and the structure is coarsened. On the other hand, when the cooling rate of primary cooling is too fast, it is easy to form a temperature distribution in the thickness direction of the steel sheet. When there is a temperature distribution in the thickness direction, the particle size of the ferrite iron and the granulated iron is changed in the center portion and the surface portion of the steel sheet, and there is a concern that the material is uneven. Therefore, the cooling rate of primary cooling is preferably set to 100 ° C / sec or less. In the case where the cooling stop temperature and the temperature range in which the air is cooled are lower than 600 ° C, the ferrite-grain iron is delayed, and a high ferrite iron fraction cannot be obtained, and the elongation is deteriorated. On the other hand, when the cooling stop temperature and the temperature range in which air cooling is performed exceed 750 ° C, TiC is coarsely precipitated in the ferrite iron, so that the precipitation strengthening of the ferrite iron cannot be sufficiently obtained, and the pulling cannot be performed. The tensile strength reaches 590 MPa. Although the intermediate air cooling has to be performed for 3 seconds or more in order to cause the ferrite iron to be metamorphosed, when the air cooling exceeds 10 seconds, the ductility and the hole expandability are deteriorated by the precipitation of the toughened iron.

After the middle air is cooled, a cooling rate of 30 ° C / sec or more is performed. The steel sheet is cooled to a secondary cooling (secondary cooling step) of 200 ° C or lower, and coiling (winding step) is performed. In the case where the cooling rate of the secondary cooling is less than 30 ° C / sec, the toughened iron metamorphosis is performed, and the granulated iron is not obtained. At this time, the tensile strength is lowered and the elongation is deteriorated. On the other hand, when the cooling rate of the secondary cooling is too fast, it is easy to form a temperature distribution in the thickness direction of the steel sheet. When there is a temperature distribution in the thickness direction, the particle size of the ferrite iron and the granulated iron is changed in the center portion and the surface portion of the steel sheet, and there is a concern that the material is uneven. Therefore, the cooling rate of the secondary cooling is preferably set to 100 ° C / sec or less. Further, when the cooling stop temperature exceeds 200 ° C, the self-tempering effect of the granulated iron is generated. When self-tempering occurs, the tensile strength is lowered and the elongation is deteriorated.

Example

The steel containing the components shown in Table 1 was melted through a converter and continuously cast to make a flat steel embryo having a thickness of 230 mm. Thereafter, the flat steel billet is heated to a temperature of 1200 ° C to 1250 ° C, subjected to rough rolling in a continuous hot rolling apparatus, finished rolling, and coiled after ROT cooling to produce a hot rolled steel sheet. Table 2 shows the steel type marks, hot rolling conditions, and plate thickness of the steel sheets used. In Table 2, "FT6" is the temperature at the end of the final completion pass, "Cooling start time" is the time from the completion rolling to the start of one cooling, and "primary cooling" is the end rolling to the intermediate air cooling temperature. The average cooling rate up to the "intermediate temperature" is the intermediate air cooling temperature after primary cooling, the "intermediate time" is the intermediate air cooling time after primary cooling, and the "secondary cooling" is from the intermediate air cooling to the winding. The average cooling rate, "curl temperature" is the temperature after the end of the secondary cooling.

With respect to the steel sheets obtained as described above, the microstructure fraction and texture analysis of the ferrite iron, the toughened iron, and the granulated iron were performed by an optical microscope. this In addition, the particle size of the granulated iron is also investigated.

The tissue fraction of the ferrite iron and the toughened iron of the steel sheet is a photograph of the structure obtained by using an optical microscope at a field of 500 × 500 μm after being corroded by a nital etchant, and image analysis is performed to obtain an area. The rate. The tissue fraction and the particle diameter of the granulated iron were subjected to image analysis using an optical microscope at a field of view of 500 × 500 μm after Lepera etching, and the area fraction and the particle diameter were determined.

The texture analysis is the X-ray random intensity of the 1/4 portion of the plate thickness from the surface to the 1/4 position in the thickness direction parallel to the rolling surface and parallel to the {211}<011> direction of the rolling direction. Than evaluation. In the EBSD (Electron Back Scattering Diffraction Pattern) method, the measurement interval of the pixels is 1/5 or less of the average particle diameter, and the measurement is performed in a region where 5,000 or more crystal grains can be measured, and the distribution is determined by the distribution of ODF (Orientation Distribution Function). X-ray random intensity ratio. Further, the X-ray random intensity ratio is considered to be 3.0 or less.

For the tensile test of the steel sheet, the JIS No. 5 test piece was taken in the rolling width direction (C direction) of the steel sheet, and the tensile strength: YP (MPa), tensile strength: TS (MPa), and elongation were determined according to JIS Z2241. :EL (%) for evaluation.

Reaming rate: For λ (%), the method specified in ISO16630 is used for evaluation.

The appearance of the steel sheet was evaluated by cutting the steel sheet into a length of 500 mm in the longitudinal direction at a position of 10 m on the outer circumference of the hot rolled coil, and measuring the area ratio of the scale pattern. The area ratio of the scale pattern of 10% or less is regarded as "G:GOOD (good it is good)". On the other hand, a person whose area ratio of the scale pattern exceeds 10% is regarded as "B: BAD (inferior)".

Table 3 shows the results of evaluation of the tissue fraction (area ratio) of each tissue, the particle size, texture, material, and appearance of the granulated iron.

As shown in Table 3, the tensile strength of the present invention is 590 MPa or more, the microstructure fraction of the ferrite iron is 90% or more, and the particle size of the granulated iron is 10 μm or less, and the tissue fraction is 1% or more and 10 Below %, the X-ray random intensity ratio parallel to the rolling surface and parallel to the {211}<011> orientation of the rolling direction is 3.0 or less. That is, any of the examples of the present invention is excellent in appearance, and balance between stretchability and expandable pores.

On the other hand, in No. 2, since the intermediate air-cooling temperature is high, Ti precipitates coarsely in the ferrite iron, and sufficient precipitation strengthening cannot be obtained, so the tensile strength is less than 590 MPa.

In No. 5, since the finishing temperature was lower than 880 ° C, the anisotropy of the texture of the steel sheet was enhanced, and the hole expandability was deteriorated.

No.8, because the time from the completion of the rolling to the start of the cooling is more than 1.0 second, the coarsening of the Osbane iron structure is carried out, and the deformation of the ferrite and iron is largely delayed, so that the elongation and the hole expandability are changed. difference.

No. 12, because the intermediate air cooling time is less than 3 seconds, the fermented iron is not sufficiently deformed, so that the elongation and the hole expandability are deteriorated.

No. 16 because the intermediate air cooling time exceeds 10 seconds, the toughened iron metamorphosis is performed, and the microstructure fraction of the granulated iron is not obtained, so that the elongation and the hole expandability are deteriorated.

No.17 cannot be obtained because the intermediate air cooling temperature is lower than 600 °C. The tissue fraction of the ferrite iron deteriorates the extensibility and the hole expandability.

No.20 will pass through the Olympics due to the completion temperature exceeding 1000 °C. The coarsening of the stellar iron structure delays the deformation of the ferrite and iron, and deteriorates the elongation and the hole expandability.

No.22 cannot obtain hemp because the coiling temperature exceeds 200 °C. The field is scattered with iron and produces toughened iron. Therefore, the tensile strength is lower than 590 MPa, and the elongation and the hole expandability are deteriorated.

No. 24, because the final pass reduction ratio is less than 20%, the granulated iron is coarsened and becomes more than 10 μm. Therefore, the hole expandability is deteriorated. Further, since the recrystallization of the Osbane iron is not sufficient, the anisotropy of the steel sheet texture is enhanced, and the hole expandability is deteriorated.

In No. 29, since the Al content is less than 0.2% by mass, the fermented iron is not deformed, and the elongation and the hole expandability are deteriorated.

In No. 30, since the Si content exceeds 0.1% by mass, a plurality of scale patterns appear on the appearance, and the area ratio of the scale pattern becomes more than 10% of the whole.

In No. 31, since the time until the start of primary cooling after completion rolling was less than 0.01 second, recrystallization was not sufficiently performed, the texture was developed, and the hole expandability was deteriorated.

In No. 32, since the cooling rate of primary cooling was less than 30 ° C / sec, the particle size of the granulated iron was more than 10 μm, and the hole expandability was lowered.

In No. 33, since the cooling rate of the secondary cooling was lower than 30 ° C / sec, the toughened iron became more than 5% during cooling. Therefore, the elongation and the hole expandability are deteriorated.

Industrial availability

According to the above aspect of the present invention, it is possible to obtain a predetermined chemical composition, and the ratio of the structure is such that the tissue fraction of the ferrite iron is 90% or more and 99% or less, and the particle size of the granulated iron is 1 μm or more and 10 μm. Hereinafter, the tissue fraction is 1% or more and 10% or less, and the X-ray random intensity ratio parallel to the rolling surface and parallel to the {211}<011> direction of the rolling direction is 3.0 or less, and the tensile strength is 590 MPa or more. Hot rolled steel sheet. This hot rolled steel sheet is excellent in appearance and balance between extensibility and hole expandability.

Claims (3)

A hot-rolled steel sheet comprising the following chemical components: C: 0.02 to 0.10%, Si: 0.005 to 0.1%, Mn: 0.5 to 2.0%, P: 0.1% or less, S: 0.01. % or less, Al: 0.2 to 0.8%, N: 0.01% or less, Ti: 0.01 to 0.11%, Nb: 0 to 0.10%, Ca: 0 to 0.0030%, Mo: 0.02 to 0.5%, Cr: 0.02 to 1.0% And the remainder is composed of Fe and impurities, and the total of the Si content and the Al content is more than 0.20% and less than 0.81%; the microstructure has 90 to 99% of the ferrite iron and 1 to 10% of the field by the area ratio. Disperse iron, and limit the toughening iron to less than 5%; the aforementioned Ma Tian loose iron particle size is 1~10μm; parallel to the rolling surface of the steel plate, and parallel to the rolling direction {211} <011> orientation X The random intensity ratio of the rays is below 3.0; The tensile strength is 590 MPa or more. The hot-rolled steel sheet according to claim 1, wherein the chemical component contains one or more of the following in terms of mass%: Nb: 0.01% to 0.10%, Ca: 0.0005 to 0.0030%, Mo: 0.02 to 0.5%, Cr: 0.02 to 1.0%. A method for producing a hot-rolled steel sheet, comprising: a casting step of continuously casting a steel material having a chemical composition as recited in claim 1 or 2 to obtain a flat steel blank; and a heating step of the flat steel preform Heating to a temperature range of 1200 ° C or higher; the rough rolling step is to carry out rough rolling of the heated flat steel preform; the finishing rolling step, after the rough rolling step, is arranged in a straight row with a plurality of The finishing rolling mill of the rolling mill is arranged in series so that the final rolling reduction ratio is 20% or more, and the finished rolling temperature is 880-1000 ° C. Obtaining a steel sheet; the primary cooling step is started from the end of the completion rolling step to 0.01 second to 1.0 second, and the steel sheet is water-cooled to a temperature range of 600 to 750 ° C at a cooling rate of 30 ° C /sec or more; Step, after the first cooling step, the steel plate is air-cooled for 3 to 10 seconds; a secondary cooling step of cooling the steel sheet to a temperature lower than 200 ° C at a cooling rate of 30 ° C /sec or more after the air cooling step; and a winding step of winding the steel sheet after the secondary cooling step .