JP5370016B2 - High-strength hot-rolled steel sheet excellent in hole expansibility and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet having excellent hole expansibility while increasing its strength. <P>SOLUTION: The steel sheet has a composition comprising, by mass, 0.01 to 0.1% C, 0.01 to 2.0% Si, 0.05 to 3.0% Mn, &le;0.1% P, &le;0.03% S, 0.005 to 0.02% Al, &le;0.005% N, 0.0005 to 0.003% Ca, &le;0.01% Nb, 0.005 to 0.3% Ti and 0.01 to 0.1% V, and the balance Fe with inevitable impurities, whose microstructure is composed of a ferritic structure, a bainitic structure or their mixed structure, and also, the random intensity ratio of the ä211} plane parallel to the rolling face is &le;2.0. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a high-strength hot-rolled steel sheet excellent in hole expansibility and a method for producing the same.

  In recent years, for the purpose of reducing the weight of various steel materials, application of increasing the strength of steel plates has been promoted. Here, not only the strength of the steel sheet itself, but also the mechanical properties required for the steel sheet are diverse, such as toughness, ductility, hole expansibility, fatigue durability or corrosion resistance. In general, as the strength of a steel plate increases, mechanical properties relating to the workability of the steel plate, such as the ductility and hole expansibility of the steel plate, tend to decrease. For this reason, it is important to exhibit the various mechanical properties in a well-balanced manner while increasing the strength of the steel sheet. Currently, hot rolled steel sheets with a tensile strength of 780 MPa are being applied to undercar parts for automobiles, and it is required that steel sheets of this strength class have both tensile strength and formability such as hole expandability.

  For example, steel sheets used for automobile members such as structural members and underbody members that account for approximately 20% of the weight of a vehicle body are cut into a predetermined shape by shearing such as punching or drilling, and then subjected to stretch flange processing. Since press molding is mainly performed, excellent hole expansibility (λ) is required.

  As a technique for improving the hole expansibility while increasing the strength of a steel sheet, for example, in Patent Document 1, by optimizing the fraction of a steel structure such as ferrite and bainite, and precipitates in the ferrite structure A technique for improving the balance between strength and hole expansibility is disclosed.

JP 2004-339606 A

  However, although a steel sheet having a tensile strength of 780 MPa and a hole expansion value of 60% or more can be obtained depending on the disclosed technique of Patent Document 1, it cannot be said that the hole expansion value is sufficient, and has sufficient strength and hole expansion property balance. It was hard to say. In order to meet the needs for automobile weight reduction, it has been desired to propose a hot-rolled steel sheet having a good balance between tensile strength and hole expansion.

  Therefore, the present invention has been devised in view of the above-mentioned problems, and the object of the present invention is to obtain a steel sheet having excellent hole expandability while keeping in mind tensile strength and hole expandability. It is an object of the present invention to provide a high-strength hot-rolled steel sheet excellent in hole expansibility and a method for producing a high-strength hot-rolled steel sheet capable of producing the steel sheet.

The high-strength hot-rolled steel sheet having excellent hole expandability according to the first invention is in mass%, C: 0.01 to 0.1%, Si: 0.01 to 2.0%, Mn: 0.05. ~3.0%, P ≦ 0.1%, S ≦ 0.0 05%, Al: 0.005~0.055%, N ≦ 0.005%, Ca: 0.0005~0.003%, Nb ≦ 0.01%, Ti: 0. 05 to 0.3% V: contains 0.01% to 0.1%, the balance being a steel sheet consisting of Fe and unavoidable impurities, from the microstructure ferrite structure, bainite structure, or a mixed structure And the X-ray random strength ratio of the {211} plane parallel to the rolling surface is 2.0 or less, and the tensile strength is 780 MPa or more.

The high-strength hot-rolled steel sheet excellent in hole expansibility according to the second invention is further mass% in the first invention,
Cu: 0.1 to 1.0%
Ni: 0.1 to 1.0%,
Cr: 0.1 to 1.0%,
REM: 0.0005 to 0.02%
Any one or two or more of them are contained.

The high-strength hot-rolled steel sheet excellent in hole expansion property according to the third invention is the first or second invention, wherein the steel sheet is adjacent to a straight line in the rolling direction in a cross section having the sheet width direction as a normal line. A group of inclusions with a circle equivalent diameter of 3 μm or more arranged at an interval of 50 μm or less with respect to other inclusions that fit, and a group of inclusions with a rolling direction length of 30 μm or more on a straight line in the rolling direction The length in the rolling direction per 1 mm ² of the cross-section with the inclusion formed at an interval of more than 50 μm with respect to other inclusions adjacent to each other, the equivalent circle diameter being 3 μm or more, and the rolling direction length being extended to 30 μm or more. The total sum is 0.25 mm or less.

The method for producing a high-strength hot-rolled steel sheet excellent in hole expansibility according to the fourth aspect of the invention heats the slab or slab having the components of the hot-rolled steel sheet according to the first or second aspect of the invention to 1200 ° C. or higher. Thereafter, rough rolling is performed, and then finish rolling is finished in a temperature range of 950 ° C. or higher, and then cooled to 600 ° C. or lower at a cooling rate of 20 ° C./second or higher, and then at a temperature of 400 ° C. or higher and 550 ° C. or lower. It is characterized by winding.

The manufacturing method of the high-strength hot-rolled steel sheet excellent in hole expansibility according to the fifth invention is a slab or steel slab having the components of the hot-rolled steel sheet according to any one of the first to fourth inventions at 1200 ° C or higher. After heating, rough rolling is performed, and then finish rolling is finished in a temperature range of 950 ° C. or higher, then cooled to 650 ° C. or lower at a cooling rate of 20 ° C./second or higher, and then 550 ° C. or higher and 650 ° C. or lower. In the temperature range, it is cooled at a cooling rate of 15 ° C./second or less for 1 second or more and 5 seconds or less , further cooled to 520 ° C. or less at a cooling rate of 20 ° C./second or more, and then wound at a temperature of 400 ° C. or more and 500 ° C. or less. It is characterized by taking.

The method for producing a high-strength hot-rolled steel sheet excellent in hole expansibility according to the sixth aspect of the invention is that in the fourth or fifth aspect, when the molten steel is melted, after adding the flux for desulfurization in the secondary refining apparatus, 3. and characterized in that zero or more rings shed.

  The hot-rolled steel sheet having the above-described configuration has an excessive increase in finish rolling finish temperature by setting the amount of V to 0.01 to 0.1% while the amount of Nb added is 0.01% or less. It is possible to reduce the X-ray random intensity ratio of the {211} plane while preventing the problem, and the hole expandability is improved.

  Furthermore, by further reducing the length in the rolling direction of stretched inclusions that deteriorate the hole-expandability and inclusion groups consisting of multiple inclusions arranged in a straight line in the rolling direction, the hole-expandability is further improved. Is planned.

It is a figure which shows the relationship between the X-ray random intensity ratio of a {211} surface parallel to the rolling surface of a steel plate, and a hole expansion rate. It is a figure which shows the relationship between the hole expansion rate and tensile strength of the hot-rolled steel plate obtained by the research which this inventor performed. It is a figure which shows the relationship between finish rolling completion | finish temperature and the X-ray random intensity ratio of a {211} surface. It is a perspective view for demonstrating a L cross section. It is a schematic diagram for demonstrating the sum total M of the rolling direction length of an inclusion. It is a figure which shows the relationship between the frequency M of the molten steel at the time of secondary refining, and the sum total M of the rolling direction length of an inclusion. It is a schematic diagram which shows the structure of RH used as a secondary refining apparatus in performing the secondary refining process which melts molten steel. It is a figure which shows a hot rolling process roughly

  Below, as a form for implementing this invention, the high intensity | strength hot-rolled steel plate excellent in hole expansibility and its manufacturing method are demonstrated in detail.

  First, the basic research results that led to the completion of the present invention will be described.

  The present inventors conducted the following examination in order to investigate the controlling factors for the hole expandability of a steel sheet mainly composed of a ferrite structure and a bainite structure.

  For the test steels A to I composed of steel components as shown in Table 1 below, the present inventors set the heating temperature to 1220 to 1265 ° C, the rolling end temperature to 925 to 1000 ° C, and the winding temperature to 420 to 480. Hot rolling, cooling, and winding were performed under the condition of a temperature range of ℃, and the microstructure, texture, and mechanical properties of the obtained hot rolled steel sheet having a thickness of 2.9 mm were examined. The tensile strength of the hot-rolled steel sheet obtained under these conditions was distributed in the range of 780 to 820 MPa.

  Here, steels A, G, and I are components in which Nb is added more than other steels, and steels B, D, E, F, and H are components in which Nb is not added and V is added, Steel C is a component obtained by adding a small amount of Nb of 0.009% and adding V. Hereinafter, steels A, G, and I are referred to as Nb-added steels, and steels B, C, D, E, F, and H are referred to as V-added steels.

  The microstructure here means a ferrite structure, a bainite structure, or the like observed by an optical microscope or the like, and a texture is representative of a steel structure at a predetermined position in a steel sheet, each crystal grain at that position. Is the average orientation of the crystal orientation. Here, the texture is a value measured at the center of the plate thickness.

  As a result, as shown in FIG. 1, it was found that the texture having a predetermined crystal orientation has an influence on the hole expandability of the hot rolled steel sheet obtained from the test steel. That is, it has been found that in the hot-rolled steel sheet, the hole expandability deteriorates as the X-ray random intensity ratio (α {211} plane strength) of the {211} plane parallel to the rolling plane increases. Here, all the microstructures of the test steels were mainly composed of a ferrite structure or a bainite structure.

  The X-ray random intensity ratio referred to here is the X-ray intensity of the hot-rolled steel sheet sample to be measured with respect to the X-ray intensity of the powder sample having a random orientation distribution in X-ray diffraction measurement, as will be described later. This means that the larger the random strength ratio, the greater the amount of texture having crystal planes in a predetermined orientation parallel to the plate surface in the steel plate.

The mechanism by which the hole expansion rate deteriorates when the α {211} plane strength is high is not necessarily clear, but is explained as follows. In a hot-rolled steel sheet, the anisotropy of the steel material increases due to the large number of α {211} surfaces. In particular, if the plastic strain ratios (r values) in the rolling direction and 45 ° direction and 90 ° direction (sheet width direction) with respect to the rolling direction are defined as r ₀ , r ₄₅ , and r ₉₀ , r _{0 in} this case And r ₄₅ and r ₉₀ are increased, and r ₉₀ is greatly decreased. This is thought to be due to the fact that, during hole-opening forming, the reduction in plate thickness increases at the end surface in the rolling direction that receives tensile strain in the plate width direction, and high stress is generated on the end surface, so that cracks are easily generated and propagated.

  FIG. 2 shows the relationship between the strength of the hot-rolled steel sheet obtained from the test steels A to I under the predetermined production conditions and the hole expansion ratio. As shown in this figure, the V-added steel has a hole expansion rate superior to that of the Nb-added steel, and the strength is equal to or higher than that. This is thought to be due to the fact that the amount of Nb added to increase the specific texture surface strength by delaying recrystallization is reduced, and V is added instead to supplement the strength.

  Further, according to the research conducted by the present inventors, the {211} plane X-ray random intensity ratio is such that, as shown in FIG. 3, the higher the finish rolling end temperature (FT7) in the hot rolling process is, It has been found to reduce. Further, as can be seen from the figure, it was also found that the α {211} plane strength is reduced by reducing the Nb addition amount and adding V.

  These reasons are considered as follows. It is known that the {211} plane X-ray random intensity ratio increases after hot rolling when rolling strain is accumulated in the steel sheet without causing recrystallization. From this, when the finish rolling finish temperature is high, recrystallization after finish finish is promoted, and this is considered to reduce the X-ray random intensity ratio of the {211} plane. In addition, in the V-added steel, since the amount of Nb added, which has a large effect of delaying recrystallization, is small, it is considered that recrystallization is promoted and the X-ray random intensity ratio on the {211} plane is reduced.

  Although the α {211} plane strength that degrades the hole expansion rate can be lowered by increasing the finish rolling end temperature, it is preferable to make the finish rolling end temperature too high because the scale wrinkles on the surface of the hot-rolled steel sheet also become remarkable. Absent. From this point of view, addition of V, which can obtain a sufficient strength without increasing the α {211} plane strength even at a relatively low finish rolling end temperature, is essential for obtaining a high-strength steel with a high hole expansion rate.

  Moreover, the present inventors investigated the inclusion by the following method about the hot-rolled steel plate obtained from these test steels A to I.

  A test piece was taken from a 300 mm portion at the center in the plate width direction, a cross section having the normal direction in the plate width direction shown in FIG. 4 (hereinafter referred to as L cross section) was mirror-polished, and × 400 using an optical microscope. The cross section of the L was observed at a magnification of 1, and the size and distribution of inclusions on the L cross section (plate thickness × 10 mm) were investigated.

  As a result of organizing the influence of inclusions on the hole expansion rate, inclusions with an equivalent circle diameter of 3.0 μm or more and a length in the rolling direction of 30 μm or more, or the length of one inclusion in the rolling direction It was found that inclusions arranged at a certain interval on the straight line in the rolling direction have a great influence on the hole expansion rate even when the thickness is 30 μm or less. As a result of further investigation on a plurality of inclusions arranged on this straight line, as shown in FIG. 5A, the inclusions are arranged with an interval of 50 μm or less with respect to other inclusions adjacent on the straight line in the rolling direction. It was found that the gathering of inclusions affected the hole expansion rate. The equivalent circle diameter here means the diameter when converted to the same area as the shape of the inclusions, and the straight line in the rolling direction is a virtual straight line extended in the rolling direction. means.

  Therefore, a group of inclusions having an equivalent circle diameter of 3.0 μm or more arranged with an interval of 50 μm or less with respect to other inclusions adjacent on the straight line in the rolling direction is referred to as one “inclusion group”. The length L1 in the rolling direction as shown in FIG. 5 (a) was determined in the L cross section of the inclusion group having a length in the rolling direction of 30 μm or more. Further, even in the case of an inclusion having an interval of more than 50 μm with respect to other inclusions adjacent on the straight line in the rolling direction, the shape has an equivalent circle diameter of 3 as shown in FIG. About the inclusion which is 0.0 micrometer or more and the length of the rolling direction is 30 micrometers or more, the length L2 of the rolling direction was measured in the L cross section. In addition, the reason why the length in the rolling direction is limited to 30 μm or more as a measurement target is because inclusions having a length in the rolling direction less than this are considered to have little influence on the deterioration of the hole expanding property. Further, the reason why the equivalent circle diameter is limited to 3.0 μm or more is that inclusions having an equivalent circle diameter less than this are considered to have a small influence on the deterioration of the hole expanding property.

  As shown in FIG. 5 (c), even if the inclusion has an equivalent circle diameter of 3.0 μm or more and a length in the rolling direction of 30 μm or more, other inclusions adjacent on the straight line in the rolling direction. On the other hand, inclusions with an interval of 50 μm or less are treated as being part of the inclusion group. Hereinafter, inclusions that are not included in the inclusion group and have an equivalent circle diameter of 3.0 μm or more and a length in the rolling direction of 30 μm or more are referred to as “stretched inclusions”.

The rolling direction length L1 of the obtained inclusion group and the rolling direction length L2 of the stretched inclusions are L1 (mm) for each inclusion group and stretched inclusion for each visual field according to the following formula (1). And L2 (mm) are summed to obtain L (mm), and based on the obtained L, a numerical value M (mm / mm ² ) is obtained according to the following formula (2), and the obtained M is expressed as a unit area (1 mm ^2). ) It was defined as the sum M of the rolling inclusion lengths per hit. In the following formula (1), L1 _i and L2 _i are the lengths in the rolling direction of each inclusion group and each stretched inclusion in one visual field, respectively, and S is the area of the observed visual field (mm ² ).

  Here, the reason for obtaining the total length M of the inclusions in the rolling direction is as follows.

  Inclusions form voids in the steel at the time of deformation of the steel sheet to promote ductile fracture, which causes deterioration of hole expandability. The influence of inclusions that deteriorates the hole-expanding property increases because the stress concentration near the inclusions increases as the shape of the inclusions increases. The present inventors have not only one stretched inclusion, but also a group of inclusions in which stretched inclusions and spherical inclusions are distributed within a range of intervals in the rolling direction, which is the crack propagation direction. As in the case of one stretched inclusion, it was found that stress concentration is generated in the vicinity of the inclusion group due to a synergistic effect of strain introduced in the vicinity of the inclusion when the steel sheet is deformed. Quantitatively, an inclusion group consisting of a collection of inclusions having an equivalent circle diameter of 3.0 μm or more arranged at an interval of 50 μm or less with respect to other inclusions adjacent on a straight line in the rolling direction, It has been found that there is the same effect as one inclusion stretched to the same length as the length of the inclusion group in the rolling direction.

  Moreover, in this test, from the obtained inclusion group and the rolling direction lengths L1 and L2 of the stretched inclusions, the arithmetic average value of these was also obtained, but this average value is 30 μm or more. In this case, no significant correlation was found with respect to the hole expansion value, and it was found that it was difficult to express the degree of the hole expansion value by the average value.

  This is considered to be based on the following reasons. That is, when the number of inclusion groups and stretched inclusions is small at the time of deformation of the steel sheet, the cracks propagate while the voids generated around these inclusion groups are interrupted. When the number is large, it is considered that surrounding voids such as inclusion groups are connected without interruption to form long continuous voids and promote ductile fracture. Such an influence of the number of inclusion groups or the like cannot be expressed by the average value of the lengths in the rolling direction of the inclusion groups or the like, but can be expressed by the sum M of the lengths in the rolling direction of the inclusion groups or the like. It is considered that the following results were obtained.

  From the above viewpoint, the sum M of the lengths in the rolling direction of inclusions per unit area was evaluated.

  Further, as shown in FIG. 1, it has been found from this investigation that, even with the same α {211} plane strength, if the total sum M in the rolling direction length of inclusions is small, the hole expandability is improved. As described above, when the total length M of the stretched inclusions in the rolling direction is small, the hole expandability is improved. As described above, the ductile fracture is promoted by the inclusion groups and the stretched inclusions, and the cracks at the end face in the hole expansion forming process. This is because the generation and propagation of the phenomenon is promoted.

  In addition, the present inventors investigated inclusions arranged on a straight line in the rolling direction, which is a factor that deteriorates hole expandability, and these are introduced for desulfurization at the steel making stage, MnS stretched by rolling, or the like. It was clarified that it was a residue of desulfurization flux. As a result of examining the manufacturing method for suppressing these, it was found that the following is important.

  That is, in order to suppress MnS, it is important to reduce the amount of S contained in the steel. From this viewpoint, in the present invention, a preferable upper limit of S (≦ 0.0050%) is set. Further, in Ti-added steel, since MnS generation is suppressed by generation of TiS at a temperature higher than the MnS generation temperature range, in order to suppress MnS, a larger amount of Ti addition is preferable. Is set for a preferable lower limit of Ti amount (Ti ≧ 0.05%).

  In addition, in order to suppress inclusions due to residual desulfurization flux, in the secondary refining process of molten steel, after the desulfurization flux is added, the molten steel is circulated within a time range that does not significantly degrade the productivity, It turned out that removal was important. Below, the molten steel reflux conditions required in order to make the sum total M of the rolling direction length of an inclusion below below predetermined are described.

  In the steelmaking process, when smelting molten steel by secondary refining, secondary refining equipment such as RH (Ruhrstahl-Heraeus) is used to remove the desulfurization flux and reduce the total length M of the inclusions in the rolling direction. It is important to circulate the molten steel 3.0 times or more in the secondary smelting apparatus after the desulfurization flux is added during the desulfurization of the molten steel using. The reason for this will be described.

FIG. 6 shows the relationship between the number of times of molten steel recirculation in the secondary refining process when melting the component steels shown in Table 1 and the total length M of inclusions in the rolling direction. As shown in FIG. 6, when the number of recirculations is 3.0 times or more, the total length M of inclusions in the rolling direction is reduced to 0.25 mm / mm ² or less, and the number of recirculations is 4.0 times. It can be seen that the total M of the inclusions in the rolling direction is reduced to 0.03 mm / mm ₂ or less in the above case.

Here, as for the number of times of recirculation of the molten steel, the recirculation speed Q (ton / min) of the molten steel, which means the amount of molten steel circulated in the secondary refining apparatus per unit time, and the recirculation time of the molten steel after adding the desulfurization flux ( min) and the amount of molten steel (ton) to be processed in the secondary refining process, can be obtained based on the following equations (3) and (4).

Ｑ：環流速度（ｔｏｎ／ｍｉｎ）、
Ｖ：環流ガス流量（Ｌ／ｍｉｎ）
Ｄ：浸漬管内径（ｍ） 、
Ｐ０：真空槽内圧力（Ｐａ）
Ｐ１：環流ガス吹込位置圧力（Ｐａ）、
ｋ：定数（二次精練装置による定数） There are various calculation formulas for the recirculation velocity Q of the molten steel described above. For example, “Refining Limit of Impurity Elements in Mass Production Scale” (Japan Steel Association High Temperature Smelting Process Group Smelting Forum Japan Society for the Promotion of Science The reaction may be determined based on the following formula (5) disclosed in the Committee Reaction Process Study Group, March 1996, pages 184 to 187). In addition, the reflux gas flow rate in the following formula (5) means the flow rate of the reflux gas per minute under the conditions of 0 ° C. and 1 atm.

Q: reflux velocity (ton / min),
V: Flowing gas flow rate (L / min)
D: inner diameter of dip tube (m),
P0: Vacuum chamber pressure (Pa)
P1: Circulating gas blowing position pressure (Pa)
k: Constant (constant by secondary scouring device)

  Based on the knowledge obtained as described above, the present inventors have intensively studied a hot-rolled steel sheet having a well-balanced tensile strength and hole-expandability and a manufacturing method thereof, and as a result, the following conditions are satisfied. The inventors have come up with a hot-rolled steel sheet and a manufacturing method thereof.

  Hereinafter, the constituent elements of the present invention will be described in detail.

  First, the reasons for limiting chemical components in the present invention will be described. Hereinafter, mass% in the composition is simply referred to as%.

C: 0.01 to 0.1%
C is an element that combines with Nb, Ti and the like to contribute to strength improvement by precipitation strengthening or the like, but if the C content is less than 0.01%, the effect of strength improvement cannot be obtained. Further, if the C content is more than 0.1%, workability and weldability deteriorate. For this reason, content of C shall be 0.01% or more and 0.1% or less.

Si: 0.01 to 2.0%
Si is an element necessary for preliminary deoxidation and an element contributing to strength improvement as a solid solution strengthening element. Si needs to be contained in an amount of 0.01% or more in order to obtain a desired strength. Further, if the Si content exceeds 2.0%, the workability deteriorates. For this reason, content of Si shall be 0.01% or more and 2.0% or less.

Mn: 0.05 to 3.0%
Mn is an element that contributes to improving the strength of the steel sheet as a solid solution strengthening element. In order to obtain a desired strength, Mn needs to be contained in an amount of 0.05% or more. Further, if the content of Mn is more than 3.0%, slab cracking during hot rolling tends to occur. For this reason, content of Mn shall be 0.05-3.0%.

P: 0.1% or less P is an impurity that is inevitably mixed during the refining of molten steel, and is an element that adversely affects workability such as toughness and weldability and decreases fatigue properties as the content increases. is there. For this reason, the lower the P content, the more desirable. When the P content exceeds 0.1%, the adverse effects on the workability and the like, and the deterioration of the fatigue characteristics as described above are significant. For this reason, content of P shall be 0.1% or less.

S: 0.0 05% or less S is an impurity unavoidably mixed during molten steel refining, when the content is too large, causing the slab cracking during hot rolling. Therefore, the S content should be minimized. S has a content of 0 . If it exceeds 005%, MnS stretched in the steel is formed, and there is a concern that the hole expandability is deteriorated by increasing the total M of the inclusions in the rolling direction. For this reason, it is preferable that the content of S is 0.005% or less.

Al: 0.005~0.0 55%
Al is an element necessary to disperse many fine oxides during deoxidation of molten steel and to refine the structure. The Al content needs to be added by 0.005% or more in order to sufficiently obtain the effect of deoxidizing molten steel. On the other hand, the content of Al is excessively added when because its effect is saturated, so the upper limit is 0.0 55%. Therefore, the Al content is 0.005% or more and 0.0 55% or less.

N: 0.005% or less N forms precipitates with Ti and Nb at a temperature higher than C, and not only reduces Ti and Nb effective for fixing C, but also causes variation in hole expansion value. A large sized Ti nitride is formed. Therefore, the N content should be reduced as much as possible, but is acceptable if it is 0.005% or less.

Ca: 0.0005 to 0.003%
Ca is an element necessary for dispersing a large number of fine oxides at the time of deoxidation of molten steel, and for refining the structure, and for desulfurization of molten steel or by fixing S in steel as spherical CaS. It is essential to add it in order to suppress the formation and improve the hole expanding property. In order to have these effects, Ca has a lower limit of 0.0005%. On the other hand, even if Ca is excessively contained in the steel, the effect is saturated and the manufacturing cost is increased, so the upper limit is made 0.003%.

Nb: 0.01% or less Nb is effective as an element for improving the strength by precipitation strengthening or structure refinement. However, when added in a large amount, the temperature of the non-recrystallized region is expanded, and the {211} plane This is not preferable because a large number of unrecrystallized rolled textures that increase the X-ray random intensity ratio remain after the hot rolling step. If the Nb content is 0.01% or less, the X-ray random intensity ratio of the {211} plane does not increase excessively and is in an acceptable range. For this reason, the upper limit of the Nb content is 0.01%.

Ti: 0. 05 to 0.3%
Ti is an element that finely precipitates as TiN and contributes to an increase in strength of the steel sheet by precipitation strengthening. Such an effect is obtained when the Ti content is 0.1%. Since the content of less than 05 % is not sufficient, the content of Ti is 0.001. 05 % or more is necessary. Ti is an element that forms carbide (TiC) and suppresses the formation of coarse cementite (Fe3C) that degrades hole expansibility. For this reason, Ti is 0. Addition of more than 05 % is necessary. If the Ti content exceeds 0.3%, not only the effect is saturated, but also the alloy cost increases. Therefore, the Ti content is 0.3% or less.

  Further, Ti is an element that suppresses precipitation of MnS that forms stretch inclusions by being precipitated as TiS in the reheating stage of the slab. In order to obtain the above effects, the Ti content is preferably 0.05% or more. Further, if the Ti content exceeds 0.3%, these effects are not only saturated, but also the alloy cost is increased. Therefore, the Ti content is preferably 0.05% or more and 0.3% or less.

V: 0.01 to 0.1%
V is one of the essential elements in the present invention. V is an element that remarkably increases the strength of the hot-rolled steel sheet by precipitation strengthening without increasing the temperature of the non-recrystallized region. If the V content is less than 0.01%, the effect of providing such a balance between the strength and the hole expanding property is not exhibited. V is an element that forms carbide (VC) and suppresses the formation of coarse cementite (Fe3C) that degrades hole expansibility. For this reason, V must be added in an amount of 0.01% or more. Further, if the V content is more than 0.1%, the slab is likely to be cracked in the steel making process during the production of the hot-rolled steel sheet. For this reason, content of V shall be 0.01% or more and 0.1% or less.

  The above is the reason for limiting the basic components of the present invention. In the present invention, Cu, Ni, Cr, and REM components may be included as necessary.

  Cu, Ni, and Cr are elements that have an effect of improving the strength of the hot-rolled steel sheet by precipitation strengthening or solid solution strengthening. One or more of these elements may be added as necessary.

  If the Cu content is less than 0.1%, the effect of improving the strength cannot be obtained, and if it exceeds 1.0%, the effect of improving the strength is saturated. Is desirable.

  Further, if the Ni content is less than 0.1%, the effect of improving the strength cannot be obtained, and if it exceeds 1.0%, the effect of improving the strength is saturated. % Or less is desirable.

  If the Cr content is less than 0.1%, the effect of improving the strength cannot be obtained. If the content of Cr exceeds 1.0%, the effect of improving the strength is saturated. Is desirable.

  Further, REM (rare earth element) is an element that is rendered harmless by changing the form of non-metallic inclusions that can be a starting point of destruction or cause deterioration of workability. If the content of REM is less than 0.0005%, the effect is not exhibited, and even if added over 0.02%, the effect is saturated and the economy is lowered. For this reason, the content of REM is set to 0.0005% or more and 0.02% or less.

  In the present invention, Zr, Sn, Co, Zn, W, and Mg may be added as necessary.

  Next, the reasons for limiting the texture and microstructure in the hot rolled steel sheet to which the present invention is applied will be described.

  In the present invention, in order to obtain a sufficient balance between the strength and the hole expansion rate, the X-ray random intensity ratio of the {211} plane parallel to the rolling surface needs to be 2.0 or less. The X-ray random intensity ratio is the intensity ratio of the X-ray intensity of the hot-rolled steel sheet sample to be measured to the X-ray intensity of the powder sample having a random orientation distribution in the X-ray diffraction measurement. The X-ray random intensity ratio is measured by measuring the X-ray diffraction intensity of the α {211} plane using a diffractometer method using an appropriate X-ray tube and comparing it with the diffraction intensity of a random sample. Shall. If the X-ray random intensity ratio of the {211} plane is 2.0 or less while satisfying the essential chemical components and the microstructure conditions described later in the present invention, the hot rolled steel sheet is being strengthened. It becomes possible to improve hole expansibility.

  The microstructure of the steel sheet in the present invention may be a ferrite structure, a bainite structure, or a mixed structure thereof in order to ensure excellent hole expansibility (burring workability). This is because in the case of these structures, the hardness of the structure becomes relatively uniform, ductile fracture is suppressed, and the hole expandability can be improved.

  Further, the microstructure of the steel sheet in the present invention allows inevitable inclusion of martensite, retained austenite and pearlite, but when these are excessively contained, they are hard and therefore ductile fracture. Therefore, the volume fraction of the combined microstructure is preferably 5% or less.

  In addition, the investigation of the microstructure of ferrite, bainite, pearlite, martensite, etc. was conducted by taking a sample cut from a 1/4 W or 3/4 W position of the steel sheet when the steel sheet width was W and the steel sheet thickness was t. The L cross section was polished, etched using a Nital reagent, and was performed with a photograph of the visual field at 1/4 t observed at a magnification of 200 to 500 times using an optical microscope.

  The term “ferrite” as used herein refers to a structure composed of BCC lattice crystal grains formed by transformation from austenite at a relatively high temperature, and includes various precipitates including TiC and some dislocations in the crystal. Shall also be included. Here, the structure composed of relatively equiaxed crystal grains having an equivalent circle diameter of 3 μm or more, which is confirmed by a nital corrosion photograph, was defined as ferrite.

  Moreover, bainite is a structure | tissue which consists of a finer and expanded structure | tissue produced | generated at low temperature than a ferrite. Here, a structure composed of elongated crystal grains having an equivalent circle diameter of 3 μm or less, which is confirmed by a nital corrosion photograph, was defined as bainite.

  Pearlite is a structure composed of ferrite and layered cementite. Here, when a layered structure was confirmed in a nital corrosion photograph, this was identified as pearlite.

  Martensite is a hard structure produced by transformation at a low temperature of approximately 350 ° C. or lower. Here, when a gray part was confirmed in the nital corrosion photograph, this was identified as martensite.

In the present invention, in order to obtain a better balance between tensile strength and hole expansibility, the total length M of inclusions in the rolling direction is preferably 0.25 mm / mm ² or less. The meaning of the sum M of the length in the rolling direction of the inclusion here is as described above. In the present invention, the feature is that not only the stretched inclusions that are known to have an effect of deteriorating the hole expandability than the conventional one, but also the length in the rolling direction of the inclusion group having a predetermined arrangement is suppressed. is there. Moreover, in order to obtain a further excellent hole expanding property, it is more desirable that the total M of the inclusions in the rolling direction is 0.03 mm / mm ² or less. Incidentally, the sum M of the lengths of inclusions in the rolling direction may be zero.

  Next, a preferable manufacturing method for obtaining a hot-rolled steel sheet to which the present invention is applied will be described. Hereinafter, the reasons for limiting each manufacturing condition will be described in detail.

  In the steelmaking process, after obtaining hot metal with a blast furnace or the like, after making it into molten steel in a converter, the obtained molten steel is melted by various secondary refining and the above-mentioned predetermined range of component contents and The components are adjusted so that

  FIG. 7 is a schematic diagram showing the configuration of RH when RH is used as a secondary refining apparatus in performing a secondary refining process for melting molten steel. The secondary refining apparatus 1 is configured by immersing two dip tubes 4 a and 4 b communicated in a degassing tank 3 in a molten steel pan 2. Moreover, this secondary refining apparatus 1 is configured such that a circulating gas such as Ar supplied from the circulating gas blowing pipe 5 to the molten steel 6 in the molten steel pan 2 is blown into the one immersion pipe 4a from below. . The molten steel 6 in the molten steel pan 2 in the secondary refining apparatus 1 rises from the molten steel pan 2 through one dip tube 4a and enters the degas tank 3, and after the degas treatment, the degas tank 3 and the other dip pipe It is comprised so that it may descend | fall and return to the molten steel pan 2 via 4b. The desulfurization flux is blown into the molten steel 6 in the molten steel pan 2 from the circulating gas blowing pipe 5 or a separately provided pipe, and the molten steel 6 is desulfurized by being stirred by the circulating gas. Become.

Here, in the present invention, when the molten steel is melted, it is preferable to control the molten steel flow so that the total sum M of the lengths of the stretched inclusions in the rolling direction becomes small as described above. That is, in order to reduce the total length M of inclusions in the rolling direction to 0.25 mm / mm ² or less, when melting molten steel, after desulfurization flux addition, during molten steel desulfurization using a secondary refining device such as RH It is preferable to recirculate the molten steel 3.0 times or more in the secondary smelting apparatus. In order to make the total length M of inclusions in the rolling direction 0.03 mm / mm ² or less, the molten steel is preferably refluxed 4.0 times or more. The above formulas (3) and (4) are used for the number of recirculation of the molten steel.

  In addition, although the example which used RH as a secondary refining apparatus was shown here, it cannot be overemphasized that other secondary refining apparatuses, such as DH (Dortmund Horde) and LF (Ladle Furnace), may be used.

  Except for the above points, the conditions are not particularly limited for other processes in the steelmaking process. After secondary refining, cast steel may be obtained by casting from molten steel obtained by secondary refining by a method such as thin continuous slab casting, in addition to normal continuous casting or ingot casting. The obtained slab is usually a steel slab cut to a predetermined length by a gas cutter or the like. Instead of obtaining hot metal in a blast furnace, iron scrap may be used as a raw material and melted in an electric furnace to obtain molten steel. In addition, when a slab or the like is obtained by continuous casting, it may be sent directly to a hot rolling mill as it is a high-temperature slab, or it is hot-rolled after being reheated in a heating furnace after being cooled to room temperature. You may do it.

  Next, the conditions in the hot rolling process in the present invention will be described. FIG. 8 is a diagram schematically showing temperature conditions in each step in the hot rolling step.

  The slab heating temperature in the slab heating step S1 for heating a slab or a steel slab needs to be, for example, 1190 ° C. or higher in order to obtain a desired random strength ratio and microstructure. The slab heating temperature is preferably 1200 ° C. or higher in order to obtain the intended tensile strength and hole expandability. This is because when a slab or the like is heated at less than 1200 ° C., precipitates containing Ti and Nb are not sufficiently dissolved in the slab and become coarse, and the precipitation strengthening ability due to the precipitates of Ti and Nb cannot be obtained. In addition, since these remain in the steel material as coarse precipitates to the end, the hole expandability is deteriorated. The upper limit of the slab heating temperature is not particularly limited, but if the slab heating temperature exceeds 1400 ° C., the amount of scale defects increases and the yield decreases, so the slab heating temperature may be 1400 ° C. or less. desirable. In addition, it does not ask in particular about the heating time in slab heating process S1.

  After slab heating process S1, it transfers to rough rolling process S2 which performs rough rolling with respect to the slab extracted from the heating furnace. The rough rolling end temperature in this rough rolling step S2 is not particularly limited, but is preferably 1150 ° C. or higher from the viewpoint of promoting dynamic recrystallization and promoting randomization of crystal orientation.

  The thickness of the coarse bar obtained by the rough rolling step S2 is not particularly limited, but is desirably small from the viewpoint of reducing rolling distortion in the finish rolling step described later, and is preferably 25 mm or less.

  Further, the time from the end of the rough rolling step S2 to the start of the finish rolling step S3 described later is not particularly limited, but it is desirable to hold it for 30 seconds or more in the meantime. This is because if the holding time is less than 30 seconds, recrystallization of austenite grains is not promoted, and a large number of unrecrystallized rolled texture remains.

  After the rough rolling step S2, the process proceeds to a finish rolling step S3 for further rolling the rough bar obtained in the rough rolling step S2. The finish rolling end temperature in this finish rolling step S3 needs to be 950 ° C. or higher in order to avoid the remaining non-recrystallized rolled texture that causes an increase in the X-ray random intensity ratio of the {211} plane. is there. The upper limit of the finish rolling end temperature is not particularly required to obtain the effect of the present invention, but is preferably set to 1000 ° C. or less because scale wrinkles may occur in operation.

  Note that the finish rolling start temperature in the finish rolling step S3 is preferably a high temperature from the viewpoint of promoting recrystallization after rolling, and is desirably 1050 ° C. or higher. Further, the rolling reduction of the final pass in the finish rolling step S3 is not particularly questioned, and the rolling speed is not particularly questioned.

  Further, in the rough rolling step S2 and the finish rolling step S3, a heating device is provided in each rolling mill or between the rolling mills so as to control the variation in temperature in the rolling direction, the plate width direction, and the plate thickness direction of the rough bar. It may be installed and controlled to reduce temperature variation during hot rolling.

  Moreover, you may make it perform descaling using a high water pressure as needed between rough rolling process S2 and finish rolling process S3.

  After finishing rolling process S3, it transfers to cooling process S4 which cools the hot-rolled steel plate obtained by finish rolling with a run-out table. In the cooling step S4, it is necessary to cool to 600 ° C. or less at a cooling rate of 20 ° C./second or more. This is because when the cooling rate is less than 20 ° C./second, a pearlite phase is generated or the precipitates of TiC are coarsened, so that the strength cannot be obtained and the hole expandability is also deteriorated. . Moreover, this is because when the cooling is stopped at over 600 ° C., the precipitate of TiC becomes coarse, so that the strength cannot be obtained and the hole expanding property is also deteriorated. This cooling is performed up to a temperature range of winding temperature to winding temperature + 20 ° C.

  In the cooling step S4, after the hot rolling, strong cooling is first performed at a cooling rate of 20 ° C./second or more, and after strong cooling, in a temperature range of 550 ° C. to 650 ° C., 15 ° C. / It is desirable to perform slow cooling at a cooling rate of less than a second. In this case, it is desirable that the slow cooling time be 1 second or more and 5 seconds or less. Thereby, precipitation of a fine precipitate is accelerated | stimulated and the intensity | strength of the hot-rolled steel plate obtained improves. The reason why the slow cooling start temperature is set to 650 ° C. or less is that when the slow cooling start temperature is higher than 650 ° C., precipitates that contribute to the strength improvement of steel materials such as TiC are likely to be coarsely precipitated, and rather the strength is lowered. This is because it is easy to invite. The reason why the end temperature of the slow cooling is set to 550 ° C. or more is that the effect of precipitating TiC finely in a short time is small and the contribution to the increase in strength is small at temperatures below that.

  The reason why the cooling rate of the slow cooling is set to 15 ° C./second or less is that when the slow cooling rate exceeds 15 ° C./second, fine precipitates are not sufficiently precipitated. The lower limit of the slow cooling rate is not particularly limited.

  The reason for the slow cooling time being 1 second or longer is that if this is less than 1 second, fine precipitates are not sufficiently deposited. Further, the reason for slow cooling is set to 5 seconds or less because if it exceeds 5 seconds, the precipitates are rather coarsely deposited, leading to a decrease in strength.

  After the slow cooling, it is necessary to perform strong cooling again at a cooling rate of 20 ° C./second or more. This is because if the strong cooling is not performed after the slow cooling, the precipitates are coarsely precipitated, which may lead to a decrease in strength. This strong cooling is performed up to a temperature range of 600 ° C. or lower, specifically, a temperature range of winding temperature to winding temperature + 20 ° C.

  Note that the strong cooling in the run-out table is performed by, for example, water cooling or mist cooling, and the slow cooling is performed by, for example, air cooling.

  After cooling process S4, it transfers to winding process S5 which winds up the hot-rolled steel plate obtained through cooling process S4. In the winding step S5, it is necessary to wind the hot-rolled steel sheet in a temperature range of 550 ° C. or lower. When the coiling temperature is higher than 550 ° C., a phase containing coarse carbides such as pearlite, which is unfavorable for workability, is generated, which may increase the X-ray random intensity ratio of the {211} plane, and the precipitates are coarse. This is not preferable because it precipitates on the surface and deteriorates the hole expansion property. Accordingly, the winding temperature in the winding step S5 is set to 550 ° C. or lower. Note that if the winding temperature in the winding step S5 is less than 400 ° C., carbides such as Ti are difficult to precipitate at the grain boundaries, and the precipitation strengthening ability by the carbides may not be sufficiently obtained. It is preferable to set it as ° C or more. In addition, after the winding process, it is gradually cooled from the winding temperature to room temperature or a predetermined temperature range.

  For the purpose of improving ductility by introducing movable dislocations and correcting the shape of the steel sheet, skin pass rolling with a rolling reduction of 0.1% or more and 2% or less may be performed after the completion of all steps. Moreover, after completion | finish of all the processes, you may make it pickle the obtained hot-rolled steel plate for the purpose of the removal of the scale adhering to the surface of the obtained hot-rolled steel plate. Further, after completion of hot rolling or after pickling, the obtained hot-rolled steel sheet may be subjected to a skin pass having a reduction rate of 10% or less or cold sweat rolling having a reduction rate of 40% or less inline or offline.

  Moreover, the hot-rolled steel sheet to which the present invention is applied is subjected to heat treatment in a hot dipping line in any case after casting, after each rolling process, and after cooling, or with respect to the hot-rolled steel sheet after completion of all the processes. You may improve the corrosion resistance of a hot-rolled steel plate by performing the surface treatment by a hot dipping method. Of course, alloying treatment may be performed in addition to hot dipping.

  Next, the Example of the high intensity | strength hot-rolled steel plate to which this invention is applied is described in detail.

  First, a steel slab of steel numbers A to K having chemical components shown in Table 2 below is obtained by secondary refining after melting a converter, and the obtained steel slab is directly or after re-casting after continuous casting. After heating, a hot rolled steel sheet having a thickness of 2.9 mm was obtained through the manufacturing conditions shown in Table 3. Table 4 shows the mechanical properties and microstructure of the hot-rolled steel sheet thus obtained.

  In addition, the numerical value and the character which attached the underline in Table 3 and Table 4 show that it is outside the range of this invention or a preferable range. Further, in obtaining the number of molten steel recirculation times in Table 3, the constant k in the above equation (5) was set to 4. Further, “Presence / absence of slow cooling” in Table 3 indicates whether or not slow cooling was performed at a cooling rate of 15 ° C./second or less in the cooling step S4, and “strong cooling rate” is a cooling rate of 20 ° C. The actual cooling rate in the strong cooling performed at a rate of more than / sec is shown, and the “slow cooling rate” indicates the actual cooling rate in the slow cooling at a cooling rate of 15 ° C./s or less. In addition, “holding time” indicates a time during which slow cooling is performed at a cooling rate of 15 ° C./second or less. In Table 3, “strong cooling 1” and “strong cooling 2” indicate the cooling conditions of “strong cooling 1” and “strong cooling 2” shown in FIG. When not performed, the cooling condition is indicated only in “strong cooling 1”.

The hole expansion rate was evaluated according to the hole expansion test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996. Λ in Table 4 is the hole expansion rate. In addition, the tensile test is performed by processing the obtained hot-rolled steel sheet into a No. 5 test piece described in JIS Z 2201, and evaluating it according to the test method described in JIS Z 2241. In Table 4, TS is tensile strength, and El is elongation at break. Each X-ray random intensity ratio is a ratio of the X-ray diffraction intensity measured from the X-ray diffraction test piece of the target sample and the X-ray diffraction intensity measured similarly in the random sample. . Here, the X-ray diffraction test piece was measured with a 20 mm × 20 mm sample ground and chemically polished to the center in the thickness direction. In addition, the microstructure should be measured by confirming the nital corrosion photograph.For example, when only ferrite is confirmed, only “ferrite” is described, and when both ferrite and bainite are confirmed. "Ferrite + bainite" was described. Resulting in the evaluation of the mechanical properties of hot-rolled steel sheets, with hole expansion ratio satisfies 90% or more conditions, the tensile strength is as Working Example is within the scope of the present invention more than 780 MPa, the hole expansion A sample whose rate and tensile strength do not satisfy these conditions was regarded as a comparative example which is outside the scope of the present invention. In Table 4, “◯” indicates that the mechanical property evaluation satisfies the above condition, and “x” indicates that the above condition is not satisfied.

Those in accordance with the present invention are steel numbers 1-11, 1 , 2, and 3. These hot-rolled steel sheets contain a predetermined amount of steel components, the microstructure is composed of a ferrite structure, a bainite structure, or a mixed structure thereof, and the [211] plane X-ray random strength ratio is 2.0 or less. It is characterized by that. These hot-rolled steel sheets have a hole expansion ratio of 90% or more. Steel Nos. 1 to 10 have a tensile strength of 780 MPa or more and a hole expansion ratio of 90% or more, and the strength and the hole expansion ratio are obtained in a well-balanced manner. Steel Nos. 8 to 10 and 23 have a particularly large number M of molten steel circulations and a particularly small total M in the rolling direction length of inclusions.

  Steel No. 11 has a slab heating temperature of less than 1200 ° C., resulting in coarse precipitates and a good balance between the tensile strength of 780 MPa or more and the hole expansion ratio of 90% or more and the hole expansion ratio. A hot-rolled steel sheet is not obtained.

In Steel No. 21, Ti is less than 0.05, which is outside the preferable range, so that a large amount of stretched MnS is contained in the steel sheet, and the total length M of inclusions in the rolling direction is 0.25 mm / there is a mm ² exceeding the preferred range. In Steel No. 22, since S is out of the preferred range of more than 0.005, a large amount of stretched MnS is contained in the steel sheet, and the total length M of inclusions in the rolling direction is 0.25 mm / there is a mm ² exceeding the preferred range.

  Steel No. 12 has a composition and a microstructure within the range of the present invention, but the {211} plane X-ray random intensity ratio is outside the range of the present invention, so that the desired hole expansion rate is obtained. Absent. This is because when the finish rolling finish temperature is less than 950 ° C., many {211} plane textures remain.

  Steel No. 13 has a composition and a texture within the scope of the present invention, but the microstructure is out of the scope of the present invention, so that the desired hole expansion rate is not obtained. This is presumably because coarse precipitates such as pearlite were produced when the cooling rate in strong cooling was less than 20 ° C./second.

  Steel No. 14 has a composition and a texture within the scope of the present invention, but the microstructure is out of the scope of the present invention, so that the desired hole expansion ratio is not obtained. This is presumably because coarse precipitates such as pearlite were produced when the coiling temperature was higher than 550 ° C.

  Steel Nos. 15 to 18 have a texture, microstructure and manufacturing process within the scope of the present invention, but because Nb is excessively contained, the texture is developed and the target hole expansion rate is Not obtained.

  Steel No. 19 has the texture, microstructure, and manufacturing process within the scope of the present invention, but does not contain V and the composition is outside the scope of the present invention, so the desired hole expansion ratio is obtained. It is not done.

  Steel No. 20 has the texture, microstructure, and manufacturing process within the scope of the present invention, but does not contain V and the composition is outside the scope of the present invention. The balance is not obtained.

Claims (6)

% By mass
C: 0.01 to 0.1%,
Si: 0.01 to 2.0%,
Mn: 0.05 to 3.0%
P ≦ 0.1%,
S ≦ 0.0 05%,
Al: 0.005 to 0.055%,
N ≦ 0.005%,
Ca: 0.0005 to 0.003%,
Nb ≦ 0.01%,
Ti: 0. 05 to 0.3%,
V: 0.01 to 0.1%
And the balance is a steel plate made of Fe and inevitable impurities,
The microstructure consists of a ferrite structure, a bainite structure or a mixed structure thereof,
A high-strength hot-rolled steel sheet having excellent hole expansibility, wherein the X-ray random strength ratio of the {211} plane parallel to the rolled surface is 2.0 or less and the tensile strength is 780 MPa or more. In addition,
Cu: 0.1 to 1.0%
Ni: 0.1 to 1.0%,
Cr: 0.1 to 1.0%,
REM: 0.0005 to 0.02%
The high-strength hot-rolled steel sheet having excellent hole expansibility according to claim 1, comprising one or more of them. Said steel sheet,
In the cross section having the normal direction in the sheet width direction, it consists of a collection of inclusions having an equivalent circle diameter of 3 μm or more arranged at an interval of 50 μm or less with respect to other inclusions adjacent on the straight line in the rolling direction, The inclusion group having a rolling direction length of 30 μm or more is spaced from the other inclusions on the straight line in the rolling direction by more than 50 μm, the equivalent circle diameter is 3 μm or more, and the rolling direction length is 30 μm. The high-strength hot rolling excellent in hole expansibility according to claim 1 or 2, characterized in that the total length in the rolling direction per 1 mm ² cross-section with the inclusions drawn as described above is 0.25 mm or less. steel sheet. The slab or steel slab having the component according to claim 1 or 2 is heated to 1200 ° C or higher, followed by rough rolling, and then finish rolling is finished in a temperature range of 950 ° C or higher, and then 20 ° C / The high-strength hot-rolled steel sheet having excellent hole expansibility according to claim 1 or 2, wherein the steel sheet is cooled to 600 ° C or lower at a cooling rate of 2 seconds or more and then wound at a temperature of 400 ° C or higher and 550 ° C or lower. Production method. The slab or steel slab having the component according to claim 1 or 2 is heated to 1200 ° C or higher, followed by rough rolling, and then finish rolling is finished in a temperature range of 950 ° C or higher, and then 20 ° C / It is cooled to 650 ° C. or less at a cooling rate of at least 2 seconds, then cooled at a cooling rate of 15 ° C./second or less at 1 to 5 seconds in a temperature range of 550 ° C. to 650 ° C., and further 20 ° C./second or more. The method for producing a high-strength hot-rolled steel sheet having excellent hole expandability according to claim 1 or 2, wherein the steel sheet is cooled at a cooling rate to 520 ° C or lower and then wound at a temperature of 400 ° C or higher and 500 ° C or lower. In the method for producing a high-strength hot-rolled steel sheet excellent in hole expansibility according to claim 4 or 5,
Upon for melting the molten steel, the method of producing a high strength hot-rolled steel sheet excellent in hole expandability according to claim 3, characterized in that circulate over 3.0 times after desulfurization flux added in secondary refining equipment .

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