JP2011058022A - High-strength hot-rolled steel sheet having excellent 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 containing, by mass, ≤0.005% S and 0.05 to 0.2% Ti and further the other components in prescribed ranges, wherein the microstructure is composed of a ferritic structure, a bainitic structure or their mixed structure, and the X-ray random intensity ratio of the ä211} face parallel to the rolling face is ≤2.2. Regarding inclusions therein, in the cross-section with a sheet width direction as a normal, they are composed of an aggregate of the inclusions with an equivalent circle diameter of ≥3 μm arranged with a space of ≤50 μm to the other inclusion adjacent on a straight line in the rolling direction, and the total of the inclusion group with a rolling direction length of ≥30 μm and the inclusions having a space of >50 μm to the other inclusion adjacent on a straight line in the rolling direction, having a circle equivalent diameter of 3 μm, and obtained by being stretched to a rolling direction length of ≥30 μm per mm<SP>2</SP>of the cross section is ≤0.25 mm. <P>COPYRIGHT: (C)2011,JPO&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 automobiles and improving fuel efficiency, the application of high strength steel sheets for automobiles has been promoted. 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 workability in a well-balanced manner while increasing the strength of the steel sheet.

  Steel sheets used for automobile members such as structural members and suspension members that account for approximately 20% of the weight of automobile bodies are cut into a predetermined shape by shearing such as punching or drilling, and then mainly stretch flange processing. Therefore, excellent hole expandability is required. The hole expandability is evaluated by expanding the punched hole in the steel plate with a conical punch, cracking occurs at the end face of the punched hole, and determining the hole diameter expansion rate (hole expansion value (λ)) when the plate penetrates the plate thickness. This is an index representing the formability of the punched end face.

Since the hole expansion value has a relatively large variation, not only the average value but also the statistical lower limit value, that is, a predetermined number of samples (test value) based on the statistics is used to improve the hole expansion property. It is necessary to increase the lower limit of the population estimated from the above to suppress variation. Specifically, the statistical lower limit value (hereinafter referred to as the lower limit value λ _min ) refers to the distribution of the population of hole expansion values as a normal distribution, and 99% of the normal distribution is This is a value obtained by estimating the lower limit of the numerical range that can be taken by the sample based on the following equation (1).
λ _min = λ _ave −2.5 × σ (1)
λ _ave : Average value of hole expansion values obtained by a predetermined number σ: Standard deviation of hole expansion values obtained by a predetermined number

Currently, in automobile undercarriage parts, hot rolled steel sheets having a tensile strength (TS) of 440 to 590 MPa are used in many cases. However, in order to reduce the fuel consumption of automobiles, it is necessary to reduce the weight of the steel sheet by about 10%. To achieve this, the tensile strength is 780 MPa or more and the average value λ _ave of the hole expansion value is 80% or more. There is a demand for a steel sheet having a lower limit value λ _min of the hole expansion value of 60% or more and a standard deviation σ of the hole expansion value of 10% or less.

However, a high-strength hot-rolled steel sheet having a material that can simultaneously achieve these goals has not been obtained. For example, Patent Document 1 discloses a technique for improving the balance between strength and hole expansibility by optimizing the fraction of a steel structure such as ferrite and bainite and precipitates in the ferrite structure. According to the disclosed technology, a steel sheet having a tensile strength of 780 MPa or more and an average value λ _ave of hole expansion values of 60% or more is obtained. However, this does not mean that the average value λ _ave of the hole expansion value is still sufficient, and further, the lower limit value λ _min of the hole expansion value and the standard deviation σ are not described, and a sufficiently good lower limit value λ _min There was no guarantee that a steel sheet with a standard deviation σ could be obtained, and it was difficult to say that it had a sufficient balance between strength and hole expansibility.

JP 2004-339606 A

Therefore, in view of the above-described problems, the present invention has been devised to improve the average value λ _{ave of} the high-strength hot-rolled steel sheet, its lower limit value λ _min and its standard deviation σ. The average value λ _bee of the hole expansion value is 80% or more, the lower limit value λ _min of the hole expansion value is 60% or more, and the standard deviation σ of the hole expansion value is 10% or less. An object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in hole expansibility that can obtain excellent tensile strength and a method for producing the same.

  In order to solve the above-described problems, the present inventors have invented the following high-strength hot-rolled steel sheet excellent in hole expansibility and a method for producing the same.

The high-strength hot-rolled steel sheet having excellent hole expansibility according to the first invention is mass%, C: 0.02 to 0.07%, Si: 0.5 to 2%, Mn: 0.5 to 1. 5%, P ≦ 0.03% (however, over 0%), S ≦ 0.005%, Al: 0.005-0.05%, N ≦ 0.005% (however, over 0%), Ti : 0.05-0.2%, Ca: 0.0005-0.01%, the balance being a steel plate made of Fe and inevitable impurities, the microstructure of which is a ferrite structure, a bainite structure, or these The X-ray random intensity ratio of the {211} plane parallel to the rolling surface is 2.2 or less, and in the cross section having the plate width direction as the normal line, other adjacent adjacent straight lines in the rolling direction The rolling method consists of a collection of inclusions with an equivalent circle diameter of 3 μm or more arranged at an interval of 50 μm or less with respect to the inclusions. An inclusion group having a direction length of 30 μm or more and an interval of more than 50 μm with respect to other inclusions adjacent on the straight line in the rolling direction, an equivalent circle diameter of 3 μm or more, and a rolling direction length of 30 μm or more A high-strength hot-rolled steel sheet excellent in hole expansibility, characterized in that the sum of the lengths in the rolling direction per 1 mm ² cross-section with the inclusions that are drawn into a diameter of 0.25 mm or less.

  The high-strength hot-rolled steel sheet excellent in hole expansibility according to the second invention is characterized in that, in the first invention, it further contains Nb ≦ 0.05% (however, more than 0%) in mass%. .

  The high-strength hot-rolled steel sheet having excellent hole expansibility according to the third aspect of the invention is the mass%, Cu ≦ 1.0% (however, over 0%), Cr ≦ 1.0 in the first or second aspect. % (However, over 0%), Ni ≦ 1.0% (however, over 0%), B ≦ 0.005% (however, over 0%) It is characterized by.

  The high-strength hot-rolled steel sheet having excellent hole expansibility according to the fourth aspect of the present invention contains, in any one of the first to third aspects, further mass% and REM: 0.0005 to 0.01%. It is characterized by.

  The method for producing a high-strength hot-rolled steel sheet excellent in hole expansibility according to the fifth aspect of the invention is to desulfurize with a vacuum degassing facility when melting molten steel containing any one of the first to fourth aspects of the invention. After the flux is added, the molten steel is circulated 3.0 times or more, and the slab obtained from the molten steel is heated to 1200 ° C. or higher, followed by rough rolling, and the subsequent finish rolling is performed at a temperature range of 960 ° C. or higher. Then, the film is cooled to a temperature range of 400 ° C. or more and 550 ° C. or less at a cooling rate of 20 ° C./sec or more, and then wound.

  The method for producing a high-strength hot-rolled steel sheet having excellent hole expansibility according to the sixth invention is the fifth invention, wherein the cooling rate is 15 ° C./sec or less during the cooling at a cooling rate of 20 ° C./sec or more. The above-described slow cooling is performed, and then the cooling is performed again at a cooling rate of 20 ° C./sec or more.

  According to the first to sixth inventions, a hot-rolled steel sheet having excellent formability, particularly hole expansibility and excellent tensile strength can be obtained. It is possible to reduce the weight of the vehicle and reduce fuel consumption.

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 X-ray random intensity ratio of {211} surface, total M of the rolling direction length of an inclusion, and average value (lambda) _ave of a hole expansion value. It is a figure which shows the relationship between X-ray random intensity ratio of {211} surface, the sum total M of the rolling direction length of an inclusion, and lower limit (lambda) _min of a hole expansion value. It is a figure which shows the relationship between the standard deviation (sigma) of a hole expansion value, and the sum total M of the rolling direction length of an inclusion. 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 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 the relationship between the temperature and time in the hot rolling process which concerns on this invention.

  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.

In order to improve the hole expansibility of a high strength hot-rolled steel sheet, the present inventors first comprise a low C component with a reduced hard phase such as pearlite that becomes the starting point of ductile fracture that degrades the hole expansibility, Furthermore, earnest examination was performed for the steel plate which consists of a ferrite structure, a bainite structure, or its mixed structure. As a result, it has been found that control of texture (suppression of anisotropy) is important in order to improve hole expansibility in such high-strength, hard-structured and component steel sheets. It was. Further, in order to improve the lower limit λ _min by reducing the standard deviation σ representing the variation of the hole expansion value, the elongated inclusions contained in the steel and the inclusion group having the same effect as this We found that suppression is important.

  The inventors set the finish rolling end temperature to 940 to 1000 ° C. and the cooling rate at the run-out table after rolling to 30 to 40 ° C. for a plurality of slabs made of various steel components in the range shown in Table 1. / Sec, hot rolling was performed under the conditions where the coiling temperature was in the range of 480 to 500 ° C., and a hot rolled steel sheet having a thickness of 2.9 mm was obtained. From the obtained hot-rolled steel sheet, in addition to the mechanical properties represented by the tensile strength, the hole expansion value, etc., the microstructure, texture, and inclusion distribution were examined.

The microstructure here means a ferrite structure, a bainite structure, etc., and a texture is a group of a plurality of crystal grains, and the crystal orientation of each crystal grain in each texture is constant. A thing that satisfies a relationship. The inclusion means a relatively coarse material that exists as a heterogeneous phase in steel and serves as a starting point for ductile fracture. For example, sulfides such as MnS and CaS in steel, CaO-Al ₂ This refers to oxides such as O ₃ -based compounds (calcium aluminate), nitrides such as TiN, or residues of desulfurization flux that is input for desulfurization in the steelmaking stage. This residue contains, for example, calcium fluoride.

  Tensile properties such as tensile strength are obtained by processing No. 5 test piece described in JIS Z 2201 parallel to the plate width direction from the center in the plate width direction of the obtained hot-rolled steel sheet, and describing the obtained test piece in JIS Z 2241. The test method was evaluated. All the tensile strengths of the hot-rolled steel sheets obtained in the above manufacturing condition range were distributed in the range of 780 MPa to 800 MPa.

  The investigation of the distribution of the microstructure, texture, and inclusions was performed on a sample cut out in the range of 300 mm in the plate width direction with the center position in the plate width direction of the steel plate as the center.

The average value λ _ave , the lower limit value λ _min, and the standard deviation σ of the hole expansion values are obtained by using 30 test pieces taken from a sample cut in a range of 300 mm in the plate width direction around the center position in the plate width direction of the steel plate. Thus, it was obtained from the hole expansion value obtained by performing the punching hole expansion test described in the Japan Iron and Steel Federation Standard JFS T 1001-1996. The average value λ _ave of the hole expansion values is the arithmetic average of the hole expansion values of the 30 test pieces obtained by the test, and the standard deviation σ of the hole expansion values is obtained based on the following formula (2). The lower limit value λ _min was determined based on the above-described formula (1). In addition, (lambda) _i in Formula (2) is a hole expansion value of each test piece.

  In the punching hole expansion test, punching is performed with an initial hole diameter of 10 mm and a punching clearance of 12.5%, and the test piece is set so that the burr caused by punching is on the opposite side of the hole expanding punch. The punched hole was expanded by a punch, and the expansion ratio of the hole diameter (inner diameter) at the time when the crack generated on the punched end face penetrated the plate thickness was obtained. Here, the punching clearance is the ratio (%) of the distance between the punch and the die in the direction perpendicular to the punch traveling direction to the plate thickness of the steel plate to be processed.

The average value λ _ave of the hole expansion value for each steel plate obtained by the above test method is distributed in a wide range of 65 to 100%, and the lower limit value λ _min of the hole expansion value is in the range of 28 to 85%. The standard deviation σ of the hole expansion value was distributed in the range of 6 to 15%. As an influence factor on the average value λ _ave of the hole expansion value, the lower limit value λ _min and the standard deviation σ, the texture and inclusions in the steel sheet were focused and arranged as described later.

  The texture was investigated by the following method. The X-ray diffraction intensity at the central portion in the plate thickness direction of a test piece cut out in a size of 20 mm in the plate width direction and 20 mm in the rolling direction with the center position in the plate width direction as the center is measured using a suitable X-ray tube. It was measured by the fractometer method. Based on the measured X-ray diffraction intensity, the intensity ratio of the X-ray diffraction intensity of the test piece to be measured with respect to the X-ray diffraction intensity of the powder sample having a random orientation distribution, that is, the X-ray random intensity ratio was obtained. Here, the X-ray random intensity ratio of the {211} plane parallel to the rolling surface was determined. The larger the X-ray random intensity ratio, the greater the amount of texture having the crystal plane in the steel sheet.

  The inclusions were investigated as follows. A test piece is taken from a sample cut out in a range of 300 mm in the plate width direction with the center in the plate width direction as a center, and a cross section (hereinafter referred to as L cross section) having the normal direction in the plate width direction shown in FIG. Then, using an optical microscope, the L section is observed at a magnification of × 400, and the length in the rolling direction is 10 mm and the length in the sheet thickness direction is within the field of view of the entire sheet thickness. The size and distribution of inclusions were investigated.

  The investigation of inclusions was conducted based on the following ideas. Inclusions form a void in the steel during deformation of the steel sheet to promote ductile fracture.The more the shape is elongated, the greater the stress concentration in the vicinity of the inclusion, resulting in ductile fracture. It is known to promote and degrade hole expansibility.

  Here, the present inventors are configured such that not only a single elongated inclusion, but also small elongated inclusions and spherical inclusions are distributed at predetermined intervals in the rolling direction, which is the crack propagation direction. It has also been found that, like one large stretched inclusion, a large concentration of stress is produced in the vicinity of the inclusion due to the synergistic effect of strain introduced in the vicinity of the inclusion when the steel sheet is deformed.

  Quantitatively, as shown in FIG. 2 (a), an inclusion 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. The inclusion group consisting of a collection of objects (hereinafter referred to as inclusion group) has the same effect as one inclusion stretched to the same length as the rolling direction length of the inclusion group. I found it. The equivalent circle diameter here means the diameter when converted into a circle having the same area as the shape of the inclusion, and the straight line in the rolling direction is a virtual straight line extending in the rolling direction. Means.

  Therefore, among the inclusion group as described above, the inclusion group having a length in the rolling direction of 30 μm or more and an inclusion having an interval of more than 50 μm with respect to other inclusions adjacent on the straight line in the rolling direction. Even so, as shown in FIG. 2 (b), the shape has an equivalent circle diameter of 3.0 μm or more and an inclusion having a length in the rolling direction of 30 μm or more is treated as an object to be measured. did. The reason for limiting the measurement target to those having a length in the rolling direction of 30 μm or more is that inclusions having a length in the rolling direction less than this are considered to have a small effect on ductile fracture. 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 effect on ductile fracture.

  As shown in FIG. 2 (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 intervening adjacent on the straight line in the rolling direction. Inclusions having an interval of 50 μm or less with respect to the object were 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”.

  These inclusions and stretched inclusions are considered to increase the stress concentration generated in the vicinity as the shape of the stretched inclusions is extended, and promote the generation and propagation of cracks during deformation of the steel sheet. In the evaluation of the properties, the lengths L1 and L2 in the rolling direction in the L section as shown in FIGS. 2 (a) to 2 (c) were measured.

  The lengths L1 and L2 in the rolling direction of the obtained inclusion groups and the drawn inclusions were determined not as an average value but as a sum M thereof based on the following reasons.

  At the time of deformation of the steel sheet, if the number of inclusion groups and stretched inclusions is small, the voids generated around these inclusion groups etc. break and the crack propagates, whereas the number of these inclusion groups etc. When the number is large, surrounding voids such as inclusions are connected without interruption, and long continuous voids are formed to promote ductile fracture. Since the influence of the number of inclusions and the like cannot be expressed by the average value of the inclusions or the like, the sum M of the inclusions in the rolling direction is determined from this point.

  In particular, in this test result, when the average value of the inclusion group etc. is 30 μm or more, there is no significant correlation with the hole expansion value, and the average value represents the degree of hole expansion. Proved difficult.

  Further, at the time of deformation of the steel sheet, it is considered that as the inclusion group and the distribution of the drawn inclusions are more distributed, the stress concentration increases due to the synergistic effect of strain, and voids are more likely to occur. For this reason, even when the average value of the rolling direction length of inclusions is small, when the number of inclusions in the steel is large, that is, when the total sum M of the lengths of inclusions in the rolling direction is large, the distribution of inclusions It becomes easy to be affected, and the variation of the hole expansion value becomes large. Also from this point, the sum M of the lengths of inclusions in the rolling direction was determined.

From the above viewpoint, the sum M of the lengths of inclusions in the rolling direction per unit area was determined. Specifically, according to the following formula (3), each inclusion group for each field of view, stretched inclusions L1 (mm) and L2 (mm) of the product were summed to obtain L (mm), and based on the obtained L, a numerical value M (mm / mm ² ) was obtained according to the following formula (4). It was decided to evaluate by M. In the following formula (3), L1 _i and L2 _i are the lengths in the rolling direction of each inclusion group and each stretched inclusion in one field of view, and S is the area of the observed field of view (mm ² ).

The relationship between the average value λ _{ave of} the hole expansion value, the lower limit value λ _min of the hole expansion value, the random strength ratio of the {211} plane, and the total length M of the inclusions in the rolling direction, obtained as a result of the above test, is shown in FIG. 3 and FIG. 4, and FIG. 5 shows the relationship between the standard deviation σ of the hole expansion value and the total sum M of the inclusions in the rolling direction. In addition, all the microstructures of the test steels obtained here had a ferrite structure or a bainite structure as the main phase.

As a result, when the {211} plane random strength ratio is 2.2 or less and the total length M of inclusions in the rolling direction is 0.25 mm / mm ² or less, the average value λ _ave of the hole expansion value is 80 It was found that a steel sheet with improved hole expandability of at least 60% at a lower limit value λ _min of the hole expansion value of 60% or more and a standard deviation σ of the hole expansion value of 10% or less can be obtained.

The mechanism by which the average value λ _ave of the hole expansion value and the lower limit value λ _min are improved as the random intensity ratio of the {211} plane is smaller 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 {211} faces. In particular, if the plastic strain ratios (r values) in the rolling direction and the 45 ° direction and 90 ° direction (sheet width direction) with respect to the rolling direction are defined as r ₀ , r ₄₅ , and r ₉₀ , respectively, The difference between ₀ and r ₄₅ and r ₉₀ is 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.

The reason why the average value λ _ave of the hole expansion value and the lower limit value λ _min decrease when the total length M of the inclusions in the rolling direction is large is as described above, and can be summarized as the following two reasons. First, as the total sum M increases, the inclusion group and the elongated inclusions have a stretched shape. As a result, the degree of stress concentration is increased, and the generation and propagation of end face cracks in hole expansion molding are promoted. Can be mentioned. Secondly, as the total sum M increases, the number of inclusions increases, and voids are easily connected during deformation. As a result, the propagation of cracks is promoted. The reason why the standard deviation σ representing the variation in the hole expansion value increases when the sum M is large is as described above. In summary, the number of inclusions increases as the sum M increases. There is a point that the variation increases due to the influence of the distribution.

  Further, as shown in FIG. 6, the present inventors show that the higher the finish rolling end temperature in the hot rolling process, the lower the {211} plane X-ray random intensity ratio, and the finish rolling temperature is 960 ° C. or higher. It has been found that the X-ray random intensity ratio intensity of the {211} plane is 2.2 or less under the above temperature range conditions.

  The reason is considered as follows. It is known that the X-ray random intensity ratio of the {211} plane increases when recrystallization is suppressed between the final rolling passes consisting of a plurality of passes or after the final pass, and rolling strain is accumulated in the steel sheet. ing. From this, when the finish rolling finish temperature is high, recrystallization will be promoted during or after finish rolling, and this will reduce the X-ray random intensity ratio of the {211} plane. It is done.

  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 mainly for MnS drawn by rolling and for desulfurization in the steelmaking stage. It was clarified that it was a residue of desulfurization flux to be introduced.

  As a result of examining the manufacturing method for suppressing these, it was found that the following is important.

  In order to suppress MnS, it is first important to reduce the amount of S contained in the steel. In addition, since Ti-added steel suppresses MnS generation due to TiS generation, Ti addition also has a great effect on MnS suppression. From this point of view, in order to suppress MnS that deteriorates hole expandability, it is necessary to provide a Ti amount lower limit (Ti ≧ 0.05%) and an S amount upper limit (S ≦ 0.005%). found. In particular, a means for reducing the upper limit of S amount without adding Ti can be considered. However, if S is reduced too much, it is necessary to use a large amount of desulfurization flux for desulfurization. As the productivity deteriorates significantly, it becomes necessary to carry out the molten steel recirculation for a long time, and the economic efficiency is impaired. For this reason, in order to suppress MnS while maintaining economic efficiency, it is important to add Ti more than a lower limit.

  In addition, in order to suppress inclusions due to residual desulfurization flux, it is important to perform sufficient recirculation after desulfurization flux addition in the secondary refining process of molten steel to remove the desulfurization flux. Will be described later.

  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: 0.02 to 0.07%
C is an element that combines with Nb, Ti, etc. and contributes to the improvement of tensile strength by precipitation strengthening or the like, but if the C content is less than 0.02%, the effect of improving the strength cannot be obtained. On the other hand, if the C content is more than 0.07%, iron carbide is generated and the hole expandability deteriorates. For this reason, content of C shall be 0.02% or more and 0.07% or less.

Si: 0.5 to 2.0%
Si is an element necessary for preliminary deoxidation and is an element that contributes to improving tensile strength as a solid solution strengthening element. If the Si content is less than 0.5%, sufficient strength improving effect is obtained. I can't get it. Further, if the Si content exceeds 2.0%, the workability deteriorates. For this reason, content of Si shall be 0.5% or more and 2.0% or less.

Mn: 0.5 to 1.5%
Mn is an element that contributes to improving the tensile strength of the steel sheet as a solid solution strengthening element. However, if the Mn content is less than 0.5%, a sufficient effect of improving the tensile strength cannot be obtained. Further, if the Mn content is more than 1.5, slab cracking during hot rolling tends to occur. For this reason, content of Mn shall be 0.5 to 1.5%.

P: 0.03% or less (however, over 0%)
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 characteristics as the content increases. For this reason, the lower the P content, the more desirable. When the P content exceeds 0.03%, the above-described adverse effects on workability and the deterioration of fatigue characteristics become significant. For this reason, the content of P is set to 0.03% or less.

S: 0.005% or less (however, over 0%)
S is an impurity that is inevitably mixed during refining of the molten steel. When the content is too large, MnS stretched in the steel is formed, and this deteriorates the hole expandability. For this reason, the content of S should be reduced as much as possible, with 0.005% being the upper limit.

Al: 0.005 to 0.05%
Al is an element necessary for deoxidation of molten steel. 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, if the Al content is excessively added, the effect is saturated, so the upper limit is made 0.05% from the viewpoint of preventing an increase in cost. For this reason, content of Al shall be 0.005% or more and 0.05% or less.

N: 0.005% or less (however, over 0%)
N forms precipitates with Ti and Nb at a higher temperature than C, and not only reduces Ti and Nb effective in strengthening the steel, but also increases the size of the hole expansion value and increases the size of Ti nitride Form things. Therefore, the N content should be reduced as much as possible, but is acceptable if it is 0.005% or less.

Ti: 0.05 to 0.2%
Ti is an element that suppresses precipitation of MnS that forms inclusions that have been stretched by precipitation as TiS in the reheating stage of the slab. Ti is an element that finely precipitates as TiC in the cooling and winding stages after rolling and contributes to an increase in the strength of the steel sheet due to precipitation strengthening. In order to obtain the above effects, the Ti content needs to be 0.05% or more. However, if the Ti content exceeds 0.2%, not only the effect is saturated, but also the alloy cost increases. Accordingly, the Ti content is set to 0.05% or more and 0.2% or less.

Ca: 0.0005 to 0.01%
Ca is an element necessary for desulfurization of molten steel. Furthermore, Ca is also an element that suppresses the formation of MnS that forms elongated inclusions by fixing S in the steel as spherical CaS. Is essential. In order to exert these effects, the lower limit of Ca is 0.0005%. On the other hand, if Ca is excessively contained in the steel, the manufacturing cost increases, so the upper limit of Ca is 0.01%.

  The above is the reason for limiting the basic component of the present invention. In the present invention, if necessary, one or more of Nb, Cu, Cr, Ni, B, and REM components are contained. You may do it. Nb, Cu, Cr, Ni, and B are elements that have an effect of improving the strength of the hot-rolled steel sheet by precipitation strengthening, solid solution strength, or structure refinement strengthening.

Nb: 0.05% or less (however, over 0%)
Nb is preferably added because a small amount of Nb has the effect of refining the structure and increasing the strength. However, if the amount of Nb added exceeds 0.05%, Nb has an effect of suppressing recrystallization, so that the non-recrystallized zone rolling texture increases and the X-ray random intensity ratio of {211} plane Becomes excessively strong, resulting in deterioration of hole expansibility. For this reason, it is preferable to add Nb with 0.05% as an upper limit.

Cu: 1.0% or less (however, over 0%)
If the Cu content is more than 1.0%, the effect of improving the strength is saturated. Therefore, it is preferable to add Cu at an upper limit of 1.0%.

Cr: 1.0% or less (however, over 0%)
If the addition amount of Cr exceeds 1.0%, the effect of improving the strength is saturated. Therefore, it is preferable to add Cr at an upper limit of 1.0%.

Ni: 1.0% or less (however, over 0%)
If the amount of Ni is more than 1.0%, the effect of improving the strength is saturated, so it is preferable to add Ni at an upper limit of 1.0%.

B: 1.0% or less (however, over 0%)
When the content of B is more than 0.005%, the effect of improving the strength is saturated, so 0.005% is preferably added as the upper limit.

REM: 0.0005 to 0.01%
REM (rare earth element) is an element that becomes 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 0.01%, the effect is saturated and the economy is lowered. For this reason, it is preferable to add REM content as 0.005% or more and 0.01% or less.

  In the present invention, one or more of 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.

X-ray random intensity ratio of {211} plane parallel to the rolling surface: 2.2 or less This is an important characteristic value in the present invention. This characteristic value is obtained by measuring the X-ray diffraction intensity using a sample obtained from a hot-rolled steel sheet subjected to hot rolling. When the X-ray random intensity ratio of the {211} plane is more than 2.2, it is not possible to obtain a steel sheet having an average value λ _ave of hole expansion values of 80% or more and a lower limit value λ _min of 60% or more.

Total M in the rolling direction length of inclusions: 0.25 mm / mm ² or less This is an important characteristic value in the present invention. In the present invention, the total length M of inclusions in the rolling direction needs to be 0.25 mm / mm ² or less in order to improve the hole expansion property. The meaning of the total sum M of the inclusions in the rolling direction here is as described above. In the present invention, not only the stretched inclusions that are known to have an effect of deteriorating the hole expanding property than the conventional one, but also the feature is that the length in the rolling direction of inclusions having a predetermined arrangement is suppressed. is there. When the total length M in the rolling direction of the inclusions is more than 0.25 mm / mm ² , the average value λ _ave of the hole expansion value is 80% or more, the lower limit value λ _min is 60% or more, and its standard deviation σ However, it is not possible to obtain a steel sheet with 10% or less.

The microstructure of the steel sheet in the present invention needs to be a ferrite structure, a bainite structure, or a mixed structure thereof in order to ensure excellent hole expandability (burring workability). From the viewpoint of obtaining better hole expansibility, a bainite single phase is preferable. This is thought to be because, in the case of a ferrite single phase structure, some cementite (iron carbide) may be generated at the grain boundary depending on the variation in composition, which promotes ductile fracture. Further, in the microstructure of the steel sheet, pearlite, martensite, and retained austenite which are inevitably included as other ferrite structure and bainite structure are allowed to be contained in an area fraction of 5% or less. For example, if pearlite is included in an area fraction exceeding 5%, a steel sheet having an average value λ _ave of hole expansion values of 80% or more and a lower limit value λ _min of 60% or more may not be obtained. There is sex.

  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 present invention, in the steelmaking process, the steel components are adjusted so as to be within the predetermined range described above, and the control of the molten steel reflux is performed so that the total sum M of the lengths in the rolling direction of the inclusions as described above becomes small. To do is essential.

  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

  Here, when melting the molten steel, in order to remove the desulfurization flux and reduce the total length M of the inclusions in the rolling direction, the molten steel is desulfurized using a secondary refining apparatus such as RH (Ruhrstahl Heraeus). After adding the desulfurization flux, it is important to recirculate the molten steel 3.0 times or more in the secondary smelting apparatus. The reason for this will be described.

FIG. 7 shows the relationship between the number of molten steel recirculations and the total length M of inclusions in the secondary refining process when melting the component steels shown in Table 1. As shown in FIG. 7, it can be seen that when the number of recirculations is 3.0 or more, the total length M of inclusions in the rolling direction is reduced to 0.25 mm / mm ² or less.

Regarding the number of times of recirculation of the molten steel, the recirculation velocity Q (ton / min) of the molten steel, which means the amount of molten steel circulated in the secondary refining device per unit time, and the molten steel recirculation time (min) after adding the desulfurization flux It can obtain | require based on the following formula | equation (5) and (6) from the amount of molten steel (ton) of the object which should be processed in a secondary refining process.

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） Here, there are various calculation formulas for the recirculation velocity Q of the molten steel described above. For example, “refining limit of impurity element in mass production scale” (Japan Steel Association High Temperature Smelting Process Group Scouring Forum Japan Society for the Promotion of Science) It may be obtained based on the following formula (7) disclosed in the Steelmaking 19th Committee Reaction Process Study Group, March 1996, pages 184 to 187). In addition, the reflux gas flow rate in the following formula (7) means the flow rate of the reflux gas per minute under the conditions of 0 ° C. and 1 atm.

Q: recirculation velocity (ton / min)
V: Circulating gas flow rate (L / min)
D: Immersion tube inner diameter (m)
P0: Vacuum chamber pressure (Pa)
P1: Circulating gas injection position pressure (Pa)
k: Constant (constant by secondary scouring device)

  FIG. 8 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. Further, the secondary refining apparatus 1 is configured such that a reflux gas such as Ar supplied from the reflux 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.

  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 subjected to a hot rolling process under conditions as shown in FIG.

  The slab heating temperature in the slab heating step for heating a slab or a steel slab obtained by cutting the slab may be, for example, 1180 ° C. or higher, but is preferably 1200 ° C. or higher. This is because when slabs are heated at less than 1200 ° C., precipitates containing Ti and Nb are not sufficiently dissolved in the slab and are coarsened, and the precipitation strengthening ability due to the precipitates of Ti and Nb cannot be obtained. This is because the tensile strength is lowered. Furthermore, it is desirable that the temperature be set to 1200 ° C. or higher from the viewpoint of suppressing MnS serving as a stretch inclusion by dissolving MnS by reheating and precipitating S as TiS. From the above viewpoint, the slab heating temperature is preferably as high as possible, and more preferably 1250 ° C. or more.

  The thickness of the slab or steel slab to be heated in the slab heating process is not particularly limited, but from the viewpoint of preventing the inclusions from being stretched in the rolling direction, the thickness is preferably 250 mm or less.

  Although the rolling end temperature in the rough rolling step performed after the slab heating step is not particularly limited, it is preferably 1150 ° C. or higher from the viewpoint of promoting dynamic recrystallization and promoting randomization of crystal orientation.

  The raw material thickness (rough bar thickness) after rough rolling is preferably thin from the viewpoint of weakening the texture orientation in finish rolling, and is desirably 35 mm or less.

  After the rough rolling process, a finish rolling process for further rolling the rough bar obtained by the rough rolling process is performed. The finish rolling finish temperature in this finish rolling process is to avoid leaving an unrecrystallized rolled texture, and to make the X-ray random intensity ratio of the {211} plane 2.2 or less and to improve the hole expansion value. Furthermore, it is necessary to set it as a temperature range of 960 degreeC or more. 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.

  In addition, the finish rolling start temperature in the finish rolling step is preferably a high temperature from the viewpoint of promoting recrystallization after rolling, and is desirably 1050 ° C. or higher.

  In addition, when performing the rough rolling process and the finish rolling process, a heating device is used in each rolling mill or between the rolling mills so as to control the variation in temperature in the rolling direction, plate width direction, and plate thickness direction of the rough bar. It may be installed and controlled to reduce temperature variation during hot rolling.

  Further, between the rough rolling process and the finish rolling process, endless rolling is performed in which a plurality of rough bars obtained by rough rolling are joined together, and finish rolling is continuously performed on the joined plurality of rough bars. May be.

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

  After the finish rolling process, a cooling process for cooling the hot-rolled steel sheet obtained by finish rolling with a run-out table is performed. The cooling rate (° C./sec) in the cooling step may be, for example, 18 ° C./sec or more, but is preferably 20 ° C./sec or more. This is because when the cooling rate is less than 20 ° C./sec, TiC precipitated during cooling on the run-out table is coarsened, so that the effect of precipitation strengthening due to fine TiC cannot be obtained sufficiently and the tensile strength decreases. It is because it ends.

  In the cooling process, in order to promote the precipitation of TiC and improve the tensile strength, as shown in FIG. 8, the cooling rate is set to 15 ° C./sec or less during cooling at a cooling rate of 20 ° C./sec or more as appropriate. It is also possible to perform the slow cooling described above, and then cool again at a cooling rate of 20 ° C./sec or more. The temperature range and time for performing this slow cooling are optimally set to a temperature range of 550 to 650 ° C. and a time of 5 seconds or less in order to promote precipitation of TiC. If the slow cooling time is too long, pearlite may be generated and the hole expandability may be deteriorated. Therefore, it is not preferable to set this time longer than necessary. In FIG. 9, cooling at a cooling rate of 20 ° C./sec or more performed in the cooling process is shown as strong cooling, and cooling at a cooling rate of 15 ° C./sec or less is shown as slow cooling.

  After the cooling step, a winding step for winding the hot-rolled steel sheet obtained through the cooling step is performed. In the winding process, it is necessary to wind the hot-rolled steel sheet in a temperature range of 400 ° C. or more and 550 ° C. or less, and in the cooling process, cooling is performed until such a temperature range is reached. When the coiling temperature is less than 400 ° C., carbides such as Ti are difficult to precipitate at the grain boundaries, and the precipitation strengthening ability by these carbides cannot be sufficiently obtained. Further, when the coiling temperature is higher than 550 ° C., a phase containing coarse carbides such as pearlite, which is not preferable for workability, may be generated. Therefore, the winding temperature in the winding process is set to 400 ° C. or more and 550 ° C. or less. In addition, after a winding process, it anneals from a winding temperature to room temperature and 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 a 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 with a reduction rate of 10% or less or cold rolling with a reduction rate of 40% or less in-line 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, examples of the high-strength hot-rolled steel sheet to which the present invention is applied and the manufacturing method thereof will be described in detail.

First, after refining the molten steel of steel numbers A to P having the chemical components shown in Table 2 below in the converter, secondary refining is performed, and recirculation for removing the desulfurization flux is performed during the secondary refining. After being obtained by the above, a steel piece was obtained by continuous casting, and then directly or reheated, and then a hot rolled steel sheet having a thickness of 2.9 mm was obtained through the production conditions shown in Table 3. Here, the desulfurization flux used was one in which CaO, CaF ₂ and MgO were mixed in substantially equal amounts by weight. The reflux was performed using RH as shown in FIG. 7 as a secondary refining device. Table 4 shows the mechanical properties and microstructure of the hot-rolled steel sheet thus obtained. Underlines in Tables 2 to 4 mean that they are outside the scope of the present invention or outside the preferred ranges. The constant k in the above equation (7) is 4. Further, in the “mechanical property evaluation” in Table 4, λ _ave is 80% or more, λ _min is 60% or more, σ is 10% or less, and a tensile strength is 780 MPa or more, and a circle is described, and λ _ave , Although λ _min and σ satisfy the same conditions, those having a tensile strength of less than 780 MPa are indicated by triangular marks, and those in which any of λ _ave , λ _min , and σ does not satisfy the same conditions are indicated by cross marks.

As described above, the hole expansion value was evaluated according to the hole expansion test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996. Here, in order to evaluate variation during the hole expansion test, an average value λ _ave and standard deviation σ of the hole expansion values were obtained. The average value λ _ave of the hole expansion values was the arithmetic average of the test values of 30 test pieces taken from a sample cut out in a range of 300 mm in the plate width direction with the center position in the plate width direction of the steel plate as the center. The lower limit value λ _min of the hole expansion value was determined based on the above equation (1), and the standard deviation σ of the hole expansion value was determined based on the above equation (2).

  As described above, the tensile test value is obtained by processing the No. 5 test piece described in JIS Z 2201 parallel to the plate width direction from the center portion in the plate width direction of the obtained hot-rolled steel sheet, and JIS Z for the obtained test piece. It was evaluated by performing the test method described in 2241.

  The texture was evaluated by determining the X-ray random intensity ratio of the {211} plane parallel to the rolled surface. In obtaining the X-ray random intensity ratio, a test piece cut out in a size of 20 mm in the plate width direction and 20 mm in the rolling direction with the center position in the plate width direction as the center was ground to a 1/2 t position with respect to the plate thickness t. Thereafter, the measurement was performed using a chemically polished sample.

  The microstructure was obtained by polishing a sample cut out from the central portion in the plate width direction so as to obtain an L cross section as shown in FIG. 1, etching it using a Nital reagent, and using a light microscope to measure the central portion of the plate thickness with an optical microscope. Observation and identification at a magnification of ˜500 times. In the column “Microstructure” in Table 4, the ferrite, bainite, and pearlite that can be confirmed by optical microscope observation are described. When two phases were confirmed among ferrite, bainite, and pearlite, the area fraction of the phase with the smaller area fraction of the two phases was described in Table 4. The inclusion evaluation method is the same as described above.

In accordance with the present invention are steel numbers 1-12, steel number 18, steel number 20 and steel number 22. These hot-rolled steel sheets contain a predetermined amount of steel components, and the {211} plane X-ray random strength ratio is 2.2 or less, and the total length M of inclusions in the rolling direction is 0.25 mm. / Mm ² or less, the average value λ _ave of the hole expansion value is 80% or more, the lower limit value λ _min of the hole expansion value is 60% or more, and most of them Excellent tensile strength of 780 MPa is obtained.

  Steel numbers 18, 20, and 22 are outside the preferred range of the present invention for the following reasons. Steel No. 18 has a tensile strength lower than 780 MPa because the slab heating temperature is lower than a predetermined value. In Steel No. 20, the cooling rate at the run-out table is lower than a predetermined value, so that the effect of precipitation strengthening by fine TiC cannot be sufficiently obtained, and the tensile strength is lowered from 780 MPa. Steel No. 22 has a coiling temperature lower than a predetermined value, so that sufficient precipitation strengthening cannot be obtained and the intended tensile strength cannot be obtained.

Steel numbers other than the above are outside the scope of the present invention for the following reasons. In Steel Nos. 13 to 17, the total number M of inclusions in the rolling direction is over 2.2 mm / mm ² because the number of recirculations in the steel making process is small, and the average value λ _{ave of} target hole expansion values The lower limit λ _min and the standard deviation σ are not obtained. Steel No. 19 has a finish rolling finish temperature lower than a predetermined value, so that the X-ray random intensity ratio of the {211} plane is higher than a predetermined value. The average value λ _{ave of the} target hole expansion value and the lower limit value λ _min are Not obtained. Steel No. 21 has a pearlite structure because the coiling temperature is higher than a predetermined value, and the average value λ _{ave of the} target hole expansion value and the lower limit value λ _min are not obtained. Steel No. 23, since S amount is larger than a predetermined rolling direction length of the sum M has become a predetermined value or more, the average value lambda _ave of hole expansion value of _interest, the lower limit value lambda _min and a standard deviation of inclusions σ is not obtained. In Steel No. 24, since the Nb amount is larger than a predetermined value, the X-ray random intensity ratio of the {211} plane is high, and the average value λ _{ave of the} target hole expansion value and the lower limit value λ _min are not obtained. . Steel No. 25 does not have the intended tensile strength because the Ti amount is lower than the predetermined value. Further, in Steel No. 25, since the Ti amount is lower than a predetermined value, the total length M of inclusions in the rolling direction is not less than a predetermined value, and the average value λ _{ave of the} target hole expansion value, the lower limit value λ _min and The standard deviation σ is not obtained.

Claims (6)

% By mass
C: 0.02 to 0.07%,
Si: 0.5 to 2.0%,
Mn: 0.5 to 1.5%
P: 0.03% or less (however, over 0%),
S: 0.005% or less (however, over 0%),
Al: 0.005 to 0.05%,
N: 0.005% or less (however, over 0%),
Ti: 0.05 to 0.2%,
Ca: 0.0005 to 0.01%
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,
The X-ray random intensity ratio of the {211} plane parallel to the rolling surface is 2.2 or less,
In a cross section having the plate width direction as a normal line, 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. A high-strength hot-rolled steel sheet excellent in hole expansibility, 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. In addition,
Nb: 0.05% or less (however, over 0%),
The high-strength hot-rolled steel sheet excellent in hole expansibility according to claim 1, comprising: In addition,
Cu: 1.0% or less (however, over 0%),
Cr: 1.0% or less (however, over 0%),
Ni: 1.0% or less (however, over 0%),
B: 0.005% or less (however, over 0%),
The high-strength hot-rolled steel sheet excellent in hole expansibility according to claim 1 or 2, characterized in that any one or more of them are contained. Further, REM in mass%: 0.0005 to 0.01%
The high-strength hot-rolled steel sheet having excellent hole expansibility according to any one of claims 1 to 3. When molten steel containing the component according to any one of claims 1 to 4 is melted, the molten steel is recirculated at least 3.0 times after the desulfurization flux is added in a secondary refining device, and then obtained from the molten steel. The obtained slab is heated to 1200 ° C. or higher, followed by rough rolling, and the subsequent finish rolling is finished in a temperature range of 960 ° C. or higher, and then 400 ° C. or higher and 550 ° C. at a cooling rate of 20 ° C./sec or higher. A method for producing a high-strength hot-rolled steel sheet excellent in hole expansibility, characterized by winding after cooling to the following temperature range. During the cooling at a cooling rate of 20 ° C./sec or more, the cooling is performed at a cooling rate of 15 ° C./sec or less, and then the cooling is performed again at a cooling rate of 20 ° C./sec or more. The manufacturing method of the high intensity | strength hot-rolled steel plate excellent in the hole expansibility of Claim 5.

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