JP5720612B2 - High strength hot rolled steel sheet excellent in formability and low temperature toughness and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet excellent in formability and low-temperature toughness used for automobile underbody members and the like, and a method for producing the same.

  In the past, attempts have been made to increase the strength of steel sheets in order to reduce the weight of the steel sheets. In general, increasing the strength of a steel sheet causes deterioration of formability such as hole expansibility, so it is important how to obtain a steel sheet having an excellent balance between tensile strength and hole expansibility.

  For example, in Patent Document 1, a steel sheet having an excellent balance between tensile strength and hole expansibility is obtained by optimizing the fraction of the microstructure in the steel such as ferrite and bainite and the precipitate in the ferrite structure. Techniques that can be used are disclosed.

  Here, the characteristic values of the steel sheet obtained by the disclosed technique of Patent Document 1 are 780 MPa or more in terms of tensile strength and 60% or more in terms of the hole expansion rate. However, for steel sheets used as, for example, automobile undercarriage members, etc., the steel sheet is excellent in balance between tensile strength and hole expansibility, with a tensile strength of 780 MPa or more and a hole expansion ratio of 70% or more. Was desired.

  In addition, a steel plate having a relatively large thickness, such as a hot-rolled steel plate, is deteriorated in low-temperature toughness and increased in ductile brittle transition temperature as its strength increases. When the ductile brittle transition temperature becomes high, when the steel sheet is formed and used as an automobile part, there is a concern that the part may be brittlely fractured when the load on the part is large. Therefore, it is required to maintain the ductile brittle transition temperature at a low temperature. From this viewpoint, it is desired to improve the balance between strength and toughness in addition to the balance between strength and formability.

JP 2004-339606 A

  Therefore, the present invention has been devised in view of the above-mentioned problems, and the object of the present invention is excellent in balance between hole expansibility and tensile strength, and further excellent in low temperature toughness. The object is to provide a high-strength hot-rolled steel sheet and a method for producing the same.

  In order to solve the above-mentioned problems, the present inventor has invented a high-strength hot-rolled steel sheet excellent in formability and low-temperature life described below and a method for producing the same, after intensive studies.

Invention 1 is mass%, C: 0.03-0.1%, Si: 0.001-2.0%, Mn: 0.5-3.0%, P: 0.1% or less, S : 0.005% or less, Al: 0.005 to 2.0%, N: 0.02% or less, Ti: 0.05 to 0.2%, with the balance being Fe and inevitable impurities A steel plate having a microstructure of a ferrite structure, a bainite structure, or a mixed structure thereof, an average crystal grain size of 6.0 μm or less, and an X-ray random intensity ratio of [211] plane parallel to the rolling surface Is equal to or less than 2.4, the ratio of the amount of Ti contained in the precipitate containing Ti of 100 nm or more in spherical phase is equal to or less than 0.036% in the steel, and the spherical phase is equal to 2 The density of precipitates containing Ti of 0.0 nm or more and 10 nm or less is 2.0 × 10 ¹⁶ pieces / cm ³ or more and 1.0 × 10 A high-strength hot-rolled steel sheet excellent in formability and low-temperature toughness, characterized by being ¹⁸ pieces / cm ³ or less.

  Invention 2 further contains, in mass%, any one or both of Ca: 0.0001 to 0.005% and REM: 0.0001 to 0.02%. A high-strength hot-rolled steel sheet with excellent formability and low-temperature toughness.

  Invention 3 is a high-strength hot-rolled steel sheet having excellent formability and low-temperature toughness according to Invention 1 or Invention 2, further comprising B: 0.0005 to 0.003% by mass.

  Invention 4 is further, in mass%, Cu: 0.001-1.0%, Cr: 0.001-1.0%, Mo: 0.001-1.0%, Ni: 0.001-1. 0.0%, Nb: 0.001 to 0.1%, V: 0.005 to 0.1%, one or more of any one of the inventions 1 to 3 A high-strength hot-rolled steel sheet having excellent formability and low-temperature toughness.

Invention 5 is a method for producing a high-strength hot-rolled steel sheet excellent in formability and low-temperature toughness according to any one of Inventions 1 to 4, and the steel slab containing the components described in Inventions 1 to 4 is set to 1200 ° C or higher. In the rough rolling process, rough rolling is performed under the conditions satisfying the following formula (1), the final rolling of the rough rolling is performed at 1100 ° C. or more, and finish rolling is started within 100 seconds from the end of the rough rolling. Finish rolling in a temperature range of 945 ° C. or higher and 1035 ° C. or lower, subsequently cooling at a cooling rate of 20 ° C./sec or higher, and then winding in a temperature range of 200 ° C. or higher and 540 ° C. or lower. A method for producing a high-strength hot-rolled steel sheet having excellent formability and low-temperature toughness.
[(Thickness before the first reduction after 1150 ° C. or lower) − (Thickness after the last reduction before 1100 ° C. or lower)] / (Before the first reduction after 1150 ° C. or lower) Plate thickness) × 100 ≧ 50 (%) (1)

  In Invention 6, when performing the cooling, cooling is performed at a cooling rate of 20 ° C./sec or more, and then cooling is performed at a cooling rate of 15 ° C./sec or less in a temperature range of 550 ° C. or more and 650 ° C. or less for 2 seconds or more. The method for producing a high-strength hot-rolled steel sheet having excellent formability and low-temperature toughness according to the invention 5, wherein the cooling is performed at a cooling rate of 20 ° C./sec or more.

  According to the first to sixth inventions, it is possible to obtain a hot-rolled high-strength steel sheet having an excellent balance between tensile strength and hole expansibility and excellent low-temperature toughness.

It is a figure which shows the relationship between the coarse deposit containing Ti, and a hole expansion rate. It is a figure which shows the relationship between rough rolling end temperature, the time from the end of rough rolling to the start of finish rolling, and the precipitation amount of the coarse precipitate containing Ti. It is a figure which shows the relationship between the X-ray random intensity ratio of a [211] surface, and a hole expansion rate. It is a figure which shows the relationship between finish rolling completion temperature and the X-ray random intensity ratio of [211] plane. It is a figure which shows the relationship between the density of the fine precipitate containing Ti, the average crystal grain diameter of a microstructure, and transition temperature. It is a figure which shows the relationship between finish rolling completion temperature, the rolling reduction in the temperature range of 1100 degreeC-1150 degreeC, and the average crystal grain diameter of a microstructure.

  Hereinafter, as a form for carrying out the present invention, a high-strength hot-rolled steel sheet excellent in the balance between strength and hole-expandability and low-temperature toughness and a manufacturing method thereof will be described.

  The present inventors conducted basic research to obtain a high-strength hot-rolled steel sheet that is excellent in the balance between strength and hole expansibility and low-temperature toughness described above.

  And in order to improve the hole expandability while maintaining high strength, it is possible to control the texture, crystal grain size, and precipitates after forming a microstructure composed of one or both of ferrite and bainite. I found it important. In particular, the inventors have newly found that the ductile fracture is promoted by the coarse precipitate containing Ti precipitated in the austenite region between the rough rolling and the finish rolling, and the hole expandability is deteriorated.

In order to secure low temperature toughness while maintaining high strength, it is important to refine the crystal grains of the microstructure and to control the amount of fine precipitates containing Ti within an appropriate range. I also found out. Specifically, the average crystal grain size of the microstructure is 6.0 μm or less, and the density of fine precipitates containing Ti (precipitates containing Ti having a spherical phase equal diameter of 2.0 nm to 10 nm) is 2 It was found that it was important to set the density to 0.0 × 10 ¹⁶ pieces / cm ³ or more and 1.0 × 10 ¹⁸ pieces / cm ³ or less. If the amount of fine precipitates containing Ti is too small, high strength cannot be obtained, and if the amount of precipitation is too large, low temperature toughness deteriorates.

  In order to make the crystal grain size of the microstructure fine, it is important to control the rolling schedule of rough rolling and lower the finish rolling temperature, and it is important to perform rolling at a low temperature and a large rolling reduction. I found out. Moreover, in order to make the density of the fine precipitate containing Ti into the predetermined range, it has been found that the coiling temperature is important to be 200 ° C. or higher and 540 ° C. or lower.

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

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

  The inventor performs hot rolling, cooling, winding and the like on the test steels composed of the steel components A and B shown in Table 1 described below under the conditions shown in Table 2 described below. A 9 mm hot rolled steel sheet was produced. The mechanical properties (strength, hole expansion rate, low temperature toughness), microstructure, and precipitates of the obtained hot rolled steel sheet were evaluated.

  For the tensile strength, a No. 5 test piece described in JIS Z 2201 was produced from the half width part of the test steel so that the longitudinal direction of the test piece was parallel to the plate width direction, and from the obtained test piece A tensile test was performed in accordance with the method described in JIS Z 2241 for measurement.

  As for the hole expanding property, a test piece having a length in the rolling direction of 150 mm and a length in the width direction of 150 mm was produced from the 1/2 sheet width part of the test steel, and described in Japan Iron and Steel Federation Standard JFS T 1001-1996. The hole expansion test was performed according to the method, and the hole expansion rate of the test piece was measured and evaluated. For the evaluation of hole expandability, 20 test pieces were manufactured from one test steel, and the hole expansion obtained by arithmetically averaging the measured values obtained by performing the hole expansion test on each manufactured test piece. The rate was determined.

In the punching hole expansion test conducted here, a punching punch having a diameter of 10 mm was used, and the punching clearance obtained by dividing the gap between the punching punch and the die hole by the plate thickness of the test piece was set to 12.5%, and the initial hole diameter ( D ₀ ) When a 10 mm punched hole is provided in the test piece, and then a conical punch with a vertex angle of 60 ° is pushed into the punched hole from the same direction as the punched punch, and a crack generated in the punched end surface penetrates in the plate thickness direction. The hole inner diameter _Df was measured at and determined from the following equation. Here, the plate thickness of the crack was visually observed.
λ (%) = [(D _f −D ₀ ) / D ₀ ] × 100

  For low temperature toughness, a 2.5 mm thick sub-size Charpy test piece having a test piece direction in the width direction and a notch direction in the longitudinal direction (TL test piece) is prepared from the central portion in the width direction of the steel sheet, and the temperature is 20 A Charpy test was performed at N = 3 at 0 ° C., 0 ° C., −30 ° C., −60 ° C., −90 ° C., −120 ° C., and −150 ° C. to determine the ductile fracture surface ratio, and the relationship between temperature and ductile fracture surface ratio From this, it was evaluated by determining a temperature (ductile brittle transition temperature) at which the ductile fracture surface ratio becomes 50%.

  For coarse precipitates containing Ti, a 30-g sample of the powder is obtained from the obtained hot-rolled steel sheet, which is dissolved by electrolysis, and then the remaining residue is vacant with a size of 0.1 μm. This was evaluated by measuring the weight of precipitates remaining on the filter paper and the amount of Ti, and determining the ratio of the amount of Ti in the precipitates containing Ti to mass% in the steel. The size of the precipitate measured by this method is approximately 100 nm or more in terms of the equivalent sphere diameter. Hereinafter, a coarse precipitate containing Ti as an object measured by such a method is also simply referred to as a coarse Ti precipitate.

  For fine precipitates containing Ti, a sample is cut out from the center in the width direction and the center of the thickness of the obtained hot-rolled steel sheet, and the three-dimensional distribution of atoms in the steel is subnanoed by the three-dimensional atom probe (3DAP) method. Obtained by the resolution of the meter, the portion where Ti atoms were densely identified was determined as a precipitate containing Ti, and the size of the portion determined as the precipitate containing Ti was determined. As this size, the equivalent sphere diameter is obtained, and the number of precipitates containing Ti in the size range of 2.0 nm or more and 10.0 nm or less is obtained per unit volume, which is a fine precipitation containing Ti. Based on this density, fine precipitates containing Ti were evaluated. Hereinafter, a fine precipitate containing Ti as an object to be measured by such a method is also simply referred to as a fine Ti precipitate.

  The microstructure was examined by cutting out and polishing the steel plate so that a cross section having the normal direction in the plate width direction (hereinafter referred to as the L cross section) was exposed from the 1/4 plate width position of the steel plate and corroding it with the Nital reagent. The ¼ plate thickness position of the steel plate was observed by using an optical microscope at a magnification of 200 to 500 times.

  Here, in investigating the microstructure, since it is known that the average crystal grain size of the microstructure has an influence on the fracture surface transition temperature, the average crystal grain size was measured. The average crystal grain size of the microstructure was determined as follows. The EBSP method (Electronron) is used to measure the crystal orientation distribution in steps of 2 μm at the center portion of the L cross section at the 1/4 sheet width position of the steel plate to be measured, which is 500 μm in the plate thickness direction and 500 μm in the rolling direction. (Back Scattering Pattern method), and connecting the points with an orientation difference of 15 ° or more as a grain boundary to obtain the number average value of the circle equivalent diameter of the crystal grain composed of the grain boundary, which is the average of the microstructure The crystal grain size was used.

  The texture was investigated by measuring the X-ray random intensity ratio. The X-ray random intensity ratio here refers to the X-ray diffraction intensity of a standard sample having a random orientation distribution without accumulation in a specific orientation and the X-ray diffraction intensity of the test steel to be measured. It means a numerical value obtained by dividing the X-ray diffraction intensity of the obtained test steel by the diffraction measurement by the X-ray diffraction intensity of the standard sample. It means that the larger the X-ray random intensity ratio in a specific orientation, the greater the amount of texture having crystal planes in that specific orientation in the steel sheet.

  X-ray diffraction measurement was performed using a diffractometer method using an appropriate X-ray tube. A sample for X-ray diffraction measurement is a half-thickness position in the thickness direction by mechanical polishing of a test piece cut to a size of 20 mm in the width direction and 20 mm in the rolling direction from a half-width position of the steel plate. Then, the strain was removed by electrolytic polishing or the like. X-ray diffraction measurement was performed for the 1/2 plate thickness position of the obtained sample.

  The knowledge obtained for steel components A and B in Table 1 will be described below.

  FIG. 1 is a diagram showing the relationship between coarse precipitates containing Ti and hole expansibility (hole expansivity λ). Thus, the smaller the amount of coarse precipitates containing Ti, the better the hole expansion property. This is because a coarse precipitate containing Ti promotes ductile fracture and causes deterioration of hole expansibility, so that the hole expansibility is improved by suppressing the precipitation of coarse precipitates containing Ti.

  FIG. 2 is a diagram showing the relationship between the rough rolling end temperature (° C.), the time from the end of rough rolling to the start of finish rolling (seconds), and the amount of coarse Ti precipitates deposited. Thus, the lower the rough rolling end temperature and the longer the time from the end of rough rolling to the start of finish rolling, the greater the amount of coarse Ti precipitates deposited. This is because strain-induced precipitation after reduction in rough rolling is promoted as the rough rolling end temperature is lower and as the time from the end of rough rolling to the start of finish rolling is longer. The rough rolling end temperature here means the temperature at which the final rolling of the rough rolling is performed.

  FIG. 3 is a diagram showing the relationship between the X-ray random intensity ratio of the {211} plane (hereinafter also referred to as {211} plane intensity) and the hole expansion property (hole expansion ratio λ). Here, the samples in the figure are plotted with the same amount of coarse Ti precipitates. Thus, when the precipitation amount of coarse Ti precipitates is the same level, the hole expansion ratio λ increases as the [211] plane strength decreases. This is because the higher the [211] plane strength, the higher the in-plane anisotropy of the r value of the steel sheet, thereby increasing the stress concentration on the end face in the rolling direction during hole expansion, and crack initiation and propagation. It is to be promoted.

  FIG. 4 is a diagram showing the relationship between the finish rolling finish temperature and the {211} plane strength. The {211} plane intensity is represented by the X-ray random intensity ratio of the {211} plane. Thus, the lower the finish rolling finish temperature, the higher the {211} plane strength. This is because when the rolling temperature is lowered, transformation from unrecrystallized γ grains occurs, and the microstructure after the transformation contains many {211} textures.

  FIG. 5 is a diagram showing the relationship between the density of fine Ti precipitates having a spherical phase equal diameter of 2.0 nm or more and 10 nm or less, the average crystal grain size of the microstructure, and the ductile brittle transition temperature. Thus, the ductile brittle transition temperature decreases as the average crystal grain size of the microstructure decreases and as the density of fine Ti precipitates decreases. The reason why the transition temperature becomes lower as the average crystal grain size of the microstructure is smaller is the conventional knowledge, but in this case, it is difficult to generate a large-sized initial crack that becomes a starting point of brittle fracture. In addition, the transition temperature decreases as the density of fine Ti precipitates decreases, because the amount of fine precipitates decreases, which facilitates the movement of dislocations and causes plastic deformation at the crack tip. This is because it becomes easier.

FIG. 6 is a graph showing the relationship between the finish rolling end temperature, the rolling reduction R in the temperature range of 1100 ° C. to 1150 ° C., and the average crystal grain size of the microstructure. Here, the rolling reduction R is represented by a numerical value on the left side in the following equation (1). Thus, the average crystal grain size of the microstructure decreases as the finish rolling finish temperature decreases and as the rolling reduction R in the temperature range of 1100 ° C. to 1150 ° C. increases. The average grain size of the microstructure decreases as the finish rolling finish temperature decreases. The recovery and recrystallization of the austenite structure after rolling slows down, and many nucleation sites remain during grain transformation and α transformation in the grain. It is because it comes to do. Moreover, the average crystal grain size of the microstructure decreases as the rolling reduction R in the temperature range of 1100 ° C. to 1150 ° C. increases. The recrystallized γ grains become finer as the rolling reduction in the low temperature γ region increases. Because.
[(Thickness before the first reduction after 1150 ° C. or lower) − (Thickness after the last reduction before 1100 ° C. or lower)] / (Before the first reduction after 1150 ° C. or lower) Plate thickness) × 100 ≧ 50 (%) (1)

In the formula (1), the reduction ratio R in the temperature range of 1100 ° C. to 1150 ° C. is used as an index for the following reason. The reason why the lower limit temperature is set to 1100 ° C. is that, in the present invention, it is assumed that rough rolling is completed at 1100 ° C. or higher. The reason why the upper limit temperature is set to 1150 ° C. is that the definition of the formula (1) is intended to make the γ grains finer, and for that purpose, the temperature is reduced as low as possible in order to avoid grain growth after recrystallization. This is because it is necessary to define a reduction rate at a low temperature.

In the present invention, the target hole expansion rate is set to 80% or more. In order to obtain the hole expansion rate, the ratio of coarse precipitates containing Ti is set to 0.036 as shown in FIG. % Or less, as shown in FIG. 3, the {211} plane strength needs to be 2.4 or less. Further, in order to make the ratio of coarse precipitates containing Ti 0.036% or less, as shown in FIG. 2, the rough rolling end temperature is 1100 ° C. or more, and the time from the end of rough rolling to the start of finish rolling is set to It must be 100 seconds or less. Moreover, in order to make {211} plane intensity | strength 2.4 or less, as shown in FIG. 4, it is necessary to make finish rolling completion | finish temperature into 945 degreeC or more. In the present invention, the target ductile brittle transition temperature is set to −80 ° C. or lower, and in order to obtain the transition temperature, the average crystal grain size of the microstructure is 6.0 μm or lower as shown in FIG. The density of fine Ti precipitates needs to be 1.0 × 10 ¹⁸ or less.

  The present invention has been made based on the above.

  Then, the reason for limitation of the component composition of the steel plate in this invention is demonstrated. Hereinafter, mass% in the component composition is simply referred to as%.

C: 0.03-0.1%
C is an element that combines with Nb, Ti, etc. and contributes to the improvement of tensile strength by precipitation strengthening. Further, the lower the content of C, the coarser the microstructure becomes, and the transition temperature increases. When the content of C is less than 0.03%, the object of the present invention cannot be obtained for these effects. From this viewpoint, the lower limit of C is 0.03%. Further, if the content of C is too large, iron carbide (Fe ₃ C), which is not preferable for hole expansibility, may be generated excessively. In order to obtain the hole expandability targeted by the present invention, the C content needs to be 0.1% or less. Accordingly, the C content is set to 0.03% or more and 0.1% or less. In order to suppress the formation of iron carbide (Fe ₃ C) and to obtain further excellent hole expandability, it is preferably 0.06% or less.

Si: 0.001 to 2.0%
Si is an element necessary for preliminary deoxidation, and in order to sufficiently obtain the effect of preliminary deoxidation, it is necessary to contain 0.001% or more. Further, Si is an element effective as a solid solution strengthening element that contributes to the improvement of tensile strength and suppresses the formation of iron carbide (Fe3C) to improve the hole expanding property. However, if the Si content is more than 2.0%, such effects are saturated and the economic efficiency is lowered. For this reason, content of Si shall be 0.001% or more and 2.0% or less.

Mn: 0.5 to 3.0%
Mn is an element that contributes to improving the tensile strength of the steel sheet as a solid solution strengthening element. Mn needs to be contained in an amount of 0.5% or more in order to obtain the intended tensile strength of the present invention. 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 0.5-3.0%.

P: 0.1% or less (excluding 0%)
P is an impurity that is inevitably mixed, and is an element that increases the amount of segregation at the grain boundary as the content increases, and causes deterioration of hole expansibility. For this reason, it is desirable that the P content is as low as possible. When the P content is 0.1% or less, these hole expandability characteristic values are acceptable. For this reason, content of P shall be 0.1% or less.

S: 0.005% or less (excluding 0%)
S is an impurity that is inevitably mixed, and if the content is too large, a large amount of MnS is produced in the steel when the steel piece is heated, and this becomes an inclusion stretched by hot rolling. The hole expandability is not obtained. For this reason, S makes the content 0.005% or less. Further, since S is difficult to reduce its content to less than 0.003% when desulfurization using a desulfurization material is not performed, the S content in this case is set to 0.003% or more. .

Al: 0.005 to 2.0%
Al is an element necessary for deoxidation of molten steel, and the content thereof needs to be added in an amount of 0.005% or more in order to sufficiently obtain the effect of deoxidizing the molten steel. Al is an element that contributes to improving the tensile strength, but even if added over 2.0%, such an effect is saturated and the economic efficiency is lowered. For this reason, content of Al shall be 0.005% or more and 2.0% or less.

N: 0.02% or less (excluding 0%)
N forms precipitates with Ti and Nb at a higher temperature than C and reduces Ti and Nb effective for fixing C, thereby causing a decrease in tensile strength. Therefore, the N content should be reduced as much as possible, but is acceptable if it is 0.02% or less. In order to more effectively suppress the decrease in tensile strength, the N content is preferably 0.005% or less.

Ti: 0.05 to 0.2%
Ti is an element that precipitates as fine precipitates containing Ti after finish rolling and contributes to the improvement of the tensile strength of the steel sheet by precipitation strengthening. In order to obtain the intended tensile strength of the present invention, it is necessary to add 0.05% or more of Ti. On the other hand, when Ti precipitates as a coarse precipitate containing Ti in the γ region, the hole expanding property is deteriorated. Further, if the amount of fine precipitates containing Ti is excessively increased, the low temperature toughness is deteriorated. From this viewpoint, the upper limit of Ti is 0.2%.

  The above is the reason for limiting the basic components of the hot-rolled steel sheet according to the present invention, and the remaining balance of the basic components is composed of Fe and inevitable impurities. Inevitable impurities include O, Zn, Pb, As, Sb, etc. Even if these are included in the range of 0.02% or less, the effects of the present invention are not lost.

  Moreover, the hot-rolled steel sheet according to the present invention may contain one or both of REM and Ca in the following content.

REM: 0.0001 to 0.02%
REM (rare earth element) is an element that spheroidizes the form of sulfides such as MnS, which causes the hole expandability to deteriorate. If the REM content is less than 0.0001%, the effect of spheroidizing a sulfide such as MnS cannot be obtained sufficiently, so the lower limit is made 0.0001%. Further, when the content of REM is more than 0.02%, such an effect is saturated and the economic efficiency is lowered. For this reason, content of REM shall be 0.02% or less.

Ca: 0.0001 to 0.005%
Ca is an element that fixes S in steel as spherical CaS, suppresses the formation of MnS, and spheroidizes the form of sulfide such as MnS. If the Ca content is less than 0.0001%, the effect of spheroidizing a sulfide such as MnS cannot be obtained sufficiently, so the lower limit is made 0.0001%. Further, if the Ca content is more than 0.005%, a large amount of calcium aluminate that tends to be inclusions in the stretched shape is generated, and on the contrary, the hole expandability is deteriorated, so the upper limit of the Ca content is 0.00. 005% or less.

  Moreover, in this invention, you may contain any 1 type or 2 types or more of B, Cu, Cr, Mo, Ni, Nb, and V by the following content as needed.

B: 0.0005 to 0.003%
B is an element that increases the strength by increasing the hardenability and increasing the hard phase. If the content of B is less than 0.0005%, the effect may not be sufficiently obtained. Further, if a large amount of B is added, the temperature of the non-recrystallized region in the hot rolling process is expanded, and the rolled texture in an unrecrystallized state that increases the X-ray random intensity ratio of the {211} plane. However, a large amount remains after the hot rolling process. Therefore, the B content needs to be 0.003% or less. From these things, when adding B, the content needs to be 0.0005%-0.003%.

  Cu, Cr, Mo, Ni, and V are elements that have an effect of improving the tensile strength of the hot-rolled steel sheet by precipitation strengthening or solid solution strengthening. However, the Cu content is less than 0.001%, the Cr content is less than 0.001%, the Mo content is less than 0.001%, the Ni content is less than 0.001%, and the V content. If it is less than 0.005%, a sufficient effect of improving the tensile strength cannot be obtained, the Cu content exceeds 1.0%, the Cr content exceeds 1.0%, and the Mo content is 1. If it exceeds 0%, the Ni content exceeds 1.0%, and the V content exceeds 0.1%, the effect of improving the tensile strength is saturated, resulting in a decrease in economic efficiency. Therefore, when one or more of these are contained, Cu is 0.001% to 1.0%, Cr is 0.001% to 1.0%, and Mo is 0.001% to 1%. 0.0% or less, Ni is 0.001% or more and 1.0% or more, and V is 0.005% or more and 0.1% or less.

Nb: 0.001 to 0.1%
Nb is effective as an element for improving the tensile strength by precipitation strengthening or refinement of the microstructure, or for obtaining the average crystal grain size of the microstructure intended by the present invention. If the amount of Nb added is too small, there is a risk that the intended tensile strength and transition temperature of the present invention may not be obtained, so 0.001% or more must be added. However, when added in a large amount, the temperature of the non-recrystallized region in the hot rolling process expands, and the unrecrystallized rolled texture that increases the X-ray random intensity ratio of the {211} plane becomes a hot rolling process. Many remain after completion. From this viewpoint, the upper limit of Nb is set to 0.1%.

  In the present invention, Zr, Sn, Co, W, and Mg may be contained in total of 1% or less as necessary.

  Next, the reasons for limiting the microstructure, texture, and inclusions of the hot rolled steel sheet according to the present invention will be described.

  The microstructure needs to be a ferrite structure, a bainite structure, or a mixed structure thereof. This is because, in the case of these microstructures, the hardness of the entire microstructure becomes relatively uniform, ductile fracture is suppressed, and the hole expansibility targeted by the present invention can be obtained. Further, in the microstructure, a microstructure called island martensite (MA) which is a mixture of martensite and retained austenite may remain slightly. Since this promotes ductile fracture and degrades the hole expansion rate, etc., it is preferable that it does not remain, but an area fraction of 3% or less is acceptable.

  The microstructure needs to have an average crystal grain size of 6 μm or less. This is because when the average crystal grain size exceeds 6 μm, the target transition temperature of the present invention cannot be obtained.

  The texture needs to have a {211} plane strength of 2.4 or less. The higher the {211} plane strength, the higher the in-plane anisotropy of the r value of the steel sheet, thereby increasing the stress concentration on the end face in the rolling direction during the hole expanding process, and the generation and propagation of cracks. This is because it is promoted and the hole expandability is deteriorated. From this point of view, the {211} plane strength needs to be 2.4 or less in order to obtain the hole expansibility desired by the present invention.

  When the amount of coarse precipitates containing Ti is large, ductile fracture is promoted by the coarse precipitates, and the hole expandability deteriorates. For this reason, from the viewpoint of improving hole expansibility, when a precipitate containing Ti having a sphere equivalent diameter of 100 nm or more is used as a coarse Ti precipitate, the coarse Ti precipitate is included in the coarse Ti precipitate. The proportion of Ti content in the steel in mass% is set to 0.036% or less. In addition, suppressing the precipitation of coarse precipitates containing Ti is preferable also from the viewpoint of promoting the precipitation of fine precipitates containing Ti and obtaining strength by precipitation strengthening.

Further, when a fine precipitate containing Ti having a sphere equivalent diameter of 2.0 nm to 10 nm is used as a fine Ti precipitate, the density of the fine Ti precipitate is 2.0 × 10 ¹⁶ pieces / cm ³ or more. Need to be. If the density of the fine Ti precipitate is smaller than this, the precipitation strengthening ability due to the fine Ti precipitate cannot be obtained sufficiently, and the tensile strength may be lowered. Further, if the density of fine Ti precipitates is 1.0 × 10 ¹⁸ pieces / cm ³ or less, the amount of fine precipitates decreases, so that dislocations easily move, and the crack tip portion Plastic deformation is likely to occur, and the transition temperature can be lowered. From such a viewpoint, the fine Ti precipitate has a density of 1.0 × 10 ¹⁸ pieces / cm ³ or less.

  Next, the manufacturing method for manufacturing the hot rolled steel sheet according to the present invention will be described.

  In the steelmaking process, for example, after obtaining hot metal in a blast furnace, etc., decarburization treatment and alloy addition are performed in a converter, and then the desulfurization treatment, deoxidation treatment, etc. are performed on the molten steel that has been produced using various secondary refining equipment. By carrying out the above, molten steel having the target component content is produced.

  After the molten steel having the target component content is produced by secondary refining, the steel slab is obtained by casting by a method such as thin slab casting in addition to normal continuous casting or casting by an ingot method. When steel slabs are obtained by continuous casting, they may be sent directly to a hot rolling mill as high-temperature steel slabs. In addition to this, they are hot-rolled after being cooled to room temperature and then reheated in a heating furnace. May be. Also, as an alternative to obtaining hot metal with a blast furnace, iron scrap may be used as a raw material, and after melting this in an electric furnace, various secondary refining may be performed to obtain molten steel with a desired component content. .

  Next, manufacturing conditions for hot rolling a steel piece obtained by continuous casting or the like will be described.

  First, a steel piece obtained by continuous casting or the like is heated in a heating furnace. The heating temperature at this time needs to be heated to 1200 ° C. or higher in order to obtain the intended tensile strength of the present invention. When the temperature is lower than 1200 ° C., precipitates containing Ti and Nb are not sufficiently dissolved in the slab, but are coarsened, and precipitation strengthening ability due to the precipitates of Ti and Nb cannot be obtained. Cannot be obtained.

Subsequently, rough rolling is performed on the steel piece extracted from the heating furnace. In the rough rolling step, it is necessary to perform rough rolling under conditions that satisfy the following expression (1). This is because the grain growth of γ grains is suppressed by carrying out a large pressure at a relatively low temperature, the recrystallized γ grains are made finer, and the crystal grains of the microstructure are thereby refined.
[(Thickness before the first reduction after 1150 ° C. or lower) − (Thickness after the last reduction before 1100 ° C. or lower)] / (Before the first reduction after 1150 ° C. or lower) Plate thickness) × 100 ≧ 50 (%) (1)

  Next, the rough rolling end temperature, which is the temperature at which the final rolling of the rough rolling is performed, needs to be 1100 ° C. or higher. This is because if the finish temperature of the rough rolling is lower than this, strain-induced precipitation after the rolling of the rough rolling is promoted, and the amount of precipitation of coarse precipitates containing Ti increases excessively.

  The time from the end of rough rolling to the start of finish rolling needs to be within 100 seconds. This is because if this time exceeds 100 seconds, the amount of coarse precipitates containing Ti excessively increases.

  Subsequently, finish rolling is performed on the steel piece obtained by rough rolling. In the finish rolling step, the end temperature needs to be in a temperature range of 945 ° C. or more and 1035 ° C. or less. The lower limit value of this temperature range was set to 945 ° C. If the finish rolling finish temperature is lower than this, transformation from unrecrystallized γ grains occurs, and the microstructure after the transformation has a [211] plane. This is because many textures that increase the strength are included, leading to a decrease in hole expansibility. In addition, the upper limit value of this temperature range is set to 1035 ° C. The reason is that by lowering the finish rolling end temperature, the recovery and recrystallization of the austenite structure after rolling are delayed, and the nucleus at the time of α transformation in the grain boundaries and grains This is because a large number of generation sites remain, and as a result, the crystal grains of the microstructure can be refined, thereby improving the low temperature toughness.

  Subsequently, the steel plate obtained by the finish rolling process is cooled by a run-out table or the like. In this cooling step, the cooling rate needs to be 20 ° C./sec or more. This is because when the cooling rate is less than 20 ° C./sec, pearlite that causes deterioration of hole expandability and the like is generated, and the average crystal grain size of the microstructure becomes large and the transition temperature is deteriorated. This is because there is a possibility that the object of the present invention cannot be obtained for these characteristic values.

  Further, in this cooling step, it is preferable to perform a three-stage cooling step as described below in order to appropriately precipitate fine precipitates containing Ti and obtain a hot-rolled steel sheet having more excellent tensile strength. In this three-stage cooling process, first stage cooling is performed at a cooling rate of 20 ° C./sec or more, then, the cooling rate is set to 15 ° C./sec or less at a temperature range of 550 ° C. to 650 ° C. Second-stage cooling for at least 2 seconds may be performed, and then third-stage cooling at a cooling rate of 20 ° C./sec or more may be performed.

  The reason why the cooling rate is set to 20 ° C./sec or more in the first stage cooling in the three-stage cooling process is that if the cooling rate is lower than this, pearlite that causes deterioration of hole expandability is generated. This is because there is a risk of it.

  The reason why the cooling rate is set to 15 ° C./sec or less in the second-stage cooling in the three-stage cooling process is that fine precipitates may not be sufficiently precipitated when the cooling rate is higher than this. . The reason why the temperature range for this cooling is set to 550 ° C. or more is that if the temperature range is lower than this, the effect of depositing fine precipitates containing Ti in a short time is reduced. In addition, if the temperature range for this cooling is set to 650 ° C. or less, if the temperature range is higher than this, precipitates containing Ti such as TiC are coarsely precipitated, and sufficient tensile strength may not be obtained. Because there is. Further, in the temperature range above 650 ° C., pearlite may be generated and the hole expanding property may be deteriorated. The reason for this cooling for 2 seconds or more is that if the cooling time is shorter than this, fine precipitates are not sufficiently deposited. The cooling time is preferably 5 seconds or less. This is because, if it exceeds 5 seconds, the precipitate is rather coarsely precipitated, and the tensile strength is lowered. Moreover, when this cooling time exceeds 5 seconds, a pearlite may produce | generate and there exists a possibility of deteriorating hole expansibility.

  The reason why the cooling rate is set to 20 ° C./sec or more in the third stage cooling in the three-stage cooling process is that if the cooling is not performed promptly after the second stage cooling, the precipitate will precipitate coarsely, This is because it may cause a decrease. In addition, when the cooling rate is less than 20 ° C./sec, pearlite is generated, and the hole expandability may be deteriorated.

  In each cooling step, a cooling rate of 20 ° C./sec or more is realized by, for example, water cooling, cooling by mist, and the like, and a cooling rate of 15 ° C./sec or less is realized by, for example, air cooling.

  Then, the steel plate cooled by the cooling process or the three-stage cooling process is wound up by a winding device or the like. In this winding process, it is necessary to wind the steel sheet in a temperature range of 200 ° C. or higher and 540 ° C. or lower. This is because when the steel sheet is wound in a temperature range of less than 200 ° C., the density of fine Ti precipitates is reduced, and as a result, the tensile strength is lowered. In addition, if the steel sheet is wound in a temperature range of more than 540 ° C., excessive precipitates containing Ti may be precipitated, and low-temperature toughness may be deteriorated. It will generate. For this reason, winding temperature shall be 200 degreeC or more and 540 degrees C or less.

  The above is the manufacturing condition of the hot rolling process according to the present invention. After the hot rolling process, skin pass rolling is performed for the purpose of improving ductility and correcting the shape of the steel sheet by introducing movable dislocations. You may do it. Moreover, you may make it pickle for the purpose of the removal of the scale adhering to the surface of the obtained steel plate after completion | finish of a hot rolling process. Further, after completion of hot rolling or after pickling, the obtained steel sheet may be subjected to skin pass rolling or cold rolling inline or offline.

  Moreover, you may make it improve the corrosion resistance of a steel plate by carrying out a plating process by the hot dipping method after completion | finish of a hot rolling process. Further, in addition to hot dipping, alloying treatment may be performed.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  First, molten steels having the steel components shown in Table 1 were produced by melting in a converter and secondary refining. A slab was obtained by continuous casting. Thereafter, hot rolling was performed under the production conditions as shown in Table 2 to obtain a steel sheet having a thickness of 2.9 mm. Table 3 shows the microstructure and texture of the obtained steel sheet and the mechanical properties of the obtained steel sheet. The measuring method of the microstructure, texture, inclusions and the measuring method of mechanical properties are as described above. The underline in Tables 1 and 2 means outside the scope of the present invention or outside the preferred range.

  Steels 1, 3, 4, 9, 16, and 17 satisfy the requirements of the invention, with a tensile strength of 780 (MPa) or higher, a hole expansion rate of 80 (%) or higher, and a fracture surface transition temperature of −80 (° C. ) The following and the characteristic values of the present invention are obtained.

  Steel Nos. 2 and 12 have a lower [211] plane strength because the finish rolling temperature is lower than a predetermined value, and as a result, the hole expandability is low.

  Steel Nos. 5 and 13 have a finish rolling temperature higher than a predetermined value, so that the average grain size of the microstructure is large, and as a result, the transition temperature is high.

  In Steel Nos. 6 and 14, since the time from the end of rough rolling to the start of finish rolling is longer than the predetermined time, the precipitation amount of coarse Ti precipitates is large, and as a result, the hole expandability is low.

  Steel Nos. 7 and 15 have a rough rolling end temperature lower than a predetermined value, so that the amount of coarse Ti precipitates is large, and as a result, the hole expandability is low.

  In Steel No. 8, since the rolling reduction R described in the formula (1) in the rough rolling step is small, the crystal grain size of the microstructure is coarsened, and as a result, the transition temperature is high.

  Steel No. 10 has a winding temperature that is too high, so that pearlite is generated, and as a result, the hole expandability is low. Moreover, since the coiling | winding temperature is too high, the steel number 10 has excessively precipitated the fine Ti precipitate, As a result, the transition temperature is high.

  Steel No. 11 has a coiling temperature that is too low, so the density of fine Ti precipitates is small, and as a result, the strength is low.

  Since the steel number 18 has S higher than a predetermined value, the hole expandability is low.

  In Steel No. 19, since C is lower than a predetermined value, the crystal grain size of the microstructure is large, and as a result, the transition temperature is excessively high. Moreover, since C is lower than a predetermined value, the density of fine Ti precipitates is reduced and the tensile strength is also reduced.

  Since steel No. 20 has C higher than a predetermined value, the hole expandability is low.

  Since steel No. 21 had Mn higher than a predetermined value, cracking occurred during hot rolling, and a hot-rolled steel sheet product could not be obtained.

Claims (6)

% By mass
C: 0.03-0.1%,
Si: 0.001 to 2.0%,
Mn: 0.5 to 3.0%
P: 0.1% or less,
S: 0.005% or less,
Al: 0.005 to 2.0%,
N: 0.02% or less,
Ti: 0.05 to 0.2%,
The balance is a steel plate made of Fe and inevitable impurities,
The microstructure is composed of a ferrite structure, a bainite structure, or a mixed structure thereof, the average crystal grain size is 6.0 μm or less, and the X-ray random intensity ratio of the [211] plane parallel to the rolling surface is 2.4 or less. The ratio of the amount of Ti contained in the precipitate containing Ti having a spherical phase equal diameter of 100 nm or more in mass% in the steel is 0.036% or less, and the spherical phase constant diameter is 2.0 nm or more and 10 nm or less. The density of the precipitate containing Ti is 2.0 × 10 ¹⁶ pieces / cm ³ or more and 1.0 × 10 ¹⁸ pieces / cm ³ or less. High strength hot rolling excellent in formability and low temperature toughness steel sheet. Furthermore, in mass%,
Ca: 0.0001 to 0.005%,
The high-strength hot-rolled steel sheet excellent in formability and low-temperature toughness according to claim 1, characterized in that it contains any one or both of REM: 0.0001 to 0.02%. Furthermore, in mass%,
B: 0.0005 to 0.003% is contained. The high-strength hot-rolled steel sheet having excellent formability and low-temperature toughness according to claim 1 or 2. Furthermore, in mass%,
Cu: 0.001 to 1.0%,
Cr: 0.001 to 1.0%,
Mo: 0.001 to 1.0%,
Ni: 0.001 to 1.0%,
Nb: 0.001 to 0.1%,
V: 0.005 to 0.1% of any one type or two or more types are contained, The high strength hot rolling excellent in formability and low temperature toughness according to any one of claims 1 to 3 steel sheet. A method for producing a high-strength hot-rolled steel sheet excellent in formability and low-temperature toughness according to any one of claims 1 to 4,
A steel slab containing the component according to any one of claims 1 to 4 is heated to 1200 ° C or higher, and in the rough rolling step, rough rolling is performed under conditions satisfying the following expression (1), and rough The final reduction of rolling is performed at 1100 ° C. or more, finish rolling is started within 100 seconds from the end of rough rolling, finish rolling is finished in a temperature range of 945 ° C. or more and 1035 ° C. or less, and then the cooling rate is 20 ° C./sec. A method for producing a high-strength hot-rolled steel sheet excellent in formability and low-temperature toughness, characterized in that cooling is performed as described above, followed by winding in a temperature range of 100 ° C to 540 ° C.
[(Thickness before the first reduction after 1150 ° C. or lower) − (Thickness after the last reduction before 1100 ° C. or lower)] / (Before the first reduction after 1150 ° C. or lower) Plate thickness) × 100 ≧ 50 (1)   When performing the cooling, the cooling is performed at a cooling rate of 20 ° C./sec or higher, followed by cooling for 2 seconds or more at a cooling rate of 15 ° C./sec or lower in a temperature range of 550 ° C. or higher and 650 ° C. or lower. The method for producing a high-strength hot-rolled steel sheet having excellent formability and low-temperature toughness according to claim 5, wherein cooling is performed at a cooling rate of 20 ° C./sec or more.

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