JP6354271B2 - High-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent in low-temperature toughness, uniform elongation and hole expansibility, and a method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent in low-temperature toughness, uniform elongation, and hole expansibility, and a method for producing the same.

  In recent years, for the purpose of improving automobile fuel efficiency, the application of high-strength steel sheets to undercarriage parts has been promoted for the purpose of reducing the weight of the vehicle body. In addition, there is a need to apply high-strength steel sheets to parts having complex shapes, which have so far only been able to use low-strength steel sheets, due to the strengthening of collision safety laws and regulations. However, in general, the higher the strength of a steel sheet, the lower the ductility and the formability deteriorate. Therefore, when applying a high-strength steel sheet to a member having a complicated shape, a steel sheet that satisfies both formability and high strength is manufactured. There is a need. In particular, in an automobile undercarriage part, a hot-rolled high-strength steel sheet is stretch-flange-formed, and therefore, strength and punching hole expansibility as an index for stretch-flange forming are important. Further, in a press-formed product, the narrowing of the steel plate becomes a problem in the strength design of the part, and therefore uniform elongation is important as the steel plate characteristics.

  In Patent Document 1, immediately after finish rolling at a temperature of “Ar3 point + 100 ° C.” or less, the average cooling rate to “Ar3 point−100 ° C.” is set to 400 ° C./s or more to extremely refine the ferrite grains. At the same time, a method for improving the in-plane anisotropy of the strength and improving the ductility and stretch flangeability by leaving a strong <111> texture has been proposed. However, when finish rolling at a temperature of Ar3 point + 100 ° C. or lower, the recrystallization of austenite does not proceed sufficiently, the ferrite structure becomes band-like, or the texture due to transformation from unrecrystallized austenite remains, and the mechanical properties It is insufficient to improve the anisotropy.

  In patent document 2, after adding B, the finishing temperature of hot rolling is increased to suppress the rolling texture, and the colony having the rolling texture is refined, and the lower limit determined by the amount of B on the runout table. By performing rapid cooling above the cooling rate, the recrystallization of austenite is promoted, the {110} plane strength of the rolling texture is reduced, the expansion of inclusions and ferrite crystal grains is suppressed, and the hole expandability is excellent. A method for suppressing the variation in hole expansibility has been proposed.

  Similarly, as a technique for improving the hole expansibility while increasing the strength of the steel sheet, for example, Patent Document 3 discloses that the X-ray random intensity ratio of the {211} plane parallel to the rolling surface is reduced; It has been found that the hole expandability can be improved in combination with the essential element, and the X-ray random intensity ratio of this {211} plane has been found to decrease as the finish rolling finish temperature in the hot rolling process increases. A method has been proposed in which finish rolling is completed at a high temperature to improve hole expansibility. However, although the inventions described in Patent Documents 2 and 3 make it possible to achieve higher strength and improved hole expansibility of the steel sheet, the structure becomes coarse, the low-temperature toughness deteriorates, and the automobile parts are cooled. It was found that anxiety remained when used on the ground.

  On the other hand, as a technique paying attention to the texture improvement by high temperature rolling, in Patent Document 4, the X-ray random intensity ratio in the {211} <011> direction is suppressed to 2.5 or less, and the coarsening by high temperature rolling is performed. In order to suppress this, a method has been proposed in which cooling is started within 1.0 seconds after the final finish rolling to improve low temperature toughness. By making the cooling start time after final finish rolling short, the coarsening of the structure is suppressed, and both low temperature toughness and hole expandability can be achieved. However, this method results in transformation from recrystallized austenite, and since there is little transformation nucleation, the ferrite structure fraction is low, and the grain size of ferrite becomes large, resulting in the recently complicated automotive parts shapes The uniform elongation was not sufficient.

JP 2004-137565 A JP 2009-24226 A JP 2010-90476 A JP 2012-136773 A

  An object of the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more that is excellent in low-temperature toughness, uniform elongation, and hole expandability, and a method for producing the same.

  The present invention, as a result of various investigations, focusing on the technology of starting rapid cooling within 1.0 seconds after high-temperature finish rolling proposed in Patent Document 4 and making fine recrystallized austenite grains, It has been found that by applying light rolling to recrystallized austenite, the low temperature toughness and hole expandability are not deteriorated, and it is possible to improve uniform elongation by suppressing ferrite grain refinement and grain size variation while maintaining good conditions. It was. Specifically, in a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, the finish temperature is increased to randomize the texture of the steel sheet, thereby maintaining good hole expandability, and then a small amount of strain caused by light rolling. Is introduced into the grain boundary of austenite. This increases the frequency of ferrite nucleating from the austenite grain boundaries, promotes ferrite transformation, and makes the ferrite grain size fine and uniform. As a result, it is possible to achieve both excellent low temperature toughness, good uniform elongation and hole expandability.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
(1) By mass%, C: 0.01 to 0.10%, Si: 0.01 to 2.0%, Mn: 0.5 to 2.5%, P: 0.1% or less, S: 0.01% or less, Al: 0.005 to 1.0%, N: 0.01% or less, Ti: 0.01 to 0.20%, consisting of the remainder Fe and inevitable impurities, and an area fraction Thus, {211} <011 in which the ferrite phase has a structure fraction of 40% or more and 90% or less and the balance is a bainite phase and is parallel to the rolling surface of the steel plate and parallel to the rolling direction. > X-ray random intensity ratio of orientation is 2.5 or less, the average grain size of ferrite grains is 5.0 μm or less, and the standard deviation of the grain size is 2.0 μm or less. High-strength hot-rolled steel sheet with a tensile strength of 780 MPa or more and excellent in uniform elongation and hole expansibility.
(2) Further, in terms of mass%, Nb: 0.001% or more, 0.10% or less, B: 0.0005% or more, 0.0030% or less, Ca: 0.0005% or more, 0.0030% or less, Low temperature toughness and uniform elongation according to (1), characterized by containing one or more of Mo: 0.02% or more, 0.5% or less, Cr: 0.02% or more, 1.0% or less High strength hot-rolled steel sheet with a tensile strength of 780 MPa or more and excellent hole expandability.
(3) A method for producing a high-strength hot-rolled steel sheet according to (1) or (2) above, wherein the continuous cast slab of the component according to (1) or (2) is heated to 1200 ° C or higher, Rolling in finish rolling is performed at a stand before the final stand at 960 ° C. or higher and 1100 ° C. or lower at a rolling rate of 20% or higher, and forced cooling is performed between the stands, and rolling at the final stand is performed at Ar 3 points or higher. , At 950 ° C. or less and at a rolling rate of 5% or more and 20% or less, and starts forced cooling within 3 seconds after the end of rolling to 600 ° C. or more and 750 ° C. or less at a cooling rate of 30 ° C./s or more. After cooling and natural cooling for 2 seconds to 10 seconds, cool to 300 ° C to 550 ° C at a cooling rate of 30 ° C / s and wind up at 300 ° C to 550 ° C. Low temperature toughness, uniform elongation and hole expansion characterized by Excellent tensile strength 780MPa above manufacturing method of the high strength hot rolled steel sheet.

  According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet excellent in low-temperature toughness, uniform elongation, and hole expansibility. Therefore, it is possible to provide a high-strength hot-rolled steel sheet suitable for pressed parts that require severe processing. it can. The high-strength hot-rolled steel sheet of the present invention can reduce the weight of automobile bodies such as automobiles, integrate parts, and shorten the machining process, improving fuel efficiency and reducing manufacturing costs. It is a high value.

  The present invention is directed to a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more. And it targets the steel plate excellent in punching hole expansion rate: λ (%), steel plate tensile strength: TS (MPa), and uniform elongation of steel plate: U-El (%) specified in ISO 16630 To do. In order to realize improvement in hole expansibility in such a high-strength steel plate, it is effective to lower the X-ray random strength ratio of the {211} plane parallel to the rolling surface as described in Patent Document 4. is there. However, as a mechanism for improving the hole expandability, not only the random intensity ratio of the {211} plane parallel to the rolling surface, but strictly speaking, the rolling direction is parallel to <011> in the {211} plane {211} It has been found that it is necessary to reduce the X-ray random intensity ratio in the <011> orientation. Specifically, in the high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, which is the subject of the present invention, good hole expansibility can be realized by setting the X-ray random strength ratio to 2.5 or less. The final finish rolling temperature in the hot rolling step is increased to 960 ° C. or higher to promote austenite recrystallization, and the required X-ray random intensity ratio can be obtained. Furthermore, after the final finish rolling temperature is increased to promote recrystallization, rapid cooling is started within 1.0 seconds after the final finish rolling is completed, thereby improving hole expandability and suppressing grain coarsening. Can be compatible.

  However, since the austenite grains are recrystallized, the dislocation density required for nucleation during ferrite transformation is very small, and it takes time for ferrite transformation and a time difference for ferrite grain growth. As a result, when controlling the structure fraction with a hot-roll runout table, it is very difficult to secure a sufficient fraction of the ferrite structure, the variation in the ferrite particle size formed is large, and the uniform elongation is It was low.

  In the present invention, focusing on ferrite nucleation sites and grain growth behavior generated during cooling after hot finish rolling, fine ferrite grains are generated, and ferrite grain refinement and grain size variation are controlled. By doing so, it is excellent in low temperature toughness, uniform elongation and hole expansibility. As the method, in the rolling before the final stand where rolling is performed by finish rolling, the austenite structure is recrystallized at a high temperature and at a high rolling rate, and then a slight strain is introduced into the austenite grain boundary by light rolling. By increasing the number of nucleation of pro-eutectoid ferrite and transforming uniformly, the variation of the average particle size and the particle size is controlled.

  Below, the individual constituent requirements of the present invention will be explained in detail. First, the reasons for limiting the components of the present invention will be described. % With respect to component content means mass%.

  C is an element effective for increasing the strength. In order to obtain the desired strength, it is necessary to contain 0.01% or more. Preferably it is 0.03% or more. However, if it exceeds 0.10%, the workability deteriorates due to the formation of carbides, so the content is made 0.10% or less.

  Si is an element necessary for preliminary deoxidation and is effective for increasing the strength as a solid solution strengthening element. In order to obtain the desired strength, it is necessary to contain 0.01% or more. However, if it exceeds 2.0%, the transformation point becomes excessively high, and it becomes difficult to secure the rolling temperature necessary for the present invention, so the upper limit is made 2.0%.

  Mn is effective for increasing the strength as a solid solution strengthening element. In order to obtain the target strength, 0.5% or more is necessary. In addition to Mn, when an element such as Ti that suppresses the occurrence of hot cracking due to S is not sufficiently added, it is desirable to add an amount of Mn that satisfies Mn / S> 20 by mass%. On the other hand, if added over 2.5%, slab cracking occurs, so the content is made 2.5% or less.

  P is an impurity element that is inevitably contained, and is desirably as low as possible. If it exceeds 0.1%, P is adversely affected on workability and weldability, and fatigue characteristics are also reduced. In order to make a material that can withstand severe molding in a part application subjected to severe processing, the content is preferably 0.02% or less.

  S is an impurity element that is inevitably contained in the same manner as P, and if it is too much, it becomes coarse inclusions such as MnS and deteriorates the moldability, so it is necessary to make it 0.01% or less. In order to make a material that can withstand severe molding in parts that undergo severe processing, the content is preferably 0.005% or less.

  Since Al is an element necessary for deoxidation of molten steel, it is necessary to contain 0.005% or more in order to obtain the effect. However, if added excessively, the transformation point is extremely raised, and it becomes difficult to secure the rolling temperature necessary for the present invention, so the upper limit is made 1.0%.

  N forms precipitates with Ti and Nb at a higher temperature than C, not only reduces Ti effective to fix C, but also forms large sized Ti nitrides that increase variation in hole expansion ratio To do. Therefore, it should be reduced as much as possible, but if it is 0.01% or less, it is an acceptable range. However, if it is excessive, coarse precipitates with Ti are deposited, so 0.005% or less is preferable.

  Ti is one of the most important elements in the present invention. That is, Ti contributes to the strength increase of the steel sheet due to the precipitation effect. However, if the content is less than 0.01%, this effect is insufficient. Even if the content exceeds 0.2%, the effect is not only saturated but also the alloy cost is increased. Therefore, the Ti content is 0.01% or more and 0.2% or less.

  Although not essential to satisfy the required characteristics, it is preferable to add the following elements in order to reduce manufacturing variability or to further improve the strength.

  In the present invention, Nb can further improve the strength by precipitation strengthening. However, if the Nb content is less than 0.001%, a sufficient strength increasing effect cannot be obtained, while if it exceeds 0.10%, the effect is saturated. For the above reasons, the Nb content is set to 0.001% or more and 0.10% or less.

  B can be added as necessary to increase the grain boundary strength and improve toughness. If the B content is less than 0.0005%, a sufficient toughness improving effect cannot be obtained. On the other hand, even if added in an amount of more than 0.003%, the effect is saturated, so the added amount of B is 0.0005% or more. 0.0030% or less.

  Ca is added as necessary to disperse many fine oxides in the deoxidation of molten steel, and is a suitable element for refining the structure. S in the steel is transformed into spherical CaS for desulfurization of the molten steel. It is an element that improves the hole expandability by suppressing the formation of stretched inclusions such as MnS. These effects are obtained when the addition amount is 0.0005%, but since saturation occurs at 0.003%, the Ca content is set to 0.0005% or more and 0.003% or less.

  Mo is an effective element for precipitation strengthening of ferrite. In order to obtain this effect, addition of 0.02% or more is desirable. However, the addition of a large amount makes the slab more susceptible to cracking and makes it difficult to handle the slab, so the upper limit is 0.5%.

  Cr is an element effective for improving the steel sheet strength. In order to obtain this effect, addition of 0.02% or more is necessary. However, the upper limit is set to 1.0% because a large amount of addition decreases ductility.

  Next, the crystal structure of the steel sheet of the present invention will be described.

  The steel sheet of the present invention needs to have an X-ray random strength ratio of 2.5 or less in the {211} <011> orientation parallel to the rolling surface and parallel to the rolling direction. The X-ray random intensity ratio is the intensity ratio of the X-ray intensity of the hot-rolled steel sheet sample to be measured to the X-ray intensity of the powder sample having a random orientation distribution in the X-ray diffraction measurement. In the hot-rolled steel sheet, the hole expandability deteriorates as the X-ray random intensity ratio in the {211} <011> orientation parallel to the rolling surface and parallel to the rolling direction increases. If the X-ray random intensity ratio is 2.5 or less, the hole expansion ratio λ (%) defined by ISO 16630 can be improved.

  The X-ray random intensity ratio is measured by measuring the X-ray diffraction intensity in the {211} <011> direction parallel to the rolling surface and parallel to the rolling direction by using a diffractometer method using an appropriate X-ray tube. And measured by comparison with the diffraction intensity of a random sample. When measurement by X-ray diffraction is difficult, use an EBSD (Electron Back Scattering Diffraction Pattern) method in a region where the measurement interval of pixels is 1/5 or less of the average grain size and 5000 or more crystal grains can be measured. The random intensity ratio may be measured from a pole figure or an ODF (Orientation Distribution Function) distribution.

In a hot-rolled steel sheet, the anisotropy of the steel material increases due to the high X-ray random strength ratio in the {211} <011> orientation parallel to the rolling surface and parallel to the rolling direction. In particular, if the plastic strain ratios (r values) in the rolling direction and the 45 ° direction and the 90 ° direction (sheet width direction) with respect to the rolling direction are defined as r ₀ , r ₄₅ , and r ₉₀ , r _{0 in} this case. And r ₄₅ and r ₉₀ are increased, and r ₉₀ is greatly decreased. As a result, at the time of hole expansion forming, the reduction in the 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 at the end surface, so that cracks are easily generated and propagated. For this reason, it is considered that the hole expansion rate deteriorates when the X-ray random intensity ratio in the {211} <011> orientation parallel to the rolling surface and parallel to the rolling direction is high.

  The steel sheet of the present invention is required to have a ferrite structure fraction of 40% or more and 90% or less. In order to ensure uniform elongation of the steel sheet, it is necessary to ensure a ferrite structure excellent in elongation. The target uniform elongation can be secured when the fraction of the ferrite structure is 40% or more. On the other hand, in a high-strength steel sheet having a tensile strength of 780 MPa or more, if the ferrite structure fraction exceeds 90%, the low-temperature toughness tends to deteriorate rapidly, so it is necessary to make it 90% or less. The balance of the steel sheet structure needs to be bainite. Since bainite has a high steel sheet strengthening ability and is more ductile than martensite, it is an indispensable structure for achieving both the strength, low-temperature toughness, elongation, and hole expandability of the present invention.

  Moreover, the average particle diameter of the ferrite of a steel plate needs to be 5.0 micrometers or less. The ductile brittle transition temperature can be set to −60 ° C. or less by refining the crystal grains of the steel sheet and setting the average particle size to 5.0 μm or less. The average grain size is measured in an area where the measurement interval of pixels is 1/5 or less of the average grain size and 1000 or more crystal grains can be measured using the EBSD method at a depth of 1/4 of the plate thickness. When the angle difference between crystal orientations between adjacent pixels is 5 degrees or more, the grain size is indicated by the diameter of a circle having the same area as the crystal grain, and the average value is calculated by the Area Fraction method. Is.

  Furthermore, the standard deviation of the ferrite grain size needs to be 2.0 μm or less. If the standard deviation is more than 2.0 μm, the tensile test piece is likely to be constricted, the uniform elongation is deteriorated, and cracking is induced at the time of hole expansion, resulting in deterioration of workability.

  Next, a manufacturing method will be described.

  The high-strength hot-rolled steel sheet of the present invention is such that the continuous cast slab having the components of the present invention is heated to 1200 ° C. or higher, and rolling at the stand before the final stand for performing rolling by finish rolling is 960 ° C. or higher and 1100 ° C. or lower. The rolling is performed at a rolling rate of 20% or more, forced cooling is performed between the stands, and rolling at the final stand is performed at an Ar3 point or higher and 950 ° C or lower, and at a rolling rate of 5% or higher and 20% or less, after the rolling is completed. Start forced cooling within 3 seconds, cool to 600 ° C or higher and 750 ° C or lower at a cooling rate of 30 ° C / second or higher, perform natural cooling for 2 seconds to 10 seconds, and then 30 ° C / s This can be achieved by cooling to 300 ° C. or more and 550 ° C. or less at the above cooling rate and performing a winding process at 300 ° C. or more and 550 ° C. or less.

  The slab needs to be homogenized and dissolved in Ti carbonitride before hot rolling. In doing this, the continuously cast slab is reheated. When the reheating temperature is less than 1200 ° C., both homogenization and dissolution become insufficient, resulting in a decrease in strength and a decrease in workability. On the other hand, when it exceeds 1350 ° C., the production cost and productivity are lowered, and the initial austenite grain size is increased, so that it tends to be finally mixed. Therefore, it is necessary to set the temperature to 1200 ° C. or higher, and preferably lower than 1350 ° C.

  After heating the slab, rough rolling is performed, and further finish rolling is performed. In order to avoid leaving an unrecrystallized rolled texture that causes an increase in the X-ray random intensity ratio of the {211} plane, rolling in the stand before the final stand is performed at 960 ° C. or higher. If the rolling temperature is too high, the austenite grain size becomes coarse, and the ferrite transformation is rapidly delayed to make it difficult to secure the ferrite fraction. Therefore, the upper limit rolling temperature is set to 1100 ° C.

  The rolling rate at the stand before the final stand is 20% or more. Thereby, a recrystallization rate can be raised at the said temperature. When the rolling rate is high, there is a concern that the rolling load is restricted and the shape of the steel sheet is deteriorated, but in order to cause recrystallization reliably, the rolling rate is preferably set to 30 to 40%. Thereafter, rolling is performed at the final stand. If the rolling temperature at the final stand is high, the strain introduced by rolling recovers, and it becomes difficult to promote the ferrite transformation and reduce the variation in grain size. Therefore, it is important that the rolling temperature at the final stand is 950 ° C. or lower. More preferably, it is 900 degrees C or less. In order to ensure the rolling temperature of the final stand, forced cooling is performed between the stands to ensure the target temperature. However, when the rolling temperature of the final stand is lower than the Ar3 point, the processed ferrite remains and the uniform elongation is significantly reduced, so the lower limit of the rolling temperature is the Ar3 point. The stand before the final stand may be, for example, the stand before the final stand, or the stand before the final stand, and when the stand before the final stand is the stage stand before the final stand, Without rolling at the stand preceding the last stand, the rolling rate at the last stand is set to 20% or more, and forced cooling is applied between either one of the two stands or between the two stands. Secure the rolling temperature of the stand.

  The rolling rate of the final stand is 5% or more and 20% or less. This rolling rate gives strain to the recrystallized austenite grain boundaries, and is effective in reducing the ferrite grain size and variation. If it is less than 5%, the strain applied to the austenite grain boundary is insufficient, and the desired ferrite fraction cannot be obtained. On the other hand, if it exceeds 20%, unrecrystallized austenite remains, so the X-ray random intensity ratio in the {211} <011> orientation increases, and the hole expandability deteriorates. Although depending on the rolling temperature and the cooling start time, in order to stably obtain the effect of light rolling, the rolling rate is preferably 7% or more and 15% or less.

  It is better to perform forced cooling as soon as possible after rolling. Strain is recovered during the period from processing to forced cooling, and grain growth tends to cause both ferrite grains and austenite grains generated by transformation to become coarse. The amount of strain recovery until the start of cooling varies depending on the rolling temperature and rolling rate, but recovery can be prevented within 3 seconds. In order to efficiently use the strain caused by rolling, it is preferably within 1 second. After the rolling, as primary cooling, cooling is performed at 600 ° C. or higher and 750 ° C. or lower at a cooling rate of 30 ° C./s or higher, and natural cooling (hereinafter referred to as “intermediate air cooling”) is performed for 2 seconds or longer and 10 seconds or shorter. In this temperature range, the nucleation frequency of ferrite is high, and ferrite having a uniform particle size is finely formed. This improves ductility and improves low temperature toughness. When the cooling rate is less than 30 ° C./s, coarsening of austenite is caused, ferrite transformation during intermediate air cooling is delayed, and the desired ferrite fraction cannot be obtained. When the intermediate air cooling start temperature exceeds 750 ° C., the ferrite fraction cannot be sufficiently obtained, the grains become too large, and low temperature toughness cannot be obtained. When the intermediate air cooling start temperature is less than 600 ° C. or the intermediate air cooling time is less than 2 seconds, a predetermined ferrite fraction cannot be obtained and ductility deteriorates. On the other hand, if the intermediate air cooling time exceeds 10 seconds, the ferrite fraction exceeds 90% and the low temperature toughness deteriorates. From the viewpoint of controlling the grain size of the ferrite, it is desirable to set it to 8 seconds or less.

  After the intermediate air cooling is completed, cooling at 30 ° C./s or more is performed. In the present invention, hole expandability and low temperature toughness can be achieved by using austenite remaining after intermediate air cooling as bainite. If it is less than 30 ° C./s, pearlite is precipitated during cooling, and the hole expandability deteriorates.

  Cooling is stopped when the steel sheet temperature is 300 ° C. or higher and 550 ° C. or lower at the cooling rate. If the cooling stop temperature exceeds 550 ° C., the ferrite is excessively transformed and the low-temperature toughness deteriorates. When the cooling stop temperature is less than 300 ° C., martensite is generated, so that the hole expandability deteriorates.

  Then, a steel plate is wound up at 300 degreeC or more and 550 degrees C or less. When the coiling temperature exceeds 550 ° C., the Ti precipitate becomes coarse and the low temperature toughness deteriorates. When the coiling temperature is less than 300 ° C., martensite is generated, so that the hole expandability deteriorates.

  Steels containing the components shown in Table 1 were melted in a converter and slabs having a thickness of 230 mm were formed by continuous casting. Thereafter, the slab was heated to a temperature of 1200 ° C. to 1250 ° C., subjected to rough rolling and finish rolling, and wound after primary cooling, intermediate air cooling, and secondary cooling to produce a hot-rolled steel sheet. Table 2 shows the steel type symbols used, finish rolling conditions, and steel plate thickness. In Table 2, “F5 rolling rate” is the rolling rate of the preceding stand before the final stand, “FT5” is the rolling temperature of the preceding stand before the final stand, “F6 rolling rate” is the rolling rate of the final stand, and “FT6” is The rolling temperature of the final stand, “S cooling amount” is the cooling amount between the last stand of the final stand and the final stand, “Cooling start” is the time from finish rolling to the start of primary cooling, “Primary cooling” is Average cooling rate from finish rolling to intermediate air cooling start temperature, “intermediate temperature” is intermediate air cooling start temperature after primary cooling, “intermediate time” is intermediate air cooling time after primary cooling, and “secondary cooling” is The average cooling rate from the intermediate air cooling to the winding, the “winding temperature” is the temperature after the end of the secondary cooling.

  The steel plate thus obtained was examined for the ferrite and bainite microstructure fraction, the ferrite grain size measurement, and the texture accumulation degree at a depth of 1/4 of the steel plate thickness using an optical microscope. .

  Regarding the structure fraction of ferrite and bainite and the grain size of ferrite in steel sheets, the area ratio of the structure and the average grain diameter and standard deviation of the ferrite are determined by image analysis using an optical microscope after a nital corrosion with a 500 × 500 μm field of view. Asked.

  For the steel sheet tensile test, JIS No. 5 test piece was taken in the rolling width direction (C direction) of the steel sheet, yield strength: YP (MPa), tensile strength: TS (MPa), uniform elongation: U-EL (%) Evaluated. A tensile strength of 780 MPa or more and a uniform elongation of 8% or more were considered good.

  The X-ray random intensity ratio is measured using an EBSD (Electron Back Scattering Diffraction Pattern) method in a region where the pixel measurement interval is 1/5 or less of the average grain size and 5000 or more crystal grains can be measured. Or the random intensity ratio was measured from the distribution of ODF (Orientation Distribution Function). Measure the X-ray diffraction intensity in the {211} <011> direction parallel to the rolling surface and parallel to the rolling direction by using a diffractometer method using an appropriate X-ray tube, etc. You may measure by comparison with.

  The hole expansion rate: λ (%) was evaluated by the method specified in ISO 16630. A hole expansion rate of 80% or more was considered good.

  About the ductile brittle transition temperature, it evaluated by the method prescribed | regulated to JISZ2242. A transition temperature of −53 ° C. or lower was considered good.

  Table 2 shows the evaluation results of the structure fraction, average particle diameter, standard deviation of particle diameter, X-ray intensity ratio, and material.

  As shown in Table 2, the examples of the present invention have a tensile strength of 780 MPa or more, a ferrite phase structure fraction of 40% or more and 90% or less, and the balance being a bainite phase, which is parallel to the rolled surface of the steel sheet. And the X-ray random intensity ratio in the {211} <011> orientation parallel to the rolling direction is 2.5 or less, the average grain size is 5.0 μm or less, and the standard deviation thereof is 2.0 μm or less. And it is a steel plate with excellent uniform elongation and hole expandability.

  On the other hand, Comparative Example 2 has a high intermediate air cooling temperature and a low ferrite fraction because the ferrite transformation did not proceed. Furthermore, since the cooling start time exceeded 3 seconds, the ferrite coarsened, the ferrite grain size exceeded 5.0 μm, and the standard deviation exceeded 2.0 μm, so the uniform elongation and low temperature toughness were inferior.

  In Comparative Example 5, since the rolling ratio of the final stand is less than 5%, the ferrite grain size is more than 5.0 μm, the standard deviation is more than 2.0 μm, the variation is large, and the uniform elongation and the low temperature toughness are inferior.

  In Comparative Example 8, since the rolling temperature of the final stand exceeded 950 ° C., austenite coarsening progressed, and the effect of light rolling was not effective, so the ferrite grain size became coarse, and the standard deviation exceeded 2.0 μm. Therefore, uniform elongation and low temperature toughness are inferior.

  In Comparative Example 12, since the intermediate air cooling time was less than 2 seconds and the ferrite transformation did not proceed sufficiently, the target structure fraction could not be obtained and the uniform elongation was inferior.

  In Comparative Example 16, since the intermediate air cooling time exceeds 10 seconds and the ferrite fraction exceeds 90%, the low temperature toughness is inferior.

  In Comparative Example 17, since the intermediate air cooling temperature is less than 600 ° C., the structure fraction of ferrite cannot be obtained, and the uniform elongation is inferior.

  In Comparative Example 20, the rolling ratio of the final stand is more than 20%, the X-ray random intensity ratio in the {211} <011> orientation is more than 2.5, and the hole expandability is inferior.

  In Comparative Example 24, since the rolling ratio at the stand before the final stand was less than 20%, the recrystallization of austenite was not sufficient, so the X-ray random intensity ratio in the {211} <011> orientation was more than 2.5. Hole expandability is inferior.

  In Comparative Example 29, Mn exceeded the upper limit of the component, and even when the hot rolling conditions were within the scope of the invention, the desired structure fraction could not be obtained due to the generation of MnS stretching inclusions, uniform elongation, hole expansibility, and low temperature. Toughness is inferior.

  In Comparative Example 30, Ti is below the component lower limit, and precipitation strengthening was not sufficient, so that a tensile strength of 780 MPa or more was not obtained.

  In this example, rolling was performed at the final stand and the previous stage stand. However, rolling was performed at the previous stage without rolling at the previous stage stand, and the final stand, the previous stage stand, and the previous stage stand were rolled. The same result can be obtained by forcing between the two stands of the preceding stage stand or between any one of the stands.

Claims (3)

C: 0.01 to 0.10% by mass%,
Si: 0.01 to 2.0%,
Mn: 0.5 to 2.5%
P: 0.1% or less,
S: 0.01% or less,
Al: 0.005 to 1.0%
N: 0.01% or less,
Ti: 0.01-0.20%,
Steel, the balance of Fe and inevitable impurities, the area fraction of the ferrite phase is 40% or more and 90% or less and the balance is a bainite phase, and the steel sheet is rolled. The X-ray random intensity ratio in the {211} <011> orientation parallel to the plane and parallel to the rolling direction is 2.5 or less, and the average grain size of ferrite grains is 5.0 μm or less. A high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent in low-temperature toughness, uniform elongation, and hole expansibility, characterized in that the deviation is 2.0 μm or less. Furthermore, Nb in mass%: 0.001% or more, 0.10% or less,
B: 0.0005% or more, 0.0030% or less,
Ca: 0.0005% or more, 0.0030% or less,
Mo: 0.02% or more, 0.5% or less,
Cr: 0.02% or more, 1.0% or less,
The high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent in low-temperature toughness, uniform elongation, and hole expansibility according to claim 1. A method for producing a high-strength hot-rolled steel sheet according to claim 1 or 2,
The continuous casting slab of the component according to claim 1 or 2 is heated to 1200 ° C or higher, and rolling at a stand before the final stand for performing rolling by finish rolling is 960 ° C or higher and 1100 ° C or lower, and a rolling rate of 20%. Performed above, forced cooling between the stands, and rolling at the final stand is performed at Ar3 point or higher and 950 ° C or lower, and at a rolling rate of 5% or higher and 20% or lower, and forced cooling within 3 seconds after rolling is completed. After cooling to 600 ° C. or more and 750 ° C. or less at a cooling rate of 30 ° C./s or more, and after allowing natural cooling for 2 seconds or more and 10 seconds or less, 300 at a cooling rate of 30 ° C./s or more. A method for producing a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more excellent in low-temperature toughness, uniform elongation and hole expansibility, which is cooled to 300 ° C. or more and 550 ° C. or less. .