Specification
Title of invention: Hot-rolled steel sheet and its manufacturing method
Technical field
[0001]
　The present invention relates to a hot-rolled steel sheet having a tensile strength of 440 MPa or more, which is suitable as a material for automobile structural parts, skeletons, and wheel discs, and has excellent stretch-flangeability and shape-freezing property, and a manufacturing method thereof.
Background technology
[0002]
　As a method for improving the mechanical properties of steel for automobiles, it is known that it is effective to refine the crystal grains in the structure of the steel. Various researches and developments have been conducted on the miniaturization of crystal grains.
[0003]
　For example, in Patent Document 1, in weight %, C: 0.05 to 0.30%, Si: 0.30 to 2.0%, Mn: 1.0 to 2.5%, Al: 0.003. To less than 0.100%, Ti: 0.05 to 0.30%, and the slab containing the balance Fe and unavoidable impurities is heated to a temperature of 950° C. or higher and 1100° C. or lower, and then the reduction rate per pass Of 20% or more is performed at least twice, and hot rolling is performed so that the finish rolling temperature is the Ar 3 transformation point or more, followed by cooling at a cooling rate of 20° C./sec or more, and 350° C. to 550° C. By winding in the temperature range of, the ultrafine particles are characterized in that polygonal ferrite having an average crystal grain size of less than 10 μm has a volume ratio of 75% or more and retained austenite has a volume ratio of 5 to 20%. It has been proposed to produce a high-strength hot-rolled steel sheet having excellent ductility, toughness, fatigue characteristics, and strength-ductility balance.
[0004]
　Further, in Patent Document 2, C: 0.01 to 0.2%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti:0 in weight %. A hot rolled steel sheet containing 0.03 to 0.2%, Al: 0.10% or less, having a composition of balance Fe and unavoidable impurities, having ferrite as a main phase, and having a main phase and second phase particles. And the average particle size of the ferrite is less than 4 μm, the second phase particles contain one or more of pearlite, martensite, bainite and retained austenite, and are represented by the following formula (1): There is proposed a hot-rolled steel sheet excellent in shape fixability, which is characterized in that the work hardening coefficient C of the stress-strain curve is 0.17 or less and the yield elongation YEL is 1.5% or less.
　σ=A×(ε+B) c　(1)
　σ: true stress (MPa), ε: true strain, A, B: constant, C: work hardening coefficient
[0005]
　Further, in Patent Document 3, C: 0.03 to 0.9%, Si: 0.01 to 1.0%, Mn: 0.01 to 5.0%, Al: 0.001 in mass%. % To 0.5%, N: 0.001 to 0.1%, Nb: 0.003 to 0.5%, Ti: 0.003 to 0.5%, the balance being Fe and inevitable impurities. And a steel slab satisfying C%+(12/14)N%≧(12/48)Ti%+(12/48)Nb%+0.03% is as-cast, without rolling or without rolling. After cooling as it is to a temperature of 500°C to room temperature, it is heated to a temperature of Ac 3 point -100°C to less than Ac 3 point and rolled or cooled to a temperature of 500°C to room temperature without rolling. Of 0.1 to 50° C./sec, and again heated to a temperature of 700° C. or lower and 550° C. or higher, and performing hot rolling at a temperature of 700° C. or lower and 550° C. or higher, the reduction ratio of one pass is 20%. As described above, after performing one pass or two or more consecutive passes with the time between passes being 10 seconds or less under the conditions that the strain rate is 1 to 200/sec and the total strain amount is 0.8 or more and 5 or less, A method for producing a high-strength steel having fine crystal grains, which is characterized by allowing to cool, has been proposed. In the example of Patent Document 3, it is specifically shown that the crystal grain size of ferrite is reduced to a minimum of 0.6 μm by this method.
Prior art documents
Patent literature
[0006]
Patent Document 1: Japanese Patent No. 3242303 Publication
Patent Document 2: Japanese Patent Laid-Open No. 2000-290750
Patent Document 3: Japanese Patent No. 4006112
Summary of the invention
Problems to be Solved by the Invention
[0007]
　Since strengthening of materials generally deteriorates material properties such as stretch flangeability and shape fixability, it is necessary to develop high strength hot rolled steel sheet without increasing these material properties. Becomes important above.
[0008]
　However, in the high-strength hot-rolled steel sheet described in Patent Document 1, the structure is a composite structure of ferrite and retained austenite, and the problem that the stretch flangeability is low due to the hardness difference between the structures and ferrite is mainly Since the phase is large, the yield point elongation is large and the shape fixability is poor.
[0009]
　Further, in the hot-rolled steel sheet described in Patent Document 2, since the structure is ferrite and the second phase particles (perlite, martensite, bainite, retained austenite, one or more types), it is caused by the hardness difference between the structures. However, there was a problem that stretch flangeability was low.
[0010]
　Further, in the method for producing high-strength steel described in Patent Document 3, precipitation of carbides and the like may be promoted by interposing a cooling step before rolling, and the subsequent reheating step also has an Ac 3 point of −100° C. Since such a relatively low temperature of less than Ac 3 point makes it difficult to form a solid solution of such a precipitate, and a coarse precipitate remains in the finally obtained structure, and as a result, it is not always sufficient. In some cases, high stretch flangeability could not be achieved.
[0011]
　It is an object of the present invention to solve the above-mentioned problems of the prior art, and to provide a hot-rolled steel sheet having a tensile strength of 440 MPa or more, which is excellent in stretch flangeability and shape fixability, and a manufacturing method thereof.
Means for solving the problem
[0012]
　In order to achieve the above-mentioned object, the inventors of the present invention have made earnest studies on refinement of crystal grains, a method of reducing a hardness difference between ferrite and a residual structure in a hot rolled steel sheet, and improvement of shape fixability. As a result, it has been found that even in a multi-phase structure steel such as ferrite and bainite that has a large hardness difference between the structures, the stretch-flangeability is improved when the average orientation difference of ferrite in the same grain is large. Further, by optimizing the rolling temperature, the strain rate, the time between passes, and the total strain amount in the manufacturing process of the hot rolled steel sheet, ferrite transformation is caused during rolling and the average grain size of ferrite is up to 5.0 μm or less. We have found that it can be miniaturized. And, since high-density dislocations are introduced into the ferrite thus generated, dislocation strengthening occurs, and since the ferrite average orientation difference within the same grain is also large, in the multi-phase structure steel of ferrite and bainite, etc. It has also been found that it is possible to have a high stretch flangeability. Furthermore, they have found that the elongation at yield is small and the shape fixability is excellent because high-density dislocations are introduced into ferrite.
[0013]
　The present invention has been completed through further studies based on such findings. That is, the gist of the present invention is as follows.
　[1]% by mass,
　C: 0.01% or more and 0.20% or less,
　Si: 1.0% or less,
　Mn: 3.0% or less,
　P: 0.040% or less,
　S: 0.004% Hereinafter,
　Al: 0.10% or less,
　N: 0.004% or less
are contained, and the balance is composed of Fe and impurities
　, and the average orientation difference within the same grain is 0.5° or more and 5.0 or more. The structure of at least one of
　bainite and a second ferrite having an average misorientation in the same grain of 0° or more and less than 0.5° , containing 30% by volume or more and 70% by volume or less of the first ferrite having a temperature of 0° or less. And the first ferrite is included in a total amount of 95% by volume or more, the
　balance structure is 5% by volume or less,
　the average crystal grain size of the first ferrite is 0.5 μm or more and 5.0 μm or less, and The average crystal grain size of at least one type of structure is 1.0 μm or more and 10 μm or less, and when the remaining structure is present, the average crystal grain size of the remaining structure is 1.0 μm or more and 10 μm or less. , Hot rolled steel sheet.
　[2] Further, in mass%,
　Nb: 0.01% or more and 0.20% or less,
　Ti: 0.01% or more and 0.15% or less,
　Mo: 0.01% or more and 1.0% or less,
　Cu: 0.01% or more and 0.5% or less, and
　Ni: 0.01% or more and 0.5% or less
, or one or more kinds selected from The hot-rolled steel sheet according to the above [1], which comprises:
　[3] (a) A steel material having the composition according to the above [1] or [2] is hot-rolled as it is without cooling after casting, or is once cooled to room temperature, and then at 1100°C or higher and 1350°C or lower. A hot rolling step of heating and hot rolling, wherein the hot rolling step includes finish rolling by continuously passing a steel material after casting through a plurality of rolling stands, the finish rolling Rolling temperature at all rolling stands is A point or higher, and continuous rolling of 2 passes or more including the final pass of the finish rolling is performed at rolling temperature: A point or higher and less than Ae 3 point, strain rate: 1.0 to 50/sec and time between passes: Hot rolling process performed under conditions of 10 seconds or less, and the total strain amount of all passes satisfying the above conditions is 1.4 or more and 4.0 or less,
　(b) finishing A cooling step of cooling the rolled steel sheet at an average cooling rate of 20° C./second or more and 50° C./second or less, the cooling step being started within 10 seconds after the hot rolling step, and
　(c ) A
method of manufacturing a hot-rolled steel sheet, comprising a winding step of winding the steel sheet in a temperature range of 300°C or higher and 600°C or lower .
　Here, point A is the temperature determined by the following (Equation 1), and point Ae 3 is the temperature determined by the following (Equation 2).
　A (°C)=910-310C-80Mn-20Cu-55Ni-80Mo (Formula 1)
　Ae 3 (°C)=919-266C+38Si-28Mn-27Ni+12Mo (Formula 2) In the
　formula, C, Si, Mn, Cu, Ni and Mo is the content (mass %) of each element.
Effect of the invention
[0014]
　According to the present invention, it is possible to obtain a hot-rolled steel sheet having high strength and excellent stretch flangeability and shape fixability, and if the present invention is applied to a structural part of an automobile or the like, in order to ensure safety of the automobile. It is possible to obtain high strength without deteriorating workability such as press moldability.
MODE FOR CARRYING OUT THE INVENTION
[0015]

　The hot-rolled steel sheet of the present invention has a predetermined composition and contains 30% by volume or more of 70% by volume of the first ferrite having an average orientation difference of 0.5° or more and 5.0° or less within the same grain. 95% by volume of at least one structure of bainite and a second ferrite having an average orientation difference within the same grain of 0° or more and less than 0.5° and the first ferrite in total of less than or equal to 95% by volume. Including the above, the balance structure is 5% by volume or less, the average crystal grain size of the first ferrite is 0.5 μm or more and 5.0 μm or less, and the average crystal grain size of the at least one structure is 1.0 μm. When the residual structure is present, the average crystal grain size of the residual structure is 1.0 μm or more and 10 μm or less.
[0016]
　Hereinafter, the hot rolled steel sheet of the present invention will be specifically described. First, the reasons for limiting the chemical components (composition) of the hot-rolled steel sheet of the present invention will be described. In addition, all the% showing the following chemical components mean the mass %.
[0017]
[C: 0.01% or more and 0.20% or less]
　C is utilized as a solid solution strengthening element for achieving desired strength. For that purpose, at least 0.01% is required. The C content may be 0.02% or more, 0.04% or more, or 0.05% or more. On the other hand, C exceeding 0.20% deteriorates workability and weldability. Therefore, the C content is 0.20% or less. The C content may be 0.18% or less, 0.16% or less, or 0.15% or less.
[0018]
[Si: 1.0% or less]
　Si is an element that suppresses coarse oxides and cementite that deteriorate toughness and contributes to solid solution strengthening, but if the content exceeds 1.0%, hot rolled steel sheet The surface properties of the are markedly deteriorated, resulting in deterioration of chemical conversion treatability and corrosion resistance. Therefore, the Si content is set to 1.0% or less. It is preferably 0.9% or less or 0.8% or less. The Si content may be 0%, for example, 0.01% or more, 0.02% or more, or 0.4% or more.
[0019]
[Mn: 3.0% or less]
　Mn is an element that forms a solid solution and contributes to an increase in strength of steel. On the other hand, when Mn exceeds 3.0%, not only the effect is saturated, but also a band-like structure is formed by solidification segregation to deteriorate workability and delayed fracture resistance. Therefore, the Mn content is 3.0% or less. Preferably it is 2.8% or less or 2.0% or less. The Mn content may be 0%, for example 0.5% or more, 1.0% or more, or 1.4% or more.
[0020]
[P: 0.040% or less]
　P is an element that forms a solid solution and contributes to the increase in strength of steel, but is an element that segregates at grain boundaries, particularly old austenite grain boundaries, and causes deterioration of low temperature toughness and workability. But also. Therefore, the P content is preferably reduced as much as possible, but the content up to 0.040% is acceptable. Therefore, the P content is 0.040% or less. It is preferably 0.030% or less, more preferably 0.020% or less. The P content may be 0%, but even if it is excessively reduced, the effect commensurate with the increase in refining cost cannot be obtained. Therefore, it is preferably 0.001%, 0.002% or more, 0.003% or more. Alternatively, it is 0.005% or more.
[0021]
[S: 0.004% or less]
　S combines with Mn to form coarse sulfides, and reduces the workability of the hot rolled steel sheet. Therefore, the S content is preferably reduced as much as possible, but the S content up to 0.004% is acceptable. Therefore, the S content is 0.004% or less. It is preferably 0.003% or less, more preferably 0.002% or less. The S content may be 0%, but even if it is excessively reduced, an effect commensurate with the increase in the refining cost cannot be obtained. Therefore, it is preferably 0.0003% or more, 0.0005% or more or 0.001%. That is all.
[0022]
[Al: 0.10% or less]
　Al acts as a deoxidizer and is an element effective in improving the cleanliness of steel. However, excessive addition of Al causes an increase in oxide inclusions, which lowers the toughness of the hot rolled steel sheet and causes defects. Therefore, the Al content is 0.10% or less. It is preferably 0.09% or less, more preferably 0.08% or less. The Al content may be 0%, but even if it is excessively reduced, an effect commensurate with the increase in refining cost cannot be obtained, and therefore, it is preferably 0.005% or more, 0.008% or more or 0.01%. That is all.
[0023]
[N: 0.004% or less]
　N is precipitated as a nitride by combining with a nitride-forming element and contributes to refinement of crystal grains. However, if it exceeds 0.004%, it comes to exist as a solid solution N, and reduces the toughness. Therefore, the N content is 0.004% or less. It is preferably 0.003% or less. The N content may be 0%, but it is preferably 0.0005% or more, 0.0008% or more or 0.001% because the effect corresponding to the increase of the refining cost cannot be obtained even if it is excessively reduced. That is all.
[0024]
　The above are the basic components of the hot-rolled steel sheet of the present invention. However, the hot-rolled steel sheet of the present invention may have Nb: 0.01% or more and 0.20 or more, if necessary, for the purpose of, for example, improving the toughness and the strength. %, Ti: 0.01% or more and 0.15% or less, Mo: 0.01% or more and 1.0% or less, Cu: 0.01% or more and 0.5% or less, and Ni: 0.01% or more. One or more selected from 0.5% or less can be contained.
[0025]
[Nb: 0.01% or more and 0.20% or less]
　Nb is an element that contributes to the increase in the strength and fatigue strength of the steel sheet through the formation of carbonitrides. In order to exhibit such effects, the Nb content needs to be 0.01% or more. For example, the Nb content may be 0.02% or more or 0.03% or more. On the other hand, when the Nb content exceeds 0.20%, the deformation resistance increases, so the rolling load of hot rolling during the production of hot-rolled steel sheet increases, and the burden on the rolling mill becomes too large, resulting in rolling operation. That can be difficult. When the Nb content exceeds 0.20%, coarse precipitates are formed and the toughness of the hot rolled steel sheet tends to be reduced. Therefore, the Nb content is 0.20% or less. For example, the Nb content may be 0.15% or less or 0.10% or less.
[0026]
[Ti: 0.01% or More and 0.15% or Less]
　Ti improves the strength and fatigue strength of the steel sheet by forming fine carbonitrides and refining the crystal grains. In order to exert such effects, the Ti content needs to be 0.01% or more. For example, the Ti content may be 0.02% or more, 0.04% or more, or more than 0.05%. On the other hand, when the Ti content exceeds 0.15% and becomes excessive, the above-mentioned effects are saturated, and coarse precipitates are increased, resulting in a decrease in toughness of the steel sheet. Therefore, the Ti content is 0.15% or less. It is preferably 0.14% or less or 0.10% or less.
[0027]
[Mo: 0.01% or more and 1.0% or less]
　Mo is an element that contributes to strengthening steel as a solid solution element. In order to obtain such an effect, the Mo content needs to be 0.01% or more. For example, the Mo content may be 0.02% or more or 0.03% or more. However, Mo has a high alloy cost, and if it exceeds 1.0%, it deteriorates the weldability. Therefore, the Mo content is 1.0% or less. It is preferably 0.5% or less or 0.4% or less.
[0028]
[Cu: 0.01% or more and 0.5% or less]
　Cu is an element that forms a solid solution and contributes to an increase in strength of steel. In order to obtain this effect, the Cu content needs to be 0.01% or more. For example, the Cu content may be 0.05% or more or 0.1% or more. However, if the Cu content exceeds 0.5%, the surface properties of the hot-rolled steel sheet deteriorate. Therefore, the Cu content is 0.5% or less. The range is preferably 0.4% or less or 0.3% or less.
[0029]
[Ni: 0.01% or more and 0.5% or less]
　Ni is an element that forms a solid solution to contribute to an increase in the strength of steel and improves toughness. To obtain these effects, the Ni content needs to be 0.01% or more. For example, the Ni content may be 0.02% or more or 0.1% or more. However, Ni has a high alloy cost, and if it exceeds 0.5%, the weldability is deteriorated. Therefore, the Ni content is 0.5% or less. It is preferably 0.4% or less or 0.3% or less.
[0030]
　Other elements may be contained within a range that does not impair the effects of the present invention. That is, the balance may be substantially iron. For example, 0.005% or less of Ca, REM (rare earth metal: Rare-Earth Metal) or the like may be contained for the purpose of improving delayed fracture resistance. A trace element or the like that improves hot workability may be contained.
[0031]
　In the hot-rolled steel sheet of the present invention, the balance other than the above components is Fe and impurities. Here, the impurities are components that are mixed by various factors of the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing the hot rolled steel sheet, and the hot rolling of the present invention. It includes those which are not intentionally added to the steel sheet. Further, the impurity is an element other than the components described above, the elements contained in the hot-rolled steel sheet at a level at which the effect specific to the element does not affect the properties of the hot-rolled steel sheet according to the present invention. It is to include.
[0032]
　Next, the reasons for limiting the structure of the hot rolled steel sheet according to the present invention will be described.
[0033]
[First ferrite having an average orientation difference within the same grain of 0.5° to 5.0°: 30% by volume to 70% by volume]
　The structure of the hot-rolled steel sheet of the present invention has an average orientation difference within the same grain. Includes 30% by volume or more and 70% by volume or less of the first ferrite having an angle of 0.5° or more and 5.0° or less.
[0034]
　Here, in the present invention, the “average orientation difference within the same grain” exists in a certain crystal grain when the orientation difference between adjacent grains is 15° or more is defined as one crystal grain. It is an index indicating the disorder of the crystal. Most ferrites produced by ordinary ferrite transformation have an average orientation difference of 0.0° within the same grain. On the other hand, when the ferrite transformation occurs during rolling as in the present invention, the ferrite is also processed, so that crystal disorder occurs in the ferrite grains and the average orientation difference in the same grains increases. In order to reduce the hardness difference with bainite and the yield point elongation, the average orientation difference within the same grain needs to be 0.5° or more. On the other hand, if the average orientation difference within the same grain exceeds 5.0°, the ductility of ferrite deteriorates. Therefore, the average orientation difference within the same grain is 0.5° or more and 5.0° or less. It is more preferably 0.7° or more and 3.0° or less.
[0035]
　In the hot-rolled steel sheet according to the present invention, when the content of the first ferrite is less than 30% by volume, the volume ratio of austenite at the final stage of finish rolling is more than 70%, and bainite and the same grain within the same grain are generated by the subsequent cooling step. Since the fraction of the second ferrite having an average orientation difference of less than 0.5° increases, the yield point elongation increases and the shape fixability decreases. Therefore, the volume ratio of the first ferrite is set to 30% by volume or more. Further, in order to increase the volume ratio of the first ferrite, it is necessary to increase the reduction ratio during hot rolling or to lower the temperature during hot rolling, but the conditions were set to exceed 70% by volume. In this case, the average orientation difference within the same grain exceeds 5.0°, the ductility of ferrite may deteriorate, and the stretch flangeability may deteriorate. Therefore, the volume ratio of the first ferrite is set to 30% by volume or more and 70% by volume or less. It is preferably at least 35% by volume, at least 40% by volume or at least 50% by volume, and/or at most 65% by volume or at most 60% by volume.
[0036]
[Bainite and at least one kind of second ferrite having an average orientation difference of 0° or more and less than 0.5° in the same grain, a total of 95% by volume or more of the first ferrite, and a residual structure of 5% by volume Hereinafter, the
　hot-rolled steel sheet according to the present invention is a total of at least one structure of bainite and second ferrite having an average orientation difference within the same grain of 0° to less than 0.5° and the first ferrite. And 95% by volume or more, preferably 98% by volume or more or 100% by volume. The balance structure is not particularly limited, but includes, for example, either one or both of martensite and retained austenite, or consists of one or both of martensite and retained austenite. If the balance microstructure exceeds 5% by volume, the decrease in stretch flangeability due to the difference in hardness between the balance microstructure and the structure of the second ferrite or bainite becomes remarkable, making it difficult to have the desired stretch flangeability. And/or, particularly, when the volume ratio of martensite as the residual structure increases, the yield ratio increases and the shape fixability decreases. Therefore, the remaining structure is 5% by volume or less. It is more preferably 2% or less, and may be 0% by volume.
[0037]
[Average crystal grain size of first ferrite: 0.5 μm or more and 5.0 μm or less] In the
　present invention, “average crystal grain size” means one crystal grain having an orientation difference of 15° or more between adjacent grains. The value calculated when defined. If the average crystal grain size of the first ferrite exceeds 5.0 μm, it becomes difficult to obtain the desired strength and the toughness deteriorates. Therefore, the average crystal grain size needs to be 5.0 μm or less. .. On the other hand, in order to make the average crystal grain size smaller than 0.5 μm, large strain processing is required during rolling, a large load is applied to the rolling mill, and the average orientation difference within the same grain exceeds 5.0°. Because it is more likely. Therefore, the average crystal grain size is 0.5 μm or more. Therefore, the average crystal grain size of the first ferrite is 0.5 μm or more and 5 μm or less, preferably 0.7 μm or more or 1.0 μm or more, and/or 4.5 μm or less or 4.0 μm or less.
[0038]
[Average grain size of at least one structure of bainite and second ferrite and residual structure: 1.0 μm or more and 10 μm or less] Average grain size of
　bainite, second ferrite, and residual structure when present Is greater than 10 μm, the strength decreases, the elongation at yield increases and the shape fixability deteriorates. Therefore, the average crystal grain size of these structures is set to 10 μm or less. However, particularly when bainite is refined to 1.0 μm or less, the strength is remarkably increased, the hardness difference from the first ferrite becomes large, and the stretch flangeability may be deteriorated. Therefore, the average crystal grain size of these structures is set to 1.0 μm or more. It is preferably 1.5 μm or more or 2.0 μm or more, and/or 9.0 μm or less, 8.0 μm or less or 5.0 μm or less.
[0039]
　In the hot-rolled steel sheet according to the present invention, the identification of each phase or structure and the calculation of the average crystal grain size are performed by image processing using a structure photograph taken by a scanning electron microscope and backscattered electron diffraction image analysis (EBSP or EBSD). Can be done by
[0040]
　More specifically, the volume ratio of the first ferrite is determined as follows. When the plate width of the steel plate is W, a cross section (width direction cross section) of the steel plate in the width direction viewed from the rolling direction is observed at a position of 1/4 W (width) or 3/4 W (width) from one end in the width direction of the steel plate. A sample is taken to be a surface, and a rectangular region of 200 μm in the width direction of the steel plate×100 μm in the thickness direction is subjected to EBSD analysis at a measurement interval of 0.2 μm at a position of ¼ depth of the plate thickness from the surface of the steel plate. Here, the EBSD analysis is performed at an analysis speed of 200 to 300 points/sec using, for example, an apparatus including a thermal field emission scanning electron microscope and an EBSD detector. Here, the orientation difference is obtained by calculating the difference in crystal orientation between adjacent measurement points based on the crystal orientation information of each measurement point measured as described above. When this orientation difference is 15° or more, the middle of the adjacent measurement points is determined as a grain boundary, and the region surrounded by this grain boundary is defined as a crystal grain in the present invention. The average orientation difference is calculated by simply averaging the orientation differences of the crystal grains within the same grain. Then, the area ratio of the crystal grains of the first ferrite is obtained, and this is taken as the volume ratio of the first ferrite. Further, the volume ratio of the second ferrite is similarly determined. The average orientation difference within the same grain can be calculated using software attached to the EBSD analysis device. In addition, bainite may have an average orientation difference of 0.5° or more within the same grain, but bainite contains carbides and has a lath-like structure, so that SEM images include carbides and lath-like structures. Bainite having the structure of is the area ratio of bainite.
[0041]
　"First ferrite having an average orientation difference of 0.5° to 5.0° in the same grain" and "second ferrite having an average orientation difference of 0° to less than 0.5° in the same grain" in the present invention The average crystal grain size of each of the "bainite" and the "remainder structure" is determined using the values ​​obtained by the above EBSD analysis. Specifically, a boundary having an orientation difference of 15° or more is set as a grain boundary, and a value calculated by the following formula is set as an average crystal grain size. In the formula, N is the number of crystal grains included in the evaluation region of the average crystal grain size, Ai is the area of ​​the i-th (i=1, 2,..., N) grain, and di is the circle of the i-th crystal grain. Indicates the equivalent diameter. These data are easily determined by EBSD analysis.
[Number 1]

[0042]
　According to the present invention, a hot rolled steel sheet having high strength and excellent stretch flangeability and shape fixability can be obtained by satisfying the above chemical components (composition) and structure. Therefore, when the hot rolled steel sheet according to the present invention is applied to a structural part of an automobile or the like, it is possible to obtain high strength required for ensuring the safety of the automobile without deterioration of workability such as press formability.
[0043]

　Next, a method of manufacturing the hot rolled steel sheet according to the present invention will be described.
[0044]
　The method for producing a hot-rolled steel sheet according to the present invention is as follows:
　(a) a steel material having the chemical composition (composition) described above is hot-rolled as it is without cooling after casting, or is once cooled to room temperature, and then 1100 A hot rolling step of heating to 1050° C. or more and 1350° C. or less and hot rolling, wherein the hot rolling step is to finish rolling by continuously passing the cast steel material through a plurality of rolling stands. Including, the rolling temperature in all the rolling stands of the finish rolling is A point or more, and two or more continuous rolling including the final pass of the finish rolling, rolling temperature: A point or more and less than Ae 3 points, strain Hot rolling performed under the conditions of speed: 1.0 to 50/sec, and time between passes: 10 seconds or less, and the total strain amount of all passes satisfying the above conditions is 1.4 or more and 4.0 or less. And
　(b) a cooling step of cooling the finish-rolled steel sheet at an average cooling rate of 20° C./second or more and 50° C./second or less, the cooling being started within 10 seconds after the hot rolling step. The method is characterized by including a cooling step and
　(c) a winding step of winding the steel sheet in a temperature range of 300° C. or higher and 600° C. or lower
.
　Here, point A is the temperature determined by the following (Equation 1), and point Ae 3 is the temperature determined by the following (Equation 2).
　A(°C)=910-310C-80Mn-20Cu-55Ni-80Mo (Formula 1)
　Ae 3 (°C)=919-266C+38Si-28Mn-27Ni+12Mo (Formula 2)
　In the formula, C, Si, Mn, Cu, Ni and Mo are the contents (mass %) of each element.
[0045]
　Hereinafter, the manufacturing method of the present invention will be described in detail.
[0046]
[(A) Hot Rolling Step] The
　hot rolling step includes finish rolling by continuously passing a cast steel material having the above-described chemical composition (composition) through a plurality of rolling stands. .. Further, descaling may be performed before the finish rolling or during the rolling between the rolling stands in the finish rolling. In the method of the present invention, finish rolling is carried out at a low strain rate to cause ferrite transformation during rolling, as will be explained later. Therefore, it is preferable that the finish rolling is performed by the direct casting rolling in which the continuous casting and the finish rolling which are easily rolled at such a low strain rate are connected. However, methods such as slab reheating-rough rolling-finish rolling, which are general hot rolling methods, may be adopted. In that case, the slab heating temperature is set to 1100° C. or higher for homogenizing the slab, and 1350° C. or lower for preventing coarsening of the austenite grain size. Further, the method for producing a steel material is not limited to a particular method, and molten steel having the above-mentioned chemical components is melted in a converter or the like and is usually used as a steel material such as a slab by a casting method such as continuous casting. Any of the above methods can be applied.
[0047]
(Rolling temperature in all rolling stands of finish rolling: A point or higher) In
　the method of the present invention, the finish rolling is performed by using the as-cast steel material, that is, the steel material immediately after casting or the steel material after heating in a plurality of rolling stands. The rolling temperature in all rolling stands of finish rolling is not less than the point A calculated by the following (Formula 1).
　A (° C.)=910-310C-80Mn-20Cu-55Ni-80Mo (Formula 1) In the
　formula, C, Mn, Cu, Ni and Mo are the contents (mass %) of each element.
　When it is less than the point A, in addition to the ferrite transformation during rolling, the ferrite transformation is accompanied by the lowering of the temperature. Ferrite produced by the latter ferrite transformation has a large crystal grain size, which causes a reduction in tensile strength and toughness. Further, the generation of such ferrite makes it difficult to control the tissue fraction. Therefore, the temperature in all rolling stands must be A point or higher. For example, the temperature in all rolling stands may be 1100°C or lower.
[0048]
(Rolling temperature of continuous rolling of two or more passes including the final pass of finish rolling: A point or more and less than Ae 3 points)
　When the rolling temperature becomes Ae 3 points or more required by the following (formula 2), ferrite is rolled during rolling. Since it becomes difficult to transform, it is set to less than Ae 3 point.
　Ae 3 (° C.)=919-266C+38Si-28Mn-27Ni+12Mo (Formula 2) In the
　formula, C, Si, Mn, Ni and Mo are the contents (mass %) of each element.
　On the other hand, when the temperature is lower than the point A, in addition to the ferrite transformation during rolling, the ferrite transformation occurs due to the lowering of the temperature. Ferrite produced by the latter ferrite transformation has a large crystal grain size, which causes a reduction in tensile strength and toughness. Further, the generation of such ferrite also makes it difficult to control the tissue fraction. Therefore, the rolling temperature of continuous rolling of two or more passes including the final pass of finish rolling is set to A point or more and less than Ae 3 point.
[0049]
(Strain rate of continuous rolling of two or more passes including the final pass of finish rolling: 1.0 to 50/sec) In
　order to cause ferrite transformation during rolling, a low strain rate is preferable. If the strain rate exceeds 50/sec, the amount of reduction necessary for ferrite transformation increases, and the load on the rolling mill increases. Further, the heat generated during processing increases, and the rolling temperature is likely to reach the Ae 3 point or higher. Therefore, the strain rate is 50/sec or less. Further, when the strain rate is less than 1.0/sec, the influence of heat removal by the rolls of the rolling mill becomes large, and the rolling temperature is likely to be less than point A. Therefore, the strain rate is set to 1.0/second or more and 50/second or less. It is more preferably 1.5/sec or more and 30/sec or less.
[0050]
(Time between two or more continuous passes including the final pass of finish rolling: 10 seconds or less) The
　time between passes influences strain recovery and recrystallization behavior between rolling stands. If the time between passes exceeds 10 seconds, recovery and recrystallization of strain between stands occur, and the strain accumulated in the previous rolling pass is released, making it difficult to cause ferrite transformation during rolling. Become. Therefore, the time between passes should be within 10 seconds. It is preferably within 8.5 seconds, within 7 seconds or within 5 seconds. For example, the inter-pass time may be 1 second or more.
[0051]
(Total strain amount: 1.4 or more and 4.0 or less)
　Continuous rolling of two or more passes including the final pass of the finish rolling, rolling temperature: A point or more and Ae less than 3 points, strain rate: 1.0 to 50 /Sec, and time between passes: The total strain amount of all passes satisfying the condition of 10 seconds or less is set to 1.4 or more and 4.0 or less. This total strain amount has a great influence on the ferrite transformation amount during rolling and the refinement of the balance bainite and ferrite. If the total strain amount is less than 1.4, it is difficult to generate a sufficient amount of ferrite transformation, and the crystal grain size of the balance bainite or ferrite becomes coarse. On the other hand, when the total strain amount exceeds 4.0, the average orientation difference within the same grain of the ferrite generated during rolling exceeds 5.0°, and the ductility of the ferrite deteriorates. Therefore, the total strain amount is 1.4 or more and 4.0 or less. It is preferably 1.6 or more and 3.5 or less.
[0052]
　If the above rolling conditions are not continuous, it becomes impossible to cause ferrite transformation during rolling and/or the ferrite produced during rolling undergoes reverse transformation to austenite, resulting in the first ferrite in the final structure. The fraction decreases and the shape fixability of the obtained hot rolled steel sheet deteriorates. Even when the final pass does not satisfy the rolling conditions, the reverse transformation from ferrite to austenite occurs in the final pass, the first ferrite fraction in the final structure decreases, and the recovery of ferrite occurs, so that the yield point elongation increases. Becomes larger and the shape fixability deteriorates. Alternatively, when the rolling temperature in the final pass becomes lower than the point A, ferrite transformation occurs due to the temperature reduction in addition to ferrite transformation during rolling, and the latter ferrite transformation has a large crystal grain size. However, the tensile strength is reduced. Therefore, continuous rolling of two or more passes including the final pass of finish rolling is performed under conditions of rolling temperature: A point or more and less than Ae 3 point, strain rate: 1.0 to 50/sec, and time between passes: 10 sec or less. It is necessary to perform so that the total strain amount of all the paths below and satisfying the condition is 1.4 or more and 4.0 or less.
[0053]
(Rough Rolling) In
　the method of the present invention, for example, in order to adjust the plate thickness and the like, rough rolling may be performed on the steel material before finish rolling. The rough rolling may be carried out as long as a desired sheet bar size can be secured, and the conditions therefor are not particularly limited.
[0054]
[(B) Cooling Step]
　According to the method of the present invention, the finish-rolled steel sheet is cooled in the cooling step at an average cooling rate of 20° C./sec or more and 50° C./sec or less, and the cooling is performed by the above heat treatment. It is started within 10 seconds after the hot rolling process. If it exceeds 10 seconds from the end of the hot rolling process to the start of cooling, ferrite recovery occurs, yield point elongation increases, and the shape fixability of the obtained hot-rolled steel sheet deteriorates. Preferably, the cooling is started within 9 seconds or within 8 seconds after the hot rolling process. On the other hand, if the average cooling rate is less than 20° C./sec, the strain in the ferrite generated during rolling recovers and softens, the yield point elongation increases, and the shape fixability deteriorates. If the cooling rate exceeds 50°C/sec, martensite is likely to be generated. Therefore, the average cooling rate of cooling after the hot rolling step is set to 20° C./sec or more and 50° C./sec or less. It is preferably 30° C./s or more and 45° C./s or less.
[0055]
[(C) Winding Step]
　The steel sheet cooled to the cooling stop temperature in the cooling step is wound in the temperature range of 300°C to 600°C in the winding step. Since the steel sheet is wound immediately after the cooling process, the winding temperature is almost equal to the cooling stop temperature. When the coiling temperature exceeds 600° C., recovery occurs in the first ferrite, the strength decreases, and the yield point elongation increases and the shape fixability decreases. On the other hand, if the temperature is lower than 300°C, martensite is generated, the yield ratio increases, and the shape fixability decreases. Therefore, the coiling temperature, which is the cooling stop temperature, is 300° C. or more and 600° C. or less. For example, the winding temperature may be 320°C or higher or 350°C or higher, and/or 580°C or lower or 550°C or lower.
[0056]
　After winding, the hot rolled steel sheet may be subjected to temper rolling according to a conventional method, or may be subjected to pickling to remove the scale formed on the surface. Alternatively, plating treatment such as hot dip galvanizing and electrogalvanizing or chemical conversion treatment may be further performed.
[0057]
　After casting a steel material having the same composition as described for the hot-rolled steel sheet of the present invention, hot rolling, and subsequent cooling and winding operations are performed as described above, whereby the first ferrite is reduced to 30%. The content of at least one of bainite and second ferrite and the first ferrite in total of 95 vol% or more, and the balance of 5 vol% or less, and When the average crystal grain size of the ferrite of No. 1 is 0.5 μm or more and 5.0 μm or less, the average crystal grain size of the at least one structure is 1.0 μm or more and 10 μm or less, and the remaining structure is present, A hot-rolled steel sheet having an average crystal grain size of the balance structure of 1.0 μm or more and 10 μm or less can be reliably manufactured. Therefore, according to the above manufacturing method, it is possible to provide a hot-rolled steel sheet having a tensile strength of 440 MPa or more, which is excellent in stretch flangeability and shape fixability.
[0058]
　Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.
Example
[0059]
　Molten steel having the chemical composition shown in Table 1 was melted in a converter. Then, a hot-rolled steel sheet having a plate thickness of 3.0 mm was produced from these steel materials by hot rolling, cooling and winding conditions shown in Table 2. The balance other than the components shown in Table 1 is Fe and impurities. In addition, the component composition obtained by analyzing the sample collected from the manufactured hot rolled steel sheet was the same as the component composition of the steel shown in Table 1.
[0060]
[table 1]

[0061]
[Table 2-1]

[0062]
[Table 2-2]

[0063]
　The "heating temperature" in Table 2 is the temperature when the slab is reheated, and the "direct feed" means that the finish rolling was performed by the direct feed rolling in which the continuous casting and the finish rolling were connected. Further, "F1" to "F7" indicate rolling stands in finish rolling, "rolling temperature" in each column indicates temperature on the stand-in side, and "interpass time" immediately after leaving the stand. Represents the time it takes to reach the next stand. Further, "T" represents the time from the hot rolling step (after finishing rolling) to the start of cooling. The cooling after finish rolling was performed by water cooling, and the steel sheet was passed through a water cooling facility having no air cooling section in the middle. The cooling rate at the time of cooling is represented by an average rate obtained by dividing the temperature drop width of the steel sheet from the introduction of the water cooling equipment to the derivation of the water cooling equipment by the required passage time of the steel sheet to the water cooling equipment.
[0064]
　Test pieces were taken from the obtained hot-rolled steel sheet, and the structure was observed (scanning electron microscope and EBSD), tensile test, and hole expansion test. The tissue observation was performed at an analysis speed of 200 to 300 points/sec using a device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL). The average orientation difference within the same grain was calculated using software (OIM Analysis ™ ) attached to the EBSD analyzer . In the hole expansion test, a 10 mmφ punched hole (initial hole: hole diameter d0=10 mm) was opened in the test piece, and a crack was formed through the plate thickness with a conical punch having a burr on the top and an apex angle of 60 degrees. The initial hole is pushed up to, the hole gauge d1mm when cracking occurs is measured, and the hole expansion rate λ(%) is calculated by the following formula. The results are shown in Table 3.
　　　λ=100×(d1-d0)/d0
[0065]
[Table 3]

[0066]
　In Table 3, “α1 phase” represents the first ferrite having an average orientation difference within the same grain of 0.5° to 5.0°, “B phase” represents bainite, and “α2 phase” is the same. Represents a second ferrite with an average grain orientation difference of less than 0.5°. Further, the "remainder structure" contained martensite and retained austenite. From Table 3, it can be seen that the hot-rolled steel sheets of the examples all have a tensile strength of 440 MPa or more and are excellent in stretch flangeability and shape fixability. The term “excellent stretch flangeability” means that λ is 90% or more, and the term “excellent shape fixability” means that the yield ratio is 70% or less and the yield point elongation is less than 1.0%. Means that.
[0067]
　On the other hand, the hot-rolled steel sheet of the comparative example which is out of the range of the present invention is deteriorated in tensile strength, stretch flangeability and/or shape fixability. In Comparative Example 4 , the ferrite transformation did not occur during rolling because the rolling temperature in the final pass of finish rolling was Ae 3 point or higher. As a result, the yield point elongation increases and the shape fixability deteriorates. In Comparative Example 5, since the cooling rate is slower than 20° C./sec, recovery occurs in the α1 phase and the fraction of the α2 phase increases, resulting in a decrease in strength, an increase in yield point elongation, and shape fixability. Is deteriorated. In Comparative Example 10, since the coiling temperature (cooling stop temperature) is less than 300° C., the martensite fraction of the remaining structure increases, that is, the remaining structure exceeds 5% by volume, resulting in a yield ratio of 70. %, and the shape fixability is deteriorated. In Comparative Example 13, more than 10 seconds have passed from the hot rolling process (completion of finish rolling) to the start of cooling, the α1 phase was recovered, the α2 phase fraction was increased, and the yield point elongation was increased. The shape freezeability is deteriorated.
[0068]
　In Comparative Example 16, the rolling temperature was less than the point A during finish rolling, and the ferrite was generated due to the temperature decrease during rolling, so the grain size of the α1 phase exceeded 5.0 μm and became large. The strength is reduced. In Comparative Example 23, the coiling temperature exceeded 600° C., the α1 phase was recovered, the α2 phase fraction was increased, the strength was decreased, and the yield point elongation was increased. Is deteriorated. In Comparative Example 28, the total strain amount is less than 1.4, the volume ratio of the α1 phase is reduced to less than 30%, and the yield point elongation is increased, so that the shape fixability is deteriorated. In Comparative Example 29, the conditions of hot rolling, cooling and winding are satisfied, but the amount of C is large, the amount of cementite in the structure is large, the hole expandability is reduced, and the stretch flangeability is increased. Is deteriorated. Similarly, in Comparative Example 30, the conditions of hot rolling, cooling and winding are satisfied, but since the Mn content is large, a band structure is formed in the structure and the hole expansibility is deteriorated. Stretch-flangeability is deteriorated.
The scope of the claims
[Claim 1]
　% By mass,
　C: 0.01% or more and 0.20% or less,
　Si: 1.0% or less,
　Mn: 3.0% or less,
　P: 0.040% or less,
　S: 0.004% or less,
　Al Content is 0.10% or less,
　N: 0.004% or less,
and the balance is Fe and impurities
　. The average orientation difference within the same grain is 0.5° or more and 5.0° or less. At least one structure of
　bainite and second ferrite having an average misorientation in the same grain of 0° or more and less than 0.5° , containing 30 vol% or more and 70 vol% or less of a certain first ferrite, and The first ferrite is contained in a total amount of 95% by volume or more, the
　residual structure is 5% by volume or less,
　the average crystal grain size of the first ferrite is 0.5 μm or more and 5.0 μm or less, and the at least one kind When the average crystal grain size of the structure is 1.0 μm or more and 10 μm or less, and the residual structure is present, the average crystal grain size of the residual structure is 1.0 μm or more and 10 μm or less. Steel plate.
[Claim 2]
　Furthermore, in mass%,
　Nb: 0.01% or more and 0.20% or less,
　Ti: 0.01% or more and 0.15% or less,
　Mo: 0.01% or more and 1.0% or less,
　Cu: 0.01 % Or
　more and 0.5% or less, and Ni: 0.01% or more and 0.5% or less
selected from 1 type, or 2 or more types are contained, The hot-rolled steel sheet of Claim 1 characterized by the above-mentioned. ..
[Claim 3]
　(A) A steel material having the composition according to claim 1 or 2 is hot-rolled as it is without cooling after casting, or is once cooled to room temperature and then heated to 1100°C or more and 1350°C or less to heat. A hot rolling step of hot rolling, wherein the hot rolling step includes finish rolling by continuously passing the cast steel material through a plurality of rolling stands, and all the rolling stands of the finish rolling. The rolling temperature in A is equal to or higher than A point, and continuous rolling of 2 passes or more including the final pass of the finish rolling is performed: rolling temperature: A point or more and Ae less than 3 points, strain rate: 1.0 to 50/sec, And time between passes: a hot rolling step performed under conditions of 10 seconds or less, and the total strain amount of all passes satisfying the above conditions is 1.4 or more and 4.0 or less,
　(b) finish rolled steel sheet A cooling step of cooling the
　steel sheet at an average cooling rate of 20° C./second or more and 50° C./second or less, wherein the cooling is started within 10 seconds after the hot rolling step, and (c) the steel sheet. A
method for producing a hot-rolled steel sheet, comprising a winding step of winding in a temperature range of 300° C. or higher and 600° C. or lower .
　Here, point A is the temperature determined by the following (Equation 1), and point Ae 3 is the temperature determined by the following (Equation 2).
　A (°C)=910-310C-80Mn-20Cu-55Ni-80Mo (Formula 1)
　Ae 3(° C.)=919-266C+38Si-28Mn-27Ni+12Mo (Formula 2) In the formula, C, Si, Mn, Cu, Ni and Mo are the contents (mass %) of each element.