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 590 MPa or more, which has excellent fatigue characteristics and stretch flangeability, and is suitable as a material for automobile structural parts, skeletons, and wheel discs, and a method for producing the same.
Background technology
[0002]
　As a method of 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, C: 0.01 to 0.20 wt%, Si: 1.00 wt% or less, Mn: 2.00 wt% or less, Al: 0.10 wt% or less, N:0. Steel ingot or slab containing 0.0070% by weight or less and Nb: 0.005 to 0.15% by weight, when Nb≦0.015% by weight, Tc=850+139000×[Nb% by weight]×[C% by weight+12 /14N% by weight] or less, and when Nb>0.015% by weight, Tc=961+51000×[Nb% by mass]×[C% by mass+12/14N% by mass] or less and 850 to Ar 3 − After hot rolling in a temperature range of 50°C, cooling at a cooling rate of 30°C/sec or more, and then winding in a temperature range of 450°C to 150°C, fine ferrite having an average grain size of 2 to 3 μm has an area ratio. Is 70% or more, the area ratio of the structure containing bainite and martensite is 20% or less, and the balance of the area ratio of the balance is a mixed-grain structure of ferrite having an average particle size of 10 μm or less, which is excellent in strength, ductility, toughness, and fatigue properties. It has been proposed to produce hot rolled high strength steel sheet.
[0004]
　Further, in Patent Document 2, the components are wt%, C: 0.01 to 0.10%, Si: 1.5% or less, Mn: more than 1.0 to 2.5%, P: 0.15. % Or less, S: 0.008% or less, Al: 0.01 to 0.08%, a total of one or two of Ti and Nb: 0.32 to 0.60%, from the balance Fe and unavoidable impurities The continuous casting slab is heated to a temperature higher than 1100° C., hot-rolled at a finish rolling temperature of Ar 3 or higher, then cooled at a cooling rate of 10 to 150° C./s, and a winding temperature of 500 to By winding at 700° C., the amount of ferrite is 95% or more in area ratio, the average crystal grain size of ferrite is 2.0 to 10.0 μm, and the structure does not contain martensite and retained austenite, It has been proposed to produce an ultrafine ferrite structure high-strength hot-rolled steel sheet having a tensile strength of 490 MPa or more and excellent stretch flangeability.
[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%, with the balance being Fe and unavoidable 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. At 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 high-strength steel with fine crystal grains characterized by allowing to cool is 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 Publication No. 6-29480
Patent Document 2: Japanese Patent No. 3725367
Patent Document 3: Japanese Patent No. 4006112
Summary of the invention
Problems to be Solved by the Invention
[0007]
　Since strengthening materials generally deteriorates material properties such as fatigue properties and stretch flangeability, it is important to develop high strength hot rolled steel sheets without degrading these material properties. Will be important.
[0008]
　However, in the hot-rolled high-tensile steel sheet described in Patent Document 1, the structure is a composite structure of ferrite, martensite, and bainite, and there is a problem that the stretch flangeability is low due to the hardness difference between the structures. It was
[0009]
　In addition, the ultrafine ferrite structure high-strength hot-rolled steel sheet described in Patent Document 2 has a problem that the ferrite is the main phase and thus has low strength, and the Nb and Ti contents are large, so that the economy is poor. ..
[0010]
　In addition, 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 590 MPa or more, which is excellent in fatigue characteristics and stretch flangeability, and a manufacturing method thereof.
Means for solving the problems
[0012]
　In order to achieve the above-mentioned object, the inventors of the present invention have a method of reducing the hardness difference between the ferrite and the residual structure in the hot rolled steel sheet, and in consideration of economy, crystal grains that do not contain Nb and Ti as essential components. We also diligently studied the miniaturization method. As a result, it was found that even in a multi-phase structure steel having a large hardness difference between the structures such as ferrite and martensite, the stretch-flangeability is improved when the average orientation difference of ferrite in the same grain is large. .. Even if Nb and Ti are not contained, 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 to produce ferrite. It was found that the average crystal grain size of can be refined to 5.0 μm or less. Then, since the high-density dislocations are introduced into the ferrite thus generated, the dislocation strengthening occurs, and the average orientation difference of the ferrite in the same grain is also large, so that it has high strength, fatigue characteristics and elongation. It was further found that it becomes possible to obtain a hot-rolled steel sheet having excellent flangeability.
[0013]
　The present invention has been completed by 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 and
　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 less. 30% by volume or more and 70% by volume or less of ferrite whose content
　is 90 ° or less, 90% by volume or more in total of the ferrite and martensite
　, and 10% by volume or less of the balance structure, and
　the average crystal grain size of the ferrite is 0. 5 μm or more and 5.0 μm or less, the average crystal grain size of the martensite is 1.0 μm or more and 10 μm or less, and 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. The hot-rolled steel sheet according to claim 1.
　[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,
[1], which contains one or more selected from 　Cu: 0.01% or more and 0.5% or less and
　Ni: 0.01% or more and 0.5% or less
. Hot-rolled steel sheet according to.
　[3] (a) A steel material having the composition according to [1] or [2] above is hot-rolled as it is without cooling after casting, or is once cooled to room temperature, and then is heated to 1100°C to 1350°C. 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 in 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, the cooling step being started within 10 seconds after the hot rolling step, and
　(c) the steel sheet at room temperature. 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 lower than 300° C.
　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.
Effect of the invention
[0014]
　According to the present invention, it is possible to obtain a hot-rolled steel sheet having high strength and extremely good stretch-flangeability and fatigue characteristics. If the present invention is applied to structural parts of automobiles, the safety of automobiles is ensured. At the same time, the weight of the vehicle body can be reduced, and the environmental load can be reduced.
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 and 70% by volume or less of ferrite having an average orientation difference of 0.5° or more and 5.0° or less within the same grain. Including 90% by volume or more in total of the ferrite and martensite, the balance structure is 10% by volume or less, the average crystal grain size of the ferrite is 0.5 μm or more and 5.0 μm or less, and the average of the martensite is When the crystal grain size 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.
[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 improves solid solution strengthening and hardenability, and generates martensite which is a low temperature transformation phase in the balance structure to secure the strength of the hot rolled steel sheet. It is an element necessary for this purpose, and at least 0.01% or more is necessary for that purpose. 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 to contribute to the strength increase of steel and enhances hardenability. 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 set to 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 in 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 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 toughness and strengthening. %, 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 exert 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. Further, when the Nb content exceeds 0.20%, coarse precipitates are formed and the toughness of the hot rolled steel sheet tends to decrease. 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 bring out such an effect, 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 enhances hardenability and contributes to high strength of the steel sheet. 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 the strength of steel. Moreover, Cu improves hardenability. In order to obtain these effects, 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 are deteriorated. Therefore, the Cu content is 0.5% or less. It 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 hardenability. In order 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%, it deteriorates the weldability. 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. In addition, the impurities are elements other than the components described above, and the elements contained in the hot-rolled steel sheet at a level at which the effect peculiar to the element does not affect the properties of the hot-rolled steel sheet according to the present invention. Includes.
[0032]
　Next, the reasons for limiting the structure of the hot rolled steel sheet according to the present invention will be described.
[0033]
[Ferrite with an average orientation difference within the same grain of 0.5° to 5.0°: 30% by volume to 70% by volume]
　The microstructure of the hot-rolled steel sheet of the present invention has an average orientation difference of 0. It contains 30% by volume or more and 70% by volume or less of ferrite that is 5° or more and 5.0° or less.
[0034]
　Here, in the present invention, the “average orientation difference within the same grain” exists within one crystal grain when the orientation difference between adjacent grains is 15° or more is defined as one crystal grain. It is an index showing 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, since the ferrite is also processed, crystal disorder occurs in the ferrite grains, and the average orientation difference within the same grain increases. In order to reduce the hardness difference from martensite, the average orientation difference within the same grain needs to be 0.5° or more. On the other hand, when 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 3.5° or less.
[0035]
　In the hot-rolled steel sheet according to the present invention, when the amount of ferrite having an average misorientation within the same grain of 0.5° or more and 5.0° or less is less than 30% by volume, the fraction of the fine grain structure decreases, which is sufficient. Since it becomes difficult to secure various fatigue characteristics, the volume ratio of the ferrite is set to 30% by volume or more. Further, in order to increase the volume ratio of the ferrite, it is necessary to increase the rolling reduction during hot rolling or lower the temperature during hot rolling. However, when the conditions exceed 70% by volume, However, there is a high possibility that the average orientation difference within the same grain will exceed 5.0°, the ductility of the ferrite will deteriorate, and the stretch-flangeability will deteriorate. Therefore, the volume ratio of ferrite having an average orientation difference of 0.5° or more and 5.0° or less within the same grain is set to 30% by volume or more and 70% by volume or less. It is preferably at least 35% by volume or at least 40% by volume, and/or at most 65% by volume or at most 60% by volume.
[0036]
[Identical grains in an average misorientation than 0.5 ° 5.0 ° or less of ferrite and martensite total of 90% by volume or more, and the remaining structure 10 vol% or less]
　hot rolled steel sheet according to the present invention, in the same particle Containing 90% by volume or more, preferably 95% by volume or more, or 100% by volume in total of ferrite and martensite having an average orientation difference of 0.5° or more and 5.0° or less. The balance structure is not particularly limited, but includes, for example, bainite, ferrite having an average orientation difference of less than 0.5° in the same grain, and retained austenite of one or more types, or bainite, of the same grain. It is composed of one or more of ferrite having an average orientation difference of less than 0.5° and retained austenite. If the residual structure is more than 10% by volume, it becomes difficult to achieve the desired strength and/or stretch flangeability, so the residual structure is 10% by volume or less. More preferably, the residual structure is 5% by volume or less, and may be 0% by volume. In addition, when the balance structure is more than 10% by volume and the bainite fraction of the balance structure is relatively high, the fatigue properties may deteriorate.
[0037]
[Average grain size of ferrite having an average orientation difference of 0.5° or more and 5.0° or less within the same grain: 0.5 μm or more and 5.0 μm or less] In the
　present invention, “average grain size” means adjacent grains. The value calculated when the crystal orientation difference of 15° or more is defined as one crystal grain. If the average grain size of ferrite having an average orientation difference of 0.5° or more and 5.0° or less in the same grain exceeds 5.0 μm, the fatigue strength and toughness deteriorate, so the average grain size is 5. It must be 0 μm or less. On the other hand, in order to reduce the average crystal grain size to less 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°. More likely. Therefore, the average crystal grain size is 0.5 μm or more. Therefore, the average grain size of ferrite having an average orientation difference of 0.5° or more and 5.0° or less within the same grain is 0.5 μm or more and 5.0 μm or less, preferably 0.7 μm or more or 1.0 μm. And/or 4.5 μm or less or 4.0 μm or less.
[0038]
[Average crystal grain size of martensite and residual structure: 1.0 μm or more and 10 μm or less]
　Martensite and, if present, the average crystal grain size of the residual structure is included in the martensite and the residual structure when it is smaller than 1.0 μm. The strength of the bainite and the like is increased, the hardness difference from the ferrite is increased, and the stretch flangeability is deteriorated. On the other hand, if it is larger than 10 μm, the fatigue characteristics and the toughness may deteriorate. Therefore, the average crystal grain size of martensite and the remaining structure, especially bainite, is 1.0 μm or more and 10 μm or less, preferably 1.5 μm or more or 2.0 μm or more, and/or 8.0 μm or less or 5.0 μm. It is the following.
[0039]
　Identification of each phase or structure and calculation of the average crystal grain size can be performed by image processing using a structure photograph taken by a scanning electron microscope or backscattered electron diffraction image analysis (EBSP or EBSD).
[0040]
　More specifically, the volume ratio of ferrite having an average orientation difference within the same grain of 0.5° or more and 5.0° or less is determined as follows. When the plate width of the steel plate is W, the 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 the orientation difference is 15° or more, the middle of the adjacent measurement points is determined to be 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 within the same grain of the crystal grains. Then, the area ratio of the ferrite crystal grains having an average orientation difference within the same grain of 0.5° or more and 5.0° or less is obtained. Volume ratio of ferrite is less than °. The average orientation difference within the same grain can be calculated using software attached to the EBSD analysis device. Martensite may also have an average orientation difference of 0.5° or more within the same grain, but martensite has a lath-like structure in shape, and therefore has a lath-like structure in an SEM image. Is martensite, and its area ratio is the volume ratio of martensite.
[0041]
　In the present invention, the average crystal grain size of each of “ferrite having an average orientation difference of 0.5° to 5.0° in the same grain”, “martensite”, and “remainder structure” is determined by the above EBSD analysis. It is determined by using the value. 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. The equivalent diameter is shown. These data are easily obtained by EBSD analysis.
[Number 1]

[0042]
　According to the present invention, a hot rolled steel sheet having high strength and excellent fatigue characteristics and stretch flangeability 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 reduce the plate thickness while ensuring the required strength, which contributes to the improvement of the fuel efficiency of the automobile.
[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 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 point, 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, the cooling step being started within 10 seconds after the hot rolling step, and
　( c) The
method is characterized by including a winding step of winding the steel sheet in a temperature range of room temperature or higher and lower than 300°C .
　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. .. 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 performed 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 carried out by the direct feed 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 equal to or higher 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, ferrite transformation occurs due to the temperature reduction in addition to the ferrite transformation during rolling. Ferrite produced by the latter ferrite transformation has a large crystal grain size, which causes reduction in tensile strength and fatigue strength. 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 obtained by the following (Equation 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 accompanying the lowering of the temperature occurs. The latter ferrite produced by the ferrite transformation has a large crystal grain size, which causes reduction in tensile strength and fatigue strength. Further, the generation of such ferrite 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/sec or more and 50/sec 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)
　Rolling temperature: A point or more and Ae less than 3 points, strain rate: 1.0-50 /Sec, and time between passes: The total strain amount of all passes satisfying the condition of 10 seconds or less is 1.4 or more and 4.0 or less. This total strain amount has a great influence on the amount of ferrite transformation that occurs during rolling, the average misorientation in the ferrite grains, and the refinement of martensite. If the total strain amount is less than 1.4, it is difficult to cause a sufficient amount of ferrite transformation, and the martensite crystal grain size becomes coarse. On the other hand, when the total strain amount exceeds 4.0, the average orientation difference within the same grain of ferrite generated during rolling exceeds 5.0, and the ductility of ferrite decreases. In addition, the amount of ferrite transformation during rolling increases and the amount of martensite decreases, resulting in insufficient strength. 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 a ferrite fraction of the final structure. It becomes small, and the elongation of the obtained hot rolled steel sheet decreases. Further, when the final pass does not satisfy the rolling conditions, the reverse transformation from ferrite to austenite occurs in the final pass, the ferrite fraction of the final structure decreases, the elongation decreases, and/or the ferrite recovers. The hardness difference from martensite becomes large, and stretch flangeability 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 and the fatigue strength are 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 performed 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 at an average cooling rate of 20°C/sec or more in the cooling step, and the cooling is performed after the hot rolling step 10 described above. Start within seconds. If it exceeds 10 seconds from the end of the hot rolling process to the start of cooling, the recovery of ferrite occurs and the hardness difference from martensite becomes large, and the stretch flange formability 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 is recovered and softened, the difference in hardness with martensite becomes large, and the stretch flangeability deteriorates. Therefore, the average cooling rate of cooling after the hot rolling step is set to 20° C./second or more. It is preferably 40° C./second or more. The upper limit of the average cooling rate is not particularly limited, but may be 100° C./second or less, for example.
[0055]
[(C) Winding Step]
　The steel sheet cooled to the cooling stop temperature in the cooling step is wound in a temperature range of room temperature or higher and lower than 300° C. in the winding step. Since the steel sheet is wound immediately after the cooling step, the winding temperature is almost equal to the cooling stop temperature. When the winding temperature is 300° C. or higher, a large amount of polygonal ferrite or bainite is generated, so that the tensile strength and the fatigue strength decrease. Therefore, the coiling temperature that is the cooling stop temperature is less than 300°C. For example, the winding temperature may be 250°C or lower or 200°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 or 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 as described above, by performing the subsequent cooling and winding operations, the average orientation within the same grain 30% by volume or more and 70% by volume or less of a ferrite having a difference of 0.5° or more and 5.0° or less, a total of 90% by volume or more of the ferrite and martensite, and a residual structure of 10% by volume or less, When the average crystal grain size of the ferrite is 0.5 μm or more and 5.0 μm or less, the average crystal grain size of the martensite is 1.0 μm or more and 10 μm or less, and when the residual structure is present, the average of the residual structure is A hot-rolled steel sheet having a crystal grain size 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 590 MPa or more, which is excellent in fatigue characteristics and stretch flangeability.
[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 equivalent to that 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 is performed by the direct feed rolling in which continuous casting and finish rolling are 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]
　Specimens were taken from the obtained hot-rolled steel sheet, and the structure was observed (scanning electron microscope and EBSD), tensile test, hole expansion test, and fatigue test by the both-sides plane bending test method. 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 (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector). The average orientation difference within the same grain was calculated using software (OIM Analysis ™ ) attached to the EBSD analyzer . Further, in the hole expansion test, a punched hole of 10 mmφ (initial hole: hole diameter d0=10 mm) is opened in the test piece, and a crack is formed through the plate thickness with a conical punch having a burr upward and an apex angle of 60 degrees. The initial hole is pushed up to, the hole gauge d1mm at the time of cracking 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 ferrite having an average orientation difference of 0.5° or more and 5.0° or less within the same grain, and “M phase” represents martensite. The "remainder structure" contained bainite, and also contained ferrite and/or retained austenite having an average orientation difference of less than 0.5° within the same grain. From Table 3, it can be seen that the hot-rolled steel sheets of the examples all have a tensile strength of 590 MPa or more and are excellent in stretch flangeability and fatigue characteristics. The term “excellent stretch flangeability” means that λ is 90% or more, and the term “excellent fatigue characteristics” means that the fatigue limit ratio (fatigue strength/tensile strength) is 0.50 or more. Means
[0067]
　On the other hand, the hot-rolled steel sheet of the comparative example, which is out of the range of the present invention, has deteriorated stretch flangeability and/or fatigue characteristics. 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, a fine grain structure of ferrite (ferrite having an average orientation difference of 0.5° to 5.0° in the same grain: 30% by volume to 70% by volume, and an average crystal grain size of the ferrite: 0.5 μm) 5.0 μm or less) is not obtained, and thus the stretch flangeability and the fatigue properties are deteriorated. In Comparative Example 5, since the cooling rate was slower than 20° C./sec, the ferrite having an average orientation difference within the same grain of 0.5° or more and 5.0° or less was recovered, and the fraction of the balance structure increased. As a result, the hardness difference from martensite is increased and the stretch flangeability is deteriorated. In Comparative Example 10, since the coiling temperature (cooling stop temperature) was 300° C. or higher, the bainite fraction of the balance structure increased, that is, the balance structure increased beyond 10% by volume, and as a result, the tensile strength and fatigue properties were increased. 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, and the average orientation difference within the same grain was recovered to be 0.5° or more and 5.0° or less. Occurs, the fraction of the remaining structure increases, the tensile strength decreases, and the difference in hardness from martensite increases and the stretch flangeability deteriorates.
[0068]
　In Comparative Example 16, the rolling temperature was less than the point A during finish rolling, and ferrite was generated due to the temperature decrease during rolling, so that the average orientation difference within the same grain was 0.5° or more and 5.0° or less. The grain size of the ferrite is larger than 5.0 μm, and the fatigue characteristics are deteriorated. In Comparative Example 28, the total strain amount is less than 1.4, the volume ratio of ferrite having an average orientation difference within the same grain of 0.5° or more and 5.0° or less is reduced to less than 30% by volume, Since the fraction of the fine grain structure is small and the grain size of martensite is coarse, the fatigue properties are deteriorated. In Comparative Example 29, the conditions of hot rolling, cooling, and winding are satisfied, but since 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. A certain ferrite is contained in an amount of 30% by volume or more and 70% by volume or less, the
　ferrite and martensite are contained in a total of 90% by volume or more, the
　balance structure is 10% by volume or less, and
　the average grain size of the ferrite is 0.5 μm or more 5 0.0 μm or less, the average crystal grain size of the martensite is 1.0 μm or more and 10 μm or less, and 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. A hot-rolled steel sheet characterized by:
[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 higher and 1350°C or lower 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 not less than A point, and continuous rolling of two or more passes including the final pass of the finish rolling is as follows: Rolling temperature: A point or more and less than Ae 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 Is a cooling step of cooling the
　steel sheet at an average cooling rate of 20° C./second or more, wherein the cooling is started within 10 seconds after the hot rolling step, and (c) the steel sheet is at room temperature or higher and lower than 300° C. A
method for manufacturing a hot-rolled steel sheet, comprising a winding step of winding in the temperature range of 1 .
　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.