JP2014043629A - Hot rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet that has a tensile strength of 590 MPa or more, high ductility and good formability, and exhibits excellent shear workability such that the hot rolled steel sheet is difficult to generate cracks in a cutting end face even when being subjected to shearing with a clearance of 5 to 30%.SOLUTION: A hot rolled steel sheet comprises: a chemical composition including, by mass%, C:0.01 to 0.20%, Si:0.01 to 2.5%, Mn:0.5 to 3.0%, P:0.02% or less, S:0.005% or less, sol.Al:0.02 to 0.5%, Ti:0.02 to 0.25% and N:0.01% or less; and a metal structure including, by area%, 80% or more of total amount of ferrite and bainite, 5% or less of pearlite, 10% or less of martensite and 3% or less of residual γ, where in a thickness center portion within ±0.12t from the center of the steel sheet, taking a thickness of the steel sheet as t, an area ratio of a crystal colony in which a <112> crystal orientation is oriented at an angle of 10° or less relative to a direction perpendicular to a steel sheet surface is 35% or less, and an area ratio of a crystal colony having a circle-equivalent diameter of 10 μm or more in the crystal colony is 30% or less.

Description

  The present invention relates to a hot-rolled steel sheet. In particular, the present invention relates to a hot-rolled steel sheet having a tensile strength of 590 MPa or more, excellent in shear workability and ductility, which is suitable as a material for structural members used in automobiles and various industrial machines.

  Due to the growing global environmental awareness, automobiles are required to be lighter. One technique for achieving weight reduction while ensuring the function as a structural member is a tailored blanks (TWB) technique in which steel sheets of different thicknesses and strengths are butt welded before forming. TWB has the advantage of being able to reduce the weight efficiently because it can partially increase the strength of a single component. Until now, TWB has been applied to upper body members, but in recent years, application of TWB is also being studied for chassis parts.

  In butt welding, the buttability between joint surfaces greatly affects the welding quality. In many cases, the joint surface is constructed by shearing such as shearing. Until now, cold-rolled steel sheets with a relatively thin plate thickness have been used for TWB. However, to apply TWB to chassis parts, it is necessary to use a thick-walled hot-rolled steel plate. In order to suppress the consumption of the mold, it is necessary to perform cutting with a wide clearance. In wide clearance cutting of thick materials, it is easy to cause defects on the end face, especially in the case of high-strength materials, cracks along the plate surface may occur on the cut end face, which may lead to poor quality of the joint due to poor butt. There's a problem. When a defect exists in the joint surface, a notch (underfill) is generated in the joint surface, and fatigue durability is greatly impaired.

  Regarding a cut end face of a hot-rolled steel sheet, for example, Patent Document 1 discloses a technique of adding Mn and P to steel in a composite manner. However, the application of this technique is limited only to low-strength steel, and an effect cannot be expected for suppressing end face cracks in high-strength steel.

  Patent Document 2 discloses a steel material having an excellent cut surface quality, but is a technology that takes into account precision cutting with a cutting clearance of 1% or less, and 5% or more that is usually used for manufacturing automobile parts. Not applicable in case of cutting clearance.

  On the other hand, Patent Documents 3 and 4 propose high-strength steel materials that limit the hard phase and cementite contents and suppress damage to the punched end face. However, what is disclosed relates to improving the punched end face properties of the round hole. In the case of cutting with a straight line, which is necessary for the manufacture of the TWB member, and with a straight cut, the shearing process imposes a heavy load on the end face in the cutting process, and therefore the method described in this document does not necessarily prevent end face cracking. There was a problem.

JP 2000-144318 A JP 2004-137607 A JP 2005-298924 A JP 2004-315857 A

  In order to solve the above-mentioned problems, the present invention has a high strength of a tensile strength of 590 MPa or more, and even when shearing is performed with a clearance of 5% to 30%, it is difficult to cause cracks on the cut end face. It aims at providing the hot-rolled steel plate excellent in workability and ductility.

  The present inventors conducted shear tests with various cutting clearances, and examined the correlation between cracks at the cut end face and the form of crystal colonies having specific crystal orientations and inclusions. As a result, <112> It has been found that end face cracking is improved by defining the form of crystal colonies oriented within 10 ° of crystal orientation, and the effect is further enhanced by suppressing coarse inclusions.

The present invention based on the above new findings is as follows.
(1) By mass%, C: 0.01% to 0.20%, Si: 0.01% to 2.5%, Mn: 0.5% to 3.0%, P: 0.00% 02% or less, S: 0.005% or less, sol.Al: 0.02% to 0.5%, Ti: 0.02% to 0.25% and N: 0.01% or less In addition, the chemical composition of the balance Fe and impurities, and in area%, ferrite and bainite: 80% or more in total, pearlite: 5% or less, martensite: 10% or less, and residual austenite: 3% or less, The area of the crystal colony in which the <112> crystal orientation is oriented within 10 ° in the direction perpendicular to the steel plate surface at the center of the plate thickness within the range of ± 0.12 t from the thickness center when the plate thickness is t The ratio is 35% or less, and the equivalent circle diameter in the crystal colony is 10 A hot-rolled steel sheet having a metal structure in which an area ratio of crystal colonies of μm or more is 30% or less and a tensile strength of 590 MPa or more.

  (2) The hot rolled steel sheet according to the above (1), wherein the area ratio of the crystal colony in which the ratio (a / b) of the short axis a to the long axis b in the crystal colony is 0.35 or less is 40% or less. .

(3) In the above (1) or (2), the total number density of inclusions, crystallized substances, and precipitates having an equivalent circle diameter of 5 μm or more existing in the center part of the plate thickness is 50 pieces / mm ² or less. The hot-rolled steel sheet described.

  (4) The hot-rolled steel sheet according to any one of the above (1) to (3), wherein the chemical composition contains mass% and Nb: 0.1% or less instead of part of Fe.

  (5) The chemical composition is mass% instead of part of Fe, V: 0.4% or less, Mo: 0.4% or less, W: 0.4% or less, and Cr: 0.4%. The hot-rolled steel sheet according to any one of the above (1) to (4), containing one or more selected from the group consisting of:

  (6) The chemical composition is selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less in mass% instead of a part of Fe. The hot-rolled steel sheet according to any one of the above (1) to (5), containing one or more kinds.

  Since the hot-rolled steel sheet according to the present invention has high strength and excellent shear workability, even if shearing is performed with a cutting clearance of 5% or more, it is difficult to cause cracks in the cut end face, and the buttability of the cut end face is excellent. Good and suitable for TWB applications. Therefore, the hot-rolled steel sheet according to the present invention can be suitably used as a material for structural members used in automobiles and various industrial machines, particularly for applications using TWB technology such as automobile chassis parts.

  The hot-rolled steel sheet of the present invention will be described in detail below. In the following description, all percentages relating to the chemical composition of steel are mass%. The balance of the chemical composition of steel is Fe and impurities. On the other hand,% which shows the ratio of each phase in the metal structure of steel is area%.

1. Chemical composition of steel C: 0.01% or more and 0.20% or less C has an effect of increasing the strength of steel. If the C content is less than 0.01%, it is difficult to ensure a tensile strength of 590 MPa or more. Accordingly, the C content is 0.01% or more. Preferably it is 0.02% or more, More preferably, it is 0.03% or more. On the other hand, if the C content exceeds 0.20%, coarse carbides are formed at the grain boundaries, and the workability is impaired. Therefore, the C content is 0.20% or less. Preferably it is 0.18% or less, More preferably, it is 0.15% or less.

Si: 0.01% or more and 2.5% or less Si has an effect of improving the balance between strength and ductility. If the Si content is less than 0.01%, it is difficult to obtain the effect by the above action. Therefore, the Si content is set to 0.01% or more. Preferably it is 0.2% or more, More preferably, it is 0.5% or more. On the other hand, if the Si content exceeds 2.5%, the effect of the above action is saturated and weldability is impaired. Therefore, the Si content is 2.5% or less. Preferably it is 2.0% or less, More preferably, it is 1.5% or less.

Mn: 0.5% or more and 3.0% or less Mn has an effect of increasing the strength of steel. If the Mn content is less than 0.5%, it is difficult to ensure a tensile strength of 590 MPa or more. Therefore, the Mn content is 0.5% or more. Preferably it is 0.8% or more, More preferably, it is 1.0% or more. On the other hand, if the Mn content exceeds 3.0%, the ferrite transformation after completion of hot rolling is excessively delayed, and it is difficult to secure a ferrite having a high formability when higher formability is desired. Become. Therefore, the Mn content is 3.0% or less. Further, if the Mn content is high, the tendency to segregate at the center of the slab becomes strong and the formability deteriorates. From this viewpoint, the Mn content is preferably 2.5% or less, and 2.2%. More preferably, it is as follows.

P: 0.02% or less P is generally contained as an impurity, segregates at the grain boundary, causes embrittlement, and promotes cracking of the cut end face. Therefore, the P content is 0.02% or less. Preferably it is 0.01% or less, More preferably, it is 0.007% or less.

S: 0.005% or less S is generally contained as an impurity, and combines with Mn, Ti, etc. to form coarse sulfide-based inclusions, which promotes cracking of the cut end face and significantly impairs workability. . Therefore, the S content is 0.005% or less. Preferably it is 0.002% or less, More preferably, it is 0.001% or less.

sol.Al: 0.02% or more and 0.5% or less Al promotes ferrite transformation and improves formability, and suppresses formation of coarse cementite and suppresses the origin of end face cracks. If the sol.Al content is less than 0.02%, it is difficult to obtain the effect of the above action. Therefore, the sol.Al content is 0.02% or more. Preferably it is 0.08% or more, More preferably, it is 0.11% or more. On the other hand, if the sol.Al content is more than 0.5%, the austenite-ferrite transformation temperature is increased and the productivity is impaired. Therefore, the sol.Al content is 0.5% or less. Preferably it is 0.3% or less, More preferably, it is 0.25% or less.

Ti: 0.02% or more and 0.25% or less Ti is effective for fine precipitation strengthening by bonding with C. If the Ti content is less than 0.02%, it is difficult to ensure a tensile strength of 590 MPa or more. Therefore, the Ti content is set to 0.02% or more. Preferably it is 0.05% or more, More preferably, it is 0.08% or more. On the other hand, if the Ti content exceeds 0.25%, the effect of the above action is saturated, leading to an increase in raw material cost, and coarse carbonitride may be formed to deteriorate formability. Therefore, the Ti content is set to 0.25% or less. Preferably it is 0.2% or less, More preferably, it is 0.17% or less.

N: 0.01% or less N is generally contained as an impurity, and combines with Ti, Nb, V, etc. to form coarse nitrides and impairs the properties of the cut end face. Therefore, the N content is 0.01% or less. Preferably it is 0.005% or less, More preferably, it is 0.003% or less.

  The elements described below are optional elements that may be optionally contained in the steel in the present invention.

Nb: 0.1% or less Nb has the effect of forming carbonitrides in steel, refining austenite grains to increase the nucleation sites of ferrite, and suppressing the coarsening of the steel structure. Moreover, when it contains in combination with V and / or Ti, it has the effect | action which forms a fine precipitate and raises the intensity | strength of steel effectively. Therefore, you may contain Nb. However, if the Nb content exceeds 0.1%, the ferrite transformation after completion of hot rolling is excessively delayed, and if it is desired to increase the area ratio of ferrite with an emphasis on ductility, ferrite with excellent formability It becomes difficult to get. Therefore, the Nb content is 0.1% or less. In order to suppress the formation of the band-like structure and further improve the properties of the cut end face, the Nb content is preferably 0.05% or less, and more preferably 0.03% or less. In addition, it is preferable to make Nb content 0.002% or more in order to acquire the effect by the said action more reliably. More preferably, the content is 0.006% or more, and particularly preferably 0.008% or more.

One or more selected from the group consisting of V: 0.4% or less, Mo: 0.4% or less, W: 0.4% or less, and Cr: 0.4% or less. Also combines with C to form fine carbides, and has the effect of refining and strengthening the steel. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if the content of any element exceeds 0.4%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Accordingly, the content of each element is as described above. The content of each element is preferably 0.35% or less, and more preferably 0.30% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to contain any element 0.01% or more. It is more preferable to contain any element in an amount of 0.05% or more, and it is particularly preferable to contain any element in an amount of 0.08% or more.

One or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less These elements all form oxides in the molten steel. It has the effect of improving the cleanliness of steel by deoxidation and contributes to the improvement of properties. In addition, since it functions as a carbonitride formation nucleus, when it is appropriately finely dispersed, it also has the effect of suppressing the formation of coarse short nitrides and suppressing cracking of the cut end face. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if any element is contained in excess of 0.01%, coarse oxides are formed, and on the contrary, the cleanliness of the steel is lowered and the formability is impaired. Accordingly, the content of each element is as described above. The content of each element is preferably 0.0050% or less, and more preferably 0.0003% or less. In addition, in order to acquire the effect by the said action | operation more reliably, it is preferable to contain any element of 0.0002% or more. It is more preferable to contain any element at 0.0005% or more, and it is particularly preferable to contain any element at 0.0001% or more.

  Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

2. Metal structure Total of ferrite and bainite: 80% or more In order to ensure formability, the total area ratio of ferrite and bainite in the metal structure is 80% or more. This area ratio is preferably 90% or more, and may be 100%. Therefore, the main phase of the metal structure is ferrite and / or bainite. When emphasizing ductility, the ferrite area ratio is preferably 10% or more, and more preferably 20% or more.

Pearlite: 5% or less If the area ratio of pearlite in the structure exceeds 5%, it becomes a starting point of cracking at the time of cutting and deteriorates the end face. Perlite is preferably 3% or less.

Martensite: 10% or less When the area ratio of martensite in the structure exceeds 10%, it becomes a starting point of cracking at the time of cutting, and deteriorates the end face. Since martensite is contained in a small amount and the hole expandability is remarkably deteriorated, it is preferably 3% or less. More preferably, it is 1% or less.

Residual austenite: 3% or less Residual austenite (residual γ) is transformed into martensite at the time of punching and deteriorates the hole expansibility, so that the area ratio in the structure is 3% or less. Residual γ is preferably set to 1% or less in order to cause cracking of the end face when the cutting clearance is increased and to deteriorate the cut end face properties.

  As for pearlite, martensite, and residual γ other than the main phase, any one or more phases may be 0%, or all three phases may be 0%. The metal structure may be, for example, a ferrite single phase structure (the area ratio of ferrite is 100%). However, considering the balance between strength and elongation, the main phase is preferably a multiphase structure of ferrite and bainite. The above-mentioned area ratio of each phase shall be measured at the plate thickness center of the steel plate (1/2 depth position of the plate thickness).

Metallic structure of the plate thickness center portion In the present invention, the “plate thickness center portion” means a range of ± 0.12 t from the plate thickness center when the plate thickness is t. That is, a region of 12% of the plate thickness from the plate thickness center to both sides in the plate thickness direction is defined as the plate thickness center portion.

1) Area ratio of crystal colonies oriented in the direction perpendicular to the steel sheet surface with <112> crystal orientation within 10 °: 35% or less, and 2) Area ratio of crystal colonies having a circle equivalent diameter of 10 μm or more in the crystal colonies : 30% or less.

  “Crystal colony” is also called an orientation colony and means a group of adjacent crystal grains having substantially the same crystal orientation. The crystal colony in the present invention is a crystal group in which the <112> crystal orientation is oriented within 10 ° in the direction perpendicular to the steel plate surface. This crystal colony can be identified by orientation analysis by the EBSD (electron beam backscatter diffraction) method of the cross section in the plate thickness direction of the steel plate.

  In the central portion of the plate thickness, the area ratio of crystal colonies (hereinafter also referred to as “<112> oriented grains”) in which the <112> crystal orientation is oriented within 10 ° in the direction perpendicular to the steel plate surface is 35% or less. In addition, by setting the area ratio of a coarse crystal colony having a circle equivalent diameter of 10 μm or more in the crystal colony to 30% or less, cracking of the cut end face can be suppressed.

  Although the mechanism by which the cracking of the cut end face is suppressed is not clear by doing in this way, the occurrence of the crack at the cut end face is strongly influenced by the structure of the central part of the plate thickness, and the above orientation is a cleavage plane of the BCC. If the angle formed with a certain {100} plane is small and there are many crystal grain colonies with such an orientation, or if the size is coarse, coarse inclusions, crystallized matter, precipitates and cracks generated from the interface with the matrix It is presumed that this is likely to facilitate the propagation of. The area ratio of the crystal colonies is preferably 30% or less, and the area ratio of crystal colonies having a circle-equivalent diameter of 10 μm or more occupied in the crystal colonies is preferably 25% or less.

3) Area ratio of crystal colonies in which the ratio (a / b) of the short axis a to the long axis b in the crystal colonies is 0.35 or less: 40% or less The aspect occupied in the crystal colonies in the center of the plate thickness When the area ratio of flat crystal colonies having a ratio (minor axis / major axis) of 0.35 or less is set to 40% or less, it is preferable because cracking of the cut end surface is further suppressed. Among the <112> oriented grains, the {112} <110> orientation is a ferrite transformation orientation from austenite in an unrecrystallized state, and tends to produce a grain colony extending in a band shape along the rolling direction. In order to remarkably deteriorate the end face properties when performed in parallel with the rolling direction, it is preferable to reduce the area ratio of such flat colonies in the steel sheet to 40% or less. The area ratio is more preferably 30% or less.

4) Total number density of inclusions, crystallized substances, and precipitates having an equivalent circle diameter of 5 μm or more present in the center portion of the plate thickness: 50 pieces / mm ² or less The equivalent circle diameter existing in the center portion of the plate thickness is 5 μm or more. Coarse inclusions, crystallized substances, and precipitates become starting points of coarse cracks between the matrix and the matrix during cutting, and deteriorate the end face properties. Therefore, it is preferable that the number density of inclusions, crystallized substances, and precipitates having an equivalent circle diameter of 5 μm or more present in the center portion of the plate thickness is 50 pieces / mm ² or less. More preferably, it is 30 pieces / mm ² or less.

  As is well known to those skilled in the art, inclusions are compounds that are already present in molten steel, crystallized products are compounds that crystallize during the solidification process, and precipitates are compounds that are formed in the solid state mainly during heat treatment. However, although the generation times are different from each other, the problems described above arise when the equivalent circle diameter becomes as large as 5 μm or more.

3. Properties The hot-rolled steel sheet according to the present invention has a high strength with a tensile strength of 590 MPa or more, and has excellent shear workability and ductility that hardly cause cracks in the cut end face even when shearing is performed with a clearance of 5 to 30%. Have

  The tensile strength is preferably 650 MPa or more. Regarding the ductility, it is preferable that the total elongation is 15% or more and the product of tensile strength × total elongation is 14000 MPa ·% or more. The product of tensile strength × total elongation, which is an index of formability, is more preferably 15500 MPa ·% or more.

4). Manufacturing conditions Although the manufacturing method of the hot-rolled steel plate which has the above-mentioned chemical composition, metal structure, and characteristic concerning this invention is not specifically limited, It is suitable to employ | adopt the following manufacturing conditions. The slab used for hot rolling is preferably manufactured by continuous casting with good work efficiency.

1) Average solidification rate at the center of the slab: 0.5 ° C / sec or more and 5 ° C / sec or less In order to suppress crystallization of coarse inclusions, the cooling rate at the center of the slab during continuous casting should be 5 ° C / sec or less. Is preferred. If the solidification rate is too high, it may cause surface cracking of the slab, so it is preferable to set it to 0.5 ° C./sec or more.

2) Slab heating temperature: 1100 ° C. or higher and 1300 ° C. or lower In the hot rolling step, the slab is preferably held at 1100 ° C. or higher for 1 hour or longer. If the temperature is lower than this, the Ti carbonitride deposited during solidification after casting remains undissolved, which not only lowers the strength but also degrades the end face properties during cutting. On the other hand, if the heating temperature is too high or the heating time is too long, the yield is deteriorated due to scale loss. Therefore, the upper limit of the heating temperature is preferably 1300 ° C. or lower.

3) Total rolling reduction at 1100 ° C. to 1000 ° C .: 35% or more In the hot rolling process, the total rolling reduction in the temperature range of 1100 ° C. to 1000 ° C. is preferably 35% or more. When the rolling reduction is lower than this, specific crystal grain colonies become coarse, and the end face properties in the shearing process are lowered. This total rolling reduction is preferably 40% or more.

4) Total rolling reduction at 1000 ° C. or less: 10% or more and 70% or less Band structure of crystal colony having an orientation that causes remarkable development of texture and reduces cutting ability when the rolling reduction is excessively increased in the hot rolling process. Therefore, the total rolling reduction in the temperature range of 1000 ° C. or lower is preferably 70% or lower. If the rolling reduction at this time is too small, it may cause a shape defect. Therefore, the total rolling reduction is preferably 10% or more.

5) Finishing temperature: 930 ° C. or higher and 980 ° C. or lower In the hot rolling step, the finish rolling temperature is preferably 930 ° C. or higher. If the finishing temperature is lower than this, banding of the structure proceeds and causes end face defects. If the finish rolling temperature is too high, austenite is coarsened, the structure after transformation is likely to be non-uniform, and the end face properties are deteriorated.

6) Air cooling time after rolling: 3 seconds or more and 10 seconds or less It is preferable that the air cooling time after rolling in the hot rolling step is 3 seconds or more. If the air cooling time is less than this, the banding of the tissue proceeds and causes end face defects. On the other hand, if the air-cooling time after rolling is too long, coarse Ti carbonitride precipitates and the strength decreases, so that it is preferably 10 seconds or less.

7) Cooling rate: 15 ° C./sec or more and 200 ° C./sec or less It is preferable to set the cooling rate after hot rolling / air cooling to 15 ° C./sec or more. When the cooling rate is slower than this, ferrite precipitates coarsely at a high temperature, and Ti carbonitride precipitates coarsely, resulting in a decrease in strength. On the other hand, if the cooling rate is too high, the surface layer portion is likely to be overcooled, and a low temperature transformation phase is formed on the surface layer portion to impair elongation. Therefore, the upper limit of the cooling rate is preferably 200 ° C./sec or less. . This cooling is called primary cooling. This primary cooling is performed by forced cooling such as water cooling.

8) Primary cooling stop temperature: 400 ° C. or higher and 750 ° C. or lower In the hot rolling step, the primary cooling stop temperature is preferably set to 400 ° C. or higher. When the primary cooling stop temperature is lower than this, a hard phase is formed in the structure, and the elongation or hole expanding property deteriorates. On the other hand, if the primary cooling stop temperature is too high, the strength decreases due to the coarsening of the Ti carbonitride that precipitates during the cooling process, and therefore, the temperature is preferably 750 ° C. or lower. The primary cooling stop temperature is more preferably 450 ° C. or higher and 730 ° C. or lower.

9) Residence time: 2 seconds or more and 15 seconds or less After the primary cooling is stopped in the above temperature range, the cooling may be temporarily interrupted to provide a residence time of 2 seconds or more at the temperature. Ferrite precipitation is stable and has the effect of improving the balance between strength, elongation and hole expansibility. If the residence time is too long, precipitation of pearlite and coarsening of Ti carbonitride occur, and the balance between strength and hole expansibility decreases, so the residence temperature is preferably 15 seconds or less. More preferably, it is 5 seconds or more and 10 seconds or less. This residence time may not be provided. In that case, air cooling may be performed immediately after reaching the winding temperature described below immediately after stopping the primary cooling.

10) Winding temperature: 400 degreeC or more and 500 degrees C or less It is preferable to implement coil winding in a hot rolling process at 400 degreeC or more. On the other hand, when the coiling temperature is too high, the properties of the cut end face are deteriorated.

11) Others Conditions other than the above may follow a general hot rolling process. For example, in order to obtain good surface properties, it is possible to appropriately perform descaling after extraction from the heating furnace until the end of finish rolling.

  Steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace using a 200 mm × 200 mm × 200 mm mold. Cooling was performed under different conditions after casting, and the solidification rate at the center of the ingot was changed as shown in Table 2. The value of the solidification rate was calculated from the interval between the secondary arms of the dendride. The obtained ingot was heated to 1200 ° C., slabs were collected from the coarse bar obtained by hot forging, and used for the experiment.

  A plurality of passes were rolled using a 3-stand hot rolling mill, and hot rolling was performed under the rolling conditions and cooling conditions shown in Table 2. In the table, MFR is the total rolling reduction in the temperature range of 1100 to 1000 ° C, and TFR is the total rolling reduction in the temperature range of 1000 ° C to finishing temperature. The steel plate temperature during rolling was measured by a radiation thermometer provided on the entry side and the exit side of each rolling mill, and MFR and TFR were controlled. After rolling, the steel sheet was cooled under the conditions shown in Table 3 to produce a hot rolled steel sheet. The thickness of the obtained hot-rolled steel sheet was in the range of 2.0 to 3.6 mm.

  Small pieces were cut out from the obtained hot-rolled steel sheet, and the metal structure and mechanical properties were examined as described below. The survey results are summarized in Table 3.

  A section in the thickness direction along the rolling direction was cut out from the produced hot-rolled steel sheet, made into a mirror surface by mechanical polishing after resin embedding, and then the metal structure at the center of the thickness was revealed by night corrosion, and an optical microscope and SEM Thus, the tissue was identified, and the area ratio of each phase was determined by image analysis.

  Separately, a cross section in the plate thickness direction along the rolling direction is cut out and made into a mirror surface by mechanical polishing, and then a sample in which polishing scratches are removed by mechanical chemical polishing with colloidal silica is prepared, and crystal grain orientation analysis is performed by EBSD analysis Went. The observation magnification was 200 times, and an area of 700 μm × 700 μm was measured at 1 μm intervals. At the time of analysis, only a boundary having an angle of 15 ° or more with an adjacent grain is regarded as a large-angle grain boundary, and a crystal colony in which the <112> crystal orientation is oriented within 10 ° in the direction perpendicular to the steel plate surface at the center of the plate thickness. The distribution of the equivalent circle diameter and aspect ratio (a / b ratio, a = minor axis length, b = major axis length) was examined.

  Using the same sample as the above-mentioned crystal colony measurement, the number of inclusions, crystallized substances and precipitates (indicated as the total number of inclusions in Table 3) having a circle equivalent diameter of 5 μm or more present at the center of the plate thickness is examined. It was. The survey was conducted using SEM at 25 magnifications at a magnification of 1000, and the number density of the total number of inclusions was determined as an average value of the total number of each visual field.

  Regarding the mechanical properties, the tensile strength was examined by carrying out a static tensile test with an Instron type tensile tester using a JIS No. 5 tensile test piece. Table 3 shows the tensile strength (TS), total elongation (EL), and TS × EL values that are indicators of moldability.

  For the evaluation of the shear end face, a small piece 25 mm long x 60 mm wide is cut out along the rolling direction, the cutting clearance is 15% of the plate thickness, and cutting is performed by shearing 25 mm long x 10 mm wide along the rolling direction. The end face properties were evaluated. The measurement was performed at n = 3, and when n = 2 or more, cracks occurred on the end face (×), when cracks occurred in one of three samples (Δ), and when no cracks occurred ( Evaluation was made at three levels. ○ is desirable, but Δ is acceptable in practice. Note that “-” indicates that the evaluation is not performed.

  In Test No. 1, since the C content is high, ferrite is not sufficiently precipitated, and the structure is mainly composed of martensite, the strength is excessive, the ductility is very low, and the TS × EL value that is an index of formability is also extremely low. The formability was inferior. Moreover, there were many crystal colonies oriented in the <112> crystal orientation in the direction perpendicular to the steel plate surface, and they were coarse. As a result, the cut end face was cracked and the shear workability was poor.

  Test No. 2 had a high content of P and S, and the moldability was unsatisfactory. In addition, there were many crystal colonies oriented in the <112> crystal orientation in the direction perpendicular to the steel sheet surface, and because they were coarse and band-shaped, cracks at the cut end face in shearing were unsatisfactory.

  In Test No. 3, the Ti content was large, and the formability became poor due to the formation of coarse inclusions. In addition, there were many crystal colonies oriented in the <112> crystal orientation in the direction perpendicular to the steel sheet surface, and because they were coarse, cracks at the cut end face in shearing were unsatisfactory.

  Test No. 4 was wound without stopping the primary cooling, so that there were many crystal colonies oriented in the <112> crystal orientation in the direction perpendicular to the steel sheet surface, and because they were coarse, the cut end face cracks in the shearing process were poor. became.

  On the other hand, in Test Nos. 4 to 12 according to the present invention, the ductility and TS × EL values are good, the moldability is excellent, and the occurrence of end face cracks is effectively suppressed even in shearing, and shear workability is improved. It was also excellent. In particular, when the area ratio of the crystal colonies having a circle equivalent diameter of 10 μm or more in the crystal colonies is 25% or less, the occurrence of end face cracks was more effectively suppressed.

Claims (6)

In mass%, C: 0.01% or more and 0.20% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.5% or more and 3.0% or less, P: 0.02% or less , S: 0.005% or less, sol.Al: 0.02% or more and 0.5% or less, Ti: 0.02% or more and 0.25% or less and N: 0.01% or less, the balance A chemical composition comprising Fe and impurities;
In area%, ferrite and bainite: 80% or more in total, pearlite: 5% or less, martensite: 10% or less, and retained austenite: 3% or less, from the thickness center when the thickness is t In the central portion of the plate thickness within the range of ± 0.12 t, the area ratio of the crystal colonies oriented in the direction perpendicular to the steel plate surface with the <112> crystal orientation within 10 ° is 35% or less, and the crystal colonies A metal structure in which the area ratio of crystal colonies having a circle equivalent diameter of 10 μm or more in the sample is 30% or less;
A tensile strength of 590 MPa or more;
A hot-rolled steel sheet characterized by comprising:   The hot rolled steel sheet according to claim 1, wherein the area ratio of the crystal colonies in which the ratio (a / b) of the minor axis a to the major axis b in the crystal colonies is 0.35 or less is 40% or less. 3. The hot-rolled steel sheet according to claim 1, wherein the total number density of inclusions, crystallized substances, and precipitates having an equivalent circle diameter of 5 μm or more existing in the center part of the plate thickness is 50 pieces / mm ² or less.   The hot-rolled steel sheet according to any one of claims 1 to 3, wherein the chemical composition contains Nb: 0.1% or less in mass% instead of part of Fe.   The chemical composition comprises, in place of a part of Fe, mass%, V: 0.4% or less, Mo: 0.4% or less, W: 0.4% or less, and Cr: 0.4% or less. The hot-rolled steel sheet according to any one of claims 1 to 4, comprising one kind or two or more kinds selected from the group.   The chemical composition may be one selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less in mass% instead of part of Fe. The hot rolled steel sheet according to any one of claims 1 to 5, comprising two or more kinds.