JP6701954B2 - High-strength hot-rolled steel sheet excellent in hole expandability and weld fatigue property and method for producing the same - Google Patents

Description

  The present invention mainly relates to a high-strength hot-rolled steel sheet used for structural parts such as automobile chassis, in particular, a high-strength hot-rolled steel sheet having excellent hole-expansion workability and excellent fatigue properties of an arc welded portion and its production It is about the method. The high-strength hot-rolled steel sheet referred to in the present invention has a strength of 780 MPa or more.

  2. Description of the Related Art In recent years, the strength of automobile steel sheets has rapidly increased in order to reduce the weight of automobiles in order to improve fuel efficiency and ensure safety in the event of a collision. Since strengthening generally causes deterioration of workability, the development of steel having a microstructure in which strength and workability are compatible is actively performed by optimizing the component composition and the manufacturing method. DP steel, TRIP steel, and precipitation strengthened steel have been developed.

  The hole expansibility is an important characteristic especially in the processing of underbody parts, and steel sheets having a relatively small strength distribution in the structure such as precipitation strengthened steel and bainite single phase steel to which Ti or Nb is added have been developed. ing.

  On the other hand, the fatigue characteristics generally improve as the strength of the base material increases. However, in particular, the improvement in the fatigue characteristics of the welded portion is smaller than the improvement in the base material strength, and in some cases, it may be reduced as compared with the low strength material. Therefore, a high-strength steel sheet having excellent weld fatigue characteristics is required.

  For example, in Patent Document 1, C: 0.005 to 0.20%, Si: 0.005 to 1.0%, Mn: 0.1 to 2.5%, and P: 0. 050 to 0.10%, S: 0.001 to 0.010%, Al: 0.005 to 0.1%, N: 0.0005 to 0.0100%, Cu: 0.10 to 0.50% , Nb: 0.01 to 0.05%, Mo: 0.1 to 0.50%, Ni: 0.05 to 0.50%, balance Fe and unavoidable impurities, and an elongation of 1.0%. Disclosed is a high-strength hot-rolled steel sheet which is excellent in corrosion resistance and weld fatigue characteristics to which strain of less than 10.0% is added.

  The high-strength hot-rolled steel sheet disclosed in Patent Document 1 is intended to suppress the softening of the heat-affected zone of the welded portion and improve the fatigue characteristics by the combined addition of Mo and Nb. Although it is shown that the softening of the part is suppressed, it has not been clarified whether or not the fatigue property is actually improved. Further, Patent Document 1 does not describe the change in texture due to the combined addition of Mo and Nb.

  In order to improve the fatigue characteristics, it is necessary to suppress the occurrence of fatigue cracks or the development of cracks, but as a thick steel plate with a low fatigue crack propagation rate, Patent Document 2 discloses that 0.015≦C≦% by weight. 0.20, 0.05 ≤ Si ≤ 2.0, 0.1 ≤ Mn ≤ 2.0, P ≤ 0.05, S ≤ 0.02, balance Fe and inevitable impurities, X-ray The (200) diffraction intensity ratio in the plate thickness direction measured in 1. is 2.0 to 15.0, and the area ratio of recovered or recrystallized ferrite grains is 15 to 40%. A steel plate with a low fatigue crack propagation rate is disclosed.

  The thick steel plate disclosed in Patent Document 2 is characterized by a (200) diffraction intensity ratio of 2.0 to 15.0 in the plate thickness direction, but does not specify the crystal orientation of the plate thickness surface layer portion.

  In Patent Document 3, in a multi-layer thick steel sheet composed of a total of three layers of a surface layer, an inner layer, and a back layer, the surface layer and the back layer are, by weight %, C: 0.005 to 0.15%, Si:0. 0.01 to less than 1.0%, Mn: 0.2 to 1.5%, P≦0.03%, S≦0.01%, Ceq≦0.24, balance Fe and inevitable impurities, and inner layer Satisfies Ceq: 0.30 to 0.70, the thickness of the surface layer and the back layer is 1.5 to 10 mm, and the ratio of the total thickness of the surface layer and the back layer to the total thickness of the steel sheet is 0.05 to. Disclosed is a high-strength multi-layer thick steel plate for welded structures, which is excellent in fatigue strength of the welded portion, which is 0.30.

  The high-strength clothing thick steel plate for welded structure disclosed in Patent Document 3 is characterized by the composition of the front and back layers and the ratio of the total thickness difference of the front and back layers to the total thickness difference of the steel plate. The purpose is to suppress the occurrence of cracks, and no texture is specified.

JP 05-195142 A Japanese Unexamined Patent Publication No. 08-199286 Japanese Patent Laid-Open No. 08-225885

  In view of the problems of conventional steel sheets, the present invention optimizes hot rolling conditions in a high-strength steel sheet, optimizes the crystal orientation of a shear fault that develops from the surface layer of the hot-rolled steel sheet to the 1/6th thick position, and the component composition. The present invention aims to provide a high-strength steel sheet and a method for manufacturing the same, which aims to significantly improve the fatigue strength of the welded portion while ensuring the hole expandability by solving the above problem.

  The present inventors diligently studied a method for solving the above problems. As a result, in the steel sheet of the composition system containing Nb and/or Ti, if the composition of the composition and the hot rolling conditions are optimized, it is possible to secure the hole expansibility and to obtain the high strength of TS780 MPa class or more excellent in the fatigue property of the welded portion. It was found that a hot-rolled steel sheet can be obtained.

  That is, the inventors of the present invention have a {110}<111> to {110}<001> orientation that significantly promotes crack propagation in welds in a surface texture that easily develops in steel containing Nb and/or Ti. It was newly found that excellent weld fatigue characteristics can be obtained by suppressing the groups as much as possible and increasing the {211}<111> orientation.

  The present invention has been made based on the above findings, and the summary thereof is as follows.

(1) The composition of the components is% by mass, C: 0.02% or more, 0.15% or less, Si: 0.01% or more, 2.00% or less, Mn: 0.50% or more, 2.50. % Or less, P: 0.001% or more, 0.100% or less, S: 0.0005% or more, 0.050% or less, Al: 0.01% or more, 0.50% or less, N: 0.0001 % Or more and 0.010% or less, and further, Ti: 0.01% or more and 0.14% or less, and Nb: 0.005% or more and 0.09% or less, one or two types are represented by the following formulas. Including in a range satisfying (1) and (2), the balance consists of iron and inevitable impurities,
Random strength ratio of {110}<111> to {110}<001> orientation groups in the region from the outermost layer to the plate thickness 1/6 is 3.5 or less, and hole expansibility and weld fatigue. High-strength hot-rolled steel sheet with excellent properties.
65≦315×[C]+40×[Mn]+11×[Si]−30×[Al]
+(35×[V]+20×[Cr]+17×[Ni]+10×[Cu]
+5×[Mo])≦150 (1)
0.1×[Mn]+9×[Mo]+2×[Al]≦1.1 (2)
[Element]:% by mass of element

  (2) The component composition is, in mass %, B: 0.0003% or more, 0.005% or less, Mo: 0.02% or more, 0.50% or less, Cr: 0.10% or more, 2.00% or less, W: 0.01% or more, 2.00% or less, Cu: 0.04% or more, 2.00% or less, Ni: 0.02% or more, 1.00% or less, V: The high-strength hot-rolled steel sheet having excellent hole expansibility and welded portion fatigue characteristics according to (1) above, containing one or more of 0.001% or more and 0.10% or less.

  (3) The component composition further contains, in mass%, one or more of Ca, Mg, Zr, and REM in a total amount of 0.0005% or more and 0.050% or less. A high-strength hot-rolled steel sheet having excellent hole expansibility and weld fatigue characteristics according to (1) or (2) above.

  (4) Any of (1) to (3) above, wherein the random intensity ratio of {211}<111> orientations in the region from the outermost layer to the plate thickness 1/6 is 2.0 or more. High-strength hot-rolled steel sheet with excellent hole expandability and weld fatigue properties described in.

(5) A production method for producing a high-strength hot-rolled steel sheet having excellent hole expandability and weld fatigue characteristics according to any one of (1) to (4) above,
(I) A steel slab having the component composition according to any one of (1) to (3) above is heated to 1150° C. or higher and 1300° C. or lower, subjected to hot rolling, and the shape ratio L in the final pass. as the sum of the shape ratio L _f-1 in the preceding stage of the path _f and final pass satisfies the following formula (3), and terminates the hot rolling in a temperature range of above 900 ° C.,
(Ii) After completion of hot rolling, the hot-rolled steel sheet is cooled (ii-1) at a cooling rate of 10°C/sec or more to a cooling stop temperature of 600 to 850°C, and (ii-2) at a cooling stop temperature. Hold for 1 to 10 seconds, and after holding (ii-3), cool again at a cooling rate of 10° C./second or more to a winding temperature of 700° C. to room temperature, and wind up. A method for producing a high-strength hot-rolled steel sheet having excellent weld fatigue characteristics.
L _f +L _f-1 ≧8.0 (3)
L _f =√{R _f ×(t _in (f)−t _out (f))}÷(2t _out (f)+t _in (f))/3
L _f : Shape ratio in final pass R _f : Roll radius in final pass (mm)
t _in (f): Inlet plate thickness (mm) at the final pass
t _out (f): Output side plate thickness (mm) at the final pass
L _f-1 =√{R _f-1 ×(t _in (f-1)−t _out (f-1))}÷(2t _out (f-1)+t _in (f-1))/3
L _f-1 : Shape ratio one step before the final pass R _f-1 : Roll radius (mm) one step before the final pass
t _in (f-1): Thickness of entrance side (mm) one step before the final pass
t _out (f-1): Thickness of exit side one step before final pass (mm)

  According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having excellent hole expandability and weld fatigue characteristics.

The crystal orientation distribution function (ODF) of the cross section of φ2=45° showing the crystal orientation of the present invention is shown. It is a figure which shows the shape of the test piece used for a plane bending fatigue test.

The high-strength hot-rolled steel sheet having excellent hole expandability and weld fatigue property of the present invention (hereinafter sometimes referred to as “hot-rolled steel sheet of the present invention”),
The composition is% by mass, C: 0.02% or more, 0.15% or less, Si: 0.01% or more, 2.00% or less, Mn: 0.50% or more, 2.50% or less, P: 0.001% or more, 0.100% or less, S: 0.0005% or more, 0.050% or less, Al: 0.01% or more, 0.50% or less, N: 0.0001% or more, 0.010% or less, further, Ti: 0.01% or more, 0.14% or less, and Nb: 0.005% or more, 0.09% or less, one or two, the following formula (1) And (2) are included in a range satisfying the condition, and the balance is iron and inevitable impurities,
The random intensity ratio of the {110}<111> to {110}<001> orientation groups in the region from the outermost layer to the plate thickness 1/6 is 3.5 or less.
65≦315×[C]+40×[Mn]+11×[Si]−30×[Al]
+(35×[V]+20×[Cr]+17×[Ni]+10×[Cu]
+5×[Mo])≦150 (1)
0.1×[Mn]+9×[Mo]+2×[Al]≦1.1 (2)
[Element]:% by mass of element

The method for producing a high-strength hot-rolled steel sheet excellent in hole expansibility and weld fatigue property of the present invention (hereinafter sometimes referred to as “the present invention production method”) is a production method for producing the hot-rolled steel sheet of the present invention. hand,
(I) A steel slab having the component composition of the hot rolled steel sheet of the present invention is heated to 1150° C. or higher and 1300° C. or lower and subjected to hot rolling, and the shape ratio L _f in the final pass and one step before the final pass. Hot rolling is completed in a temperature range of 900° C. or higher so that the sum of the shape ratios L _f-1 in
(Ii) After completion of hot rolling, the hot-rolled steel sheet is cooled (ii-1) at a cooling rate of 10°C/sec or more to a cooling stop temperature of 600 to 850°C, and (ii-2) at a cooling stop temperature. It is characterized in that it is held for 1 to 10 seconds, and after (ii-3) holding, it is again cooled at a cooling rate of 10° C./second or more to a winding temperature of 700° C. to room temperature and wound up.
L _f +L _f-1 ≧8.0 (3)
L _f =√{R _f ×(t _in (f)−t _out (f))}÷(2t _out (f)+t _in (f))/3
L _f : Shape ratio in final pass R _f : Roll radius in final pass (mm)
t _in (f): Inlet plate thickness (mm) at the final pass
t _out (f): Output side plate thickness (mm) at the final pass
L _f-1 =√{R _f-1 ×(t _in (f-1)−t _out (f-1))}÷(2t _out (f-1)+t _in (f-1))/3
L _f-1 : Shape ratio one step before the final pass R _f-1 : Roll radius (mm) one step before the final pass
t _in (f-1): Thickness of entrance side (mm) one step before the final pass
t _out (f-1): Thickness of exit side one step before final pass (mm)

  The hot rolled steel sheet of the present invention and the manufacturing method of the present invention will be described below.

  First, the reasons for limiting the component composition of the hot-rolled steel sheet of the present invention will be described. Hereinafter, “%” relating to the component composition means “mass %”.

Component composition C: 0.02% or more and 0.15% or less C is an element effective in improving strength. If C is less than 0.02%, the required strength cannot be secured, so C is set to 0.02% or more. It is preferably 0.03% or more, more preferably 0.05% or more.

  On the other hand, when C exceeds 0.15%, the strength is excessively increased, the ductility is reduced, and the weldability is reduced, so C is set to 0.15% or less. It is preferably 0.12% or less, more preferably 0.09% or less.

Si: 0.01% or more and 2.00% or less Si is an element that contributes to the improvement of strength. If Si is less than 0.01%, the effect of addition is not sufficiently obtained, so Si is set to 0.01% or more. From the viewpoint of weldability, the content is preferably 0.05% or more, more preferably 0.10% or more.

  On the other hand, if Si exceeds 2.00%, the workability is deteriorated and the surface properties are deteriorated, so Si is 2.00% or less. It is preferably 1.80% or less, more preferably 1.50% or less.

Mn: 0.50% or more and 2.50% or less Mn is an element that enhances hardenability and contributes to improvement of strength. If Mn is less than 0.50%, the effect of addition cannot be sufficiently obtained, so Mn is set to 0.50% or more. It is preferably 1.00% or more, more preferably 1.30% or more.

  On the other hand, if Mn exceeds 2.50%, weld cracking susceptibility increases, so Mn is set to 2.50% or less. It is preferably 2.20% or less, more preferably 2.00% or less.

P: 0.001% or more and 0.100% or less P is an element that contributes to the improvement of strength. If P is less than 0.001%, the effect of addition is not sufficiently obtained, so P is made 0.001% or more. It is preferably 0.005% or more, more preferably 0.010% or more.

  On the other hand, if P exceeds 0.100%, it segregates to the grain boundaries and inhibits local ductility, weldability, and toughness, so P is made 0.100% or less. It is preferably 0.070% or less, more preferably 0.050% or less.

S: 0.0005% or more and 0.050% or less S is an element that produces MnS and inhibits local ductility, weldability, and toughness. If S is reduced to less than 0.0005%, the steelmaking cost rises significantly, so S is made 0.0005% or more. It is preferably 0.0010% or more, more preferably 0.0050% or more.

  On the other hand, if S exceeds 0.050%, the local ductility, weldability, and toughness deteriorate significantly, so S is set to 0.050% or less. It is preferably 0.030% or less, more preferably 0.010 or less.

Al: 0.01% or more and 0.50% or less Al is an element that functions as a deoxidizer. If Al is less than 0.01%, the effect of addition is not sufficiently obtained, so Al is made 0.01% or more. It is preferably at least 0.05%, more preferably at least 0.08%.

  On the other hand, when Al exceeds 0.50%, the steel becomes brittle and promotes the development of the {110}<111> to {110}<001> orientation groups of the surface layer to hinder the fatigue strength of the welded portion. Therefore, Al is 0.50% or less. It is preferably 0.30% or less, more preferably 0.20% or less.

N: 0.0001% or more and 0.010% or less N is an element that forms a nitride and inhibits ductility and hole expansibility, and also causes blowholes during welding, resulting in weld fatigue. It is an element that impedes the properties.

  If N is reduced to less than 0.0001%, the steelmaking cost will increase significantly, so N is made 0.0001% or more. It is preferably 0.0015% or more, more preferably 0.0035% or more.

  On the other hand, if N exceeds 0.010%, ductility, hole expandability, and weld fatigue characteristics are significantly reduced, so N is made 0.010% or less. It is preferably 0.008% or less, more preferably 0.006% or less.

Ti: 0.01% or more and 0.14% or less Ti is an element that, as TiC, precipitates in ferrite or bainite during cooling or winding and contributes to the improvement of strength. If Ti is less than 0.01%, TiN is consumed in austenite, and a sufficient precipitation strengthening effect cannot be obtained. Therefore, Ti is set to 0.01% or more. It is preferably 0.03% or more, more preferably 0.05% or more.

  On the other hand, if the Ti content exceeds 0.14%, the toughness and weldability deteriorate, so the Ti content is set to 0.14% or less. It is preferably 0.12% or less, more preferably 0.10% or less.

Nb: 0.005% or more and 0.09% or less Nb, like Ti, precipitates as NbC and contributes to the improvement of strength, and also contributes to the improvement of ductility and hole expandability by refining crystal grains. It is an element. If Nb is less than 0.005%, the effect of addition is not sufficiently obtained, so Nb is set to 0.005% or more. It is preferably 0.008% or more, more preferably 0.010% or more.

  On the other hand, if Nb exceeds 0.09%, the toughness and ductility decrease, so Nb is set to 0.09% or less. It is preferably 0.07% or less, more preferably 0.05% or less.

  The composition of the hot-rolled steel sheet of the present invention is (a) one or more of B, Mo, Cr, W, Cu, Ni and V, and/or (b) Ca for improving steel sheet properties. You may contain 1 type(s) or 2 or more types of Mg, Zr, and REM (rare earth element).

(a) Group element B: 0.0003% or more and 0.005% or less B is an element that enhances hardenability and contributes to improvement of strength. If B is less than 0.0003%, the effect of addition cannot be sufficiently obtained, so B is preferably 0.0003% or more. More preferably, it is 0.0005% or more.

  On the other hand, if B exceeds 0.005%, the effect of addition is saturated while the toughness deteriorates, so B is preferably 0.005% or less. It is more preferably 0.003% or less.

Mo: 0.02% or more, 0.50% or less Cr: 0.10% or more, 2.00% or less W: 0.01% or more, 2.00% or less Mo, Cr, W are all quenched. It is an element that enhances the property and forms carbide to contribute to the improvement of strength. If Mo is less than 0.02%, Cr is less than 0.10%, or W is less than 0.01%, the effect of addition is not sufficiently obtained, so Mo is 0.02% or more and Cr is 0%. 10% or more and W is preferably 0.01% or more. More preferably, Mo is 0.05% or more, Cr is 0.13% or more, and W is 0.05% or more.

  On the other hand, when Mo exceeds 0.50% and Cr exceeds 2.00% or W exceeds 2.00%, ductility and weldability deteriorate, so Mo is 0.50% or less and Cr Is preferably 2.00% or less and W is preferably 2.00% or less. More preferably, Mo is 0.30% or less, Cr is 1.50% or less, and W is 1.50% or less.

Cu: 0.04% or more and 2.00% or less Cu is an element that contributes to the improvement of strength and enhances corrosion resistance and scale releasability. If Cu is less than 0.04%, the effect of addition is not sufficiently obtained, so Cu is preferably 0.04% or more. It is more preferably 0.10% or more. On the other hand, when Cu exceeds 2.00%, surface defects occur, so Cu is preferably 2.00% or less. It is more preferably 1.50% or less.

Ni: 0.02% or more and 1.00% or less Ni is an element that contributes to improvement of strength and toughness. If Ni is less than 0.02%, the effect of addition is not sufficiently obtained, so Ni is preferably 0.02% or more. It is more preferably 0.10% or more. On the other hand, if Ni exceeds 1.00%, the ductility decreases, so Ni is preferably 1.00% or less. It is more preferably 0.60% or less.

V: 0.001% or more and 0.10% or less V is an element that contributes to the improvement of strength. If V is less than 0.001%, the effect of addition is not sufficiently obtained, so V is preferably 0.001% or more. It is more preferably 0.010% or more. On the other hand, if V exceeds 0.10%, toughness decreases, so V is preferably 0.10% or less. It is more preferably 0.07% or less.

(b) Group element Ca, Mg, Zr, and one or more of REM: 0.0005% or more and 0.050% or less in total Ca, Mg, Zr, and REM are sulfides or oxides. It is an element that controls the shape of the material and contributes to the improvement of toughness.

  If the total content of one or more of Ca, Mg, Zr, and REM is less than 0.0005%, the effect of addition is not sufficiently obtained, so one of Ca, Mg, Zr, and REM is used. Alternatively, the total of two or more is preferably 0.0005% or more. More preferably, it is 0.0010% or more.

  On the other hand, if the sum of one or more of Ca, Mg, Zr, and REM exceeds 0.050%, the workability decreases, so one or two of Ca, Mg, Zr, and REM are used. The total of seeds or more is preferably 0.050% or less. It is more preferably 0.030% or less.

  In the composition of the hot rolled steel sheet of the present invention, the balance excluding the above elements is iron and inevitable impurities. The unavoidable impurities are elements (for example, Sn, As, etc.) that are inevitably mixed from the steel raw material and/or in the steelmaking process and remain within the range that does not impair the characteristics of the hot-rolled steel sheet of the present invention.

  Next, formulas (1) and (2) below, which are required to satisfy the composition of the hot-rolled steel sheet of the present invention, will be described.

65≦315×[C]+40×[Mn]+11×[Si]−30×[Al]
+(35×[V]+20×[Cr]+17×[Ni]+10×[Cu]
+5×[Mo])≦150 (1)
0.1×[Mn]+9×[Mo]+2×[Al]≦1.1 (2)
[Element]:% by mass of element

Formula (1) above
“315×[C]+40×[Mn]+11×[Si]−30×[Al]+(35×[V]+20×[Cr]+17×[Ni]+10×[Cu] in the above formula (1). “+5×[Mo])” (hereinafter sometimes referred to as “index X”) is a value obtained by comprehensively evaluating the degree (contribution degree) of each element contributing to the strength improvement, and contributing to the strength improvement. In the hot rolled steel sheet of the invention, it is an index showing the range of components necessary for making the base material strength and the fatigue strength of the arc welded portion compatible with each other.

  When the index X is less than 65, it is difficult to secure a strength of 780 MPa or more even if the precipitation strengthening by Ti and Nb is utilized, so the index X is set to 65 or more. It is preferably 75 or more, more preferably 85 or more.

  On the other hand, when the index X exceeds 120, the structure of the heat-affected zone after arc welding deteriorates, and even if the texture of the surface layer part is optimized, the fatigue strength of the weld part decreases, so the index X is It should be 150 or less. It is preferably 130 or less, more preferably 110 or less.

Formula (2) above
“0.1×[Mn]+9×[Mo]+2×[Al]” (hereinafter sometimes referred to as “index Y”) in the above formula (2) is {110}<111> of the plate thickness surface layer portion. ~ {110}<001> Orientation group is promoted and the degree of inhibition of elements that inhibit weld fatigue characteristics is comprehensively evaluated to secure required weld fatigue characteristics in the hot-rolled steel sheet of the present invention. It is an index to do.

  When the index Y exceeds 1.1, the {110}<111> to {110}<001> orientation groups of the plate thickness surface layer portion are strongly developed, the {112}<111> orientation is weakened, and the weld portion fatigue strength is increased. Therefore, the index Y is set to 1.1 or less. It is preferably 1.0 or less, more preferably 0.9 or less. The lower limit of the index Y is determined from the lower limits of Mn, Al, and Mo, and is not particularly limited.

  Although the mechanism by which Mn, Al, and/or Mo influences the formation of the texture of the surface layer is not clear, these elements influence the crystal rotation accompanying shear deformation during rolling, It is considered that the balance of the two bearing groups changes due to the influence. From this viewpoint, the index Y is preferably 1.0 or less, and more preferably 0.9 or less, as described above.

  Next, the random intensity ratio of the {110}<111> to {110}<001> orientation groups in the region from the outermost layer to the plate thickness 1/6 will be described.

Random strength ratio of {110}<111> to {110}<001> orientation groups in the region from the outermost layer to the plate thickness 1/6: 3.5 or less The fatigue strength of the weld is the crystal of the base metal before welding. It also changes depending on the bearing. In a steel sheet containing Nb, Ti, B, etc., a non-recrystallized shearing/transformation texture develops in the region from the outermost layer to the sheet thickness 1/6, and {110}<111> to {110}<001>. The direction group becomes stronger.

  In this azimuth group, the {100} plane, which is an open wall, is oriented perpendicularly to the thickness direction, and when the number of crystal grains whose {100} plane is oriented perpendicularly to the thickness direction increases, brittle fracture occurs. At this time, cracks in the plate thickness direction progress at once.

  When the random intensity ratio of {110}<111> to {110}<001> orientation groups in the region from the outermost layer to the plate thickness 1/6 exceeds 3.5, the {100} plane becomes perpendicular to the plate thickness direction. Since the number of facing crystal grains increases and the risk of brittle fracture increases, the random strength ratio of the {110}<111> to {110}<001> orientation groups in the region from the outermost layer to the plate thickness 1/6 is 3.5. Below. It is preferably 3.3 or less, more preferably 3.1 or less.

  The random strength ratio of the {110}<111> to {110}<001> orientation groups depends on the component composition, hot rolling conditions, and cooling conditions after hot rolling. Therefore, the lower limit is not specified, but it is 0 by definition. including.

  Further, similar to the {110}<111> to {110}<001> orientation group, the {211}<111> orientation, which is the main orientation of the unrecrystallized shear/transformation texture, contributes to the improvement of fatigue strength. It is the direction to do. Therefore, the random intensity ratio of {211}<111> orientation in the region from the outermost layer to the plate thickness 1/6 is preferably 2.0 or more. It is more preferably 2.5 or more.

  The random strength ratio of the {211}<111> orientation depends on the component composition, hot rolling conditions, and cooling conditions after hot rolling, so the upper limit is not specified, but no particular effect is obtained at 4.0 or higher. Therefore, 4.0 is a practical upper limit.

  The random intensity ratios of the {100} to {111}<011> orientation groups and the random intensity ratios of the {211}<111> orientations are obtained by using azimuth data measured by an EBSD (Electron Back Scattering Diffraction) method as a spherical harmonic function. It can be obtained from a crystal orientation distribution function (Orientation Distribution Function [ODF]) displaying a three-dimensional texture.

  FIG. 1 shows a crystal orientation distribution function (ODF) of a φ2=45° cross section showing the crystal orientation of the present invention. Strictly speaking, the {110}<111> to {110}<001> azimuth groups are Φ=90° and Φ1=35.26 to 90, as indicated by the black dots on the axis of Φ=90° in FIG. Refers to the range of °.

  However, since there is a measurement error due to the processing of the test piece and the setting of the sample, the maximum value of the random intensity ratio of the {110}<111> to {110}<001> orientation groups is shown by the hatched portion in FIG. , Φ=85 to 90°, φ1=35 to 90°, the maximum random intensity ratio.

  In the φ2=45° cross section of the three-dimensional texture, the {112}<111> orientation is in the range of Φ=30 to 40° and φ1=85 to 90° with the black dot shown in FIG. 1 as the center. The maximum value of the random intensity ratio is defined as the random intensity ratio of the above orientation.

  Regarding the crystal orientation, the orientation perpendicular to the plate surface is usually represented by [hkl] or {hkl}, and the orientation parallel to the rolling direction is represented by (uvw) or <uvw>. {Hkl} and <uvw> are generic terms for equivalent planes, and [hkl] and (uvw) refer to individual crystal planes. That is, since the hot rolled steel sheet of the present invention is targeted for the bcc structure, for example, (111), (-111), (1-11), (11-1), (-1-11), (-11) -1), (1-1-1), and (-1-1-1) are equivalent surfaces and are indistinguishable. In such a case, these orientations are collectively referred to as {111}.

  Since the ODF is also used to display the orientation of a crystal structure having low symmetry, it is usually displayed with φ1=0 to 360°, φ=0 to 180°, φ2=0 to 360°, and each orientation is [ hkl](uvw). However, since the hot-rolled steel sheet of the present invention is targeted for the bcc crystal structure having high symmetry, Φ and φ2 are displayed in the range of 0 to 90°.

  The range of φ1 changes depending on whether or not the change in symmetry due to deformation is taken into consideration when performing the calculation. In the hot rolled steel of the present invention, the calculation is performed in consideration of the symmetry (orthotropic), and φ1 =0 to 90° is displayed. That is, the average value of the same azimuth at φ1=0 to 360° is displayed on the ODF of 0 to 90°. In this case, [hkl](uvw) and {hkl}<uvw> are synonymous. For example, the (110)[1-11] random intensity ratio of the ODF in the φ2=45° cross section shown in FIG. 1 is the {110}<111> orientation random intensity ratio.

  The test piece is prepared so that the plate thickness cross section becomes the polished surface. The polishing method is not limited to a specific polishing method. For example, it is polished with colloidal silica or the like so as to have a smooth metal surface, or after mechanical grinding, electrolytic polishing is further performed. It is preferable to use a polishing method such as grinding by ion milling that causes as little distortion as possible on the surface.

  The measurement range is from the outermost surface of the plate thickness to the position where the plate thickness is ⅙, and the data measured on both sides are evaluated together. In the plate width direction, a wide area of 500 μm or more is measured. The measurement pitch is not particularly set, but it is preferable to set the measurement pitch so that the number of data is 4000 or more.

  Next, the manufacturing method of the present invention will be described.

The manufacturing method of the present invention is
(I) A steel slab having the component composition of the hot rolled steel sheet of the present invention is heated to 1150° C. or higher and 1300° C. or lower and subjected to hot rolling, and the shape ratio L _f in the final pass and one step before the final pass. Hot rolling is completed in a temperature range of 900° C. or higher so that the sum of the shape ratios L _f-1 in the pass of (4) satisfies the following formula (3), and
(Ii) After completion of hot rolling, the hot-rolled steel sheet is cooled to (ii-1) a cooling stop temperature of 600 to 850° C. at a cooling rate of 10° C./sec or more, and (ii-2) at a cooling stop temperature. It is characterized in that it is held for 1 to 10 seconds, and after (ii-3) holding, it is again cooled at a cooling rate of 10° C./second or more to a winding temperature of 700° C. to room temperature and wound up.
L _f +L _f-1 ≧8.0 (3)
L _f =√{R _f ×(t _in (f)−t _out (f))}÷(2t _out (f)+t _in (f))/3
L _f : Shape ratio in final pass R _f : Roll radius in final pass (mm)
t _in (f): Inlet plate thickness (mm) at the final pass
t _out (f): Output side plate thickness (mm) at the final pass
L _f-1 =√{R _f-1 ×(t _in (f-1)−t _out (f-1))}÷(2t _out (f-1)+t _in (f-1))/3
L _f-1 : Shape ratio one step before the final pass R _f-1 : Roll radius (mm) one step before the final pass
t _in (f-1): Thickness of entrance side (mm) one step before the final pass
t _out (f-1): Thickness of exit side one step before final pass (mm)

  The process conditions will be described below.

Steel piece A molten steel having the composition of the hot-rolled steel sheet of the present invention is cast by an ordinary method to produce a steel piece to be subjected to hot rolling. The steel slab to be subjected to hot rolling may be one obtained by forging or rolling a steel ingot, but a steel slab manufactured by continuous casting is preferable from the viewpoint of productivity. A steel slab produced by a thin slab caster may be used, or a steel slab produced by immediately subjecting a continuously cast slab to continuous casting-direct rolling (CC-DR) may be used.

Hot rolling Steel billet heating temperature: 1150° C. or higher, 1300° C. or lower When the cooled and cast steel billet is heated again for hot rolling, the steel billet is heated to 1150° C. or higher and 1300° C. or lower. If the heating temperature is lower than 1150° C., Ti and Nb do not form a solid solution in austenite sufficiently, and the effect of suppressing recrystallization cannot be sufficiently obtained. Therefore, the heating temperature is set to 1150° C. or higher. It is preferably 1180° C. or higher.

  On the other hand, if the heating temperature exceeds 1300°C, the crystal grains of the steel sheet become coarse and the workability deteriorates, so the heating temperature is set to 1300°C or less. It is preferably 1270° C. or lower.

Finish rolling temperature: 900° C. or higher The finish rolling temperature in hot rolling is 900° C. or higher. If the finish rolling temperature is lower than 900° C., the number of rolling passes in the non-recrystallized region increases, and the {110}<111> to {110}<001> orientation groups of the surface layer develop too much. The temperature is 900°C or higher. It is preferably 930° C. or higher.

  The upper limit of the finish rolling temperature is not particularly specified, but when finish rolling is finished at a temperature of more than 1000° C., the strength decreases, the scale thickness of the surface layer increases, and the surface texture of the finish rolled material deteriorates. The temperature is preferably 1000° C. or lower.

L _f +L _f-1 : 8.0 or more The heat is adjusted so that the sum of the shape ratio L _f in the final pass and the shape ratio L _f-1 in the pass one step before the final pass is 8.0 or more. Finish the hot rolling. When L _f +L _f-1 is less than 8.0, the required random strength ratio of {211}<111> orientations, which contributes to the improvement of fatigue strength, in the region from the outermost layer to the plate thickness 1/6 is required. Since it is difficult to secure a level of preferably 2.0 or more, L _f +L _f-1 is set to 8.0 or more. It is preferably 8.5 or more.

The upper limit of _Lf + _Lf-1 is not particularly set, but if it exceeds 12.0, {110}<111> to {110}<001> orientation groups develop and the weld fatigue strength deteriorates. _f +L _f-1 is preferably 12.0 or less. It is more preferably 10.0 or less.

Cooling/winding after hot rolling (ii-1) Cooling rate: 10°C/sec or more Cooling stop temperature: 600 to 850°C

  After the hot rolling is finished, the hot rolled steel sheet is cooled to a cooling stop temperature of 600 to 850°C at a cooling rate of 10°C/sec or more. If the cooling rate is less than 10° C./second, ferrite transformation proceeds during cooling and the strength decreases, so the cooling rate is set to 10° C./second or more. Although the upper limit of the cooling rate is not particularly defined, in order to secure the cooling rate of 100° C./sec or more, excessive equipment investment is required, and a particular cooling effect cannot be obtained. Therefore, the cooling rate is 100° C. /Sec or less is realistic.

  If the cooling stop temperature exceeds 850° C., coarse TiC or NbC is generated during the holding at the cooling stop temperature to reduce the strength and the hole expandability. Therefore, the cooling stop temperature is set to 850° C. or lower. It is preferably 800° C. or lower.

  On the other hand, if the cooling stop temperature is lower than 600° C., TiC or NbC does not precipitate while maintaining the cooling stop temperature, the strength is lowered, and the hole expandability is also lowered. Therefore, the cooling stop temperature is 600° C. or higher. . It is preferably 650° C. or higher.

Hold of (ii-2) Hold time at cooling stop temperature: 1 to 10 seconds Hold time at cooling stop temperature is 1 to 10 seconds. If the holding time is less than 1 second, TiC or NbC will not precipitate, the strength will decrease, and the hole expandability will also decrease, so the holding time is set to 1 second or more. It is preferably 3 seconds or more.

  On the other hand, if the holding time exceeds 10 seconds, coarse TiC or NbC is generated, the strength decreases, and the hole expandability also decreases, so the holding time is set to 10 seconds or less. It is preferably 7 seconds or less. The holding at the cooling stop temperature is preferably held in the air-cooled state.

Cooling/winding of (ii-3) Cooling rate: 10°C/sec or more Winding temperature: 700°C to room temperature

  If the cooling rate up to the coiling temperature is less than 10° C./second, ferrite transformation may progress during cooling and the strength may decrease, so the cooling rate is set to 10° C./second or more. Although the upper limit of the cooling rate is not particularly defined, as described above, in order to secure the cooling rate of 100° C./second or more, excessive equipment investment is required, and a particular cooling effect cannot be obtained, A cooling rate of 100° C./sec or less is realistic.

  The winding temperature is 700° C. to room temperature. When the coiling temperature exceeds 700°C, the strength decreases, so the coiling temperature is 700°C or less. It is preferably 650°C or lower. The lower limit of the coiling temperature is room temperature. Since cooling to room temperature or lower requires excessive equipment and no particular effect can be obtained, the coiling temperature is set to room temperature or higher.

  As described above, according to the manufacturing method of the present invention, by accurately controlling the texture of the surface layer, excellent hole expandability is secured, and a high-strength hot-rolled steel sheet with excellent weld fatigue properties is manufactured. can do.

  Next, an example of the present invention will be described. The condition in the example is one condition example adopted for confirming the practicability and effect of the present invention, and the present invention is based on this one condition example. It is not limited. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example 1)
Steel having the chemical composition shown in Table 1 was melted, hot-rolled under the conditions shown in Table 2, and the characteristics of the obtained hot-rolled steel sheet were measured and evaluated.

  The tensile test and the hole expansion test were carried out in accordance with JIS Z 2241 and the iron ream standard JFS T1001, respectively.

  The fatigue property (fatigue strength) of the welded part was evaluated by the following method. Overlap fillet arc welding was performed under conditions where the shape of the welded part did not change significantly, and as shown in FIG. 2, the test piece was collected so that the toe of the weld bead was in the center of the test piece, and the collected test piece Was subjected to a plane bending fatigue test in which the bending displacement was constant in both-side bending with a stress ratio of -1. The stress amplitude at which the test piece did not break even when the stress was repeatedly applied 2 million times was evaluated.

  Random intensity ratio of {110}<111> to {110}<001> orientation groups in the region from the outermost layer of the hot rolled steel sheet to the sheet thickness 1/6 and the random intensity ratio of the same {112}<111> orientation. Was determined by measuring the plate thickness cross section of the test piece polished with colloidal silica by EBSD.

  Table 3 shows the measurement/evaluation results.

  In the invention examples, the product TS·λ (MPa·%) of the tensile strength TS and the hole expansibility index λ exceeds 50000, and the weld fatigue strength is 0.40 or more in specific strength. And shows excellent characteristics.

  Manufacturing No. Nos. 27 to 34 are steel Nos. whose composition is outside the range of the present invention. It is a comparative example using ah. Manufacturing No. In Comparative Examples 27 and 34, since the Mn amount and the C amount are too high, the formula (1) is not satisfied, the hardenability is too high, and even if the texture of the surface layer is controlled, the propagation of fatigue cracks is suppressed. Can not.

  Manufacturing No. In Comparative Examples 28, 29, and 32, the amount of Mo or the amount of Al was too high, or the total amount of Mn, Mo, and Al was too high, and the formula (2) was not satisfied, and {110}< The 111> to {110}<001> orientation groups have developed, the {211}<111> orientation has become weak, and the fatigue strength ratio has decreased.

  Manufacturing No. In Comparative Examples 30 and 33, the amount of C or the amount of Mn was too low, and sufficient strength was not obtained. Manufacturing No. In the comparative example of No. 31, the amount of Si was too high, the workability was remarkably reduced, and fracture occurred during the tensile test.

  Manufacturing No. The comparative examples of 4, 7, 12, 14, 16, 18, 20, 22, and 24 are comparative examples whose manufacturing conditions are outside the scope of the present invention. Manufacturing No. In Comparative Example No. 4, the holding time was too long, and during the holding, coarse TiC was precipitated, the strength was lowered, and the hole expandability was also lowered.

  Manufacturing No. In Comparative Example No. 7, the heating temperature was too low, Ti and/or Nb could not be sufficiently re-dissolved during heating, the precipitation strengthening effect could not be obtained, the strength was lowered, and λ was also lowered. Manufacturing No. In Comparative Example No. 12, the finishing hot rolling temperature is too low, the {110}<111> to {110}<001> orientation groups of the surface layer develop, fatigue crack propagation becomes easy, and fatigue strength deteriorates. ..

  Manufacturing No. In Comparative Example No. 14, the holding temperature was too low, TiC could not be sufficiently precipitated, the strength was lowered, and the hole expandability was also lowered. In the comparative example of Production No. 16, the shape ratio in the second and second stages of hot rolling was too high, the texture of the surface layer was too strong, fatigue crack propagation was facilitated, and fatigue strength was reduced. Manufacturing No. In Comparative Example No. 18, the cooling rate was too slow, the strength was remarkably lowered, and the hole expandability was also lowered.

  Manufacturing No. In Comparative Example No. 20, the cooling stop temperature was too high, coarse TiC was deposited, and the strength and hole expansibility were reduced. Manufacturing No. In Comparative Example No. 22, the holding time is too short and the hole expandability is deteriorated. Manufacturing No. In Comparative Example No. 24, the winding temperature is too high, and the strength and the hole expandability are deteriorated.

  As described above, according to the manufacturing method of the present invention, it is possible to provide a high-strength hot-rolled steel sheet having excellent hole expandability and weld fatigue characteristics.

  As described above, according to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having excellent hole expandability and weld fatigue characteristics. When the high-strength hot-rolled steel sheet of the present invention is applied to, for example, an underbody part of an automobile, the vehicle body can be remarkably reduced in weight and fuel consumption is improved while ensuring safety. Therefore, the present invention greatly contributes to society. Therefore, it is highly applicable to the steel sheet manufacturing industry and the automobile industry.

Claims (5)

The composition is% by mass, C: 0.02% or more, 0.15% or less, Si: 0.01% or more, 2.00% or less, Mn: 0.50% or more, 2.50% or less, P: 0.001% or more, 0.100% or less, S: 0.0005% or more, 0.050% or less, Al: 0.01% or more, 0.50% or less, N: 0.0001% or more, 0.010% or less, further, Ti: 0.01% or more, 0.14% or less, and Nb: 0.005% or more, 0.09% or less, one or two, the following formula (1) And (2) are included in a range satisfying the condition, and the balance is iron and inevitable impurities,
A hole expansion characterized by a random strength ratio of the {110}<111> to {110}<001> orientation groups in the plate thickness cross section in the region from the outermost layer to the plate thickness 1/6 thickness is 3.5 or less. High-strength hot-rolled steel sheet with excellent weldability and weld fatigue characteristics.
65≦315×[C]+40×[Mn]+11×[Si]−30×[Al]
+(35×[V]+20×[Cr]+17×[Ni]+10×[Cu]
+5×[Mo])≦150 (1)
0.1×[Mn]+9×[Mo]+2×[Al]≦1.1 (2)
[Element]:% by mass of element   The composition of the components is, in mass %, B: 0.0003% or more, 0.005% or less, Mo: 0.02% or more, 0.50% or less, Cr: 0.10% or more, 2.00. % Or less, W: 0.01% or more, 2.00% or less, Cu: 0.04% or more, 2.00% or less, Ni: 0.02% or more, 1.00% or less, V: 0.001 % Or more and 0.10% or less of one type or two or more types, and the high-strength hot-rolled steel sheet having excellent hole expansibility and welded portion fatigue characteristics according to claim 1.   The component composition further contains, in mass%, one or more of Ca, Mg, Zr, and REM in a total amount of 0.0005% or more and 0.050% or less. A high-strength hot-rolled steel sheet having excellent hole expandability and weld fatigue characteristics as described in 1 or 2. 4. The random intensity ratio of {211}<111> orientations of the plate thickness cross section in the region from the outermost layer to the plate thickness 1/6 position is 2.0 or more. A high-strength hot-rolled steel sheet excellent in hole expandability and weld fatigue property as described in the item. A manufacturing method for manufacturing a high-strength hot-rolled steel sheet having excellent hole expandability and weld fatigue characteristics according to any one of claims 1 to 4,
(I) A steel slab having the chemical composition according to any one of claims 1 to 3 is heated to 1150°C or higher and 1300°C or lower, subjected to hot rolling, and the shape ratio L _f in the final pass. And so that the sum of the shape ratios L _f-1 in the pass one step before the final pass satisfies the following formula (3) and the hot rolling is completed in the temperature range of 900° C. or higher,
(Ii) After completion of hot rolling, the hot-rolled steel sheet is cooled to (ii-1) a cooling stop temperature of 600 to 850° C. at a cooling rate of 10° C./sec or more, and (ii-2) at a cooling stop temperature. Hold for 1 to 10 seconds, and after (ii-3) holding, cooling at a cooling rate of 10° C./second or more to 700° C. to room temperature and then winding and winding. A method for producing a high-strength hot-rolled steel sheet having excellent weld fatigue characteristics.
L _f +L _f-1 ≧8.0 (3)
L _f =√{R _f ×(t _in (f)−t _out (f))}÷(2t _out (f)+t _in (f))/3
L _f : Shape ratio in final pass R _f : Roll radius in final pass (mm)
t _in (f): Inlet plate thickness (mm) at the final pass
t _out (f): Output side plate thickness (mm) at the final pass
L _f-1 =√{R _f-1 ×(t _in (f-1)−t _out (f-1))}÷(2t _out (f-1)+t _in (f-1))/3
L _f-1 : Shape ratio one step before final pass R _f-1 : Roll radius (mm) one step before final pass
t _in (f-1): Thickness of the entry side (mm) one step before the final pass
t _out (f-1): Output side plate thickness one step before the final pass (mm)