JP2017206764A - High-strength hot-rolled steel sheet with excellent hole expansibility and weld fatigue properties and method for producing the same - Google Patents

Abstract

[Problem] To provide a high-strength steel sheet in which the fatigue strength of a weld zone is enhanced while ensuring hole expansibility. [Solution] Mass %, C: 0.02-0.15%, Si: 0.01-2%, Mn: 0.50-2.5%, P: 0.001-0.1%, S: 0.0005-0.05%, Al: 0.01-0.5%, N: 0.0001% to 0.01%, Ti: 0.01% to 0.14%, and Nb: 0.005% to 0.09%, including one or two in the range that satisfies the following formulas (1) and (2), and a thickness of 1 from the outermost layer A high-strength hot-rolled steel sheet characterized by having a random strength ratio of {110}<111> to {110}<001> orientation group up to /6 thickness of 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) [Selection diagram] Fig. 1

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 excellent in hole-expanding workability and fatigue characteristics of arc welded parts, and its manufacture. It is about the method. The high-strength hot-rolled steel sheet as used in the present invention refers to one having a strength of 780 MPa or more.

  2. Description of the Related Art In recent years, steel sheets for automobiles have been rapidly strengthened in order to achieve both weight reduction for improving automobile fuel efficiency and ensuring safety in the event of a collision. Higher strength generally results in deterioration of workability, so the development of steel having a microstructure with both strength and workability is actively performed by optimizing the component composition and manufacturing method. DP steel, TRIP steel, and precipitation strengthened steel have been developed.

  Hole expansibility is an especially important characteristic in the processing of undercarriage parts, and steel sheets with a relatively small strength distribution in the structure, such as precipitation strengthened steel and bainite single phase steel added with Ti and Nb, have been developed. ing.

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

  For example, Patent Document 1 discloses, in terms of% by weight, C: 0.005 to 0.20%, Si: 0.005 to 1.0%, Mn: 0.1 to 2.5%, P: 0.00. 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% Remaining Fe and inevitable impurities, 1.0% in elongation A high-strength hot-rolled steel sheet excellent in corrosion resistance and weld fatigue characteristics to which strain of less than 10.0% is applied is disclosed.

  The high-strength hot-rolled steel sheet disclosed in Patent Document 1 is a composite addition of Mo and Nb that suppresses softening of the heat-affected zone of the weld and improves fatigue characteristics. Although suppression of the softening of a part is shown, it is not clarified whether the fatigue characteristics actually improved. Further, Patent Document 1 does not describe a 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 growth of cracks. However, as a thick steel plate having a low fatigue crack propagation rate, Patent Document 2 discloses 0.015 ≦ C ≦% by weight. 0.20, 0.05 ≦ Si ≦ 2.0, 0.1 ≦ Mn ≦ 2.0, P ≦ 0.05, S ≦ 0.02, comprising balance Fe and inevitable impurities, X-ray The (200) diffraction intensity ratio in the plate thickness direction measured in (2) is 2.0 to 15.0, and the area ratio of recovered or recrystallized ferrite grains is 15 to 40%. A thick steel plate having 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 define the crystal orientation of the plate thickness surface layer portion.

  In Patent Document 3, in a multi-layered steel plate 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 C: 0.005 to 0.15%, Si: 0% by weight. 0.01 to less than 1.0%, Mn: 0.2 to 1.5%, P ≦ 0.03%, S ≦ 0.01%, Ceq ≦ 0.24, remaining Fe and inevitable impurities, inner layer However, Ceq: 0.30 to 0.70 was satisfied, the thickness of the surface layer and the back layer was 1.5 to 10 mm, respectively, and the ratio of the total thickness of the surface layer and the back layer to the total thickness of the steel sheet was 0.05 to A high-strength multilayer steel sheet for welded structure having excellent fatigue strength of the welded portion, characterized by being 0.30, is disclosed.

  The high-strength clothing steel plate for welded structure disclosed in Patent Document 3 is characterized by the component composition of the front and back layers and the ratio of the total thickness difference between the front and back layers to the total thickness difference of the steel plate. The purpose is to suppress the occurrence of cracks, and the texture is not defined.

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

  The present invention is based on the problems of conventional steel plates, and in high-strength steel plates, optimizes the hot-rolling conditions, optimizes the crystal orientation of the shear layer that develops from the surface layer of the hot-rolled steel plate to 1/6 thickness, and the component composition It is an object of the present invention to provide a high-strength steel sheet and a method for producing the same that can solve the problem by optimizing the above-described problems and remarkably increasing the fatigue strength of the welded portion while ensuring hole expandability.

  The present inventors diligently studied a method for solving the above problems. As a result, in the component steel sheet containing Nb and / or Ti, if the component composition and hot rolling conditions are optimized, the hole expandability is ensured and the fatigue strength of the welded portion is high, and the strength is higher than the TS780 MPa class. It has been found that a hot-rolled steel sheet can be obtained.

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

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

(1) Component composition is 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 %: 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, Ti: 0.01% or more, 0.14% or less, and Nb: 0.005% or more, 0.09% or less, Inclusive of (1) and (2), the balance consists of iron and inevitable impurities,
Hole expandability and weld fatigue, wherein the 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 High-strength hot-rolled steel sheet with excellent 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]: Mass% of element

  (2) The component composition further includes, in mass%, B: 0.0003% or more and 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 weld fatigue properties as described in (1) above, comprising one or more of 0.001% or more and 0.10% or less.

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

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

(5) A production method for producing a high-strength hot-rolled steel sheet excellent in hole expansibility and weld fatigue properties according to any one of (1) to (4),
(I) A steel slab having the component composition according to any one of (1) to (3) is heated to 1150 ° C. or higher and 1300 ° C. or lower, subjected to hot rolling, and a shape ratio L in the final pass. Hot rolling is finished in a temperature range of 900 ° C. or higher so that the sum of the shape ratio L _f-1 in the pass one stage before _f and the final pass satisfies the following formula (3):
(Ii) After the hot rolling is completed, the hot-rolled steel sheet is cooled to a cooling stop temperature of 600 to 850 ° C. at a cooling rate of (ii-1) 10 ° C./second or more, and (ii-2) at the cooling stop temperature. Hold for 1 to 10 seconds, and (ii-3) After holding, the hole expandability is characterized in that it is cooled again to a winding temperature of 700 ° C. to room temperature at a cooling rate of 10 ° C./second or more and wound. A method for producing a high-strength hot-rolled steel sheet with excellent weld fatigue properties.
L _f + L _f-1 ≧ 8.0 (3)
L _f = √ {R _f × (t _in (f) −t _out (f))} ÷ (2 t _out (f) + t _in (f)) / 3
L _f : Shape ratio in the final pass R _f : Roll radius in the final pass (mm)
t _in (f): Thickness (mm) of entry side in the final pass
t _out (f): Outboard thickness in the final pass (mm)
L _f−1 = √ {R _f−1 × (t _in (f−1) −t _out (f−1))} ÷ (2 t _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): Incoming plate thickness (mm) one step before the final pass
t _out (f-1): Outboard thickness (mm) one step before the final pass

  According to the present invention, a high-strength hot-rolled steel sheet excellent in hole expansibility and welded portion fatigue characteristics can be provided.

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

The high-strength hot-rolled steel sheet (hereinafter sometimes referred to as “the hot-rolled steel sheet of the present invention”) having excellent hole expansibility and weld fatigue characteristics according to the present invention,
Component composition is 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, Ti: 0.01% or more, 0.14% or less, and Nb: 0.005% or more, 0.09% or less, represented by the following formula (1) And (2) in a range satisfying, with the balance being iron and inevitable impurities,
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 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]: Mass% of element

The method for producing a high-strength hot-rolled steel sheet excellent in hole expansibility and weld zone fatigue properties (hereinafter sometimes referred to as “the present invention production method”) of the present invention is a production method for producing the present hot-rolled steel sheet. And
(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 stage before the final pass The hot rolling is finished in a temperature range of 900 ° C. or higher so that the sum of the shape ratio L _f-1 in the pass of
(Ii) After the hot rolling is completed, the hot-rolled steel sheet is cooled to a cooling stop temperature of 600 to 850 ° C. at a cooling rate of (ii-1) 10 ° C./second or more, and (ii-2) at the cooling stop temperature. Hold for 1 to 10 seconds, (ii-3) After holding, it is cooled again to a winding temperature of 700 ° C. to room temperature at a cooling rate of 10 ° C./second or more, and is wound.
L _f + L _f-1 ≧ 8.0 (3)
L _f = √ {R _f × (t _in (f) −t _out (f))} ÷ (2 t _out (f) + t _in (f)) / 3
L _f : Shape ratio in the final pass R _f : Roll radius in the final pass (mm)
t _in (f): Thickness (mm) of entry side in the final pass
t _out (f): Outboard thickness in the final pass (mm)
L _f−1 = √ {R _f−1 × (t _in (f−1) −t _out (f−1))} ÷ (2 t _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): Incoming plate thickness (mm) one step before the final pass
t _out (f-1): Outboard thickness (mm) one step before the final pass

  Hereinafter, the hot-rolled steel sheet of the present invention and the production method of the present invention will be described.

  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 “% by mass”.

Component composition C: 0.02% or more and 0.15% or less C is an element effective for improving the strength. If C is less than 0.02%, the required strength cannot be ensured, so C is 0.02% or more. Preferably it is 0.03% or more, More preferably, it is 0.05% or more.

  On the other hand, if C exceeds 0.15%, the strength increases excessively, ductility decreases, and weldability decreases, so C is made 0.15% or less. Preferably it is 0.12% or less, More preferably, it is 0.09% or less.

Si: 0.01% or more and 2.00% or less Si is an element that contributes to improvement in strength. If Si is less than 0.01%, the effect of addition cannot be sufficiently obtained, so Si is made 0.01% or more. Preferably, from the viewpoint of weldability, it is 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 lowered, so Si is made 2.00% or less. Preferably it is 1.80% or less, More preferably, it is 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 in strength. If Mn is less than 0.50%, the effect of addition cannot be sufficiently obtained, so Mn is 0.50% or more. Preferably it is 1.00% or more, More preferably, it is 1.30% or more.

  On the other hand, if Mn exceeds 2.50%, the weld cracking sensitivity increases, so Mn is set to 2.50% or less. Preferably it is 2.20% or less, More preferably, it is 2.00% or less.

P: 0.001% or more and 0.100% or less P is an element contributing to improvement in strength. If P is less than 0.001%, the effect of addition cannot be obtained sufficiently, so P is made 0.001% or more. Preferably it is 0.005% or more, More preferably, it is 0.010% or more.

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

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

  On the other hand, if S exceeds 0.050%, the local ductility, weldability, and toughness are remarkably reduced, so S is made 0.050% or less. Preferably it is 0.030% or less, More preferably, it is 0.010 or less.

Al: 0.01% or more and 0.50% or less Al is an element that functions as a deoxidizing material. If Al is less than 0.01%, the effect of addition cannot be sufficiently obtained, so Al is made 0.01% or more. Preferably it is 0.05% or more, More preferably, it is 0.08% or more.

  On the other hand, if 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 inhibit the fatigue strength of the weld. Therefore, Al is made 0.50% or less. Preferably it is 0.30% or less, More preferably, it is 0.20% or less.

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

  If N is reduced to less than 0.0001%, the steelmaking cost increases significantly, so N is made 0.0001% or more. Preferably it is 0.0015% or more, More preferably, it is 0.0035% or more.

  On the other hand, when N exceeds 0.010%, ductility, hole expansibility, and welded portion fatigue characteristics are remarkably deteriorated, so N is made 0.010% or less. Preferably it is 0.008% or less, More preferably, it is 0.006% or less.

Ti: 0.01% or more and 0.14% or less Ti is an element that precipitates as ferrite or precipitates on ferrite or bainite during cooling or winding, and contributes to improvement in strength. When Ti is less than 0.01%, it is consumed as TiN in austenite, and a sufficient precipitation strengthening effect cannot be obtained, so Ti is made 0.01% or more. Preferably it is 0.03% or more, More preferably, it is 0.05% or more.

  On the other hand, if Ti exceeds 0.14%, the toughness and weldability deteriorate, so Ti is made 0.14% or less. Preferably it is 0.12% or less, More preferably, it is 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, refines the crystal grains, and contributes to the improvement of ductility and hole expansibility. It is an element. If Nb is less than 0.005%, the effect of addition cannot be sufficiently obtained, so Nb is made 0.005% or more. Preferably it is 0.008% or more, More preferably, it is 0.010% or more.

  On the other hand, if Nb exceeds 0.09%, the toughness and ductility deteriorate, so Nb is made 0.09% or less. Preferably it is 0.07% or less, More preferably, it is 0.05% or less.

  The component composition of the hot-rolled steel sheet of the present invention is (a) one or more of B, Mo, Cr, W, Cu, Ni, V and / or (b) Ca, One or more of Mg, Zr, and REM (rare earth elements) may be included.

(a) Group element B: 0.0003% or more and 0.005% or less B is an element that improves hardenability and contributes to improvement in 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, but the toughness deteriorates, so B is preferably 0.005% or less. More preferably, it is 0.003% or less.

Mo: 0.02% to 0.50% Cr: 0.10% to 2.00% W: 0.01% to 2.00% Mo, Cr, and W are all quenched. It is an element that contributes to the improvement of strength by forming carbides as well as improving the properties. If Mo is less than 0.02%, Cr is less than 0.10%, or W is less than 0.01%, the effect of addition cannot be 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, if Mo exceeds 0.50%, Cr exceeds 2.00%, or W exceeds 2.00%, ductility and weldability deteriorate, so Mo is 0.50% or less, 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 improvement in strength and improves corrosion resistance and scale peelability. If Cu is less than 0.04%, the effect of addition cannot be obtained sufficiently, so Cu is preferably 0.04% or more. More preferably, it is 0.10% or more. On the other hand, if Cu exceeds 2.00%, surface defects are generated, so Cu is preferably 2.00% or less. More preferably, it is 1.50% or less.

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

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

(b) Group element One or more of Ca, Mg, Zr and REM: 0.0005% or more and 0.050% or less in total Ca, Mg, Zr and REM are sulfides and oxides It is an element that contributes to improving toughness by controlling the shape of the object.

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

  On the other hand, if the total 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 The total of the seeds or more is preferably 0.050% or less. More preferably, it is 0.030% or less.

  In the component composition of the hot-rolled steel sheet of the present invention, the balance excluding the above elements is iron and inevitable impurities. Inevitable 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 properties of the hot-rolled steel sheet of the present invention.

  Next, the following formulas (1) and (2) that are required to satisfy the component 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]: 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 also referred to as“ index X ”) is a comprehensive evaluation of the degree (contribution) of each element contributing to strength improvement to contribute to strength improvement. Invented hot-rolled steel sheet is an index indicating the component range necessary for achieving both the base metal strength and the fatigue strength of the arc welded portion.

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

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

The above formula (2)
“0.1 × [Mn] + 9 × [Mo] + 2 × [Al]” (hereinafter sometimes referred to as “index Y”) of the above formula (2) is {110} <111> of the plate thickness surface layer portion. ~ {110} <001> Acceleration of the development of orientation group, comprehensive evaluation of the degree of inhibition of elements that inhibit weld fatigue properties, ensuring the required weld fatigue properties in the hot-rolled steel sheet of the present invention It is an indicator to do.

  When the index Y exceeds 1.1, the {110} <111> to {110} <001> orientation groups of the plate thickness surface portion develop strongly, the {112} <111> orientation weakens, and the weld fatigue strength Therefore, the index Y is set to 1.1 or less. Preferably it is 1.0 or less, More preferably, it is 0.9 or less. The lower limit of the index Y is not particularly limited because it is determined from the lower limits of Mn, Al, and Mo.

  The mechanism by which Mn, Al, and / or Mo influence the formation of the texture of the surface layer is not clear, but these elements affect the crystal rotation accompanying shear deformation during rolling, and It is thought that the balance between the two orientation groups changes due to the influence. In this respect, the index Y is preferably 1.0 or less, more preferably 0.9 or less, as described above.

  Next, 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 will be described.

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

  In this orientation group, when the {100} plane, which is a wall open surface, is oriented perpendicular to the plate thickness direction and the number of crystal grains whose {100} plane is oriented perpendicular to the plate thickness direction increases, brittle fracture occurs. When cracked, cracks in the thickness direction progress at a stretch.

  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 is perpendicular to the plate thickness direction. Since the number of crystal grains facing 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. The following. Preferably it is 3.3 or less, More preferably, it is 3.1 or less.

  The random intensity ratio of the {110} <111> to {110} <001> orientation groups depends on the component composition, hot rolling conditions, and cooling conditions after hot rolling, and therefore the lower limit is not particularly defined, but is 0 on definition. including.

  Further, like the {110} <111> to {110} <001> orientation groups, the {211} <111> orientation, which is the main orientation of the non-recrystallized shear / transformation texture, contributes to the improvement of fatigue strength. It is a 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. More preferably, it is 2.5 or more.

  The {211} <111> orientation random strength ratio depends on the component composition, hot rolling conditions, and cooling conditions after hot rolling, so the upper limit is not particularly defined, but a special effect is not obtained at 4.0 or higher. Therefore, 4.0 is a practical upper limit.

  The {100} to {111} <011> orientation group random intensity ratio and the {211} <111> orientation random intensity ratio are obtained by measuring the azimuth data measured by the 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]) that displays a three-dimensional texture calculated and calculated using.

  FIG. 1 shows a crystal orientation distribution function (ODF) of a φ2 = 45 ° cross section displaying the crystal orientation of the present invention. Strictly speaking, the {110} <111> to {110} <001> orientation groups are represented by a black dot on the axis of Φ = 90 ° in FIG. 1, and Φ = 90 ° and φ1 = 35.26-90. Refers to the range of °.

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

  In the cross section of φ2 = 45 ° of the three-dimensional texture, the {112} <111> orientation is in the range of φ = 30-40 ° and φ1 = 85-90 ° centered on the black spot shown in FIG. The maximum value of the random intensity ratio is defined as the random intensity ratio in the above direction.

  As for the crystal orientation, the orientation perpendicular to the plate surface is usually indicated by [hkl] or {hkl}, and the orientation parallel to the rolling direction is indicated by (uvw) or <uvw>. {Hkl} and <uvw> are generic names of equivalent planes, and [hkl] and (uvw) indicate individual crystal planes. That is, since the hot-rolled steel sheet of the present invention targets 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 cannot be distinguished. In such a case, these orientations are collectively referred to as {111}.

  Since ODF is also used for displaying the orientation of a crystal structure with low symmetry, it is usually displayed at φ1 = 0 to 360 °, φ = 0 to 180 °, φ2 = 0 to 360 °, and the individual orientations are [ hkl] (uvw). However, since the hot rolled steel sheet of the present invention is intended for a highly symmetrical bcc crystal structure, Φ and φ2 are displayed in the range of 0 to 90 °.

  The range of φ1 varies depending on whether or not the change in symmetry due to deformation is taken into account when performing the calculation. However, in the hot rolled steel of the present invention, the calculation is performed in consideration of the symmetry, and φ1 Displayed at 0 to 90 °. That is, the average value in the same direction 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 random intensity ratio of (110) [1-11] of the ODF in the φ2 = 45 ° cross section shown in FIG. 1 is the random intensity ratio of the {110} <111> orientation.

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

  The measurement range is from the outermost surface of the plate thickness to the position of 1/6 thickness, 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 production method of the present invention comprises:
(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 stage before the final pass The hot rolling is finished in a temperature range of 900 ° C. or higher so that the sum of the shape ratio L _f-1 in the pass of
(Ii) After the hot rolling is completed, the hot-rolled steel sheet is cooled to a cooling stop temperature of 600 to 850 ° C. at a cooling rate of (ii-1) 10 ° C./second or more, and (ii-2) at the cooling stop temperature. Hold for 1 to 10 seconds, (ii-3) After holding, it is cooled again to a winding temperature of 700 ° C. to room temperature at a cooling rate of 10 ° C./second or more, and is wound.
L _f + L _f-1 ≧ 8.0 (3)
L _f = √ {R _f × (t _in (f) −t _out (f))} ÷ (2 t _out (f) + t _in (f)) / 3
L _f : Shape ratio in the final pass R _f : Roll radius in the final pass (mm)
t _in (f): Thickness (mm) of entry side in the final pass
t _out (f): Outboard thickness in the final pass (mm)
L _f−1 = √ {R _f−1 × (t _in (f−1) −t _out (f−1))} ÷ (2 t _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): Incoming plate thickness (mm) one step before the final pass
t _out (f-1): Outboard thickness (mm) one step before the final pass

  Hereinafter, process conditions will be described.

Steel slab The molten steel which has the component composition of this invention hot-rolled steel plate is cast by a conventional method, and the steel slab which uses for hot rolling is manufactured. Although the steel slab used for hot rolling may be a forged or rolled steel ingot, a steel slab manufactured by continuous casting is preferable from the viewpoint of productivity. It may be a steel slab produced by a thin slab caster, or a steel slab produced by subjecting a continuous cast slab to continuous casting-direct rolling (CC-DR) immediately.

Hot rolling Heating temperature of steel slab: 1150 ° C. or higher and 1300 ° C. or lower When the steel slab cooled after casting is heated again and subjected to hot rolling, the steel slab is heated to 1150 ° C. or higher and 1300 ° C. or lower. If the heating temperature is less than 1150 ° C., Ti and Nb are not sufficiently dissolved in austenite, and a sufficient effect of suppressing recrystallization cannot be obtained. Therefore, the heating temperature is set to 1150 ° C. or higher. Preferably it is 1180 degreeC or more.

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

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

  The upper limit of the finish rolling temperature is not particularly specified, but when finish rolling is finished at a temperature exceeding 1000 ° C., the strength is reduced and the scale thickness of the surface layer is increased, and the surface properties of the finished rolled material are lowered. The temperature is preferably 1000 ° C. or lower.

L _f + L _f-1 : 8.0 or more Heat is applied 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. The hot rolling is finished. When L _f + L _f-1 is less than 8.0, the {211} <111> orientation random strength ratio that contributes to the improvement of fatigue strength in the region from the outermost layer to the plate thickness ６ Since it is difficult to secure 2.0 or more at the level, L _f + L _f-1 is set to 8.0 or more. Preferably it is 8.5 or more.

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

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

  After the hot rolling, the hot-rolled steel sheet is cooled to a cooling stop temperature of 600 to 850 ° C. at a cooling rate of 10 ° C./second 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. The upper limit of the cooling rate is not particularly defined, but in order to secure a cooling rate of 100 ° C./second or more, excessive capital investment is required, and since a special cooling effect cannot be obtained, 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, the strength is lowered, and the hole expandability is also lowered. Therefore, the cooling stop temperature is set to 850 ° C. or less. Preferably it is 800 degrees C or less.

  On the other hand, when the cooling stop temperature is less than 600 ° C., TiC or NbC does not precipitate during holding at the cooling stop temperature, the strength is lowered, and the hole expandability is also lowered. Therefore, the cooling stop temperature is 600 ° C. or more. . Preferably it is 650 degreeC or more.

(Ii-2) Holding Time at the cooling stop temperature: 1 to 10 seconds The holding time at the cooling stop temperature is 1 to 10 seconds. If the holding time is less than 1 second, TiC or NbC does not precipitate, the strength is lowered, and the hole expansibility is also lowered, so the holding time is 1 second or longer. Preferably it is 3 seconds or more.

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

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

  If the cooling rate to the coiling temperature is less than 10 ° C./second, there is a concern that the ferrite transformation progresses 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, as described above, in order to secure a cooling rate of 100 ° C./second or more, excessive capital investment is required, and a special cooling effect cannot be obtained. The cooling rate is practically 100 ° C./second or less.

  The winding temperature is 700 ° C. to room temperature. If the coiling temperature exceeds 700 ° C, the strength decreases, so the coiling temperature is set to 700 ° C or less. Preferably it is 650 degrees C or less. The lower limit of the coiling temperature is room temperature. Cooling to room temperature or lower requires an excessive facility, and a special effect cannot be obtained. Therefore, the winding temperature is set to room temperature or higher.

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

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt 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 component composition shown in Table 1 was melted and hot-rolled under the conditions shown in Table 2, and the properties of the obtained hot-rolled steel sheet were measured and evaluated.

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

  The fatigue characteristics (fatigue strength) of the welded portion were evaluated by the following method. Lap fillet arc welding is performed under the condition that the shape of the welded portion does not change greatly, and as shown in FIG. 2, the test piece is collected so that the toe of the weld bead is at the center of the test piece, and the collected specimen Was subjected to a plane bending fatigue test in which the bending displacement was constant by a double swing bending with a stress ratio of -1. The stress amplitude at which the specimen does not break even when stress was repeatedly applied 2 million times was evaluated.

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

  Table 3 shows the measurement / evaluation results.

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

  Production No. Nos. 27 to 34 are steel Nos. Whose composition is outside the scope of the present invention. It is a comparative example using ah. Production No. In the comparative examples of 27 and 34, the amount of Mn and C is too high, so the formula (1) is not satisfied, the hardenability is too high, and even if the surface texture is controlled, the propagation of fatigue cracks is suppressed. Can not.

  Production No. In the comparative examples of 28, 29, and 32, the amount of Mo and Al is too high, or the total amount of Mn, Mo, and Al is too high to satisfy the formula (2), and {110} < 111>-{110} <001> orientation groups are developed, {211} <111> orientations are weakened, and the fatigue strength ratio is reduced.

  Production No. In the comparative examples 30 and 33, the amount of C or the amount of Mn is too low, and sufficient strength is not obtained. Production No. In Comparative Example 31, the amount of Si is too high, the workability is remarkably lowered, and the sample is broken during the tensile test.

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

  Production No. In the comparative example No. 7, the heating temperature is too low, and Ti and / or Nb cannot be sufficiently re-dissolved during heating, the precipitation strengthening effect cannot be obtained, the strength is lowered, and λ is also lowered. Production No. In the comparative example of 12, the finish 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. .

  Production No. In Comparative Example 14, the holding temperature is too low, TiC cannot be sufficiently precipitated, the strength is lowered, and the hole expansibility is also lowered. In the comparative example of Production No. 16, the shape ratio in the second stage after hot rolling is too high, the texture of the surface layer becomes too strong, fatigue crack propagation becomes easy, and the fatigue strength is reduced. Production No. In the comparative example of 18, the cooling rate is too slow, the strength is remarkably lowered, and the hole expandability is also lowered.

  Production No. In the comparative example of 20, the cooling stop temperature is too high, coarse TiC is precipitated, and the strength and hole expansibility are lowered. Production No. In the comparative example of 22, the holding time is too short and the hole expansibility is lowered. Production No. In the comparative example of 24, the coiling temperature is too high, and the strength and hole expansibility are reduced.

  As described above, according to the production method of the present invention, a high-strength hot-rolled steel sheet excellent in hole expansibility and weld zone fatigue characteristics can be provided.

  As described above, according to the present invention, a high-strength hot-rolled steel sheet excellent in hole expansibility and weld fatigue characteristics can be provided. When the high-strength hot-rolled steel sheet of the present invention is applied to, for example, an undercarriage part of an automobile, the vehicle body can be significantly reduced in weight while ensuring safety, and fuel efficiency is improved. Therefore, the present invention greatly contributes to society. It is highly usable in the steel plate manufacturing industry and the automobile industry.

Claims (5)

Component composition is 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, Ti: 0.01% or more, 0.14% or less, and Nb: 0.005% or more, 0.09% or less, represented by the following formula (1) And (2) in a range satisfying, with the balance being iron and inevitable impurities,
Hole expandability and weld zone characterized in that the 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 High-strength hot-rolled steel sheet with excellent 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]: Mass% of element   The component composition is further 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 %: 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 The high-strength hot-rolled steel sheet having excellent hole expansibility and weld fatigue properties according to claim 1, wherein the high-strength steel sheet has excellent hole expansibility and weld fatigue properties.   The component composition further includes one or more of Ca, Mg, Zr, and REM in a mass% of 0.0005% or more and 0.050% or less in total. A high-strength hot-rolled steel sheet having excellent hole expansibility and weld fatigue characteristics according to 1 or 2.   4. The random intensity ratio of the {211} <111> orientation in the region from the outermost layer to the plate thickness 1/6 position is 2.0 or more. 5. High-strength hot-rolled steel sheet with excellent hole expansibility and weld fatigue properties. A manufacturing method for manufacturing a high-strength hot-rolled steel sheet excellent in hole expansibility and weld fatigue properties according to any one of claims 1 to 4,
(I) A steel slab having the component 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 a shape ratio L _f in the final pass. And the hot rolling is finished in a temperature range of 900 ° C. or higher so that the sum of the shape ratio L _f-1 in the pass one stage before the final pass satisfies the following formula (3):
(Ii) After the hot rolling is completed, the hot-rolled steel sheet is cooled to a cooling stop temperature of 600 to 850 ° C. at a cooling rate of (ii-1) 10 ° C./second or more, and (ii-2) at the cooling stop temperature. Hold for 1 to 10 seconds, and (ii-3) After holding, the hole expandability is characterized in that it is cooled again to a winding temperature of 700 ° C. to room temperature at a cooling rate of 10 ° C./second or more and wound. A method for producing a high-strength hot-rolled steel sheet with excellent weld fatigue properties.
L _f + L _f-1 ≧ 8.0 (3)
L _f = √ {R _f × (t _in (f) −t _out (f))} ÷ (2 t _out (f) + t _in (f)) / 3
L _f : Shape ratio in the final pass R _f : Roll radius in the final pass (mm)
t _in (f): Thickness (mm) of entry side in the final pass
t _out (f): Outboard thickness in the final pass (mm)
L _f−1 = √ {R _f−1 × (t _in (f−1) −t _out (f−1))} ÷ (2 t _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): Incoming plate thickness (mm) one step before the final pass
t _out (f-1): Outboard thickness (mm) one step before the final pass