JP6668662B2 - Steel sheet excellent in fatigue characteristics and formability and method for producing the same - Google Patents

Description

  The present invention relates to a steel sheet having excellent fatigue characteristics and formability.

  Recently, for the purpose of reducing the weight of an automobile body, the thickness of underbody parts or structural parts of the body has been reduced by increasing the strength. However, even if the tensile strength and proof stress are improved, fatigue strength, which is an important property in automobiles, is not sufficiently improved, and high strength is achieved by using structural and organizational discontinuities such as notches and welds. There is a problem that the fatigue crack propagation resistance from the steel is lowered.

It is known that as a technique for improving the fatigue strength other than increasing the strength, it is effective to refine the structure. For example, Patent Documents 1 and 2 disclose a hot-rolled steel sheet having ultrafine ferrite grains having an average particle size of less than 2 μm as hot-rolled and having bainite or the like as a second phase. Is excellent in ductility, toughness, fatigue strength and the like, and is considered to have small anisotropy of these properties.
Further, since fatigue cracks occur near the surface, it is also effective to refine the structure near the surface. Patent Document 3 has a crystal grain size gradient structure in which the average crystal grain size of polygonal ferrite, which is the main phase, gradually decreases from the center of the plate thickness toward the surface layer, and bainite or the like is used as a second phase in a volume fraction of 5%. % Is described. Further, the refinement of the martensite structure is also effective for improving the fatigue characteristics. Patent Document 4 discloses that 80% or more of the area fraction of the microstructure is martensite, the average block diameter of the martensite structure is 3 μm or less, and the maximum block diameter is 1 to 3 times the average block diameter. Is described. Further, Patent Literature 4 describes that carbon is uniformly dispersed as lower bainite or martensite by hot rolling a structure of an ingot before pipe forming. However, while grain refinement suppresses the generation of fatigue cracks, it has the disadvantage of deteriorating fatigue crack propagation characteristics, and as a result, there has been a problem of deteriorating fatigue characteristics including notches and welding defects.

  On the other hand, it has been reported that complex organization is effective in suppressing fatigue crack propagation. In Patent Document 5, the crack propagation speed is reduced by dispersing hard bainite or martensite in a structure mainly composed of fine ferrite. Patent Documents 6 and 7 report that the crack propagation speed can be reduced by increasing the aspect ratio of martensite in a composite structure. However, these are effective methods for slowing the fatigue crack propagation speed, but for improving the lower limit stress intensity factor range (ΔKth), which is the minimum stress intensity factor range at which the crack does not propagate. Is not mentioned. In a steel sheet having a small thickness and a short crack propagation life in the fatigue life, it is often more important to improve ΔKth than the crack propagation speed.

Japanese Patent Application Laid-Open No. H11-92959 JP-A-11-152544 JP-A-2004-212199 JP 2010-70789 A JP-A-04-337026 JP 2005-320619 A JP-A-07-90478

  The present invention has been devised in view of the above-described problems, and aims to provide a steel sheet having excellent fatigue characteristics and formability, in which the sheet thickness is small, and the crack propagation life occupying the fatigue life is short. It is another object of the present invention to improve the lower limit stress intensity factor range (ΔKth) also in a steel sheet.

The present inventors have conducted intensive studies in order to solve the above-mentioned problems, and have obtained the following findings. That is, by optimizing the composition and the manufacturing conditions of the steel sheet and controlling the structure of the steel sheet, a steel sheet excellent in fatigue characteristics and formability was successfully manufactured. The summary is as follows.
(1)
Chemical composition in mass%
C: 0.002 to 0.100%,
Si: 2.00% or less,
Mn: 2.00% or less,
Al: 2.000% or less,
N: 0.0100% or less,
O: 0.0100% or less,
Ti: 0.200% or less,
The impurities P and S are
P: 0.100% or less,
S: limited to 0.0300% or less,
The balance is Fe and a steel plate that is an unavoidable impurity,
Using Tief calculated from the following (formula a),
The effective carbon amount Ceff calculated by the following (formula b) is 0.002% to 0.050%,
When a crystal grain is defined as a region surrounded by a grain boundary having an orientation difference of 15 ° or more from an adjacent crystal and having an equivalent circle diameter of 0.3 μm or more in the region, the orientation in the crystal grain is defined as Containing 90% or more in terms of area ratio of crystal grains having an average difference of 0 ° to 0.5 °,
The sum of the area ratios of the hard phase composed of martensite or tempered martensite or retained austenite is 2% or less, and the average of the misorientation is 0 ° or more and 0.5 ° or less. A steel sheet having excellent fatigue characteristics and formability, characterized in that the amount of solute carbon is 20 ppm or more and 200 ppm or less.
Tief = [Ti] −48 / 14 × [N] −48 / 32 × [S] (Formula a)
Ceff = [C] −12 / 48 × Tieff (Equation b)
However, [Ti], [N], [S], and [C] indicate the mass% of Ti, N, S, and C in the steel sheet, respectively, and 0% is substituted when they are not contained. .
When Tief ≦ 0, calculation is performed assuming that Tief = 0 in (Equation b).
The following additional elements are added in place of part of the Fe.
(2)
The steel sheet having excellent fatigue properties and formability according to (1), wherein the density of TiC is 1.0 × 10 ¹⁶ pieces / cm ³ or less.
(3)
In further mass%,
Nb: 0.100% or less,
V: 0.300% or less,
Cu: 1.20% or less,
Ni: 0.60% or less,
Cr: 2.00% or less,
Mo: 1.00% or less,
(1) or (2), wherein the steel sheet has excellent fatigue characteristics and formability.
(4)
In further mass%,
Mg: 0.0100% or less,
Ca: 0.0100% or less,
REM: 0.1000% or less,
The steel sheet having excellent fatigue properties and formability according to any one of (1) to (3), characterized by containing one or more of the following.
(5)
In further mass%,
B: 0.0020% or less,
The steel sheet according to any one of (1) to (4), which is excellent in fatigue characteristics and formability.
(6)
The fatigue according to any one of (1) to (5), further comprising 1% by mass or less of one or more of Sn, Zr, Co, Zn, and W. Steel sheet with excellent properties and formability.
(7)
A method for producing a steel sheet having excellent fatigue properties and formability according to any one of (1) to (6), comprising the component composition according to any one of (1) to (6). A heating step of heating the steel ingot to a temperature T1 (° C.) calculated from the following (formula c) or 1100 ° C., whichever is larger, and 1300 ° C. or less;
The hot ingot is subjected to rough rolling of the heated ingot, and then subjected to finish rolling by multi-stage continuous rolling to obtain a hot-rolled steel sheet,
Having a cooling step of cooling the obtained hot-rolled steel sheet,
In the finish rolling by the multi-stage continuous rolling, the rolling temperature at the rearmost stage among the stages with a draft of 5% or more is higher than the lower temperature of Ar3 or 1000 ° C. which is calculated by the following (formula d). Temperature
In the cooling step, the hot-rolled steel sheet is formed on the basis of Ae1 calculated by the following (Equation e):
Ae1-30 ° C. or more, stay in a temperature range of Ae1 + 30 ° C. or less for 8 seconds or more,
Ae1 The cooling rate from 30 ° C. to 300 ° C. is 100 ° C./sec or more,
Further, a method for producing a steel sheet having excellent fatigue characteristics and formability, wherein the cooling rate from 300 ° C. to 30 ° C. is 30 ° C./sec or more.
T1 (° C.) = 7000 / {2.75-log ([Ti] × [C])} − 273 (Formula c)
Ar3 (° C.) = 868-396 × [C] −68.1 × [Mn] + 24.6 × [Si] −36.1 × [Ni] −24.8 × [Cr] −20.7 × [Cu ] + 250 × [Al] (formula d)
Ae1 (° C.) = 723-10.7 × [Mn] + 29.1 × [Si] −16.9 × [Ni] + 16.9 × [Cr] + 70 × [Al] (formula e)
However, [element name] in the formula indicates the content (% by mass) of the element in the steel sheet, and 0% is substituted when the element is not contained.

  According to the present invention, it is possible to provide a steel sheet having excellent fatigue characteristics and formability. If this steel sheet is used, it is possible to extend the fatigue life of a component having a complicated shape applied to an undercarriage component of a material for an automobile, and the industrial contribution is remarkable.

  Hereinafter, the contents of the present invention will be described in detail. The present invention can be suitably used for a steel plate having a thickness of 12 mm or less. In particular, the thinner the thickness, the more the effect is exhibited. Therefore, the present invention is preferably applied to a steel sheet having a final product having a thickness of preferably 8 mm or less, more preferably 6 mm or less, and still more preferably 3 mm or less.

[Chemical composition of steel sheet]
First, the reasons for limiting the chemical components of the steel sheet of the present invention will be described. In addition, unless otherwise specified,% of content shows mass% and ppm shows mass ppm (0.0001 mass%).
In addition, the designation of [element name] used in each formula in this specification indicates the content (% by mass) of the element in the steel sheet, and 0% is substituted when the element is not contained. . For example, [C] indicates the content (% by mass) of C (carbon) and [Ti] indicates the content (% by mass) of Ti (titanium).

(C: 0.002% to 0.100%)
C is one of the important elements in the present invention. In the present invention, as will be described later, the lower limit stress intensity factor range (ΔKth) can be improved by controlling the amount of solid solution carbon (solid solution C) in the crystal grains to a predetermined amount.
Therefore, carbon (C) is preferably added at 0.002% or more. Further, in order to ensure workability, the content of carbon (C) is preferably set to 0.100% or less.

(Effective carbon amount (Ceff): 0.002 to 0.050%)
C precipitates as TiC when Ti is present in steel, so the effective carbon amount (Ceff) that can effectively act as carbon (C) varies depending on the amount of TiC (Ti carbide).
On the other hand, Ti carbide is generated at a lower temperature than Ti nitride or Ti sulfide. For this reason, if N or S in the steel is large, Ti nitride or Ti sulfide is preferentially generated, so that the amount of Ti (Tief) that can be precipitated as TiC is calculated by the following (formula a). Used as an indicator.

  When Tief is equal to or less than 0 (negative or zero), the total carbon amount in the steel becomes the effective carbon amount.

On the other hand, when Tief becomes a value greater than 0 (positive), part of C precipitates as TiC, so the effective carbon amount is lower than the total carbon amount in the steel by the amount precipitated as TiC. . In such a case, the effective carbon amount may be determined in consideration of the amount of C precipitated as TiC. That is, the effective carbon amount Ceff may be calculated by (Equation b).
Tief = [Ti] −48 / 14 × [N] −48 / 32 × [S] (Formula a)
Ceff = [C] −12 / 48 × Tieff (Equation b)
However, [Ti], [N], [S], and [C] indicate the mass% of Ti, N, S, and C in the steel sheet, respectively, and 0% is substituted when they are not contained. .
When Tief ≦ 0, calculation is performed assuming that Tief = 0 in (Equation b).
In the present invention, the lower limit stress intensity factor range (ΔKth) can be improved by controlling the amount of solid solution carbon (solid solution C) in the crystal grains to be described later to a predetermined amount. In order to obtain the effect of improving ΔKth, the effective carbon amount is preferably set to 0.002% or more. To ensure this effect, the lower limit of the effective carbon amount is preferably set to 0.003%, and more preferably 0.004%.

  On the other hand, if the effective carbon amount exceeds 0.050%, the area ratio of the low-temperature transformation product, which is the hard second phase, increases, and the hole expandability decreases. Therefore, the effective carbon content is set to 0.050% or less. From the viewpoint of ensuring the hole expanding property, the content is desirably 0.040% or less, and more desirably 0.030% or less.

  The above are the basic chemical components of the steel sheet of the present invention, and may further contain the following components.

(Si: 2.00% or less)
Since Si is an element capable of improving tensile strength without significantly impairing workability, Si may be added. However, if added in excess of 2.00%, toughness and formability decrease, so the Si content is set to 2.00% or less. Further, if added in excess of 0.50%, the surface properties are significantly deteriorated, and the productivity in the pickling step is extremely deteriorated. Therefore, the content of Si is desirably 0.50% or less. More desirably, the content is set to 0.30% or less.
On the other hand, since Si is naturally contained as an impurity, setting the content to less than 0.01% is not desirable from the viewpoint of cost. Therefore, the lower limit of the Si content is desirably set to 0.01%.

(Mn: 2.00% or less)
Mn may be added as a solid solution strengthening element. If the Mn content is more than 2.00%, a segregation zone of Δn is formed at the center in the thickness direction of the steel sheet, and the segregation zone becomes a starting point of cracking, so that the hole expansion ratio is reduced. Therefore, the content of Mn is set to 2.00% or less.
On the other hand, since Mn is naturally contained as an impurity, setting the content to less than 0.01% is not desirable from the viewpoint of cost. Therefore, the lower limit of the Mn content is desirably set to 0.01%.

(P: 0.100% or less)
P is an impurity contained in the hot metal, is an element that segregates at the grain boundary and decreases the low-temperature toughness with an increase in the content. Therefore, the smaller the P content, the better. If the content of P exceeds 0.100%, workability and weldability are adversely affected, so the content is limited to 0.100% or less. In particular, in consideration of weldability, the P content is preferably limited to 0.030% or less.

(S: 0.0300% or less)
S is an impurity contained in the hot metal, and when its content is too large, it is an element that not only causes cracks during hot rolling, but also generates inclusions such as MnS that deteriorates hole expanding properties. For this reason, the content of S should be reduced as much as possible. However, if the content is 0.0300% or less, it is within an allowable range, so the content is limited to 0.0300% or less. However, when a certain degree of hole expanding property is required, the S content is preferably limited to 0.0100% or less, more preferably 0.0050% or less.

(Al: 2.000% or less)
Al is an element effective as a deoxidizing agent for molten steel, and has the effect of stabilizing the yield of molten steel of other elements, and thus may be added. To obtain this effect, the lower limit of the Al content is desirably set to 0.010%.
On the other hand, if the Al content exceeds 2.000%, cracks may occur during rolling. Therefore, the upper limit of the Al content is set to 2.000%. If the Al content exceeds 1.000%, the weldability and toughness start to deteriorate, so the upper limit of the Al content is desirably 1.000%, and the more desirable upper limit of the Al content is 0.1%. 500%.

(N: 0.0100% or less)
N may be added because it exists as TiN and contributes to improvement in low-temperature toughness through refinement of the crystal grain size during heating of the ingot. However, since the nitride in the steel lowers the hole expansion ratio, it is required to be 0.0100% or less. Desirably, it is 0.0050% or less.
On the other hand, since it is not economically desirable to make the content less than 0.0005%, it is preferable to make the content 0.0005% or more.

(O: 0.0100% or less)
O forms an oxide and deteriorates the formability, so its content must be suppressed. In particular, if O exceeds 0.0100%, this tendency becomes remarkable, so it is necessary to set the content to 0.0100% or less.
On the other hand, since it is not economically preferable to make the content less than 0.0010%, it is desirable to make the content 0.0010% or more.

(Ti: 0.200% or less)
Ti may be added because it exists as TiC and contributes to the strengthening of the steel sheet through precipitation strengthening. However, even if added in excess of 0.200%, this effect is saturated and also causes nozzle clogging during casting. Therefore, the Ti content is set to 0 to 0.200%.
When the content of Ti is more than 0.050% as described later, the density of TiC becomes 1.0 × 10 ¹⁶ / cm ³ or more, and the formability is reduced. For this reason, the desirable content of Ti may be set to 0.050% or less.
On the other hand, if the content of Ti is less than 0.001%, the effect of precipitation strengthening may not be sufficiently obtained, so it is desirable to add 0.001% or more. In order to ensure the effect of precipitation strengthening, it is more desirable to add 0.010% or more.

(Nb: 0.100% or less)
Nb may be added because carbonitride or solute Nb delays grain growth during hot rolling, thereby making it possible to reduce the grain size of the hot-rolled sheet and improve low-temperature toughness. Even if the Nb content exceeds 0.100%, the above effect is saturated and the economic efficiency is reduced.
If the Nb content is less than 0.010%, the above effects cannot be sufficiently obtained. Therefore, when Nb is contained as necessary, the Nb content is desirably 0.010% or more.

(V: 0.300% or less)
V is an element having the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening, and may be added. Even if the V content exceeds 0.300%, the above effect is saturated and the economic efficiency is reduced. Therefore, when V is contained, the V content is desirably 0.300% or less.
If the V content is less than 0.010%, the above effects cannot be sufficiently obtained. Therefore, when V is contained, the V content is desirably 0.010% or more.

(Cu: 2.00% or less)
Cu is an element having the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening, and may be added. Even if the Cu content exceeds 2.00%, the above effect is saturated and the economic efficiency is reduced. Therefore, when Cu is contained, the Cu content is desirably 2.00% or less. Further, if the content of Cu exceeds 1.20%, scale-induced scratches may occur on the surface of the steel sheet. Therefore, it is more preferable that the Cu content be 1.20% or less.
If the Cu content is less than 0.01%, the above effects cannot be sufficiently obtained. Therefore, when Cu is contained, the Cu content is desirably 0.01% or more.

(Ni: 2.00% or less)
Ni is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening, and may be added. Even if the Ni content exceeds 2.00%, the above effect is saturated and the economic efficiency is reduced. Therefore, when Ni is contained, the Ni content is desirably 2.00% or less. If the Ni content exceeds 0.60%, ductility starts to deteriorate. Therefore, the Ni content is desirably set to 0.60% or less.
If the Ni content is less than 0.01%, the above effects cannot be sufficiently obtained. Therefore, when Ni is contained as necessary, the Ni content is desirably 0.01% or more.

(Cr: 2.00% or less)
Cr is an element having the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening, and may be added. Even if the Cr content exceeds 2.00%, the above effect is saturated and the economic efficiency is reduced. Therefore, when Si is contained, the Si content is desirably 2.00% or less.
On the other hand, if the Cr content is less than 0.01%, the above effects cannot be sufficiently obtained. Therefore, when Cr is contained as necessary, the Cr content is preferably 0.01% or more.

(Mo: 1.00% or less)
Mo is an element having the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening, and may be added. Even if the Mo content exceeds 1.00%, the above effect is saturated and the economic efficiency is reduced. Therefore, when Mo is contained, the Mo content is desirably 1.00% or less.
On the other hand, if the content of Mo is less than 0.01%, the above effects cannot be sufficiently obtained. Therefore, when Mo is contained as required, the Mo content is desirably 0.01% or more.

(Mg: 0.0100% or less)
Mg may be added because it is an element that controls the form of nonmetallic inclusions that serve as a starting point of destruction and causes deterioration of workability and improves workability. Even if the content of Mg exceeds 0.0100%, the above effect is saturated and the economic efficiency is reduced. Therefore, when Mg is contained, the Mo content is desirably 1.00% or less.
The effect becomes remarkable when the content of Mg is 0.0005% or more. Therefore, when Mg is contained as necessary, it is desirable that the Mg content be 0.0005% or more.

(Ca: 0.0100% or less)
Ca may be added because Ca is an element that controls the form of non-metallic inclusions that become the starting point of fracture and causes deterioration of workability and improves workability. Even if the content of Ca exceeds 0.0100%, the above effect is saturated and the economic efficiency is reduced. Therefore, when Ca is contained, the Ca content is desirably 0.0100% or less.
The effect becomes remarkable when the content of Ca is 0.0005% or more. Therefore, when Ca is contained as necessary, the Ca content is desirably 0.0005% or more.

(REM: 0.1000% or less)
REM (rare earth element) may be added because it is an element that controls the form of non-metallic inclusions that become a starting point of destruction and causes deterioration of workability and improves workability. Even if the content of REM exceeds 0.1000%, the above effect is saturated and the economic efficiency is reduced. Therefore, when REM is contained, the REM content is desirably 0.1000% or less.
The effect becomes significant when the content of REM is 0.0005% or more. Therefore, when REM is contained as necessary, the REM content is desirably 0.0005% or more.

(B: 0.0100% or less)
B segregates at the grain boundary and improves the low-temperature toughness by increasing the grain boundary strength. For this reason, they may be added. When the content of B exceeds 0.0100%, the effect is not only saturated, but also economical. Therefore, when B is contained, the Ca content is desirably 0.0100% or less. This effect is remarkable when the B content in the steel sheet is 0.0002% or more.
B is a strong quenching element, and when added in an amount exceeding 0.0020%, the area of a crystal grain in which the average of the misorientation in the crystal grain important in this study is 0 to 0.5 °. The rate may be reduced. Therefore, when B is contained as necessary, the B content is desirably 0.0002% or more.

  It has been confirmed that the effects of the present invention are not impaired even when Sn, Zr, Co, Zn, and W are contained in a total amount of 1% or less for other elements. Of these elements, Sn is desirably 0.05% or less because a flaw may occur during hot rolling.

[Microstructure of steel sheet]
The microstructure of the steel sheet will be described.
The steel sheet of the present invention is a region surrounded by a grain boundary in which the azimuth difference between adjacent crystals is 15 ° or more , and when a region whose equivalent circle diameter of the region is 0.3 μm or more is defined as a crystal grain, The crystal grains having an average of the misorientation in the crystal grains of 0 to 0.5 ° are included in an area ratio of 90% or more. The orientation difference in a crystal grain can be measured by using the EBSD method (electron beam backscatter diffraction pattern analysis method) often used for crystal orientation analysis. Crystal grains having such a misorientation in crystal grains have high ductility, uniform deformability, and low yield ratio. Therefore, by increasing the ratio, the formability can be improved.

Here, the formability refers to three of high ductility expressed by total elongation, high stretch flangeability expressed by hole expansion ratio, and desirably low yield ratio.
If the total elongation is small, the thickness of the sheet tends to decrease due to necking during press molding, which causes press cracking. In order to ensure press formability, it is preferable that the product of the total elongation (El) and the tensile strength (TS): (TS) × (El) ≧ 10000 MPa% is satisfied. Here, the tensile strength (TS) indicates the tensile strength according to JIS Z 2241 2011, and the total elongation (El) indicates the total elongation at break of JIS Z 2241 2011.
Further, when the hole expansion ratio according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996 is (λ), the steel sheet excellent in formability in the present patent should satisfy λ ≧ 150%. If the steel sheet satisfies λ ≧ 150%, the stretch flange portion of a normal underbody part can be formed without any problem.
Further, when the yield ratio is YR, a steel sheet satisfying YR ≦ 0.80 often has a low press load for tensile strength and is excellent in formability.

[Measurement method of crystal grain orientation]
The ratio of crystal grains having an average of the misorientation in crystal grains of 0 to 0.5 ° can be measured, for example, by the following method.
When the width of the steel sheet is W, a cross section (width direction cross section) of the steel sheet viewed from the rolling direction is observed at a position of 1 / 4W (width) or 3 / 4W (width) from one end in the width direction of the steel sheet. A sample is taken so as to be a plane, and EBSD analysis is performed on a rectangular area of 200 μm in the width direction × 100 μm in the thickness direction of the steel sheet at a measurement interval of 0.2 μm at a position １ ／ of the thickness of the steel sheet from the surface of the steel sheet.
Here, the EBSD analysis is performed using a device composed of a thermal field emission scanning electron microscope (for example, JSM-7001F made by JEOL) and an EBSD detector (for example, a HIKARI detector made by TSL) at a rate of 200 to 300 points / sec. Perform at analysis speed.

  The azimuth difference is obtained by calculating the difference in crystal azimuth between adjacent measurement points based on the crystal azimuth information of each measurement point measured as described above. When the azimuth difference is 15 ° or more, the middle between adjacent measurement points is determined to be a grain boundary, and when the area surrounded by the grain boundary is 0.3 μm or more in circle equivalent diameter, this is referred to as a crystal. Defined as grains. The average orientation difference is calculated by simply averaging the orientation differences in the crystal grains. Then, the area ratio of the crystal grains having an average orientation difference of 0 to 0.5 ° in the crystal grains is obtained. The definition of the crystal grains and the calculation of the average azimuth difference in the crystal grains can be obtained by using software “for example, OIM Analysis ™” attached to the EBSD analyzer.

[A crystal grain having an average of misorientation in crystal grains of 0 to 0.5 ° is 90% or more in area ratio]
When the crystal grains having an average of the misorientation in the crystal grains of 0 to 0.5 ° are less than 90% in area ratio, the ductility is deteriorated, which is a standard of the automobile underbody steel sheet having good ductility (TS ) × (El) ≧ 10000 MPa%. Therefore, it is preferable that the area ratio of crystal grains having an average of the orientation difference in the crystal grains of 0 to 0.5 ° is 90% or more. Since the ductility is improved as the area ratio of the crystal grains in which the average of the orientation difference in the crystal grains is 0 to 0.5 ° is higher, the area ratio is desirably 95% or more, more desirably 98% or more. Good to do.

  In the present embodiment, the crystal grains having an average of the orientation difference in the crystal grains of 0 to 0.5 ° and the ferrite defined from the observation result of the optical microscope are not directly related. In other words, for example, even if there is a steel sheet having a ferrite area ratio of 90% or more, the proportion of crystal grains having an average of the misorientation in crystal grains of 0 to 0.5 ° is not necessarily 90% or more. Absent. Therefore, the characteristics equivalent to the steel sheet according to the present embodiment cannot be obtained only by controlling the ferrite area ratio.

[Martensite + tempered martensite + retained austenite ≤ 2%]
The hard phase composed of martensite or tempered martensite or retained austenite becomes a source of voids during deformation and causes a decrease in the hole expansion ratio (λ). If the area fraction of the hard phase is more than 2%, λ ≧ 150% cannot be satisfied, so that the total structure of martensite, tempered martensite, and retained austenite is 2% or less in area fraction. Good. A method for obtaining the area fraction of martensite, tempered martensite and austenite constituting the steel sheet structure of the present invention as described above will be described below.

  The area fraction of martensite and tempered martensite constituting the steel sheet structure of the present invention is defined as a cross section (width) of the steel sheet in the width direction viewed from the rolling direction at a position of 1 / 4W or 3 / 4W of the sheet width W from one end of the steel sheet. A sample is taken so that the observation surface is the observation surface, and the observation surface is polished and nital-etched. (Field emission scanning electron microscope). When observed by FE-SEM, a structure having a lath (thin and long plate-like) structure and having no carbide precipitated was taken as martensite. A lath-like structure in which carbides were precipitated in a multi-variant form (cementite was precipitated in various directions) was tempered martensite. In addition, the thing which the carbide precipitates in a single variant (the cementite precipitates in one direction) was determined to be bainite. Using a FE-SEM, an area of 120 μm × 100 μm was measured at a magnification of 1000 times in 10 fields of view for each of the 板, ／, and 厚 thickness ranges. The area fraction of martensite and tempered martensite was determined for each visual field, and the average value thereof was used as a typical area fraction of martensite and tempered martensite.

  Similar to martensite and tempered martensite, the area fraction of retained austenite can be calculated from a cross section (width direction cross section) of the steel sheet in the width direction at a 1 / 4W or 3 / 4W position of the sheet width W from one end of the steel sheet. ) Was taken as an observation surface, the observation surface was polished, the processed layer was removed by electrolytic polishing, and then measured using the EBSD method (electron beam backscatter diffraction pattern analysis method). Calculate the angle difference of all combinations of six or more bands (corresponding to crystal planes) obtained by backscattering, for example, using a data file in “OIM Analysis ™” or a data file created by measuring an appropriate sample. In comparison, when the angle difference between the bands is closer to the angle difference between the bands when the data file crystal grains are austenite than when the angle difference between the bands when the data file crystal grains are ferrite, The grains were defined as retained austenite. Using the EBSD method, a region of 100 μm × 100 μm or more was observed in each range of １ ／, ／ and １ ／ of the plate thickness. The area fraction of retained austenite was determined for each range, and the average value thereof was used as a representative area fraction of retained austenite.

[Amount of solute carbon in crystal grains having an average of misorientation of 0 to 0.5 ° is 20 to 200 ppm]
Next, the solid solution C in the crystal grain in which the average of the misorientation in the crystal grain is 0 to 0.5 ° will be described. The solid solution C in the crystal grains is important for improving the lower limit stress intensity factor range (ΔKth). ΔKth indicates the range of the stress intensity factor at which the fatigue crack stops and does not propagate, and contributes to the improvement of the fatigue limit of the smooth material and the improvement of the fatigue characteristics of the punched material and the notched material. In order for a fatigue crack to propagate, dislocations must be activated at the tip of the fatigue crack. As a result of intensive studies, the inventors have found that if the solute C in the crystal grains is 20 ppm or more, the movement of dislocations is suppressed, and ΔKth is improved. The effect is more remarkable as the amount of solute C in the crystal grains increases, and preferably 50 ppm or more, more preferably 100 ppm or more. It is thought that the cause of the dissolution C suppressing dislocation movement is the dynamic strain aging effect. Although the upper limit of the amount of solid solution C is not specified, it is difficult in principle to exceed the solubility of C in ferrite since it is about 200 ppm even at the temperature of Ae1 at which the solubility is maximum.

Means for measuring the amount of solute C in the crystal grains in which the average of the misorientation in the crystal grains is 0 to 0.5 ° is not particularly specified. For example, measurement using a 3D-AP (three-dimensional atom probe) is not possible. It is possible. Specifically, while cutting the sample so that the crystal grain to be measured is at a measurable position, and performing electropolishing, a needle-like sample is prepared through processing by a focused ion beam processing method as necessary. . Next, a plurality of two-dimensional distribution images of the atoms of the created needle-shaped sample are acquired in the depth direction of the needle-shaped sample with a three-dimensional atom probe, and the obtained two-dimensional distribution images are reconstructed to obtain a real space. The three-dimensional distribution image of the atoms is obtained at the above step, and the amount of solute C is measured. In this study, at least 10 or more visual fields having a size of 20 nm × 20 nm × 50 nm of the sample were observed, and when a total of 5 or more carbon atoms were contained in a volume of 1 nm ³ , these were aggregated (carbon compound). And clusters), and the value obtained by subtracting the carbon contained in the aggregate from all the carbons in the observed region is defined as the solid solution C amount. Is defined as the amount of solute carbon (the amount of solute C).

[The density of TiC is 1.0 × 10 ¹⁶ / cm ³ or less]
Next, the amount of TiC in the structure will be described. TiC is a precipitate and has an effect of increasing the yield ratio (YR) by precipitation strengthening. Since a steel sheet having a low YR tends to have a small load at the time of press forming, the sheet pressing force at the time of pressing can be reduced, which is advantageous in press formability. According to the study by the inventors, when the density of TiC is more than 1.0 × 10 ¹⁶ / cm ³ , the effect of increasing the YR by TiC increases, and YR> 0.80. Therefore, the density of TiC is desirably 1.0 × 10 ¹⁶ / cm ³ or less. However, TiC as defined in this patent includes not only titanium carbide but also a compound compound of nitrogen and niobium combined with titanium carbide such as Ti (CN), (TiNb) C and (TiNb) (CN).

  The means for identifying TiC and measuring the density is not particularly specified. For example, the positions of Ti and C in the steel sheet are measured using a 3D-AP (three-dimensional atom probe), and the positions of Ti and C match. By defining this case as TiC, the number of TiC particles having a small particle size can be measured with high accuracy. At least 10 or more fields of view of the sample having a size of 20 nm × 20 nm × 50 nm are observed, and the density of precipitates is calculated from the relationship between the range of the observation field and the number of observed TiCs.

  The steel sheet of the present invention having the above structure and composition may be manufactured by hot rolling or cold rolling. The corrosion resistance can be improved by providing the surface with a hot-dip galvanized layer by a hot-dip galvanizing treatment, or further, by applying an alloying treatment after plating to an alloyed galvanized layer. Further, the plating layer is not limited to pure zinc, and elements such as Si, Mg, Al, Fe, Mn, Ca, and Zr may be added to further improve the corrosion resistance. Providing such a plating layer does not impair the excellent punching fatigue characteristics and workability of the present invention. In addition, the effects of the present invention can be obtained regardless of any surface treatment layer formed by organic film formation, film lamination, organic salt / inorganic salt treatment, non-color treatment, or the like.

[Steel sheet manufacturing method]
Formability The method for producing the steel sheet according to the present invention is not particularly limited, and examples thereof include the following method.

  The production method prior to hot rolling is not particularly limited. That is, various secondary smelting may be performed after smelting using a blast furnace, an electric furnace, or the like to adjust the composition to the above-described composition, and then casting may be performed by a method such as ordinary continuous casting or thin slab casting. At that time, scrap may be used as a raw material as long as it can be controlled within the component range of the present invention.

  The cast ingot (slab) is heated to a predetermined temperature before starting hot rolling (heating step). In the case of continuous casting, it may be cooled once to a low temperature, then heated again and then hot-rolled, or it may be heated and then hot-rolled without continuous cooling.

When Ti is added in an amount of 0.001% or more, the ingot heating temperature of the hot rolling is set to T1 (° C.) or more represented by (Expression c). However, when T1 is lower than 1100 ° C. or when Ti is not added, the ingot heating temperature is set to 1100 ° C. or higher. When ordinary casting is performed, the temperature of the ingot temporarily lowers to or below the Ar3 temperature after casting, so that TiC precipitates in the structure when Ti is added. If the ingot heating temperature is lower than T1, the temperature of the heating furnace is set to T1 or higher because TiC precipitated in the ingot does not sufficiently turn into solution and the amount of solid solution C cannot be controlled. Further, although the upper limit of the ingot heating temperature is not particularly defined, the effect of the present invention is exhibited, but if the heating temperature is excessively high, the surface of the ingot is oxidized and becomes scale, which is not economically preferable. For this reason, the upper limit of the ingot heating temperature is desirably 1300 ° C. or less. In addition, Ar3 is a temperature at which ferrite transformation starts when cooled from austenite, and T1 represents a temperature at which TiC turns into a solution.
T1 (° C.) = 7000 / {2.75-log ([Ti] × [C])} − 273 (Formula c)
Here, [Ti] and [C] indicate mass% of Ti and C, respectively.

After the heating step, the ingot extracted from the heating furnace is subjected to a hot rolling rough rolling step and a subsequent finish rolling step to obtain a hot-rolled steel sheet (hot rolling step). Finish rolling is usually performed by continuous rolling of multiple stages (for example, 6 stages or 7 stages). In the finish rolling performed in this multi-stage continuous rolling, the rolling reduction is higher in the upstream side (upstream side) than in the downstream side (downstream side), and the downstream side (downstream side) is reduced in rolling rate. Sometimes. In the present invention, in the finish rolling performed by this multi-stage continuous rolling, among the finishing rolling stages in which the rolling reduction is 5% or more, the rolling temperature (the rolling reduction) in the last stage (downstream side) finishing rolling stage. (Also referred to as a final rolling temperature at a rate of 5% or more) is set to a lower temperature of Ar3 (° C.) or 1000 ° C., whichever is lower, which is represented by (Formula d). Rolling at less than Ar3 (° C.) results in two-phase rolling, and the area fraction of crystal grains having an average of misorientation in crystal grains of 0 to 0.5 ° cannot be made 90% or more. However, if the final rolling temperature at a rolling reduction of 5% or more is 1000 ° C. or more, even if the final rolling temperature at a rolling reduction of 5% or more is Ar3 (° C.) or less, the crystal is formed by ferrite recrystallization after two-phase rolling. The area fraction of crystal grains in which the average of the misorientation in grains is 0 to 0.5 ° can be 90% or more.
Ar3 (° C.) = 868-396 × [C] −68.1 × [Mn] + 24.6 × [Si] −36.1 × [Ni] −24.8 × [Cr] −20.7 × [Cu ] + 250 × [Al] (formula d)
The rolling reduction is obtained by the following equation for each stage.
Reduction rate = (thickness of the entrance side of the rolling mill−thickness of the exit side of the rolling mill) / (thickness of the entrance side of the rolling mill) × 100%

Next, in the cooling step after rolling, the steel sheet is kept for 8 seconds or more in the range of Ae1 ± 30 (° C.) represented by (Equation e). The residence time is preferably at least 10 seconds, more preferably at least 12 seconds.
Ae1 (° C.) = 723-10.7 × [Mn] + 29.1 × [Si] −16.9 × [Ni] + 16.9 × [Cr] + 70 × [Al] (formula e)
Ae1 is the eutectoid transformation start temperature of austenite to ferrite and cementite in an equilibrium state.

  The cooling rate when cooling from the temperature after rolling to Ae1 + 30 (° C.) is not particularly limited. It has been confirmed that if the cooling rate is 1 ° C./s or more corresponding to air cooling and 500 ° C./s or less corresponding to rapid cooling, no problem occurs in characteristics. If the residence time in the range of Ae1 ± 30 (° C.) is short, the area fraction of crystal grains in which the average of the misorientation in crystal grains is 0 to 0.5 ° cannot be made 90% or more.

  After staying, the cooling rate from Ae1-30 (° C) to 300 ° C is set to 100 ° C / s or more, and the cooling rate from 300 ° C to 30 ° C is set to 30 ° C / s or more. If these cooling rates cannot be realized, the amount of solute C in the crystal grains in which the average of the misorientation in the crystal grains is 0 to 0.5 ° precipitates as carbides such as cementite. No more. Desirably, the cooling rate from 300 ° C. to 30 ° C. is set to 50 ° C./s or more.

Even if a part of the process such as pickling, which is a process associated with the normal hot rolling process, is omitted and the production is performed, excellent fatigue characteristics and moldability, which are the effects of the present invention, can be ensured . Also, by performing appropriate cold-annealing, also in a cold-rolled steel sheet, a region where the orientation difference between adjacent crystals surrounded by grain boundaries is more than 15 °, the circle equivalent diameter of the area 0 When a crystal grain having a grain size of 0.3 μm or more is defined as a crystal grain, the crystal grain having an average of the misorientation in the crystal grain of 0 to 0.5 ° is included in an area ratio of 90% or more, and the martensite or the tempered martensite is contained. Alternatively, a structure in which the sum of the area ratios of the hard phases composed of retained austenite is 2% or less, and the amount of solute carbon in the crystal grains whose average of the misorientation is 0 to 0.5 ° is 20 to 200 ppm. It is possible to produce

Test results for confirming the operation and effect of the present invention will be described.
Table 1 shows the components of the steel subjected to the test.
Table 2 shows the steel types of the test pieces subjected to the test and the manufacturing conditions.
The finish rolling was performed by a seven-stage continuous rolling.
Table 3 shows the evaluation results of each test piece.
Among the mechanical properties, the tensile strength characteristics (tensile strength, total elongation, yield ratio) are as follows: when the width of the plate is W, the position is 1/4 W or 3/4 W from one end of the plate in the width direction. Using a No. 5 test piece of JIS Z 2241 2011 sampled with the direction (width direction) perpendicular to the rolling direction taken as the longitudinal direction, evaluation was performed in accordance with JIS Z 2241 2011. The yield stress used for the yield ratio was the descending yield stress. The hole expansion ratio was determined by taking a test piece from the same position as the tensile test piece taking position and following the test method described in the Japan Iron and Steel Federation Standard JFST 1001-1996. Further, the steel sheet having excellent formability in the present invention is a steel sheet satisfying (TS) × (El) ≧ 10000 MPa%, satisfying (λ) ≧ 150%, and desirably satisfying (YR) ≦ 0.80. Here, (TS) is the tensile strength, (El) is the total elongation, (λ) is the hole expansion ratio, and (YR) is the yield ratio.

In the present invention, in order to evaluate the lower limit stress intensity factor range, ASTM E647-08 A1. Is set so that the direction perpendicular to the rolling direction from the same position as the tensile test specimen collection position is the crack propagation direction. Was collected and subjected to a crack propagation test by a method in accordance with ASTM E647-08. When the stress ratio was set to 0.01 and the stress intensity factor range ΔK was reduced by the gradual decrease method, the decrease in the fatigue crack propagation speed was measured, and the crack propagation speed was 1.0 × 10 ⁻¹⁰ (m / cycle). ), That is, ΔK which is 1.0 (Å (angstrom) / cycle) (= 100 pm / cycle) is defined as ΔKth. Other test conditions at this time are as follows.
Test method: Using an electro-hydraulic servo type ± 10 ton fatigue tester, the crack length is measured by a computer-controlled compliance method with a gradual load reduction method and a K value reduction method (the load is automatically reduced as the crack progresses) ΔKth is measured by the method) Test environment: room temperature, in the air Control method: load control Stress ratio: R = 0.01
Frequency: 10-20Hz
The steel sheet having excellent fatigue characteristics in the present invention is a steel sheet satisfying ΔKth ≧ 5 (MPa · m ^1/2 ).

  As shown in Table 3, the steel sheet according to the present invention had excellent fatigue characteristics and formability.

  On the other hand, in steel Nos. 27 and 31, the final rolling temperature at a rolling reduction of 5% or more was less than Ar 3 ° C. and less than 1000 ° C. represented by (Formula d), and the average of the misorientation in the crystal grains was 0 to 0.5. °, the area fraction of the crystal grains was less than 90%, and (TS) × (El) ≧ 10000 MPa% could not be satisfied.

  In the cooling process after rolling, steel numbers 28 and 32 had a residence time in the range of Ae1 ± 30 ° C. represented by (formula e) of less than 8 seconds, so that the average of the misorientation in the crystal grains was 0 to 0. The area fraction of crystal grains of 0.5 ° was less than 90%, and (TS) × (El) ≧ 10000 MPa% could not be satisfied.

Steel Nos. 29 and 33 had a cooling rate from Ae1-30 ° C. to 300 ° C. less than 100 ° C./s. The carbon content was less than 20 ppm, and ΔKth ≧ 5 (MPa · m ^1/2 ) could not be satisfied.

In steel numbers 30 and 34, the cooling rate from 300 ° C. to 30 ° C. was less than 30 ° C./s, so the amount of solute carbon in the crystal grains in which the average of the misorientation in the crystal grains was 0 to 0.5 ° Was less than 20 ppm, and ΔKth ≧ 5 (MPa · m ^1/2 ) could not be satisfied.

Steel No. 35 had a heating temperature of T1 ° C. or lower specified by (formula c), and thus the amount of solute carbon in a crystal grain having an average of misorientation in a crystal grain of 0 to 0.5 ° was less than 20 ppm. ΔKth ≧ 5 (MPa · m ^1/2 ) could not be satisfied.

  In steel No. 36, since the Tief represented by (Formula a) was a negative value and the C content was more than 0.050%, the area ratio of the low-temperature transformation product as the hard second phase was more than 2%. And it was not possible to satisfy (λ) ≧ 150%.

In steel No. 37, since the content of C was less than 0.002%, the amount of solute carbon in crystal grains having an average of misorientation in crystal grains of 0 to 0.5 ° was less than 20 ppm, and ΔKth ≧ 5 (MPa · m ^1/2 ) could not be satisfied.

For Steel Nos. 37 and 45, Tief represented by (Formula a) was a positive value and ([C] −12 / 48 × Tief) was less than 0.002%. Was 0 to 0.5 °, the amount of solute carbon in the crystal grains was less than 20 ppm, and ΔKth ≧ 5 (MPa · m ^1/2 ) could not be satisfied.

  In steel No. 38, since the content of Si was more than 2.00%, the workability was lowered, and (TS) × (El) ≧ 10000 MPa% could not be satisfied.

  In steel No. 39, since the Mn content was more than 2.00%, the workability was reduced, and (λ) ≧ 150% could not be satisfied.

  In steel No. 40, since the content of P was more than 0.100%, the workability was lowered, and (TS) × (El) ≧ 10000 MPa% could not be satisfied.

  Steel No. 41 had a S content of more than 0.0300%, so cracks occurred during rolling, and a hot-rolled sheet could not be obtained.

  Steel No. 42 had an Al content of more than 2.000%, so cracks occurred during rolling, and a hot-rolled sheet could not be obtained.

  Steel No. 43 had an N content of more than 0.010%, so the workability was reduced, and (λ) ≧ 150% could not be satisfied.

  Steel No. 44 had a Ti content of more than 0.200%, so nozzle blockage occurred during casting, and a hot-rolled sheet could not be obtained.

  INDUSTRIAL APPLICATION This invention can be utilized as steel materials for machine structures. In particular, it can be applied to underbody parts of automobiles and structural parts of vehicle bodies.

Claims (7)

Chemical composition in mass%
C: 0.002% to 0.100%,
Si: 2.00% or less,
Mn: 2.00% or less,
Al: 2.000% or less,
N: 0.0100% or less,
O: 0.0100% or less,
Ti: 0.200% or less,
The impurities P and S are
P: 0.100% or less,
S: limited to 0.0300% or less,
The balance is Fe and a steel plate that is an unavoidable impurity,
Using Tief calculated from the following (formula a),
The effective carbon amount Ceff calculated by the following (formula b) is 0.002% to 0.050%,
When a crystal grain is defined as a region surrounded by a grain boundary having an orientation difference of 15 ° or more from an adjacent crystal and having an equivalent circle diameter of 0.3 μm or more in the region, the orientation in the crystal grain is defined as Containing 90% or more in terms of area ratio of crystal grains having an average difference of 0 ° to 0.5 °,
The sum of the area ratios of the hard phase composed of martensite or tempered martensite or retained austenite is 2% or less, and the average of the misorientation is 0 ° or more and 0.5 ° or less. A steel sheet having excellent fatigue characteristics and formability, characterized in that the amount of solute carbon is 20 ppm or more and 200 ppm or less.
Tief = [Ti] −48 / 14 × [N] −48 / 32 × [S] (Formula a)
Ceff = [C] −12 / 48 × Tieff (Equation b)
However, [Ti], [N], [S], and [C] indicate the mass% of Ti, N, S, and C in the steel sheet, respectively, and 0% is substituted when they are not contained. .
When Tief ≦ 0, calculation is performed assuming that Tief = 0 in (Equation b). The steel sheet having excellent fatigue properties and formability according to claim 1, wherein the density of TiC is 1.0 × 10 ¹⁶ pieces / cm ³ or less. In further mass%,
Nb: 0.100% or less,
V: 0.300% or less,
Cu: 1.20% or less,
Ni: 0.60% or less,
Cr: 2.00% or less,
Mo: 1.00% or less,
The steel sheet having excellent fatigue properties and formability according to claim 1 or 2, characterized by containing one or more of the following. In further mass%,
Mg: 0.0100% or less,
Ca: 0.0100% or less,
REM: 0.1000% or less,
The steel sheet having excellent fatigue properties and formability according to any one of claims 1 to 3, characterized by containing one or more of the following. In further mass%,
B: 0.0020% or less,
The steel sheet according to any one of claims 1 to 4, wherein the steel sheet has excellent fatigue characteristics and formability.   The fatigue according to any one of claims 1 to 5, further comprising 1 mass% or less of one or more of Sn, Zr, Co, Zn, and W. Steel sheet with excellent properties and formability. A method for producing a steel sheet having excellent fatigue properties and formability according to any one of claims 1 to 6, having the component composition according to any one of claims 1 to 6. A heating step of heating the steel ingot to a temperature T1 (° C.) calculated from the following (formula c) or 1100 ° C., whichever is larger, and 1300 ° C. or less;
The hot ingot is subjected to rough rolling of the heated ingot, and then subjected to finish rolling by multi-stage continuous rolling to obtain a hot-rolled steel sheet,
Having a cooling step of cooling the obtained hot-rolled steel sheet,
In the finish rolling by the multi-stage continuous rolling, the rolling temperature at the rearmost stage among the stages with a draft of 5% or more is higher than the lower temperature of Ar3 or 1000 ° C. which is calculated by the following (formula d). Temperature
In the cooling step, the hot-rolled steel sheet is formed on the basis of Ae1 calculated by the following (Equation e):
Ae1-30 ° C. or more, stay in a temperature range of Ae1 + 30 ° C. or less for 8 seconds or more,
Ae1 The cooling rate from 30 ° C. to 300 ° C. is 100 ° C./sec or more,
Further, a method for producing a steel sheet having excellent fatigue characteristics and formability, wherein the cooling rate from 300 ° C. to 30 ° C. is 30 ° C./sec or more.
T1 (° C.) = 7000 / {2.75-log ([Ti] × [C])} − 273 (Formula c)
Ar3 (° C.) = 868-396 × [C] −68.1 × [Mn] + 24.6 × [Si] −36.1 × [Ni] −24.8 × [Cr] −20.7 × [Cu ] + 250 × [Al] (formula d)
Ae1 (° C.) = 723-10.7 × [Mn] + 29.1 × [Si] −16.9 × [Ni] + 16.9 × [Cr] + 70 × [Al] (formula e)
However, [element name] in the formula indicates the content (% by mass) of the element in the steel sheet, and 0% is substituted when the element is not contained.