JP6332570B1 - Hot-rolled steel sheet, steel forged parts and method for producing them - Google Patents

Abstract

The chemical composition of the steel sheet is mass%, C: 0.020-0.070%, Si: 0.05-1.70%, Mn: 0.60-2.50%, Al: 0.010-1.000%, N> 0-0.0030%, P ≦ 0.050%, S ≦ 0.005%, Ti: 0.015-0.170%, Nb: 0-0.100%, V: 0-0.300%, Cu: 0-2.00%, Ni: 0-2.00%, Cr: 0-2.00%, Mo: 0 -1.00%, B: 0-0.0100%, Mg: 0-0.0100%, Ca: 0-0.0100%, REM: 0-0.1000%, Zr: 0-1.000%, Co: 0-1.000%, Zn: 0- 1.000%, W: 0-1.000%, Sn: 0-0.050%, balance: Fe and impurities, metal structure of steel sheet in area%, ferrite: 5-70%, bainite: 30-95%, residual γ ≦ 2%, martensite ≦ 2%, pearlite ≦ 1%, ferrite + bainite ≧ 95%, and the number density of fine Ti precipitates in the ferrite grains is 1.0 × 1016-50.0 × 1016 / cm3 The average equivalent circle diameter of TiN precipitates in the steel sheet is 1.0-10.0μm, the average value of the shortest distance between adjacent TiN precipitates is 10.0μm or more, Hot-rolled steel sheet with a quasi-deviation of 1.00 GPa or less.

Description

  The present invention relates to a hot-rolled steel sheet, a steel forged part, and a method for producing them.

  Steel sheets used in automobile body structures are required to have high strength and high press workability from the viewpoint of improving safety and reducing weight. In response to such a demand, a high-strength steel sheet excellent in hole expansibility (high burring property) better than before has been proposed. For example, as a steel sheet excellent in hole expansibility (λ value), a steel sheet of a ferrite main phase that is precipitation strengthened by fine precipitates such as Ti and Nb and a manufacturing method thereof have been reported.

  Patent Document 1 discloses a hot-rolled steel sheet having high strength and excellent stretch flangeability. Patent Document 2 discloses a high-formability, high-tensile hot-rolled steel sheet having excellent material uniformity. Furthermore, Patent Document 3 discloses a high-tensile hot-rolled steel sheet having excellent elongation and stretch flangeability.

JP 2002-105595 A JP 2002-322540 A JP 2002-322541 A

  By the way, with the complication of the car body structure and the complexity of the part shape of automobiles, the processing of steel sheets for automobiles is not limited to conventional press processing elements, but new processing is applied to conventional press processing elements such as plate forging. Elements have been combined in a complex way. Conventional press working elements include, for example, deep drawing, hole expansion, stretch forming, bending, and ironing.

  However, in recent press work represented by plate forging, forging work elements such as upsetting, for example, by further applying a compressive load to the conventional press work elements by further distributing the press load. Further, processing elements such as thickening (thickening) processing have been added. That is, plate forging is a press work having a composite working element including a working element peculiar to forging, in addition to a working element when pressing a conventional steel plate.

  By performing such plate forging, the plate thickness of the steel plate remains the original plate thickness, or the steel plate is deformed while being reduced (thinned) by conventional press processing, while the part is being molded, In parts that have undergone forging by applying a compressive force, the thickness of the steel sheet is increased (thickening) so that it can be efficiently deformed so that it has the thickness of the steel sheet necessary for its function. The strength of the parts can be ensured.

  However, Patent Documents 1 to 3 do not mention any processing including a composite processing element represented by plate forging. Moreover, the winding conditions for manufacturing the hot-rolled steel sheet described in Patent Document 1 are very strict and unrealistic. Furthermore, since the hot-rolled steel sheets described in Patent Documents 2 and 3 contain 0.07% or more of Mo, which is an expensive alloy element, there is a problem that the manufacturing cost is high.

  High burring steel is known to exhibit good formability in conventional pressing. However, it has been found that in plate forging, which is a forming method that includes elements of forging in the conventional press working, cracks may occur in the steel sheet even when the degree of processing is small.

  In other words, in conventional pressing, press cracks occur in the areas where sheet thickness constriction (reduction in sheet thickness) occurs, but even in processes that do not involve sheet thickness constriction, such as sheet forging, the material cracks. It has been found that the product may not be obtained due to breakage.

  It is not well known about what kind of properties of the steel sheet the limit of cracking in such plate forging is controlled and how it can be improved. Therefore, there has been a demand for a high burring steel that does not break even after plate forging while effectively utilizing the functions of conventional high burring steel, such as deep drawing workability, hole expansibility, and stretch forming workability. .

  The present invention has been made in order to solve the above-mentioned problems, and while maintaining the basic function as a high burring steel, the crack limit of the portion subjected to forging by applying a partial compression force is achieved. It aims at providing the hot rolled steel plate excellent in the plate forgeability which can be improved.

  This invention is made | formed in order to solve the said subject, and makes a summary the following hot-rolled steel plate and steel forged components, and those manufacturing methods.

(1) The chemical composition of the steel sheet is mass%,
C: 0.020 to 0.070%,
Si: 0.05 to 1.70%,
Mn: 0.60 to 2.50%,
Al: 0.010 to 1.000%
N: more than 0% to 0.0030% or less,
P: 0.050% or less,
S: 0.005% or less,
Ti: 0.015-0.170%,
Nb: 0 to 0.100%,
V: 0 to 0.300%,
Cu: 0 to 2.00%,
Ni: 0 to 2.00%,
Cr: 0 to 2.00%
Mo: 0 to 1.00%,
B: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Ca: 0 to 0.0100%,
REM: 0 to 0.1000%,
Zr: 0 to 1.000%,
Co: 0 to 1.000%,
Zn: 0 to 1.000%
W: 0 to 1.000%
Sn: 0 to 0.050%, and
Balance: Fe and impurities,
In the cross section perpendicular to the rolling direction of the steel sheet, when the width and thickness of the steel sheet are W and t, respectively, 1/4 W or 3/4 W from the end face of the steel sheet and 1 from the surface of the steel sheet / 4t or 3 / 4t, the metallographic structure is area%,
Ferrite: 5 to 70%,
Bainite: 30-95%
Residual austenite: 2% or less,
Martensite: 2% or less, and
Perlite: 1% or less, and
Total of ferrite and bainite: 95% or more,
The ferrite has a precipitate containing Ti in the grains,
The number density of the precipitates containing Ti is 1.0 × 10 ^{16 to} 50.0 × 10 ¹⁶ pieces / cm ³ ,
TiN precipitates are included in the steel sheet,
The average equivalent circle diameter of the TiN precipitate is 1.0 to 10.0 μm,
The average value of the shortest distance between the adjacent TiN precipitates is 10.0 μm or more,
The standard deviation of nano hardness is 1.00 GPa or less,
Hot rolled steel sheet.

(2) The average equivalent circle diameter of the precipitate containing Ti is 1.00 to 3.00 nm.
The hot-rolled steel sheet according to (1) above.

(3) Tensile strength is 780 MPa or more,
The product of uniform elongation and tensile strength is 7000 MPa ·% or more,
The product of the hole expansion ratio and the tensile strength is 50000 MPa ·% or more,
The hot-rolled steel sheet according to (1) or (2) above.

(4) A method for producing a hot-rolled steel sheet according to any one of (1) to (3) above,
For the slab having the chemical composition described in (1) above, a heating process, a continuous hot rolling process, a first cooling process, a second cooling process, and a winding process are performed in order,
In the heating step, the slab is heated to a temperature of SRTmin ° C. or higher and 1260 ° C. or lower represented by the following formula (i):
The continuous hot rolling step includes rough rolling and multi-stage finish rolling of three or more stages,
The end temperature of the rough rolling is 1100 ° C. or higher,
The cumulative strain in the final three stages of rolling in the multistage finish rolling is 0.01 to 0.10,
The rolling end temperature of the multistage finish rolling is a temperature of Ar ₃ + 30 ° C. or higher obtained by the following formula (ii):
In the first cooling step, after the multi-stage finish rolling is finished, cooling is started after 1.00 to 5.00 s, and from the rolling finish temperature to a temperature range of 650 to 750 ° C., 10 ° C./s or more. Cool at average cooling rate, then hold in air for 1-10 s,
In the second cooling step, after being held in the atmosphere, cooling is performed at an average cooling rate of 10 ° C./s or more from a temperature range of 600 to 740 ° C.,
In the winding step, winding is performed at a winding temperature of 450 to 650 ° C.
Manufacturing method of hot rolled steel sheet.
SRTmin = 7000 / {2.75−log (Ti × C)} − 273 (i)
Ar ₃ = 970-325 × C + 33 × Si + 287 × P + 40 × Al-92 × (Mn + Mo + Cu) −46 × (Cr + Ni) (ii)
However, the element symbol in the said formula represents content (mass%) in the hot-rolled steel plate of each element, and shall substitute 0 when not containing.

(5) Obtained from the hot-rolled steel sheet according to any one of (1) to (3) above,
Forged steel parts.

(6) At least forging processing is performed on the hot-rolled steel sheet according to any one of (1) to (3) above.
Manufacturing method of steel forged parts.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to obtain the hot rolled steel plate excellent in plate forgeability, maintaining the favorable hole expansibility which is a basic function as high burring steel.

It is a schematic diagram explaining a simple shear test. Fig.1 (a) is a figure which shows the test piece of a simple shear test. FIG.1 (b) is a figure which shows the test piece after a simple shear test.

  The present inventors diligently studied to solve the above-mentioned problems, and obtained the following knowledge.

(A) Equivalent plastic strain Plate forging includes deformation in a strain range (high strain range) exceeding the fracture strain in the conventional tensile test. Moreover, since plate forging is a complex process, it cannot be evaluated simply by tensile test and shear test data. Therefore, the present inventors introduced “equivalent plastic strain” as an index, and established a new evaluation method.

  By using this equivalent plastic strain as an index, it is possible to evaluate compositely the tensile stress and tensile strain at break when a tensile test is performed, and the shear stress and shear strain at break when a shear test is performed. I found it.

Equivalent plastic strain converts the relationship between the shear stress σs and the shear plastic strain εsp in the simple shear test into the relationship between the tensile stress σ and the tensile strain ε in the uniaxial tensile test with different deformation modes. . Then, assuming the relationship between the isotropic hardening rule and the plastic work conjugate, the conversion can be performed as shown in the following equation by using a constant conversion coefficient (κ). After calculating the conversion coefficient (κ) by the method described later, the equivalent plastic strain is derived.
Tensile stress σ in uniaxial tensile test = Shear stress σs × κ in simple shear test
Tensile strain in uniaxial tensile test ε = Shear plastic strain in simple shear test εsp / κ

(B) Multistage shear test In order to obtain the equivalent plastic strain, it is necessary to obtain the relationship between the tensile stress and the tensile strain by the tensile test and the relationship between the shear stress and the shear strain by the shear test. However, plate forging includes deformation in a high strain region. Therefore, when a test is performed once using a commonly used shear test apparatus, a crack progresses from the portion holding the test piece to the test piece. As a result, it is often impossible to test deformation up to a high strain range. Accordingly, there is a need for a method of reproducing a process that does not cause a reduction in thickness (thinning and constriction) of a steel plate, such as plate forging.

  Therefore, the shear test is performed in multiple stages, and after each stage of the shear test, the starting point of the crack of the test piece generated in the part holding the test piece is machined to crack the test piece. The test results were evaluated by connecting these shear test results in series. By applying this test method, it becomes possible to obtain a shear test result up to a high strain region, and a relationship between shear stress and shear strain up to the high strain region can be obtained.

  On the other hand, conventional tensile test methods can be applied to tensile stress and tensile strain. For example, a JIS No. 5 test piece based on JIS Z2241 (2011) can be used.

(C) Crack generation mechanism By adopting the multi-stage shear test described above, an evaluation method using equivalent plastic strain, and micro-investigation of the steel sheet before and after plate forging, the following knowledge about the crack generation mechanism was obtained. Obtained.

  As a microstructure of high burring steel, in order to obtain excellent hole expandability, a structure mainly composed of ferrite (precipitation strengthened ferrite) strengthened by precipitation with fine precipitates such as Ti and Nb is used. On the other hand, when Ti is added, coarse TiN precipitates unless a special manufacturing method is used (hereinafter, the deposited TiN is also simply referred to as “TiN”). This is a compound in which TiN is thermodynamically very stable and preferential to other compounds at high temperatures such as casting during the steel sheet manufacturing process, hot rolling heating, or rough rolling initial stage. This is because of crystallization or precipitation.

  TiN is hard enough to be used as a coating for cutting tools, machine parts, plastic molds, sporting goods, ornaments, etc., and its hardness is known to be about Hv 2000 to 2300, and it is a very hard precipitate. It is a thing. Therefore, when subjected to deformation in a high strain region such as plate forging, voids are likely to occur at the interface due to the difference in deformability from the matrix structure.

  Due to the difference in deformability between the hard precipitate (TiN) and the moderately soft matrix structure (ferrite or bainite), voids (minute cavities) are generated at the interface between the two phases. Thereafter, as the forging distortion increases, voids grow, bond to adjacent voids, become cracks, and break. Thus, it has been found that the generation of cracks can be suppressed by preventing the generation of voids and suppressing the bonding with adjacent voids even when the voids grow. However, in that case, it is also important not to impair the original function as high burring steel.

  The following items were found from these findings.

(I) Limiting the average diameter of TiN.
That is, since voids are generated at the TiN grain boundaries of hard precipitates, the generation of voids can be reduced by limiting the average diameter of TiN.

(Ii) Limiting the distance between TiNs.
In other words, since voids are generated at the TiN grain boundaries, disposing TiNs apart from each other makes it difficult to bond even when voids grow.

(Iii) To reduce variations in nano hardness.
That is, generation of voids can be reduced by reducing the difference in hardness between the hard tissue and the soft tissue as much as possible.

(Iv) The equivalent plastic strain at break is 0.90 (90%) or more.
By satisfying the above conditions (i) to (iii), the equivalent plastic strain at the time of fracture becomes 0.90 (90%) or more, and a certain workability is obtained even in complex machining such as plate forging. Confirmed that it is possible to secure.

(D) Effective cumulative strain In order to obtain the structures (i) to (iv) above, in multi-stage finish rolling performed in multi-stage (for example, 6-stage or 7-stage) continuous rolling of 3 or more stages in hot rolling, the final It is necessary to perform the final finish rolling so that the cumulative strain in the three-stage rolling (hereinafter sometimes referred to as “effective cumulative strain”) is 0.01 to 0.10.

  The effective cumulative strain is an index considering the temperature during rolling, the recovery of crystal grains due to the rolling reduction of the steel sheet by rolling, recrystallization, and grain growth. Therefore, when obtaining the effective cumulative strain, a constitutive law expressing a static recovery phenomenon over time after rolling was used. Considering that the grains recover statically over time after rolling, the release of energy accumulated as strain in the grains after rolling is due to static recovery due to the disappearance of dislocations in the thermal grains. Because it happens. The disappearance of this thermal dislocation is influenced by the rolling temperature and the elapsed time after rolling. Therefore, taking this static recovery into account, we introduced an index that describes the temperature during rolling, the rolling reduction (logarithmic strain) of the steel sheet due to rolling, and the elapsed time after rolling as parameters, and this is called “effective cumulative strain”. Defined.

  Thus, by limiting the effective cumulative strain, the desired microstructure can be obtained and the variation in nano hardness is reduced, so by suppressing the generation of voids at the interface between the hard tissue and the soft tissue, Since cracking does not occur even after plate forging, a steel plate excellent in plate forgeability can be obtained.

  The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The reason for limitation of each element is as follows. In the following description, “%” for the content means “% by mass”.

C: 0.020 to 0.070%
C combines with Nb, Ti, etc. to form precipitates in the steel sheet, and contributes to strength improvement by precipitation strengthening. When the C content is less than 0.020%, the effect by the above action cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.070%, the iron-based carbide that becomes the starting point of cracking during hole expansion processing increases, and the hole expansion value deteriorates. Therefore, the C content is 0.020 to 0.070%. The C content is preferably 0.025% or more, and more preferably 0.030% or more. Moreover, it is preferable that C content is 0.060% or less, and it is more preferable that it is 0.050% or less.

Si: 0.05 to 1.70%
Si has a deoxidizing effect and an effect of suppressing precipitation of iron-based carbides such as cementite in the material structure and contributing to improvement of ductility and hole expandability. However, when the content is excessive, ferrite transformation is likely to occur at high temperatures, and accordingly, carbides containing Ti are likely to precipitate at high temperatures. Precipitation of carbides at high temperatures tends to cause variations in the amount of precipitation, resulting in material variations such as strength and hole expandability. Therefore, the Si content is set to 0.05 to 1.70%.

  The Si content is preferably 0.06% or more, and more preferably 0.08% or more, from the viewpoint of suppressing the occurrence of scale defects such as scales and spindle scales. Moreover, it is preferable that Si content is 1.50% or less, and it is more preferable that it is 1.00% or less from a viewpoint of improving chemical conversion property and corrosion resistance after coating.

Mn: 0.60 to 2.50%
Mn is an element that contributes to strengthening ferrite and improving hardenability. On the other hand, if it is contained in a large amount, the hardenability is increased more than necessary, and sufficient ferrite cannot be secured, and slab cracking occurs during casting. Therefore, the Mn content is 0.60 to 2.50%. The Mn content is preferably 1.00% or more, and more preferably 1.50% or more. Moreover, it is preferable that Mn content is 2.00% or less, and it is more preferable that it is 1.80% or less.

Al: 0.010 to 1.000%
Al, like Si, has a deoxidizing effect and an effect of generating ferrite. On the other hand, if the content is excessive, embrittlement is caused and the tundish nozzle is easily closed during casting. Therefore, the Al content is made 0.010 to 1.000%. The Al content is preferably 0.015% or more or 0.020% or more, and more preferably 0.025% or more or 0.030% or more. Further, the Al content is preferably 0.800% or less, 0.700% or less or 0.600% or less, more preferably 0.500% or less or 0.400% or less.

N: More than 0% to 0.0030% or less When N is contained in a large amount, not only does solute nitrogen remain and ductility decreases, but also TiN precipitates and decreases hole expansibility. Therefore, the N content is 0.0030% or less. The N content is preferably 0.0025% or less.

P: 0.050% or less P is an impurity contained in the hot metal, and since it segregates at the grain boundaries, it degrades local ductility and weldability. Therefore, the P content is limited to 0.050% or less. The P content is preferably 0.030% or less or 0.020% or less. In particular, it is not necessary to specify a lower limit, and the lower limit is 0%. However, excessively reducing the content increases the cost during refining, so the lower limit may be made 0.001%.

S: 0.005% or less,
S is also an impurity contained in the hot metal, and forms MnS to deteriorate local ductility and weldability. Therefore, the S content is limited to 0.005% or less. In order to improve ductility or weldability, the S content may be 0.003% or less or 0.002% or less. In particular, it is not necessary to specify a lower limit, and the lower limit is 0%. However, excessively reducing the content increases the cost during refining, so the lower limit may be made 0.0005%.

Ti: 0.015-0.170%
Ti has the effect that carbonitride or solute Ti delays grain growth during hot rolling, thereby reducing the grain size of the hot-rolled sheet and improving low-temperature toughness. Further, by finely dispersing in the ferrite as TiC, it contributes to increasing the strength of the steel sheet through precipitation strengthening. However, when the content is excessive, in addition to saturation of the effect, TiN that is a hard precipitate is likely to be precipitated. Therefore, the Ti content is set to 0.015 to 0.170%. The Ti content is preferably 0.030% or more, 0.045% or more, or 0.060% or more, 0.070% or more, 0.080% or more, 0.090% or more, or 0.100% or more. It is more preferable that The Ti content is preferably 0.160% or less, 0.150% or less, 0.140% or less, 0.130% or less, or 0.120% or less.

Nb: 0 to 0.100%
Nb has the effect of reducing the grain size of the hot-rolled sheet and improving low-temperature toughness by delaying grain growth during hot rolling by carbonitride or solute Nb. Moreover, by existing as NbC, it contributes to the strengthening of a steel plate through precipitation strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Nb content is 0.100% or less. If necessary, the Nb content may be 0.080% or less, 0.060% or less, or 0.050% or less. The lower limit is 0%, but the lower limit may be 0.001% or 0.010% in order to sufficiently obtain the above effect.

V: 0 to 0.300%
V is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the V content is set to 0.300% or less. If necessary, the V content may be 0.200% or less, 0.100% or less, or 0.060% or less. The lower limit is 0%, but the lower limit may be 0.001% or 0.010% in order to sufficiently obtain the above effect.

Cu: 0 to 2.00%
Cu is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Cu content is 2.00% or less. In addition, if the Cu content is large, scratches due to scale may occur on the surface of the steel sheet. Therefore, the Cu content may be 1.20% or less, 0.80% or less, 0.50% or less, or 0.25% or less. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Cu content may be 0.01%.

Ni: 0 to 2.00%
Ni is an element having an effect of improving the strength of the steel sheet by solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Ni content is 2.00% or less. Moreover, when Ni content is contained abundantly, there exists a possibility that ductility may deteriorate. Therefore, the Ni content may be 0.60% or less, 0.35% or less, or 0.20% or less. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Ni content may be 0.01%.

Cr: 0 to 2.00%
Cr is an element having an effect of improving the strength of the steel sheet by solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Cr content is 2.00% or less. In order to further improve economy, the upper limit may be set to 1.00%, 0.60%, or 0.30%. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Cr content may be 0.01%.

Mo: 0 to 1.00%
Mo is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Mo content is set to 1.00% or less. In order to further improve economy, the upper limit may be set to 0.60%, 0.30%, or 0.10%. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Mo content may be 0.005% or 0.01%.

B: 0 to 0.0100%
B segregates at the grain boundaries and improves the low temperature toughness by increasing the grain boundary strength. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the B content is 0.0100% or less. Further, B is a strong quenching element, and if its content is large, ferrite transformation does not proceed sufficiently during cooling, and sufficient retained austenite may not be obtained. Therefore, the B content may be 0.0050% or less, 0.0020% or less, or 0.0015%. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the B content may be 0.0001% or 0.0002%.

Mg: 0 to 0.0100%
Mg is an element that improves the workability by controlling the form of non-metallic inclusions that become the starting point of fracture and cause the workability to deteriorate. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Mg content is 0.0100% or less. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Mg content may be 0.0001% or 0.0005%.

Ca: 0 to 0.0100%
Ca is an element that improves the workability by controlling the form of non-metallic inclusions that become the starting point of fracture and cause the workability to deteriorate. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Ca content is 0.0100% or less. The lower limit is 0%, but in order to sufficiently obtain the above effects, the Ca content is preferably 0.0005% or more.

REM: 0 to 0.1000%
REM (rare earth element) is an element that improves the workability by controlling the form of non-metallic inclusions that become the starting point of destruction and cause the workability to deteriorate. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the REM content is 0.1000% or less. If necessary, the upper limit may be 0.0100% or 0.0060%. The lower limit is 0%, but the lower limit of the REM content may be 0.0001% or 0.0005% in order to sufficiently obtain the above effect.

  Here, in the present invention, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. The lanthanoid is industrially added in the form of misch metal.

Zr: 0 to 1.000%
Co: 0 to 1.000%
Zn: 0 to 1.000%
W: 0 to 1.000%
It has been confirmed that even if Zr, Co, Zn, and W are each in the range of 1.000% or less, the effects of the present invention are not impaired. These upper limits may be set to 0.300% or 0.10%. The total content of Zr, Co, Zn and W is preferably 1.000% or less or 0.100%. These contents are not essential, and the lower limit is 0%, but the lower limit may be 0.0001% if necessary.

Sn: 0 to 0.050%
It has been confirmed that the effect of the present invention is not impaired even if Sn is contained in a small amount. However, if it exceeds 0.05%, wrinkles may occur during hot rolling. Therefore, the Sn content is 0.050% or less. The content of Sn is not essential, and the lower limit is 0%, but the lower limit may be 0.001% if necessary.

  In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities.

  Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when industrially manufacturing steel sheets, and are permitted within a range that does not adversely affect the present invention. Means something.

(B) Metal structure The metal structure of the steel plate of this invention is demonstrated. In the present invention, the metallographic structure is 1/4 W or 3/4 W from the end face of the steel sheet when the width and thickness of the steel sheet are W and t, respectively, in a cross section perpendicular to the rolling direction of the steel sheet, and The structure at a position of 1/4 t or 3/4 t from the surface of the steel sheet. In the following description, “%” means “area%”.

Precipitation strengthened ferrite: 5-70%
Supersaturation of Ti carbides when fine precipitates containing Ti (such as finely precipitated Ti carbides, hereinafter also referred to as “fine Ti precipitates”) undergo a γ → α transformation during cooling after rolling. With the degree of driving force, Ti carbide is precipitated at the phase interface or homogeneously nucleated in the ferrite, and the pro-eutectoid ferrite in which the Ti carbide is finely dispersed is precipitation strengthened ferrite (hereinafter also referred to as “precipitation strengthened ferrite”). ). Precipitation strengthened ferrite is a structure necessary for achieving both excellent uniform elongation and strength.

  However, if the area ratio of the precipitation strengthened ferrite is less than 5%, it is difficult to achieve both uniform elongation and strength. On the other hand, if it exceeds 70%, the uniform elongation is excellent, but the local ductility deteriorates. Therefore, the area ratio of precipitation strengthened ferrite is set to 5 to 70%. From the viewpoint of securing a balance between uniform elongation and strength, the area ratio of precipitation strengthened ferrite is preferably 7% or more, and more preferably 10% or more. Further, the area ratio of the precipitation strengthened ferrite is preferably 65% or less, and more preferably 60% or less.

Here, in the present invention, the precipitation strengthened ferrite means a ferrite in which the number density of fine Ti precipitates contained in the grains is 1.0 × 10 ^{16 to} 50.0 × 10 ¹⁶ pieces / cm ^3. . If the number density of fine Ti precipitates contained in the ferrite grains is less than 1.0 × 10 ¹⁶ pieces / cm ³ , the effect of precipitation strengthening cannot be sufficiently obtained. On the other hand, when the number density of fine Ti precipitates exceeds 50.0 × 10 ¹⁶ pieces / cm ³ , not only the strength is saturated but also the ductility is lowered.

That is, the area ratio of precipitation-strengthened ferrite being 5 to 70% means that the area ratio of ferrite is 5 to 70% and the number density of fine Ti precipitates contained in ferrite grains is 1.0. means that a × ^{10 16} ~50.0 × ¹⁰ 16 atoms / cm ^3.

  Furthermore, the average equivalent circle diameter of the fine Ti precipitates contained in the grains of the precipitation strengthened ferrite is preferably 1.00 to 3.00 nm. If the average equivalent circle diameter of the fine Ti precipitate is less than 1.00 nm, it is difficult to obtain the effect of precipitation strengthening. On the other hand, if the average equivalent circle diameter is more than 3.00 nm due to coarse grains, a sufficient amount of fine Ti precipitate is precipitated. This is because things cannot be secured.

Bainite: 30-95%
Bainite is a structure necessary for obtaining a balance between strength and local ductility, and has an effect of suppressing crack propagation. However, if the amount of bainite increases too much, ferrite decreases, and the local elongation is excellent, but the uniform elongation is significantly deteriorated. Therefore, the area ratio of bainite is set to 30 to 95%. The area ratio of bainite is preferably 80% or less, and more preferably 70% or less when the uniform elongation is important.

Residual austenite: 2% or less The high burring steel is characterized by ensuring high strength and ensuring both strength and workability while ensuring workability due to the presence of precipitation strengthened ferrite and bainite. However, the presence of thermodynamically stable retained austenite that did not cause martensitic transformation in the steel sheet means that the C concentration of the retained austenite is high, and the retained austenite is formed by work-induced transformation during sheet forging. The hardness of the site becomes too high and promotes the generation of voids. Therefore, the retained austenite is preferably as small as possible, and the area ratio is 2% or less. The area ratio of retained austenite is preferably 1.5% or less, 1% or less, or 0.5% or less. In particular, there is no need to define a lower limit, and the lower limit is 0%, and 0% is most preferable.

Martensite: 2% or less High burring steel is characterized by ensuring both high strength and workability while ensuring workability due to the presence of precipitation strengthened ferrite and bainite. However, when the area ratio of martensite, which is a hard structure, exceeds 2%, voids are likely to be generated at the boundary between martensite and ferrite with an increase in distortion of the steel sheet due to plate forging, and breakage is likely to occur. Therefore, the area ratio of martensite is 2% or less. The area ratio of martensite is preferably 1.5% or less, 1% or less, or 0.5% or less. In particular, it is not necessary to specify a lower limit, and the lower limit is 0%.

Pearlite: 1% or less Since pearlite is the starting point of fracture during hole expansion molding, its area ratio is 1% or less. The area ratio of pearlite is preferably 0.5% or less. The area ratio of pearlite is preferably reduced as much as possible, and is preferably 0%.

Total of precipitation strengthening ferrite and bainite: 95% or more High burring steel has precipitation strengthening ferrite that achieves both excellent uniform elongation and strength, and bainite that satisfies both strength and local ductility. This provides excellent strength, uniform elongation and local ductility. If the total area ratio of precipitation strengthened ferrite and bainite is less than 95%, these characteristics deteriorate. Therefore, the total area ratio of precipitation strengthened ferrite and bainite is 95% or more. The total area ratio is preferably 97% or more, and more preferably 98% or more.

  Here, in the present invention, the area ratio of the metal structure is obtained as follows. As described above, first, a sample is taken from a position of 1/4 W or 3/4 W from the end surface of the steel plate and from a position of 1/4 t or 3/4 t from the surface of the steel plate. And the rolling direction cross section (what is called L direction cross section) of this sample is observed.

  Specifically, the sample is subjected to nital etching, and observation is performed in a 300 μm × 300 μm visual field using an optical microscope after the etching. Then, by performing image analysis on the obtained structure photograph, the area ratio A of ferrite, the area ratio B of pearlite, and the total area ratio C of bainite, martensite and retained austenite are obtained.

  Next, the nital-etched portion is repeller-etched and observed with a 300 μm × 300 μm field of view using an optical microscope. And the total area ratio D of a retained austenite and a martensite is computed by performing image analysis with respect to the obtained structure | tissue photograph. Furthermore, the volume fraction of retained austenite is obtained by X-ray diffraction measurement using a sample that is chamfered from the normal direction of the rolling surface to ¼ depth of the plate thickness. Since the volume ratio is substantially equal to the area ratio, the volume ratio is defined as the area ratio E of retained austenite. The area ratio of bainite is determined from the difference between the area ratio C and the area ratio D, and the area ratio of martensite is determined from the difference between the area ratio E and the area ratio D. By this method, the area ratios of ferrite, bainite, martensite, retained austenite, and pearlite can be obtained.

  Moreover, the area ratio of precipitation strengthening ferrite can be calculated | required by Kernel SP (OAMTM) method equipped with EBSP-OIMTM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy).

  In the KAM method, six regular hexagonal pixels in the measurement data (first approximation), 12 outside (second approximation), or 18 outside (third approximation) pixels. The difference between the directions is averaged, and the calculation is performed for each pixel with the average value as the value of the center pixel. By performing this calculation so as not to cross the grain boundary, a map expressing the orientation change in the grain can be created. That is, this map represents a strain distribution based on local orientation changes in the grains.

  The analysis condition of precipitation strengthened ferrite in the present invention is EBSP-OIMTM, where the average orientation difference between adjacent pixels is calculated by the third approximation, and the portion where this orientation difference is calculated to be 1 ° or less is precipitation strengthened. Ferrite was used.

  The formation temperature range of the precipitation-strengthened ferrite of the present invention is that the Ti carbide precipitates at the phase interface or homogeneously nucleates in the ferrite with the driving force as the supersaturation degree of the Ti carbide during γ → α transformation during cooling after rolling. It matches the temperature range. Polygonal pro-eutectoid ferrite transformed at high temperature is formed by diffusion transformation, so that the dislocation density is small and the intra-granular strain is small, so the intra-granular difference in crystal orientation is small. Therefore, the crystal orientation difference of the precipitation strengthened ferrite is similarly reduced. According to various investigation results conducted by the inventors so far, the polygonal ferrite area ratio obtained by optical microscope observation and the area where the azimuth difference in the third approximation measured by the KAM method is 1 ° or less can be obtained. This is because the area ratios are almost the same.

  The measurement of the area fraction of precipitation strengthened ferrite was carried out in detail as follows. A sample collected in the same manner as described in the structure observation was polished with a colloidal silica abrasive for 30 to 60 minutes, and EBSP measurement was performed under measurement conditions of 400 times magnification, 160 μm × 256 μm area, and measurement step of 0.5 μm. The EBSP-OIMTM method irradiates an electron beam onto a highly inclined sample in a scanning electron microscope (SEM), images the Kikuchi pattern formed by backscattering with a high-sensitivity camera, and processes the computer image. It consists of a device and software that measure the crystal orientation of the glass in a short time.

  The EBSP method can quantitatively analyze the microstructure and crystal orientation of the surface of the bulk sample, and the analysis area is an area that can be observed with an SEM. Depending on the resolution of the SEM, analysis can be performed with a minimum resolution of 20 nm. The analysis takes several hours and is performed by mapping tens of thousands of points to be analyzed in a grid at equal intervals. In a polycrystalline material, the crystal orientation distribution and crystal grain size in the sample can be seen.

  In this way, the portion where the orientation difference in the above third approximation was calculated to be 1 ° or less was defined as precipitation strengthened ferrite, and the area of precipitation strengthened ferrite was obtained. Asked.

  The fine Ti precipitate was observed by the three-dimensional atom probe measurement method as follows.

  First, a needle-like sample is prepared from a sample to be measured by cutting and electrolytic polishing using a focused ion beam processing method in combination with an electrolytic polishing method as necessary. In the three-dimensional atom probe measurement, the accumulated data can be reconstructed and obtained as an actual distribution image of atoms in real space. In the case of a fine Ti precipitate having a Na—Cl structure, the unit cell is 4.33 angstroms, and therefore the interatomic distance between Ti and Ti is 4.33 × √2 = 6.1 angstroms.

  Therefore, when there are a plurality of Ti atoms at almost the same coordinate position (7 angstroms or less), it is determined that these Ti atoms are in the same precipitate, and if they are in the same precipitate, The number of Ti atoms judged was counted, and when this number was 50 or more, this precipitate was defined as a fine Ti precipitate.

  The size of the fine Ti precipitate is an equivalent circle diameter calculated from the number of Ti atoms constituting the fine Ti precipitate and the lattice constant of the fine Ti precipitate, assuming that the fine Ti precipitate is spherical.

  A method for obtaining the equivalent circle diameter (diameter) R of the precipitate using the number of Ti atoms of the fine Ti precipitate obtained by the three-dimensional atom probe measurement method is shown below.

  Although the number N of all atoms in the target sample is measured by the three-dimensional atom probe measurement method, in reality, the number N of all atoms in the target sample cannot be detected by the three-dimensional atom probe measurement method. Since there is an atom detection rate α (= number of detected atoms / total number of atoms) unique to each device, the number N of atoms that would have existed is calculated from the actual measurement value n. That is, the total number of atoms N = n / α. Note that the detection rate α of the instrument measured in the present invention was 0.35.

Next, with respect to the number N of atoms, in the case of a Ti precipitate having a Na—Cl structure, there are 8 Ti atoms in the unit cell, and the lattice constant a of the Na—Cl structure is 4. Assuming that it is 33 Å, the equivalent circle diameter is calculated by the following equation.
Circle equivalent diameter (diameter) R = {(6/8) · (1 / π) · N · a ³ } ^(1/3)

  For example, when the number of Ti is 50, the equivalent circle diameter is calculated to be approximately 1 nm. In the present invention, the equivalent circle diameter (diameter) of 30 or more fine Ti precipitates is arbitrarily measured, and the average value is obtained.

The number density of fine Ti precipitates is determined using the measurement field of view as the denominator and the number of fine Ti precipitates as the numerator. In the measurement of the number density, 5 or more fields of view of 10 nm (plate thickness direction t) × 40 nm (plate width direction W) × 60 nm (plate longitudinal direction L) were measured, and the number density (pieces / cm ³ ) The average value was obtained.

  In the present invention, the existence state of TiN is also defined as follows.

The average equivalent circle diameter of TiN: 1.0-10.0 μm
When TiN is large, voids existing at the grain boundaries are easily combined with an increase in distortion of the steel sheet due to plate forging. Therefore, the average equivalent circle diameter of TiN is set to 10.0 μm or less. In order to ensure these effects more reliably, the average equivalent circle diameter of TiN is preferably 8.0 μm or less, and more preferably 5.0 μm or less.

  Since TiN is preferably as small as possible, it is not necessary to originally provide a lower limit for the average equivalent circle diameter of TiN. However, in the TiN observation method described later, if the equivalent circle diameter of TiN is less than 1.0 μm, it is difficult to determine whether it is TiN. Therefore, in the present invention, only those having an equivalent circle diameter of 1.0 μm or more are set as TiN as measurement targets. Therefore, the average equivalent circle diameter of TiN is 1.0 μm or more.

  The average equivalent circle diameter (diameter) of TiN is obtained as follows. As described above, first, a sample is taken from a position of 1/4 W or 3/4 W from the end surface of the steel plate and from a position of 1/4 t or 3/4 t from the surface of the steel plate. Then, the rolling direction cross section (so-called L direction cross section) of the sample is polished and observed without being etched. Specifically, a microstructure photograph is taken at a magnification of 1000 times using an optical microscope, and the microstructure photograph is observed visually or with an image processing apparatus or the like.

  In a microstructural photograph, the equivalent circle diameter (diameter) is obtained for those that can be identified as TiN, and only those having an equivalent circle diameter (diameter) of 1.0 μm or more are defined as TiN. Then, the visual field of 60 μm (rolling direction L) × 40 μm (sheet thickness direction t) was observed for 20 or more visual fields, and the average of the equivalent circle diameter (diameter) of TiN was averaged. Diameter).

Average value of the shortest distance between adjacent TiN: 10.0 μm or more In order to prevent voids generated at the interface between TiN and ferrite from growing and bonding to each other to form a larger void, the distance between TiN It is necessary to secure a certain amount. Therefore, the average value of the distance between adjacent TiNs is set to 10.0 μm or more.

  From the viewpoint of suppressing the occurrence of cracks due to the growth of voids, the average value is preferably 15.0 μm or more, and more preferably 20.0 μm or more. Although the upper limit is not particularly set, it is inevitable that TiN is deposited to some extent. Therefore, the average value of the shortest distance between adjacent TiNs is preferably set to 1000 μm or less.

  The average value of the shortest distance between adjacent TiNs is obtained as follows. Twenty arbitrary TiNs are selected, the distance to the nearest TiN is measured, and the average value is calculated. In addition, the measurement of the shortest distance between TiN is calculated | required similarly to the measurement of an average equivalent circle diameter.

(C) Mechanical properties Standard deviation of nano hardness: 1.0 GPa or less By reducing the difference in deformability between hard and soft tissues, the number of voids generated at the interface between both tissues is reduced, and the void interval is further increased. , It becomes possible to prevent the voids from being combined and growing into cracks. Therefore, generation of voids can be suppressed by reducing the nano hardness difference corresponding to the difference in deformability between the hard tissue and the soft tissue as much as possible. In the present invention, the standard deviation of the nano hardness in the cross section of the sample is adopted as an index of the hardness difference between the soft tissue and the hard tissue.

  The nano hardness can be measured, for example, using a TriboScope / TriboIndenter manufactured by Hystron. The nano hardness of 100 points or more can be arbitrarily measured at a load of 1 mN, and the standard deviation of the nano hardness can be calculated from the result.

  In order to reduce the hardness difference between the soft tissue and the hard tissue and suppress the generation of voids, the standard deviation of the nano hardness is preferably small and is set to 1.0 GPa or less. The standard deviation of nano hardness is preferably 0.8 GPa or less.

Tensile strength: 780 MPa or more The steel sheet according to the present invention preferably has a tensile strength of 780 MPa or more equivalent to that of conventional high burring steel. The upper limit of the tensile strength is not particularly required, but may be 1200 MPa, 1150 MPa, or 1000 MPa. However, the tensile strength indicates the tensile strength of JIS Z 2241 (2011).

Product of uniform elongation and tensile strength: 7000 MPa ·% or more If the uniform elongation is small, the plate thickness tends to decrease due to necking during press molding, which causes press cracking. In order to ensure press formability, it is preferable to satisfy the product of uniform elongation (u-EL) and tensile strength (TS): TS × u-EL ≧ 7000 MPa%. However, the uniform elongation is a nominal value at which the value obtained when the nominal stress σn is differentiated by the nominal strain εn is zero in the relationship between the nominal stress σn and the nominal strain εn in the test specified by JIS Z 2241 (2011). When the strain is εn0, it is expressed by the following formula.
Uniform elongation (u-EL) = ln (εn0 + 1)

Product of hole expansion rate and tensile strength: 50000 MPa ·% or more If the hole expansion property is poor, the material flowability may be poor and cracking may occur when stretch flange processing is performed. Therefore, in order to ensure the hole expansion property, it is preferable to satisfy the product of the hole expansion rate (λ) and the tensile strength (TS): (TS) × (λ) ≧ 50000 MPa%. However, the hole expansion rate (λ) represents (λ) of the hole expansion rate according to a test method based on JIS Z 2256 (2010).

Equivalent plastic strain: 0.9 or more Equivalent plastic strain is the relationship between the shear stress σs and the shear plastic strain εsp in the simple shear test, and the tensile stress σ and tensile strain ε in the uniaxial tensile test with different deformation modes. Assuming the relationship between the isotropic hardening rule and the plastic work conjugate, the relationship is converted using a constant conversion coefficient (κ).

  Here, the isotropic hardening rule is a work hardening law that assumes that the shape of the yield curve does not change even when strain progresses (that is, expands to a similar shape). The relation of plastic work conjugation is a relation that work hardening is described as a function of only plastic work, and shows the same work hardening amount when given the same plastic work (σ × ε) regardless of the deformation form.

Thereby, the shear stress and the shear plastic strain in the simple shear test can be converted into the tensile stress and the tensile strain in the uniaxial tensile test, respectively. This relationship is shown below.
Tensile stress σ (conversion) in uniaxial tensile test = Shear stress σs × κ in simple shear test
Tensile strain ε (transformation) in uniaxial tensile test = shear plastic strain εsp / κ in simple shear test

  Next, the conversion coefficient κ is determined so that the relationship between the shear stress and the shear plastic strain is similar to the relationship between the tensile stress and the tensile strain. For example, the conversion coefficient κ can be obtained by the following procedure. First, the relationship between tensile strain ε (actual value) and tensile stress σ (actual value) in a uniaxial tensile test is obtained. Subsequently, the relationship between the shear strain εs (actual value) and the shear stress σs (actual value) in the uniaxial shear test is obtained.

  Next, by changing κ, the tensile strain ε (conversion) obtained from the shear strain εs (actual value) and the tensile stress σ (conversion) obtained from the shear stress σs (actual value) are obtained in advance. The tensile stress σ (conversion) is determined when the strain ε (conversion) is between 0.2% and uniform elongation (u-EL). At this time, an error between the tensile stress σ (conversion) and the tensile stress σ (measured value) is obtained, and κ that minimizes the error is obtained using the least square method.

  The equivalent plastic strain εeq is defined as a value obtained by converting the shear plastic strain εsp (rupture) at the time of rupture in the simple shear test into the tensile strain ε in the simple tensile test using the obtained κ.

  The steel sheet according to the present invention is characterized by good processing characteristics in a high strain region represented by plate forging, and the equivalent plastic strain εeq satisfies 0.50 or more. Since the equivalent plastic strain of the conventional TRIP steel is at most about 0.30, it was confirmed that the plate forgeability of the steel sheet according to the present invention is good.

(D) Dimensions Plate thickness: 1.0-4.0mm
The steel plate according to the present invention is mainly used for automobiles and the like, and its thickness range is mainly 1.0 to 4.0 mm. For this reason, the plate thickness range may be 1.0 to 4.0 mm. The lower limit is 1.2 mm, 1.4 mm, or 1.6 mm, and the upper limit is 3.6 mm, 3.2 mm, or 2. It may be 8 mm.

(E) Manufacturing method The inventors have shown that the hot-rolled steel sheet of the present invention can be manufactured by conducting the manufacturing steps from (a) to (f) shown below in order according to previous research. I have confirmed. Hereinafter, each manufacturing process will be described in detail.

(A) Melting process The manufacturing method preceding hot rolling is not particularly limited. That is, it adjusts so that it may become the component composition mentioned above by performing various secondary smelting following melting by a blast furnace or an electric furnace. Then, what is necessary is just to manufacture a slab by methods, such as normal continuous casting and thin slab casting. At that time, scrap or the like may be used as a raw material as long as it can be controlled within the component range of the present invention.

(B) Heating process The manufactured slab is hot-rolled to obtain a hot-rolled steel sheet. When performing hot rolling, first, the slab is heated. In the heating step, the slab is heated to a temperature of SRTmin ° C. or higher and 1260 ° C. or lower represented by the following formula (i). In the case of continuous casting, after cooling to a low temperature once, it may be heated again, or may be heated subsequent to continuous casting without cooling. Here, SRTmin means the solution temperature of TiC.
SRTmin = 7000 / {2.75−log (Ti × C)} − 273 (i)
However, the element symbol in the said formula represents content (mass%) in the hot-rolled steel plate of each element, and shall substitute 0 when not containing.

(C) Continuous hot rolling step After heating, the slab extracted from the heating furnace is subjected to rough rolling and subsequent multistage finish rolling. The end temperature of rough rolling is 1100 ° C. or higher so that precipitates containing Ti do not precipitate. Further, as described above, the multistage finish rolling is performed by continuous rolling of three or more stages (for example, six stages or seven stages). Then, multistage finish rolling is performed so that the cumulative strain (effective cumulative strain) in the final three-stage rolling is 0.01 to 0.10.

  As described above, the effective cumulative strain is the change in crystal grain size due to rolling temperature, rolling reduction of the steel sheet due to rolling, and change in crystal grain size where the crystal grains recover statically over time after rolling. It is an index that takes into account. The effective cumulative strain (εeff) can be obtained by the following equation.

Effective cumulative strain (εeff) = Σεi (ti, Ti) (iii)
Σ in the above formula (iii) represents the sum for i = 1 to 3.
However, i = 1 is the first-stage rolling (that is, final-stage rolling) from the last in multi-stage finish rolling, i = 2 is the second-stage rolling from the last, and i = 3 is the third-stage rolling from the last. Are shown respectively.

Here, in each rolling indicated by i, εi is expressed by the following equation.
εi (ti, Ti) = ei / exp ((ti / τR) ^2/3 ) (iv)
ti: Time from the last i-th rolling to the start of primary cooling after the last rolling (s)
Ti: Rolling temperature (K) of the i-th rolling from the end
ei: Logarithmic strain when rolled down by rolling of the i-th stage from the end ei = | ln {1− (thickness of inlet side of i-th stage−thickness of outlet side of i-th stage) / (thickness of inlet side of i-th stage) } ｜
= | Ln {(i-th exit side plate thickness) / (i-th entry side plate thickness)} | (v)
τR = τ0 · exp (Q / (R · Ti)) (vi)
τ0 = 8.46 × 10 ⁻⁹ (s)
Q: constant of activation energy relating to transfer of Fe dislocation = 183200 (J / mol)
R: Gas constant = 8.314 (J / (K · mol))

  By defining the effective accumulated strain thus derived, the desired microstructure can be obtained and the variation in nano hardness can be reduced. As a result, it suppresses the growth of voids generated at the interface between the hard and soft tissues, makes it difficult to bond even when voids grow, and does not generate cracks even after plate forging. Steel plate can be obtained.

The end temperature of multi-stage finish rolling, that is, the end temperature of the continuous hot rolling process, may be set to Ar ₃ (° C.) + 30 ° C. or higher using Ar ₃ obtained by the following equation (ii). This is because the intended precipitation-strengthened ferrite and bainite are obtained in the present invention.
Ar ₃ = 970-325 × C + 33 × Si + 287 × P + 40 × Al-92 × (Mn + Mo + Cu) −46 × (Cr + Ni) (ii)
However, the element symbol in the said formula represents content (mass%) in the hot-rolled steel plate of each element, and shall substitute 0 when not containing.

(D) 1st (acceleration) cooling process Cooling of the hot-rolled steel sheet obtained after 1.00-5.00 s is started after completion | finish of multistage finishing rolling. And it cools with the average cooling rate of 10 degrees C / s or more from rolling completion temperature to the temperature of 650-750 degreeC, and hold | maintains 1-10 seconds in air | atmosphere after that.

  When cooling is started in less than 1.00 s after the end of the continuous hot rolling process, ferrite transformation is promoted, and the desired bainite area ratio cannot be obtained in the final microstructure. The effect of can not be obtained. On the other hand, when cooling is started beyond 5.00 s, the ferrite transformation is delayed and the desired area ratio of the precipitation strengthened ferrite cannot be obtained.

  Moreover, it becomes easy to produce | generate pearlite in the 1st cooling process that the average cooling rate is less than 10 degrees C / s. On the other hand, the upper limit of the cooling rate is not particularly limited, but may be 300 ° C./s or less in order to avoid overcooling. Further, when the holding temperature in the atmosphere is lower than 650 ° C., bainite is easily generated, and the bainite area ratio is increased. On the other hand, when the holding temperature in the atmosphere exceeds 750 ° C., pearlite is easily generated.

  Here, holding in the atmosphere includes that the hot-rolled steel sheet is limited to air cooling or cooling in the cooling facility to a minimum, and the lower limit of the cooling rate at this time is ideally 0 ° C./s. The upper limit is 8 ° C./s.

(E) 2nd (acceleration) cooling process After hold | maintaining in air | atmosphere, it cools with the average cooling rate of 10 degrees C / s or more from the temperature range of 600-740 degreeC. When the cooling start temperature is less than 600 ° C., the ferrite transformation does not proceed sufficiently and the precipitation of fine Ti precipitates becomes insufficient. On the other hand, when the cooling start temperature exceeds 740 ° C., the ferrite transformation proceeds excessively, and pearlite is generated, which may deteriorate the hole expandability. Moreover, there exists a possibility that a fine Ti precipitate may coarsen and intensity | strength may fall.

  Also, when the average cooling rate is less than 10 ° C./s, pearlite may be generated and the hole expanding property may be deteriorated. The upper limit of the average cooling rate is not particularly limited, but it may be 1000 ° C./s or less because there is a concern that the steel sheet warps due to thermal strain due to thermal deviation.

(F) Winding process Thereafter, the cooled hot-rolled steel sheet is wound at a winding temperature of 450 to 650 ° C. The conditions after the winding process are not particularly limited.

(F) Steel forged parts Since the hot-rolled steel sheet obtained as described above is excellent in plate forgeability, it could not be achieved by forging the hot-rolled steel sheet by plate forging or the like. A forged part having a complicated shape requiring high strength can be obtained.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steel having the chemical composition shown in Table 1 was melted to produce a slab. The slab was hot-rolled under the conditions shown in Table 2 and then cooled and wound to produce a hot-rolled steel sheet. Table 3 shows the thickness of the obtained hot-rolled steel sheet.

[Metal structure]
The metal structure of the obtained hot rolled steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, in the cross section perpendicular to the rolling direction of the steel sheet, when the width and thickness of the steel sheet are W and t, respectively, 1/4 W from the end face of the steel sheet and 1 from the surface of the steel sheet A specimen for observing the metal structure was cut out from the position of / 4t.

  And the rolling direction cross section (what is called L direction cross section) of said test piece was nital-etched, and it observed in the 300 micrometer x 300 micrometer visual field using the optical microscope after the etching. Then, by performing image analysis on the obtained structure photograph, the area ratio A of ferrite, the area ratio B of pearlite, and the total area ratio C of bainite, martensite and retained austenite were obtained.

  Next, the nital-etched portion was repeller-etched and observed with a 300 μm × 300 μm field of view using an optical microscope. And the total area rate D of a retained austenite and a martensite was computed by performing image analysis with respect to the obtained structure | tissue photograph. Furthermore, the volume ratio of the retained austenite was calculated | required by the X-ray-diffraction measurement using the sample which faced from the rolling surface normal direction to 1/4 depth of board thickness. Since the volume ratio is substantially equal to the area ratio, the volume ratio is defined as the area ratio E of retained austenite. The area ratio of bainite was determined from the difference between the area ratio C and the area ratio D, and the area ratio of martensite was determined from the difference between the area ratio E and the area ratio D. By this method, the area ratios of ferrite, bainite, martensite, retained austenite, and pearlite were determined.

  As described above, the area ratio of precipitation-strengthened ferrite was measured by EBSP after polishing the test piece with a colloidal silica abrasive and measuring a field of view of 160 × 256 μm at a magnification of 400 × under a measurement step of 0.5 μm. I asked for.

  As described above, the fine Ti precipitate was also subjected to electrolytic polishing of the test piece and measured by a three-dimensional atom probe measurement method, and the equivalent circle diameter and number density were obtained.

  As described above, for TiN, the test specimen was observed at 20 magnifications at a magnification of 1000 times and a 60 × 40 μm visual field, and the average equivalent circle diameter of TiN was determined by image processing. Further, the shortest distance between TiNs was obtained by observing the same part as that in the structure investigation with a 500 times metal microscope.

[Mechanical properties]
Among the mechanical properties, the tensile strength properties (tensile strength (TS), uniform elongation (u-EL), hole expansion rate (λ)) are 1 / (in the plate width direction from one end of the plate). Using either No. 5 test piece of JIS Z 2241 (2011) sampled with the direction perpendicular to the rolling direction (width direction) as the longitudinal direction at either 4W or 3 / 4W position, the JIS Z 2241 (2011) It was evaluated in conformity. The hole expansion ratio was evaluated based on the test method described in JIS Z 2256 2010 by collecting a test piece from the same position as the tensile test piece collection position.

  Furthermore, a simple shear test was performed according to the following procedure, and an equivalent plastic strain was determined based on the result.

  The test piece of the simple shear test is a direction (width direction) orthogonal to the rolling direction at a position of 1/4 W or 3/4 W from one end of the plate to the plate width direction when the plate width of the steel plate is W. Is taken as the longitudinal direction. An example of a test piece is shown to Fig.1 (a). The test piece of the simple shear test shown in FIG. 1 has a rectangular thickness of 23 mm in the width direction of the steel plate and 38 mm in the rolling direction of the steel plate so that both sides are evenly ground so that the thickness is 2.0 mm. It processed so that it might become a test piece.

  The chucking part 2 on both sides is chucked by 10 mm toward the long piece side (rolling direction) of the test piece in the short piece direction (width direction), and a shear width of 3 mm (shear deformation generating part 1) is formed at the center of the test piece. It was made to provide. In addition, when the plate thickness was less than 2.0 mm, the plate thickness was tested as it was without grinding. Moreover, the center of the test piece was marked with a straight line with a pen or the like in the short piece direction (width direction).

Then, by moving the chucked long side in the long piece direction (rolling direction) so as to be opposite to each other, a shear stress σs was applied, and shear deformation was applied to the test piece. FIG. 1B shows an example of a test piece subjected to shear deformation. The shear stress σs is a nominal stress obtained by the following formula.
Shear stress σs = shear force / (length of specimen in rolling direction of steel sheet × thickness of specimen)

In the shear test, since the length and thickness of the test piece do not change, it may be considered that shear nominal stress≈shear true stress. During the shear test, a straight line drawn at the center of the test piece was photographed with a CCD camera, and the inclination θ was measured (see FIG. 1B). From this inclination θ, the shear strain εs generated by shear deformation was determined using the following equation.
Shear strain εs = tan (θ)

  In the simple shear test, a simple shear tester (maximum displacement 8 mm) was used. Therefore, there is a limit on the stroke (displacement) of the testing machine. In addition, due to the occurrence of cracks at the end of the test piece or at the chuck part, in one shear test, the test may not be performed until the test piece breaks. Therefore, as described above, the “multi-stage shear test method” is adopted, which repeats a series of operations such as loading of the shear test load, unloading of the load, cutting the end of the chuck part of the test piece in a straight line, and reloading of the load did.

In order to connect these multi-stage shear test results in series and evaluate them as one continuous simple shear test result, the shear modulus is taken into account from the shear strain (εs) obtained in each stage of the shear test. The shear plastic strain (εsp) obtained by subtracting the shear elastic strain (εse) was determined as follows, and the shear plastic strain (εs) at each stage was combined and joined together.
Shear plastic strain εsp = shear strain εs−shear elastic strain εse
Shear elastic strain εse = σs / G
σs: Shear stress G: Shear elastic modulus Here, G = E / 2 (1 + ν) ≈78000 (MPa).
E (Young's modulus (longitudinal elastic modulus)) = 206000 (MPa)
Poisson's ratio (ν) = 0.3

  In the simple shear test, the test is performed until the test piece breaks. In this way, the relationship between the shear stress σs and the shear plastic strain εsp can be traced. The shear plastic strain when the test piece breaks is εspf.

  From the relationship between the shear stress σs obtained in the simple shear test and the shear plastic strain εspf when the test piece broke, the equivalent plastic strain εeq was determined using the conversion coefficient κ by the method described above.

  Next, the standard deviation of nano hardness was measured. The specimen for observing the metallographic structure was ground again, and at a load of 1 mN (loading 10 s, unloading 10 s), a 1/4 depth position (1 / 4t part), a measurement area of 25 μm × 25 μm was measured at intervals of 5 μm. From the results, the average value of nano hardness and the standard deviation of nano hardness were calculated. The measurement of nano hardness was carried out using a Triscope or TriboIndenter manufactured by Hystron.

  These measurement results are also shown in Table 3.

  As apparent from Table 3, if the hot rolled steel sheet according to the present invention, the tensile strength (TS) is 780 MPa or more, the product of uniform elongation u-EL and tensile strength TS (TS × u-EL). ) Is 7000 MPa ·% or more, the product of the hole expansion ratio λ and the tensile strength TS (TS × λ) is 50000 MPa ·% or more, and a hot-rolled steel sheet having well-balanced characteristics is obtained. Moreover, it was confirmed that the hot-rolled steel sheet according to the present invention is a steel sheet that has an equivalent plastic strain exceeding 0.90 (90%) and can withstand high strain region processing such as plate forging.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to obtain the hot rolled steel plate excellent in plate forgeability, maintaining the favorable hole expansibility which is a basic function as high burring steel. Therefore, the hot-rolled steel sheet according to the present invention can be widely used for machine parts and the like. In particular, by applying it to the processing of a steel plate having processing in a high strain region such as plate forging, the remarkable effect can be obtained.

1 Shear deformation generation part 2 Chucking part

Claims (5)

The chemical composition of the steel sheet is
C: 0.020 to 0.070%,
Si: 0.05 to 1.70%,
Mn: 0.60 to 2.50%,
Al: 0.010 to 1.000%
N: more than 0% to 0.0030% or less,
P: 0.050% or less,
S: 0.005% or less,
Ti: 0.015-0.170%,
Nb: 0 to 0.100%,
V: 0 to 0.300%,
Cu: 0 to 2.00%,
Ni: 0 to 2.00%,
Cr: 0 to 2.00%
Mo: 0 to 1.00%,
B: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Ca: 0 to 0.0100%,
REM: 0 to 0.1000%,
Zr: 0 to 1.000%,
Co: 0 to 1.000%,
Zn: 0 to 1.000%
W: 0 to 1.000%
Sn: 0 to 0.050%, and
Balance: Fe and impurities,
In the cross section perpendicular to the rolling direction of the steel sheet, when the width and thickness of the steel sheet are W and t, respectively, 1/4 W or 3/4 W from the end face of the steel sheet and 1 from the surface of the steel sheet / 4t or 3 / 4t, the metallographic structure is area%,
Ferrite: 5 to 70%,
Bainite: 30-95%
Residual austenite: 2% or less,
Martensite: 2% or less, and
Perlite: 1% or less, and
Total of ferrite and bainite: 95% or more,
The ferrite has a precipitate containing Ti in the grains,
The number density of the precipitates containing Ti is 1.0 × 10 ^{16 to} 50.0 × 10 ¹⁶ pieces / cm ³ ,
TiN precipitates are included in the steel sheet,
The average equivalent circle diameter of the TiN precipitate is 1.0 to 10.0 μm,
The average value of the shortest distance between the adjacent TiN precipitates is 10.0 μm or more,
Standard deviation of nano-hardness Ri der following 1.00GPa,
The average equivalent circle diameter of the precipitate containing Ti is 1.00 to 3.00 nm.
Hot rolled steel sheet. The tensile strength is 780 MPa or more,
The product of uniform elongation and tensile strength is 7000 MPa ·% or more,
The product of the hole expansion ratio and the tensile strength is 50000 MPa ·% or more,
The hot rolled steel sheet according to claim 1 . A method for producing the hot-rolled steel sheet according to claim 1 or 2 ,
The slab having the chemical composition according to claim 1 is subjected to a heating process, a continuous hot rolling process, a first cooling process, a second cooling process, and a winding process in order,
In the heating step, the slab is heated to a temperature of SRTmin ° C. or higher and 1260 ° C. or lower represented by the following formula (i):
The continuous hot rolling step includes rough rolling and multi-stage finish rolling of three or more stages,
The end temperature of the rough rolling is 1100 ° C. or higher,
The cumulative strain in the final three stages of rolling in the multistage finish rolling is 0.01 to 0.10,
The rolling end temperature of the multistage finish rolling is a temperature of Ar ₃ + 30 ° C. or higher obtained by the following formula (ii):
In the first cooling step, after the multi-stage finish rolling is finished, cooling is started after 1.00 to 5.00 s, and from the rolling finish temperature to a temperature range of 650 to 750 ° C., 10 ° C./s or more. Cool at average cooling rate, then hold in air for 1-10 s,
In the second cooling step, after being held in the atmosphere, cooling is performed at an average cooling rate of 10 ° C./s or more from a temperature range of 600 to 740 ° C.,
In the winding step, winding is performed at a winding temperature of 450 to 650 ° C.
Manufacturing method of hot rolled steel sheet.
SRTmin = 7000 / {2.75−log (Ti × C)} − 273 (i)
Ar ₃ = 970-325 × C + 33 × Si + 287 × P + 40 × Al-92 × (Mn + Mo + Cu) −46 × (Cr + Ni) (ii)
However, the element symbol in the said formula represents content (mass%) in the hot-rolled steel plate of each element, and shall substitute 0 when not containing. Obtained from the hot-rolled steel sheet according to claim 1 or 2 ,
Forged steel parts. The hot-rolled steel sheet according to claim 1 or claim 2 is subjected to at least forging.
Manufacturing method of steel forged parts.

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