JP6131872B2 - High strength thin steel sheet and method for producing the same - Google Patents

Description

  The present invention includes undercarriage members such as lower arms of automobiles, skeletal members such as pillars and members and their reinforcing members, various automobile members such as door impact beams and seat members, vending machines, desks, home appliances / OA devices, building materials, etc. The present invention relates to a high-strength thin steel sheet excellent in punchability and toughness, which is optimal for structural members used in manufacturing, and a manufacturing method thereof.

In recent years, in response to growing interest in the global environment, there has been an increasing demand for reducing the amount of steel sheets with large CO ₂ emissions during manufacturing. In addition, in the automobile field and the like, there is an increasing need to reduce the exhaust gas by improving the fuel consumption by reducing the body. Therefore, production of high-strength steel sheets and their application to automobile parts are being actively promoted. According to the high-strength steel plate, the steel plate can be thinned while ensuring a desired rigidity. And as a result of being able to reduce the thickness of the steel sheet while ensuring the desired rigidity, the amount of steel sheet used can be significantly reduced. In addition, by applying a thin high-strength steel sheet to automobile parts, it is possible to reduce the weight of the automobile body while maintaining a desired strength, thereby improving fuel consumption and reducing exhaust gas amount.

However, since the punchability and toughness of the steel sheet are reduced as the strength of the steel sheet is increased, it is difficult to apply the high-strength steel sheet particularly to automobile parts in which a large impact is applied to the end surface portion.
Many techniques for improving the punchability and toughness of high-strength steel sheets have been proposed to date.

  For example, in Patent Document 1, the steel plate composition is C = 0.04 to 0.15 wt%, Si ≧ 1.0 wt%, Mn ≧ 1.0 wt%, Nb ≧ 0.005 wt%, Al = 0.005 to 0.10 wt%, S ≦ 0.01 wt. % And Fe as the main component, steel sheet structure is mainly composed of ferrite and martensite, ferrite space factor> 50%, ferrite average particle size ≦ 5 μm, and martensite average particle size ≦ 5 μm A technology has been proposed. In Patent Document 1, by defining the composition and structure as described above, tensile strength> 590 MPa, yield ratio ≦ 70%, strength-ductility balance (tensile strength × total elongation) ≧ 18000 MPa ·%, hole It is described that a high-strength hot-rolled steel sheet excellent in workability, fatigue characteristics, and low-temperature toughness having an expansion ratio ≧ 1.2, fatigue limit ratio ≧ 0.40, and fracture surface transition temperature ≦ −40 ° C. is described.

  Patent Document 2 discloses that the steel sheet composition is C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.1%, P: 0.025% or less, S: 0.005% or less, and Al: mass%. It contains 0.005 to 0.10%, and is composed of the balance Fe and inevitable impurities, the steel sheet structure is the main phase of bainite phase or tempered martensite phase, and the average grain size of the prior austenite grains is parallel to the rolling direction. There has been proposed a technique for forming a structure having a cross section of 20 μm or less and a cross section perpendicular to the rolling direction of 15 μm or less. In Patent Document 2, by defining the composition and the structure as described above, it has excellent toughness and excellent bending properties even though it has a high strength of yield strength YS: 960 MPa or more. It is described that a high-strength hot-rolled steel sheet provided with

  Further, in Patent Document 3, the steel sheet composition is C: 0.06% or more and 0.13% or less, Si: 0.5% or less, Mn: less than 0.5%, P: 0.03% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.01% or less, Ti: 0.14% or more and 0.25% or less, V: 0.01% or more and 0.5% or less, [Ti] / 48 + [V] / 51 + [W] / 184 + [Mo] / 96 + [ Nb] /93≧0.0043 ([Ti], [V], [W], [Mo], [Nb]: content of each element (% by mass)) Steel sheet structure, the ferrite phase area ratio is 95% or more, the ferrite phase average grain size is 10μm or less, the carbide average grain size in the ferrite phase grains is less than 10nm The technology which makes it an organization is proposed. Patent Document 3 describes that a high-strength hot-rolled steel sheet having a tensile strength of 850 MPa or more and excellent punchability can be obtained by defining the composition and structure as described above.

Japanese Patent Laid-Open No. 7-150294 JP 2013-1117068 A JP 2013-124395 A

  However, the technique proposed in Patent Document 1 has a problem that surface properties such as chemical conversion treatment and plating properties are not good because the amount of Si contained in the steel sheet is large. Since many automobile parts are subjected to plating treatment or chemical conversion treatment for the purpose of imparting corrosion resistance, a steel plate having a problem in surface properties is not suitable as a material for automobile parts. Moreover, in the technique proposed by patent document 1, the punchability of a steel plate is not examined.

  In the technique proposed in Patent Document 2, the steel sheet structure has a bainite phase or a tempered martensite phase as a main phase and does not contain ferrite. Thus, in a structure that does not contain ferrite that is excellent in workability, the formability of the steel sheet is inferior, and a steel sheet that is excellent in punchability cannot be obtained. On the other hand, with the technique proposed in Patent Document 3, although a high-strength steel sheet excellent in punchability can be obtained, the toughness has not been sufficiently studied and there is room for improvement.

As described above, in the conventional technology, it is difficult to achieve both the punchability and toughness of the steel sheet while ensuring the desired steel sheet strength.
The object of the present invention is to solve the problems of the prior art and provide a high-strength thin steel sheet excellent in punchability and toughness and a method for producing the same.

  As a technique for achieving both the strength and workability of a steel sheet, a technique is known in which the steel sheet structure is a ferrite single-phase structure with good workability, and fine precipitates such as Ti carbide are precipitated in the ferrite. Therefore, the present inventors paid attention to a steel sheet having a ferrite single-phase structure with such precipitation strengthening, and intensively studied various factors affecting the strength, punchability, and toughness. As a result, by adding one or more of carbide-forming elements Ti, Nb, and V to the steel sheet, and optimizing the amount of these elements added to precipitate fine carbides, it is possible to increase the strength of the steel sheet. The inventors have found that a sufficient amount of fine precipitates having a particle diameter of less than 20 nm contributing can be secured, and that, for example, a high strength thin steel sheet having a tensile strength of TS: 780 MPa or more can be obtained.

  In addition, the present inventors have obtained an unprecedented new finding that a strong correlation is found between the punchability and toughness of a steel sheet and the specific resistance of the steel sheet. As a result of further investigation, by suppressing the specific resistance of the steel sheet at −196 ° C. to a predetermined value or less, a high-strength thin steel sheet excellent in punchability and toughness can be obtained while maintaining the strength of the steel sheet. It became clear.

  Based on the above examination results, the present inventors sought a means for reducing the specific resistance of a high-strength thin steel sheet at −196 ° C. And in the manufacturing process, the knowledge that the steel sheet in which specific resistance at -196 ° C was controlled was obtained by fully releasing the distortion accumulated in the steel sheet due to precipitation of Ti and the like was obtained. . Specifically, when producing a high-strength thin steel plate, the strain accumulated in the steel plate is obtained by subjecting the hot-rolled plate obtained by subjecting the steel material to hot rolling by annealing treatment under predetermined conditions. It was found that a high-strength thin steel sheet having a significantly reduced specific resistance at −196 ° C. can be obtained.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.036% to 0.250%, Si: 0.30% or less, Mn: 0.1% to 3.0%, P: 0.10% or less, S: 0.030% or less, Al: 0.10% or less, N : 0.010% or less, Ti: 0.01% or more and 1.00% or less, Nb: 0.01% or more and 1.00% or less, V: 0.01% or more and 1.00% or less C ^* represented by the formula (1) is contained so as to satisfy the following formulas (2) and (3), the balance is composed of Fe and inevitable impurities, and the area ratio of the ferrite phase is The amount of Ti, Nb, and V in the precipitate having a particle size of less than 20 nm among the precipitates is 95% or more, and the C ^* expressed by the following formula (1) A high-strength steel sheet having a structure satisfying the formula (4), having a tensile strength TS of 780 MPa or more, and a specific resistance at −196 ° C. of 12 μΩ · cm or less.
C ^* = (Ti / 48 + Nb / 93 + V / 51) x 12 ... (1)
(Ti, Nb, V is the content of each element (mass%))
0.9 ≦ C / C ^* ≦ 1.5… (2)
(C is the C content (% by mass))
C ^* ≧ 0.04 ... (3)
_{([Ti] 20/48 +} [Nb] 20/93 + [V] 20/51) × 12 ≧ C * × 0.60 ... (4)
([Ti] ₂₀ , [Nb] ₂₀ , [V] ₂₀ is the amount of Ti, Nb, V (% by mass) in the precipitate having a particle size of less than 20 nm.
))

[2] By mass%, C: 0.036% to 0.250%, Si: 0.30% or less, Mn: 0.1% to 3.0%, P: 0.10% or less, S: 0.030% or less, Al: 0.10% or less, N : Containing 0.010% or less, Ti: 0.01% or more and 1.00% or less, Nb: 0.01% or more and 1.00% or less, V: one or more selected from 0.01% or more and 1.00% or less, and Mo: One or more selected from 0.005% to 0.500%, Ta: 0.005% to 0.500%, W: 0.005% to 0.500%, C ^* represented by the following formula (5) ^* Is contained so as to satisfy the following formulas (6) and (7), the balance is composed of Fe and inevitable impurities, the area ratio of the ferrite phase is 95% or more, and the precipitate is The amount of Ti, Nb, V, Mo, Ta, and W in the precipitate having a particle diameter of less than 20 nm is precipitated with respect to C ^** represented by the following formula (5): (8) Structure in which the amount of Ti, Nb, V, Mo, Ta and W in the precipitate having a particle diameter of 100 nm or more satisfies the following formula (9) with respect to C ^** represented by the following formula (5) A high strength thin steel sheet characterized by having a tensile strength TS: 780 MPa or more and a specific resistance at −196 ° C. of 12 μΩ · cm or less.
C ^** = (Ti / 48 + Nb / 93 + V / 51 + Mo / 96 + Ta / 181 + W / 184) x 12 ... (5)
(Ti, Nb, V, Mo, Ta, W are the contents of each element (% by mass))
0.9 ≦ C / C ^** ≦ 1.5… (6)
(C is the C content (% by mass))
C ^** ≧ 0.04 ... (7)
_{([Ti] 20/48 +} [Nb] 20/93 + [V] 20/51 + [Mo] 20/96 + [Ta] 20/181 + [W] 20/184) × 12 ≧ C ** × 0.60 ... (8)
([Ti] ₂₀ , [Nb] ₂₀ , [V] ₂₀ , [Mo] ₂₀ , [Ta] ₂₀ , [W] ₂₀ are Ti, Nb, V, Mo in the precipitates whose particle diameter is less than 20 nm. , Ta, W amount (% by mass))
_{([Ti] 100/48 +} [Nb] 100/93 + [V] 100/51 + [Mo] 100/96 + [Ta] 100/181 + [W] 100/184) × 12 ≦ C ** × 0.10 ... (9)
([Ti] ₁₀₀ , [Nb] ₁₀₀ , [V] ₁₀₀ , [Mo] ₁₀₀ , [Ta] ₁₀₀ , [W] ₁₀₀ are Ti, Nb, V, Mo in the precipitate having a particle diameter of 100 nm or more. , Ta, W amount (% by mass))

[3] A high-strength thin steel sheet according to [1] or [2], further containing, in addition to the composition, B: 0.0003% to 0.0050% by mass.

[4] In any one of the above [1] to [3], in addition to the above composition, Cr: 0.01% to 1.00%, Ni: 0.01% to 1.00%, Cu: 0.01% or more in mass% A high-strength thin steel sheet containing one or more selected from 1.00% or less.

[5] The high-strength thin steel sheet according to any one of [1] to [4], further containing, in addition to the composition, Sb: 0.005% to 0.050% by mass.

[6] In any one of the above [1] to [5], in addition to the above composition, the mass is further selected from Ca: 0.0005% to 0.0100%, REM: 0.0005% to 0.0100%. A high-strength thin steel sheet characterized by containing one or two kinds.

[7] The high-strength thin steel sheet according to any one of [1] to [6], wherein the steel sheet surface has a plating layer.

[8] the steel material having the composition according to [1], then subjected to hot rolling to the finish rolling delivery temperature and 800 ° C. or higher 1000 ° C. or less, up to 700 ° C. from the finish rolling delivery temperature Cool at an average cooling rate of 30 ° C / s or higher in the temperature range, wind up at a winding temperature of 500 ° C or higher and 700 ° C or lower, cool to room temperature, and then 550 ° C at an average heating rate of 200 ° C / h or lower above 700 ° C. was heated to a soak temperature, mixture was allowed to stay in the homogeneous heat temperature 100h below, and facilities the annealing cooling to room temperature at an average cooling rate of below 200 ° C. / h, the area ratio of the ferrite phase at least 95%, precipitates are precipitated, the following with respect to C ^* Ti, the Nb and V content of precipitates particle size less than 20nm of the precipitation distillate is represented by the following formula (1) (4) has a satisfactory tissue type, tensile strength TS: provided with a strength above 780 MPa, this resistivity at -196 ° C. to a thin steel sheet or less 12μΩ · cm And a method for producing a high-strength thin steel sheet.
C ^* = (Ti / 48 + Nb / 93 + V / 51) x 12 ... (1)
(Ti, Nb, V is the content of each element (mass%))
_{([Ti] 20/48 +} [Nb] 20/93 + [V] 20/51) × 12 ≧ C * × 0.60 ... (4)
([Ti] ₂₀ , [Nb] ₂₀ , [V] ₂₀ is the amount of Ti, Nb, V (% by mass) in the precipitate having a particle size of less than 20 nm.
))
[9] After subjecting the steel material having the composition described in [2] above to hot rolling at a finish rolling exit temperature of 800 ° C. to 1000 ° C., the finish rolling exit temperature to 700 ° C. Cool at an average cooling rate of 30 ° C / s or higher in the temperature range, wind up at a winding temperature of 500 ° C or higher and 700 ° C or lower, cool to room temperature, and then 550 ° C at an average heating rate of 200 ° C / h or lower After heating to a soaking temperature of 700 ° C. or less and retaining for 100 h or less at the soaking temperature, an annealing treatment is performed to cool to room temperature at an average cooling rate of 200 ° C./h or less, and the area ratio of the ferrite phase is 95 %, And the amount of Ti, Nb, V, Mo, Ta and W in the precipitate having a particle size of less than 20 nm is expressed by the following formula (5). ^** satisfies the following formula (8), and among the precipitates, the amount of Ti, Nb, V, Mo, Ta and W in the precipitate having a particle diameter of 100 nm or more is represented by the following formula (5). to C ^** is In addition, the steel sheet has a structure satisfying the following formula (9), has a tensile strength TS: 780 MPa or more, and has a specific resistance at −196 ° C. of 12 μΩ · cm or less. To produce a high strength thin steel sheet.
C ^** = (Ti / 48 + Nb / 93 + V / 51 + Mo / 96 + Ta / 181 + W / 184) x 12 ... (5)
(Ti, Nb, V, Mo, Ta, W are the contents of each element (% by mass))
_{([Ti] 20/48 +} [Nb] 20/93 + [V] 20/51 + [Mo] 20/96 + [Ta] 20/181 + [W] 20/184) × 12 ≧ C ** × 0.60 ... (8)
([Ti] ₂₀ , [Nb] ₂₀ , [V] ₂₀ , [Mo] ₂₀ , [Ta] ₂₀ , [W] ₂₀ are Ti, Nb, V, Mo in the precipitates whose particle diameter is less than 20 nm. , Ta, W amount (% by mass))
_{([Ti] 100/48 +} [Nb] 100/93 + [V] 100/51 + [Mo] 100/96 + [Ta] 100/181 + [W] 100/184) × 12 ≦ C ** × 0.10 ... (9)
([Ti] ₁₀₀ , [Nb] ₁₀₀ , [V] ₁₀₀ , [Mo] ₁₀₀ , [Ta] ₁₀₀ , [W] ₁₀₀ are Ti, Nb, V, Mo in the precipitate having a particle diameter of 100 nm or more. , Ta, W amount (% by mass))
[10] The method for producing a high-strength thin steel sheet according to [8] or [9], further comprising B: 0.0003% to 0.0050% by mass% in addition to the composition.
[11] In any one of the above [8] to [10], in addition to the above composition, Cr: 0.01% to 1.00%, Ni: 0.01% to 1.00%, Cu: 0.01% or more in mass% A method for producing a high-strength thin steel sheet, comprising one or more selected from 1.00% or less.
[12] The method for producing a high-strength thin steel sheet according to any one of [8] to [11], further comprising, in addition to the composition, Sb: 0.005% to 0.050% by mass% .
[13] In any one of the above [8] to [12], in addition to the composition, the mass is further selected from Ca: 0.0005% to 0.0100% and REM: 0.0005% to 0.0100%. A method for producing a high-strength thin steel sheet, comprising one or two kinds.

[ 14 ] In any one of [8] to [13] , following the annealing treatment, reheating to a temperature range of 500 ° C. or more and 700 ° C. or less, and 420 ° C. or more and 500 ° C. or less in the cooling process after reheating. A method for producing a high-strength thin steel sheet, characterized by performing a plating treatment immersed in a galvanizing bath.

[ 15 ] In the above [ 14 ], following the plating treatment, an alloying treatment is performed by reheating to a temperature range of 460 ° C. to 600 ° C. and retaining in the temperature range for 1 s or longer. A method of manufacturing a steel sheet.

[ 16 ] The method for producing a high-strength thin steel sheet according to any one of [8] to [13] , wherein the annealing is performed at a sheet thickness reduction rate of 0.1% to 3.0% after the annealing treatment.

[ 17 ] The method for producing a high-strength thin steel sheet according to [ 14 ], wherein, after the plating treatment, processing is applied at a sheet thickness reduction rate of 0.1% to 3.0%.

[ 18 ] The method for producing a high-strength thin steel sheet according to [ 15 ], wherein after the alloying treatment, processing is applied at a sheet thickness reduction rate of 0.1% to 3.0%.

  According to the present invention, it is optimal for parts of transportation machinery including automobiles, structural steel materials such as construction steel materials, etc., for example, high tensile strength TS: 780 MPa or more and excellent punchability, and further excellent A high strength thin steel sheet having toughness is obtained. In addition, since the excellent steel sheet characteristics can be obtained as described above, the present invention enables further application development of the high-strength thin steel sheet, and has a remarkable industrial effect.

It is a figure which shows the relationship between the average heating rate in the annealing treatment process of a steel plate, and the specific resistance in -196 degreeC of a steel plate. It is a figure which shows the relationship between the specific resistance in -196 degreeC of a steel plate, the punchability of a steel plate, and a ductility-brittle transition temperature (toughness).

Hereinafter, the present invention will be specifically described.
First, the reasons for limiting the component composition of the high strength thin steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% (mass%) unless there is particular notice.

C: 0.036% to 0.250%
C is an element that contributes to increasing the strength of the steel sheet by forming carbides with Ti, Nb, V, and precipitating in the steel sheet as fine precipitates. C is an element that contributes to the refinement of the carbide. Carbides such as Ti that contribute to increasing the strength of the steel sheet are precipitated at the same time as the γ → α transformation during the cooling and winding process after hot rolling in the steel sheet manufacturing process. Here, when the γ → α transformation point becomes a high temperature, carbides such as Ti are precipitated in a high temperature region, so that the carbides become coarse. In response to such a problem, C has an effect of lowering the temperature at which the ferrite transformation starts in the cooling process, thereby lowering the precipitation temperature of the carbide and suppressing the coarsening of the carbide. Furthermore, C increases the grain boundary strength due to segregation of solid solution C at the grain boundaries, and contributes to the improvement of punching and toughness of the steel sheet by making the grains finer.

  In order to fully exhibit the above effects, the C content needs to be 0.036% or more. Preferably it is 0.05% or more. On the other hand, when the C content is excessively high, the ferrite transformation of the steel is suppressed and the transformation to bainite or martensite is promoted, resulting in a decrease in formability of the steel sheet. Further, excess C that does not form carbides with Ti, Nb, and V becomes cementite, which lowers the formability and toughness of the steel sheet. In order to avoid these problems, the C content needs to be 0.250% or less. Preferably, it is 0.200% or less.

Si: 0.30% or less
When a large amount of Si is added, ferrite transformation is promoted in the cooling process after hot rolling in the steel sheet manufacturing process, and as a result of the carbide precipitation temperature rising, the carbide precipitates coarsely. Moreover, since it becomes easy to produce | generate the oxide of Si on the surface, it is easy to produce a chemical conversion treatment defect in a hot-rolled sheet, and defects, such as a non-plating, easily arise in a plating plate. Therefore, the Si content needs to be 0.30% or less. Preferably it is 0.10% or less, More preferably, it is 0.05% or less, More preferably, it is 0.03% or less.

Mn: 0.1% to 3.0%
Mn has the effect of reducing the precipitation temperature of carbides and reducing the carbides by reducing the temperature at which ferrite transformation starts in the cooling process after hot rolling in the steel sheet manufacturing process. Mn has the effect of improving the punchability and toughness of the steel sheet by making ferrite finer. Furthermore, Mn contributes to increasing the strength of the steel sheet by solid solution strengthening and also has the effect of detoxifying harmful steel S as MnS. In order to obtain such an effect, the Mn content needs to be 0.1% or more. Preferably it is 0.5% or more, More preferably, it is 1.0% or more, More preferably, it is 1.3% or more. On the other hand, if the Mn content becomes excessively high, ferrite transformation is suppressed in the cooling process after hot rolling, and bainite transformation and martensitic transformation are promoted, thereby suppressing the formation of fine carbides of Ti, Nb, and V. Will be. Therefore, the Mn content needs to be 3.0% or less. Preferably it is 2.5% or less, More preferably, it is 2.0% or less.

P: 0.10% or less
P is an element that segregates at the grain boundaries and degrades the ductility and toughness of the steel sheet. P is also an element that accelerates ferrite transformation, raises the precipitation temperature of carbides, and precipitates carbides coarsely in the cooling process after hot rolling in the steel sheet manufacturing process. In order to avoid these problems, the P content needs to be 0.10% or less. Preferably it is 0.05% or less, More preferably, it is 0.03% or less, More preferably, it is 0.01% or less.

S: 0.030% or less
S significantly reduces the hot ductility, thereby inducing hot cracking and significantly deteriorating the surface properties of the steel sheet. S not only contributes to the strength of the steel, but also reduces the ductility and stretch flangeability of the steel sheet by forming coarse sulfides as impurity elements. These problems become significant when the S content exceeds 0.030%. Therefore, the S content is 0.030% or less. Preferably it is 0.010% or less, More preferably, it is 0.003% or less, More preferably, it is 0.001% or less.

Al: 0.10% or less
Al is an element that promotes ferrite transformation, raises the precipitation temperature of carbides, and precipitates carbides coarsely in the cooling process after hot rolling in the steel plate manufacturing process. Further, the addition of a large amount of Al leads to an increase in aluminum oxide in the steel, which causes a reduction in the ductility of the steel sheet. In order to avoid these problems, the Al content needs to be 0.10% or less. Preferably it is 0.06% or less. In addition, although the minimum of Al content is not prescribed | regulated in particular, even if it contains 0.01% or more as Al killed steel, it is satisfactory.

N: 0.010% or less
N forms coarse nitrides at high temperatures with Ti, Nb, and V, but these nitrides do not contribute much to the strength of the steel sheet. Therefore, N becomes a factor that reduces the effect of increasing the strength of the steel sheet by Ti, Nb, and V. If N is contained in a large amount, slab cracking may occur during hot rolling, and surface flaws may occur in the steel sheet. Therefore, the N content needs to be 0.010% or less. Preferably it is 0.005% or less, More preferably, it is 0.003% or less, More preferably, it is 0.002% or less.

One or more selected from Ti: 0.01% to 1.00%, Nb: 0.01% to 1.00%, V: 0.01% to 1.00%
Ti, Nb, and V form fine carbides with C and precipitate in the steel sheet as fine precipitates, thereby contributing to high strength of the steel sheet. In order to obtain a desired steel plate strength (preferably TS: 780 MPa or more), it is necessary to contain one or more selected from Ti, Nb and V, and these contents are both 0.01 % Or more is necessary. Preferably it is 0.03% or more. On the other hand, even if Ti, Nb and V are each contained in a large amount exceeding 1.00%, the effect of increasing the strength of the steel sheet is not obtained, but the production cost is increased. Therefore, the contents of Ti, V and Nb are all 1.00% or less. Preferably it is 0.9% or less.

Further, in the present invention, one or more selected from Ti, Nb and V is C ^* represented by the following formula (1) is represented by the following formulas (2) and (3): It is necessary to make it contain so that it may satisfy. Ti, Nb, and V on the right side of the formula (1) are the contents (mass%) of each element in the steel sheet. In addition, when a steel plate does not contain Ti, C ^* shall be calculated by setting Ti of the right side of (1) Formula to 0 (zero). The same applies when the steel sheet does not contain Nb or does not contain V. Further, C in the middle of the formula (2) is the content (mass%) of C in the steel sheet.

C ^* = (Ti / 48 + Nb / 93 + V / 51) x 12 ... (1)
(Ti, Nb, V is the content of each element (mass%))
0.9 ≦ C / C ^* ≦ 1.5… (2)
(C is the C content (% by mass))
C ^* ≧ 0.04 ... (3)

0.9 ≦ C / C ^* ≦ 1.5
As the amount of added C decreases with respect to the added amount of Ti, Nb, and V, the amount of Ti, Nb, and V precipitated as carbides decreases, and the amount that remains in solid solution increases. Solid solution Ti, Nb, and V do not contribute to increasing the strength of the steel sheet. Therefore, C / C ^* needs to be 0.9 or more. Preferably it is 1.0 or more, More preferably, it is 1.1 or more. On the other hand, if the amount of added C is too large relative to the added amount of Ti, Nb, and V, excess C that does not precipitate as carbides of Ti, Nb, and V increases, resulting in an increase in cementite, and the forming of the steel sheet. The nature will decline. Therefore, C / C ^* needs to be 1.5 or less. Preferably it is 1.3 or less.

C ^* ≧ 0.04
If the total amount of Ti, Nb, and V that contribute to the formation of carbides is small, the amount of carbides decreases and it becomes difficult to increase the strength of the steel sheet. Therefore, C ^* needs to be 0.04 or more. Preferably it is 0.07 or more. However, if the value of C ^* is excessively high, the strength of the steel sheet is extremely increased, and the punchability and toughness may be lowered. Therefore, C ^* is preferably set to 0.30 or less.

  In addition to the above elements, the high-strength thin steel sheet of the present invention can further contain the following elements for the purpose of improving strength, toughness and the like.

Mo: 0.005% or more and 0.500% or less, Ta: 0.005% or more and 0.500% or less, W: 0.005% or more and 0.500% or less
Mo, Ta, and W are elements that contribute to increasing the strength of the steel sheet by forming fine carbides with C and precipitating in the steel sheet as fine precipitates. In order to obtain such an effect, when Mo, Ta, and W are contained, it is preferable to contain one or more of each 0.005% or more. More preferably, it is 0.03% or more. On the other hand, when these elements are added in a large amount, not only the above effects are saturated, but also the cost is increased. Therefore, when Mo, Ta, and W are contained, it is preferable to contain one or more of them at 0.50% or less. More preferably, it is 0.40% or less.

In the case of a composition containing one or more of Mo, Ta and W, C ^** represented by the following formula (5) instead of the formulas (1) to (3) ^: However, it is necessary to satisfy the following expressions (6) and (7). Ti, Nb, V, Mo, Ta, and W on the right side of the formula (5) are the contents (mass%) of each element in the steel sheet. When the steel sheet does not contain Ti, C ^** is calculated by setting Ti on the right side of Equation (5) to 0 (zero). The same applies when no other elements (Nb, V, Mo, Ta and W) are contained. Further, C in the middle of the formula (6) is the content (% by mass) of C in the steel sheet.

C ^** = (Ti / 48 + Nb / 93 + V / 51 + Mo / 96 + Ta / 181 + W / 184) x 12 ... (5)
(Ti, Nb, V, Mo, Ta, W are the contents of each element (% by mass))
0.9 ≦ C / C ^** ≦ 1.5… (6)
(C is the C content (% by mass))
C ^** ≧ 0.04 ... (7)

0.9 ≦ C / C ^** ≦ 1.5
As the amount of added C decreases with respect to the added amounts of Ti, Nb, V, Mo, Ta, and W, the amount of these elements precipitated as carbides decreases, and the amount remaining in solid solution increases. Solid solution Ti, Nb, V, Mo, Ta, and W do not contribute to increasing the strength of the steel sheet. Therefore, C / C ^** needs to be 0.9 or more. Preferably it is 1.0 or more, More preferably, it is 1.1 or more. On the other hand, if the amount of added C is too large relative to the added amount of Ti, Nb, V, Mo, Ta, W, excess C that does not precipitate as carbides will increase, resulting in an increase in cementite, and forming the steel sheet. The nature will decline. Therefore, C / C ^** needs to be 1.5 or less. Preferably it is 1.4 or less.

C ^** ≧ 0.04
If the total amount of Ti, Nb, V, Mo, Ta, and W that contributes to the formation of carbides is small, the amount of carbides decreases and it becomes difficult to increase the strength of the steel sheet. Therefore, C ^** needs to be 0.04 or more. Preferably it is 0.07 or more. However, if the value of C ^** is excessively high, the strength of the steel sheet is extremely increased, and the punchability and toughness may be lowered. Therefore, C ^** is preferably set to 0.30 or less.

B: 0.0003% or more and 0.0050% or less
B is an element that lowers the temperature at which the ferrite transformation starts in the cooling process after hot rolling in the steel sheet manufacturing process, thereby lowering the carbide precipitation temperature and finely depositing the carbide. B is an element that contributes to improving the toughness of the steel sheet by segregating at the grain boundaries to increase the grain boundary strength. In order to obtain such an effect, the B content is preferably 0.0003% or more. More preferably, it is 0.0005% or more, More preferably, it is 0.0010% or more. On the other hand, a large amount of B raises the deformation resistance value of the hot steel and makes rolling difficult. Therefore, when B is added, its content is preferably 0.0050% or less. More preferably, it is 0.0030% or less, More preferably, it is 0.0020% or less.

One or more selected from Cr: 0.01% to 1.00%, Ni: 0.01% to 1.00%, Cu: 0.01% to 1.00%
Cr, Ni, and Cu are elements that contribute to increasing the strength of the steel sheet by refining the structure. In order to obtain such an effect, when Cr, Ni, or Cu is added, it is preferable to contain one or more of each 0.01% or more. More preferably, it is 0.02% or more. On the other hand, when Cr, Ni and Cu are added in a large amount, not only the above effects are saturated but also the cost is increased. Therefore, when adding Cr, Ni, and Cu, it is preferable to contain 1 type or 2 types or more at 1.00% or less, respectively. More preferably, it is 0.50% or less.

Sb: 0.005% or more and 0.050% or less
Since Sb segregates on the surface during hot rolling, it has the effect of preventing the formation of coarse nitrides by preventing the nitriding of steel materials such as slabs. When Sb is added to obtain such an effect, the content is preferably 0.005% or more. On the other hand, since the cost increases when a large amount of Sb is added, when Sb is added, its content is preferably 0.050% or less. More preferably, it is 0.008% or more and 0.030% or less.

Ca: 0.0005% or more and 0.0100% or less, REM: One or two selected from 0.0005% or more and 0.0100% or less
Ca and REM have the effect of improving the ductility and stretch flangeability of the steel sheet by controlling the form of sulfide in the steel. In order to obtain such an effect, when adding Ca and REM, it is preferable to contain one or two or more of 0.0005% or more. More preferably, it is 0.0010% or more. On the other hand, adding a large amount of Ca and REM not only saturates the above effect but also increases the cost. Therefore, when adding Ca and REM, it is preferable to contain 1 type, or 2 or more types at 0.0100% or less. More preferably, it is 0.0070% or less.

  The balance is Fe and inevitable impurities. Inevitable impurities include Sn, Mg, Co, As, Pb, Zn, and the like. If the total content of these elements is 0.5% or less, the steel plate characteristics are not affected. Further, O (oxygen) does not affect the steel sheet characteristics if its content is 0.01% or less.

Next, the reason for limiting the structure and specific resistance value of the high strength thin steel sheet of the present invention will be described.
The high-strength thin steel sheet of the present invention has a ferrite phase area ratio of 95% or more and a structure in which precipitates are precipitated.

Ferrite phase area ratio: 95% or more If a low-temperature transformation phase such as bainite or martensite is present in the structure of the steel sheet, formability deteriorates. Therefore, in the high-strength thin steel sheet of the present invention, it is necessary that the ferrite phase reinforced with fine precipitates is the main phase and the structure fraction is 95% or more in terms of the area ratio with respect to the entire metal structure. Preferably it is 98% or more, More preferably, it is 100%.

  In the high-strength thin steel sheet of the present invention, examples of the structure other than the ferrite phase that can be contained in the structure of the steel sheet include cementite, pearlite, bainite phase, martensite phase, and retained austenite phase. Since these structures adversely affect the formability of the steel sheet, it is preferable to reduce them as much as possible. However, it is acceptable if the total area ratio relative to the entire metal structure is 5% or less. Preferably it is 2% or less, more preferably 0 (zero)%.

Precipitates Precipitates precipitated on the high-strength thin steel sheet of the present invention are carbides mainly containing one or more selected from Ti, Nb and V, specifically Ti carbides and Nb carbides. , V carbide, Ti-Nb composite carbide, Ti-V composite carbide, Nb-V composite carbide, Ti-Nb-V composite carbide, or one or more thereof. Further, when the steel sheet contains one or more of Mo, Ta and W, the precipitates precipitated on the high-strength thin steel sheet of the present invention are mainly Ti, Nb, V, Mo, Ta and W. It is the carbide | carbonized_material containing 1 type, or 2 or more types selected from these. In the present invention, the strength of the steel sheet is increased by ensuring a sufficient amount of precipitates having a particle diameter of less than 20 nm among such precipitates.

In the present invention, among the precipitates, Ti, Nb and V amount in the precipitate having a particle size of less than 20 nm, the following formula (4) with respect to C ^* represented by the formula (1) Will be satisfied. [Ti] ₂₀ , [Nb] ₂₀ , [V] _{20 on the} left side of the formula (4) is the amount of Ti deposited as a precipitate having a particle diameter of less than 20 nm among the precipitates precipitated in the steel sheet ( Mass%), Nb content (mass%), and V content (mass%). Note that the left side of the formula (4) represents a carbon amount converted value of the total amount of Ti, Nb, and V in the precipitate having a particle diameter of less than 20 nm.
_{([Ti] 20/48 +} [Nb] 20/93 + [V] 20/51) × 12 ≧ C * × 0.60 ... (4)

Of the precipitates precipitated on the steel sheet, coarse precipitates do not contribute to increasing the strength of the steel sheet. In order to increase the strength of the steel sheet, it is necessary to suppress coarse precipitates and to precipitate a large amount of precipitates having a particle diameter of less than 20 nm. If the proportion of Ti, Nb, and V deposited as precipitates with a particle diameter of less than 20 nm is less than the amount of Ti, Nb, and V added to the steel sheet, the efficiency of increasing the strength is poor, leading to an increase in manufacturing costs. End up. Further, it is not possible to sufficiently secure the precipitation amount of fine precipitates that contribute to increasing the strength of the steel sheet, and it becomes impossible to obtain a desired steel sheet strength, for example, a steel sheet strength of tensile strength TS: 780 MPa or more. Here, since the precipitate deposited on the high-strength thin steel sheet of the present invention is a carbide mainly containing one or more selected from Ti, Nb and V, in the present invention, the particles The carbon amount converted value of the total amount of Ti, Nb, and V in the precipitate having a diameter of less than 20 nm is set to 0.60 or more times C ^* represented by the formula (1). Preferably, it is 0.70 or more times C ^* , more preferably 0.80 or more times C ^* , and even more preferably 0.90 or more times C ^* .

Further, when the steel sheet contains one or more of Mo, Ta and W, the amount of Ti, Nb, V, Mo, Ta and W in the precipitate having a particle diameter of less than 20 nm, Instead of the formula (4), the following formula (8) is satisfied with respect to C ^** represented by the formula (5). [Ti] ₂₀ , [Nb] ₂₀ , [V] ₂₀ , [Mo] ₂₀ , [Ta] ₂₀ , and [W] _{20 on the} left side of the formula (8) are particles out of the precipitates precipitated in the steel sheet. Ti amount (mass%), Nb amount (mass%), V amount (mass%), Mo amount (mass%), Ta amount (mass%), W amount (mass%) as precipitates having a diameter of less than 20 nm %). Note that the left side of the formula (8) represents the carbon amount converted value of the total amount of Ti, Nb, V, Mo, Ta, and W in the precipitate having a particle diameter of less than 20 nm.
_{([Ti] 20/48 +} [Nb] 20/93 + [V] 20/51 + [Mo] 20/96 + [Ta] 20/181 + [W] 20/184) × 12 ≧ C ** × 0.60 ... (8)

When the proportion of Ti, Nb, V, Mo, Ta, and W precipitated as precipitates with a particle diameter of less than 20 nm is small relative to the amount of Ti, Nb, V, Mo, Ta, and W added to the steel sheet The efficiency of increasing the strength is poor and the manufacturing cost is increased. Moreover, the precipitation amount of the fine precipitate which contributes to the strengthening of a steel plate cannot be ensured enough, and desired steel plate strength cannot be obtained. Therefore, in the present invention, the carbon amount converted value of the total amount of Ti, Nb, V, Mo, Ta, and W in the precipitate having a particle diameter of less than 20 nm is represented by the formula (5) of C ^** . 0.60 times or more. Preferably it is 0.70 or more times C ^** , more preferably 0.80 or more times C ^** , and even more preferably 0.90 or more times C ^** .

Further, when the steel sheet contains one or more of Mo, Ta and W, the amount of Ti, Nb, V, Mo, Ta and W in the precipitate having a particle diameter of 100 nm or more, The following formula (9) is satisfied with respect to C ^** represented by the formula (5). (9) [Ti] ₁₀₀ , [Nb] ₁₀₀ , [V] ₁₀₀ , [Mo] ₁₀₀ , [Ta] ₁₀₀ , and [W] _{100 on the} left side of the formula are the particle diameters of the precipitates precipitated in the steel sheet. Ti amount (mass%), Nb amount (mass%), V amount (mass%), Mo amount (mass%), Ta amount (mass%), W amount (mass%) ). Note that the left side of the formula (9) represents the carbon amount converted value of the total amount of Ti, Nb, V, Mo, Ta, and W in the precipitate having a particle diameter of 100 nm or more.
_{([Ti] 100/48 +} [Nb] 100/93 + [V] 100/51 + [Mo] 100/96 + [Ta] 100/181 + [W] 100/184) × 12 ≦ C ** × 0.10 ... (9)

The proportion of Ti, Nb, V, Mo, Ta, and W deposited as precipitates with a particle diameter of 100 nm or more is higher than the amount of Ti, Nb, V, Mo, Ta, and W added to the steel sheet. As a result, the number of precipitates having a particle size of 100 nm or more increases. And since such a coarse precipitate becomes a starting point of a crack at the time of punching of a steel plate, punching property and toughness fall significantly. Therefore, in the present invention, the carbon amount converted value of the total amount of Ti, Nb, V, Mo, Ta, and W in the precipitate having a particle diameter of 100 nm or more is expressed as C ^** expressed by the formula (5). 0.10 times or less. Preferably it is 0.02 times or less of C ^** .

Specific resistance at −196 ° C .: 12 μΩ · cm or less As the specific resistance increases, the strain inside the steel sheet increases and the punchability and toughness decrease. Therefore, the specific resistance at −196 ° C. should be 12 μΩ · cm or less. Preferably, it is 10 μΩ · cm or less. Although the mechanism by which the punchability and toughness of the steel sheet improve as the specific resistance decreases is still unclear, the stress generated inside the steel sheet due to coherent precipitation when carbides finely precipitate in the ferrite region is long-lasting. It is estimated that the punchability and toughness are improved by reducing the specific resistance by reducing the heat treatment.

By defining the composition, structure and specific resistance as described above, a high-strength thin steel sheet having desired strength and excellent punchability and toughness can be obtained. The strength of the high-strength thin steel sheet of the present invention is not particularly specified, but the tensile strength TS is preferably 780 MPa or more, and more preferably 980 MPa or more. The thickness of the high-strength thin steel sheet of the present invention is not particularly specified, but the thickness is preferably 4.0 mm or less, more preferably 0.8 mm or more and 3.0 mm or less.
In addition, for the purpose of imparting corrosion resistance to the steel sheet, even if a plating layer is provided on the surface of the high strength thin steel sheet of the present invention, or a film is formed on the upper layer of the plating layer by chemical conversion or the like, the effects of the present invention described above Will not be damaged.

  In the present invention, the type of the plating layer provided on the surface of the steel sheet is not particularly limited, and any of an electroplating layer and a hot dipping layer can be applied. The alloy component of the plating layer is not particularly limited, and a hot dip galvanized layer, an alloyed hot dip galvanized layer, and the like can be cited as suitable examples, but of course not limited thereto. In addition to zinc, composite plating of zinc and Al, composite plating of zinc and Ni, Al plating, composite plating of Al and Si, and the like may be performed. By forming a plating layer on the surface, the corrosion resistance of the high-strength thin steel sheet is improved, and it becomes possible to apply it to automobile parts and the like used in severe corrosive environments.

Next, the manufacturing method of the high intensity | strength thin steel plate of this invention is demonstrated.
In the present invention, the steel material having the above composition is subjected to hot rolling at a finish rolling exit temperature of 800 ° C. or more and 1000 ° C. or less, and then average cooling in a temperature range from the finish rolling exit temperature to 700 ° C. Cool at a rate of 30 ° C / s or higher, wind at a winding temperature of 500 ° C or higher and 700 ° C or lower, cool to room temperature, and then 550 ° C or higher and 700 ° C or lower at an average heating rate of 200 ° C / h or lower. Heating to a soaking temperature, retaining for 100 h or less at the soaking temperature, and then performing an annealing process for cooling to room temperature at an average cooling rate of 200 ° C./h or less.

  In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, a slab (steel material) is cast from molten steel, but it is preferable to use a continuous casting method to obtain a slab (steel material) from the viewpoint of productivity. However, the slab may be formed by a known casting method such as ingot-bundling rolling or continuous slab casting.

  The steel material obtained as described above is hot-rolled. When hot rolling is performed on a steel material that has become a hot piece or a cold piece, the steel material is reheated to an austenite single-phase region prior to hot rolling. If the steel material before hot rolling is not heated to the austenite single phase region, the re-dissolution of precipitates present in the steel material will not proceed, and precipitates after rolling (mainly containing Ti, Nb, V, etc.) Fine precipitation of carbides is not achieved.

  For the reasons described above, in the present invention, prior to hot rolling, the steel material is reheated to an austenite single phase region, preferably 1200 ° C. or higher. However, if the reheating temperature of the steel material becomes excessively high, there is a concern that the amount of oxidation increases and the yield decreases, so the reheating temperature is preferably 1300 ° C. or less. In addition, when hot-rolling a steel raw material, when the steel raw material (slab) after casting is the temperature of an austenite single phase region, you may carry out direct rolling without heating a steel raw material.

  Hot rolling is usually composed of rough rolling and finish rolling, but the conditions for rough rolling are not particularly limited in the present invention. In particular, when a thin slab casting method is employed, rough rolling may be omitted. Finish rolling is performed under the following conditions.

Finishing rolling exit temperature: 800 ° C. or more and 1000 ° C. or less When the finishing rolling exit temperature is lowered, ferrite transformation is promoted in cooling after rolling, and carbides are largely precipitated. Moreover, when the exit temperature of finish rolling is in the ferrite region, coarse carbides are precipitated due to strain-induced precipitation. Therefore, the finish rolling exit temperature, that is, the finish final rolling exit temperature needs to be 800 ° C. or higher, preferably 850 ° C. or higher. On the other hand, when the finish rolling exit temperature becomes excessively high, the ferrite grains after transformation become large, and the punchability and toughness of the steel sheet are lowered. Therefore, the finish rolling exit temperature needs to be 1000 ° C. or less. Preferably it is 950 degrees C or less.
After the hot rolling is completed, accelerated cooling is performed under the following conditions, and carbides are finely precipitated in the steel sheet.

Average cooling rate in the temperature range from the finish rolling exit temperature to 700 ° C .: 30 ° C./s or more When the cooling rate from the finish rolling to 700 ° C. is low, ferrite transformation is promoted and carbides are largely precipitated. Therefore, the average cooling rate in the temperature range from the finish rolling exit temperature to 700 ° C. needs to be 30 ° C./s or more. Preferably, it is 50 ° C./s or higher, more preferably 80 ° C./s or higher, and even more preferably 100 ° C./s or higher. However, if the average cooling rate in the above temperature range is excessively large, it is difficult to control the coiling temperature and it may be difficult to obtain a stable steel sheet strength.

Winding temperature: 500 ° C. or higher and 700 ° C. or lower If the winding temperature is too high, the carbides become coarse. Therefore, the winding temperature needs to be 700 ° C. or less. Preferably it is 650 degrees C or less. On the other hand, when the coiling temperature is too low, low-temperature transformation phases such as bainite and martensite are generated. Therefore, the coiling temperature needs to be 500 ° C. or higher. Preferably it is 550 degreeC or more.

  Subsequently, the hot-rolled sheet (hot-rolled coil) after winding up to room temperature is heated to a soaking temperature under the following conditions, retained at the soaking temperature for a predetermined time, and then cooled to room temperature. Apply. Note that room temperature usually means a temperature of 20 ± 5 ° C, but in an actual hot rolling line, the temperature environment around the coil (hot rolled coil) after winding is approximately 0 ° C to 100 ° C. It may vary within the range.

Average heating rate from room temperature to soaking temperature: 200 ° C / h or less When heating rate from room temperature to soaking temperature is large, strain generated by precipitation of carbides in the accelerated cooling or winding process may be released. This is not possible, and the punchability and toughness of the steel sheet are reduced. Therefore, the average heating rate from room temperature to the soaking temperature needs to be 200 ° C./h or less. Preferably it is 150 degrees C / h or less, More preferably, it is 100 degrees C / h or less. The lower limit of the average heating rate is not particularly set, but 10 ° C./h or more is sufficient from the viewpoint of productivity.

Soaking temperature: not less than 550 ° C and not more than 700 ° C If the soaking temperature is too low, the strain caused by the precipitation of carbides cannot be released. Therefore, the soaking temperature needs to be 550 ° C. or higher. Preferably it is 600 degreeC or more. On the other hand, when the soaking temperature is excessively high, precipitates are coarsened and the steel sheet strength is reduced. Therefore, the soaking temperature needs to be 700 ° C. or lower. Preferably it is 650 degrees C or less.

Residence time at soaking temperature (soaking time): 100 h or less If the soaking time is too long, the precipitates become coarse and the steel sheet strength decreases. Therefore, the soaking time needs to be 100 hours or less. Preferably it is 70 hours or less, More preferably, it is 50 hours or less. The lower limit of the soaking time is not particularly specified, but from the viewpoint of sufficiently releasing the strain, the soaking time is preferably 1.0 hour or more, and more preferably 10 hours or more.

Average cooling rate from the soaking temperature to room temperature: 200 ° C./h or less If the cooling rate from the soaking temperature to room temperature is too large, the strain caused by precipitation of carbides cannot be released. Therefore, the average cooling rate from the soaking temperature to room temperature needs to be 200 ° C./h or less. Preferably it is 150 degrees C / h or less, More preferably, it is 100 degrees C / h or less. There is no particular lower limit for the average cooling rate from the soaking temperature to room temperature, but from the viewpoint of increasing productivity, it is sufficient to set it at 10 ° C./h or more.

In addition, when performing an annealing process, what is necessary is just to perform what is called batch annealing which heats with a hot-rolled coil wound up. Moreover, the pickling may be performed before the annealing treatment, or the pickling after the annealing treatment may be performed.
As described above, a high-strength thin steel sheet having a desired structure and specific resistance, high strength (for example, TS: 780 MPa or more) and excellent punchability and toughness can be obtained.

  In the present invention, the high-strength thin steel sheet produced as described above may be plated to form a plating layer on the steel sheet surface. As the plating treatment, either electroplating or hot dipping can be applied. Further, the alloy component of the plating layer is not particularly limited, and a hot dip galvanized layer, an alloyed hot dip galvanized layer and the like can be mentioned as suitable examples, but of course, it is not limited thereto. In addition to zinc, composite plating of zinc and Al, composite plating of zinc and Ni, Al plating, composite plating of Al and Si, and the like may be performed.

  In the case of forming the hot dip galvanized layer, after the annealing treatment, a heat treatment for reheating to the following temperature may be performed, and then a plating treatment for immersing in a plating bath in the cooling process from the reheating temperature may be performed.

Reheating temperature: 500 ° C. or more and 700 ° C. or less If the reheating temperature at the time of plating is too high, the precipitates may become coarse and the steel sheet strength may be reduced. Therefore, the reheating temperature is preferably 700 ° C. or lower. More preferably, it is 680 ° C. or lower. The lower limit of the reheating temperature is not particularly provided, but is preferably 500 ° C. or higher from the viewpoint of making the plating surface beautiful. After heating to the reheating temperature and retaining at the reheating temperature for a predetermined time (for example, 30 seconds to 200 seconds), cooling is performed. And in the cooling process, it is immersed in a zinc plating bath at 420 ° C. or more and 500 ° C. or less to form a hot dip galvanized layer. When the steel sheet is immersed in the galvanizing bath, it is preferable to immerse the steel sheet in the plating bath at a stage where the steel sheet temperature is cooled to a temperature range of 380 ° C. or higher and 550 ° C. or lower.

  In the case of forming an alloyed hot-dip galvanized layer, it is preferable to perform an alloying treatment that is reheated to a temperature range of 460 ° C. to 600 ° C. and stays in the temperature range for 1 s or more following the plating treatment. If the reheating temperature at the time of alloying treatment of Zn and Fe becomes too high, alloying proceeds too much and the plated layer becomes brittle. Accordingly, the reheating temperature during the alloying treatment is preferably 600 ° C. or lower. More preferably, it is 570 ° C. or lower. On the other hand, if the reheating temperature during the alloying treatment is too low, alloying does not proceed. More preferably, it is 480 ° C. or higher. Although there is no particular upper limit for the residence time in the temperature range of 460 ° C. or more and 600 ° C. or less, the precipitate in the steel sheet becomes coarse when the residence time is prolonged, and therefore it is preferably 10 s or less. More preferably, it is 5 s or less.

  The steel plate after the annealing treatment, plating treatment, and alloying treatment can increase the number of movable dislocations by light processing, and can improve punchability and toughness. When light processing is added, it is preferable to apply processing with a plate thickness reduction rate of 0.1% or more, and it is more preferable to apply processing with a plate thickness reduction rate of 0.3% or more. On the other hand, if the plate thickness reduction rate becomes too large, the ductility of the steel plate is reduced by work hardening. Therefore, when processing is applied, the plate thickness reduction rate is preferably 3.0% or less. More preferably, it is 2.0% or less, More preferably, it is 1.0% or less. When the light processing is applied, the light processing may be applied by pressing with a rolling roll, or the light processing may be applied by tension that applies tension to the steel sheet. Furthermore, light processing may be imparted by a combination of reduction and tension by the rolling roll.

Molten steel was melted and continuously cast by a generally known method to obtain a slab (steel material) having a thickness of 250 mm having the composition shown in Table 1. These slabs were heated to 1250 ° C., roughly rolled, subjected to finish rolling under the conditions shown in Table 2, accelerated cooling, winding, and hot rolled sheets (hot rolled coils) with a thickness of 1.6 to 3.6 mm. . Subsequently, the hot-rolled coil cooled to room temperature was annealed under the conditions shown in Table 2 to obtain a thin steel plate. In addition, some of the hot-rolled coils were annealed under the conditions shown in Table 2, and then reheated and allowed to stay at the reheating temperature for 30 to 200 seconds, after which the steel sheet was cooled to 380 to 550 ° C. A thin steel plate (GI material) that is plated with a hot-dip galvanized layer with a coating weight of 45 g / m ^{2 on} one side and is immersed in a 460 ° C plating bath (plating composition: 0.20 mass% Al-Zn) ). Furthermore, for some hot rolled coils, following the annealing treatment under the conditions shown in Table 2, the plating treatment was performed under the same conditions as above, and a hot dip galvanized layer with an adhesion amount of 45 g / m ² per side was applied to the surface of the steel sheet. After the formation, an alloying treatment was performed by reheating under the conditions shown in Table 2 and retaining at the reheating temperature to obtain a thin steel plate (GA material). Further, some of the thin steel plates were subjected to processing at a sheet thickness reduction rate shown in Table 2 after the annealing treatment or alloying treatment.

  Specimens were collected from the thin steel plates obtained above (non-plated thin steel plates, GI materials, GA materials) and subjected to structure observation, electrolytic extraction, tensile test, punching test, Charpy impact test, and specific resistance measurement. . The structure observation method, electrolytic extraction method, various test methods, and specific resistance measurement methods were as follows.

(I) Microstructure observation Specimens were collected from the obtained thin steel sheet, embedded in resin, and the cross section in the rolling direction was subjected to nital corrosion. Then, the structure of the 300 × 300 μm ² region at the center of the plate thickness was measured with an optical microscope (magnification: 500 times). Observations were made. The area ratio of the ferrite phase was measured by determining the ratio of the area occupied by the ferrite phase to the entire observed area (300 × 300 μm ² area).

(Ii) Electrolytic extraction By electrolytic extraction, the amount of Ti, Nb, V, Mo, Ta, and W in the precipitate having a particle diameter of less than 20 nm (that is, [Ti] in the above formula (4) or (8) ₂₀ , [Nb] ₂₀ , [V] ₂₀ , [Mo] ₂₀ , [Ta] ₂₀ , [W] ₂₀ ) were calculated, and a carbon amount converted value of these total amounts was calculated.
Samples were taken from the obtained thin steel sheet (parts other than the plating layer in the case of GI and GA materials), and in 10% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride / methanol) Constant current electrolysis was performed at a current density of 20 mA / cm ² . The test piece after electrolysis was taken out from the electrolytic solution and immersed in a solution having dispersibility, and the deposits adhering to the surface of the test piece were dispersed in the solution. Next, the solution was filtered using a 20 nm pore size filter, and the filtrate (the solution after filtration) was analyzed using ICP-MS. Ti, Nb, V, Mo, Ta, W contained in the filtrate were analyzed. The mass of was measured. The amount of Ti in the precipitate having a particle size of less than 20 nm, that is, [Ti] ₂₀ is obtained by using the mass M _{20 of} Ti contained in the filtrate and the mass Ms of the test piece electrolyzed by constant current electrolysis. ] ₂₀ = M ₂₀ / Ms × 100 (mass%) [Nb] ₂₀ , [V] ₂₀ , [Mo] ₂₀ , [Ta] ₂₀ , and [W] ₂₀ were determined in the same manner. Using [Ti] ₂₀ , [Nb] ₂₀ , [V] ₂₀ , [Mo] ₂₀ , [Ta] ₂₀ , and [W] ₂₀ obtained in this way, in the precipitate having a particle diameter of less than 20 nm. Ti, Nb, V, Mo, Ta, W the total amount of carbon equivalence, that the equation (4) left _{(([Ti] 20/48} + [Nb] 20/93 + [V] 20/51) × 12 ) or (8) of the left-hand side _{(([Ti] 100/48} + [Nb] 100/93 + [V] 100/51 + [Mo] 100/96 + [Ta] 100/181 + [W] 100/184) × 12 ) Was calculated.
Further, by electrolytic extraction, the amount of Ti, Nb, V, Mo, Ta, and W in the precipitate having a particle diameter of 100 nm or more (that is, [Ti] ₁₀₀ , [Nb] ₁₀₀ , [ V] ₁₀₀ , [Mo] ₁₀₀ , [Ta] ₁₀₀ , [W] ₁₀₀ ) were calculated, and the carbon amount converted value of these total amounts was calculated.
Samples were taken from the obtained thin steel sheet (parts other than the plating layer in the case of GI and GA materials), and in 10% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride / methanol) Constant current electrolysis was performed at a current density of 20 mA / cm ² . The test piece after electrolysis was taken out from the electrolytic solution and immersed in a solution having dispersibility, and the deposits adhering to the surface of the test piece were dispersed in the solution. Next, the solution is filtered using a 100 nm pore size filter, and the extraction residue remaining in the 100 nm filter is analyzed using ICP-MS. The mass of Ti, Nb, V, Mo, Ta, and W in the extraction residue is analyzed. It was measured.
Ti in the precipitate having a particle diameter of 100 nm or more, that is, [Ti] ₁₀₀ uses the mass M ₁₀₀ of Ti in the extraction residue and the mass Ms of the test piece electrolyzed by constant current electrolysis, and [Ti] ₁₀₀ = M was determined by ₁₀₀ / Ms × ₁₀₀ (% by weight). [Nb] ₁₀₀ , [V] ₁₀₀ , [Mo] ₁₀₀ , [Ta] ₁₀₀ , and [W] ₁₀₀ were determined in the same manner. Using [Ti] ₁₀₀ , [Nb] ₁₀₀ , [V] ₁₀₀ , [Mo] ₁₀₀ , [Ta] ₁₀₀ , and [W] ₁₀₀ obtained in this way, in the precipitate having a particle diameter of 100 nm or more. Ti, Nb, V, Mo, Ta, W the total amount of carbon equivalence, namely the (9) of the left-hand side _{(([Ti] 100/48} + [Nb] 100/93 + [V] 100/51 + [Mo] _{100/96 + [Ta] 100} /181 + [W] 100/184) was calculated value of × 12).

(Iii) Tensile test From the obtained thin steel sheet, a JIS No. 5 tensile test piece with the direction perpendicular to the rolling direction as the tensile direction was cut out and subjected to a tensile test in accordance with JIS Z 2241, yield strength (yield point YP), tensile Strength (TS) and total elongation (El) were measured.

(Iv) Punching test A 20 mmφ hole was punched into the obtained thin steel sheet with a clearance of 20% on one side, and the appearance of the punched end face was visually observed. When no edge roughness or cracks were observed, “Punchability: Good (◯)”, when edge roughness was observed but no cracks were observed, “Punchability: Slightly poor (△)”, edge The case where roughness and end face cracks were observed was evaluated as “punchability: poor (×)”.

(V) Charpy impact test (toughness evaluation)
From the obtained thin steel sheet, a test piece was collected and subjected to a Charpy impact test, and the fracture surface transition temperature was measured. The test piece was a V-notch test piece conforming to JIS Z 2242 except that the plate thickness was kept at the original thickness, and was prepared so that the longitudinal direction was the rolling direction. The Charpy impact test compliant with JIS Z 2242 was performed three times at each 10 ° C pitch to determine the brittle fracture surface ratio, and the ductile-brittle transition temperature (the temperature at which the brittle fracture surface ratio was 50%) was determined. . When the ductile-brittle transition temperature was −40 ° C. or lower, the toughness was evaluated as good.

(Vi) Measurement of specific resistance From the obtained thin steel plate, a test piece of 4 mm x 80 mm was collected, and a specific resistance value at -196 ° C was measured by a four-terminal method. Regarding the GI material and the GA material, the specific resistance was measured after removing the plating layer formed on the surface.

  The results obtained as described above are shown in Table 3. Moreover, about the test body No. 8-12 of Table 3, the relationship between the average heating rate of an annealing process and the specific resistance obtained by said (vi) is shown in FIG. Furthermore, regarding specimen Nos. 8 to 12 in Table 3, FIG. 2 shows the relationship between the specific resistance obtained by (vi) and the punchability and toughness obtained by (iv) and (v). In FIG. 2, a case where the punching property is good is indicated by ◯, a case where the punching property is slightly poor is indicated by Δ, and a case where the punching property is poor is indicated by ×.

  As shown in Table 3, the specimen of the present invention has a strength of TS: 780 MPa or more, a specific resistance at −196 ° C. of 12 μΩ · cm or less, and good punchability (◯). The ductile-brittle transition temperature is −40 ° C. or less, and it is a high-strength thin steel sheet with excellent toughness. On the other hand, the specimen of the comparative example is a high-strength thin steel sheet in which either one or both of punchability and toughness is insufficient. Moreover, it can be understood from FIG. 1 that the specific resistance of the steel sheet becomes lower as the average heating rate of the annealing process is lower, that is, as the strain accumulated in the steel sheet is smaller. Furthermore, it can be understood from FIG. 2 that the punchability and toughness of the steel sheet improve as the specific resistance of the steel sheet decreases.

Claims (18)

% By mass
C: 0.036% or more and 0.250% or less, Si: 0.30% or less,
Mn: 0.1% to 3.0%, P: 0.10% or less,
S: 0.030% or less, Al: 0.10% or less,
N: Contains 0.010% or less,
Ti: 0.01% to 1.00%, Nb: 0.01% to 1.00%,
V: Contains one or more selected from 0.01% or more and 1.00% or less so that C ^* represented by the following formula (1) satisfies the following formulas (2) and (3) And the balance has a composition consisting of Fe and inevitable impurities,
The area ratio of the ferrite phase is 95% or more, precipitates are precipitated, and the Ti, Nb and V amounts in the precipitates having a particle diameter of less than 20 nm among the precipitates are represented by the following formula (1). It has a structure satisfying the following formula (4) for C ^* ,
Tensile strength TS: A high-strength thin steel sheet having a strength of 780 MPa or more and a specific resistance at −196 ° C. of 12 μΩ · cm or less.
Record
C ^* = (Ti / 48 + Nb / 93 + V / 51) x 12 ... (1)
(Ti, Nb, V is the content of each element (mass%))
0.9 ≦ C / C ^* ≦ 1.5… (2)
(C is the C content (% by mass))
C ^* ≧ 0.04 ... (3)
_{([Ti] 20/48 +} [Nb] 20/93 + [V] 20/51) × 12 ≧ C * × 0.60 ... (4)
([Ti] ₂₀ , [Nb] ₂₀ , [V] ₂₀ are Ti, Nb, V amount (mass%) in the precipitate having a particle size of less than 20 nm) % By mass
C: 0.036% or more and 0.250% or less, Si: 0.30% or less,
Mn: 0.1% to 3.0%, P: 0.10% or less,
S: 0.030% or less, Al: 0.10% or less,
N: Contains 0.010% or less,
Ti: 0.01% to 1.00%, Nb: 0.01% to 1.00%,
V: one or more selected from 0.01% to 1.00%, and
Mo: 0.005% to 0.500%, Ta: 0.005% to 0.500%,
W: One or more selected from 0.005% or more and 0.500% or less so that C ^** expressed by the following formula (5) satisfies the following formulas (6) and (7) Containing, the balance has a composition consisting of Fe and inevitable impurities,
The area ratio of the ferrite phase is 95% or more, precipitates are precipitated, and the amount of Ti, Nb, V, Mo, Ta and W in the precipitates having a particle diameter of less than 20 nm among the precipitates is as follows (5 The following formula (8) is satisfied with respect to C ^** represented by the formula), and the amount of Ti, Nb, V, Mo, Ta and W in the precipitate having a particle diameter of 100 nm or more among the precipitates is It has a structure that satisfies the following formula (9) for C ^** represented by the following formula (5),
Tensile strength TS: A high-strength thin steel sheet having a strength of 780 MPa or more and a specific resistance at −196 ° C. of 12 μΩ · cm or less.
Record
C ^** = (Ti / 48 + Nb / 93 + V / 51 + Mo / 96 + Ta / 181 + W / 184) x 12 ... (5)
(Ti, Nb, V, Mo, Ta, W are the contents of each element (% by mass))
0.9 ≦ C / C ^** ≦ 1.5… (6)
(C is the C content (% by mass))
C ^** ≧ 0.04 ... (7)
_{([Ti] 20/48 +} [Nb] 20/93 + [V] 20/51 + [Mo] 20/96 + [Ta] 20/181 + [W] 20/184) × 12 ≧ C ** × 0.60 ... (8)
([Ti] ₂₀ , [Nb] ₂₀ , [V] ₂₀ , [Mo] ₂₀ , [Ta] ₂₀ , [W] ₂₀ are Ti, Nb, V, Mo in the precipitates whose particle diameter is less than 20 nm. , Ta, W amount (% by mass))
_{([Ti] 100/48 +} [Nb] 100/93 + [V] 100/51 + [Mo] 100/96 + [Ta] 100/181 + [W] 100/184) × 12 ≦ C ** × 0.10 ... (9)
([Ti] ₁₀₀ , [Nb] ₁₀₀ , [V] ₁₀₀ , [Mo] ₁₀₀ , [Ta] ₁₀₀ , [W] ₁₀₀ are Ti, Nb, V, Mo in the precipitate having a particle diameter of 100 nm or more. , Ta, W amount (% by mass))   The high-strength thin steel sheet according to claim 1 or 2, further comprising B: 0.0003% or more and 0.0050% or less by mass% in addition to the composition.   In addition to the above composition, one or more selected from Cr: 0.01% to 1.00%, Ni: 0.01% to 1.00%, Cu: 0.01% to 1.00% by mass% The high-strength thin steel sheet according to any one of claims 1 to 3, which is contained.   The high-strength thin steel sheet according to any one of claims 1 to 4, further comprising, in addition to the composition, Sb: 0.005% or more and 0.050% or less by mass%.   2. In addition to the composition, the composition further comprises one or two selected from Ca: 0.0005% to 0.0100% and REM: 0.0005% to 0.0100% in mass%. The high-strength thin steel sheet according to any one of 5 to 5.   The high-strength thin steel sheet according to any one of claims 1 to 6, wherein the steel sheet surface has a plating layer. The steel material having the composition according to claim 1 is subjected to hot rolling at a finish rolling exit temperature of 800 ° C. or higher and 1000 ° C. or lower, and then an average temperature range from the finish rolling exit temperature to 700 ° C. Cool at a cooling rate of 30 ° C / s or higher, wind up at a winding temperature of 500 ° C or higher and 700 ° C or lower, cool to room temperature, and then 550 ° C or higher and 700 ° C or lower at an average heating rate of 200 ° C / h or lower was heated to the soaking temperature, mixture was allowed to stay below 100h to the homogeneous heat temperature, and facilities the annealing cooling to room temperature at an average cooling rate of below 200 ° C. / h,
The area ratio of the ferrite phase is 95% or more, precipitates are precipitated, and the Ti, Nb and V amounts in the precipitates having a particle diameter of less than 20 nm among the precipitates are represented by the following formula (1). It has a structure satisfying the following formula (4) for C ^* ,
A method for producing a high-strength thin steel sheet, characterized in that the steel sheet has a tensile strength TS: 780 MPa or more and a specific resistance at −196 ° C. of 12 μΩ · cm or less .
Record
C ^* = (Ti / 48 + Nb / 93 + V / 51) x 12 ... (1)
(Ti, Nb, V is the content of each element (mass%))
_{([Ti] 20/48 +} [Nb] 20/93 + [V] 20/51) × 12 ≧ C * × 0.60 ... (4)
([Ti] ₂₀ , [Nb] ₂₀ , [V] ₂₀ are Ti, Nb, V amount (mass%) in the precipitate having a particle size of less than 20 nm) The steel material having the composition according to claim 2 is subjected to hot rolling at a finish rolling exit temperature of 800 ° C. or more and 1000 ° C. or less, and then an average temperature range from the finish rolling exit temperature to 700 ° C. Cool at a cooling rate of 30 ° C / s or higher, wind up at a winding temperature of 500 ° C or higher and 700 ° C or lower, cool to room temperature, and then 550 ° C or higher and 700 ° C or lower at an average heating rate of 200 ° C / h or lower After being heated to a soaking temperature of 100 ° C. for 100 hours or less and then subjected to an annealing treatment for cooling to room temperature at an average cooling rate of 200 ° C./h or less,
The area ratio of the ferrite phase is 95% or more, precipitates are precipitated, and the amount of Ti, Nb, V, Mo, Ta and W in the precipitates having a particle diameter of less than 20 nm among the precipitates is as follows (5 C represented by ^** In contrast, the following formula (8) is satisfied, and among the precipitates, the amount of Ti, Nb, V, Mo, Ta and W in the precipitate having a particle diameter of 100 nm or more is represented by the following formula (5): C ^** With a structure that satisfies the following formula (9):
A method for producing a high-strength thin steel sheet, characterized in that the steel sheet has a tensile strength TS: 780 MPa or more and a specific resistance at −196 ° C. of 12 μΩ · cm or less.
                                Record
      C ^** = (Ti / 48 + Nb / 93 + V / 51 + Mo / 96 + Ta / 181 + W / 184) × 12… (5)
        (Ti, Nb, V, Mo, Ta, W are the contents of each element (% by mass))
     ([Ti] ₂₀ / 48 + [Nb] ₂₀ / 93 + [V] ₂₀ / 51 + [Mo] ₂₀ / 96 + [Ta] ₂₀ / 181 + [W] ₂₀ / 184) × 12 ≧ C ^** × 0.60… (8)
     ([Ti] ₂₀ , [Nb] ₂₀ , [V] ₂₀ , [Mo] ₂₀ , [Ta] ₂₀ , [W] ₂₀ Is the Ti, Nb, V, Mo, Ta, W amount (mass%) in the precipitate whose particle size is less than 20nm
     ([Ti] ₁₀₀ / 48 + [Nb] ₁₀₀ / 93 + [V] ₁₀₀ / 51 + [Mo] ₁₀₀ / 96 + [Ta] ₁₀₀ / 181 + [W] ₁₀₀ / 184) × 12 ≦ C ^** × 0.10… (9)
     ([Ti] ₁₀₀ , [Nb] ₁₀₀ , [V] ₁₀₀ , [Mo] ₁₀₀ , [Ta] ₁₀₀ , [W] ₁₀₀ Is Ti, Nb, V, Mo, Ta, W amount (mass%) in precipitates with particle size of 100nm or more) The method for producing a high-strength thin steel sheet according to claim 8 or 9, further comprising B: 0.0003% or more and 0.0050% or less by mass% in addition to the composition. In addition to the above composition, one or more selected from Cr: 0.01% to 1.00%, Ni: 0.01% to 1.00%, Cu: 0.01% to 1.00% by mass% It contains, The manufacturing method of the high strength thin steel plate in any one of Claims 8 thru | or 10 characterized by the above-mentioned. The method for producing a high-strength thin steel sheet according to any one of claims 8 to 11, further comprising Sb: 0.005% or more and 0.050% or less by mass% in addition to the composition. 9. In addition to the above composition, the composition further contains one or two selected from Ca: 0.0005% to 0.0100% and REM: 0.0005% to 0.0100% in mass%. The manufacturing method of the high intensity | strength thin steel plate in any one of thru | or 12. Following the annealing treatment, reheating to a temperature range of 500 ° C. or more and 700 ° C. or less, and performing a plating treatment soaking in a zinc plating bath of 420 ° C. or more and 500 ° C. or less in the cooling process after reheating. Item 14. A method for producing a high-strength thin steel sheet according to any one of Items 8 to 13 . The high-strength thin steel sheet according to claim 14 , wherein, following the plating treatment, re-heating to a temperature range of 460 ° C or higher and 600 ° C or lower is performed, and an alloying treatment is performed so as to stay in the temperature range for 1 second or longer. Method. The method for producing a high-strength thin steel sheet according to any one of claims 8 to 13, wherein the annealing is performed at a sheet thickness reduction rate of not less than 0.1% and not more than 3.0%. The method for producing a high-strength thin steel sheet according to claim 14 , wherein after the plating treatment, processing is applied at a sheet thickness reduction rate of 0.1% or more and 3.0% or less. The method for producing a high-strength thin steel sheet according to claim 15 , wherein after the alloying treatment, processing is applied at a sheet thickness reduction rate of 0.1% or more and 3.0% or less.

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