JP2013227657A - Method for producing high-strength cold-rolled steel sheet with excellent steel sheet shape - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method by which a high-strength cold-rolled steel sheet with an excellent steel sheet shape can be produced with good productivity.SOLUTION: A production method includes an annealing step of heating a steel product satisfying a prescribed constituent composition in an austenite single phase region for 15-600 seconds to anneal, a primary cooling step of, after annealing, gradually cooling down to a primary cooling stop temperature in the temperature range of 650-800°C at an average cooling rate of 10°C/second or lower (excluding 0°C/second), a secondary cooling step of gradually cooling down from the primary cooling stop temperature to a secondary cooling stop temperature in the temperature range from a temperature of an Ms point or above to 500°C or below at an average cooling rate of 20-100°C/second, a tertiary cooling step of rapidly cooling from the secondary cooling stop temperature to room temperature at an average cooling rate of higher than 100°C/second, and an over aging treatment step of heating up to the temperature region of 150-300°C and maintaining for 30-1,500 seconds, in the order.

Description

  The present invention relates to a method for producing a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, and specifically relates to a method for producing a high-strength cold-rolled steel sheet having an excellent steel plate shape.

  In recent years, there has been an increasing demand for safety improvement of automobiles, and a lightweight and sufficient reinforcing member (for example, bumper reinforcement, door impact bar) using a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more. Etc.) are actively trying to install. It is also important that the high-strength cold-rolled steel sheet used for such applications has good workability (particularly elongation).

As the high-strength cold-rolled steel sheet, a structure strengthened using a hard low-temperature transformation structure typified by martensite is used. A high-strength cold-rolled steel sheet using a hard low-temperature transformation structure can be produced using a water quenching type continuous annealing facility. For example, it can be produced by heating and holding a steel material in a recrystallization temperature range of Ac ₁ point or higher for a short time, followed by water quenching or cooling to a predetermined temperature range, followed by water quenching and then performing an overaging treatment. .

By the way, from the viewpoint of workability in the forming line, it is necessary that the cold rolled steel sheet has a good shape. Patent Document 1 proposes a technique for reducing the flatness of a steel plate as a cold rolled steel plate shape. The cold-rolled steel sheet disclosed in this document is characterized in that the metal structure is a martensite single phase, the tensile strength is 980 MPa or more, and the flatness of the steel sheet is 10 mm or less. This cold-rolled steel sheet has an average cooling rate of 20 ° C./second or more from the soaking temperature above the Ac ₃ transformation point to the Ms point (Martensite transformation start temperature) to Ms point + 200 ° C. temperature range after cold rolling. In the above temperature range for 0.1 to 60 seconds, followed by secondary cooling to 100 ° C. or less at an average cooling rate of 100 ° C./second or more. The holding in the temperature range of Ms point to Ms point + 200 ° C. is performed in order to make the temperature in the steel plate uniform, and temperature unevenness occurs due to the difference in cooling rate in the plate thickness direction or width direction of the steel plate. And it is described that the stress in the steel sheet is not reduced and the shape of the steel sheet deteriorates. However, in order to maintain in the temperature range of Ms point to Ms point + 200 ° C., it is necessary to newly provide a salt bath, a metal bath, an induction heating device, or the like, resulting in an increase in cost. Moreover, since it hold | maintained in the predetermined | prescribed temperature range, productivity was bad.

JP 2011-202195 A

  The present invention has been made paying attention to the circumstances as described above, and its purpose is to provide a high-strength cold-rolled steel sheet excellent in steel sheet shape with less warpage when the cold-rolled steel sheet is placed on a surface plate. The object is to provide a method capable of producing with high productivity.

With the manufacturing method of the high strength cold-rolled steel plate excellent in the steel plate shape which concerns on this invention which was able to solve the said subject, C: 0.1-0.20% (meaning mass%. Hereinafter, it is the same about a component. ), Si: 0.2-2%, Mn: 1.0-3%, P: 0.05% or less (not including 0%), S: 0.01% or less (not including 0%), Satisfying Ti: 0.001-0.2%, Al: 0.01-0.1%, B: 0.0002-0.01%, and N: 0.01% or less (excluding 0%) The balance is composed of iron and inevitable impurities, and the ratio to the entire metal structure is 65 area% or more for tempered martensite, 5 area% or less for retained austenite (including 0 area%), and 20 area% or less for ferrite (0 Bainite has a high strength satisfying 10 area% or less (including 0 area%). A method for producing a rolled steel sheet. And the manufacturing method of this invention is the annealing process which heats and anneals the steel materials which satisfy | fill the said component composition in an austenite single phase area for 15 to 600 seconds, and the primary cooling stop temperature in the temperature range of 650-800 degreeC after annealing. Primary cooling step of slow cooling at an average cooling rate of 10 ° C./second or less (excluding 0 ° C./second), and a temperature of Ms point or higher calculated by the following formula (1) from the primary cooling stop temperature, 500 ° C. or lower Secondary cooling step of cooling at an average cooling rate of 20 to 100 ° C./second to a secondary cooling stop temperature in the temperature range of the above, and tertiary cooling rapidly cooling from the secondary cooling stop temperature to room temperature at an average cooling rate of over 100 ° C./second It has a gist in that it includes a process and an overaging treatment process in which heating is performed in a temperature range of 150 to 300 ° C. and holding for 30 to 1500 seconds in this order. In the following formula (1), [] means the content (% by mass) of each element.
Ms = 561-474 × [C] −33 × [Mn] −17 × [Ni] −17 × [Cr] −21 × [Mo] (1)

The steel material, as another element,
(A) Cu: 1% or less (not including 0%) and / or Ni: 1% or less (not including 0%),
(B) Cr: 1% or less (not including 0%) and / or Mo: 1% or less (not including 0%),
Etc. may be contained.

  In the present specification, “excellent in steel plate shape” means that there is little warpage when a cold-rolled steel plate is installed on a surface plate.

  According to the present invention, when a steel material satisfying a predetermined component composition is heated and annealed in an austenite single-phase region and then cooled to room temperature and then subjected to an overaging treatment, in the cooling process, the predetermined temperature region is exceeded. The cooling rate is changed in three stages. And in this invention, since it cools gradually from the austenite single phase area | region to the primary cooling stop temperature in the temperature range of 650-800 degreeC, it can cool uniformly, without producing temperature distribution in steel materials. As a result, as in the above-mentioned Patent Document 1, it is possible to maintain a temperature range of Ms point to Ms point + 200 ° C., and to provide a high steel plate shape without providing new equipment for uniformizing the temperature in the steel plate. Strength cold-rolled steel sheets can be manufactured with high productivity.

FIG. 1 is a graph showing the relationship between the rapid cooling start temperature and the warp height of the steel sheet.

  The present inventors have earnestly provided a method for producing a high-strength cold-rolled steel sheet excellent in the shape of a steel sheet with little warpage when the cold-rolled steel sheet is placed on a surface plate without making a new capital investment. I have been studying it. As a result, a steel material satisfying a predetermined component composition is heated and annealed in an austenite single phase region, and then cooled to room temperature and then subjected to an overaging treatment. In the cooling process, from the austenite single phase region to 650 to 800 ° C. After the gradual cooling to the primary cooling stop temperature in the temperature range of 10 ° C./second or less (hereinafter sometimes referred to as primary cooling), the temperature of the Ms point or higher and 500 ° C. or lower from the primary cooling stop temperature. Cooling is performed at an average cooling rate of 20 to 100 ° C./second until the secondary cooling stop temperature in the temperature range (hereinafter sometimes referred to as secondary cooling), and then the average cooling rate of 100 ° C./second from the secondary cooling stop temperature to room temperature. It is important to perform supercooling (hereinafter also referred to as tertiary cooling), and if three-stage cooling is performed in this way, the temperature is maintained within a predetermined temperature range as in Patent Document 1 described above. Found can be produced with good productivity high strength cold rolled steel sheet excellent in without steel shape providing a new equipment for equalizing the temperature in steel plate, thereby completing the present invention.

  In particular, in the present invention, it is important to gradually cool from the austenite single phase region to the primary cooling stop temperature in the temperature region of 650 to 800 ° C. By gradually cooling the temperature range, temperature unevenness in the steel material can be eliminated and the temperature distribution can be made uniform. And, by rapidly cooling the steel material having a uniform temperature distribution from the secondary cooling stop temperature to room temperature, it is possible to generate martensite while uniformly generating transformation strain associated with martensitic transformation. Even without providing a new facility for maintaining a constant temperature as in Document 1, there is no unevenness of transformation strain in the longitudinal direction of the steel sheet, the occurrence of warpage can be suppressed, and the steel sheet shape is improved.

  Hereinafter, the manufacturing method of the high intensity | strength cold-rolled steel plate which concerns on this invention is demonstrated.

  The high-strength cold-rolled steel sheet of the present invention has C: 0.1 to 0.20%, Si: 0.2 to 2%, Mn: 1.0 to 3%, P: 0.05% or less (0% Not included), S: 0.01% or less (not including 0%), Ti: 0.001 to 0.2%, Al: 0.01 to 0.1%, B: 0.0002 to 0.01 % And N: 0.01% or less (excluding 0%), the balance is made of iron and inevitable impurities, and the metal structure has tempered martensite, residual austenite, ferrite, and bainite. The ratio of the entire metal structure is 65 area% or more for the tempered martensite, 5 area% or less (including 0 area%) for the retained austenite, and 20 area% or less (including 0 area%) for the ferrite. The bainite is 10 area% or less (including 0 area%). It is those that are happy.

Such a high-strength cold-rolled steel sheet includes an annealing process in which a steel material satisfying the above-described composition is annealed by heating for 15 to 600 seconds in an austenite single-phase region, and an average cooling rate to a primary cooling stop temperature of 650 to 800 ° C. after annealing. A primary cooling step of gradual cooling at 10 ° C./second or less (not including 0 ° C./second), and a temperature not lower than the temperature of Ms point calculated by the following formula (1) from the primary cooling stop temperature and not higher than 500 ° C. A secondary cooling step of cooling at an average cooling rate of 20 to 100 ° C./second to a secondary cooling stop temperature in the region, and a tertiary cooling step of rapidly cooling from the secondary cooling stop temperature to room temperature at an average cooling rate of over 100 ° C./second; And an over-aging treatment step of heating in a temperature range of 150 to 300 ° C. and holding for 30 to 1500 seconds in this order. In the following formula (1), [] means the content (% by mass) of each element.
Ms = 561-474 × [C] −33 × [Mn] −17 × [Ni] −17 × [Cr] −21 × [Mo] (1)

  First, a procedure for manufacturing a steel material to be subjected to the annealing process will be described.

  What is necessary is just to prepare what the said steel materials manufactured according to the conventional method, for example, hot-rolling the steel piece obtained by adjusting a component so that the said component composition may be satisfied, and then performing cold rolling, and manufacturing. do it. That is, after the steel slab satisfying the above component composition is heated to, for example, 1100 to 1300 ° C., the hot rolling is performed at a finish rolling temperature (hot rolling end temperature) of, for example, 850 to 950 ° C. What is necessary is just to manufacture a hot-rolled steel plate by winding up temperature as 400-700 degreeC, for example.

  The obtained hot-rolled steel sheet may be pickled according to a conventional method to remove the oxide scale on the surface, and then cold-rolled to produce a cold-rolled steel sheet. Cold rolling may be performed at a rolling reduction (cold rolling ratio) of 30 to 80%, for example.

  The hot-rolled steel sheet is assumed to be manufactured through ordinary steelmaking, casting, and hot rolling processes, but some or all of the hot rolling process is omitted, for example, by thin casting. May be manufactured.

  Next, the annealing process will be described in order.

[Annealing process]
In the annealing step, the steel material satisfying the above component composition is annealed by soaking for 15 to 600 seconds in the austenite single phase region.

  When the annealing temperature is too low to reach the austenite single phase region, carbides in the steel material are not sufficiently dissolved or recrystallization of ferrite is not completed, and desired strength and ductility cannot be obtained. Accordingly, the annealing temperature is set to the austenite temperature range.

The austenite single-phase region is a temperature region of Ac ₃ point or higher, and the Ac ₃ point represents the component composition of the steel material and the “Leslie Steel Materials Science” (Maruzen Co., Ltd., issued May 31, 1985, p. 273) can be calculated from the following formula (a). In the following formula (a), [] indicates the content (% by mass) of each element, and the content of elements not included in the steel material may be calculated as 0% by mass.
Ac ₃ (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] −11 × [Cr] + 31.5 × [Mo] −20 × [Cu] −15 2 × [Ni] + 400 × [Ti] + 104 × [V] + 700 × [P] + 400 × [Al] (a)

  The upper limit of the annealing temperature is not particularly limited, but when it exceeds 950 ° C., the growth of austenite grains becomes remarkable, and the structure produced by subsequent cooling may be coarsened. When the structure becomes coarse, the ductility, stretch flangeability, toughness, etc. of the steel sheet may deteriorate. Accordingly, the annealing temperature is preferably 950 ° C. or less, more preferably 930 ° C. or less, and further preferably 920 ° C. or less.

  If the annealing time is less than 15 seconds, the heating time in the austenite single phase region is too short, so that the carbide in the steel material is not sufficiently dissolved or the recrystallization of ferrite is not completed, Strength and ductility may not be obtained. Therefore, the annealing time is 15 seconds or longer, preferably 30 seconds or longer, more preferably 60 seconds or longer. However, even if the annealing time is 600 seconds or more, the effect is saturated, and a large amount of energy is consumed, resulting in an increase in cost. Therefore, the annealing time is 600 seconds or less, preferably 500 seconds or less, more preferably 400 seconds or less.

[Primary cooling process]
In the primary cooling step, after annealing, annealing is performed at an average cooling rate of 10 ° C./second or less (not including 0 ° C./second) from the annealing temperature to the primary cooling stop temperature in the temperature range of 650 to 800 ° C. By gradually cooling at an average cooling rate of 10 ° C./second or less, temperature unevenness in the steel material can be reduced, and the temperature distribution in the steel plate can be made uniform. As a result, transformation strain due to martensitic transformation can be introduced uniformly in the tertiary cooling step described later. Therefore, as in Patent Document 1, a high-strength cold-rolled steel sheet having an excellent steel plate shape can be manufactured without providing a new holding facility.

  When the primary cooling stop temperature is lower than 650 ° C., ferrite is excessively generated, and a desired martensite generation amount cannot be ensured at the time of quenching, so that the strength of the steel sheet decreases. Therefore, the primary cooling stop temperature is 650 ° C. or higher, preferably 670 ° C. or higher, more preferably 680 ° C. or higher. However, when the primary cooling stop temperature exceeds 800 ° C., the effect of reducing the temperature unevenness in the steel material is hardly obtained. Therefore, even if other manufacturing conditions are appropriately controlled, the transformation strain introduced in the tertiary cooling process described later. Therefore, a high-strength cold-rolled steel sheet having a good steel plate shape cannot be produced. Therefore, the primary cooling stop temperature is 800 ° C. or lower, preferably 780 ° C. or lower, more preferably 750 ° C. or lower.

  The primary cooling stop temperature may be set, for example, in a temperature range on the low temperature side of 650 to 700 ° C. in order to improve the ductility of the steel sheet by generating ferrite and suppress the generation of ferrite to the steel sheet. When improving the intensity | strength of, it is recommended to set in the temperature range of the high temperature side of 700-800 degreeC, for example.

  If the average cooling rate up to the primary cooling stop temperature exceeds 10 ° C./second, temperature unevenness occurs in the steel material, and the temperature distribution in the steel sheet does not become uniform. Since the transformation strain introduced in the tertiary cooling step is not uniform, a high-strength cold-rolled steel sheet having a good steel sheet shape cannot be produced. Therefore, the average cooling rate in the primary cooling step is 10 ° C./second or less, preferably 8 ° C./second or less, more preferably 5 ° C./second or less.

  The lower limit value of the average cooling rate in the primary cooling step is not particularly limited as long as it does not cause a problem in productivity.

[Secondary cooling process]
In the secondary cooling step, the average cooling rate is 20 to 100 ° C./second from the primary cooling stop temperature to the secondary cooling stop temperature in the temperature range of the Ms point calculated by the above formula (1) to 500 ° C. or less. Cool with. This secondary cooling suppresses the growth of ferrite produced in the primary cooling step, and austenite is kept in a supercooled state, causing a martensitic transformation (temperature range above the Ms point and below 500 ° C.) ) And can control the metal structure appropriately.

  When the average cooling rate up to the secondary cooling stop temperature is less than 20 ° C./second, ferrite is generated and grows during cooling, and the strength of the steel sheet is reduced. Further, bainite is generated and grows during cooling, and the strength of the steel sheet varies, and a high-strength cold-rolled steel sheet with a stable material cannot be manufactured. Therefore, the average cooling rate in the secondary cooling step is 20 ° C./second or more, preferably 25 ° C./second or more, more preferably 30 ° C./second or more.

  However, when the average cooling rate in the secondary cooling step exceeds 100 ° C./second, the steel material cannot be cooled uniformly, so that temperature unevenness occurs in the steel material, and temperature distribution occurs. Therefore, since the transformation strain introduced along with the martensitic transformation is introduced non-uniformly in the tertiary cooling step described later, the steel plate shape cannot be improved. Therefore, the average cooling rate in the secondary cooling step is 100 ° C./second or less, preferably 80 ° C./second or less, more preferably 60 ° C./second or less.

  When the secondary cooling stop temperature is lower than the temperature at the Ms point, martensitic transformation occurs during secondary cooling, and martensite is generated. Since this martensite is lower in strength than martensite generated during the third cooling described later, the desired strength cannot be obtained. Accordingly, the secondary cooling stop temperature is set to the temperature of the Ms point or higher, preferably the Ms point + 5 ° C. or higher, more preferably the Ms point + 10 ° C. or higher.

  However, if the secondary cooling stop temperature exceeds 500 ° C., the amount of temperature drop of the steel material becomes too large at the time of tertiary cooling described later, so that the thermal strain introduced into the steel material at the time of cooling becomes large. Accordingly, warpage occurs and the steel plate shape cannot be improved. Therefore, the secondary cooling stop temperature is 500 ° C. or lower, preferably Ms point + 50 ° C. or lower, more preferably Ms point + 45 ° C. or lower.

  The secondary cooling may be performed by gas jet cooling in which a gas is blown onto the steel material for cooling. As the gas to be sprayed, an inert gas (for example, nitrogen gas) may be used.

[Third cooling process]
In the tertiary cooling step, rapid cooling is performed from the secondary cooling stop temperature to room temperature (27 ° C.) at an average cooling rate exceeding 100 ° C./second. By quenching this temperature range, the supercooled austenite can be transformed into martensite. At this time, in the present invention, since the temperature unevenness in the steel material is reduced and the temperature distribution in the steel material is made uniform in the primary cooling step, the transformation strain accompanying the martensitic transformation can be uniformly introduced into the steel material. As a result, since the material of the steel material is uniform, warpage is unlikely to occur and the steel plate shape is good. Moreover, the strength of the steel sheet can be increased by forming martensite. The average cooling rate may be a rate at which a martensite structure is obtained, and is preferably 150 ° C./second or more, more preferably 200 ° C./second or more. In addition, the upper limit of the average cooling rate in a tertiary cooling process is not specifically limited.

  The quenching cooling method may be any method such as water quenching, water cooling roll cooling, air-water cooling, and gas jet cooling. For example, in the case of water quenching in which a steel material is immersed in a water tank, it is only necessary to spray jet water onto the steel material from a nozzle immersed in the water tank, and the average cooling rate of the steel material can be controlled by adjusting the amount of jet water to be sprayed. . In addition, the cooling rate can be controlled by changing the above-described cooling method.

[Overaging process]
In the overaging treatment step, after cooling to room temperature in the tertiary cooling step, heating to a temperature range of 150 to 300 ° C. and holding for 30 to 1500 seconds, an overaging treatment (low temperature tempering treatment) is performed. By holding for a predetermined time in this temperature range, martensite can be tempered, and a steel material that has a lot of dissolved C and is thermally unstable can be stabilized. That is, when a large amount of solute C is present, it is thermally unstable, so that the solute C forms carbides and precipitates during storage at room temperature for a long time, and the steel sheet shape changes, It causes the mechanical properties to change. Therefore, in the present invention, it is necessary to perform overaging treatment.

  If the holding temperature is less than 150 ° C., the overaging treatment becomes insufficient, and the thermally unstable steel material cannot be stabilized. Accordingly, the holding temperature is 150 ° C. or higher, preferably 160 ° C. or higher, more preferably 170 ° C. or higher. However, when the holding temperature exceeds 300 ° C., martensite is softened, and the strength of the steel material is rapidly reduced. Accordingly, the holding temperature is 300 ° C. or lower, preferably 250 ° C. or lower, more preferably 230 ° C. or lower.

  If the holding time is less than 30 seconds, the overaging treatment becomes insufficient, and the thermally unstable steel material cannot be stabilized. Accordingly, the holding time is 30 seconds or longer, preferably 60 seconds or longer, more preferably 120 seconds or longer, and even more preferably 180 seconds or longer. However, even if the holding time exceeds 1500 seconds, the effect is saturated and productivity is only lowered. Accordingly, the holding time is set to 1500 seconds or shorter, preferably 1200 seconds or shorter, more preferably 900 seconds or shorter, and even more preferably 600 seconds or shorter.

[Others]
According to the production method of the present invention, it is not necessary to perform temper rolling for the purpose of correcting the shape of the steel material after the overaging treatment, but in order to adjust the surface roughness of the steel material or to adjust the material of the steel material Of course, temper rolling may be performed if necessary.

  Next, the component composition and metal structure of the high-strength cold-rolled steel sheet according to the present invention obtained by the above production method will be described in detail.

<Ingredient composition>
The high-strength cold-rolled steel sheet of the present invention contains C, Si, Mn, P, S, Ti, Al, B, and N in the ranges shown below.

[C: 0.1 to 0.20%]
C is an element indispensable for increasing the strength of the steel sheet. When the C content is less than 0.1%, it is difficult to ensure both the strength of the steel sheet and the ductility. Therefore, the C content is 0.1% or more, preferably 0.115% or more, more preferably 0.120% or more. However, if the amount of C exceeds 0.20% and becomes excessive, the tensile strength becomes too high, and the steel plate shape cannot be improved. Moreover, since tensile strength becomes high too much, elongation falls. On the other hand, when the amount of C is excessive, the welded part and the heat-affected part are markedly cured, and the weldability deteriorates. Therefore, the C content is 0.20% or less, preferably 0.18% or less, more preferably 0.17% or less.

[Si: 0.2-2%]
Si is an element that acts on the solid solution strengthening of ferrite, and is an element that acts to secure the hardness of the ferrite and increase the elongation of the steel sheet. Therefore, Si is 0.2% or more, preferably 0.3% or more, more preferably 0.4% or more. However, when Si is contained excessively, the occurrence of red scale or the like causes deterioration of surface properties, deterioration of plating adhesion, and deterioration of plating adhesion. Therefore, the Si amount is 2% or less, preferably 1.5% or less, more preferably 1.4% or less.

[Mn: 1.0 to 3%]
Mn is an element that acts on the strengthening of the steel sheet. Further, it is an element necessary for securing the amount of tempered martensite that is a hard phase. Therefore, the amount of Mn is 1.0% or more, preferably 1.3% or more, more preferably 1.50% or more, and still more preferably 1.7% or more. However, if the amount of Mn exceeds 3% and excessively contained, the productivity is lowered, for example, the castability is deteriorated. Accordingly, the Mn content is 3% or less, preferably 2.7% or less, more preferably 2.5% or less.

[P: 0.05% or less (excluding 0%)]
P is an element that acts to reinforce the steel sheet and increase the elongation, but if contained excessively, it causes embrittlement by grain boundary segregation and degrades impact characteristics. Therefore, the P content is 0.05% or less, preferably 0.03% or less, more preferably 0.01% or less.

[S: 0.01% or less (excluding 0%)]
S is an element that is unavoidably contained, and forms sulfide-based inclusions such as MnS to deteriorate the impact resistance or cause cracking along the metal flow of the welded portion, so that it is reduced as much as possible. There is a need. Therefore, considering the manufacturing cost, in the present invention, it is 0.01% or less, preferably 0.007% or less, more preferably 0.005% or less.

[Ti: 0.001 to 0.2%]
Ti is an element that exhibits finer crystal grains and an effect of suppressing grain growth by forming fine carbides and nitrides. Further, fine carbides and nitrides of Ti act as trap sites for trapping diffusible hydrogen inside the steel sheet, thereby reducing the sensitivity to hydrogen embrittlement of the steel sheet. Further, fine carbides and nitrides of Ti act to densify the generated rust and improve the corrosion resistance. Therefore, Ti is 0.001% or more, preferably 0.01% or more, more preferably 0.03% or more. However, when Ti is contained excessively, the carbide is coarsened and the strength is lowered. Therefore, the Ti content is 0.2% or less, preferably 0.15% or less, more preferably 0.1% or less.

[Al: 0.01 to 0.1%]
Al is an element that acts as a deoxidizer, and in the present invention, it is necessary to contain 0.01% or more. Preferably it is 0.02% or more, More preferably, it is 0.03% or more. However, when Al is contained excessively, many inclusions such as alumina are generated in the steel sheet, and the workability of the steel sheet deteriorates. Therefore, the Al content is 0.1% or less, preferably 0.09% or less, more preferably 0.08% or less.

[B: 0.0002 to 0.01%]
B is an element having an action of suppressing the formation and growth of ferrite from the austenite grain boundary, and is an element necessary and important for improving the steel plate shape. Therefore, the B content is 0.0002% or more, preferably 0.0005% or more, more preferably 0.0010% or more. However, if it contains B excessively and exceeds 0.01%, workability deteriorates. Therefore, the B content is 0.01% or less, preferably 0.007% or less, more preferably 0.005% or less.

[N: 0.01% or less (excluding 0%)]
N is an element that is inevitably contained, and when it is excessively contained, nitride is formed and the workability is deteriorated. In particular, when BN precipitates are formed by combining with B in the steel sheet, the effect of suppressing the formation of ferrite from the austenite grain boundary is not sufficiently exhibited, so that the shape of the steel sheet cannot be improved. Further, when BN precipitates are formed, the effect of improving hardenability by B is hindered. Therefore, N is 0.01% or less, preferably 0.008% or less, more preferably 0.006% or less.

  The component composition of the high-strength cold-rolled steel sheet according to the present invention is as described above, and the balance is inevitable impurities other than iron and S and N.

  The high-strength cold-rolled steel sheet of the present invention may further contain other elements such as (a) Cu and / or Ni, (b) Cr and / or Mo as other elements.

[(A) Cu: 1% or less (excluding 0%) and / or Ni: 1% (excluding 0%)]
Cu and Ni are elements having an effect of increasing the strength of the steel sheet. In order to exhibit such an action effectively, Cu is preferably contained in an amount of 0.05% or more, more preferably 0.08% or more, and further preferably 0.1% or more. Ni is preferably contained in an amount of 0.05% or more, more preferably 0.08% or more, and further preferably 0.1% or more. However, if the amount of Cu exceeds 1%, it may be easy to generate surface flaws during hot rolling, resulting in poor productivity and poor workability of the steel sheet. Accordingly, the Cu content is preferably 1% or less, more preferably 0.9% or less, and still more preferably 0.8% or less. On the other hand, if the Ni content exceeds 1%, the workability of the steel sheet may deteriorate. Therefore, the Ni content is preferably 1% or less, more preferably 0.9% or less, and still more preferably 0.8% or less. In addition, when Cu is contained alone, there is a concern that hot embrittlement may occur, so it is recommended to use it together with Ni. Since Ni is an expensive element, it is recommended to add it only when it is necessary to strengthen the steel sheet.

[(B) Cr: 1% or less (excluding 0%) and / or Mo: 1% or less (excluding 0%)]
Cr and Mo are elements that act to increase the strength of the steel sheet. Moreover, Cr and Mo also have the effect | action which suppresses the production | generation of the carbide | carbonized_material which degrades the balance of intensity | strength and ductility. In particular, Mo also acts to prevent softening of the weld heat affected zone. In order to effectively exhibit such an action, Cr is preferably contained in an amount of 0.005% or more, more preferably 0.05% or more, and further preferably 0.1% or more. Mo is preferably contained in an amount of 0.005% or more, more preferably 0.01% or more, and still more preferably 0.05% or more. However, when it contains excessively, the ductility of a steel plate will be deteriorated. Therefore, Cr is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. Mo is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. Cr and Mo may be contained alone or in combination. For example, the total amount when Cr and Mo are used in combination is preferably 1.5% or less, and more preferably 1% or less.

<Metallic structure>
The high-strength cold-rolled steel sheet of the present invention has tempered martensite, residual austenite (hereinafter sometimes referred to as residual γ), ferrite, and bainite, and the ratio to the entire metal structure is as follows. It is.

[Tempered martensite: 65 area% or more]
The tempered martensite means a tempered structure formed by heating and tempering after completion of the martensite transformation by water quenching. The said tempered martensite is a structure | tissue required in order to improve a steel plate shape and to stabilize a characteristic. Moreover, tempered martensite is a hard phase and contributes to high strength of the steel sheet. Accordingly, the tempered martensite is 65 area% or more, preferably 75 area% or more, more preferably 85 area% or more, particularly preferably 90.0 area% or more, most preferably 100 area%, based on the entire metal structure. is there.

[Residual austenite (residual γ): 5 area% or less (including 0 area%)]
Residual γ is transformed into hard martensite at the time of forming and reduces the stretch flangeability of the steel sheet. Therefore, it is preferable to reduce the residual γ as much as possible, and it is acceptable up to 5 area%. Preferably it is 3 area% or less, More preferably, it is 2.5 area% or less, Most preferably, it is 0 area%.

[Ferrite: 20 area% or less (including 0 area%)]
In order to improve the steel sheet shape, it is necessary to reduce ferrite as much as possible and increase the amount of tempered martensite produced. Moreover, when ferrite produces | generates excessively, the production | generation amount of the tempered bainite which is a hard phase cannot be ensured, but the intensity | strength of a steel plate will fall. Accordingly, ferrite needs to be 20 area% or less, preferably 15 area% or less, more preferably 13.0 area% or less, and most preferably 0 area%.

  In addition, ferrite has the effect | action which raises the elongation of a steel plate and improves workability. Therefore, when improvement in workability is required, ferrite may be positively contained, preferably 1 area% or more, more preferably 3 area% or more, still more preferably 5 area% or more, particularly preferably. 10 area% or more.

[Bainite: 10 area% or less (including 0 area%)]
Bainite is a hard phase that contributes to increasing the strength of the steel sheet, like the tempered martensite. However, the characteristics vary greatly depending on the temperature range where bainite is generated, which may cause variations in material. Therefore, it is recommended to reduce bainite as much as possible, and up to 10 area% is acceptable. Preferably it is 5 area% or less, More preferably, it is 3.0 area% or less, Most preferably, it is 0 area%.

  For the structural fractions of the above tempered martensite, ferrite, and bainite, the cross-section parallel to the rolling direction is exposed, mirror-polished, and then subjected to corrosion by nital, and metal using an optical microscope or scanning electron microscope The calculation may be performed by observing the tissue, taking a photograph, performing image analysis, and the like.

  The structural fraction of the residual austenite may be measured by an X-ray diffraction method using a Cr tube.

  The high-strength cold-rolled steel sheet of the present invention is used, for example, for parts having impact absorption capability such as bumper reinforcement and reinforcing members such as door impact bars.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steel was melted by vacuum melting to produce steel pieces having the composition shown in Table 1 below (the balance being iron, inevitable impurities other than S and N). The temperature at the Ms point calculated by the above formula (1) based on the component composition shown in Table 1 below is also shown in Table 1 below. The obtained steel slab was heated to 1250 ° C., hot rolled at a finish rolling temperature of 900 ° C., and then wound at 650 ° C. to produce a hot-rolled steel sheet having a thickness of 2.8 mm. The obtained hot-rolled steel sheet was pickled and cold-rolled to produce a cold-rolled steel sheet (steel material) having a thickness of 1.4 mm.

  Next, the obtained steel material was heat-treated in a water-quenching type heat treatment facility. As the heat treatment equipment, a steel plate heat treatment simulator (model number: CCT-AQV) manufactured by ULVAC-RIKO Inc. was used. A test piece obtained by cutting the steel material into a thickness of 1.4 mm, a width of 150 mm, and a length of 250 mm was used for the heat treatment. The heat treatment was carried out with the thermal pattern shown in Table 2 below by placing the test piece on a dedicated jig. That is, after heating the said test piece to 920 degreeC of annealing temperature and hold | maintaining at this temperature for the time shown in following Table 2, it shows in following Table 2 by the average cooling rate (degreeC / sec) shown in following Table 2 as primary cooling. Cooled to the primary cooling stop temperature (° C.), and from this primary cooling stop temperature, the secondary cooling stop temperature (° C.) shown in the following Table 2 at the average cooling rate (° C./second) shown in the following Table 2 as the secondary cooling. After cooling to room temperature, jet water was sprayed on the test piece, and water quenching was performed to room temperature (27 ° C.) at an average cooling rate (° C./second) shown in Table 2 below to perform tertiary cooling.

  In addition, No. shown in Table 2 below. Nos. 7, 20, and 26 were annealed and then cooled to the primary cooling stop temperature, and were then quenched from this temperature to the room temperature by water quenching under the same conditions as described above. This final cooling is called tertiary cooling for convenience.

  No. shown in Table 2 below. After annealing, Nos. 8, 21, and 27 were subjected to water quenching from this temperature under the same conditions as described above, and finally cooled to room temperature. This final cooling is called tertiary cooling for convenience.

  In Table 2 below, for convenience of explanation, temperatures at the time of starting the tertiary cooling (final cooling) (rapid cooling start temperature) are collectively shown.

  After cooling to room temperature, the mixture was heated to the tempering temperature shown in Table 2 below, and maintained at this temperature for the time shown in Table 2 below to perform an overaging treatment (low temperature tempering treatment). About the test piece which performed the overaging process, the metal structure, the tensile characteristic, and the steel plate shape were evaluated.

<Metallic structure>
For the structural fraction of tempered martensite, ferrite, and bainite, a cross section parallel to the rolling direction of the test piece subjected to the tempering treatment was exposed, mirror-polished, and then subjected to corrosion by nital, an optical microscope or A metal structure at a 1/4 position with respect to the plate thickness was observed using a scanning electron microscope, photographed, and image analysis was performed for calculation. The ratio of each tissue was determined by binarizing a photograph taken using an image analysis apparatus.

  The structural fraction of retained austenite was measured using a RINT 1500 X-ray diffraction measurement apparatus manufactured by Rigaku. The measurement was performed using a surface obtained by electropolishing a surface obtained by reducing the thickness of the steel sheet to ¼ thickness in the thickness direction. Using Co Kα rays as a measurement source and diffraction peaks of α-Fe (110), (200), (211) and γ-Fe by X-ray diffraction at a scan rate of 1.2 ° / min. Measure the integrated intensity of the peaks (111), (200), (220), and (311) planes, calculate the area ratio of residual γ for each combination from the obtained integrated intensity of each plane, and calculate the average value. The amount of residual γ was taken.

  The fraction (area%) of each structure with respect to the entire metal structure is shown in Table 3 below.

<Tensile properties>
A JIS No. 5 tensile test piece was cut out so that the direction perpendicular to the rolling direction of the test piece subjected to overaging treatment was the longitudinal direction, and 0.2% proof stress (YS), tensile based on JIS Z2241 Strength (TS) and elongation at break (EL) were measured. Moreover, the yield ratio (YR) was calculated based on YS and TS. The results are shown in Table 3 below. In the present invention, the case where YS is 900 MPa or more is passed, the case where it is less than 900 MPa is rejected, the case where TS is 980 MPa or more is passed, the case where it is less than 980 MPa is rejected, and the case where EL is 8.5% or more The case where it passed and less than 8.5% was determined to be unacceptable. In this invention, the case where both YS and TS were determined to be passed was evaluated as “high strength”, and the case where EL was determined to be passed was evaluated as “excellent in workability”.

<Steel shape>
The steel plate shape was evaluated based on the warp height measured in the same manner as in FIG. 1 of Patent Document 2 except that the size of the test piece was changed. The warp height was measured using a contact displacement meter in which a tempered test piece was placed on a surface plate so that the warp was up, and the stylus moved over the object to be measured. Specifically, the shape of the steel sheet was continuously measured at the center position in the width direction and at a position 25 mm away from both ends in the width direction, and the maximum value from the surface plate surface was measured as the warp height. The measurement was performed over the entire length of the test piece (thickness 1.4 mm × width 150 mm × length 250 mm). The measurement results are shown in Table 3 below. In the present invention, the case where the warp height is 3 mm or less and high flatness is determined to be acceptable, and the case where the warp height exceeds 3 mm and the flatness is low is determined to be unacceptable. In the present invention, a material whose warp height was determined to be acceptable was evaluated as “excellent in steel plate shape”.

  In this example, a case where all of YS, TS, EL, and warp height were determined to be acceptable was evaluated as an invention example, and a case where even one was determined to be unacceptable was evaluated as a comparative example.

  The following Table 1 to Table 3 can be considered as follows. No. 1-4, 9, 12, 13, 15-18, 22, 24, 28, 30, 40, 41, 43-45, 47, 48, 50 all satisfy the requirements defined in the present invention. It can be seen that a high-strength cold-rolled steel sheet excellent in steel sheet shape can be produced with high productivity.

  On the other hand, no. 5, 10, 14, 19, 42, 46, 49, 51 are examples in which the secondary cooling stop temperature is too high, and since a large amount of thermal strain was introduced in the tertiary cooling process, the warp height increased, and the steel plate shape Could not be improved. No. 6 and no. No. 11 is an example in which the secondary cooling stop temperature is too low. Since bainite was excessively generated, YS was too low. No. Nos. 7, 20, and 26 are examples of water quenching as they were from the primary stop temperature, and since the rapid cooling start temperature was too high, the thermal strain increased, the warp height increased, and the steel plate shape could not be improved.

  No. Nos. 8, 21, and 27 are examples in which water annealing was performed as it was after annealing by holding at 920 ° C. for 180 seconds. Since the steel material was cooled with the temperature distribution generated, transformation strain accompanying martensitic transformation was introduced non-uniformly. Further, since water quenching was performed from 920 ° C., the thermal strain also increased. Therefore, the warp height was increased and the steel plate shape could not be improved. No. 23, 25, 29, and 31 are examples in which the cooling rate in the secondary cooling step is too small and the secondary cooling stop temperature is too high. Therefore, since a large amount of thermal strain was introduced in the tertiary cooling process, the warp height increased and the steel plate shape could not be improved.

  No. 32 to 39 are examples in which the composition of the steel slab does not satisfy the requirements defined in the present invention. No. No. 32 is an example in which the amount of B is too small. Since ferrite was generated excessively, the amount of tempered martensite generated could not be secured, and YS and TS decreased. No. No. 33 is an example in which the amount of B is too small and the secondary cooling stop temperature is too high. Since ferrite was excessively generated, YS and TS were lowered and the steel plate shape could not be improved.

  No. No. 34 is an example in which the amount of C is too much, and since TS became too high, the steel plate shape could not be improved. Moreover, since TS became too high, EL became low. No. No. 35 is an example in which the amount of C is too much and the secondary cooling stop temperature is too high. TS was too high, so the steel plate shape could not be improved. Moreover, since TS became too high, EL became low.

  No. 36 and no. 37 is an example in which the amount of Si is too small, and the EL was low. In particular, no. In No. 37, since the secondary cooling rate was too low and the secondary cooling stop temperature was too high, a large amount of thermal strain was introduced in the tertiary cooling step, and the steel plate shape could not be improved.

  No. 38 and no. No. 39 is an example in which the amount of Mn is too small. Since ferrite was excessively generated, the amount of tempered martensite generated could not be ensured, and TS decreased. In particular, no. In No. 39, since the secondary cooling stop temperature was too high, bainite was generated excessively, the warp height was increased, and the steel plate shape could not be improved.

  Next, the graph which shows the relationship between the rapid cooling start temperature in a tertiary cooling process, and curvature height is shown in FIG. In FIG. 1, No. 1 shown in Table 3 below is shown. Only data of 1 to 31 and 40 to 51 are plotted, and examples (No. 32-39) in which the composition of the steel slab does not satisfy the requirements defined in the present invention are not plotted. As is clear from FIG. 1, it can be seen that by setting the rapid cooling start temperature in the tertiary cooling step to 500 ° C. or less, the warp height can be 3 mm or less, and the steel plate shape can be improved.

Claims (3)

C: 0.1 to 0.20% (meaning mass%, hereinafter the same for the components),
Si: 0.2-2%
Mn: 1.0-3%
P: 0.05% or less (excluding 0%),
S: 0.01% or less (excluding 0%),
Ti: 0.001 to 0.2%,
Al: 0.01 to 0.1%,
B: 0.0002 to 0.01%, and N: 0.01% or less (not including 0%),
The balance consists of iron and inevitable impurities,
The ratio to the whole metal structure is
Tempered martensite is more than 65 area%,
Residual austenite is 5 area% or less (including 0 area%),
Ferrite is 20 area% or less (including 0 area%),
Bainite is a method for producing a high-strength cold-rolled steel sheet that satisfies 10 area% or less (including 0 area%),
An annealing process in which a steel material satisfying the above component composition is annealed by heating for 15 to 600 seconds in an austenite single phase region;
After the annealing, a primary cooling step of gradually cooling at an average cooling rate of 10 ° C./second or less (not including 0 ° C./second) to a primary cooling stop temperature in a temperature range of 650 to 800 ° C .;
A secondary cooling step of cooling at an average cooling rate of 20 to 100 ° C./second from the primary cooling stop temperature to the secondary cooling stop temperature in the temperature range of Ms point or higher and 500 ° C. or lower calculated by the following formula (1). When,
A tertiary cooling step of rapidly cooling from the secondary cooling stop temperature to room temperature at an average cooling rate exceeding 100 ° C./second;
The manufacturing method of the high strength cold-rolled steel plate excellent in the steel plate shape characterized by including the overaging process process heated to a 150-300 degreeC temperature range, and hold | maintaining for 30-1500 seconds in this order.
Ms = 561-474 × [C] −33 × [Mn] −17 × [Ni] −17 × [Cr] −21 × [Mo] (1)
[In Formula (1), [] means content (mass%) of each element. ] The steel material, as another element,
The manufacturing method according to claim 1, containing Cu: 1% or less (excluding 0%) and / or Ni: 1% or less (excluding 0%). The steel material, as another element,
The production method according to claim 1 or 2, comprising Cr: 1% or less (excluding 0%) and / or Mo: 1% or less (excluding 0%).

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