JP2009270126A - Cold rolled steel sheet, hot dip plated steel sheet and method for producing the steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength cold rolled steel sheet and a hot dip plated steel sheet, which attain compatibility between ductility and bendability. <P>SOLUTION: The steel sheet has a chemical composition comprising, by mass, 0.08 to 0.25% C, ≤0.7% Si, 1.0 to 2.6% Mn, ≤1.5% Al, ≤0.02% P, ≤0.02% S and ≤0.01% N, wherein the relation between Si and Al satisfies 1.0%≤Si+Al≤1.8%, and the balance Fe with impurities, and preferably has a steel structure including a retained γ phase of ≥5 vol.%, and in which, regarding the region of a depth from the surface to 0.1 mm directly below the surface, the difference between the maximum hardness and the minimum hardness is ≤10 in Vickers hardness. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a cold-rolled steel sheet and a hot-dip plated steel sheet that are excellent in formability, specifically ductility and bendability, among high-tensile steel sheets mainly used as members for automobiles, buildings, electrical equipment, and the like, and The present invention relates to a method for manufacturing these steel plates.

  In recent years, in the technical field of automobiles, development for ensuring collision safety while reducing the weight of a vehicle body has been actively performed. If the steel plate thickness is reduced to reduce the weight of the vehicle body, a soft steel plate with good formability cannot maintain safety. For this reason, the demand for high-strength steel sheets with increased strength is increasing.

  However, when the strength is increased, generally, a decrease in ductility and a decrease in bendability tend to occur. For this reason, in the conventional high-tensile steel sheet, cracks are likely to occur during part processing, and the degree of freedom in part design cannot be increased. Therefore, there is a need for a steel sheet that is high in strength and excellent in ductility and bendability.

  Here, increasing the Si content in the steel is very effective from the viewpoint of securing a good strength-ductility balance, and further contains a large amount of the ferrite-forming element Al and the austenite-forming element Mn. Thus, development of a high ductility, high-tensile steel sheet using the TRIP effect of retained austenite has been carried out.

  A steel sheet which is intended to improve ductility by such a TRIP effect ensures high ductility by generating martensite in plastic working. However, since the generated martensite becomes an obstacle in bending, it is common that such a steel sheet cannot provide good bendability. For this reason, high-strength steel sheets that have both bendability and ductility have not been developed so far, despite the fact that bendability is one of the important characteristics, centering on automobile undercarriage parts and reinforcement parts. .

  For example, Patent Document 1 discloses a residual γ-type high-strength steel sheet of Si and Al addition type, and the examples disclose tensile strength TS (MPa) × total elongation El (%). No mention is made of the bendability important for processing, and it is unclear whether it is excellent in part workability.

  On the other hand, in response to market demands for improved corrosion resistance and appearance, the use of surface-treated steel sheets for members has been progressing. Currently, plated steel sheets with plated coatings formed by hot dipping are used for many parts. ing. However, in a normal hot dip galvanizing process, when a steel sheet with a high Si content is used as a base material, the oxide of Si is concentrated on the steel sheet surface even under a normal reducing atmosphere, resulting in a decrease in plating wettability. And as a result, the plating adhesion is lowered. Therefore, since there is a concern about deterioration of plating adhesion in ordinary Si-containing steel, application to applications requiring high corrosion resistance has not progressed. On the other hand, the demand for corrosion resistance has become stricter year by year, as seen in the movement of rust prevention of car bodies in Europe for 20 years, and the development of high-tensile hot-dip galvanized steel sheets with sufficient corrosion resistance is urgently desired. .

In order to meet such demands, high-strength steel sheets including a viewpoint of plating adhesion have been studied so far, but there is no example that considers bendability.
For example, Patent Document 2 discloses the relationship between hot-dip galvanizing manufacturing conditions and characteristics, but the hot-dip galvanizing manufacturing conditions for improvement of TS × El have been studied in the same manner as in the above-mentioned patent literature. However, no study has been made from the viewpoint of improving the bendability.
JP 2003-105486 A JP-A-11-131145

  As described above, no technology has yet been disclosed that can efficiently solve the problem of achieving both good formability, particularly ductility and bendability, while having high strength, and promotes the application of high-tensile steel sheets. In doing so, there was a need to resolve these issues.

  An object of the present invention is to solve this problem and provide a cold-rolled steel sheet and a hot-dip steel sheet having a tensile strength of 590 MPa or more, which are excellent in both formability, particularly ductility and bendability, and a method for producing these steel sheets. .

  In order to solve the above problems, the present inventors have investigated in detail the influence of the chemical composition and production conditions on the material of the steel sheet, and as a result, have obtained the following knowledge.

  (A) In order to realize high ductility due to the TRIP effect, the ratio of the residual γ phase in the steel sheet needs to be 5% by volume or more.

  (B) In order to realize a residual γ amount of 5% by volume or more, it is necessary to control the sum of the contents of Si and Al within a predetermined range in the chemical composition of the steel sheet.

(C) Furthermore, a steel sheet that satisfies the following two conditions can achieve both good ductility and bendability:
TS × El ≧ 17500 (TS: tensile strength (MPa), El: total elongation (%)),
ρ ≦ 1.5 × t (ρ: critical bending radius (mm), t: plate thickness (mm)).

  (D) Since the phase having different hardness or the interface of the structure becomes a crack starting point, it is possible to reduce the critical bending radius by suppressing the hardness variation of the structure.

  (E) In order to stabilize the residual γ phase, the main phase (refers to the phase or structure having the largest volume ratio in the composite structure) is ferrite, and the second phase (phase and structure other than the main phase described above) is referred to. ) Is preferably bainite, and in this case, the critical bending radius can be reduced by reducing the hardness of bainite.

(F) Particularly important control temperatures in the method of stably producing a steel sheet having the above characteristics are as follows:
A) Finishing temperature,
B) Winding temperature,
C) Soaking temperature,
D) Alloying temperature.

Based on the above findings, the inventors have completed the following invention.
(1) By mass%, C: 0.08 to 0.25%, Si: 0.7% or less, Mn: 1.0 to 2.6%, Al: 1.5% or less, P: 0.03 %, S: 0.02% or less and N: 0.01% or less, and the relationship between Si and Al satisfies 1.0% ≦ Si + Al ≦ 1.8%, and the balance Fe and impurities TS ≧ 590 (TS: tensile strength (MPa)), TS × El ≧ 17500 (El: total elongation (%)), and ρ ≦ 1.5 × t (ρ: critical bending radius) (mm), t: Cold-rolled steel sheet having mechanical properties satisfying sheet thickness (mm).

  Here, the measurement of the limit bending radius is performed as follows. After bending to 170 ° by the bending method described in JIS Z2248 (1996), 180 ° bending and close contact bending are performed at a plurality of different bending radii, and the surface outside the bent portion is observed. . The minimum bending radius at which no cracks and necking (necking) are observed is taken as the limit bending radius. In addition, said "necking" means what is accompanied by the local thickness reduction of 20% or more compared with the surrounding thickness.

  (2) It has a steel structure containing 5% by volume or more of the residual γ phase, and the difference between the maximum hardness and the minimum hardness is 10 or less in terms of Vickers hardness in a region from the surface to a depth of 0.1 mm immediately below the surface. The cold-rolled steel sheet according to (1), which has a hardness distribution.

Here, regarding the cross section from the surface in the plate thickness direction of the steel plate to the 1/4 plate thickness depth position, the difference between the maximum hardness and the minimum hardness is preferably 10 or less in terms of Vickers hardness.
The residual γ amount is preferably 20% by volume or less, particularly preferably 15% by volume or less.

  Furthermore, as a steel structure, it is preferable that the main phase is ferrite and the second phase is bainite and residual γ phase.

  (3) The chemical composition (1) or (1) above, wherein the chemical composition contains one or two of Ni: 1.0% or less and Cu: 0.5% or less in mass% instead of a part of Fe. The cold-rolled steel sheet according to (2).

  (4) The chemical composition is selected from the group consisting of Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.2% or less in mass% instead of part of Fe. The cold-rolled steel sheet according to any one of (1) to (3), comprising seeds or two or more kinds.

  (5) The chemical composition is mass% in place of part of Fe, Co: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.01% The cold-rolled steel sheet according to any one of (1) to (4), containing one or more selected from the group consisting of:

  (6) The cold-rolled steel sheet according to any one of (1) to (5), wherein the chemical composition contains Ca: 0.01% or less in mass% instead of part of Fe.

  (7) On the surface of the cold-rolled steel sheet according to any one of (1) to (6) above, zinc is added to the surface of the cold-rolled steel sheet having a chemical composition in which Si of the chemical composition is mass% and Si: 0.6% or less. A hot-dip galvanized steel sheet comprising a hot-dip plated layer.

  Here, the “hot-dipped layer containing zinc” may be a hot-dipped layer made only of zinc, or may be a hot-dipped layer made of an alloy containing, for example, aluminum in addition to zinc. Moreover, the alloying process with a steel plate structural element may be performed after the hot dipping process. Thus, the “hot-dipped steel sheet” includes one in which an alloy containing zinc is plated during the hot-dipping process and one in which plating is alloyed after the hot-dipping process. Moreover, the case where content of zinc in a hot dipping layer is 50 mass% or less is included.

  (8) The hot-dip galvanized steel sheet according to the above (7), wherein the hot-dip plated layer is an alloyed hot-dip galvanized layer.

(9) A slab having the chemical composition described in any one of (1) to (6) above, finishing temperature: Ar ₃ points to (Ar ₃ points + 80 ° C.), coiling temperature: heat of 450 to 680 ° C. Hot-rolled steel sheet is subjected to hot rolling, the hot-rolled steel sheet is pickled and cold-rolled to be cold-rolled steel sheet, and the cold-rolled steel sheet is held in a two-phase coexisting temperature range for 30 seconds or more, then A method for producing a cold-rolled steel sheet, wherein the steel sheet is cooled to a temperature range of 350 to 600 ° C. at a cooling rate of 3 ° C./s or more, and is subjected to a continuous annealing treatment for 5 seconds or more.

Here, the “two-phase coexistence temperature range” is a temperature range from Ac ₁ point to Ac ₃ point where two phases of ferrite (α) / austenite (γ) coexist.
In addition, it is good also considering the cooling rate in the cooling after hold | maintaining in the said two-phase coexistence temperature range as 5-30 degrees C / s. Moreover, you may additionally provide the conditions which make the start temperature of finishing rolling 1050 degrees C or less.

(10) To a slab having the chemical composition described in any one of (1) to (6) above, a finishing temperature: Ar ₃ points to (Ar ₃ points + 80 ° C.), a coiling temperature: 450 to 680 ° C. Hot-rolled steel sheet is subjected to hot rolling, the hot-rolled steel sheet is pickled and cold-rolled to be cold-rolled steel sheet, and the cold-rolled steel sheet is held in a two-phase coexisting temperature range for 30 seconds or more, then It is cooled to a temperature range of 450 to 600 ° C. at a cooling rate of 3 ° C./s or more, maintained in the temperature range for 5 seconds or more, and further subjected to continuous hot dipping treatment to form a hot dipping layer containing zinc. A method for producing a hot-dip galvanized steel sheet.

  (11) The method for producing a hot-dip galvanized steel sheet according to (10), wherein the hot dip galvanized steel sheet obtained by the continuous hot dip galvanizing process is subjected to an alloying process at an alloying temperature of 580 ° C. or lower. .

  The lower limit of the alloying treatment temperature is determined by the plating composition, the steel plate composition, the target alloy composition, and further the economy, and for example, 450 ° C. can be mentioned.

  According to the present invention, a cold-rolled steel sheet and a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and having particularly good ductility and bendability excellent in formability are provided. This steel sheet is suitable as a material for members for automobiles, buildings, electrical equipment, etc., particularly structural members around automobile suspensions and members for reinforcing equipment, and is extremely useful in each technical field.

  The best mode of the present invention, the range of manufacturing conditions, and the reasons for setting them will be described below. In the present specification, “%” indicating a chemical composition is “% by mass” unless otherwise specified.

1. Chemical Composition First, the chemical composition of the steel according to this embodiment will be described.
C: The steel according to the present embodiment improves the strength-ductility balance by containing C and generating a residual γ phase. The C content may be adjusted according to the target strength, but in order to achieve the target tensile strength of 590 MPa or more, the steel according to the present embodiment needs to be at least 0.08% or more. is there. Preferably it is 0.10% or more. On the other hand, the upper limit is assumed to be 0.25% or less from the viewpoint of spot weldability because it is assumed that automobile undercarriage parts and reinforcing equipment are typical uses of the steel according to the present embodiment. Preferably it is 0.20% or less.

  Si: Si is a ferrite forming element, and is an element effective for concentrating C in austenite and adjusting the stability of austenite to achieve high ductility due to the TRIP effect. However, since it is easily segregated on the steel sheet surface layer and is also an easily oxidizable element, when the content is large, the surface properties after pickling are lowered. Therefore, the upper limit is made 0.7% or less. Moreover, when performing hot dipping, since the wettability of the plating is extremely lowered, the upper limit is made 0.6% or less.

Al: Al, like Si, is a ferrite-forming element, and is an important element for realizing high ductility by the TRIP effect by concentrating C in austenite and adjusting the stability of austenite. However, even if contained in excess, the effect is saturated, and Al ₂ O ₃ precipitates as inclusions at the molten metal interface during DC butt welding or the like, and the welding strength is significantly reduced. Therefore, the upper limit is 1.5% or less.

  Sum of Si and Al: As described above, Si and Al are ferrite forming elements, and are essential elements for stabilizing the residual γ phase. In order to make it more than%, the total content of these elements needs to be made 1.0% or more. Preferably it is 1.2% or more. On the other hand, if the sum exceeds 1.8%, the effect is saturated. Rather, there is an adverse effect (a decrease in surface properties, a decrease in plating properties, a decrease in welding strength) that is a concern when the contents of Si and Al are large. The risk of materialization increases. Therefore, the upper limit in the sum of the contents of Si and Al is set to 1.8% or less. Considering economic efficiency, it is preferably 1.7% or less.

  Mn: Mn is an element that not only increases the strength of the steel sheet but also acts to stabilize austenite. In addition, there is an effect of suppressing the generation of carbide during cooling from a high temperature and improving the bendability. In order to exert these effects, it is necessary to contain at least 1.0% or more of Mn, and the content of Mn may be adjusted to the lower limit or more according to the target strength. On the other hand, the upper limit is set to 2.6% or less from the viewpoint of suppressing formation of a band-like structure that adversely affects bendability, and further from the viewpoint of embrittlement suppression and economic efficiency. Preferably it is 2.1% or less.

  P: P content is preferably as low as possible. In particular, when the content exceeds 0.03%, the spot weldability of the steel sheet is remarkably deteriorated and the ductility is deteriorated. Therefore, the P content is 0.03% or less.

  S: S content is preferably as low as possible. In particular, when the content exceeds 0.02%, Mn contained as an austenite stabilizing element is consumed as a precipitate (MnS), which greatly affects the characteristics. Moreover, this precipitate becomes a crack factor and reduces the bendability of the steel sheet. Therefore, the S content is 0.02% or less.

  If N: N exceeds 0.01%, the amount of Al consumed as AlN increases, and the above-described effect of Al is reduced and the properties of the steel sheet are deteriorated. Furthermore, the deterioration of ductility due to the generated AlN is likely to be manifested. Therefore, the N content is 0.01% or less.

The steel plate according to this embodiment may contain the following elements as optional components.
Ni, Cu: These elements, like Mn, are austenite-generating elements, which act to stabilize austenite and at the same time have the effect of improving strength. Further, it is an element that improves the plating adhesion and the wettability of plating during hot dipping. And since it is an element which is harder to oxidize than Fe, it concentrates on a steel plate surface layer and suppresses the plating adhesiveness and wettability fall by oxidation of Si. Therefore, one or more of these elements can be contained. However, if the Ni content is excessive, a thick and highly adhesive scale is generated in the hot rolling stage, which causes surface flaws on the steel sheet. Further, if the Cu content is excessive, cracking occurs during hot rolling. For this reason, the Ni content is 1.0% or less, and preferably 0.8% or less in consideration of economy. Moreover, Cu content shall be 0.5% or less.

  Ti, Nb, V: These elements not only improve the strength, but also have an effect of improving the alloying speed when the alloying treatment is performed by hot-dip plating containing zinc. Accordingly, one or more of these elements can be contained. However, excessive addition causes a large amount of precipitates such as TiC to precipitate, and such precipitates not only cause deterioration of ductility but also deterioration of bendability. Moreover, since these elements also have the effect | action which increases Fe% in a plating layer, excessive addition will deteriorate powdering property, when alloying hot dip galvanization is given. Therefore, when Ti and Nb are contained, the content is less than 0.1%. About V, since the effect at the time of making it contain the same amount compared with Ti and Nb is small, the content shall be less than 0.2%.

  Co, Cr, Mo, B: Although pearlite that may be generated in the cooling process from a high temperature adversely affects bendability, these elements are effective in improving bendability because they suppress the formation of pearlite. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if the Co and Cr contents exceed 1.0%, the Mo content exceeds 1.0%, and the B content exceeds 0.01%, the effect is only saturated. This is not preferable from an economic viewpoint. Therefore, the upper limit of the content of these elements is 1.0% or less for Co and Cr, 1.0% or less for the Mo content, and 0.01% or less for the B content.

  Ca: Ca has the effect | action which controls the form of an inclusion and improves bendability. However, even if the content exceeds 0.01%, the effect is saturated and economically disadvantageous. For this reason, when it contains Ca, the content shall be 0.01% or less.

  Components other than the above are Fe and impurities.

2. Mechanical characteristics, steel structure, hardness distribution Next, the mechanical characteristics, steel structure, and hardness distribution of the steel sheet according to the present embodiment will be described.

(1) Strength, ductility and residual γ phase The steel sheet according to the present embodiment has the above-described chemical composition, has a tensile strength (TS) of 590 MPa or more, and further exhibits excellent ductility due to the TRIP effect. And satisfies the following formula:
TS × El ≧ 17500 (TS: tensile strength (MPa), El: total elongation (%)).

  For this reason, it is necessary for the γ phase to remain in the steel sheet, and the volume ratio of the residual γ phase is at least 5% by volume, preferably 10% by volume or more. On the other hand, when the residual γ phase is excessive, the volume ratio of martensite generated from the residual γ phase increases, and there is a concern that the bendability may be adversely affected. Therefore, the upper limit of the volume ratio of the residual γ phase is preferably 20% or less, particularly preferably 15% or less.

  In addition, even if the residual γ phase is unstable even if it is a steel plate made of a chemical composition in which many residual γ phases are likely to remain, specifically, when the carbon concentration dissolved in the residual γ phase is low, For example, there is a concern that the residual γ phase is decomposed during heat treatment in the production process (specifically, alloying treatment can be mentioned). Therefore, it is preferable that the steel sheet according to the present embodiment has bainite so that the carbon concentration of the residual γ phase becomes high.

Therefore, a particularly preferable aspect of the steel structure of the steel sheet according to the present embodiment is that the main phase is ferrite and the second phase is bainite and a residual γ phase of 5 to 20% by volume.
(2) Bendability and hardness distribution When the steel sheet according to the present embodiment has the main phase and the second phase as described above, it is possible to maintain the good bendability by reducing the hardness of the bainite in the second phase. Is important to do.

  Compared with ferrite, which is the main phase, the hardness of bainite of the second phase is likely to vary depending on the concentration of carbon contained. For this reason, when the hardness of bainite is high, the difference in hardness from ferrite becomes large, and the interface between the two phases becomes a crack starting point during bending, and cracking is likely to occur. Cracks resulting from this hardness difference are particularly prominent in a region near the surface where compressive stress and tensile stress are significantly applied during bending.

  Accordingly, the hardness variation in the region from the surface to a depth of 0.1 mm immediately below the surface is set to 10 or less in terms of Vickers hardness. Preferably, the difference between the maximum hardness and the minimum hardness is 10 or less in terms of Vickers hardness with respect to the cross section from the surface in the plate thickness direction of the steel plate to the 1/4 plate thickness depth position. It is particularly preferable if the hardness difference is 10 or less for the entire steel plate.

  At this time, ρ ≦ 1.5 × t (TS: tensile strength (MPa), ρ: limit bending radius (mm), t: plate thickness (mm)) is obtained as bending performance, and has excellent bendability. Obtaining a steel plate is realized.

  As described above, even if the steel sheet according to the present embodiment is not a case where it has a ferrite as a main phase and a bainite as a second phase, the hardness variation in the surface layer region in the steel sheet is suppressed. In the same manner, it exhibits excellent bendability.

3. Manufacturing Method The steel sheet according to the present embodiment has the above-described chemical composition and structural characteristics, and has a tensile strength of 590 MPa and further has mechanical properties defined by the above two formulas. The production method is not particularly limited. However, if the following manufacturing method is adopted, it is possible to efficiently and stably obtain the steel plate according to the present embodiment.

(1) Hot rolling a) Until rough rolling A steel having the above chemical composition is cast by a conventional method, or is further divided and rolled, and the resulting slab is roughly rolled. The slab is heated and rough rolled by a conventional method and then used for finish rolling. However, when the slab obtained by continuous casting is directly fed or the slab after partial rolling is used for rough rolling quickly. In addition, when the slab temperature after casting or partial rolling is high and a finishing temperature in finish rolling described later can be secured, rough rolling may be performed while omitting slab heating. Further, when a thin slab is obtained by a known method such as a thin slab CC, rough rolling may be omitted.

B) Finish rolling It is preferable that the hot-rolled sheet in manufacturing the steel sheet according to the present embodiment has a uniform steel structure as much as possible so as not to generate a band-like structure that deteriorates bendability.

If the finishing temperature is a temperature less than the Ar ₃ point, a band-shaped structure is formed. This structure also affects the steel structure after being subjected to cold rolling and continuous annealing or continuous hot dipping, and forms a non-uniform steel structure, resulting in deterioration of bendability. Therefore, it is preferable that the finishing temperature is ₃ points or more at Ar. On the other hand, from the viewpoint of promoting the ferrite transformation after finish rolling to make the steel structure uniform, it is preferable that the amount of rolling strain introduced into the austenite in the finish rolling is larger. Therefore, the finishing temperature is preferably lower. Therefore, the finishing temperature is preferably (Ar ₃ points + 80 ° C.) or less.

In finish rolling, it is desirable to use auxiliary heating means as necessary so as to ensure the above-mentioned finishing temperature over the entire length of the steel material. When the steel material is long, the steel material temperature is lowered during rolling, and the lower limit (Ar ₃ points) of the finishing temperature may not be ensured in the latter stage of hot rolling. Therefore, it is preferable to supplementarily reheat at the entrance of finish rolling. Although this auxiliary reheating method is not limited, an electromagnetic induction heating method in which the heating amount can be easily controlled according to the temperature distribution of the steel material on the entry side of the finish rolling is preferable.

C) Winding temperature The steel structure determined by the winding temperature is a cold-rolled steel sheet obtained by subsequent cold rolling and continuous annealing treatment, and hot-dip plating obtained by cold rolling and continuous hot-dip plating treatment. It greatly affects the properties of the steel sheet. For this reason, control of coiling temperature is important.

  Specifically, it is preferable to increase the winding temperature. By increasing the coiling temperature, the microscopic carbon concentration distribution in the steel sheet tends to vary. This microscopically high carbon concentration region undergoes rapid austenite transformation when held in a two-phase coexisting temperature range in continuous annealing or continuous hot dipping after cold rolling, and C is discharged from the ferrite phase. And the concentration of C in the austenite phase is promoted. Furthermore, the bainite transformation at the time of holding in a predetermined temperature range after cooling is also promoted, and the discharge of C from the bainite structure and the concentration of C into the austenite phase are promoted. As a result, a stable residual γ phase is formed. Further, C discharge from the bainite structure is promoted, the bainite structure becomes softer, and the hardness difference from the adjacent ferrite phase is reduced, so that the bendability is improved.

  Conversely, when the coiling temperature is low, the hardness of the bainite structure in the final product increases. When such hard bainite is formed, the difference in hardness from the adjacent ferrite phase becomes large, and the interface between the phases having different hardness becomes a crack starting point and causes a decrease in bendability.

Note that when the coiling temperature is lowered, the bainite in the hot-rolled steel sheet is hardened, which causes a manufacturing problem that subsequent cold rolling becomes difficult.
The coiling temperature is preferably 450 ° C. or higher as a temperature that can avoid the above problems.

  On the other hand, when the coiling temperature is excessively increased, the surface of the steel sheet is decarburized and the surface quality is deteriorated. In order to prevent this, the coiling temperature is desirably 680 ° C. or lower.

(3) Pickling / cold rolling The hot-rolled steel sheet obtained by the hot rolling process is descaled by pickling and then cold-rolled to form a cold-rolled steel sheet. Pickling and cold rolling may be performed in a conventional manner. However, if the rolling reduction in cold rolling is excessively increased, plate breakage occurs due to work hardening, and the production efficiency decreases. Therefore, the rolling reduction in cold rolling is preferably 45% or more and 85% or less.

(4) Continuous annealing treatment In order to realize the steel sheet according to the present embodiment, it is preferable to hold for 30 seconds or more in the two-phase coexisting temperature range of ferrite (α) / austenite (γ). This step promotes C discharge from ferrite and C concentration to austenite. Specifically, it is heated to a two-phase coexistence temperature range of Ac ₁ point to Ac ₃ point in a reducing atmosphere and held for 30 seconds or more. A particularly preferable holding time is 60 seconds or more. As the reducing atmosphere at this time, it is preferable that hydrogen is 1 to 30% by volume, the balance is nitrogen and unavoidable trace moisture, and the moisture content is within a range of −60 to 0 ° C. as a dew point. Good. Particularly preferred is a case where hydrogen is 2 to 15% by volume, the balance is nitrogen and an inevitable minute amount of water, and the amount of water is in the range of −50 to −0 ° C. as a dew point. The upper limit of the holding time is not particularly required, but holding for a long time leads to a decrease in productivity and an increase in the length of the continuous annealing equipment, so it is preferably 600 seconds or less, and preferably 300 seconds or less. Further preferred. Further, as described above, the temperature may be maintained in the two-phase coexistence temperature range, and there may be a temperature change such as temperature rise or temperature fall within the two-phase coexistence temperature range. Furthermore, before holding in the two-phase coexistence temperature range, it may be once heated to a temperature range exceeding Ac ₃ points.

  It is preferable to perform the cooling process following the holding | maintenance in the said two-phase coexistence temperature range as follows. Since the steel sheet according to the present embodiment contains a predetermined amount of residual γ phase in the final product, it is necessary to avoid generation of pearlite during cooling from the two-phase coexisting temperature range. Therefore, it is preferable to cool at 3 ° C./s or higher to a temperature range of 350 to 600 ° C., and a particularly preferable cooling rate is 5 ° C./s or higher. The upper limit of the cooling rate is not particularly required, but is practically 100 ° C./s or less, particularly preferably 50 ° C. or less.

  In this way, cooling is performed while avoiding the formation of pearlite, and the bainite is generated in the temperature range of 350 to 600 ° C., thereby increasing the C concentration in the γ phase, and in the cooling process to room temperature, The γ phase is stabilized so as not to decompose into carbides. Moreover, discharge | emission of C from a bainite is accelerated | stimulated, a bainite is made softer and a bendability is improved. Therefore, if the holding time is too short, bainite is not sufficiently generated, so the time is set to 5 seconds or more. When using a continuous annealing facility with an overaging zone, it is usually 60 seconds or longer. The upper limit of the holding time is not particularly required, but if it is excessively long, the productivity is remarkably reduced or the length of the continuous annealing equipment is increased. Therefore, it is preferably 300 seconds or less, and 180 seconds or less. More preferably.

(5) Continuous hot dipping treatment The conditions and reasons for maintaining in the two-phase coexisting temperature range and subsequent cooling in the continuous hot dipping treatment are the same as described in the section of the continuous annealing treatment. The upper limit of the cooling rate in the cooling following the maintenance of the two-phase coexisting temperature range is not particularly required, but in the case of continuous hot dipping unlike the case of continuous annealing, it is 30 ° C./s or less due to equipment constraints.

  The upper limit of the temperature range held after the cooling in the continuous hot dipping process, the lower limit of the holding time, and the reason thereof are also the same as described in the section of the continuous annealing process. Unlike the case of continuous annealing, the lower limit of the temperature range is preferably set to 450 ° C. or more from the viewpoint of thermal efficiency when performing the subsequent hot dipping process. The upper limit of the holding time is not particularly required as in the case of continuous annealing, but in the case of continuous hot dipping, it is practical to set it to 90 seconds or less.

What is necessary is just to perform the hot dipping process in a continuous hot dipping process in accordance with a conventional method. The material of the hot dip plating may be only zinc or an alloy containing aluminum, for example.
When alloying is performed after plating, the alloying temperature is preferably 580 ° C. or lower. When the alloying temperature exceeds 580 ° C., the stabilized γ phase is decomposed into ferrite and carbide, and the ductility and bendability tend to be extremely lowered as the characteristics of the steel sheet. As described above, since the hardness of the bainite after the heat treatment in the continuous hot-dip galvanizing treatment can be controlled by the hot rolling conditions, particularly the winding conditions, the alloying treatment temperature is set to 580 ° C. or lower. By appropriately setting the hot rolling conditions while controlling, it is possible to obtain desired bendability.

The lower limit of the alloying treatment temperature is determined by the plating composition, the steel plate composition, the target alloy composition, and further the economy, and for example, 450 ° C. can be mentioned.
Moreover, what is necessary is just to cool on normal cooling conditions after a plating process. At this time, cooling may be performed using N ₂ and industrial gas, and further normal mist cooling may be performed.

(6) Others By the above manufacturing method, it is realized that the cold-rolled steel sheet and the hot-dip plated steel sheet according to the present embodiment are obtained stably and efficiently.

  In addition, what is necessary is just to manufacture by a well-known method about manufacturing processes other than the above. For example, after continuous annealing or hot dipping, temper rolling may be performed by a known method for the purpose of surface roughness adjustment or flattening.

  In addition, the cold-rolled steel sheet according to the present embodiment may be subjected to some treatment on the surface as long as it has the above-described mechanical characteristics and steel structure characteristics. Examples of the treatment include chemical conversion treatment, electroplating treatment, and coating treatment. The chemical conversion treatment may be a zinc phosphate chemical conversion treatment or a manganese phosphate chemical conversion treatment. The electroplating process may be zinc-based or tin-based. The coating process may be a simple immersion in a coating solution or an electrodeposition coating process.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

1. Production of Cold Rolled Steel Sheet and Alloyed Hot Dip Galvanized Steel Sheet The production conditions of the examples shown in Table 1 will be described.

  A slab having the chemical composition shown in Table 1 is heated to 1100 ° C. to 1350 ° C. and subjected to rough hot rolling to form a rough bar, and the rough bar is appropriately subjected to auxiliary heating using an induction heating device and finished. A hot-rolled steel sheet obtained by rolling was wound into a coil. Table 2 shows the finishing temperature and the winding temperature.

  The hot-rolled steel sheet thus manufactured was pickled and cold-rolled at a reduction rate of 50% until the plate thickness shown in Table 1 was reached.

Subsequently, the cold-rolled steel sheet was subjected to continuous annealing treatment or continuous hot-dip plating treatment under the following conditions.
(Continuous annealing treatment)
Soaking temperature: see Table 2 Soaking atmosphere: dew point -30 ° C, 3% H ₂ -97% N ₂
Soaking time: 60 seconds Cooling rate: 60 ° C / s
Low temperature holding temperature: see Table 2 Low temperature holding time: 110 seconds Low temperature holding atmosphere: dew point −30 ° C., 3% H ₂ −97% N ₂
In the continuous annealing treatment, after soaking under the soaking conditions, a heat treatment was performed by cooling to the low temperature holding temperature at the cooling rate and holding the low temperature under the low temperature holding conditions.

(Hot plating process)
Soaking temperature: see Table 2 Soaking atmosphere: dew point -30 ° C, 10% H ₂ , balance N ₂
Soaking time: 120 seconds Cooling rate: 20 ° C / s (gas cooling)
Low temperature holding temperature: see Table 2 Low temperature holding time: 20 seconds Low temperature holding atmosphere: dew point −40 ° C., 7 to 12% H ₂ −88 to 93% N ₂
In the continuous annealing treatment, after soaking under the soaking condition, the heat treatment was performed by cooling to the low temperature holding temperature at the cooling rate and holding at the low temperature under the low temperature holding condition. Furthermore, it was cooled to a temperature substantially equal to the bath temperature of 460 ° C. of the galvanizing bath, and hot dip galvanizing was performed while controlling the adhesion amount to be in the range of 30 to 50 g / m ² . Subsequently, at the alloying treatment temperature shown in Table 2, the alloying treatment was performed while adjusting various alloying treatment times so that the Fe concentration in the plating layer became 10%. Then, it cooled to 250 degrees C or less with the cooling rate of 20 degrees C / s, and obtained the galvannealed steel plate.

  The obtained cold-rolled steel sheet and alloyed hot-dip galvanized steel sheet were subjected to skin pass to make fine adjustments such as flatness.

2. Evaluation Evaluation of the various cold-rolled steel sheets and galvannealed steel sheets obtained was performed as follows.

  The steel structures of cold-rolled steel sheets and alloyed hot-dip galvanized steel sheets are analyzed with an optical microscope, SEM, and X-ray diffractometer, and the structure is evaluated based on the obtained optical microscope images, electron microscope images, and crystal structure data. went.

The mechanical test values (TS: tensile strength, El: total elongation) were obtained by carrying out a tensile test in accordance with JIS Z2241, using a No. 5 test piece defined in JIS Z2201.
The bending test was performed according to the method defined in JIS Z2248. After bending to 170 ° by the push bending method, 180 ° bending and contact bending were performed at a plurality of different bending radii, and the surface outside the bent portion was observed. The minimum bending radius at which no cracks and necking (necking) were observed was taken as the limit bending radius.

  The hardness was measured by a Vickers method using a micro Vickers hardness measuring device (measurement load: 9.8 kN). The hardness measurement range is a range of the plate width direction: 5 mm and the plate thickness direction: surface to 0.1 mm depth position with respect to the cross section of the steel plate to be measured, with an interval of 0.5 mm in the plate width direction and 0.01 mm in the plate thickness direction. Hardness measurements were taken at intervals. Each measurement point was measured three times, and the average value was taken as the hardness of the measurement point. Among the hardnesses of the cross sections thus measured, the maximum hardness and the minimum hardness were extracted to determine the difference between them, and the difference in hardness between the steel sheets was determined.

3. Results Table 3 shows the results of evaluating the various cold-rolled steel sheets and galvannealed steel sheets obtained by the above-described methods.

Claims (11)

In mass%, C: 0.08 to 0.25%, Si: 0.7% or less, Mn: 1.0 to 2.6%, Al: 1.5% or less, P: 0.03% or less, S: 0.02% or less and N: 0.01% or less, and
The relationship between Si and Al satisfies 1.0% ≦ Si + Al ≦ 1.8%,
Having a chemical composition comprising the balance Fe and impurities,
TS ≧ 590 (TS: tensile strength (MPa)),
TS × El ≧ 17500 (El: total elongation (%)) and ρ ≦ 1.5 × t (ρ: critical bending radius (mm), t: plate thickness (mm))
A cold-rolled steel sheet characterized by having mechanical properties satisfying Having a steel structure containing 5% by volume or more of residual γ phase,
For a region with a depth from the surface to 0.1 mm just below the surface, the difference between the maximum hardness and the minimum hardness is 10 or less in Vickers hardness,
The cold-rolled steel sheet according to claim 1.   3. The chemical composition according to claim 1, wherein the chemical composition contains one or two of Ni: 1.0% or less and Cu: 0.5% or less in mass% instead of a part of Fe. Cold rolled steel sheet.   The chemical composition is one or two selected from the group consisting of Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.2% or less in mass% instead of part of Fe. The cold-rolled steel sheet according to any one of claims 1 to 3, comprising seeds or more.   The chemical composition comprises, in place of a part of Fe, in mass%, Co: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.01% or less. The cold-rolled steel sheet according to any one of claims 1 to 4, comprising one or more selected from the group.   The cold-rolled steel sheet according to any one of claims 1 to 5, wherein the chemical composition contains Ca: 0.01% or less in mass% instead of part of Fe.   The surface of the cold-rolled steel sheet according to any one of claims 1 to 6, further comprising: A hot dipped galvanized steel sheet.   The hot-dip plated steel sheet according to claim 7, wherein the hot-dip plated layer is an alloyed hot-dip galvanized layer. A slab having the chemical composition described in any one of claims 1 to 6 is subjected to hot rolling at a finishing temperature: Ar ₃ points to (Ar ₃ points + 80 ° C.) and a winding temperature: 450 to 680 ° C. It is made into a rolled steel plate, pickled and cold-rolled to the hot-rolled steel plate to make a cold-rolled steel plate, and kept in the cold-rolled steel plate for 30 seconds or more in a two-phase coexisting temperature range, and then 3 ° C / s or more A method for producing a cold-rolled steel sheet, which is subjected to a continuous annealing treatment in which cooling is performed at a cooling rate to a temperature range of 350 to 600 ° C. and maintained in the temperature range for 5 seconds or more. A slab having the chemical composition described in any one of claims 1 to 6 is subjected to hot rolling at a finishing temperature: Ar ₃ points to (Ar ₃ points + 80 ° C.) and a winding temperature: 450 to 680 ° C. It is made into a rolled steel plate, pickled and cold-rolled to the hot-rolled steel plate to make a cold-rolled steel plate, and kept in the cold-rolled steel plate for 30 seconds or more in a two-phase coexisting temperature range, and then 3 ° C / s or more A hot-dip galvanized steel sheet which is cooled to a temperature range of 450 to 600 ° C. at a cooling rate, maintained in the temperature range for 5 seconds or more, and further subjected to a continuous hot-dip plating process to form a hot-dip plating layer containing zinc. Production method.   The method for producing a hot-dip galvanized steel sheet according to claim 10, wherein the hot-dip galvanized steel sheet obtained by the continuous hot dip galvanizing process is alloyed at an alloying temperature of 580 ° C or lower.