JP2004307992A - Dual-phase cold-rolled steel sheet superior in surface distortion resistance and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dual-phase cold-rolled steel sheet which is superior in surface distortion resistance, has a tensile strength of 340-590 MPa and a high formability, and is applicable to automotive inner and outer plates and to provide its manufacturing method. <P>SOLUTION: The microstructure of the dual-phase cold-rolled steel sheet is a dual-phase structure consisting of a ferrite phase and a second phase containing at least 60% martensite phase. The average size (d) of the martensite phase particles is 1.5 μm or smaller. The ratio (a/b) of the number (a) of the martensite phase particles to the number (b) of the ferrite phase particles in a unit volume is 0.7 to 2.4. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a cold rolled steel sheet having a composite structure, which has a good appearance after press forming and has excellent surface distortion resistance, which is suitable for fields such as automobiles and home appliances.

  2. Description of the Related Art In recent years, steel sheets for automobiles have been made thinner for the purpose of improving fuel efficiency by reducing the weight of the vehicle body and have been strengthened for the purpose of improving safety. In general, since high strength of a steel sheet causes deterioration of formability, excellent formability is required. In particular, external parts have been required to have a good appearance after pressing. What matters in the appearance of the molded article is the surface shape such as undulation, that is, the surface distortion, the stretcher strain on the steel sheet surface, the plating adhesion and the chemical conversion treatment. From the viewpoint of surface strain, it is widely known that it is effective to promote strain propagation in a low strain region, that is, to suppress the yield strength to a low level, but the yield strength increases with the increase in strength. There is a big problem.

  In order to solve such problems, for ultra-low carbon steel, techniques for improving the surface strain resistance by reducing the yield strength and improving the press formability by increasing the r-value and elongation are disclosed in Patent Documents 1 and 2. It is proposed in Document 2 and the like.

  Patent Literature 3 and Patent Literature 4 have low strength compared to the conventional multi-structure steel sheet by reducing the carbon and properly controlling the content of Mn, Cr or Mo or B to secure the hardenability. Techniques for obtaining steel sheets have been proposed. Since the steel sheet obtained by these techniques is a composite structure steel sheet containing martensite, it has a high yield strength, a low yield ratio, and excellent surface distortion resistance, even though it has high strength. Further, since the transformation strengthening is utilized, high strength can be achieved without depending on the solid solution strengthening ability, so that the contents of Si and P, which adversely affect plating adhesion and chemical conversion treatment, can be reduced.

  However, since the conventional technology is based on ultra-low carbon steel, since the strengthening mechanism depends on solid solution strengthening, from the viewpoint of surface properties, the amount of solid solution strengthening elements such as P and Si is naturally limited, It is difficult to stably produce a steel plate having a substantial strength level of 390 MPa or more. In addition, since Ti is added to the base of an ultra-low carbon steel sheet to impart bake hardenability, there remains a problem in aging resistance.

  In the technique described in Patent Document 3, since a large amount of nitrogen is contained in order to obtain high bake hardenability, ductility is degraded due to an increase in the amount of solute N, and aging is apt to occur. Cracking is a major problem.

  The technology described in Patent Document 4 is a technology for improving overhang formability by increasing the number of martensite phase particles in a unit volume, but a large amount of martensite phase particles is a starting point of voids generated during deformation. And the moldability is reduced. In addition, the winding temperature after hot rolling is set to 550 ° C. or less to form fine carbides, thereby making the structure after annealing fine and shortening the distance between martensite grains. Since the yield strength is increased, there is a problem that surface distortion resistance is deteriorated.

JP-A-7-62209 JP-A-5-78784 JP 2001-323337 A JP-A-2002-322537

  The present invention has been made in view of the above problems, and has a tensile strength of 340 MPa or more and 590 MPa or less, a composite structure cold-rolled steel sheet having excellent formability and excellent surface distortion resistance applicable to automotive inner and outer panels. And a method for producing the same.

  In order to solve the above problems, the present invention provides a composite structure in which the microstructure is composed of a ferrite phase and a second phase having a martensite phase ratio of 60% or more, and the average particle size d of the martensite phase particles is 1. 5 μm or less, and the ratio a / b of the number a of the martensite phase particles to the number b of the ferrite particles per unit volume is 0.7 to 2.4, and is excellent in surface distortion resistance. Provide a composite structure cold rolled steel sheet.

  The present invention provides the above-mentioned cold rolled composite structure steel sheet, in which, by mass%, C: 0.005 to 0.05%, Si: 1.5% or less, Mn: 3.0% or less, P: 0. 10% or less, S: 0.03% or less, Al: 0.01 to 0.1%, N: less than 0.008%, and the balance can be substantially composed of Fe.

  The present invention provides the above-mentioned cold rolled steel sheet with a composite structure, in which, as a chemical component, Cr: 1.0% or less; Mo: 1.0% or less; V: 1.0% or less; % Or less, Ti: 0.1% or less, and Nb: 0.1% or less.

In addition, the present invention melts steel having any one of the above chemical components, then performs hot rolling, and then performs cold rolling, and obtains the obtained steel sheet in a temperature range of ₁ point or more and ₃ points or less of Ac. After annealing and cooling as primary cooling at a cooling rate of more than 3 ° C./sec and less than 10 ° C./sec and a primary cooling stop temperature in a temperature range of 450 to 700 ° C., continuously cooling at 10 ° C./sec or more After the secondary cooling at a speed and a cooling stop temperature of less than 450 ° C., the overaging process is started in a temperature range of 100 to 400 ° C., and the processing time of the overaging process is 150 seconds or more, and the overaging process is performed. Provided is a method for producing a cold rolled steel sheet having a composite structure excellent in surface distortion resistance, characterized in that the treatment end temperature is less than 350 ° C.

  In order to obtain a steel sheet having a strength of 340 to 590 MPa having high workability and excellent surface distortion resistance, the present invention having such a configuration is extremely difficult in the prior art. It was made as a result of an intensive study focusing on. In other words, by utilizing transformation strengthening as a strengthening mechanism and reducing the martensite phase fraction as much as possible, it is possible to obtain excellent surface distortion resistance and resistance while having a strength range of 340 to 590 MPa, which was difficult with IF steel base. Along with the aging property, by further uniformly dispersing fine martensite phase particles in a certain number of ferrite grounds, a low YR and an improvement in formability are achieved, and a cold rolled composite structure having excellent surface shape after press forming. They found that a steel sheet could be obtained, and completed the present invention.

  According to the present invention, by appropriately controlling the structure of the martensite phase, excellent surface distortion resistance and moldability can be obtained in composite molding of automobile inner and outer panels and the like, which is industrially extremely significant.

Hereinafter, the present invention will be described in detail.
The microstructure cold-rolled steel sheet according to the present invention has a microstructure in which the ratio of a ferrite phase and a martensite phase is 60% or more of a second phase, and the average particle size d of the martensite phase particles is 1. The ratio a / b of the number a of the martensitic phase particles per unit volume to the number b of the ferrite particles in a unit volume of 5 μm or less is 0.7 to 2.4.

(A) Proportion of martensite phase in second phase: 60% or more In the martensite phase, a large number of mobile dislocations are introduced at the time of transformation, so that the strain dispersing ability becomes high. Therefore, in the present invention, a certain amount of the martensite phase is essential, and the ratio of the martensite phase in the second phase is set to 60% or more. Preferably it is 80% or more. Other than the martensite phase, a residual γ phase, a bainite phase, and a carbide may be contained. The fact that a certain amount of martensite phase is essential means that the second phase fraction of 0% is not included in the present invention, and in order to stably obtain a high strain dispersibility, the second phase fraction is required. The rate is preferably 1% or more. The second phase fraction is preferably at most 10%, more preferably at most 8%.

(B) Average particle size d of martensite phase particles: 1.5 μm or less From the viewpoint of moldability, it is effective to make the martensite phase particles fine. Therefore, the average particle size d of the martensite phase particles is specified to be 1.5 μm or less. Preferably it is 1.2 μm or less.

(C) Ratio a / b of the number a of the martensite phase particles to the number b of the ferrite particles in a unit volume a / b: 0.7 to 2.4.
Increasing a / b results in a low yield ratio because the nucleus for deformation increases. Therefore, a certain number of martensite phase particles is required to lower the yield strength, and a / b is at least 0.7 or more. 1 (a) and 1 (b) show microstructure photographs of the steel of the present invention and a comparative steel in which a / b is out of the range of the present invention. When a / b exceeds 2.4, the martensite phase is continuous and dense, so it is difficult to say that the martensite phase is finely dispersed, and the high moldability targeted by the present invention cannot be obtained. Therefore, a / b is set to 2.4 or less, preferably 2.0 or less, more preferably 1.8 or less from the viewpoint of moldability. In order for the martensite phase to sufficiently act as a nucleus for deformation, it is preferable that the martensite phase exists at the grain boundary.

  Incidentally, the structure of the steel of the present invention, it is important that a certain number of fine martensitic phase particles are present at the grain boundaries, but when the martensitic phase is dense, it adversely affects the formability, It is preferable that the distance between the martensitic phase particles is equal to or greater than a certain distance, and the average distance between the martensitic phase particles is more than 2.7 μm.

In the present invention, the desired characteristics can be obtained as long as the above-mentioned structure is formed. However, in order to obtain such a structure, the chemical components are expressed as mass% and C: 0.005 to 0.05. %, Si: 1.5% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.03% or less, Al: 0.01 to 0.1%, N: 0.008 %, The balance being substantially composed of Fe. Further, in order to obtain still other desired characteristics, in addition to the above components, in mass%, Cr: 1.0% or less, Mo: 1.0% or less, V: 1.0% or less, B: 0. It may contain one or more of 01% or less, Ti: 0.1% or less, and Nb: 0.1% or less.
Hereinafter, the reasons for these limitations will be described.

C: 0.005 to 0.05%
C is one of the extremely important elements in the present invention, and is very effective in forming a martensite phase and increasing the strength. However, when the C content exceeds 0.05%, the workability is significantly reduced, and the weldability is further deteriorated. Therefore, the C content is set to 0.05% or less. Preferably, it is at most 0.04%. On the other hand, in order to form a martensite phase having a constant volume ratio, it is necessary to contain a constant amount of C, so that the content is at least 0.005% or more, preferably more than 0.010%.

Si: 1.5% or less Si is an effective element for stably obtaining a composite structure. However, if the Si content exceeds 1.5%, the surface properties and the chemical conversion treatment properties are significantly reduced. Therefore, the amount of Si is set to 1.5% or less. Preferably it is 1.0% or less.

Mn: 3.0% or less Mn is a very important element for the formation of a martensitic phase, and by improving hardenability and fixing S in steel as MnS, the grain boundary of S is reduced. Since it has an effect of preventing slab cracking during hot rolling caused by oxidizing action, it is necessary to add a certain amount, preferably 1.0% or more. However, when Mn is added in excess of 3.0%, the slab cost is significantly increased and workability is deteriorated. Therefore, the Mn content is set to 3.0% or less. Preferably it is 2.5% or less.

P: 0.10% or less P is an element effective for increasing the strength and stabilizing the martensite phase. However, if the P content exceeds 0.10%, the alloying speed of the galvanized layer is reduced, which causes poor plating and non-plating, and segregates at the grain boundaries of the steel sheet to deteriorate the secondary work brittleness resistance. Let it. Therefore, the P content is set to 0.10% or less.

S: 0.03% or less S segregates at the grain boundaries during hot rolling and generates slab cracks, so that the rate of occurrence of surface flaws increases. Therefore, by adding Mn, S is fixed as MnS. However, excess MnS becomes a starting point of a void at the time of working, which causes a decrease in workability. Therefore, the content of S is desirably small, and if the S content exceeds 0.03%, the workability is significantly deteriorated. Therefore, the S content is set to 0.03% or less.

Al: 0.01 to 0.1%
Al has a function of reducing inclusions in steel as a deoxidizing element. However, if the Al content is less than 0.01%, the above-mentioned effects cannot be obtained stably. On the other hand, when the amount of Al exceeds 0.1%, the amount of cluster-like alumina-based inclusions increases, thereby deteriorating the workability. Therefore, the Al content is in the range of 0.01 to 0.1%.

N: less than 0.008% N is preferably small from the viewpoint of workability and aging. When the N content is 0.008% or more, ductility and toughness are deteriorated due to excessive nitride formation. Therefore, the N content is set to less than 0.008%.

Cr, Mo, V: 1.0% or less when added Cr, Mo, V are elements for improving hardenability, and are added to stably generate a martensite phase. However, even if it is added in excess of 1.0%, the effect is not only saturated, but also disadvantageous in cost. Therefore, when Cr, Mo, and V are added, the content of each is set to 1.0% or less.

B: When added, 0.01% or less B is an element effective for improving hardenability, and is added to stably obtain a martensite phase. However, even if it is added in excess of 0.01%, an effect commensurate with cost cannot be obtained. Therefore, when B is added, the content is 0.01% or less.

Ti, Nb: When added, 0.1% or less, respectively. Ti and Nb are effective elements for forming carbonitrides, reducing the amount of dissolved C and N, and improving deep drawability. However, even if any of them is added in excess of 0.1%, the effect is saturated, and the recrystallization temperature at the time of annealing is increased, so that the productivity is reduced. Therefore, when adding Ti and Nb, each is set to 0.1% or less.

  In addition, "the balance substantially consists of Fe" means that those containing other trace elements including unavoidable impurities are included in the scope of the present invention, as long as the function and effect of the present invention are not impaired. I do.

Next, the manufacturing conditions will be described.
In order to produce the cold-rolled steel sheet of the present invention, a steel having the above chemical composition is melted, then hot-rolled, and then cold-rolled, and the obtained steel sheet is obtained from _one point of Ac to _three points of Ac or less. After annealing in a temperature range and cooling at a cooling rate of more than 3 ° C./sec and less than 10 ° C./sec as a primary cooling and a primary cooling stop temperature in a temperature range of 450 to 700 ° C., continuously 10 ° C./sec. After secondary cooling at a cooling rate of at least 2 seconds and a cooling stop temperature of less than 450 ° C., an overaging process is started in a temperature range of 100 to 400 ° C., and the processing time of the overaging process is 150 seconds or more. And the overaging treatment end temperature is set at less than 350 ° C.

(A) Annealing temperature: not less than Ac ₁ point and not more than Ac ₃ points The annealing temperature needs to be heated to an appropriate temperature in order to obtain a microstructure of ferrite phase + second phase. If the annealing temperature is less than _one point of Ac, two phases are not separated, so that a martensite phase cannot be obtained. On the other hand, when the annealing temperature exceeds the Ac ₃ point, the entire amount of the ferrite phase is austenite, so that the recrystallization structure is initialized and the two-phase separation does not sufficiently proceed, so that properties such as moldability deteriorate. Therefore, the annealing temperature is set to _one or more Ac and _three or less Ac. It is preferable that the Ac ₁ point or more Ac ₁ point + 50 ℃ or less.

(B) Primary cooling rate: more than 3 ° C./sec and less than 10 ° C./sec, primary cooling stop temperature: 450 to 700 ° C.
The cooling rate of the primary cooling needs to be appropriately controlled in order to suppress pearlite precipitation and secure the volume ratio of austenite. If the primary cooling rate is 3 ° C./second or less, pearlite precipitates, and the formability deteriorates. Therefore, the primary cooling rate is set to more than 3 ° C./sec. When the cooling rate exceeds 10 ° C./sec, the two-phase separation of the ferrite phase and the austenite phase does not sufficiently proceed, and the martensite phase may be insufficient. In that case, desired characteristics cannot be obtained, so the cooling rate of the primary cooling is set to less than 10 ° C./sec.

  On the other hand, when the cooling stop temperature is higher than 700 ° C., the size of the martensitic phase particles increases, leading to deterioration in moldability. In addition, since the cooling width up to the secondary cooling stop temperature is increased, the shape of the steel sheet is inferior, which adversely affects productivity and quality. On the other hand, when the cooling stop temperature is lower than 450 ° C., the temperature exceeds the Ms point, and a martensite phase may not be obtained. Therefore, the cooling stop temperature of the primary cooling is set in the range of 450 to 700 ° C. Preferably it is in the range of 500 to 600 ° C.

(C) Secondary cooling rate: 10 ° C./sec or more, secondary cooling stop temperature: less than 450 ° C. In order to obtain a martensite phase, it is necessary to cool at an appropriate cooling rate during transformation. Therefore, the secondary cooling is performed at a cooling rate of 10 ° C./sec or more, and the cooling stop temperature is set to less than 450 ° C. More preferably, it is lower than 400 ° C.

(D) Overaging treatment start temperature: 100 to 400 ° C
Various properties required for the steel sheet aimed at by the present invention include ductility and aging characteristics in addition to the surface distortion resistance. By optimizing the hardness of the martensite phase and the amount of solute C in ferrite by overaging treatment, these characteristics can improve ductility and aging characteristics while maintaining good surface distortion resistance. Become. However, when heat treatment is performed at a temperature exceeding 450 ° C., mobile dislocations introduced during the martensitic transformation are fixed, and a large number of carbides are precipitated, so that the YP increases and the surface strain resistance decreases.
If the temperature is lower than 100 ° C., the above-mentioned effects cannot be sufficiently obtained. Therefore, the start of the overaging treatment is performed within a temperature range of 100 to 400 ° C.

(E) Holding time of overage treatment: 150 seconds or more If the holding time is less than 150 seconds, the processing time is short and the above-mentioned effects cannot be sufficiently obtained, so the holding time is 150 seconds or more. Preferably it is 200 seconds or more. However, holding for more than 10 minutes not only saturates the effect, but is also undesirable from the viewpoint of cost and productivity. Therefore, the holding time is desirably 10 minutes or less.

(F) Overaging treatment end temperature: less than 350 ° C. Controlling the end temperature is very important as controlling the start temperature. If the overaging treatment end temperature exceeds 350 ° C., the YP increases and the surface distortion resistance decreases, so the overaging treatment end temperature is set to less than 350 ° C. Preferably it is 320 ° C. or lower, more preferably 300 ° C. or lower. However, if the end temperature is lower than 100 ° C., a sufficient effect cannot be obtained. Therefore, the end temperature is preferably 100 ° C. or higher.

  For example, in order to prevent a decrease in the surface strain resistance, heat treatment is started at 400 ° C., which is the upper limit of the overaging treatment start temperature within the range of the present invention, then gradually cooled in the overaging zone, and overaged at less than 350 ° C. Thus, avoiding staying in the high-temperature region for a long time is an effective means for preventing the reduction of the surface distortion resistance. Of course, it is also effective to start the heat treatment from a low temperature range and keep the temperature as it is.

  It goes without saying that the intended effect can be obtained even if the electro-galvanizing or hot-dip galvanizing is applied to the cold-rolled steel sheet obtained as described above. In the case of a hot-dip galvanized steel sheet, an alloying treatment may be performed. Further, these plated steel sheets may be further subjected to an organic film treatment after plating.

In the present invention, in hot rolling the slab, the slab may be rolled after reheating in a heating furnace, or may be directly rolled without heating. Further, the hot rolling finish rolling temperature is preferably not lower than the Ar ₃ transformation point, and the winding temperature is preferably higher than 550 ° C. The cooling rate may be set to 50 to 85% within the normal operation range.

Table 1 shows the steel numbers No. 1 to No. After smelting the steel No. 12, the slab was made into a slab by continuous casting, heated to 1200 ° C., and then subjected to finish rolling at a temperature of _three or more points of Ar to produce a hot-rolled steel sheet at a winding temperature of more than 550 ° C. and 650 ° C. or less. The hot rolled sheet was pickled and cold rolled. Subsequently, continuous annealing was performed under the conditions shown in Table 2, and 1 to No. 24 annealed plates were obtained. The microstructure of the obtained annealed plate was observed in the following manner, and its performance was evaluated.

  In the microstructure observation, the test piece was corroded with nital, and the central part of the plate thickness was continuously observed at a magnification of 2000 times with a SEM observation of a visual field of 100 μm × 200 μm to measure the size and number of martensite phase particles and the number of ferrite particles. . When the whole grain cannot be observed at the end of the visual field, the number was determined to be 個, and when the martensitic phase particles were connected, the number was measured as one for each. Mechanical properties were evaluated by sampling JIS No. 5 tensile test pieces.

  The surface roughness after press molding, surface distortion, after collecting a sample with a blank angle of 160 mm, press-molding with a punch of 800 mmR cylindrical surface, grind the resulting Kamaboko panel, grind it with a visual observation, excellent sample by visual observation, slight Samples in which surface distortion was observed and which could be improved by optimizing the molding conditions, and samples in which surface shape defects and cracks were clearly observed were evaluated in three stages of ○, Δ, and ×, respectively. Table 2 shows the results of the above surveys.

  FIG. 2 is a diagram in which the evaluation results of the rough surface condition and the surface strain after press molding are arranged by the average particle size of the martensite phase particles and a / b. As is clear from FIG. 2, it can be seen that by setting the microstructure of the steel sheet within the range of the present invention, a composite structure cold-rolled steel sheet having an excellent surface shape can be obtained. On the other hand, in the comparative example, clear surface shape defects and cracks were observed, and it was found that the moldability and the surface distortion resistance were inferior.

  INDUSTRIAL APPLICABILITY The present invention can be widely applied not only to home and exterior panels for home appliances and automobiles, but also to general steel sheet manufacturing techniques requiring high formability and excellent surface distortion resistance.

(A) and (b) are photographs illustrating the metal structures of a typical steel sheet of the present invention and a comparative steel sheet. The figure which shows the influence on the surface shape after a press of the average particle diameter of the martensitic-phase particle | grain, and the ratio of the number of the martensitic-phase particle | grains in a unit volume, and the number of ferrite particles.

Claims (4)

  The microstructure is a composite structure composed of a second phase in which the ratio of the ferrite phase and the martensite phase is 60% or more, and the average particle size d of the martensite phase particles is 1.5 μm or less, and further the martensite in a unit volume is A composite structure cold-rolled steel sheet having excellent surface strain resistance, wherein the ratio a / b of the number a of phase particles to the number b of ferrite particles is 0.7 to 2.4.   Chemical component is mass%, C: 0.005 to 0.05%, Si: 1.5% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.03% or less , Al: 0.01 to 0.1%, N: less than 0.008%, and the balance is substantially made of Fe, and the structure has excellent surface distortion resistance according to claim 1. Composite structure cold rolled steel sheet.   Further, as a chemical component, in mass%, Cr: 1.0% or less, Mo: 1.0% or less, V: 1.0% or less, B: 0.01% or less, Ti: 0.1% or less, Nb The cold-rolled steel sheet with a composite structure according to claim 1, wherein the steel sheet has at least one of 0.1% or less and has excellent surface distortion resistance. The steel having the chemical composition according to claim 2 or 3 is melted, then hot-rolled, and then cold-rolled, and the obtained steel sheet is subjected to a temperature range of ₁ point Ac to ₃ points Ac. After annealing and cooling as primary cooling at a cooling rate of more than 3 ° C./sec and less than 10 ° C./sec and a primary cooling stop temperature in a temperature range of 450 to 700 ° C., continuously cooling at 10 ° C./sec or more After the secondary cooling at a speed and a cooling stop temperature of less than 450 ° C., the overaging process is started in a temperature range of 100 to 400 ° C., and the processing time of the overaging process is 150 seconds or more, and the overaging process is performed. A method for producing a cold rolled steel sheet having a composite structure excellent in surface distortion resistance, wherein the treatment end temperature is lower than 350 ° C.

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