JP2004197156A - High strength thin steel plate of excellent secondary working brittleness resistance, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin steel plate having the tensile strength of ≥340 MPa and <590 MPa, with press-workability applicable to inner and outer plates for automobile, and excellent secondary working brittleness resistance, and a manufacturing method therefor. <P>SOLUTION: The steel has a composition consisting of, by mass, ≤0.05% C, ≤2.0% Si, ≤3.0% Mn, ≤0.1% P, ≤0.03% S, ≤0.1% Al, and ≤0.01% N, and the balance substantially iron. A microstructure is formed of ferrite and a low-temperature transformation phase of <10% volume fraction, and an inequality d <-0.5×Vm+16 (Vm<10%) is satisfied, where d (μm) denotes the grain size of ferrites, and Vm(%) is the low-temperature transformation phase fraction. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００４３】
【表２】

【００４４】
【発明の効果】
以上説明したように、本発明によれば、化学成分組成を特定の範囲に制御するとともに、フェライト＋低温変態相からなる組織とし、さらに粒径と低温変態相分率の関係を最適化することで、高い成形性を有し、かつ耐二次加工脆性に優れた高強度薄鋼板が得られる。このため、本発明の鋼板は、自動車用鋼板を始め、家電等広い分野で適用することが可能である。
【図面の簡単な説明】
【図１】実施例の鋼板の低温変態相分率とフェライト粒径の関係に対する耐二次加工脆性の優劣を示す図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength cold-rolled steel sheet applied to fields such as automobiles and home appliances, and particularly to a high-strength thin steel sheet having a strength of 340 MPa or more and less than 590 MPa and excellent in secondary working brittleness resistance, which is suitable for inner and outer panels of automobiles.
[0002]
[Prior art]
2. Description of the Related Art In recent years, steel sheets for automobiles have been increasing in strength for the purpose of improving fuel efficiency by reducing the weight of a vehicle body. In addition, with the complexity of the single unit design, excellent moldability is required. In order to satisfy such demands for compatibility between formability and high strength, IF steel obtained by adding a carbonitride forming element such as Ti or Nb to ultra-low carbon steel is dissolved in P, Si, Mn or the like. Strengthened, high-strength IF steels have been developed.
[0003]
However, in the IF steel, since C is precipitated and fixed by Ti or Nb, crystal grain boundaries are very clean, and secondary working cracks due to grain boundary fracture after forming are likely to occur. Further, when P is added as a solid solution strengthening element, there is a problem that secondary working embrittlement is more likely to occur due to segregation of P at the grain boundary. Further, in the case of a high-strength IF steel, the inside of the grains is strengthened by a solid solution strengthening element, and the relative grain boundary strength is significantly reduced. It has been reported that the transition temperature significantly deteriorates.
[0004]
Several methods have been proposed to solve these problems. For example, in Patent Document 1, based on a Ti-added IF steel, the amount of P added is reduced as much as possible, and a large amount of Si and Mn is added for the purpose of avoiding the deterioration of secondary work brittleness due to grain boundary segregation. Thus, a technique for obtaining a high-tensile steel sheet having excellent secondary work brittleness resistance has been proposed.
[0005]
Further, Patent Literature 2 proposes a technique in which B is added in addition to Ti and Nb by using ultra-low carbon steel to increase the strength of grain boundaries and increase the resistance to secondary working brittleness. .
[0006]
[Patent Document 1]
JP-A-5-59491 [Patent Document 2]
JP-A-6-57373
[Problems to be solved by the invention]
However, in Patent Document 1, Si and Mn have a lower solid solution strengthening ability than P, and a desired strength cannot be obtained unless they are added in a large amount. There are points. Further, the addition of a large amount of Si causes non-plating and poor alloying when producing a plated steel sheet, and also causes deterioration in surface properties such as poor chemical conversion treatment when producing a cold-rolled steel sheet.
[0008]
Further, in Patent Document 2, since the recrystallization temperature is increased by the addition of B, there is a problem that the manufacturing cost is increased and the moldability is reduced.
[0009]
Furthermore, the two prior arts described above mainly rely on solid solution strengthening as the strengthening mechanism because they are based on IF steel. Therefore, the amount of P added is naturally limited in order to avoid the deterioration of the secondary working brittleness due to the segregation of P at the grain boundary, and it is extremely difficult to stably produce a steel sheet having a substantial strength level of 390 MPa or more. It is.
[0010]
The present invention has been made in view of the above circumstances, and has a tensile strength of 340 MPa or more and less than 590 MPa, has press formability applicable to automotive inner and outer panels, and has excellent secondary work brittleness resistance. And a method for producing the same.
[0011]
[Means for Solving the Problems]
The present inventors have intensively studied to obtain a high-strength thin steel sheet having excellent press formability and excellent secondary work brittleness resistance, which was extremely difficult with the conventional technology. As a result, by actively utilizing transformation strengthening as a strengthening mechanism to secure strength, the solid solution strengthening elements such as P, which adversely affect conventional secondary work brittleness resistance, are reduced as much as possible, and the ferrite grain size is further reduced. By controlling the low temperature transformation phase fraction, it was found that the resistance to secondary working embrittlement can be further improved.
[0012]
Specifically, unlike the conventional composite structure steel, the low-temperature transformation phase fraction is reduced, and the low-temperature transformation phase is uniformly and finely dispersed to suppress the promotion of cracking due to the low-temperature transformation phase. It has been found that refining the crystal grain size by the pinning effect of the low-temperature transformation phase is a key to improving the resistance to secondary working embrittlement.
[0013]
The present invention has been completed on the basis of these findings of the present inventors. In mass%, C: 0.05% or less, Si: 2.0% or less, Mn: 3.0% or less. , P: 0.1% or less, S: 0.03% or less, Al: 0.1% or less, N: 0.01% or less, the balance is substantially composed of iron, and the microstructure is ferrite and volume fraction. It consists of a low-temperature transformation phase of less than 10%, and the ferrite grain size d (μm) and the low-temperature transformation phase fraction Vm (%) satisfy the relationship of d <−0.5 × Vm + 16 (Vm <10%). To provide a high-strength thin steel sheet having excellent secondary work brittleness resistance.
[0014]
In addition to the above, in mass%, Cr: 1% or less, Mo: 1% or less, V: 1% or less, B: 0.01% or less, Ti: 0.1% or less, Nb: 0.1% One or more of the following may be contained.
[0015]
Further, the present invention, after melting the steel having the above component composition, hot rolling, after cold rolling the resulting hot-rolled steel sheet, and then annealed in a temperature range of Ac ₁ point or more Ac ₃ points or less, A secondary working brittle resistance characterized by successively cooling at a rate of more than 3 ° C / s to a temperature range of 450 to 700 ° C and then at a rate of 10 ° C / s or more to a temperature below the Ms point. Provided is a method for manufacturing a high-strength thin steel sheet excellent in quality.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
First, the component composition will be described.
The high-strength thin steel sheet according to the present invention has a mass% of C: 0.05% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0. 03% or less, Al: 0.1% or less, N: 0.01% or less, and the balance substantially consists of iron. Further, Cr: 1% or less, Mo: 1% or less, V: 1% or less, B: 0.01% or less, Ti: 0.1% or less, Nb: 0.1% or less May be.
[0017]
C: 0.05% or less C is one of the extremely important elements in the present invention. Although it is a very effective element for generating a low-temperature transformation phase and achieving high strength, if it is added in excess of 0.05%, the workability is significantly reduced and the weldability is also deteriorated. , C amount is 0.05% or less. Particularly, since the present invention is mainly applied to the inner and outer panels of an automobile, extremely high formability (deep drawability, overhangability, etc.) is essential. It is known that their formability generally decreases with an increase in the amount of C. When the amount of C exceeds 0.05%, the amount of solid solution C in ferrite increases, and the above-mentioned formability decreases. I am not satisfied. Further, if the C content exceeds 0.05%, scale surface defects are likely to occur during the hot rolling step, deteriorating the final surface properties after galvanizing, and obtaining a surface quality at the level of the inner and outer panels of an automobile. Therefore, the C content is restricted to 0.05% or less. When extremely high formability is required, it is desirable to reduce the content to 0.04% or less. However, in order to form a low-temperature transformation phase with a constant volume ratio, it is essential to contain a certain amount. Therefore, although depending on the content of other elements, the C content is desirably 0.01% or more.
[0018]
Si: 2.0% or less Si is an effective element for stably obtaining a low-temperature transformation phase. However, when the content is high, the surface properties and the chemical conversion property are significantly deteriorated. 0% or less.
[0019]
Mn: 3.0% or less Mn is a very important element for the formation of a low-temperature transformation phase, and in the present invention, it is necessary to add a certain amount, preferably 0.5% or more, in order to improve hardenability. is there. However, if added excessively, the slab cost is significantly increased and workability is deteriorated. Therefore, the Mn content is set to 3.0% or less.
[0020]
P: 0.1% or less P is an element effective for stabilizing the low-temperature transformation phase like Si, but when added in a large amount, the grain boundary is embrittled by the grain boundary segregation of P. In addition, the alloying speed of zinc plating is slowed down, resulting in poor plating and non-plating. Therefore, the P content is set to 0.1% or less.
[0021]
S: 0.03% or less S is segregated at the grain boundary during hot rolling to generate slab cracks and increases the rate of occurrence of surface flaws. Therefore, the content of S is preferably small. On the other hand, if the content exceeds 0.03%, MnS precipitates and workability deteriorates. Therefore, the amount of S is set to 0.03% or less.
[0022]
Al: 0.1% or less Al has a function of reducing inclusions in steel as a deoxidizing element. However, when the Al content exceeds 0.1%, cluster-like alumina-based inclusions increase and ductility decreases. Therefore, the amount of Al is set to 0.1% or less. In order to exhibit the effect of reducing inclusions, the content is desirably 0.01% or more.
[0023]
N: 0.01% or less N should be small in content from the viewpoint of processability and aging. If added in an amount exceeding 0.01%, ductility and toughness deteriorate due to excessive nitride formation. Therefore, the N content is set to 0.01% or less.
[0024]
Cr, Mo, V: 1% or less respectively Cr, Mo, V are quenching improving elements, and are added as necessary to stably generate a low-temperature transformation phase. However, even if it is added excessively, the effect is not only saturated, but also disadvantageous in cost. Therefore, when Cr, Mo, and V are added, each content is set to 1% or less.
[0025]
B: 0.01% or less B is an element effective for strengthening the grain boundary. In addition, it contributes to the improvement of hardenability, and is added as needed to stably obtain a low-temperature transformation phase. However, even if added in an amount exceeding 0.01%, an effect commensurate with cost cannot be obtained. Therefore, if added, the content is set to 0.01% or less.
[0026]
Ti, Nb: 0.1% or less Ti, Nb is an element effective for forming carbonitride, reducing the amount of solute C and N, and improving the deep drawability, and is added as necessary. I do. However, the effect is saturated even if the content of any of them exceeds 0.1%, and the recrystallization temperature during annealing after cold rolling becomes high, so that the productivity is deteriorated. Therefore, when they are added, each is set to 0.1% or less.
[0027]
In the present invention, in addition to the above components, the balance may be substantially iron, and unavoidable impurities and other trace elements within a range that does not impair the function and effect of the present invention are acceptable.
[0028]
Next, the microstructure will be described.
The high-strength thin steel sheet according to the present invention has the above component composition, a microstructure comprising ferrite and a low-temperature transformation phase having a volume fraction of less than 10%, and a ferrite grain size d (μm) and a low-temperature transformation phase fraction Vm ( %) Satisfies the relationship d <−0.5 × Vm + 16 (Vm <10%).
[0029]
Low-temperature transformation phase fraction: less than 10% Since the low-temperature transformation phase is the starting point of crack generation, it is necessary to reduce the low-temperature transformation phase fraction. Furthermore, since the present invention is mainly intended for automobile outer panels and the like that require high formability, it is very important to reduce the low-temperature transformation phase fraction as much as possible and to secure workability. Therefore, the low-temperature transformation phase fraction is specified as less than 10%. In order to further improve the resistance to secondary working brittleness and formability, the low-temperature transformation phase fraction is desirably less than 7%. On the other hand, in order to secure a desired strength, it is desirable that the low-temperature transformation phase fraction be 2% or more. Here, the low-temperature transformation phase mainly includes a martensite phase, but may further include a residual γ phase, a bainite phase, and a carbide.
[0030]
Ferrite grain size d (μm), low-temperature transformation phase fraction Vm (%): d <−0.5 × Vm + 16 (Vm <10%)
In order to obtain excellent secondary work brittleness resistance, it is extremely important that the ferrite grain size d and the low-temperature transformation phase fraction Vm be in the optimum ranges. That is, when the low-temperature transformation phase fraction is large, it is necessary to further reduce the ferrite grain size. As described above, when the ferrite grain size is larger than −0.5 × Vm + 16 in the range of Vm <10%, the ferrite grain size is not sufficiently small with respect to the low-temperature transformation phase. Brittleness cannot be obtained. Therefore, the ferrite grain size d is defined as a range satisfying the relational expression of d <−0.5 × Vm + 16 when Vm <10%. In order to further improve the resistance to secondary working brittleness, it is more preferable that the ferrite grain size be d <−0.5 × Vm + 11.
[0031]
Next, the manufacturing method of the present invention will be described.
In the present invention, as a manufacturing method capable of obtaining a high-strength thin steel sheet excellent in the above-mentioned secondary working brittleness resistance, a steel having the above-described component composition is melted, and then hot-rolled. After cold-rolling the rolled steel sheet, it is annealed in a temperature range of at least Ac ₁ point and not more than Ac ₃ points, and is then primarily cooled at a speed of more than 3 ° C./s to a temperature range of 450 to 700 ° C. The secondary cooling is performed at a speed of not more than the Ms point.
[0032]
In the present invention, since the role of the low-temperature transformation phase is very important, a fine and hard low-temperature transformation phase mainly composed of martensite is generated under the above manufacturing conditions. That is, in order to make the final microstructure into a ferrite + low-temperature transformation phase, annealing is performed in a range of _one Ac or more and _three Ac or less. As described above, it is effective to lower the low-temperature transformation phase fraction in order to further improve the secondary work brittleness resistance and the formability. Therefore, the annealing temperature, Ac ₁ point or more, _Ac 1 + 50 ° C. The following ranges is desirable.
[0033]
With respect to the cooling rate and the stop temperature of the subsequent primary cooling, it is necessary to perform primary cooling at a rate of more than 3 ° C./s to a temperature range of 450 to 700 ° C. in order to avoid deterioration in formability due to pearlite precipitation. However, in the present invention, since the low-temperature transformation phase is used, if the primary cooling rate is set to 30 ° C./s or more, the two-phase separation does not proceed sufficiently, so that the hard low-temperature transformation phase is not generated and the desired characteristics are obtained. May not be obtained. Therefore, it is desirable that the primary cooling rate be less than 30 ° C./s. In order to obtain a martensite phase more stably, it is desirable to perform primary cooling to a temperature range of 500 to 650 ° C.
[0034]
In the subsequent secondary cooling, it is necessary to cool to a temperature below the Ms point at a rate of 10 ° C./s or more in order to stably generate a low-temperature transformation phase. In order to secure a predetermined strength and obtain better characteristics, it is desirable to set the secondary cooling rate to more than 20 ° C./s. After the secondary cooling, an overaging treatment may be performed.
[0035]
Needless to say, the high-strength thin steel sheet obtained by the above description can achieve the intended effect even when it is used as an electro-galvanized steel sheet or a hot-dip galvanized steel sheet. 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.
[0036]
In the present invention, when 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 performed at an Ar ₃ transformation point or higher. The cooling rate may be set to 60 to 85% within the normal operation range.
[0037]
【Example】
Table 1 shows the steel numbers No. 1 to No. After smelting 15 steels, slabs were produced by continuous casting. As shown in Table 1, steel No. Nos. 1 to 11 are steels of the present invention in the range of the component composition specified in the present invention. Reference numerals 12 to 15 are comparative steels in which the C content, the Si content, the Mn content, and the P content exceed the upper limit and are out of the component range of the present invention.
[0038]
After heating these slabs to 1200 ° C., finish rolling was performed at a finishing temperature of at least _three points of Ar, and hot-rolled steel sheets were produced within the range of the winding temperature of the normal operation. The hot-rolled steel sheet was pickled and cold-rolled. Subsequently, annealing was performed at a soaking temperature and a cooling rate shown in Table 2 to obtain an annealed plate. The microstructure of the obtained annealed sheet was observed, the ferrite grain size and the low-temperature transformation phase fraction were measured, and the performance was evaluated.
[0039]
The tensile strength was measured by performing a tensile test on a JIS No. 5 tensile test piece. The vertical crack transition temperature was evaluated by the following method. First, a circular plate having a diameter of 100 mm was collected from each of the obtained annealed plates, and subjected to deep drawing at a drawing ratio of 2.0 to form a cylindrical cup having a diameter of 50 mm. Next, the ears of these cylindrical cups were removed to prepare a sample having a height of 30 mm. Thereafter, the sample prepared above was placed on a 60-degree truncated cone-shaped mold with the bottom surface facing upward, the entire tester was cooled to a predetermined temperature, and held for a certain period of time. The critical temperature at which brittle cracking occurred on the side wall of the cylindrical cup was determined and defined as the vertical crack transition temperature. Table 2 shows the test results together with the measurement results of the ferrite grain size and the low-temperature transformation phase fraction.
[0040]
In Table 2, No. Nos. 1 to 4, 7, 8, 10, 11, 13 to 16 are examples of the present invention in which the chemical composition range, low-temperature transformation phase fraction, and ferrite grain size all satisfy the present invention. The production conditions of the heat temperature, the primary cooling rate, and the secondary cooling rate are also satisfied. On the other hand, No. 5, 6, 9, 12, 17 to 24 are comparative examples in which any one of the chemical composition range, low-temperature transformation phase fraction, and ferrite particle size is out of the range of the present invention.
[0041]
FIG. 1 is a diagram showing the relationship between the low-temperature transformation phase fraction and the ferrite grain size shown in Table 2, and also shows the results of a longitudinal crack transition temperature investigation of each steel sheet. However, in the case of No. 3 whose chemical component range is out of the range of the present invention. 21 to 24 (Steel No., 12 to 15) are not shown. As shown in this figure, the steel sheets of the examples of the present invention produced under the conditions specified in the present invention all had an excellent vertical crack transition temperature of -80 ° C or higher. Further, when the ferrite grain size is smaller than -0.5 * Vm + 11, the vertical crack transition temperature is -100 [deg.] C. or lower, which is more preferable. On the other hand, when the ferrite grain size is larger than −0.5 × Vm + 16, the transition temperature of the vertical crack is −50 ° C. or lower, and it is found that the secondary work brittleness resistance is reduced.
[0042]
[Table 1]

[0043]
[Table 2]

[0044]
【The invention's effect】
As described above, according to the present invention, the chemical composition is controlled to a specific range, the structure is composed of ferrite + low-temperature transformation phase, and the relationship between the grain size and the low-temperature transformation phase fraction is optimized. Thus, a high-strength thin steel sheet having high formability and excellent secondary work brittleness resistance can be obtained. Therefore, the steel sheet of the present invention can be applied to a wide range of fields, such as steel sheets for automobiles and home appliances.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the low-temperature transformation phase fraction and the ferrite grain size of a steel sheet according to an example and the resistance to secondary working embrittlement.

Claims (3)

mass%, C: 0.05% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.03% or less, Al: 0.1% Hereinafter, N: 0.01% or less, the balance substantially consisting of iron, microstructure consisting of ferrite and a low-temperature transformation phase having a volume fraction of less than 10%, and ferrite grain size d (μm) and low-temperature transformation phase component A high-strength thin steel sheet excellent in secondary work brittleness resistance, wherein the rate Vm (%) satisfies the relationship of d <−0.5 × Vm + 16 (Vm <10%). Further, in mass%, Cr: 1% or less, Mo: 1% or less, V: 1% or less, B: 0.01% or less, Ti: 0.1% or less, Nb: 0.1% or less The high-strength thin steel sheet excellent in secondary work brittleness resistance according to claim 1, comprising at least one kind. After smelting the steel having the component composition according to claim 1 or 2, hot rolling is performed, and the obtained hot-rolled steel sheet is cold-rolled, and then annealed in a temperature range of ₁ point of Ac or more and ₃ points of Ac or less, A secondary working brittle resistance characterized by successively cooling at a rate of more than 3 ° C / s to a temperature range of 450 to 700 ° C and then at a rate of 10 ° C / s or more to a temperature below the Ms point. Method for producing high-strength thin steel sheet with excellent quality.

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