JPH06145788A - Production of high strength steel sheet excellent in press formbility - Google Patents

Abstract

PURPOSE:To produce a high strength steel sheet excellent in press formability by utilizing the transformation-induced plasticity of residual austenite. CONSTITUTION:A cold rolled sheet of a steel having a composition which contains 0.06-0.22% C, 0.05-1.0% Si, 0.5-2.0% Mn, and 0.25-1.5% Al and also contains, if necessary, 0.03-0.3% Mo and where the relaion among Al, Si, and C satisfies 0.6Si(%)<=Al(%)<=3-12.5C(%) is annealed in a two phase coexiting temp. region of 650-900 deg.C for 10sec-3min, cooled down to 350-600 deg.C at (4 to 200) deg.C/s cooling rate, held at the temp. in this range for 5sec-10min, and then cooled down to <=250 deg.C at >=5 deg.C/s cooling rate, by which the high strength steel sheet containing residual austenite by 3-20% by volume ratio and excellent in press formability can be obtained. By this method, this steel sheet can be easily produced by using continuous annealing equipment and can also be produced even by using hot dipping equipment, and further, the surface corrosion resistance of the high strength steel sheet can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high strength steel sheet having good press formability.

[0002]

2. Description of the Related Art The position of automobiles as a convenient and comfortable means of transportation in the people's lives is increasing year by year, and fuel consumption is reduced and fossil fuel consumption is suppressed in order to prevent environmental damage and global warming. Is becoming more important than ever. For this reason, it is required to improve the engine performance and reduce the weight of the vehicle body, and it is required to further increase the strength and the surface corrosion resistance of the steel sheet, which is a main vehicle body constituent material, without impairing the formability. . The index value of formability includes n value and r value including elongation in a tensile test, but simplification of the pressing process by integral molding has been an issue in recent years, and the n value corresponding to uniform elongation is large. Above all, it is becoming important.

For this reason, utilization of transformation-induced plasticity of retained austenite is proposed, and 0.0
Only 7 to 0.4% C, 0.3 to 2.0% Si and 0.2 to 2.5% Mn are the basic alloying elements, and 300 to 300% after annealing in the two-phase region. A steel sheet containing retained austenite in its metallographic structure by heat treatment characterized by performing bainite transformation at temperatures inside and outside 450 ° C.
No. 0715 and Japanese Patent Laid-Open No. 2-217425. This type of steel sheet is not only a cold-rolled steel sheet manufactured by continuous annealing but also a hot-rolled steel sheet obtained by controlling the cooling and winding temperature at a run-out table as in JP-A-1-79345. Widespread practical application is expected.

[0004]

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention
In the steel sheets disclosed in Japanese Patent No. 30715 and Japanese Patent Laid-Open No. 2-217425, 0.3 to 2.0% of Si is added, and retained austenite is secured by utilizing the unique bainite transformation. Unless the cooling after annealing in the two-phase coexisting temperature range and the holding in the temperature range of 300 to 450 ° C. are controlled fairly strictly, the intended metal structure cannot be obtained, and the strength and elongation deviate from the target ranges. Although this thermal history is industrially realized in continuous annealing equipment, it is 450-60
At 0 ° C, the transformation of austenite is completed promptly, so control to shorten the residence time at 450 to 600 ° C is particularly required, and the metal structure remarkably changes depending on the holding time at 350 to 450 ° C. If the condition is not met, only unobtrusive strength and elongation can be obtained. Again 45
Since it stays at 0 to 600 ° C for a long time and contains Si as an alloying element that deteriorates the plating property, it cannot be passed through a hot dip plating facility to form a plated steel plate, and its surface corrosion resistance is inferior. Was being hampered. The present invention solves such problems and provides a method capable of easily producing a high-strength steel sheet having good press formability by continuous annealing equipment and also by hot dipping equipment for improving surface corrosion resistance.

[0005]

[Means for Solving the Problems] In order to solve the above problems, a ferrite has a matrix and has a metal structure in which bainite, martensite and 3 to 20% of retained austenite are mixed, and the transformation-induced plasticity of retained austenite is utilized. In order to manufacture high strength steel plates with good formability without difficulty using continuous annealing equipment or hot dip coating equipment, the amount of Si added should be reduced and C or S
The present inventors have found that it is important to add Al while maintaining a constant relationship with the amount of i. The present invention has been made by paying attention to the fact that it is also effective to add Mo if necessary. That is, in the present invention, C: 0.06 to 0.22% and Si: 0.05 to 1.0 by weight%.
%, Mn: 0.5 to 2.0%, Al: 0.25 to 1.5
%, And the relationship between Al, Si, and C satisfies 0.6Si (%) ≦ Al (%) ≦ 3-12.5C (%), and Mo: 0.03 to 0 if necessary. .3
%, And the balance of Fe and unavoidable impurities in a cold-rolled steel sheet in a two-phase coexistence temperature range of 650 to 900 ° C. for 10 seconds to
After annealing for 3 minutes, 350 at a cooling rate of 4 to 200 ° C./s
To 600 ° C., hold in a temperature range of 350 to 600 ° C. for 5 seconds to 10 minutes, and then cool to 250 ° C. or less at a cooling rate of 5 ° C./s or more, in a metallographic structure. Is a method for producing a high-strength steel sheet having good press formability, containing 3 to 20% by volume of retained austenite.

First, the reasons for limiting the composition range of the steel targeted by the present invention will be described. First, C is an austenite stabilizing element, which moves from the ferrite in the two-phase coexistence temperature range and the bainite transformation temperature range and is concentrated in the austenite. As a result, the chemically stabilized austenite remains 3 to 20% after cooling to room temperature, and the transformation-induced plasticity improves the formability. 3% if C is less than 0.06%
It is difficult to secure the above retained austenite,
I can't achieve my purpose. On the other hand, if it exceeds 0.22%, the weldability is deteriorated and it must be avoided.

Si does not form a solid solution in cementite and suppresses its precipitation, thereby delaying the transformation from austenite at 350 to 600 ° C. During this period, the concentration of C in the austenite is promoted, so that the chemical stability of the austenite is increased, transformation-induced plasticity is caused, and retained austenite that contributes to good moldability can be secured. If the amount of Si is less than 0.05%, the effect cannot be found. On the other hand, excessive addition of more than 1.0% causes scale during hot rolling to the extent that pickling performance is deteriorated, and significantly deteriorates plating property, and therefore must be avoided.

Mn is an austenite-forming element, and since it prevents austenite from decomposing into pearlite during cooling to 350 to 600 ° C. after annealing in the two-phase coexisting temperature range, it has a metallic structure after cooling to room temperature. Include retained austenite. Addition of less than 0.5% is not appropriate because it is necessary to increase the cooling rate so that it cannot be industrially controlled to suppress decomposition into pearlite. On the other hand, if it exceeds 2%, the band structure becomes remarkable and the characteristics are deteriorated, and the spot welds are easily broken in the nugget, which is not preferable.

Al, like Si, does not form a solid solution with cementite, and suppresses the precipitation of cementite during holding at 350 to 600 ° C. and delays the progress of transformation. However, since the ferrite forming ability is stronger than that of Si, the transformation starts earlier, and C is more concentrated in the austenite than in the annealing in the two-phase coexistence temperature range even if it is held for a very short time, and the chemical stability is increased. There is only a small amount of martensite that deteriorates the formability in the metal structure after cooling to room temperature. For this reason, when coexisting with Si, changes in strength and elongation due to holding conditions at 350 to 600 ° C. are small, and high strength and good press moldability are easily obtained. If the amount is less than 0.25% or less than 0.6 times the amount of Si added, such an effect is not recognized. On the other hand, if it exceeds 1.5%, the embrittlement of the steel becomes remarkable, so it must be avoided. Also 3-12.5
When Al is contained in the steel sheet in excess of C (%), austenite is present after cooling to room temperature, but it becomes chemically stable and does not transform during press forming, so transformation-induced plasticity cannot be obtained and formability is good. It can not be said.

The steel sheet of the present invention has the above-mentioned basic components,
In addition to these elements and Fe, P, S, N and other elements inevitably mixed with general steel are included.
Further, Mo, like Mn, prevents austenite from decomposing into pearlite in the course of cooling to 350 to 600 ° C. after annealing in the two-phase coexisting temperature range, and 350 to 600 ° C. as a ferrite forming element like Al. In that case, the transformation starts early, so that when it coexists with an element that does not form a solid solution with cementite, such as Si or Al, it is easy to generate retained austenite. Therefore, the inclusion of 0.03 to 0.3% achieves the object of the present invention. It is preferable above. The amount is 0.03
If it is less than%, the effect is not clear. However, if it exceeds 0.3%, carbides become visible in the metal structure, the press formability deteriorates, and the cost increases, which is not industrially preferable.

Next, the reasons for limitation in the process will be described. The purpose of the process is to adjust the volume ratio of retained austenite to 3 to 20%, and as a result, good formability can be obtained with high strength due to transformation-induced plasticity. Volume ratio of retained austenite is 3%
If it is less than, no clear effect is observed. On the other hand, when the volume ratio of retained austenite exceeds 20%, when extremely difficult forming is performed, a large amount of martensite may exist in the press-formed state, which causes a problem in secondary workability and impact resistance. Sometimes.

The cold rolled steel sheet is first annealed in the two phase coexisting temperature range of 650 to 900 ° C. for 10 seconds to 3 minutes. By this annealing, a fine recrystallized structure in which ferrite and austenite coexist is formed, and at the same time, austenite stabilizing elements such as C and Mn are concentrated to some extent in austenite, and stabilize the austenite during the microstructural change accompanying a series of subsequent heat treatments. Make it easy. If the annealing temperature is lower than 650 ° C, a sufficient amount of austenite is not formed, and recrystallization may not be completed in some cases. On the other hand, annealing at a temperature over 900 ° C. requires a large amount of energy and is uneconomical. Further, since the distribution ratio of C between austenite and ferrite is small, the chemical stability of austenite deteriorates. Since the process is severely restricted, the intended metallographic structure cannot be easily obtained. If the annealing time is less than 10 seconds, the carbide is not sufficiently solid-dissolved, and only a small amount of austenite is formed. Annealing for more than 3 minutes not only wastes energy, but also reduces productivity in a continuous line.

The steel sheet after annealing is subsequently kept at 4 to 200 ° C. /
It is cooled to a range of 350 to 600 ° C. at a cooling rate of s, and its purpose is to bring austenite formed during the two-phase region annealing to the bainite transformation region without decomposing it into pearlite. Cooling rate in this case is 4 ℃ / s
If the amount is less than the above, pearlite is generated and precipitates as a carbide, so that the solid solution C that stabilizes austenite most effectively decreases, and the intended metallographic structure cannot be obtained. On the other hand, 200
If it exceeds ℃ / s, it is difficult to stop cooling at the target temperature, and even if it can be achieved, the shape of the steel sheet is bad and it cannot be used for press forming. Further, when cooling is stopped at a temperature exceeding 600 ° C., decomposition into pearlite occurs rapidly and austenite cannot remain. On the other hand, when the cooling end temperature is lower than 350 ° C., a majority of austenite is transformed into martensite, so that the press formability deteriorates although the strength becomes high.

After this, in the present invention, 350-60.
Hold in the temperature range of 0 ° C for 5 seconds to 10 minutes, then 5 ° C /
It is cooled to 250 ° C. or lower at a cooling rate of s or more, but the purpose is to convert C into untransformed austenite during bainite transformation.
Is further promoted to obtain retained austenite that causes transformation-induced plasticity at room temperature. As described above, since Si and Al are contained in combination, a metal structure that provides high strength and good press formability can be stably obtained within such a relatively large variation range of temperature and time. However, when the temperature is maintained above 600 ° C., pearlite is generated, and retained austenite is not contained in the metal structure.
On the other hand, if the temperature is lower than 350 ° C., the diffusion of C is extremely slow even with the bainite transformation, and since the shear transformation becomes dominant, the retained austenite exhibiting transformation-induced plasticity cannot be effectively obtained after cooling to room temperature. Further, if the holding time is less than 5 seconds, bainite is not sufficiently formed although Al is contained, C concentration in untransformed austenite is insufficient, and martensite is formed during cooling to room temperature, resulting in press formability. Make it worse. On the other hand, holding for more than 10 minutes leads to waste of energy, reduction in productivity of continuous lines, and deterioration of both strength and press formability due to carbide precipitation and disappearance of untransformed austenite. Cooling after this holding is 5 ℃
For the same reason, it is also necessary to avoid stopping at less than / s or stopping at a temperature exceeding 250 ° C.

In this series of heat treatments, the holding temperature does not have to be constant as long as it is within the specified temperature range, and the fact that the cooling rate changes within the specified range during cooling does not impair the gist of the present invention. In particular, holding in the temperature range of 350 to 600 ° C. for 5 seconds to 10 minutes may be such that the material is supercooled and then reheated within this temperature range, any of which is illustrated in FIG. 1. Also, as long as the heat history is satisfied,
The steel sheet may be heat-treated by any equipment such as continuous annealing equipment and hot dipping equipment. It does not hinder the purpose of the present invention to perform temper rolling for shape correction after heat treatment, or to coat the surface layer with electroplating or the like. Note that the material of the present invention is generally manufactured by steelmaking, casting, hot rolling process, but may be manufactured by thin casting and omitting a part or all of the hot rolling process. There is no problem. Hot rolling conditions also do not matter.

[0016]

[Examples] Steels whose components are shown in Table 1 are hot rolled and pickled, and then 75
Cold rolling at a rolling rate of 1.0% to obtain a cold rolled steel sheet having a thickness of 1.0 mm, and then a series of heat treatments and 0.6% temper rolling configured under the series of conditions shown in Tables 2 and 3. And then J
When an IS5 tensile test piece was sampled and subjected to a room temperature tensile test at a gauge length of 50 mm and a tensile speed of 10 mm / min, the tensile strength and total elongation as shown in Tables 2 and 3 were obtained. Tables 2 and 3 also show the volume ratio of the retained austenite measured by X-ray diffraction after chemical polishing of the inner layer of 1/4 of the plate thickness from the surface layer. Sample N of the present invention sample
o. Nos. 2 to 4, 9 and 13 contained 3 to 20% by volume of retained austenite in the metal structure and had a tensile strength of 500M.
The total elongation is 35% or more even though it is Pa or more, and deep drawability, hole expandability, bendability, etc. are excellent, and both high strength and good press formability are achieved.

On the other hand, S outside the range of the components of the present invention
Steel f containing Al in an amount not more than 0.6 times the addition amount of i was selected as Sample No. 1 if heat treatment conditions were appropriately selected and heat treatment was performed under conditions very close to the set conditions. As in No. 19, high strength and press formability comparable to those of the sample of the present invention can be obtained, but if the conditions are slightly different, the properties obtained as in Sample 20 become obsolete. Further, steels a, e, g which deviate from the other components of the present invention other than f
Also in sample j. As described in Nos. 1, 18, 21 to 24, either one of high strength and good press moldability is not satisfied, and in some cases, both are not satisfied. On the other hand, even if the steel of the present invention has one or more inappropriate processing conditions, Sample N
o. As described in 5-8, 10-12, and 14-17, either high strength or good press formability is not satisfied in some cases, and in some cases, both problems cannot be solved.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

[0021]

As is apparent from the above examples, according to the present invention, the retained austenite is contained in the metal structure in a volume ratio of 3%.
A high-strength steel sheet having a good press formability of up to 20% can be easily produced by continuous annealing equipment and can be produced by hot dip equipment because it improves surface corrosion resistance, and its industrial effect is extremely remarkable.

[Brief description of drawings]

FIG. 1 is a diagram illustrating some of the heat treatment conditions in the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshikazu Matsumura 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd.

Claims (2)

[Claims] 1. By weight%, C: 0.06 to 0.22% Si: 0.05 to 1.0% Mn: 0.5 to 2.0% Al: 0.25 to 1.5% And Al and Si,
The relationship of C satisfies 0.6 Si (%) ≤ Al (%) ≤ 3-12.5 C (%), and the cold-rolled steel sheet made of the balance Fe and unavoidable impurities is subjected to two-phase heating at 650 to 900 ° C. After annealing for 10 seconds to 3 minutes in the coexistence temperature range, 35 at a cooling rate of 4 to 200 ° C / s.
Cooling to 0 to 600 ° C, holding in a temperature range of 350 to 600 ° C for 5 seconds to 10 minutes, and then cooling to 250 ° C or less at a cooling rate of 5 ° C / s or more, a metallographic structure. A method for producing a high-strength steel sheet having good press formability, containing retained austenite in a volume ratio of 3 to 20%. 2. By weight%, C: 0.06 to 0.22% Si: 0.05 to 1.0% Mn: 0.5 to 2.0% Al: 0.25 to 1.5% Mo: 0.03 to 0.3% and Al and Si,
The relationship of C satisfies 0.6 Si (%) ≤ Al (%) ≤ 3-12.5 C (%), and the cold-rolled steel sheet made of the balance Fe and unavoidable impurities is subjected to two-phase heating at 650 to 900 ° C. After annealing for 10 seconds to 3 minutes in the coexistence temperature range, 35 at a cooling rate of 4 to 200 ° C / s.
Cooling to 0 to 600 ° C, holding in a temperature range of 350 to 600 ° C for 5 seconds to 10 minutes, and then cooling to 250 ° C or less at a cooling rate of 5 ° C / s or more, a metallographic structure. A method for producing a high-strength steel sheet having good press formability, containing retained austenite in a volume ratio of 3 to 20%.