JP2660644B2 - High strength steel sheet with good press formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art As a convenient and comfortable means of transportation, the position of automobiles in the national life is increasing year by year, and to reduce fuel consumption and curb fossil fuel consumption to prevent environmental destruction and global warming. Are 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 constituent material of the vehicle, without impairing the formability. . Index values of formability include n-values and r-values, including elongation in a tensile test, but simplification of the pressing process by integral molding has become an issue in recent years. It is becoming especially important.

[0003] For this reason, the utilization of transformation-induced plasticity of retained austenite has been proposed.
Only about 7 to 0.4% of C, about 0.3 to 2.0% of Si and about 0.2 to 2.5% of Mn are used as basic alloying elements, and after annealing in a two-phase region, A steel sheet containing retained austenite in the metal structure by heat treatment characterized by performing bainite transformation at a temperature of 450 ° C.
0715 and JP-A-2-217425. This type of steel sheet can be obtained not only from a cold-rolled steel sheet manufactured by continuous annealing but also from a hot-rolled steel sheet by controlling the cooling and winding temperature at a run-out table as disclosed in JP-A-1-79345. Extensive practical use is expected.

[0004]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open No. 1-2
The steel sheets disclosed in Japanese Patent No. 30715 and Japanese Patent Application Laid-Open No. 2-217425 add 0.3 to 2.0% of Si and utilize the peculiar bainite transformation to secure the retained austenite. Unless cooling after annealing in the two-phase coexisting temperature range and holding in the temperature range of 300 to 450 ° C. are considerably strictly controlled, the intended metallographic structure cannot be obtained, and the strength and elongation are out of the target ranges. Although this heat history is industrially realized in a continuous annealing facility or a run-out table and a winding step after hot rolling, the transformation of austenite is quickly completed at 450 to 600 ° C.
Control is required to shorten the time for staying at ~ 600 ° C in particular, and the metal structure changes remarkably depending on the time for holding at 350-450 ° C, so that if it is out of the intended conditions, only stale strength and elongation can be obtained. .

[0005] Further, since the residence time at 450 to 600 ° C is long and Si, which deteriorates the plating property, is contained as an alloying element, it cannot be passed through a hot-dip plating facility to form a plated steel sheet. Industrial use was hindered. The present invention solves such a problem, and can be easily manufactured by continuous annealing equipment or cooling and subsequent winding at a run-out table after hot rolling, and can also be manufactured by hot-dip plating equipment to improve surface corrosion resistance. It has been found that the composition and the metal structure of a high-strength steel sheet having good press formability have been found.

[0006]

To solve the above-mentioned problems, the present invention has a metal structure in which ferrite is used as a matrix and 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 sheets with good formability by continuous annealing equipment, hot-dip plating equipment or cooling at the run-out table after hot rolling and subsequent winding without difficulty, reduce the amount of Si added,
The present inventors have found that it is important to add Al while maintaining a constant relationship with the amounts of C and Si. The present invention has been made by paying attention to the fact that addition of Mo is effective as needed. That is, in the present invention, C: 0.06 to 0.22% by weight, Si: more than 0.5,
1.0% or less , Mn: 0.5 to 2.0%, Al: 0.2
5 to 1.5%, and the relationship between Al, Si and C satisfies 0.6Si (%) ≦ Al (%) ≦ 3-12.5C (%). 0.03-0.3
%, The balance being Fe and unavoidable impurities, and containing 3 to 20% by volume of retained austenite in the metal structure and having good press formability.

[0007] First, the reasons for limiting the range of components of the steel to which the present invention is applied will be described. First, C is an austenite stabilizing element, which migrates from ferrite in the two-phase coexisting temperature range and the bainite transformation temperature range and becomes concentrated in austenite. As a result, 3 to 20% of chemically stabilized austenite remains even after cooling to room temperature, and improves formability by transformation induced plasticity. 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, when the content exceeds 0.22%, the weldability is deteriorated, so that it must be avoided.

[0008] Si does not form a solid solution in cementite, but delays the transformation from austenite at 350 to 600 ° C by suppressing its precipitation. 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 which contributes to improving the formability can be secured. If the amount of Si is less than 0.5 %, the effect cannot be found. On the other hand, an excessive addition exceeding 1.0% must be avoided because it causes scale during hot rolling so as to deteriorate the pickling property and significantly deteriorates the plating property.

Mn is an austenite-forming element and prevents austenite from decomposing into pearlite during cooling to 350 to 600 ° C. after annealing in a two-phase coexisting temperature range. In order to include retained austenite. If the addition is less than 0.5%, it is necessary to increase the cooling rate so that industrial control cannot be performed to suppress the decomposition to pearlite, which is not appropriate. On the other hand, if it exceeds 2%, the band structure becomes remarkable and the characteristics are deteriorated, and the spot welded portion is easily broken in the nugget, which is not preferable.

Al, like Si, does not form a solid solution in cementite, and suppresses the precipitation of cementite when held at 350 to 600 ° C., thereby delaying the progress of transformation. However, since the transformation start is early due to the stronger ferrite forming ability than Si, C is concentrated in austenite and the chemical stability is increased even during holding in a two-phase coexisting temperature range even if holding for a very short time. There is only a small amount of martensite which 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 can be 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 the content exceeds 1.5%, the steel becomes significantly embrittled and must be avoided. 3-12.5
If Al exceeds C (%) in the steel sheet, although austenite is present after cooling to room temperature, it is chemically extremely stable and does not transform even during press forming, and transformation-induced plasticity is not obtained, so that formability is good. It can not be said.

The steel sheet of the present invention has the above basic components,
In addition to these elements and Fe, P, S, N and other elements inevitably mixed into general steel are included.
Mo prevents austenite from being decomposed into pearlite during cooling to 350 to 600 ° C. after annealing in a two-phase coexisting temperature range, similarly to Mn, and has a ferrite forming element of 350 to 600 ° C., similar to Al. In this case, since the transformation is started early, if it coexists with an element that does not form a solid solution in cementite, such as Si or Al, it is easy to generate retained austenite. Therefore, including 0.03-0.3% achieves the object of the present invention. Preferred above. The amount is 0.03
%, The effect is not clear. However, if the content exceeds 0.3%, carbides become apparent in the metal structure, the press formability deteriorates, and the cost increases, which is not industrially preferable.

A hot-rolled steel sheet containing these components can be easily converted into a matrix by using ferrite as a bainite, a martensite, or a cold-rolled steel sheet or a hot-dip coated steel sheet by appropriately selecting the heat history during continuous annealing or hot-dip coating. Sites and 3-2
It becomes a metal structure in which 0% of retained austenite is mixed.
The retained austenite contained in the metal structure of such a steel sheet is stable when not deformed, but transforms into martensite when deformed and exhibits transformation-induced plasticity, so good formability and high strength are achieved. Is obtained. When the volume fraction of retained austenite is less than 3%, no clear effect is observed. On the other hand, if the volume fraction of retained austenite exceeds 20%, when extremely severe forming is performed, a large amount of martensite may be present in a pressed state, which may cause problems in secondary workability and impact properties. is there.

In the case of a hot-rolled steel sheet, especially 700 to 1000
After finishing at 4 ° C, 4-20
After cooling at a rate of 0 ° C./s and winding at 350 to 600 ° C., and particularly in a cold-rolled steel sheet or a hot-dip coated steel sheet, after annealing in a two-phase coexisting temperature range of 650 to 900 ° C. for 10 seconds to 3 minutes, 350-60 at a cooling rate of 4-200 ° C / s
After cooling to 0 ° C., maintaining the temperature in a temperature range of 350 to 600 ° C. for 5 seconds to 10 minutes, and cooling at a cooling rate of 5 ° C./s or more to 250 ° C.
Cooling to not more than ℃ is desirable for improving the formability by transformation induced plasticity. In addition, as long as the metal structure contains 3 to 20% by volume of retained austenite in the metal structure, the surface layer may be hot-dipped with a plating layer of any composition or may be electroplated. The spirit of the invention is not impaired.

[0014]

EXAMPLES Hot-rolled steel sheets, cold-rolled steel sheets, hot-dip coated steel sheets, and electroplated steel sheets as shown in Table 2 were obtained from the steels shown in Table 1 and JIS No. 5 tensile test pieces were collected. When a room temperature tensile test was performed at 50 mm and a tensile speed of 10 mm / min, tensile strength and total elongation as described in the table were obtained. In the same table, the volume ratio of retained austenite measured by X-ray diffraction after chemically polishing a 1/4 inner layer from the surface layer is also described. In the sample No. of the present invention. 2,
3, 5 to 8 have a tensile strength of 500M regardless of the type of hot-rolled steel sheet, cold-rolled steel sheet, hot-dip coated steel sheet, and electro-plated steel sheet.
The total elongation is 35% or more while being Pa or more, and it has excellent deep drawability, hole expandability, bendability, etc., and achieves both high strength and good press moldability.

On the other hand, even if the composition of the present invention falls within the range of the composition, the sample No. A high-strength steel sheet having good press formability as in No. 4 is not obtained. In addition, Sample No. out of the range of the component of the present invention. Sample Nos. 1 and 9 to 14 are samples No. 1 if the production conditions are strictly controlled. Although a steel sheet having a relatively high strength and good press formability as in the case of Sample Nos. 10 to 14 is obtained, it is difficult to obtain it industrially stably.
In many cases, only stale characteristics such as 1, 9, 11 to 13 can be obtained, so that the object of the present invention cannot be achieved.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

As is clear from the above examples, the steel sheet having the components and the metal structure according to the present invention has a high quality in any of hot-rolled steel sheet, cold-rolled steel sheet, hot-dip coated steel sheet and electroplated steel sheet. Good press moldability despite strength
Since it can be easily manufactured industrially, it has an extremely remarkable effect on industry.

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yoshikazu Matsumura 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (56) References JP-A-5-247586 (JP, A)

Claims (2)

(57) [Claims] 1. C: 0.06-0.22% by weight% Si: more than 0.5 , 1.0% or less Mn: 0.5-2.0% Al: 0.25-1.5% And Al and Si,
The relationship of C satisfies 0.6Si (%) ≦ Al (%) ≦ 3-12.5C (%), and the balance consists of Fe and inevitable impurities. High strength steel sheet with good press formability. 2. C: 0.06-0.22% by weight% Si: more than 0.5, 1.0% or less Mn: 0.5-2.0% Al: 0.25-1.5% Mo: 0.03 to 0.3%, and Al and Si,
The relationship of C satisfies 0.6Si (%) ≦ Al (%) ≦ 3-12.5C (%), and the balance consists of Fe and inevitable impurities. High strength steel sheet with good press formability.