JPH05195056A - Production of steel sheet having high ductility and high strength - Google Patents

Abstract

PURPOSE:To produce a high strength steel sheet reduced in Si content and having high ductility by coiling a hot rolled plate of a steel having a composition containing specific amounts of C, Mn, etc., and then subjecting this steel plate, in the as-hot-rolled state or after cold rolling, to annealing at specific temp. CONSTITUTION:A slab of a steel having a composition which contains, by weight, 0.10-0.40% C, 2.0-4.0% Mn, 0.01-0.10% Al, and at least one kind among <=1.0% (preferably <=0.5%) Si, <=0.10% P, and <=0.015% S or further contains >=at least one kind among specific small amounts of Ni, Cr, Mo, Cu, Nb, Ti, V, REM, Ce, Ca, Zr, and B and where the total content of C and Mn/5 is regulated to >=0.60% is hot-rolled into a plate. The hot rolled plate is coiled at 300-750<=, by which a structure free from polygonal ferrite is formed. This steel plate is held, in the as-hot-rolled state or after cold rolling, at 620-750 deg.C for >=60sec to undergo annealing treatment, by which retained austenite of >3% volume percentage can be formed.

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-ductile high-strength steel sheet suitable for the field of press-forming thin steel sheet such as automobiles and construction.

[0002]

2. Description of the Related Art In forming automobile parts such as wheels and rear cross members, steel sheets are required to have high ductility. In recent years, from the viewpoint of reducing the weight of automobiles, these parts have been made high tensile strength, and development of hot-rolled and cold-rolled high-tensile steel sheets having excellent elongation has been earnestly desired.

As such a high-tensile steel sheet, conventionally, a two-phase structure consisting of ferrite and manrutensite structure is used.
(DP) steel and retained austenite with a volume fraction of 3% or more
A γR steel containing (γR) or a three-phase (Tri · P) steel composed of ferrite, bainite and mantensite has been developed and put into practical use. All of these achieve high ductility by containing a relatively large amount (0.7% or more) of Si.

On the other hand, plated steel sheets are frequently used for the purpose of improving the anticorrosive ability of automobile parts, and there is an increasing need for plating of the above-mentioned high ductility steel.
For steel, electro-Zn plating (EG) and hot-dip Zn plating (G)
(I, GA), it is not in practical use at present. That is, it is necessary to reduce Si for plating, but on the other hand, there is a problem that ductility is significantly reduced due to the reduction of Si.

An object of the present invention is to solve the above problems of the prior art and to provide a method capable of producing a high strength steel sheet having a low Si system and high ductility.

[0006]

DISCLOSURE OF THE INVENTION The inventors of the present invention have conducted extensive studies on low Si steels in order to solve the above problems, and as a result, produce a certain amount of retained austenite in the low Si steels. It has been found that the manufacturing conditions can be realized in the current thin steel plate manufacturing technology, and a high-ductility and high-strength steel plate of TS590N / mm ² or more can be obtained.

That is, according to the present invention, C: 0.1 to 0.4.
Steel containing 0%, Mn: 2.0 to 4.0%, Al: 0.01 to 0.10%, and having a composition satisfying C + (Mn / 5): 0.60% or more, After hot rolling, a steel sheet containing no polygonal ferrite is obtained by winding the coil in the range of 300 to 750 ° C., and as-rolled or after cold rolling, it is held in the range of 620 to 750 ° C. for 60 seconds or more. Thus, the gist is a method for producing a high-ductile high-strength steel sheet, which is characterized in that retained austenite having a volume ratio of 3% or more is generated.

The present invention will be described in more detail below.

[0009]

[Action]

Retained austenite is known to improve the ductility of steel through the TRIP (strain-induced transformation plasticity) phenomenon, and relatively large amount of Si, which is the basic idea of conventional steel, is used for generation of retained austenite. Is common. That is, the inclusion of Si promotes the C concentration in austenite and stabilizes the austenite, thereby generating retained austenite. It is known that Mn in steel also contributes to the formation of residual austenite by concentrating it in austenite, but an annealing process is required for Mn enrichment, and it has been difficult to use it in the laboratory because of the low market needs so far. It goes beyond the basic research at the level.

The inventors of the present invention have conducted a research study on the formation of retained austenite in low-Si steel while taking into consideration the weldability, paintability, etc. that should be provided as a steel sheet for automobiles. The present invention has been completed.

First, the following are important examination items that have led to the present invention. (1) Appropriate chemical composition in consideration of weldability of steel sheet, magnet handling of steel material at steelmaking site, hot ductility, rolling load, etc. (2) A hot-rolled sheet structure for stably producing an appropriate amount of retained austenite after annealing within the above constraints. (3) Annealing conditions for stabilizing the material.

The above problem (1) is extremely important in terms of manufacturing equipment technology and practical application of products. Especially Mn exceeding 5%
In terms of the amount, there is a problem in magnet handling, slitting of hot ductile steel sheet, etc., and it is necessary to keep the amount of Mn relatively low. Further, since various resistance welding and arc welding are carried out in the assembly of parts at the automobile production site, it is particularly important to keep the C content of steel low. An important issue is how to stably generate retained austenite in a region where the amount of Mn and C is low.

The problem (2) is to clarify the hot rolling conditions within the component restrictions (1). In the previous reports, when low Si steel was heat treated in a laboratory to complete martensite, or as a part of DP steel production, Mn to the second phase (austenite phase in the heat treatment stage) in the polygonal ferrite matrix was This is a description of concentration, and there is no research on a coil-rolled hot rolled sheet. On the other hand, in the present invention, a structure different from any of these structures (that is, a structure containing complete martensite or polygonal ferrite) is used as a hot rolled sheet structure, and can be achieved by a hot rolled coil.

The above-mentioned problem (3) is to anneal for finally producing retained austenite, and to define an appropriate condition for continuous annealing or batch annealing. By annealing under predetermined conditions, 3% or more of retained austenite is generated and a high ductility high tensile steel sheet can be obtained. It is also possible to apply annealing after cold rolling the hot rolled sheet.

The reasons for limiting the chemical composition of steel and the production conditions in the present invention will be described below.

C, Mn: C is less than 0.10%, Mn is 2.0
If it is less than%, the formation of retained austenite cannot be substantially expected. On the other hand, when C exceeds 0.40% and Mn exceeds 4.0%, there are problems that cracking occurs due to deterioration of hot ductility and magnet handling is impossible. Therefore, the C content is set to 0.10 to 0.40% and the Mn content is set to 2.0 to 4.0%. However, it is necessary to satisfy the relationship of C + (Mn / 5) ≧ 0.60%. This is a necessary condition for stable production of retained austenite in actual production, and is important for stabilizing the microstructure of hot-rolled sheet and achieving production of 3% or more retained austenite throughout the coil. ..

Al: Al may be in a range that normally acts as Al-killed steel. This range is 0.01 to 0.10%.

In addition to the above components, at least one of the following elements for the purpose of increasing the strength of the product, improving the workability by controlling the non-metallic inclusions, and promoting the bainization of the hot rolled sheet structure.
The inclusion of seeds does not hinder the effect of the present invention.

Si ≤ 1.0%, P ≤ 0.10%, S ≤ 0.0
15%, Ni ≦ 1.0%, Cr ≦ 1.0%, Mo ≦ 0.5%,
Cu ≦ 1.0%, REM ≦ 0.05%, Ce ≦ 0.05%,
Ca≤0.05%, Zr≤0.05%, Nb≤0.08%, T
i ≦ 0.08%, V ≦ 0.08%, B ≦ 0.0030%. The reasons for limiting each component are as follows.

The present invention mainly aims to form retained austenite in a low Si steel, but it is possible to add Si up to 1.0% in applications where there is no restriction of low Si. If the Si content exceeds 1.0%, the load on the rolling mill during hot rolling increases due to the increased deformation resistance of the steel, and annealing at a higher temperature is required, which is disadvantageous in addition to the cost of adding Si. Therefore, it is not preferable.

The upper limit of P content (0.1%) is defined by the hot ductility of steel. It is desirable that S is as low as possible in order to maintain the workability of steel, and is made 0.015% or less. Further, addition of elements such as REM, Ca, Ce, and Zr, which are elements for spheroidizing sulfide-based inclusions, is more effective for controlling the shape of non-metallic inclusions, but shape control is possible without deteriorating the cleanliness of steel. In order to achieve this, the upper limit is 0.05%.

Ni, Cu, Cr and Mo are effective not only for solid solution strengthening of steel but also for controlling the transformation structure, and can be added depending on the amount of base components such as C and Mn. However, excessive addition increases the susceptibility to cracking in slabbing and hot rolling and may impair the stable formation of retained austenite.
The upper limits of u and Cr are 1.0%, and the upper limit of Mo is 0.5%.
And

Nb, Ti, V and B are effective in controlling the structure of the hot rolled sheet. It suppresses the formation of polygonal ferrite in the cooling process after hot rolling, and promotes the formation of a fine bainite structure as described later. This effect saturates up to 0.08% for Nb, Ti, and V and 0.0030% for B, and it is not necessary to add more than these amounts.

Incidentally, Si ≦ 1.0%, P ≦ 0.10%, S
≦ 0.015% group, Ni ≦ 1.0%, Cr ≦ 1.0
%, Mo ≦ 0.5%, Cu ≦ 1.0%, REM ≦
0.05%, Ce ≦ 0.05%, Ca ≦ 0.05%, Zr ≦
Group consisting of 0.05%, Nb ≤ 0.08%, Ti ≤ 0.08
%, V.ltoreq.0.08%, B.ltoreq.0.0030%, and at least one selected from any one of these groups or two or more groups is preferably added.

Hot rolling conditions: The steps from melting of the steel having the above chemical composition to hot rolling are not particularly limited. For example, HDR,
Technologies such as bar casting can be applied.

In hot rolling, it is important to suppress the formation of polygonal ferrite by cooling after finish rolling.
For that purpose, it is necessary to wind the coil at a temperature below the Bs point. When the production of polygonal ferrite is large, it is not possible to expect the production of retained austenite of 3% or more after annealing. Although the Bs point of steel differs depending on the chemical composition, in the steel within the scope of the present invention, when a large amount of austenite forming elements such as C, Mn, Ni, and Cr are contained, the formation of a polygonal ferrite structure is avoided even at a coiling temperature of 750 ° C. can do. If such elements are relatively small, it is necessary to wind the coil at a temperature of about 500 ° C. On the other hand, 300
When coiling at a temperature of less than ℃, depending on the cooling pattern of the steel, the structure may consist of martensite only, which significantly impairs the flatness of the steel sheet. Disappear. Further, in such low-temperature winding, even a very slight variation in cooling conditions during cooling of the steel sheet causes a large change in the microstructure in the coil, which impairs the stability of product quality after annealing, which is not preferable. Therefore, the proper range of the winding temperature is 300 to 7
It is 50 ° C.

From the above-mentioned viewpoint, it goes without saying that the cooling rate after hot finish rolling is a rate that suppresses the formation of polygonal ferrite and pearlite. In general, steel Ar ₃
The average cooling rate in the vicinity of the point needs to be higher for steel having a smaller value represented by the formula (C + (Mn / 5)), but the minimum value of the formula (C + (Mn / 5)) of the present invention is 0. Even about 60% steel, about 30 ° C / s is sufficient.

The hot rolled steel sheet is annealed as it is or after cold rolling. The annealing may be either continuous annealing or batch annealing, but it is necessary to hold at a temperature of 620 to 750 ° C. for 60 seconds or more. If annealing is carried out at a lower temperature and for a shorter time than this, the formation of a predetermined amount (3% or more) of retained austenite cannot be achieved. At an annealing temperature of more than 750 ° C, a large amount of martensite is finally produced, and the production of retained austenite is suppressed, which is not preferable. The upper limit of the annealing time in the above temperature range is not particularly specified, but 240 minutes or less is preferable from the viewpoint of productivity. Cooling after annealing may be air cooling or furnace cooling.

The steel sheet thus produced can be used as it is as annealed, and can also be used as an original plate for plating. However, for plating applications, it is important to design the Si content of steel to be low, and 0.50% or less is preferable. Furthermore, it is possible to carry out the annealing process together with the plating process.

Next, examples of the present invention will be described.

[0032]

Example: Melted by a conventional method

[Table 1] ,

[Table 2] Steels having various chemical compositions shown in Table 1 were roughly rolled into slabs with a thickness of 30 mm. In hot rolling, a slab heating temperature of 1200 ° C. and a finishing temperature of about 900 ° C. were used to form a steel plate having a thickness of 3.2 mm, and the average cooling rate and the winding temperature were changed. The front and back surfaces of this steel sheet were cut and annealed under various conditions to a steel sheet having a thickness of 2.0 mm, and the materials were investigated. Some hot-rolled steel sheets were pickled, cold-rolled to 0.8 mm, and then annealed to examine the material. Those results

[Table 3] ,

[Table 4] Shown in.

As is clear from Tables 3 and 4, the hot rolled steel sheets of the examples of the present invention each had a product of TS and El of 21000N.
/ Mm ² ·% or more shows excellent high strength and high ductility. The cold rolled steel sheet of the present invention also has a TS × El of 190
High strength and high ductility at over 00 N / mm ² ·%. It can be seen that, in both the hot-rolled steel sheet and the cold-rolled steel sheet, the steel sheet of the comparative example which does not satisfy the conditions of the present invention has a remarkably small product value of TS and El.

[0034]

As described in detail above, according to the present invention,
Even low-Si steels can produce a certain amount of retained austenite and ensure high ductility, and TS590N / mm
High strength of ² or more can be obtained. Since it is a low Si type, it has good plating properties. The effects that can be realized with the current thin steel sheet manufacturing technology are great.

Claims (4)

[Claims] 1. In weight% (hereinafter the same), C: 0.10
0.40%, Mn: 2.0-4.0%, Al: 0.01-0.1
A steel containing 0% and having a composition satisfying C + (Mn / 5): 0.60% or more contains polygonal ferrite by hot rolling and coiling the steel in the range of 300 to 750 ° C. A high-ductile high-strength characterized by producing retained austenite with a volume ratio of 3% or more by making the steel sheet not to be used and as-rolled or cold-rolled for 60 seconds or more in the range of 620 to 750 ° C. Steel plate manufacturing method. 2. The steel further comprises Si ≦ 1.0% and P ≦ 0.1.
The method according to claim 1, containing at least one of 10% and S ≦ 0.015%, and the balance being Fe and inevitable impurities. 3. The steel further comprises Ni ≦ 1.0% and Cr ≦ 1.0.
0%, Mo ≦ 0.5%, Cu ≦ 1.0%, Nb ≦ 0.08%,
Ti ≤ 0.08%, V ≤ 0.08%, REM ≤ 0.05%,
Ce ≦ 0.05%, Ca ≦ 0.05%, Zr ≦ 0.05%, B
The method according to claim 1 or 2, containing at least one of ≤ 0.0030%, and the balance consisting of Fe and inevitable impurities. 4. The content of Si ≦ 0.5%,
The method according to 2 or 3.