NL8006404A - METHOD FOR MANUFACTURING STEEL SHEET; PRODUCTS. - Google Patents

Description

VO 121 + 9

Title: Method for manufacturing steel sheet; products

The invention relates to a method for manufacturing a two-phase, cold-rolled steel sheet with a low rupture point, a low rupture ratio and a high ductility, yet 2 a tensile strength of 1 + 0-80 kg / mm. Steel strips and cut steel plates 5 are commonly referred to herein as plates.

Recently, much effort has been made in the automotive industry to reduce the weight of car bodies, mainly to reduce the fuel consumption of the car. Since a reduction in weight necessitates a reduction in the thickness of the steel plates of the body, it is essential that very strong steel plates are used to achieve this goal as well as to ensure safety. The use of high tensile steel plates is more common in the automotive industry today.

In order to meet such requirements in the automotive industry, it is necessary for the steel industry to provide steel plates with better properties than known cold-rolled high strength steel plates at a lower cost.

Known two-phase cold-rolled high tensile steel sheets and a method of manufacturing them are described, for example, in Japanese Patent Application Laid-Open No. 50-981 + 19, according to which publication a carbon and manganese and optionally 0.1 - 0.7 $ silicon-containing steel is heated to the two-phase (a + +; γ) co-existence zone, as determined in the equilibrium phase diagrams, and then cooled relatively quickly to 500 ° C with an average cooling rate of 0.5 to 30 ° C / sec to obtain a steel structure, which is mainly composed of ferrite and the. transformation product, which is formed by the rapid cooling and partly contains the residual austenite.

The object of the invention is to provide a method for producing a two-phase, cold-rolled, high-tensile cold-rolled steel sheet at low cost, which is excellently workable by a combination of the cooling conditions following the soaking stage, in particular in a continuous tempering treatment with the chemical composition of the steel raw materials.

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The steel sheet obtained according to the invention has a low point of failure, yet a tensile strength of 1 + 0-80 kg / mm and a high elongation. Therefore, the steel sheet according to the invention has advantages in the following points.

The failure point is related to the rebound phenomenon of the steel sheet when subjected to press molding. The low failure point (low failure ratio) therefore ensures a better fit of the steel in the shape of the press and thus a better shaped shape, so that the loads of the machine forming the press 10 can be significantly reduced. The invention can help meet the automotive industry's demand for high tensile steel plates at lower prices because a two-phase high strength steel plate can be manufactured at lower production costs with smaller amounts of alloying elements.

The high ductility of the steel sheet obtained according to the invention can, of course, ensure success of a high degree of processing. In this regard, the invention is extremely advantageous.

The steel plates commonly used for the exterior of the automobile bodies have a thickness of about 0.8 mm. As has been said, many attempts have been made to reduce the weight of the automobile bodies, one of which is aimed at reducing the thickness of the steel plates.

However, in attempting to reduce the thickness of the steel sheet, a problem has occurred with regard to dent resistance, namely, resistance to local denting of the sheet. The dent resistance depends on the thickness and strength of the steel plate. The main reason for using a thin high strength steel sheet for the exterior of the automotive bodies is to improve this dent resistance. The invention therefore has the important technical advantage that a steel plate with better dent resistance with a high tensile strength is obtained while maintaining a satisfactory machinability with a low point of failure and a high elongation, and the invention is therefore of great commercial importance because the steel sheet obtained according to the invention can well meet the automotive industry's demand for replacement of the known mild steel plates which are used as the exterior of the automobile bodies.

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The invention will be explained in more detail with reference to the drawing and a number of specific examples.

Figures 1a, b and c show the tendency to form a two-phase structure, respectively, depending on different carbon-5 and manganese contents in combination, at a given cooling rate.

One of the essential features of the invention, as will be explained hereinafter, is to apply a very high cooling rate of 30-300 ° C / sec for cooling to a temperature of no more than 250 ° C in a continuous annealing process. In order to achieve the desired objectives of the invention with such rapid cooling, the steel stock material must meet the following chemical composition conditions.

When the carbon content is less than $ 0.01, the structure that can be obtained by the rapid cooling is not sufficient, but when, on the other hand, the carbon content is more than $ 0.12, the amount of the rapid cooling due transformation product excessive so that it is difficult to obtain a two phase structure with high ductility as desired according to the invention. In this case, the transformation product consists of martensite 20 and untransformed austenite.

Manganese increases the hardenability of the γ phase, promotes the transformation product of the rapid cooling during the cooling stage, and strengthens the. ferrite matrix, therefore increases ductility. However, manganese contents of less than 1.0% are not sufficient to achieve the desired curability. On the other hand, manganese contents in excess of $ 1.8 are not desirable because it reduces weldability and poses certain economic disadvantages.

Aluminum is essential for the deoxidation of steel, but less than 0.0: 1 $ aluminum is sufficient for that purpose. However, aluminum contents of no more than 0.10% are sufficient for this purpose.

Silicon is not essential, and no more than 0.1 $ silicon is sufficient, but to improve the ductility of the two-phase structure, it can be added in an amount of no more than 1.2 $. Silicon contents in excess of $ 1.2 will adversely affect the paintability and corrosion resistance of the resulting steel sheets. The upper limit for the silicon content is therefore 1.2 / ¾.

For further enhancement of strength, chromium, copper and nickel may each be added in amounts of more than 1%, and to improve flexibility, calcium, rare earth 5 'and zirconium may be added in amounts of not more than 1%. 0.1% are added.

The starting material having a chemical composition set as indicated above is processed into slabs ("slabs") using a conventional continuous casting or casting crushing method, after which the plates are hot rolled and cold rolled into cold rolled plates ("sheets"). The cold-rolled plates thus obtained are subjected to the continuous unloading process which is one of the features of the invention.

The soaking in the continuous annealing process is preferably performed in a temperature range of 730-800 ° C for a period of time from 20 sec to 2 minutes for the following reasons.

Soaking below T30 ° C and less than 20 seconds does not result in a sufficient amount of γ and a sufficient carbon, manganese, etc. concentration in γ that is necessary to obtain a desired residual austenite, and even when rapid cooling is performed 20, a desired two-phase structure is not obtained, making it impossible to obtain a steel sheet with well-balanced strength and ductility. On the other hand, soaking at a temperature above 800 ° C will result in an excessive amount of γ and the manganese concentration in γ will be diluted, so that even with rapid cooling a desired amount of residual austenite cannot be obtained and rather the amount of hard martensite becomes increases. The steel sheet obtained will then not have a good balance between strength and ductility. The upper limit for the week duration is set at 2 minutes, although a longer week duration can be applied. However, a longer duration requires a longer oven and is not desirable for reasons of capital cost and economy.

The cooling step subsequent to the above-mentioned cooling is carried out at a cooling rate of 30-300 ° C / sec, preferably 100-300 ° C / sec to 250 ° C. The reason for setting an end point of the rapid cooling at 250 ° C is that when the rapid cooling is terminated at temperatures above 250 ° C, the residual austenite transforms into marten site so that a two-phase structure cannot be obtained.

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Regarding the cooling rate, it is noted when the cooling rate is less than 30 ° C / sec, it is not possible to obtain a two-phase structure with the low-alloy steel composition as defined according to the invention, and when the cooling rate is more than 300 ° C / sec, the ductility of the steel obtained is low. As for the ductility, it does not change substantially when the cooling rate is in the range of 30-300 ° C / sec. The cooling rate controlled in this region is difficult to achieve by liquid cooling or water immersion cooling, but can easily be obtained by blowing vapor-liquid cooling through a mixture of vapor and liquid against the steel sheet. An additional advantage of this vapor-liquid cooling is that a uniform cooling effect can be realized across the width of the steel strip, and therefore a uniform quality of the material can be processed.

The steel sheets are stretched after the continuous annealing process, usually with a reduction of about 10%.

Some examples to illustrate the invention will now follow.

Steel plates with different steel compositions, as shown in Table A, were manufactured and hot rolled to hot rolled steel strips of 2.5 mm thickness, and further cold rolled to cold rolled steel strips of 0.7 mm thickness. These cold-rolled steel strips were subjected to a continuous annealing treatment, soaking at 770 ° C for ho seconds, and cooling to ordinary temperatures at various cooling rates ranging from 10 ° C to about 1000 ° C / sec (quenching with water). The properties of the obtained steel plates are also shown in Table A.

According to the usual continuous annealing treatment, the cooling rate after the soaking step was about 10 ° C / sec. With such a slow cooling rate, it was impossible to obtain a two-phase steel sheet with a low point of failure and a low failure ratio, unless the amount of alloying elements (Mn) is large as in steel D. Furthermore, this standard technique has the disadvantage that when manganese content increases, the weldability deteriorates and production costs increase.

If the cooling rate was kept in the range of 30-300 ° C / sec 800 6 40 4-6, as directed according to the invention, a satisfactory two-phase structure can be obtained even with a small amount of alloying elements, and does not substantially change the resulting ductility, as shown by samples A - C and E - I.

On the other hand, when the cooling rate was further increased to 350 ° C / sec (spraying with water) or 1000 ° C / sec (immersion in water), the obtained ductility became considerably worse and the desired object of the invention could not be achieved.

In Figures 1a, b and c, which show the relationship between the cooling rate and the formation of the two-phase structure at different levels of carbon and manganese, the numbers shown indicate the failure ratio in percent, and a failure ratio of 50 $ has been set as a boundary line for the two-phase structure.

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Claims (3)

Method for the production of a two-phase, cold-rolled steel sheet with high tensile strength, which is excellently workable, characterized in that a steel containing 0.01 - 0.12% carbon, not more than 0.1% silicon, 1, 0 - 1.8% manganese, 0.01 - 0.10% sol.Al, and the balance contains iron and 5 impurities, is rolled hot; the hot-rolled steel sheet is cold-rolled, the cold-rolled steel sheet is subjected to. a continuous annealing treatment, soaking in a temperature range of 730-800 ° C for 20 seconds to 2 minutes, and cooling to 250 ° C or lower at a cooling rate of 30-300 ° C / sec. 2. Process according to claim 1, characterized in that the silicon content in the steel is more than 0.1%, but not more than 1.2%. Method according to claim 1 or 2, characterized in that the cooling speed is 100 - 300 ° C / sec. The method according to a lofimer of claims 1-3, characterized in that the cooling is carried out with a vapor-water mixture. Articles produced using steel produced by the method according to one or more of claims 1 - h. 1 8 00 6 40 4