KR20090090301A - Process for producing flat steel products from steel forming martensitic structures - Google Patents

Abstract

The invention relates to a method which allows extremely high-tensile flat steel products to be manufactured with less effort in a wide range of geometric dimensions. In order to do so, a steel that forms a martensitic structure and contains 0.15 to 0.19 percent of C, 0.80 to 1.20 percent of Mn, up to 0.030 percent of P, up to 0.004 percent of S, 0.60 to 1.00 percent of Si, up to 0.05 percent of Al, up to 0.0060 percent of N, 0.30 to 0.60 percent of Cr, 0.040 to 0.070 percent of Nb, the remainder being composed of iron and inevitable impurities, is cast into a cast strip having a thickness of 1 to 4 mm, the cast strip is hot-rolled in-line into a hot-rolled strip having a thickness of 0.5 to 3.2 mm in a continuous process at a final hot-rolling temperature ranging from 900 to 1050‹C, the shaping degree being greater than 20 percent, and the hot-rolled strip is coiled at a maximum coiling temperature of 350‹C so as to obtain a hot-rolled strip which has a minimum tensile strength Rm of 1400 MPa and a minimum breaking elongation A80 of 5 percent. ® KIPO & WIPO 2009

Description

METHOD FOR MANUFACTURING FLAT STEEL PRODUCTS FROM A STEEL FORMING A MARTENSITIC STRUCTURE}

The present invention relates to a method for producing flat steel products such as strip or sheet metal materials from high-tensile martensitic steels. Such MS (Martensitic) steels belong to the group of multi-phase steels. This relates to the steel whose properties are determined by the shape, amount and arrangement of the phases of the microstructure. Thus, at least two phases (eg, ferrite, martensite, bainite) are present in the microstructure. As a result, polyphase steels have a superior combination of strength / formability than conventional steels.

The high strength of polyphase steels on the one hand allows the use of smaller material thicknesses, while at the same time reducing the weight of the vehicle, and on the other hand, improving the safety (collision behavior) of the vehicle body in the event of a collision. As a result, these special characteristics have attracted considerable attention, particularly for automobile structures. Thus, polyphase steels having at least the same strength of the entire body enables a reduction in sheet metal thickness of components made of such polyphase steels as compared to bodies made of conventional steels.

Generally, multiphase steels are melted in a converter steel mill and cast into slabs or thin slabs in a continuous casting machine, and then hot rolled and wound into hot rolled strips. In this case, the mechanical properties of the hot rolled strip can be changed by selective controlled cooling of the hot rolled strip after hot rolling for the purpose of adjusting the specific microstructure fraction. The hot rolled strip may be cold rolled into a cold rolled strip to obtain a thinner sheet metal thickness (EP 0 910 675 B1, EP 0 966 547 B1, EP 1 169 486 B1, EP 1 319). 725 B1 and EP 1 398 390 A1).

Another problem in the manufacture of plate-shaped products with high tensile strength polyphase steels with tensile strengths exceeding 800 MPa is that high rolling forces must be applied when rolling such steels. According to these requirements, high-tensile strength hot rolled strips made of the type of steel in question are often used in conventional manufacturing equipment, which is now generally available, in width and thickness, which often do not meet the requirements required today in the automotive industry. It can only be manufactured as. In particular, thin and wide strips cannot be easily manufactured in conventional installations. It has also been found by conventional methods that it is practically difficult to produce cold rolled strips with strengths exceeding 800 MPa from polyphase steels.

An alternative method for producing steel strips from polyphase steels is proposed in EP 1 072 689 B1 (DE 600 09 611 T2). According to this known method for producing thin steel strips, first, 0.05% and 0.25% of C, in total 0.5% to 3% of Mn, Cu and Ni, in total of 0.1% to 4% of Si and Al, P, Sn, As and Sb up to 0.1% in total, Ti, Nb, V, Zr and REM less than 0.3% in total, Cr, Mo and V, remainder Fe and inevitable impurities less than 1% each The molten steel is cast into a casting strip of thickness 0.5mm to 10mm, especially 1mm to 5mm. The cast strip is then hot rolled into the hot rolled strip inline in one or more passes, with a degree of deformation in the range of 25% to 70%. The final hot rolled temperature is in this case higher than the Ar ₃ temperature. The hot rolled strip obtained at the end of hot roll is then cooled in two stages. In the first stage of this cooling, a cooling rate of 5 ° C.-100 ° C. per second is maintained until a temperature is reached in the range 400 ° C.-550 ° C. The hot rolled strip is then held at this temperature for the holding time necessary to cause bainite transformation of the steel with a residual austenite content greater than 5%. Formation of pearlite should be avoided in this case. After a sufficient holding time to obtain the required microstructure, the transformation process is stopped by initiation of the second cooling step, and the hot rolled strip is cooled to a temperature below 400 ° C. and then wound into a coil at a winding temperature below 350 ° C.

In the process described in EP 1 072 689 B1, a hot rolled strip having a bainite microstructure fraction in a simple manner from a multiphase steel with TRIP properties ["TRIP" = "Transformation Induced Plasticity"]. It should be possible to manufacture Such steels are relatively high in strength and good in formability. However, strength is not sufficient for many applications, especially in the field of automotive structures.

It is therefore an object of the present invention to provide a method that enables plate steel products having extremely high tensile strength to be easily manufactured in a wide range of geometric dimensions.

This object is based on the prior art described above, according to the present invention, forming martensite microstructures (in weight%) of 0.15% to 0.19% C, 0.80% to 1.20% Mn, 0.030% or less of P, 0.004 1mm thick steel containing up to S, 0.60% to 1.00% Si, up to 0.05% Al, up to 0.0060% N, 0.30% to 0.60% Cr, 0.040% to 0.070% Nb and residual Fe and unavoidable impurities Cast to a cast strip of ˜4 mm, and the cast strip is hot rolled into a hot rolled strip of 0.5 mm to 3.2 mm in thickness at a final hot rolling temperature in the range of 900 ° C. to 1050 ° C. in an in-line continuous process, A method of manufacturing flat steel products having a hot rolled strip having a minimum tensile strength (R _m ) of 1400 MPa at a maximum breaking elongation (A ₈₀ ) of greater than 20% and winding at a winding temperature of 350 ° C. or less. Is achieved by.

The present invention particularly exploits the possibility of strip casting to process martensitic steels with high tensile strength into hot rolled strips. Since the cast strips themselves are already small in this case, only relatively low strains should be maintained during the hot rolling of such strips, especially in order to produce thin-walled flat products, as is required in the field of automotive construction. Thus, by defining the corresponding initial thickness of the casting strip by the method according to the invention, it is possible to produce a hot rolled strip having an optimum distribution of properties and a maximum thickness of 1.5 mm without any problem, for example from a hot rolled strip Support structural components can be made.

Due to the low degree of deformation in hot rolling, the rolling force required for this is lower than the force required for hot rolling of slabs or thin slabs in the conventional method, so that the width is substantially larger than the width of the hot rolled strip of the same strength and thickness cast in the conventional manner. Hot rolled strips can be produced without any problem by the method according to the invention. The present invention thus makes it possible to reliably produce high-tensile strength hot rolled strips consisting of martensitic steels of the composition defined and treated according to the invention and having a width greater than 1,200 mm, in particular greater than 1,600 mm. do.

The application of the strip casting process according to the invention for the processing of high-tensile strength steel of the type constructed in accordance with the invention, in addition to the advantages described above, is characterized by the process parameters (e.g. hot rolling final temperature, cooling, Winding temperature) and properties give the possibility to reliably cast critical steel compositions of the type treated according to the invention in terms of solidification behavior. Thus, by rapid solidification of the casting strip, which is characteristic of strip casting, the hot rolled strip produced according to the present invention is particularly uniform in cross section and length, so that the characteristic distribution and microstructure are particularly uniform over the cross section and length. The risk of occurrence is substantially reduced.

According to another particular advantage of the process according to the invention, the hot rolled strips produced according to the invention are maintained, for example, by the need for cooling interruption between the end of the hot roll and the winding described in EP 1 072 689 B1. It has a high strength of at least 1400 MPa, without the special cooling cycle of the hot rolled strip to be made. In carrying out the process according to the invention, it is only necessary to ensure that the hot rolling is terminated within a relatively narrow range of temperature window and the winding is carried out within a precisely set temperature range. In between, a single-stage cooling takes place.

According to another advantage of the method according to the invention, on the basis of one steel analysis, by changing the cooling and rolling conditions, an extension of the range of mechanical properties of the strip produced according to the invention can be achieved. have.

Hot rolled strips made according to the invention are particularly suitable for subsequent processing into cold rolled strips. Thus, in one practical embodiment of the present invention, there is provided a configuration in which the hot rolled strip is cold rolled into a cold rolled strip having a thickness of 0.5 mm to 1.4 mm, in particular 0.7 mm to 1.3 mm, which is required for the vehicle body configuration. In order to eliminate hardening occurring during cold rolling, the cold rolled strip may be annealed at annealing temperatures of 750 ° C to 850 ° C. For cold rolled strips thus produced from hot rolled strips produced according to the invention, a minimum tensile strength of 800 MPa can be reliably ensured. At the same time, it is also reliably guaranteed that the minimum elongation at break A ₅₀ of the cold rolled strip is 10%.

According to another preferred embodiment of the invention, the cold rolled strip is provided with a metal coating layer in an essentially known manner, in which case, for example, the coating layer may be a zinc coating layer.

Hereinafter, the present invention will be described in detail based on the exemplary embodiments.

In the tests conducted to illustrate the effects of the present invention, two types of steels A and B composed of the compositions shown in Table 1 were dissolved in accordance with the present invention, each 1.6 mm in a typical two-roll casting machine. Cast into cast strips of thickness.

TABLE 1 (wt% data)

C Mn P S Si Al N Cr Nb A 0.157 1.25 0.006 0.005 0.72 0.021 0.0049 0.46 0.060 B 0.157 1.08 0.006 0.004 0.77 0.022 0.0060 0.48 0.064

Immediately after the strips were cast from steels A and B, the casting strips were hot rolled into a hot rolled strip having a thickness of 1.25 m at the final hot rolled temperature (WET) in-line. Thereafter, each hot rolled strip obtained was cooled and wound up to the winding temperature HT directly in the cooling step. After winding, each hot rolled strip made from steel A and steel B was subjected to the tensile strength (R _m ) shown in Table 2, depending on the final hot rolling temperature (WET) and winding temperature (HT) maintained in each case during manufacture. Elongation at break (A ₈₀ ).

TABLE 2

River WET [℃] HT [℃] R _m [MPa] A ₈₀ [%] A 900 290 1414 5.2 B 980 290 1405 6.0

The hot rolled strip made from Steel A was cold rolled into a 0.7 mm thick cold rolled strip after winding and pickling, and annealed in-line at a temperature of 780 ° C. for recrystallization of the strip.

The cold rolled strip thus obtained had a break elongation (A ₅₀ ) of 21.8% and a tensile strength (R _m ) of 654 MPa.

Claims (9)

In weight percent, C: 0.15%-0.19%, Mn: 0.80%-1.20%, P: ≤ 0.030%, S: ≤ 0.004%, Si: 0.60%-1.00%, Al: <0.05%, N: ≤ 0.0060%, Cr: 0.30%-0.60%, Nb: 0.040%-0.070%, A steel forming a martensite microstructure of the composition consisting of the balance Fe and unavoidable impurities is cast into a casting strip having a thickness of 1 mm to 4 mm, Hot-rolled the casting strip into an in-line continuous process with a hot rolled strip in the range of 0.5 mm to 3.2 mm in thickness at a final hot rolled temperature in the range from 900 ° C. to 1050 ° C., with a deformation rate in excess of 20%, The hot rolled strip is wound at a winding temperature of 350 ° C. or less, A method for producing a flat steel product characterized by obtaining a hot rolled strip having a minimum tensile strength (R _m ) of 1400 MPa at a minimum breaking elongation (A ₈₀ ) of 5%. The method of claim 1, A hot rolled strip has a width of greater than 1,200 mm, in particular greater than 1,600 mm. The method according to any one of the preceding claims, A hot rolled strip has a thickness of up to 1.5 mm. The method according to any one of the preceding claims, A method for producing a flat steel product, wherein the hot rolled strip is cold rolled into a cold rolled strip having a thickness of 0.5 mm to 1.4 mm. The method of claim 4, wherein A cold rolled strip is annealed at an annealing temperature of 750 ° C to 850 ° C. The method according to claim 4 or 5, Cold rolled strip has a minimum tensile strength of 600MPa flat plate steel products manufacturing method. The method according to any one of claims 4 to 6, Cold rolled strip has a minimum elongation at break (A ₅₀ ) of 15%. The method according to any one of the preceding claims, A method for producing a flat steel product, characterized by coating a metal coating layer on a hot rolled strip or a cold rolled strip. The method of claim 8, A method for producing a flat steel product, characterized in that the metal coating layer is a zinc coating layer.