WO2008052919A1 - Method for manufacturing flat steel products from a multiphase steel microalloyed with boron - Google Patents

Abstract

The invention relates to a method which allows 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 multiphase structure and contains 0.08 to 0.12 wt.-% of C, 1.70 to 2.00 wt.-% of Mn, up to 0.030 wt.-% of P, up to 0.004 wt.-% of S, up to 0.20 wt.-% of Si, 0.01 to 0.06 wt.-% of Al, up to 0.0060 wt.-% of N, 0.20 to 0.50 wt.-% of Cr, 0.010 to 0.050 wt.-% of Ti, 0.0010 to 0.0045 wt.-% of B, 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 800 to 1100°C, the shaping degree being greater than 20 percent, and the hot-rolled strip is coiled at a coiling temperature ranging from 250 to 570°C so as to obtain a hot-rolled strip which has a minimum tensile strength R<SUB>m</SUB> of 800 MPA and a minimum breaking elongation A<SUB>80</SUB> of 5 percent.

Description

 Method of producing steel flat products from boron microalloyed multiphase steel

The invention relates to a method for producing steel flat products, such as strips or sheet metal blanks, from high-strength, boron microalloyed steels. Such steels belong to the group of multiphase steels. These are usually steels whose properties are determined by the type, amount and arrangement of the phases of the structure. The structure therefore has at least two phases (eg ferrite, martensite, bainite). This gives them a strength / formability combination that is superior to conventional steels.

This preparation route presents problems in particular when casting peritectically solidifying compositions. With these steel grades there is the danger of the formation of longitudinal cracks during continuous casting. The formation of such longitudinal cracks can reduce the quality of the hot strips produced from the cast slabs or thin slabs so much that they become unusable. In order to prevent this danger, extensive measures, such as increased insulation costs, are required, which can go so far that the processing of such steel grades becomes uneconomical. When casting steels with high Al contents, there are also undesirable interactions with the casting powder, which also adversely affect the quality of a flat product made from these steels. Because of these peculiarities, multiphase steels are of particular interest for the automotive industry because of their high strength on the one hand the use of lower material thicknesses and concomitantly reduce the vehicle weight and on the other hand improve the safety of the vehicle body in the event of a collision (crash behavior) , Thus, multiphase steels with at least constant strength of the overall body allow a reduction in the sheet thickness of a component produced from such multiphase steels compared to a body produced from conventional steels.

Usually, multiphase steels are melted in the converter steelworks and cast on a continuous casting plant into slabs or thin slabs, which are then hot rolled into hot strip and coiled. By a controlled controlled cooling of the hot strip after hot rolling with the aim of adjusting certain microstructures, the mechanical properties of the hot strip can be varied. In addition, the hot strips can be cold-rolled to cold-rolled strip in order to provide thinner sheet-metal thicknesses (EP 0 910 675 B1, EP 0 966 547 B1, EP 1 169 486 B1, EP 1 319 725 B1, EP 1 398 390 A1).

A problem in the production of flat products made of high-strength multiphase steels with tensile strengths of more than 800 MPa is that high rolling forces must be applied when rolling such steels. As a consequence of this requirement, high-strength hot strips made of steels of the type normally used in the currently available production plants are generally used Speech standing type often only in a width and thickness can be made available that are no longer fully meet the demands made in the field of automotive engineering today. Above all, tapes of small thicknesses with sufficient widths can be poorly represented on conventional systems. It also turns out in conventional practice in practice difficult to produce multiphase steels cold strips with strengths of more than 800 MPa.

An alternative way of producing steel strips from a multiphase steel has been proposed in EP 1 072 689 B1 (DE 600 09 611 T2). According to this known method, for the production of thin steel strips, first of all a molten steel containing (in% by weight) 0.05 and 0.25% C, in total 0.5-3% Mn, Cu and Ni, in total 0, 1 to 4% Si and Al, in total up to 0.1% P, Sn, As and Sb, in total less than 0.3% Ti, Nb, V, Zr and REM and in each case less than 1% Cr, Mo and V, balance iron and unavoidable impurities, cast into a cast strip having a thickness of 0.5 - 10 mm, especially 1 - 5 mm. The cast strip is then hot rolled in-line in one or more passes of between 25% and 70% strain to a hot strip. The final temperature of hot rolling is above the Ar ₃ temperature. After the end of the hot rolling, the hot strip obtained is then cooled in two stages. In the first stage of this cooling, a cooling rate of 5 -

100 ° C / s maintained until a temperature between 400 - 550 ⁰ C is reached. At this temperature, the hot strip is allowed to sit for a break time needed to achieve a bainitic transformation of the steel to a residual austenite portion of more than 5%. The formation of perlite should be avoided. After sufficient for the adjustment of the required microstructure pause time of the conversion process is stopped by the beginning of the second stage of cooling, in which the hot strip is brought to a temperature below 400 ⁰ C, to then at a lying below 350 ⁰ C reel temperature to a Wind coil.

With the procedure described in EP 1 072 689 Bl, a simple production of hot strip with bainitic microstructure parts from a multiphase steel should be possible which has TRIP properties ("TRIP" = "Transformation Induced Plasticity"). Such steels have relatively high strengths with good ductility. However, the strength is not sufficient for many applications, especially in the field of automotive engineering.

The object of the invention was therefore to provide a method by means of which high-strength steel flat products can be produced over a wide range of geometric dimensions with reduced manufacturing outlay.

Based on the above-described prior art, this object has been achieved by a method for producing steel flat products, in which a multiphase structure forming steel, the (in wt .-%) 0.08 - 0.12% C, 1 , 70-2.00% Mn, up to 0.030% P, up to 0.004% S, up to 0.20% Si, 0.01-0.06% Al, up to 0.0060% N, 0.20 - 0.50% Cr, 0.010 - 0.050% Ti, 0.0010 - 0.0045% B and the remainder contains iron and unavoidable impurities to a cast strip with a thickness of 1 - 4 mm is cast in which the cast strip in a continuous operation with a degree of deformation of more than 20% in-line lying in a range of 800 - 1100 ⁰ C.

Hot rolling end temperature is hot rolled to a hot strip having a thickness of 0.5 - 3.2 mm and in which the hot strip at a coil temperature 250 - 570 ⁰ C is coiled, so that a hot strip is obtained whose tensile strength R _m at least 800 MPa an elongation at break A ₈ o of at least 5%.

The invention uses the possibility of strip casting to process a particularly high-strength, possibly peritectically solidifying multiphase steel into a hot strip. Since the cast strip itself already has a small thickness, in the course of hot rolling of this strip only relatively small degrees of deformation must be maintained in order to produce flat products with small thicknesses, as are required in particular in the automotive industry. Thus, by specifying a corresponding output thickness of the cast strip, it is easily possible to produce hot strips with the method according to the invention, which have a maximum property distribution of at most 1.5 mm and from which, for example, elements for the support structure of an automobile can be produced.

Due to the low degrees of deformation during hot rolling, the rolling forces required for this are low compared with the forces required in the hot rolling of slabs or thin slabs in the conventional approach, so that hot strips of large width are easily produced by the method according to the invention which are well above the width of conventionally producible hot strips of the same strength and thickness class. Thus, the invention makes it possible to manufacture high-strength hot strips consisting of a martensitic steel of the specified composition processed according to the invention, whose width is more than 1,200 mm, in particular more than 1,600 mm.

The use according to the invention of the strip casting method in the processing of high-strength steels of the type assembled according to the invention offers the possibility, in addition to the above-mentioned advantages due to its process-specific properties and manipulated variables (eg hot rolling end temperature, cooling, coiling temperature), of also critical steel compositions according to the invention with regard to their solidification behavior to safely shed processed species. Thus, the very rapid solidification of the cast strip, which is characteristic of strip casting, leads to a significantly reduced risk of the formation of center segregations compared with conventional production, with the result that the hot strip produced according to the invention has a particularly uniform distribution of properties and microstructure over its cross section and its length.

A further particular advantage of the procedure according to the invention is that the hot strip produced according to the invention has high strengths of at least 800 MPa, without having to observe a special cooling cycle of the hot strip between the end of the hot rolling and the hasp, as described, for example, in EP 1 072 689 bl through the Necessity of a cold break is required. When carrying out the method according to the invention, it merely has to be ensured that the hot rolling ends in a relatively narrow temperature window and that the reeling is also carried out in a precisely defined temperature range. In between there is a one-stage cooling down.

A further advantage of the procedure according to the invention is that an extension of the range of mechanical properties of the strip produced according to the invention based on only one steel analysis can be achieved by varying the cooling and rolling conditions.

Hot strips produced according to the invention are particularly suitable for further processing into cold rolled strip. Accordingly, a practice-oriented embodiment of the invention provides that the hot strip is cold rolled to a cold strip having a thickness of 0.5-1.4 mm, in particular 0.7 mm to 1.3 mm, as is required for the construction of automobile bodies. In order to eliminate solidification during cold rolling, the cold strip can be annealed at an annealing temperature of 750-850 ° C. For cold rolled strip produced in this way from the hot strip produced according to the invention, tensile strengths of at least 800 MPa can be reliably ensured. The breaking elongation A _{50 of} the cold strip is just as safe at least 10%.

According to a further advantageous embodiment of the invention, the cold strip in a conventional manner provided with a metallic coating, which may be, for example, a galvanizing.

The strength and elongation values according to the invention produced hot strips can be adjusted over a wide range by an appropriate vote of the Hotwalzend- and reel temperatures. For example, to hot rolled strips are produced which have a tensile strength R _m of at least 800 MPa at an elongation at break A ₈₀ of the hot strip obtained by at least 10%, this can be achieved that the hot rolling 900-1000 ⁰ C and the coiling temperature 420 to 510 ⁰ C amount.

If, on the other hand, a hot strip having a guaranteed higher tensile strength R _m of at least 1000 MPa and an elongation at break A ₈ o of at least 5% is produced, then the hot rolling end temperature in the range of 900-1100 ° C. and the coiling temperature in the range of 450-570 ^{0 are used} C is chosen.

Even higher tensile strengths R _{m of} the resulting hot strip of at least 1200 MPa at an elongation at break A ₈₀ of at least 5% can be achieved by the hot rolling end temperature 800 - 1000 ⁰ C and the reel temperature 250 - 550 ⁰ C.

The invention will be explained in more detail by means of exemplary embodiments.

In experiments carried out to demonstrate the effect of the invention are two composite according to the invention Steels A and B having the composition shown in Table 1 were melted and cast in a conventional two-roll casting machine into cast strip 1.6 mm thick.

C Mn P Si Si Al N Cr Ti B

A o, 1 02 1, 7 6 0, 005 0, 004 0, 14 0, 014 0, 0057 0, 24 0, 016 0, 0027

B 0, 0 98 1, 8 1 0, 005 0, 003 0, 19 0, 060 0, 0048 0, 37 0, 045 0, 0044

Table 1 (in% by weight)

The tapes cast from steels A and B were hot rolled into a hot strip whose thickness was 1.25 mm in six different trials immediately following in-line strip casting at a hot rolling end temperature WET. Subsequently, each hot strip obtained has been cooled directly in a cooling step to a coiler temperature HT and coiled. After coiling, the hot strips produced from steels A and B each had a tensile strength R _m and an elongation at break A ₈₀ , which, like the hot rolling end temperature WET and reel temperature HT, respectively, maintained in their preparation, are given in Table 2.

Table 2

After hot reeling and pickling, the hot strip of steel B produced according to test 4 was cold-rolled to a 0.7 mm thick cold-rolled strip and annealed at a temperature of 800 ^° C. in order to recrystallize the strip.

At an elongation at break A ₅₀ of 11.5%, the tensile strength R _{m of} the cold strip thus obtained was 835 MPa.

Claims

PATENT APPLICATIONS 1. Process for producing steel flat products, in which a steel forming a multi-phase structure of the following composition (% by weight) C: 0, 08 - 0, 12% Mn: 1,70 - 2,00% P: <0.030% S: <0.004% Si: <0.20% Al: 0.01-0.06% N: <0.0060% Cr: 0.20 - 0.50% Ti: 0.010-0.050% B: 0.0010-0.0045% Remaining iron and unavoidable impurities cast into a cast strip with a thickness of 1 - 4 mm, in which the cast strip is hot rolled to a hot strip 0.5 to 3.2 mm thick in a continuous operation with a degree of deformation greater than 20% in-line, at a hot rolling end temperature of between 800 and 1100 ^° C and - in which the hot-rolled strip is wound at a reel temperature of 250 - 570 ⁰ C, - So that a hot strip is obtained whose tensile strength R _{m is} at least 800 MPa at an elongation at break A ₈₀ of at least 5%. 2. The method of claim 1, wherein the width of the hot strip is more than 1,200 mm, in particular more than 1,600 mm. 3. The method according to one of the preceding claims, wherein the thickness of the hot strip is at most 1.5 mm. 4. Method according to one of the preceding claims, characterized in that the hot strip is cold rolled to a cold strip having a thickness of 0.5-1.4 mm. 5. The method according to claim 4, characterized in that the cold strip is annealed at an annealing temperature of 750 - 850 ⁰ C. 6. The method according to claim 4 or 5, characterized in that the tensile strength of the cold strip is at least 800 MPa. 7. The method according to any one of claims 4 to 6, characterized in that the cold strip has an elongation at break A ₅₀ of at least 10. 8. Method according to one of the preceding claims, characterized in that the hot strip or the cold strip is provided with a metallic coating. 9. The method of claim 8, wherein the metallic coating is a galvanizing. 10. The method according to any one of the preceding claims, characterized in that in a breaking elongation A ₈₀ of the obtained hot-rolled strip of at least 10% the hot rolling 900 - amount to 490 ⁰ C - 1020 ⁰ C and the coiling temperature 420th 11. The method according to any one of claims 1 to 9, characterized in that at a tensile strength R _{m of} the obtained hot strip of at least 1000 MPa, the hot rolling end temperature 900 - 1100 ⁰ C and the reel temperature 450 - 570 ⁰ C. 12. The method according to any one of claims 1 to 9, characterized in that at a tensile strength R _{m of} the obtained hot strip of at least 1200 MPa, the hot rolling end temperature 800 - 1000 ⁰ C and the reel temperature 250 - 550 ⁰ C.