EP1918406B1 - Process for manufacturing steel flat products from boron microalloyed multi phase steel - Google Patents

Abstract

The method for the production of flat steel products useful for automotive industry, comprises casting a steel into a cast strip having a thickness of 1-4 mm, hot-rolling the cast strip in-line into a hot-rolled strip having a thickness of higher than 1.5 mm in a continuous process at a final hot-rolling temperature at 800-1100[deg] C and coiling the hot-rolled strip at a coiling temperature of 250-570[deg] C to obtain a hot-rolled strip, which has a minimum tensile strength R m of 800 MPa and a minimum breaking elongation A 8 0 of 5%. The method for the production of flat steel products useful for automotive industry, comprises casting a steel into a cast strip having a thickness of 1-4 mm, hot-rolling the cast strip in-line into a hot-rolled strip having a thickness of higher than 1.5 mm in a continuous process at a final hot-rolling temperature of 800-1100[deg] C and coiling the hot-rolled strip at a coiling temperature of 250-570[deg] C to obtain a hot-rolled strip, which has a minimum tensile strength R m of 800 MPa and a minimum breaking elongation A 8 0 of 5%. The steel forms a complex phase structure. The shaping degree is greater than 20%. The width of the hot-rolled strip is more than 1.600 mm. The hot-rolled strip is cold-rolled with a thickness of 0.5-1.4 mm at 750-850[deg] C to obtain a cold-rolled strip, which has a minimum tensile strength of 800 MPa and a minimum breaking elongation A 5 0 of 10%. The cold or hot-rolled strip is provided with a metallic coating, which is galvanizing. The hot-rolled temperature is 900-1020[deg] C and the coiling temperature is 420-490[deg] C, when the minimum breaking elongation A 8 0 of the obtained hot-rolled strip is 10%. The hot-rolled temperature is 900-1100[deg] C and the coiling temperature is 450-570[deg] C, when the minimum tensile strength of the obtained hot-rolled strip is 1000 MPa.

Description

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, they permit the use of lower material thicknesses and concomitantly a reduction in vehicle weight and, secondly, 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 strip to provide thinner sheet 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 strip from a multiphase steel is in the EP 1 072 689 B1 ( DE 600 09 611 T2 ) has been proposed. 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 is maintained until a temperature between 400 - 550 ° 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 a sufficient for the adjustment of the required microstructure pause time, 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 ° C, then at a lying below 350 ° C reel to a Wind coil.

With the in the EP 1 072 689 B1 described procedure is a simple production of hot strip with bainitic microstructural parts of a multi-phase steel is possible, the TRIP properties ("TRIP =" T ransformation I nduced P lasticity ") has. 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 according to claim 1 for producing steel flat products, in which a steel forming a multi-phase structure, 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 1 to 4 mm thick, in which the cast strip is made into a hot strip with a thickness of 0 in a continuous operation with a degree of deformation of more than 20% in-line at a hot rolling end temperature ranging from 800 to 1100 ° C , 5 - 3.2 mm, and in which the hot strip is rewound at a coiler temperature of 250 - 570 ° C to obtain a hot strip whose tensile strength R _{m is} at least 800 MPa with an elongation at break A ₈₀ 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 low 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 thickness of at most 1.5 mm with an optimum distribution of properties 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 procedure, 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.

Another 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 coiling, as described, for example, in US Pat EP 1 072 689 B1 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. If, for example, hot strips are to be produced which have a tensile strength R _m of at least 800 MPa at an elongation at break A _{80 of} the resulting hot strip of at least 10%, this can be achieved by setting the hot rolling end temperature 900-1000 ° C. and the reel temperature 420-510 ° C.

If, on the other hand, a hot strip with a guaranteed higher tensile strength R _m of at least 1000 MPa and an elongation at break A ₈₀ of at least 5% are to be produced, then the hot rolling end temperature in the range of 900-1100 ° C. and the coiling temperature in the range of 450-570 ° C. selected.

Even higher tensile strengths R _{m of} the obtained 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 ° C and the reel temperature 250 - 550 ° 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. Table 1 (in% by weight) C Mn P S Si al N Cr Ti B A 0, 102 1.76 0.005 0,004 0.14 0,014 0.0057 0.24 0.016 0.0027 B 0.098 1.81 0.005 0,003 0.19 0,060 0.0048 0.37 0,045 0.0044

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 attempt stole WET [° C] HT [° C] R _m [MPa] A ₈₀ [%] 1 B 950 500 878 11.3 2 B 1050 480 1073 5.5 3 A 830 285 1234 6.2 4 B 950 540 1041 5.3 5 B 950 510 1263 5.5 6 A 950 440 1244 5.1

After hot-rolling 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 a continuous process 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 (12)

1. Method for manufacturing flat steel products,

   - wherein a steel that forms a multi-phase microstructure with the following composition (in wt. %) 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 % remainder iron and unavoidable impurities
   is cast into a cast strip having a thickness of 1 - 4 mm,

   - wherein the cast strip is hot-rolled in-line into a hot-rolled strip having a thickness ranging from 0.5 to 3.2 mm in a continuous process at a final hot-rolling temperature ranging from 800 to 1100 °C, the deformation degree being greater than 20 %, cooled in one step and

   - wherein 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_m of 800 MPa at a minimum breaking elongation A₈₀ of 5 %.
2. Method according to Claim 1, characterized in that the width of the hot-rolled strip is greater than 1,200 mm, in particular greater than 1,600 mm.
3. Method according to any one of the preceding claims, characterized in that the thickness of the hot-rolled strip is 1.5 mm at most.
4. Method according to any one of the preceding claims, characterized in that the hot-rolled strip is cold-rolled into cold-rolled strip having a thickness of 0.5 - 1.4 mm.
5. Method according to Claim 4, characterized in that the cold-rolled strip is annealed at an annealing temperature of 750 - 850 °C.
6. Method according to Claim 4 or 5, characterized in that the minimum tensile strength of the cold-rolled strip is 800 MPa.
7. Method according to any one of Claims 4 to 6, characterized in that the cold-rolled strip has a minimum breaking elongation A₅₀ of 10 %.
8. Method according to any one of the preceding claims, characterized in that the hot-rolled strip or cold-rolled strip is provided with a metallic coating.
9. Method according to Claim 8, characterized in that the metallic coating is a zinc coating.
10. Method according to any one of the preceding claims, characterized in that with a minimum breaking elongation A₈₀ of the obtained hot-rolled strip of 10 %, the final hot-rolling temperature is 900 - 1020 °C and the coiling temperature is 420 - 490 °C.
11. Method according to any one of Claims 1 to 9, characterized in that with a minimum tensile strength R_m of the obtained hot-rolled strip of 1000 MPa, the final hot-rolling temperature is 900 - 1100 °C and the coiling temperature is 450 - 570 °C.
12. Method according to any one of Claims 1 to 9, characterized in that with a minimum tensile strength R_m of the obtained hot-rolled strip of 1200 MPa, the final hot-rolling temperature is 800 - 1000 °C and the coiling temperature is 250 - 550 °C.