WO2008052920A1

WO2008052920A1 - Method for manufacturing flat steel products from a multiphase steel alloyed with aluminum

Info

Publication number: WO2008052920A1
Application number: PCT/EP2007/061391
Authority: WO
Inventors: Brigitte Hammer; Thomas Heller; Johann Wilhelm Schmitz; Jochen Wans
Original assignee: Thyssenkrupp Steel Ag
Priority date: 2006-10-30
Filing date: 2007-10-24
Publication date: 2008-05-08
Also published as: JP2010508436A; US20100065162A1; JP5350254B2; ATE432374T1; EP1918404A1; ES2325963T3; EP1918404B1; PL1918404T3; KR101461584B1; CN101528966A; CN101528966B; DE502006003832D1; KR20090090302A

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 0.10 to 0.14 wt.-% of C, 1.30 to 1.70 wt.-% of Mn, up to 0.030 wt.-% of P, up to 0.004 wt.-% of S, 0.10 to 0.30 wt.-% of Si, 0.90 to 1.2 wt.-% of Al, up to 0.0070 wt.-% of N, 0.070 to 0.130 wt.-% of Ti, 0.040 to 0.060 wt.-% of Nb, 0.140 to 0.260 wt.-% of Mo, 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 850 to 1000°C, the shaping degree being greater than 20 percent, and the hot-rolled strip is coiled at a coiling temperature ranging from 350 to 480°C so as to obtain a hot-rolled strip which has a minimum tensile strength Rm of 800 MPa and a minimum breaking elongation A80 of 5 percent.

Description

Process for producing steel flat products from aluminum alloyed multiphase steel

The invention relates to a method for producing steel flat products, such as strips or sheet metal blanks, from high-strength, aluminum-alloyed 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.

SI / cs 051420WO Because of these peculiarities, multiphase steels are of particular interest for the automotive industry because of their high strength, they allow 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-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. This requirement has the consequence that high-strength hot strips made of steels of the type in question can usually only be made available in width and thickness with the currently customarily available production facilities, which do not meet the demands made today in the field of automobile construction

SI / cs 051420WO more fully. 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-rolled strip is left to rest for a period of time required to allow bainitic transformation of the steel with a residual austenite content greater than 5%. The formation of perlite should be avoided. After a sufficient pause time for the adjustment of the required microstructure, the conversion process by the start of the second stage of

SI / cs 051420WO Cooling stopped, in which the hot strip is brought to a temperature below 400 ⁰ C, to then wrap it at a reel temperature lying below 350 ⁰ C to form a 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 I_nduced P _^ lasticity"). 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-mentioned 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 .-%) 0.10 - 0.14% C, 1 , 30 - 1.70% Mn, up to 0.030% P, up to 0.004% S, 0.10 - 0.30% Si, 0.90 - 1.2% Al, up to 0.0070% N, 0.070 0.130% Ti, 0.040-0.060% Nb, 0.140-0.260% Mo and the remainder contains iron and unavoidable impurities, is cast into a cast strip with a thickness of 1-4 mm, in which the cast strip in a continuous operation a degree of deformation of more than 20% in-line at a lying in the range of 850 - 1000 ⁰ C hot rolling end temperature is hot rolled to a hot strip with a thickness of 0.5 - 3.2 mm and the

SI / cs 051420WO the hot strip at a 350 - 480 ⁰ C forming amount coiling temperature is coiled, so that a hot strip is obtained, the tensile strength R _m of at least 800 MPa at 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, 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 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 required rolling forces are low compared to the forces required in the hot rolling of slabs or thin slabs in the conventional approach, so that can be easily produced by the inventive method hot strips of large width, well above the width of be produced in a conventional manner 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.

SI / cs 051420WO 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 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 in EP 1 072 689 Bl is required by the need for a cooling break. 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.

Another 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 by a

SI / cs 051420WO Variation of the cooling and rolling conditions can be achieved.

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 arising during cold hardening, the cold strip is at an annealing temperature from 750 to 850 ⁰ C to be annealed. 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%.

The flat product produced according to the invention is characterized by a particularly good coatability. According to a further advantageous embodiment of the invention, the cold strip is provided in a conventional manner 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 hot rolling 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

SI / cs 051420WO Reel temperature 400 - 480 ⁰ C. If, however, a hot strip with guaranteed higher tensile strength R _m of at least 1100 MPa at an elongation at break A ₈ o generated by at least 5%, they will be, the hot-rolling temperature in the range 850-1100 ⁰ C and the coiling temperature in the range 350-400 ⁰ C is chosen.

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

In tests carried out to demonstrate the effect of the invention, two steels A and B of the invention 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.

SI / cs 051420WO C Mn PS Si Al N Ti Nb Mo

A o, 12 6 1, 62 0, 00 6 0, 005 0, 28 1, 110 0, 007 0 0, 122 0, 048 0, 20

B 0, 10 9 1, 59 0, 00 6 0, 004 0, 24 1, 010 0, 004 1 0, 076 0, 050 0, 20

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 four different trials immediately after 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

SI / cs 051420WO The hot strip produced from the steel A was cold rolled after the coiling to a 0.74 mm thick cold strip and annealed at an annealing temperature of 800 ⁰ C in the flow to recrystallize the strip.

At an elongation at break A ₅₀ of 11.1%, the tensile strength R _{m of} the cold strip thus obtained was 985 MPa.

SI / cs 051420WO

Claims

PATENT APPLICATIONS

1. Process for producing steel flat products,

in which a multiphase structure steel of the following composition (in% by weight)

C: 0, 10 - 0, 14% Mn: 1.30 - 1.70%

P: <0.030%

S: <0.004%

Si: 0, 10 - 0.30%

Al: 0, 90 - 1.2%

N: <0.0070%

Ti: 0.070 - 0.130%

Nb: 0.040 - 0.060%

Mo: 0.140 - 0.260%

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 in a continuous operation with a degree of deformation of more than 20% in-line at a lying in the range of 850 - 1000 ⁰ C hot rolling to a hot strip with a thickness of 0.5 - 3.2 mm and

SI / cs 051420WO - in which the hot-rolled strip is wound at a reel temperature of 350 - 480 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. A process according to claim 4 or 5, wherein the tensile strength of the cold-rolled strip is at least 800 MPa.

SI / cs 051420WO

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 at a breaking elongation A _{80 of} the obtained hot strip of at least 10%, the hot rolling end temperature 900 - 1000 ⁰ C and the reel temperature 390 - 480 ⁰ C.

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

850 - 1000 ⁰ C and the reel temperature 350 - 390 ⁰ C.

SI / cs 051420WO