Method for producing a flat steel product with an amorphous, partially amorphous or fine-crystalline microstructure and flat steel product with such characteristics

Abstract

A method is provided for producing a 0.8-4.5 mm thick steel strip with an amorphous, partially amorphous or fine-crystalline microstructure with grain sizes in the range of 10-10000 nm and also a flat steel product made therefrom. A molten steel is cast into a cast strip in a casting device and cooled down at an accelerated rate. Along with Fe and impurities that are unavoidable for production-related reasons, the molten material contains at least two elements belonging to the group Si, B, C and P. In this case, the following applies for the contents of these elements (in % by weight) Si: 1.2-7.0%, B: 0.4-4.0%, C: 0.5-4.0%, P: 1.5-8.0%. With a corresponding composition and a microstructure with corresponding characteristics, a flat steel product according to the invention has a HV0.5 hardness of 760-900.

Claims

1. A method for producing a flat steel product with an amorphous, a partially amorphous, or a fine-crystalline microstructure, the fine-crystalline microstructure having grain sizes in the range of 10-10000 nm, comprising: casting molten steel into a cast strip in a casting device comprising two rolls rotating counter to one another wherein a molten pool of metal feeds a gap between the two rolls; cooling said molten steel at an accelerated rate in a casting region defined by the gap between the two rolls to form a cast strip; further cooling the cast strip leaving the casting region using an additional cooling device, wherein the molten steel is cooled down at a cooling rate of at least 200 K/s to a temperature below the glass transition temperature T.sub.G; and hot-rolling the cast strip at an initial hot-rolling temperature lying in the range between the glass transition temperature T.sub.G and the crystallization temperature T.sub.x to form a flat steel product, wherein the thickness of the cast strip is 0.8-4.5 mm and the molten steel comprises, along with iron and unavoidable impurities, 1.2-7.0% Si and at least one element selected from the group consisting of B, C and P, wherein (in % by weight): B: 0.4-4.0%, C: 0.5-4.0%, and/or P: 1.5-8.0% and also optionally one or more elements selected from the group consisting of Cu, Cr, Al, N, Nb, Mn, Ti and V, wherein (in % by weight): Cu: up to 5.0%, Cr: up to 10.0%, Al: up to 10.0%, N: up to 0.5%, Nb: up to 2.0%, Mn: up to 3.0%, Ti: up to 2.0%, and/or V: up to 2.0%.

2. The method as claimed in claim 1, wherein the molten steel is cooled at a cooling rate of up to 1100 K/s.

3. The method as claimed in claim 1, wherein the casting region of the casting device is formed on at least one longitudinal side by a wall that moves in a casting direction and is cooled during the casting operation, and wherein the molten steel is cooled by contact with the moving and cooled wall at a cooling rate of at least 200 K/s.

4. The method as claimed in claim 3, wherein, after leaving the casting region, the cast strip continues to be cooled at a cooling rate of at least 200 K/s by the additional cooling device.

5. The method as claimed in claim 3, wherein the cast strip leaving the casting region is cooled continuously until its temperature is below the glass transition temperature T.sub.G of the respective steel.

6. The method as claimed in claim 3, further comprising hot-rolling the cast strip at an initial hot-rolling temperature of 500-1000 C. to form a hot strip.

7. The method as claimed in claim 3, further comprising annealing the cast strip leaving the casting device and having an amorphous or partially amorphous microstructure at an annealing temperature T.sub.anneal corresponding at least to the crystallization temperature T.sub.x of the respective steel.

8. The method as claimed in claim 7, wherein the annealing temperature T.sub.anneal lies in the range of 500-1000 C.

9. The method as claimed in claim 1, wherein the molten steel contains at least one element selected from the group consisting of Cu, Cr, Al, N, Nb, Mn, Ti and V.

10. The method as claimed in claim 1, wherein, for at least one of the elements selected from the group consisting of B, C, and P, at least one of the following respectively applies (in % by weight): B: 0.4-3.0%, C: 0.5-3.0% and/or P: 2.0-6.0%.

11. The method as claimed in claim 1, wherein the molten steel comprises (in % by weight) at least one element selected from the group consisting of Cu, Cr, Al and N, wherein (in % by weight): Cu: 0.1-5.0%, Cr: 0.5-10.0%, Al: 1.0-10.0%, and/or N: 0.005-0.5%.

12. The method as claimed in claim 1, wherein a strip speed at which the cast strip leaves the gap is 0.3-1.7 m/s.

13. The method as claimed in claim 1, wherein the Si is 2.0-6.0%.

14. A flat steel product made according to the method of claim 1 with a thickness of 0.8-4.5 mm, comprising a steel that comprises, along with iron and unavoidable impurities, 1.2-7.0% Si and at least one element selected from the group consisting of B, C and P, wherein (in % by weight): B: 0.4-4.0%, C: 0.5-4.0%, and P: 1.5-8.0%, and optionally one or more elements selected from the group consisting of Cu, Cr, Al, N, Nb, Mn, Ti and V, wherein (in % by weight): Cu: up to 5.0%, Cr: up to 10.0%, Al: up to 10.0%, N: up to 0.5%, Nb: up to 2.0%, Mn: up to 3.0%, Ti: up to 2.0%, and/or V: up to 2.0%, and having an amorphous, partially amorphous or fine-crystalline microstructure with grain sizes that lie in the range of 10-10000 nm, wherein the HV0.5 hardness of the flat steel product is 760-900.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in more detail below on the basis of a drawing representing an exemplary embodiment. The single FIGURE schematically shows a device for producing a cast strip in a lateral view.

DETAILED DESCRIPTION OF THE INVENTION

(2) The installation 1 for producing a cast strip B comprises a casting device 2, which is constructed as a conventional two-roll casting device, and accordingly comprises two rolls 3, 4 rotating counter to one another about axes X1, X2 aligned axially parallel to one another and at the same height. The rolls 3, 4 are arranged at a distance from one another establishing the thickness D of the cast strip B to be produced and thus bound at their longitudinal sides a casting region 5, which is formed as a casting gap and in which the cast strip B is formed. On its narrow sides, the casting region 5 is sealed off in a similarly known way by side plates that are not visible here, which are pressed against the end faces of the rolls 3, 4.

(3) During the casting operation, the intensively cooled rolls 3, 4 rotate and in this way form longitudinal walls of a casting mold that is formed by the rolls 3, 4 and the side plates, which walls move along continuously during the casting operation. The direction of rotation of the rolls 3, 4 is in this case directed in the direction of gravitational force R into the casting region 5, so that, as a consequence of the rotation, molten material S is transported from a molten pool in the space above the casting region 5 between the rolls 3, 4 into the casting region 5. The molten material S thereby solidifies when it comes into contact with the circumferential surface of the rolls 3, 4, on account of the intensive heat removal taking place there, to form a respective shell. The shells adhering to the rolls 3, 4 are transported by the rotation of the rolls 3, 4 into the casting region 5 and compressed there under the effect of a strip-forming force K into the cast strip B. The cooling output effective in the casting region 5 and the strip-forming force K are in this case made to match one another in such a way that the cast strip B continuously leaving the casting region 5 is to the greatest extent completely solidified.

(4) In order to suppress crystallization effects, after the casting region 5 the cast strip B runs into a cooling device 7, which applies a cooling medium to the cast strip B, so that it cools down further. The cooling down by the cooling device 7 directly follows on here after the casting region 5 and in this case takes place so intensely that the temperature T of the cast strip B continuously decreases, until it lies below the glass transition temperature T.sub.G of the respectively cast molten material S. Any crystallization of the microstructure of the cast strip B is thus suppressed, so that, as before, it is in an amorphous state when it reaches the transporting section 6.

(5) The strip B leaving the casting region 5 is initially transported away vertically in the direction of gravitational force R and subsequently deflected in a known way in a continuously curved arc into a horizontally aligned transporting section 6.

(6) On the transporting section 6, the cast strip B may subsequently run through a heating-up device 8, in which the strip B is heated up throughout at an annealing temperature T.sub.anneal, lying above the crystallization temperature T.sub.x of the respectively cast molten steel S, over an annealing time t.sub.anneal. The aim of this heat treatment is the controlled formation in the cast strip B of a fine-crystalline microstructure with grain sizes that lie in the range of 10-10000 nm. The cast strip B heat-treated in this way is subsequently hot-rolled into hot strip WB in a hot-rolling stand 9.

(7) In the installation 1, a cast strip B has been respectively produced from three molten steels S with the compositions Z1, Z2, Z3 stated in Table 1. For each composition Z1, Z2, Z3, the thickness D of the strips B cast from the respective molten steel S, the cooling-down rate AR respectively achieved in the cooling down of the molten material S in the casting region 5, the cooling-down rate ARZ respectively achieved in the cooling down of the cast strip B leaving the casting region 5 in the additional cooling device 7, and also the target temperature T.sub.Z of the additional cooling down are stated. Furthermore, the microstructural state and the possibly present constituents of the microstructure of the strip obtained are presented in Table 2.

(8) Different heat treatments have been carried out in the heating-up device 8 on two specimens of the cast strip B produced in the way explained above from the molten steel S with the composition Z1. The annealing temperature T.sub.anneal being set and the annealing time t.sub.anneal of the heat treatment, respectively, are compared in Table 3.

(9) It was found that, before the heat treatment, the cast strip B already had a fine-crystalline microstructure of -Fe, Fe.sub.2B, Fe.sub.3B and Fe.sub.3Si with an HV0.5 hardness of 840-900. Also after the heat treatment, the microstructure consisted of -Fe, Fe.sub.2B, Fe.sub.3B and Fe.sub.3Si, but then the HV0.5 hardness was 760-810.

(10) It goes without saying that the described heat treatment by means of the heating-up device 8 and also the hot rolling with the hot-rolling stand 9 are only optional method steps.

(11) The invention consequently provides methods for producing a steel strip B with an amorphous, partially amorphous or fine-crystalline microstructure with grain sizes in the range of 10-10000 nm and also a flat steel product with corresponding characteristics. According to the invention, for this purpose molten steel is cast into a cast strip (B) in a casting device (2) and cooled down in an accelerated manner. Along with Fe and impurities that are unavoidable for production-related reasons, the molten material contains at least two further elements belonging to the group Si, B, C and P. According to a first variant of the method, the following applies for the contents of these elements (in % by weight) Si: 1.2-7.0%, B: 0.4-4.0%, C: 0.5-4.0%, P: 1.5-8.0%. According to a second variant of the method, the molten steel containing Si, B, C and P is cast into a cast strip (B) in a casting device (2), the casting region (5) of which is formed on at least one of its longitudinal sides by a wall that moves in the casting direction (G) and is cooled during the casting operation, the molten steel (S) being cooled down by contact with the moving and cooled wall at a cooling-down rate of at least 200 K/s.

DESIGNATIONS

(12) 1 Installation for producing a cast strip B 2 Casting device 3.4 Rolls of the casting device 2 5 Casting region 6 Horizontally aligned transporting section 7 Cooling device 8 Heating-up device 9 Hot-rolling stand B Cast strip D Thickness of the cast strip B R Direction of gravitational force S Molten material K Strip forming force X1,X2 Axes of rotation of the rolls 3, 4

(13) TABLE-US-00001 TABLE 1 C Si Mn P Al Cr Cu Nb Ti V B Z1 0.038 5.5 0.44 3.3 0.005 0.3 0.133 0.059 0.11 0.048 2.0 Z2 0.041 3.3 0.51 0.025 0.005 0.4 0.09 0.001 0.09 0.055 2.2 Z3 1.5 3.0 0.64 0.030 1.30 0.4 0.08 0.002 0.08 0.045 1.6 Figures are given in % by weight, the remainder iron and unavoidable impurities

(14) TABLE-US-00002 TABLE 2 D AR ARZ Tz [mm] [K/s] [K/s] [ C.] Microstructure Z1 1.2 900 900 400 amorphous Z2 1.2 1050 600 600 fine-crystalline -Fe, Fe.sub.2B, Fe.sub.3B, Fe.sub.3Si Z3 1.1 700 500 500 fine-crystalline, -Fe, Fe.sub.2C, Fe.sub.2B, Fe.sub.3B, Fe.sub.3Si

(15) TABLE-US-00003 TABLE 3 D T.sub.anneal t.sub.anneal [mm] [ C.] [ C.] Microstructure Z1 1.2 600 C. 1 min partially amorphous (amorphous + -Fe, Fe.sub.2B, Fe.sub.3B, Fe.sub.3Si) Z1 1.2 600 C. 20 min fine-crystalline -Fe, Fe.sub.2B, Fe.sub.3B, Fe.sub.3Si