ES2325962T3 - PROCEDURE FOR MANUFACTURING STEEL FLAT PRODUCTS FROM A MICROALEATED MULTIPHASIC STEEL WITH BORO. - 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

Procedure for manufacturing flat products of steel from a multi-phase microalloyed steel with boron.

The invention relates to a method for manufacture flat steel products, such as strapping or cutting sheet, made from microalloyed steels with high resistance boron. Steels of this type belong to the group of steels multi-phase In this case it is usually steels whose properties are determined by type, quantity and disposition of the phases of the structure. Therefore, in the structure are at least two phases present (for example, ferrite, martensite, bainita). This way they have a combination of superior strength / malleability compared to steels conventional.

This manufacturing route causes problems especially in the casting of compositions that solidify so  peritectic In the case of these qualities of steel there is the risk of the formation of longitudinal cracks during casting keep going. The appearance of longitudinal cracks of this type can reduce the quality of laminated strips so strongly in hot produced from flat roughing or roughing thin cast planes that become unusable. To prevent This risk requires extensive measures, such as a high cost of the flame, which can go so far that the processing of Steel grades of this type become unprofitable. In the Steel casting with high Al contents also occurs unwanted interactions with laundry powder for which also the quality of a flat product is negatively influenced manufactured from these steels.

Due to these peculiarities, steels Multiphasics are of great interest especially for construction of cars because, due to their high resistance, they allow part, the use of smaller material thicknesses and a reduction in weight of the car that this is associated with and, on the other hand, improve the safety of the car body in case of a shock (behavior to shock). Therefore, steels multiphase make possible with at least constant resistance of the total body a reduction of the thickness of sheet of a piece manufactured from such multiphase steels compared to a body made from conventional steels.

Normally, multi-phase steels melt in steelworks of converters and sneak into a laundry plant continue to give flat slabs or thin flat slabs that then they are hot rolled to give a hot rolled strip And they wind up. In this case, the mechanical properties of the strip hot rolled can be varied by cooling specifically controlled hot rolled strip after of hot rolling with the aim of adjusting certain proportions of the structure. Hot rolled strips they can also be cold rolled to give a cold rolled strip to also make thinner sheet thicknesses available (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 manufacture of products flat from high strength multi-phase steels with tensile strengths of more than 800 MPa is that at laminate steels of this type should apply high forces of laminate. This requirement has the consequence that generally with the manufacturing plants that are currently available normally rolled strips can be made available in hot high strength steels of the type in question in many cases only in a width and thickness that no longer satisfy totally the requirements currently demanded in the sector of the car construction Especially the minor strips thicknesses with sufficient widths can hardly be obtained in conventional plants. It has also proved difficult in the practice during conventional way of manufacturing strapping cold rolled from multi-phase steels with resistors of more than 800 MPa.

An alternative route to manufacture strapping of steel from a multiphase steel is proposed in the document EP 1 072 689 B1 (DE 600 09 611 T2). According to this procedure known, to manufacture thin steel strips initially it strain a melt of steel containing (in% by weight) 0.05 and 0.25% of C, in sum 0.5-3% of Mn, Cu and Ni, in sum 0.1-4% of Si and Al, in sum up to 0.1% of P, Sn, As and Sb, in sum less than 0.3% of Ti, Nb, V, Zr and REM, as well as respectively less than 1% of Cr, Mo and V, the rest iron and unavoidable impurities to give a cast strip with a thickness of 0.5-10 mm, especially 1-5 mm. He cast strip is then hot rolled in line in one or several passes with a degree of deformation that is between 25% and 70% to give a hot rolled strip. The final hot rolling temperature is in this case above the temperature of Ar 3. After finishing the hot rolled, the hot rolled strip obtained is then cool in two stages. In the first stage of this cooling  a cooling rate of 5-100ºC / s until a temperature is reached that It is between 400-550ºC. At this temperature, the hot rolled strip is then allowed to stand for a resting time that is needed to make possible a Bainitic transformation of steel with a proportion of austenite residual of more than 5%. In this case, the formation of perlite After a sufficient rest time for adjustment of the required structure, the transformation process is interrupted by the start of the second stage of cooling in the that the hot rolled strip is brought to a temperature below 400 ° C and then roll it into a coil at a winding temperature that is below 350 ° C.

With the procedure described in EP 1 072 689 B1 it should be possible to make a simple hot rolled strip with proportions of bainitic structure from a multiphase steel that has TRIP properties ("TRIP" = " T ransformation I nduced P lasticity "(transformation-induced plasticity)). Steels of this type have relatively high strengths with good deformability. However, resistance is not enough for many application cases, especially in the car construction sector.

Therefore, the objective of the invention was in making available a procedure with which they could produce high strength steel flat products with a great diversity of geometric dimensions with expenses of reduced manufacturing

Starting from the prior art explained, this objective is achieved by a procedure according to claim 1 for manufacturing flat steel products in the that a steel that forms a multiphasic structure that contains (in % by weight) 0.08-0.12% C, 1.70-2.00% of Mn, up to 0.030% of P, up to 0.004% of S, up to 0.20% of Si, 0.01-0.06% of Al, up to 0.0060% of N, 0.20-0.50% of Cr, 0.010-0.050% of Ti, 0.0010-0.0045% of B and as rest iron and inevitable impurities sneaks to give a cast strip with a thickness of 1-4 mm, in which cast strip is hot rolled in line in one operation continues with a degree of deformation of more than 20% at a final hot rolling temperature found in the 800-1100 ° C range to give a laminated strip hot with a thickness of 0.5-3.2 mm and in the that the hot rolled strip is wound at a temperature of winding that amounts to 250-570 ° C so that obtains a hot rolled strip whose tensile strength R_ {m} amounts to at least 800 MPa at an elongation at break A_ {80} of at least 5%.

The invention also exploits the possibility of strapping to process a multiphase steel of especially high resistance that solidifies possibly peritectic in a hot rolled strip. As the strained strip already has a small thickness in this case, in the Hot rolling course of this strap should only keep relatively small degrees of deformation to produce flat products with small thicknesses as needed especially in the automotive industry sector. By therefore, by the advantage of a corresponding starting thickness of the cast strip it is possible to manufacture without problems with the method according to the invention hot rolled strips which at an optimal distribution of properties have a thickness of as 1.5 mm maximum and from which they can be manufactured, for example, elements for the supporting structure of a car.

Due to the small degrees of deformation During hot rolling, the necessary rolling forces for this compared to the necessary forces in the laminate hot flat slabs or thin flat slabs in the conventional way of proceeding are small, so that with the procedure according to the invention can occur without problems hot rolled strips of greater width than clearly found above the width of hot rolled strips of the same strength and thickness class that can occur from conventional way. Therefore, the invention allows manufacturing of Safely form high strength hot rolled strips constituted by a martensitic steel of the composition Specified processed according to the invention whose width amounts to more 1,200 mm, especially more than 1,600 mm.

The application according to the invention of strip casting procedure in the processing of steels high strength of the compound type according to the invention offers, In addition to the previously mentioned advantages, the possibility, also in relation to its solidification behavior, of safely strain critical steel compositions of the type processed according to the invention due to its specific properties of the procedure and magnitudes of adjustment (for example, temperature hot rolled finish, cooling, temperature winding). Therefore, the very fast solidification characteristic of Strapping of strapping straps leads to, compared to a conventional manufacturing, a clearly reduced risk of occurrence of segregations centered with the consequence that the Hot rolled strip produced according to the invention features along its cross section and its length a distribution of especially homogeneous properties and structure.

Another special advantage of the way to proceed according to the invention is that the hot rolled strip produced according to the invention exhibits high resistance of at least 800 MPa, without also having to maintain a cooling cycle Special hot rolled strip between the laminate finish hot and winding as this is written previously, by example, in EP 1 072 689 B1 because of the need for a cooling phase In carrying out the procedure according to the invention should only ensure that hot rolling end in a relatively limited temperature window narrow and that the winding is also performed in a range of exactly defined temperature. In between a one stage cooling.

Another advantage of the way to proceed according to the invention is that an extension of the variety of mechanical properties of the strap produced according to the invention which only based on an analysis of steel can be achieved by a variation of the cooling and rolling conditions.

Hot rolled strips produced according to the invention they are especially suitable for processing  back to give a cold rolled strip. A configuration of the invention according to practice correspondingly provides that the hot rolled strip is cold rolled to give a strap cold rolled with a thickness of 0.5-1.4 mm, especially 0.7 mm to 1.3 mm, as needed for construction of car bodies. To eliminate hardening which occur during cold rolling, the strip rolled in cold can be annealed at an annealing temperature of 750-850 ° C. For cold rolled strip produced in this way from the hot rolled strip manufactured according to the invention, resistance can be guaranteed safely at tensile of at least 800 MPa. In this case, the elongation to A 50 breakage of the cold rolled strip also amounts to security at least 10%.

According to another advantageous configuration of the invention, the cold rolled strip can be provided in a manner known in itself of a metallic coating in which case it can be, for example, a galvanized.

The resistance and elongation values of hot rolled strips produced according to the invention can adjust to great diversity through adaptation corresponding of the final hot rolling temperature and winding If, for example, rolled strips must be produced in hot than at an elongation at breakage A_ {80} of the strip hot rolled obtained from at least 10% present a tensile strength R m of at least 800 MPa, then this can be achieved by raising the final rolling temperature in heat to 900-1000 ° C and winding temperature to 420-510 ° C.

If, on the contrary, there should be a strap hot rolled with superior tensile strength R m Guaranteed at least 1000 MPa at elongation at break A_ {80} of at least 5%, so for this you choose the final hot rolling temperature in the range of 900-1100ºC and the winding temperature in the range of 450-570 ° C.

Still greater resistance to the traction R m of the hot rolled strip obtained from at at least 1200 MPa at an elongation at break A 80 of at least 5% increasing the final hot rolling temperature to 800-1000 ° C and winding temperature at 250-550 ° C.

The following explains in more detail the invention by examples of embodiment.

In experiments performed to check the effect of the invention, two steels A and B compounds according to the invention with the composition specified in table 1 melted and sneaked into a conventional two roller casting machine to give a cast strip having a thickness of 1.6 mm.

TABLE 1

(data in% in weight)

one

Straps cast from steels A and B they were hot rolled online in six different experiments immediately after the strip casting to a TFL hot rolled final temperature to give a strap hot rolled whose thickness amounted to 1.25 mm. TO then the hot rolled strip respectively obtained was cooled directly in a cooling stage until a TB winding temperature and it was wound. After winding, hot rolled strips produced from A steels  and B respectively presented a tensile strength R_ {m} and an elongation at break A_ {80} specified in table 2 as well as the final rolling temperature in Hot TFL and TB winding temperature respectively maintained during its manufacture.

TABLE 2

2

The hot rolled strip produced according to the experiment 4 is cold rolled after winding and pickling to give a 0.7 mm thick cold rolled strip and it is counted at a temperature of 800 ° C continuously to recrystallize the strip.

At an elongation at break A 50 of the 11.5%, the tensile strength R m of the laminated strip in cold thus obtained amounted to 835 MPa.

Claims (12)

1. Procedure for manufacturing flat products of steel,

-

at that a steel that forms a multiphasic structure of the following composition (in% by weight)

C:

0, 08-0.12%

Mn:

1.70-2.00%

P:

≤ 0.030%

S:

≤ 0.004%

Yes:

? 0.20%

To the:

0.01-0.06%

N:

le0.0060%

Cr:

0.20-0.50%

You:

0.010-0.050%

B:

0.0010- 0.0045%

the rest iron and inevitable impurities

it sneaks in for give a strained strip with a thickness of 1-4 mm

-

at that the cast strip is hot rolled in line in one operation continues with a degree of deformation of more than 20% at a final hot rolling temperature found in the 800-1100 ° C range to give a laminated strip hot with a thickness of 0.5-3.2 mm, it cools in one stage and

-

at that the hot rolled strip is wound at a temperature of winding that amounts to 250-570ºC,

-

from so that a hot rolled strip is obtained whose resistance at traction R_m amounts to at least 800 MPa at elongation at break A 80 of at least 5%.

2. Method according to claim 1, characterized in that the width of the hot rolled strip is more than 1,200 mm, especially more than 1,600 mm. Method according to one of the preceding claims, characterized in that the thickness of the hot rolled strip amounts to a maximum of 1.5 mm. Method according to one of the preceding claims, characterized in that the hot rolled strip is cold rolled to give a cold rolled strip with a thickness of 0.5-1.4 mm. 5. Method according to claim 4, characterized in that the cold rolled strip is recoated at an annealing temperature of 750-850 ° C. Method according to claim 4 or 5, characterized in that the tensile strength of the cold rolled strip amounts to at least 800 MPa. Method according to one of claims 4 to 6, characterized in that the cold rolled strip has an elongation at break A 50 of at least 10%. Method according to one of the preceding claims, characterized in that the hot rolled strip or the cold rolled strip is provided with a metallic coating. 9. The method according to claim 8, characterized in that the metallic coating is galvanized. Method according to one of the preceding claims, characterized in that at an elongation at break A 80 of the hot rolled strip obtained at least 10% the final hot rolling temperature is 900-1020 ° C and the temperature of winding at 420-490 ° C.

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Method according to one of the claims 1 to 9, characterized in that at a tensile strength R m of the hot rolled strip obtained at least 1000 MPa the final hot rolling temperature is 900-1100 ° C and the temperature winding at 450-570 ° C. 12. Method according to one of claims 1 to 9, characterized in that at a tensile strength R m of the hot rolled strip obtained from at least 1200 MPa the final hot rolling temperature is 800-1000 ° C and the temperature winding at 250-550 ° C.