MX2011012920A

MX2011012920A - Method for producing high-strength, low alloy steel with copper.

Info

Publication number: MX2011012920A
Application number: MX2011012920A
Authority: MX
Inventors: Arth Gregor; Bernhard Christian; Eckerstorfer Gerald; Hohenbichler Gerarld; Linzer Bernd
Original assignee: Siemens Vai Metals Tech Gmbh
Priority date: 2010-12-02
Filing date: 2011-12-02
Publication date: 2012-06-11
Also published as: RU2589961C2; UA112150C2; CN102485362B; RU2011149098A; EP2460596A1; CN102485362A; EP2460596B1; ES2433425T3

Abstract

A method for the production of high-strength, low-alloy steel with the addition of copper to reduce the formation of a second of the steel ductility minimum is provided in that the steel has a copper content of 0.05% up to 0.5%, in particular in the range of 0.15% brought to 0.35%, and is then poured into a strand or strip casting a strand or a band with a maximum thickness of 130 mm and solidified, wherein the casting speed 4.5 m at least / min, preferably at least 5 m / min, and the strand or strip min followed by continuous rolling or semi-endless rolling is less than 5.8, especially less than 4.5 minutes, it is rolled to the desired final thickness..

Description

PROCESS TO PRODUCE LOW HIGH ALLOY STEEL RESISTANCE FIELD OF THE INVENTION The invention relates to a process for producing low alloy and high strength steel with the addition of copper.

The low alloy and high strength steel is also referred to as HSLA steel. HSLA steel supports better mechanical and processing properties than C-alloyed steel of the same strength. HSLA steels have a carbon content of 0.05 to 0.25% by weight and contain up to 2% by weight of manganese and small fractions of other alloying elements such as copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare earth elements or zirconium. The deformation limit is between 250-590 MPa and can even reach 700 MPa.

The niobium, titanium and vanadium alloy elements are required for a specific steel strength but, despite their small proportion (which is different depending on the grade of the steel), they constitute a large fraction of the steel costs. Therefore, there have been and are tests to reduce the use of these alloying elements.

BACKGROUND OF THE INVENTION Document O 2004/026497 Al discloses a process for producing a steel strip by means of which a yarn is produced and then further processed through continuous rolling in a coarse rolling mill and a finishing rolling mill, with cooling (not defined in greater detail) occurring downstream of the finishing mill.

It is also known that copper can be added to reduce expensive alloying elements for HSLA steel, see for example, published patent application JP 2009-280902 A. This document suggests the addition of copper to the 1 to 2% extension for the resistance produced, see for example paragraph 22 of the automatic translation into English. The proportion of niobium is in the range of 0.01-0.05%, the proportion of vanadium is 0.01 to 0.1% and the proportion of titanium is 0.01-0.03%.

In these quantities, copper has the property of producing a precipitation of particles that are stable in the deformation and heating temperature range and have one. grain size distribution that prevents grain growth, and thus simultaneously provide an increase in the strength of the steel or in the strip of steel or finished rolled steel.

In some processes to produce steel, copper is often already present in steel, for example if low value pieces with a high copper content are used in an arc furnace.

However, copper has the disadvantage that, mainly in connection with the sulfide, this can lead to the formation of a second minimum ductility of the steel, which emerges as a result of cracking during deformation in the mill train, for example in the form of edge cracks. This phenomenon is associated with accumulation mechanisms, which are reinforced mainly by slow solidification speeds and long residence times of the steel in the reheating furnaces.

Therefore, an object of the invention is to reduce these phenomena that lead to the formation of a second minimum ductility.

SUMMARY OF THE INVENTION This objective is achieved by the process claimed in claim 1, according to which the steel is provided with a copper content of 0.15% to 0.35%, and is then melted into a wire or a strip having a thickness maximum of 130 mm in a strip casting or continuous and solidified installation, where the casting speed is at least 4.5 m / min, preferably at least 5 m / min, and the thread or strip is then laminated to the thickness desired finish by continuous rolling or semi-continuous rolling in less than 5.8 min, in particular less than 4.5 min, and where the rolling to the desired final thickness is followed by cooling of 15-90 K / s, preferably 25-60 K / s, at a temperature below 650 ° C, preferably below 600 ° C.

The percentages provided in the claims represent the percentage by weight.

The duration of the rolling operation is measured from the end of the strip or continuous casting installation, i.e., when the fused wire or the cast strip has left the last guide device (thus a pair of guide rolls) of The casting or continuous casting facility. Here, the duration of 5.8 min corresponds to a melting rate of about 3-3.5 m / min, and the duration of 4.5 min corresponds to a melting rate of about 4.8-5.4 m / min.

In contrast to JP 2009-280902 A, mentioned in the introduction, precipitations of about 20 nm or higher are produced in the case of the present invention in consideration of the relatively low copper content of the alloy and the different production process. This is too high to lead to the hardening of the precipitation, because the effect is very small due to these sizes of precipitations and the dissolved proportion in the matrix is decreased. However, precipitation in the 20 nm region provides outstanding help with the production of a uniform microstructure with small grain sizes.

In the case of the alloys in JP 2009-280902 ?, copper is described as strengthening the solid solution and as a contribution to the hardening of the precipitation. Given the aforementioned copper content, welding problems and cracking would be expected.

The resulting high strength low alloy steel in particular may assume the form of a sub-peritectic steel having a carbon content of 0.5 wt% to 0.1 wt% or of a medium carbon steel having a carbon content from 0.05% by weight to 0.25% by weight.

The continuous rolling or semi-continuous rolling according to the invention preferably occurs in a two-stage rolling mill, consisting of a coarse rolling mill and a finishing rolling mill.

The duration of the rolling operation of less than 5.8 min begins with the emergence of the yarn or strip from the strip or continuous casting facility and ends with the emergence of the steel, usually in the form of a steel strip , from the last active rolling station (ie in coupling with the steel) of the finishing rolling mill. It is understood that a rolling mill means the direct juxtaposition of rolling stations with a distance between each of the stations of less than 7 m, preferably less than 6 m. The rolling stations separated by a greater distance form part of the next rolling mill or are individual stations.

The process according to the invention involves the use of a form of flat steel strip production where the melting process and the rolling process are linked.

The casting process occurs in a casting facility, where a liquid steel wire emerging from a mold of a casting facility is guided through a wire guide apparatus directly following the mold. This apparatus comprises a plurality (generally three to fifteen) of guiding segments, each guiding segment comprising one or more (usually three to ten) pairs of guiding elements preferably in the form of yarn support rollers. The support rollers are rotatable about an axis extending orthogonally to the direction in which the yarn is transported. Instead of yarn support rollers, individual guiding elements could also be formed as static components, for example, fixed pulley type. Regardless of the specific shape of the guide elements, these are accommodated on both sides of the broad sides of the yarn, so that the yarn is guided through the upper and lower series of the guiding elements.

The yarn emerges from the mold substantially vertically downwards and is deviated towards the horizontal. The yarn guide apparatus then has a profile that is curved substantially over an angular range of 90 °.

Reference is made to "continuous rolling" when a casting facility is connected to a rolling facility so that the melted wire in the casting facility or the cast strip is guided directly, without separation of the part of the wire or strip that has just it has been melted and without intermediate storage, towards a rolling installation, where it is laminated to the final thickness. The start of the yarn or the strip can then already be finished with the laminate to the desired thickness to form a steel strip, while the casting facility continues to carry out the casting in the same yarn or in the same strip, ie There is not yet the end of the thread or the strip. Reference is also made to the directly coupled operation or continuous operation of the casting and rolling installation.

In the case of the so-called "semi-continuous lamination", the melted yarn is divided into plates after casting or the strips are divided after casting and the plates or divided strips are supplied to the rolling mill without intermediate storage and Cooling at room temperature. This separation can be effected in such a way that the plate head of the preceding plate is being or has already been laminated in the next rolling mill, or has not yet been detected due to the relatively large distances of the first rolling station.

The yarn that emerges from the melting facility is generally desalinated and roughly rolled in the coarse mill, and the intermediate strip produced in the process is reheated in a furnace generally at temperatures of approximately 1200 ° C and laminated for finishing in the Rolling mill finishing. The hot rolling is generally carried out in the finishing mill, that is, the rolling material remains in the austenite region during rolling. The final rolling temperatures are in the range of 780-850 ° C, preferably in the range of 800-830 ° C.

In consideration of the continuous rolling or semi-continuous rolling, the cooling of the steel after the casting operation is avoided by further intermediate processing in the coarse rolling mill. In contrast to this, in conventional laminators, plates are often stored after they have been produced and have to be reheated just upstream of the coarse rolling mill. However, this activates the mechanisms of undesirable accumulation.

In addition, the maximum thicknesses of the cast or 130 mm strips or plates, which occurs during continuous or semi-continuous rolling, also have a positive effect on undesirable precipitation, because the copper particles are deposited in a form faster and, therefore, are restricted to a small, specific, diameter that depends mainly on the speed of solidification.

Tests carried out on the strip material from the continuous casting facility have shown that the copper precipitations have a diameter of about 20-40 nm if the casting rate is set to be at least greater than 4.5 m / min and the lamination is then carried out in two stages (each with three or five rolling stations) in 4.3 min. In this case, the content of the selected alloy elements was as follows: 0. 3% copper (Cu) 0. 025% niobium (Nb).

The range of size of the precipitation of 20-40 nm also corresponds to the range of size of the precipitations that are the objective of the micro-alloy elements (titanium, niobium), to have an effect that influences the microstructure and also increases the resistance .

Given the cooling rates of 15-90 K / s, preferably 25-60 K / s, at a temperature below 650 ° C, preferably below 600 ° C, in particular within a maximum of 35 seconds, of preference within a maximum of 15 seconds, after rolling to the desired final thickness, it is possible to achieve tensile strength up to 925 MPa or a deformation limit up to 700 MPa, relatively high values preferably achieved through cooling fast (50-90 K / s) immediately after the last deformation step and through cooling to less than 500 ° C.

Depending on the cooling strategy and, therefore, also on the resistance class, the ambient temperature microstructure obtained mainly consists of ferrite or pearlite or bainite.

Comparable HSLA steels produced in a conventional manner contain approximately 0.07% vanadium, 0.15% titanium and 0.07% niobium.

According to the invention, then it can be established that the proportion of vanadium (V) added to the steel is below 0.03%, in particular below 0.01%, and / or that the proportion of niobium (Nb) added to the steel is below 0.055%, preferably below 0.045%, particularly preferably below 0.03%.

In the case of a continuous casting installation, it can be established that the cast plate has a preferred thickness of 40-130 mm, particularly preferably 40-105 mm, in particular about 80 mm.

In the case of a strip casting installation, it can be established that the melted and solidified strip has a preferred thickness of 1-4.5 mm, in particular about 3 mm. If the rolling is also carried out after the casting of the strip, division into a coarse rolling mill and a finishing rolling mill naturally does not occur.

In the process according to the invention, the thickness of the intermediate strip, that is, of the steel between the coarse rolling mill and the finishing rolling mill, is preferably 5-25 mm, preferably 10-18 mm.

The coarse rolling mill should comprise at least two, better yet three, rolling stations; The finishing laminating mill should comprise at least four, better yet five, rolling stations.

The final thickness of the product with laminated finish is in the range of 0.6-12 mm, preferably 1-6 mm.

The process according to the invention supports the advantage that it is also possible to use pieces of lower quality and, therefore, that they contain copper for the production of steel and, in addition, the addition of alloying elements can be reduced, in particular microalloying elements (niobium, titanium, vanadium).

The process according to the invention can be used for uncoated metal sheets for the automotive industry, for electrogalvanized metal sheets and for hot galvanized metal sheets in the automotive sector. For use in the automotive sector, the silicon content of steel should not exceed 0.9% by weight; a range of 0.1% to 0.9% is preferred.

DETAILED DESCRIPTION OF THE INVENTION In the case of production in a continuous rolling process, a wire having a thickness of about 70-100 mm is melted in a casting machine. The melting machine is immediately followed by three stations of the coarse rolling mill, where a large reduction in the thickness of the middle strip is achieved, to approximately 15 mm. These stations are followed by a deslagging facility, and thereafter, the material passes through an inductive heater and a five-station finishing mill, where the thickness can be reduced to 0.6 mm. This is followed by the cooling of the strip in the cooling section, for example, by application of water, in order to establish the properties of the material, and this cooling section is followed by a winder in which the strip is wound to form coils, which concludes the production process.

By installing a plurality of shears (downstream of the coarse mill, upstream of the finishing mill and upstream of the coiler), it would be possible to operate the mill in one piece mode, the intermediate strips being cut downstream of the mill. Rolling mill train and are individually laminated in the finishing mill. In the case of continuous rolling, the strips that have been laminated to the final thickness are cut just before the winder, and the rolling operation occurs continuously. The main advantage of this installation is the low amount of energy required to produce the steel strip. While approximately 2 GJ of energy is required to produce one ton of hot strip in conventional hot mills, this value drops to 0.4 GJ per ton of hot strip in a continuous rolling or semi-continuous rolling mill facility.

In the cooling section for the laminated finishing strip, the cooling of 15-90 K / s, preferably 25-60 K / s, at a temperature below 650 ° C, preferably below 600 ° C , occurs within a maximum of 35 seconds, preferably within a maximum of 15 seconds, after rolling to the desired final thickness.

Claims

NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following is claimed as a priority: CLAIMS

1. - A process for producing high strength low alloy steel with the addition of copper, characterized in that the steel is provided with a copper content of 0. 15% to 0.35%, and then it is melted into a wire or strip having a maximum thickness of 130 mm in a continuous or solidified strip casting facility, where the casting speed is at least 4.5 m / min, preferably at least 5 m / min, and the yarn or the strip is then laminated to the desired final thickness by continuous rolling or semi-continuous lamination in less than 5.8 min, in particular less than 4.5 min, and where the lamination to the thickness The desired final is followed by cooling of 15-90 K / s, preferably 25-60 K / s, at a temperature below 650 ° C, preferably below 600 ° C.

2. - The process in accordance with the claim 1, characterized in that the continuous rolling or semi-continuous rolling occurs in a two-stage laminator, consisting of a coarse rolling mill and a finishing rolling mill.

3. - The process according to claim 1, characterized in that the cooling occurs within a maximum of 35 seconds, preferably within a maximum of 15 seconds, after rolling to the desired final thickness.

4. - The process according to one of claims 1 to 3, characterized in that the proportion of vanadium (V) added to the steel is below 0.03%, in particular below 0.01%.

5. - The process according to one of claims 1 to 4, characterized in that the proportion of niobium (Nb) added to the steel is below 0.0551, preferably below 0.045%, particularly below 0.03%.

6. - The process according to one of claims 1 to 5, characterized in that the melted and solidified yarn has a preferred thickness of 40-130 mm, particularly preferably 40-105 mm, in particular about 80 mm.

7. - The process according to one of claims 1 to 5, characterized in that the fused and solidified strip has a preferred thickness of 1-4.5 mm, in particular about 3 mm.

8. - The process according to one of claims 1 to 6, characterized in that the thickness of the intermediate strip is 5-25 mm, preferably 10-18 mm.

9. - The process according to one of claims 2 to 6 and 8, characterized in that the coarse rolling mill comprises at least two, preferably three, rolling stations.

10. - The process according to one of claims 2 to 6, 8 and 9, characterized in that the finishing mill comprises at least four, preferably five, rolling stations.

11. - The process according to one of claims 1 to 10, characterized in that the final thickness is in the range of 0.6-12 mm, preferably 1-6 mm.