EP2924140B1 - Method for manufacturing a high strength flat steel product - Google Patents

Description

The invention relates to a process for producing a flat steel product with a yield strength of at least 700 MPa and with at least 70% by volume bainitic structure.

Flat steel products of the type in question are typically rolled products, such as steel strips or sheets, as well as blanks and blanks made therefrom.

In particular, the invention relates to a method for producing high-strength so-called "heavy plates" which have a thickness of at least 3 mm.

All information on contents of the steel compositions given in the present application are by weight, unless expressly stated otherwise. All unspecified "% figures" in connection with a steel alloy are therefore to be understood as statements in "% by weight".

High-strength flat steel products, in particular in the field of commercial vehicle construction an increasing importance, as they reduce the dead weight of the vehicle and a Increase the payload. A low weight not only contributes to the optimal use of the technical performance of the respective drive unit, but also supports resource efficiency, cost optimization and climate protection.

A significant reduction in the dead weight of steel sheet constructions can be achieved by increasing the mechanical properties, in particular the strength of each processed flat steel product. In addition to high strength, modern toughened steel products intended for commercial vehicle construction are also expected to have good toughness properties, good brittle fracture resistance behavior and optimum suitability for cold forming and welding.

It is known that this combination of properties can be achieved by choosing a suitable alloying concept and a specific manufacturing method. In conventional processes for producing high-strength plates with a minimum yield strength of 700 MPa, the procedure is as follows. First, the slabs are hot rolled and cooled after rolling in air. Thereafter, the sheets are reheated, cured and subjected to a tempering treatment. The process thus contains several stages in order to achieve the mechanical properties. The large number of associated production steps leads to comparatively high production costs. Also, exact process control is required to achieve the desired toughness properties and surface qualities.

From the EP 2 130 938 A1 For example, there is known a method for producing a hot-rolled steel flat product in which a melt is cast into slabs which contain, in addition to iron and unavoidable impurities (in% by weight) 0.01-0.1% by weight C, 0.01-0 , 1 wt% Si, 0.1-3 wt% Mn, not more than 0.1 wt% P, not more than 0.03 wt% S, 0.001-1 wt% Al, not more than 0.01 wt .-% N, 0.005 - 0.08 wt .-% Nb and 0.001 to 0.2 wt .-% Ti, wherein for the respective Nb content% Nb and the respective C Content% C is:% Nb x% C ≤ 4.34 x 10 ^-3 .

After pouring and solidification of the melt in the known method, the steel slab is reheated to a temperature range whose lower limit is determined depending on the C and Nb contents of each potted steel and whose upper limit is 1170 ° C. Subsequently, the reheated slab is pre-rolled at a final temperature which is 1080-1150 ° C. After a pause of 30-150 seconds, during which the pre-rolled slab is maintained at 1000-1080 ° C, the pre-rolled slab is then hot-rolled to a hot-rolled strip. The degree of deformation of the last pass of the hot rolling should be 3 - 15%.

According to the known method, the hot rolling is completed at a hot rolling end temperature which is at least the Ar3 temperature of the processed steel and is at most 950 ° C. After the end of the hot rolling, the hot strip obtained is cooled at a cooling rate of more than 15 ° C / s to a coiling temperature of 450 - 550 ° C, where it is coiled into a coil.

In the hot strip thus produced, the grain boundary density of the carbon present in solid solution should be 1 - 4.5 atoms / nm ² and the size of the grains of cement precipitated at the grain boundaries should not be more than 1 μm. The flat steel products produced in this way and produced by the known method should have tensile strengths of more than 780 MPa and have yield strengths of up to 726 MPa at sufficiently high-dose alloy contents. In this way, the hot strip produced in the known manner should have a combination of properties which is particularly suitable for use in automobile construction. Optimum surface finish is achieved by limiting the reheat temperature to which the slab is heated prior to hot rolling to the above-mentioned temperature range and thus avoiding excessive scale formation which would be incorporated into the hot strip surface during hot rolling.

In addition to the above-described prior art is from the EP 2 436 797 A1 a high strength steel sheet comprising (in mass%) 0.03 to 0.10% C, 0.01 to 1.5% Si, 1.0 to 2.5% Mn, 0.1% or less P, 0.02% or less S, 0.01-1.2% Al, 0.06-0.15% Ti, 0.01% or less N and the balance iron and unavoidable impurities, its tensile strength being 590 MPa or is more and the ratio of tensile strength and yield strength is 0.80 or more. The steel sheet should have a microstructure with at least 40 area% bainite, the remainder ferrite and martensite. In order to achieve this, a precursor cast from an appropriately alloyed steel is heated to 1150 - 1280 ° C, at a Hot rolling end temperature, which is between the Ar3 temperature and 1050 ° C, hot rolled and cooled at a high cooling rate, for example, 45 ° C / s to a coiler temperature of less than 600 ° C, with a reel temperature of 300 - 500 ° C is set, if a purely bainitic structure is desired.

In addition, from the US 2013/167985 A1 a method of producing a steel sheet comprising 0.05-0.15% C, 0.2-1.2% Si, 1.0-2.0% Mn, not more than 0.04% P, not more than 0.0030% S, 0.005 - 0.10% Al, not more than 0.005% N and 0.03 - 0.13% Ti, and the remainder being Fe and unavoidable impurities. It should consist of less than 80 area% of the structure of bainite and the rest of ferrite. For its production, a correspondingly composed melt is cast into a precursor, which is hot rolled at a hot rolling end temperature of 800 - 1000 ° C and then cooled first at least 55 ° C / s and then at least 120 ° C / s to at most 500 ° C. , The tensile strength of the steel sheet thus obtained should be 780 MPa.

Against the background of the prior art described above, the object of the invention was to provide a method with which high-strength steel sheets can be produced in a practical manner with mechanical properties optimized with regard to use in automobile construction and with an equally optimized surface finish.

The invention solves this problem by the method specified in claim 1.

Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail.

1. a) Erschmelzen einer Stahlschmelze, die (in Gew.-%) aus
   • C: 0,05 - 0,08 %,
   • Si: 0,015 - 0,500 %,
   • Mn: 1,60 - 2,00 %,
   • P: bis zu 0,025 %,
   • S: bis zu 0,010 %,
   • Al: 0,020 - 0,050 %,
   • N: bis zu 0,006 %,
   • Cr: bis zu 0,40 %,
   • Nb: 0,060 - 0,070 %,
   • B: 0,0005 - 0,0025 %,
   • Ti: 0,090 - 0,130 %,
   • sowie aus technisch unvermeidbaren Verunreinigungen, zu denen bis zu 0,12 % Cu, bis zu 0,100 % Ni, bis zu 0,010 % V, bis zu 0,004 % Mo und bis zu 0,004 % Sb gehören, und
   • als Rest aus Eisen
   besteht;
2. b) Vergießen der Schmelze zu einer Bramme;
3. c) Wiedererwärmen der Bramme auf eine
   Wiedererwärmungstemperatur von 1200 - 1300 °C;
4. d) Vorwalzen der Bramme bei einer 950 - 1250 °C betragenden Vorwalztemperatur und einer über das Vorwalzen erzielten Gesamtstichabnahme von mindestens 50 %;
5. e) Fertigwarmwalzen der vorgewalzten Bramme, wobei das Fertigwarmwalzen bei einer Warmwalzendtemperatur von 800 - 880 °C beendet wird;
6. f) innerhalb von höchstens 10 s nach dem Fertigwarmwalzen einsetzendes intensives Kühlen des fertig warmgewalzten Stahlflachprodukts mit einer Abkühlgeschwindigkeit von mindestens 40 K/s auf eine 550 - 620 °C betragende Haspeltemperatur;
7. g) Haspeln des fertig warmgewalzten Stahlflachprodukts.

Accordingly, a process according to the invention for producing a flat steel product having a yield strength of at least 700 MPa and having a bainitic structure of at least 70% by volume comprises the following operations:

1. a) melting a molten steel, the (in wt .-%) from
   • C: 0.05-0.08%,
   • Si: 0.015-0.500%,
   • Mn: 1.60-2.00%,
   • P: up to 0.025%,
   • S: up to 0.010%,
   • Al: 0.020-0.050%,
   • N: up to 0.006%,
   • Cr: up to 0.40%,
   • Nb: 0.060-0.070%,
   • B: 0.0005 - 0.0025%,
   • Ti: 0.090-0.130%,
   • and technically unavoidable impurities including up to 0.12% Cu, up to 0.100% Ni, up to 0.010% V, up to 0.004% Mo and up to 0.004% Sb, and
   • as the remainder of iron
   consists;
2. b) pouring the melt into a slab;
3. c) reheating the slab to one
   Reheating temperature of 1200 - 1300 ° C;
4. d) pre-rolling the slab at a roughing temperature of 950 - 1250 ° C and a total reduction of at least 50% in the roughing;
5. e) finish hot rolling the pre-rolled slab, finishing the finish hot rolling at a hot rolling end temperature of 800 - 880 ° C;
6. (f) intensive cooling of the finished hot rolled flat steel product commencing within a maximum of 10s after finish hot rolling, at a cooling rate of at least 40 K / s to a reel temperature of 550-620 ° C;
7. g) Coiling the finished hot-rolled flat steel product.

The method according to the invention is based on a steel alloy whose alloying constituents and alloy contents are matched to one another within narrow limits in such a way that maximized mechanical properties and optimized surface textures are achieved in a procedure which must be carried out safely.

• C: Der Kohlenstoffgehalt des erfindungsgemäß verarbeiteten Stahls beträgt 0,05 - 0,08 Gew.-%. Um die gewünschten Festigkeitseigenschaften zu erreichen, ist ein C-Gehalt von wenigstens 0,05 Gew.-% erforderlich. Falls jedoch der Kohlenstoffgehalt zu hoch ist, werden die Zähigkeitseigenschaften bzw. die Schweißbarkeit und die Umformbarkeit des erfindungsgemäß verarbeiteten Stahls beeinträchtigt. Aus diesem Grund ist der Kohlenstoffgehalt auf höchstens 0,08 Gew.-% begrenzt.
• Si: Silizium wird bei dem erfindungsgemäß verarbeiteten Stahl als Desoxidationsmittel sowie zum Verbessern der Festigkeitseigenschaften eingesetzt. Wenn jedoch der Siliziumgehalt zu hoch ist, werden die Zähigkeitseigenschaften, insbesondere die Zähigkeit in der Wärmeeinflusszone von Schweißverbindungen, stark beeinträchtigt. Aus diesem Grund soll der Siliziumgehalt des erfindungsgemäß verarbeiteten Stahls 0,50 Gew.-% nicht überschreiten. Zur sicheren Vermeidung von Störungen der Oberflächenqualität kann der Siliziumgehalt auf max. 0,25 Gew.-% beschränkt werden.
• Mn: Mangan wird zur Einstellung der gewünschten Festigkeitseigenschaften bei guten Zähigkeitseigenschaften dem erfindungsgemäß verwendeten Stahl in Gehalten von 1,6 - 2,0 Gew.-% zugegeben. Wenn der Mangangehalt weniger als 1,60 Gew.-% beträgt, werden die geforderten Festigkeitseigenschaften nicht mit der ausreichenden Sicherheit erreicht. Durch die Beschränkung des Mn-Gehalts auf max. 2,00 Gew.-% wird eine Verschlechterung der Schweißbarkeit, der Zähigkeitseigenschaften, der Umformbarkeit und des Seigerungsverhaltens vermieden.
• P: Das Begleitelement Phosphor verschlechtert die Kerbschlagarbeit und die Umformbarkeit. Der Phosphorgehalt soll daher die Obergrenze von 0,025 Gew.-% nicht überschreiten. Optimaler Weise ist der P-Gehalt auf weniger als 0,015 Gew.-% beschränkt.
• S: Schwefel verschlechtert die Kerbschlagarbeit und die Umformbarkeit eines erfindungsgemäß verarbeiteten Stahls infolge von MnS-Bildung. Aus diesem Grund darf der S-Gehalt eines erfindungsgemäß verarbeiteten Stahls höchstens 0,010 Gew.-% betragen. Ein derart niedriger Schwefelgehalt kann in an sich bekannter Weise z. B. durch eine CaSi-Behandlung erzielt werden. Um die negativen Einflüsse von Schwefel auf die Eigenschaften des erfindungsgemäß verarbeiteten Stahls sicher auszuschließen, kann der S-Gehalt auf max. 0,003 Gew.-% beschränkt sein.
• Al: Aluminium wird ebenfalls als Desoxidationsmittel verwendet und behindert infolge von AlN-Bildung die Vergröberung des Austenitkorns beim Austenitisieren. Liegt der Aluminiumgehalt unter 0,020 Gew.-%, laufen die Desoxidationsprozesse nicht vollständig ab. Übersteigt der Aluminiumgehalt jedoch die Obergrenze von 0,050 Gew.-%, so können sich Al₂O₃-Einschlüsse bilden. Diese wirken sich negativ auf den Reinheitsgrad und die Zähigkeitseigenschaften aus.
• N: Das Begleitelement Stickstoff bildet mit Aluminium AlN oder mit Titan TiN. Wenn jedoch der Stickstoffgehalt zu hoch ist, werden die Zähigkeitseigenschaften verschlechtert. Um dies zu verhindern, ist bei einem erfindungsgemäß verarbeiteten Stahl die Obergrenze für den Stickstoff-Gehalt auf 0,006 Gew.-% festgesetzt.
• Cr: Chrom kann einem erfindungsgemäß verarbeiteten Stahl optional zugegeben sein, um seine Festigkeitseigenschaften zu verbessern. Wenn der Chromgehalt zu hoch ist, werden allerdings die Schweißbarkeit und Zähigkeit in der Wärmeeinflusszone negativ beeinflusst. Daher ist bei einem erfindungsgemäß verarbeiteten Stahl die obere Grenze für den Chromgehalt auf 0,40 Gew.-% festgesetzt.
• Nb: Niob ist in einem erfindungsgemäß verarbeiteten Stahl enthalten, um die Festigkeitseigenschaften durch Kornfeinung der Austenitstruktur beim temperaturgesteuerten Walzen bzw. durch Ausscheidungshärtung beim Haspeln zu unterstützen. Hierzu sind im erfindungsgemäß verarbeiteten Stahl 0,060 - 0,070 Gew.-% Nb vorhanden. Liegt der Niobgehalt unterhalb dieses Bereichs, werden die Festigkeitseigenschaften nicht erreicht. Liegt der Nb-Gehalt über der Obergrenze dieses Bereichs, verschlechtert sich die Schweißbarkeit und die Zähigkeit in der Wärmeeinflusszone einer Schweißung.
• B: Der Borgehalt eines erfindungsgemäß verarbeiteten Stahls beträgt 0,0005 - 0,0025 Gew.-%. B wird zur Unterstützung der Festigkeitseigenschaften und zur Verbesserung der Härtbarkeit verwendet. Zu hohe Borgehalte verschlechtern jedoch die Zähigkeitseigenschaften.
• Ti: Titan trägt ebenfalls zur Verbesserung der Festigkeitseigenschaften durch Verhinderung des Kornwachstums beim Austenitisieren bzw. durch Ausscheidungshärtung beim Haspeln bei. Um dies zu gewährleisten, betragen die Ti-Gehalte eines erfindungsgemäß verarbeiteten Stahls 0,09 - 0,13 Gew.-%. Liegt der Titangehalt unter 0,09 Gew.-%, werden die erfindungsgemäß angestrebten Festigkeitswerte nicht erreicht. Wird die Obergrenze des vorgegebenen Ti-Gehaltsbereichs überschritten, verschlechtern sich die Schweißbarkeit und die Zähigkeit in der Wärmeeinflusszone einer Schweißung.

As explained below, alloying constituents and alloy contents of the steel alloy melted in step a) are selected such that a hot rolled flat steel product having a combination of properties can be reliably produced while adhering to the invented steps, which makes it suitable for use in lightweight steel construction, in particular in commercial vehicle construction , especially suitable:

• C: The carbon content of the steel processed according to the invention is 0.05-0.08% by weight. To achieve the desired strength properties, a C content of at least 0.05% by weight is required. However, if the carbon content is too high, the toughness properties and weldability and formability of the steel processed according to the present invention are impaired. For this reason, the carbon content is limited to at most 0.08 wt .-%.
• Si: Silicon is used in the inventively processed steel as a deoxidizer and to improve the strength properties. However, if the silicon content is too high, the toughness properties, especially the toughness in the heat affected zone of welded joints, are greatly impaired. For this reason, the silicon content of the steel processed according to the invention should not exceed 0.50% by weight. To safely avoid surface quality problems, the silicon content can be reduced to max. 0.25 wt .-% be limited.
• Mn: Manganese is added to the desired strength properties with good toughness properties of the steel used in the invention in amounts of 1.6 to 2.0 wt .-%. If the manganese content is less than 1.60 wt%, the required strength properties are not achieved with sufficient certainty. By limiting the Mn content to max. 2.00 wt.%, Deterioration of weldability, toughness properties, formability and segregation behavior is avoided.
• P: The accompanying element phosphor deteriorates the impact work and the formability. The phosphorus content should therefore not exceed the upper limit of 0.025 wt .-%. Optimally, the P content is limited to less than 0.015 wt%.
• S: Sulfur deteriorates the impact work and formability of a steel processed according to the present invention due to MnS formation. For this reason, the S content of a steel processed according to the invention may not exceed 0.010% by weight. Such a low sulfur content can be in a conventional manner z. B. can be achieved by a CaSi treatment. In order to reliably exclude the negative effects of sulfur on the properties of the steel processed according to the invention, the S content can be reduced to max. Be limited to 0.003 wt .-%.
• Al: Aluminum is also used as a deoxidizer and hinders austenitizing coarsening of austenite due to AlN formation. If the aluminum content is below 0.020% by weight, the deoxidation processes do not proceed completely. However, if the aluminum content exceeds the upper limit of 0.050% by weight, Al ₂ O ₃ inclusions may form. These have a negative effect on the degree of purity and the toughness properties.
• N: The accompanying element nitrogen forms AlN with aluminum or TiN with titanium. However, if the nitrogen content is too high, the toughness properties are deteriorated. In order to prevent this, in a steel processed according to the invention, the upper limit for the nitrogen content is set at 0.006% by weight.
• Cr: Chromium may optionally be added to a steel processed according to the present invention to improve its strength properties. If the chromium content is too high, however, the weldability and toughness in the heat affected zone are adversely affected. Therefore, in an inventively processed steel, the upper limit of the chromium content is set to 0.40 wt%.
• Nb: Niobium is included in a steel processed in accordance with the present invention to promote the strength properties by grain refining of the austenite structure in temperature controlled rolling or by precipitation hardening during coiling. For this purpose, 0.060-0.070% by weight Nb are present in the steel processed according to the invention. If the niobium content is below this range, the strength properties are not achieved. If the Nb content exceeds the upper limit of this range, the weldability and toughness in the heat affected zone of a weld are deteriorated.
• B: The boron content of a steel processed according to the invention is 0.0005-0.0025% by weight. B is used in support of strength properties and Improvement of hardenability used. Excessive boron contents, however, degrade the toughness properties.
• Ti: Titanium also contributes to the improvement of the strength properties by preventing the grain growth during austenitization or by precipitation hardening during coiling. In order to ensure this, the Ti contents of a steel processed according to the invention are from 0.09 to 0.13% by weight. If the titanium content is below 0.09% by weight, the strength values desired according to the invention are not achieved. If the upper limit of the given Ti content range is exceeded, the weldability and toughness in the heat affected zone of a weld deteriorate.

Cu, Ni, V, Mo and Sb occur as accompanying elements, which enter the steel processed according to the invention as a technically unavoidable impurity in the steelmaking process. Their contents are limited to amounts which are ineffective in relation to the properties of the steel processed according to the invention. For this, the Cu content is limited to max. 0.12 wt .-%, the Ni content to less than 0.1 wt%, the V content to at most 0.01 wt .-%, the Mo content to less than 0.004 wt .-%, and the Sb content is also limited to less than 0.004 wt%.

• mit %C = jeweiliger C-Gehalt in Gew.-%,
• %Mn = jeweiliger Mn-Gehalt in Gew.-%,
• %Cr = jeweiliger Cr-Gehalt in Gew.-%,
• %Mo = jeweiliger Mo-Gehalt in Gew.-%,
• %V = jeweiliger V-Gehalt in Gew.-%,
• %Cu = jeweiliger Cu-Gehalt in Gew.-%,
• %Ni = jeweiliger Ni-Gehalt in Gew.-%,

berechnete Kohlenstoffäquivalent CE gilt: $$\mathrm{CE}\le 0,5\mathrm{Gew}\mathrm{.}-\%$$

In order to achieve a good weldability, the C, Mn, Cr, Mo, V, Cu and Ni content of the steel according to the invention can be determined within the According to the invention predetermined limits are set so that for the according to the formula $$\mathrm{CE}=\%\mathrm{C}+\%\mathrm{Mn}/\mathrm{6}+\left(\%\mathrm{Cr}+\%\mathrm{Not\; a\; word}+\%\mathrm{V}\right)/\mathrm{5}+\left(\%\mathrm{Cu}+\%\mathrm{Ni}\right)/\mathrm{15}$$

• with% C = respective C content in% by weight,
• % Mn = respective Mn content in% by weight,
• % Cr = respective Cr content in% by weight,
• % Mo = respective Mo content in wt.%,
• % V = respective V content in wt%,
• % Cu = respective Cu content in% by weight,
• % Ni = respective Ni content in wt.%,

Calculated carbon equivalent CE: $$\mathrm{CE}\le 0.5\mathrm{weight}\mathrm{,}-\%$$

After the slab has been cast, it is reheated to an austenitizing temperature which is 1200-1300 ° C. The upper limit of the temperature range to which the slab is heated to austenitise should not be exceeded in order to avoid coarsening of the austenite grain and increased scale formation. In the present invention, the rewarming temperature range of 1200 - 1300 ° C does not yet result in the increased formation of Rotzunder that would reduce the surface quality of the steel flat product produced according to the invention. Rotzunder forms in the processing according to the invention composite slabs exclusively during the hot rolling process (steps d), e) of the method according to the invention), if after Reheating too much primary scale is present on the slab surface.

The lower limit of the reheating temperature, however, is set so that the desired homogenization of the structure is ensured with a uniform temperature distribution. From this temperature, a largely complete dissolution of the coarse Ti and Nb carbonitride precipitates present in the respective slab begins in the austenite. During the final coiling of the finished hot-rolled flat steel product (step g) of the process according to the invention, fine Ti or Nb carbonitride precipitations can then be newly formed, which, as explained, make a significant contribution to increasing the strength properties. In this way, it is ensured that the flat steel products produced and assembled according to the invention regularly have a minimum yield strength of 700 MPa.

According to the invention, the reheating temperature during austenitisation of the respective slab is at least 1200 ° C., in order to achieve the desired effect of the most complete possible dissolution of the TiC and NbC precipitates. On the other hand, when the austenitizing temperature is below 1200 ° C., the amount of carbide precipitates of Ti and Nb dissolved in austenite is so small that the effects used according to the invention do not occur. A rewarming temperature below 1200 ° C. would therefore have in the processing of flat steel products, which corresponds to the optimized alloy selection according to the invention are composed, with the result that the required strength properties are not achieved. The most complete possible dissolution of the TiC and NbC precipitates can be ensured with particular certainty if the reheating temperature is at least 1250 ° C.

A flat steel product meeting the highest quality requirements for its surface finish can be produced by completely removing the scale present on the slab before rough rolling. This can be done by completely descaling the slab surface after the furnace discharge and, if possible, immediately before the rough rolling. For this purpose, the slab can go through a conventional scale scrubber.

In order to produce a flat steel product with optimized surface finish, the time t_1, the transfer of the slab from the workstation ("reheating (step c)") or the optional "post-reheating" removal of the primary scale (step c ') "to start of finish hot rolling (step e)) is required, limited to a maximum of 300 s. This optimally includes pre-rolling. In such a short transfer time, only such a small amount of primary scale is newly formed that the red scale forming therefrom during hot rolling is harmless to the quality of the surface of the flat steel product obtained after hot rolling. In the case that descaling is carried out before roughing, the transport time between the descaling unit and the roughing stand should not exceed 30 s. With a so short transport time can thus form no or at most a harmless thin oxide layer on the previously descaled slab.

In step d), the respectively processed slab is pre-rolled at a rough rolling temperature of 950-1250 ° C. The total reduction in pre-rolling amounts to at least 50%. The total loss Δhv is the ratio formed by the difference between the thicknesses of the slab before (thickness dVv) and after (thickness dNv) the pre-rolling and the thickness dVv of the slab before rough-rolling $$\left(\mathrm{\Delta hv}\left[\%\right]=\left(\mathrm{dVv}-\mathrm{DNV}\right)/\mathrm{dVv}\times 100\%\right)\mathrm{,}$$

The lower limit of the predetermined for the rough rolling temperature range and the minimum value of the total stitch decrease Δhv are set so that the recrystallization processes in the respective pre-rolled slab can run completely. In this way, the formation of a fine-grained austenitic structure is ensured before the finish rolling, whereby optimized toughness and elongation at break properties of the steel flat product produced according to the invention are achieved.

The dwell and pause time t_2 between rough rolling and finish rolling is limited to 50 seconds to avoid undesirable austenite grain growth.

Pre-rolling is followed, in step e), by hot rolling of the pre-rolled slab into a hot-rolled flat steel product having a hot strip thickness of typically 3-15 mm. Flat steel products with such thicknesses are referred to in the jargon as "heavy plate".

The final temperature of hot rolling is 800 - 880 ° C. By maintaining this hot rolling final temperature range, a highly stretched austenite grain is achieved in the microstructure of the obtained hot strip. The comparatively low hot rolling end temperature enhances the effect of hot rolling. In the structure of the hot strip obtained is dislocation rich austenite. After intensive cooling (step f)), this transforms to a dislocated, finely structured bainite, so that the yield strength is increased. The upper limit of the range of the hot rolling finish temperature is set so that no recrystallization of the austenite takes place during rolling in the hot rolling finishing line. This also contributes to the expression of a fine-grained structure. The lower limit temperature is at least 800 ° C, so that no ferrite forms during rolling.

The total reduction achieved during finish rolling .DELTA.hf is at least 70%, here the drop decrease .DELTA.hf according to the formula .DELTA.hf = (dVf-dNf) / dVf x 100% (with dVf = thickness of the rolling stock at the inlet to the final hot-rolling stand and dNf = thickness of the rolling stock at the outlet of the Fertigwarmwalzstaffel) is calculated. Due to the high reduction Δhf, the phase transformation takes place from strongly formed austenite. This has a positive effect on the fine grain, so that low in the structure of the steel flat product produced according to the invention Grain sizes are present.

After the finished hot-rolled flat steel product emerged from the last stand of the finished hot rolling mill, intensive cooling takes place within at most 10 seconds during which the hot-rolled flat steel product is cooled to a coiling temperature of 550-620 ° C. with a cooling rate dT of at least 40 K / s becomes.

The cooling break after hot rolling is at most 10 seconds to prevent undesirable microstructural changes between hot rolling and controlled accelerated cooling.

By observing the predetermined range of reel temperature according to the invention, the conditions for the formation of a bainitic structure of the steel flat product produced according to the invention are created.

At the same time, the choice of reel temperature has a decisive influence on precipitation hardening. For this purpose, the reel temperature range according to the invention is chosen so that it is on the one hand below the Bainitstarttemperatur, on the other hand in the excretion maximum for the formation of Karbonitridausscheidungen. However, a reel temperature which is too low would mean that the precipitation potential would no longer be usable and thus the required minimum yield strength would no longer be reached. The cooling conditions are inventively chosen so that the hot rolled flat steel product immediately before reeling a bainitic structure having a phase content of at least 70 vol .-%. A further bainite formation then takes place in the reel. With regard to the required combination of properties, it proves to be optimal if the microstructure of the hot-rolled flat steel product produced according to the invention, after coiling, consists entirely of bainite in the technical sense. This is achieved by observing the inventively predetermined range of reel temperature.

The high cooling rate avoids the formation of unwanted phase components. In order to obtain an optimally flat flat steel product, the cooling rate of the cooling after hot rolling can be limited to 150 K / s.

The yield strength of the hot-rolled flat steel products produced according to the invention in the manner explained above is reliably 700-850 MPa. Their elongation at break is in each case at least 12%. Equally regularly, steel flat products according to the invention achieve tensile strengths of 750-950 MPa. The notched impact work determined for products according to the invention is in the range from 50 to 110 J at -20 ° C. and in the range from 30 to 110 J at -40 ° C.

Steel flat products produced according to the invention have a fine-grained structure with a mean grain size of at most 20 μm, in order to achieve good elongation at break and toughness.

In this case, in the procedure according to the invention, the abovementioned properties lie with a hot-rolled flat steel product in the rolling state after Reel in front. A further heat treatment for the adjustment or expression of certain important properties for the intended use as a high-strength sheet in commercial vehicle construction is not necessary.

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

Steel melts A - E with the composition given in Table 1 have been melted and cast in a known manner into slabs 1 - 26.

Subsequently, the slabs made of the steels A - E have been heated to a reheating temperature TW.

From the reheating furnace, the reheated slabs have been transported in less than 30 s to a scale washer in which the primary scale adhering to them has been removed from the slabs.

The slabs emerging from the scale scrubber have then been transported to a roughing stand, where they have been pre-rolled with a rough-rolling temperature TVW and a total reduction in stitches Δhv over the rough-rolling.

Subsequently, the pre-rolled slabs were finished hot rolled in a finished hot rolling mill to hot strip with a thickness of BD and a width BB. The hot rolling has been completed with a total decrease in the finished heat transfer scale Δhf at a hot rolling end temperature TEW. The time between the exit from the Tinder scrubber and the beginning of the Fertigwarmwalzens has passed, each was less than 300 s.

The finished hot-rolled flat steel product exiting from the last stand has been cooled to a coiling temperature HT by intensive cooling with water at a cooling rate dT of 50-120 K / s after a pause t_p of 1-7 seconds in which it has cooled slowly in air , After cooling, the flat steel products already had at least 70% by volume bainitic structure.

At this reel temperature HT, the hot strips obtained have each been coiled into a coil. In the course of the cooling of the flat steel products in the coil, the complete transformation of the microstructure into bainite occurred, so that the resulting flat steel products had, in the technical sense, 100% by volume of a banitic microstructure.

In Tables 2a, 2b, the process parameters reheating temperature TW, rough-rolling temperature TVW set in the processing of the slabs 1-26, total decay Δhv over the roughing, time t_1 between descaling after reheating and pre-rolling, and start of final hot rolling are shown. Time t_2 Time between rough rolling and hot rolling, total reduction achieved via finish rolling Δhf, final rolling temperature TEW, cooling pause t_p between the end of hot rolling and the beginning of forced cooling, cooling rate dT, reel temperature HT, belt thickness BD and belt width BB.

The mechanical properties as well as the microstructure of the obtained hot strips have been investigated.

The tensile tests for determining the yield strength ReH, the tensile strength Rm and the elongation at break A were carried out according to DIN EN ISO 6892-1 on longitudinal samples of the hot strips.

The notched bar impact tests to determine the impact energy Av at -20 ° C and -40 ° C and -60 ° C were carried out on longitudinal samples according to DIN EN ISO 148-1.

The structural investigations were carried out by light microscope and scanning electron microscope. For this purpose, the samples were taken from a quarter of the bandwidth, prepared as a longitudinal section and etched with Nital (ie alcoholic nitric acid containing a proportion of 3% by volume of nitric acid) or sodium disulfite. The determination of the structural components was carried out by means of area analysis in sample position 1/3 sheet thickness, as in H. Schumann and H. Oettel "Metallography" 14th edition, 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim described.

The mechanical properties and the structural constituents of the hot strips produced according to the invention are given in Table 3. The band sheets produced according to the method of the present invention have high strength properties with good toughness properties and good elongation at break.

The yield strengths of the hot strips produced in the above manner are between 700 MPa and 790 MPa. The elongation at break is at least 12% and the Tensile strength 750 - 880 MPa. The notch impact work at -20 ° C is in the range 60 to 100 J. At -40 ° C, the notch impact work is 40 to 75 J and at -60 ° C, the impact energy is at 30 - 70 J. Table 1 stole C Si Mn P S al N Cr Nb B Ti Cu Ni V Not a word sb A 0,060 0.42 1.77 0,012 0.0010 0.034 0.0046 0.04 0.062 0.0020 0,110 0.02 0.03 0,010 0,004 0,004 B 0.053 0.49 1.75 0,015 0.0014 0.034 0.0049 0.06 0.066 0.0020 0.091 0.02 0.03 0.005 0,004 0,004 C 0,061 0.22 1.79 0,014 0.0021 0,050 0.0047 0.04 0.063 0.0019 0.097 0.02 0.02 0,003 0,004 0,004 D 0,065 0.20 1.8 0,014 0.0021 0,040 0.0047 0.04 0,065 0.0005 0,110 0.02 0.02 0,003 0,004 0,004 e 0,070 0.03 1.89 0.011 0.0014 0,042 0.0051 0.04 0,060 0.0005 0.130 0.02 0.03 0,008 0,004 0,004 Data in wt .-%, balance iron and unavoidable impurities No. stole TW Δhv TVW t_1 t_2 △ Hf TEW t_p dT HT BD BB [° C] [%] [° C] [s] [s] [%] [° C] [s] [K / s] [° C] [mm] [mm] 1 A 1293 85 1070 220 40 90 905 1 100 600 4 1525 2 A 1296 80 1065 220 40 92 915 1 100 600 4 1525 3 A 1288 80 1045 225 40 92 895 2 100 605 4 1525 4 A 1287 85 1045 230 42 90 880 2 100 605 4 1530 5 A 1269 82 1055 230 40 91 890 2 100 600 4 1525 6 A 1300 82 1050 240 45 82 835 3 70 600 8th 1545 7 A 1296 82 1050 245 41 82 810 4 70 600 8th 1545 8th A 1305 76 1060 240 42 86 825 4 70 600 8th 1755 9 A 1247 76 1040 260 44 83 800 6 50 580 10 1530 10 B 1291 80 1060 230 40 90 910 2 100 600 5 1630 11 B 1309 80 1110 240 44 90 870 2 100 610 5 1630 12 B 1288 85 1070 230 40 88 890 2 100 600 5 1540 13 B 1304 76 1055 240 40 90 860 2 90 600 6 1540 14 B 1285 85 1030 255 42 75 800 5 50 590 10 1550 15 B 1296 85 1100 210 40 93 850 2 120 600 3 1280 16 B 1298 82 1090 200 40 93 900 1 120 580 3 1275 No. stole TW Δhv TVW t_1 t_2 △ Hf TEW t_p dT HT BD BB [° C] [%] [° C] [s] [s] [%] [° C] [s] [K / s] [° C] [mm] [mm] 17 B 1206 82 1067 205 40 93 870 1 120 610 3 1275 18 C 1289 85 1040 260 45 75 800 6 50 550 10 1550 19 C 1291 85 1090 235 42 85 880 2 90 605 6 1535 20 C 1214 82 1070 230 40 91 865 2 100 600 4 925 21 D 1290 85 1090 205 40 93 890 1 120 620 3 1280 22 D 1285 82 1080 200 40 93 900 1 120 575 3 1275 23 e 1290 76 1060 260 43 83 800 6 50 598 10 1550 24 e 1290 78 1090 235 40 89 860 3 90 615 6 1535 25 e 1290 80 1040 260 45 76 800 7 50 590 12 1530 26 e 1285 78 1045 260 45 73 822 7 50 570 15 1530 No. stole Location on the coil Tensile test, longitudinal Charpy impact test, longitudinal Structural constituents Deer rm A Av-20 ° C Av-40 ° C Av-60 ° C [MPa] [MPa] [%] [J] [J] [J] Vol% 1 A Beginning 770 852 19.0 nb nb nb 100 bainite 2 A Beginning 762 837 17.0 nb nb nb 100 bainite 3 A Beginning 749 819 18.0 nb nb nb 100 bainite 4 A Beginning 754 818 21.0 nb nb nb 100 bainite 5 A Beginning 737 809 24.0 nb nb nb 100 bainite 6 A Beginning 736 834 20.3 70 44 31 100 bainite 7 A Beginning 739 842 15.7 81 62 31 100 bainite 8th A Beginning 716 817 17.2 62 40 31 100 bainite 9 A Beginning 733 832 23.5 79 68 65 100 bainite 10 B Beginning 750 852 16.0 nb nb nb 100 bainite 11 B Beginning 752 841 22.0 nb nb nb 100 bainite 12 B Beginning 736 829 20.0 nb nb nb 100 bainite 13 B Beginning 734 860 17.0 99 48 33 100 bainite 14 B Beginning 717 846 18.0 84 58 30 100 bainite 15 B Beginning 782 864 23.0 nb nb nb 100 bainite 16 B Beginning 779 857 24.0 nb nb nb 100 bainite 17 B Beginning 720 819 23.0 nb nb nb 100 bainite 18 C Beginning 705 813 19.1 97 73 30 100 bainite 19 C Beginning 718 783 24.0 80 60 31 100 bainite 20 C Beginning 710 790 24.0 nb nb nb 100 bainite 21 D Beginning 720 850 22.0 nb nb nb 100 bainite 22 D Beginning 760 823 22.0 nb nb nb 100 bainite 23 e Beginning 712 820 20.0 97 73 30 100 bainite 24 e Beginning 713 825 23.0 80 60 31 100 bainite 25 e Beginning 733 809 21.0 72 53 42 100 bainite 26 e Beginning 727 821 19.2 83 76 67 100 bainite "nb" = "not determined"

Claims (13)

1. Method for producing a flat steel product having a yield strength of at least 700 MPa and having a structure which is up to at least 70 vol.% bainitic, comprising the work steps of:

   a) melting a steel melt which consists (in wt.%) of

   C: 0.05-0.08 %,

   Si: 0.015-0.500 %,

   Mn: 1.60-2.00 %,

   P: up to 0.025 %,

   S: up to 0.010 %,

   Al: 0.020-0.050 %,

   N: up to 0.006 %,

   Cr: up to 0.40 %,

   Nb: 0.060-0.070 %,

   B: 0.0005-0.0025 %,

   Ti: 0.090-0.130 %,

   and of technically unavoidable impurities, including up to 0.12 % Cu, up to 0.100 % Ni, up to 0.010 % V, up to 0.004 % Mo and up to 0.004 % Sb, and the remainder consisting of iron;

   b) casting the melt into a slab;

   c) reheating the slab to a reheating temperature of from 1200-1300 °C;

   d) pre-rolling the slab at a pre-rolling temperature of from 950-1250 °C and with a total pass reduction achieved by means of the pre-rolling of at least 50 %;

   e) finish-hot-rolling the pre-rolled slab, the finish-hot-rolling being terminated at a hot-rolling end temperature of from 800-880 °C;

   f) within at most 10 s after the finish-hot-rolling, starting the intensive cooling of the finish-hot-rolled flat steel product at a cooling speed of at least 40 K/s to a coiling temperature of from 550-620 °C;

   g) coiling the finish-hot-rolled flat steel product,

   wherein the elongation at break of the hot-rolled flat steel product obtained after the coiling is at least 12 %.
2. Method according to Claim 1, characterised in that, for the carbon equivalent CE of the steel melt melted in the work step a), which is calculated according to the formula $$\mathrm{CE}=\%\mathrm{C}+\%\mathrm{Mn}/\mathrm{6}+\left(\%\mathrm{Cr}+\%\mathrm{Mo}+\%\mathrm{V}\right)/\mathrm{5}+\left(\%\mathrm{Cu}+\%\mathrm{Ni}\right)/\mathrm{15},$$

   

   where %C = respective C content in wt.-%,

   %Mn = respective Mn content in wt.%,

   %Cr = respective Cr content in wt.%,

   %Mo = respective Mo content in wt.%,

   %V = respective V content in wt.%,

   %Cu = respective Cu content in wt.%,

   %Ni = respective Ni content in wt.%,

   the following applies: $$\mathrm{CE}\le 0.5\mathrm{wt}\mathrm{.}\%$$

   
3. Method according to any of the preceding claims, characterised in that the reheating temperature is from 1250-1300 °C.
4. Method according to any of the preceding claims, characterised in that, in a work step c', which is passed through between the reheating (work step c)) and the pre-rolling (work step d)), furnace scales adhering to the respectively processed slab are removed.
5. Method according to any of the preceding claims, characterised in that the transport time, which elapses for the transportation of the slab from the work station previously passed through in each case (work step c) or optionally work step c')) to the finish-hot-rolling (work step e)), is restricted to a maximum of 300 s.
6. Method according to any of the preceding claims, characterised in that the holding time which elapses between the pre-rolling (work step d)) and the finish-hot-rolling (work step e)) is at most 50 s.
7. Method according to any of the preceding claims, characterised in that the cooling speed in the case of the cooling in work step f) is at most 150 K/s.
8. Method according to any of the preceding claims, characterised in that the thickness of the hot-rolled flat steel product obtained after the hot-rolling is 3-15 mm.
9. Method according to any of the preceding claims, characterised in that the yield strength of the hot-rolled flat steel products obtained after the coiling is from 700-850 MPa.
10. Method according to any of the preceding claims, characterised in that the tensile strength of the hot-rolled flat steel products obtained after the coiling is from 750-950 MPa.
11. Method according to any of the preceding claims, characterised in that the notch impact energy of the hot-rolled flat steel products obtained after the coiling, at - 20 °C, is in the range of from 50-110 J.
12. Method according to any of the preceding claims, characterised in that the hot-rolled flat steel products obtained after the coiling have an exclusively bainitic structure except for miscellaneous structural constituents which are technically unavoidable.
13. Method according to any of the preceding claims, characterised in that the average grain diameter of the structure of the hot-rolled flat steel products obtained after the coiling is at most 20 µm.