DE102022124366A1 - Process for producing a hot-rolled flat steel product for use in pipe production - Google Patents

Abstract

The present invention relates to a method for producing a hot-rolled flat steel product for use in pipe production.

Description

The invention relates to a method for producing a hot-rolled flat steel product for use in pipe production.

Hot-rolled flat steel products (hot strip) for spiral-seam or longitudinal-seam welded pipes with sheet thicknesses of up to 25.4 mm are usually produced on hot-strip mills or finishing rolling mills using thermomechanical rolling with subsequent accelerated cooling. Thermomechanical rolling (TM rolling) requires temperatures of 780 - 900 °C in the finishing rolling mill of a hot rolling mill, which results in an extremely fine-grained transformation structure and thus the desired combination of high strength and high resistance to brittle fracture. A very fine ferritic microstructure is important for achieving the required mechanical properties for high-strength pipe steels. To achieve this, TM rolling, with its low temperature in the finishing rolling mill below the recrystallization temperature, has been the most common process to date. This process requires significant pendulum times to cool down a pre-strip in front of the finishing rolling mill and precise adherence to the specified rolling temperatures. Depending on the local conditions (e.g. temperature inhomogeneities and belt speed), differences in the local degree of recrystallization are also to be expected. However, TM rolling leads to high rolling stand loads and, compared to normal production, noticeably lower rolling performance in the hot strip mill (up to 30-40%) as a result of pronounced oscillation of the pre-strip in front of the finishing rolling mill. Furthermore, TM rolling carries the risk of high anisotropy of the mechanical properties and strong scattering of the brittle fracture properties due to partial rolling in the two-phase region.

A generic method for producing a hot-rolled flat steel product for pipe production is exemplified in DE 10 2013 107 010 A1 disclosed. Also stated therein, steels for the production of thick-walled pipes typically consist of an alloy in which, in addition to iron and unavoidable impurities, in % by weight C: <0.18%, Si: <1.5%, Mn: <2.5 %, P: 0.005-0.1%, S: < 0.03%, N: < 0.02%, Cr: < 0.5%, Cu: < 0.5%, Ni: < 0.5% , Mo: < 0.5%, Al < 2%, up to a total of 0.3% of one or more of the elements B, Nb, Ti, V, Sn and Ca. These steels also include those under the designation “X70” and “X80” known steel grades.

Furthermore, from the US 2011/253267 A1 discloses a process for producing a steel sheet of pipe band grade “X65”.

The object of the invention is to provide a method for producing hot-rolled flat steel products for use in pipe production on a hot strip mill, with which the same or improved mechanical properties with high product homogeneity and unchanged chemical composition can be set on the hot-rolled flat steel product with lower roll stand load and improved rolling performance.

1. a) Erschmelzen eines Stahls enthaltend in Gew.-%:
   • C: < 0,18%, insbesondere 0,0010 bis 0,16%, vorzugsweise 0,010 bis 0,15%,
   • Si: < 1,50%, insbesondere 0,010 bis 1,0%, vorzugsweise 0,10 bis 0,85%,
   • Mn: < 2,50%, insbesondere 0,10 bis 2,20%, vorzugsweise 0,50 bis 2,10%,
   • P: < 0,10%, insbesondere 0,0001 bis 0,070%, vorzugsweise 0,0005 bis 0,030%,
   • S: < 0,030%, insbesondere bis 0,010%, vorzugsweise bis 0,0050%,
   • N: < 0,020%, insbesondere 0,0001 bis 0,0150%, vorzugsweise 0,0005 bis 0,010%,
   • Cr: < 0,50%, insbesondere 0,0001 bis 0,45%, vorzugsweise 0,0002 bis 0,35%,
   • Cu: < 0,50%, insbesondere 0,0010 bis 0,20%, vorzugsweise 0,0020 bis 0,20%,
   • Ni: < 0,50%, insbesondere 0,0010 bis 0,40%, vorzugsweise 0,0020 bis 0,30%,
   • Mo: < 0,50%, insbesondere 0,0010 bis 0,30%, vorzugsweise 0,0020 bis 0,20%,
   • Al: < 2,0%, insbesondere 0,0010 bis 1,0%, vorzugsweise 0,0020 bis 0,50%,
   • Nb: < 0,15%, insbesondere 0,0005 bis 0,10%, vorzugsweise 0,0010 bis 0,080%,
   • Ti: < 0,10%, insbesondere 0,0005 bis 0,090%, vorzugsweise 0,0010 bis 0,080%,
   • Ca: < 0,010%, insbesondere bis 0,0050%, vorzugsweise 0,0001 bis 0,0040%,
   bis insgesamt 0,20%, insbesondere bis insgesamt 0,10% in Summe von einem oder mehreren der Elemente B, V, Sn, Rest Fe und unvermeidbare Verunreinigungen;
2. b) Vergießen der Schmelze zu einem Vorprodukt;
3. c) Vorwärmen des Vorprodukts auf eine Temperatur und/oder Halten des Vorprodukts bei einer Temperatur zwischen 1100 und 1350°C;
4. d) Warmwalzen des Vorprodukts zu einem warmgewalzten Stahlflachprodukt in einer Fertigwalzstaffel umfassend mindestens vier und höchstens neun Walzgerüsten mit einer in die Fertigwalzstaffel eintretenden Vorprodukttemperatur zwischen 1000 und 1100°C und einer aus der Fertigwalzstaffel austretenden warmgewalzten Stahlflachtemperatur zwischen 750 und 950°C;
5. e) Abkühlen des erhaltenen warmgewalzten Stahlflachprodukts auf eine zwischen 360 und 600°C betragende Haspeltemperatur.

The teaching of the invention relates to a method for producing a hot-rolled flat steel product comprising the steps:

1. a) Melting a steel containing in% by weight:
   • C: <0.18%, in particular 0.0010 to 0.16%, preferably 0.010 to 0.15%,
   • Si: <1.50%, in particular 0.010 to 1.0%, preferably 0.10 to 0.85%,
   • Mn: <2.50%, in particular 0.10 to 2.20%, preferably 0.50 to 2.10%,
   • P: <0.10%, in particular 0.0001 to 0.070%, preferably 0.0005 to 0.030%,
   • S: <0.030%, in particular up to 0.010%, preferably up to 0.0050%,
   • N: <0.020%, in particular 0.0001 to 0.0150%, preferably 0.0005 to 0.010%,
   • Cr: <0.50%, in particular 0.0001 to 0.45%, preferably 0.0002 to 0.35%,
   • Cu: <0.50%, in particular 0.0010 to 0.20%, preferably 0.0020 to 0.20%,
   • Ni: <0.50%, in particular 0.0010 to 0.40%, preferably 0.0020 to 0.30%,
   • Mo: <0.50%, in particular 0.0010 to 0.30%, preferably 0.0020 to 0.20%,
   • Al: <2.0%, in particular 0.0010 to 1.0%, preferably 0.0020 to 0.50%,
   • Nb: <0.15%, in particular 0.0005 to 0.10%, preferably 0.0010 to 0.080%,
   • Ti: <0.10%, in particular 0.0005 to 0.090%, preferably 0.0010 to 0.080%,
   • Ca: <0.010%, in particular up to 0.0050%, preferably 0.0001 to 0.0040%,
   up to a total of 0.20%, in particular up to a total of 0.10% in total of one or more of the elements B, V, Sn, the balance Fe and unavoidable impurities;
2. b) casting the melt into a preliminary product;
3. c) preheating the precursor to a temperature and/or maintaining the precursor at a temperature between 1100 and 1350°C;
4. d) hot rolling the preliminary product into a hot-rolled flat steel product in a finishing rolling mill comprising at least four and a maximum of nine rolling stands with a preliminary product temperature entering the finishing rolling mill between 1000 and 1100 ° C and a hot-rolled steel flat temperature emerging from the finishing rolling mill between 750 and 950 ° C;
5. e) cooling the hot-rolled flat steel product obtained to a coiling temperature of between 360 and 600 ° C.

The information in % in connection with the aforementioned alloying elements refers to % by weight.

The elements (B, V, Sn) can be approved individually or in combination, whereby in particular these are not added to alloys but, if present and/or measurable, are present as accompanying elements: B: <0.0050%, in particular up to 0.0015%, preferably up to 0.0010%, V: <0.150%, in particular up to 0.10%, preferably up to 0.080%, Sn: <0.020%, in particular up to 0.015%, preferably up to 0.012%.

The molten steel with an alloy composition within the ranges specified above is cast into a preliminary product, which in the classic production route can be a slab of standard dimensions. However, the steel can also be produced by direct hot rolling of a continuous casting in a casting-rolling plant as a preliminary product of a thin slab or in a strip casting plant as a preliminary product of a cast preliminary strip. For example, in a casting-rolling plant or strip casting plant, the preliminary product can be further processed directly, i.e. coming directly from the casting heat, so that the preliminary product is kept at a temperature or, if necessary, preheated to a temperature, for example in an equalization or preheating furnace, at which one is possible Complete homogenization is guaranteed and in which any precipitates that may have formed during casting dissolve (again) as completely as possible. If, for example, the melt is cast into a preliminary product in a continuous casting plant, the cast and completely solidified strand is separated into several slabs of finite dimensions and finally the slabs are allowed to cool down to ambient temperature, in particular through natural cooling. The preliminary product or the slab is reheated to a temperature for further processing, for example in a walking beam furnace or using other suitable means. Otherwise, the slab will be reheated directly after casting without completely cooling to ambient temperature.

The temperature when preheating and/or holding the preliminary product is at least 1100 ° C, in particular at least 1150 ° C, preferably at least 1200 ° C, in order to ensure the most complete possible dissolution of any undesirable precipitates in the form of carbides/carbonitrides and/or nitrides in the preliminary product . The temperature for preheating and/or holding should not exceed 1350 °C in order to avoid partial melting and/or excessive scaling of the preliminary product. For ecological and economic reasons, the temperature for preheating and/or holding is limited to a maximum of 1290 °C.

The preliminary product is hot-rolled into a hot-rolled flat steel product in a finishing rolling mill comprising at least four and a maximum of nine rolling stands with a preliminary product temperature entering the finishing rolling mill between 1000 and 1100 ° C and a hot-rolled flat steel temperature emerging from the finishing rolling mill between 750 and 950 ° C.

The temperature of the preliminary product is recorded, for example, at the entrance to the finishing rolling mill in front of the first rolling stand using suitable means. The preliminary product temperature can in particular be at least 1010 ° C, preferably at least 1020 ° C and in particular at most 1080 ° C, preferably at most 1060 ° C. The selected preliminary product temperatures can reduce the rolling forces by at least 10%; the moments are reduced by at least 10% and the power consumption by at least 10% in the finishing rolling train compared to the generic reference method.

The temperature of the hot-rolled flat steel product emerging from the finishing rolling mill can, for example, be equated with the final hot rolling temperature, the temperature being in particular at least 820 ° C, preferably at least 840 ° C and in particular at most 900 ° C, preferably at most 890 ° C.

The hot-rolled flat steel product obtained is cooled to a coiling temperature of between 360 and 600 ° C. The coiling temperature can in particular be at least 400 ° C, preferably at least 430 ° C, in order to prevent martensite formation and to promote the formation of a structure of bainite, bainitic ferrite and / or ferrite in the hot-rolled flat steel product. In order to reduce the diffusion of oxygen-affinous alloy elements to the surface during the coiling process, the coiling temperature is limited to a maximum of 600 ° C, in particular a maximum of 580 ° C, preferably a maximum of 540 ° C, preferably a maximum of 520 ° C.

The hot-rolled flat steel product cooled to the coiling temperature is preferably coiled into a coil.

The hot-rolled flat steel product has a thickness of at least 3.0 mm, in particular at least 7.0 mm, preferably at least 10.0 mm and at most 25.4 mm.

The hot-rolled flat steel product (hot strip) wound onto a coil is further processed in such a way that it is used for pipe production, especially large pipes. Pipe production is preferably carried out from the coil by uncoiling the hot-rolled flat steel product and feeding it to a production unit in which spiral-seam or longitudinal-seam welded pipes are produced.

The pipes produced from the hot-rolled flat steel product are used in particular for durable and robust pipelines for the transport of water, oil, gas and hydrogen.

According to one embodiment, at least in the first three roll stands of a finishing rolling mill, hot rolling is carried out in the recrystallizing temperature range with a degree of recrystallization of the austenitic microstructure of at least 40%.

According to a further embodiment, the hot rolling is carried out in the recrystallizing temperature range in the first roll stand of a finishing rolling mill at a degree of recrystallization of the austenitic microstructure of at least 50%, in particular at least 60%, preferably at least 70%, preferably at least 80%.

The degree of recrystallization can be calculated using semi-empirical regression equations, which can be validated on the basis of double impact tests, see for example “Control of precipitation sequences during hot rolling to improve product uniformity of titanium containing high strength steels (PRETICONTROL)”, European Commission, EUR 30529 EN, including on pages 88 to 96 in Chapter 6.1.

Rolling in the recrystallizing temperature range leads to grain refinement in the austenite structure.

In particular, rolling takes place in the subsequent rolling stands of a finishing rolling mill in the non-recrystallizing temperature range.

Preferably, the decrease in thickness in the first three roll stands is at least 65%, in particular at least 70%, preferably at least 75%, preferably at least 80, in relation to the existing number of active roll stands, which means that a further decrease in thickness is provided in the further roll stands % and at most 98%, in particular at most 95%.

According to one embodiment, the hot-rolled flat steel product is cooled after exiting the finishing rolling mill with a first average cooling rate to a temperature below A _r3 and optionally then to the coiling temperature with a second average cooling rate, which is lower than the first average cooling rate. The determination of the A _r3 temperature depends on the alloy and can be approximated by the formula A _r3 = 868 - 396 C - 68.1 Mn + 24.6 Si - 36.1 Ni - 24.8 Cr - 20.7 Cu determine, whereby the specified alloying elements in wt.% are taken into account in the formula, see for example “STEEL FORMING AND HEAT TREATING HANDBOOK”, Gorni, Edition: December 13, 2019, page 42. The temperature to be achieved using the first average cooling rate is below A _r3 is a maximum of the reel temperature + 80 K, in particular a maximum of the reel temperature + 60 K, preferably a maximum of the reel temperature + 40 K. The temperature below A _r3 can particularly preferably be a maximum of 610 ° C.

For example, the temperature is measured without contact, usually using a pyrometer; this corresponds at least to the temperature on the surface of the hot-rolled flat steel product, although the temperature inside or in the core can be higher than the measured temperature on the surface. By means of heat conduction within the flat steel product, reheating and thus an increase in the temperature on the surface could take place.

The first average cooling rate is at least 10 K/s and is necessary to largely avoid the formation of cementite and the formation of coarse precipitates.

The first average cooling rate can in particular be at least 15 K/s, 20 K/s, 30 K/s, preferably at least 40 K/s, 45 K/s, 50 K/s, preferably at least 55 K/s, 60 K/s, 65 K/s, 70 K/s. This can be limited to a maximum of 200 K/s, in particular to a maximum of 180 K/s.

If the cooling rate is chosen to be sufficiently high, a bainitic microstructure can be achieved. Alternatively, a ferritic-perlitic microstructure is preferably used.

The second average cooling rate can be optional and be at most 10 K/s in order to be able to set, for example, a fine bainitic or alternatively a fine ferritic-pearlitic microstructure. The second average cooling rate can in particular be at most 8 K/s, 6 K/s, 5 K/s, preferably at most 4 K/s, 3 K/s. If a second average cooling rate is required, this can be at least 0.1 K/s, in particular at least 0.2 K/s, preferably at least 0.3 K/s. If necessary, optional cooling with a second average cooling rate can also take place passively, for example by disposition in air (environment).

The “average” cooling rate is defined as the quotient of the difference between an initial temperature (actual temperature) and a target temperature (target temperature) and the time required between the initial temperature and reaching the target temperature. As a rule, the cooling rate is not a constant value.

On the one hand, the hot-rolled flat steel product can have a ferritic-pearlitic microstructure with a proportion of at least 90% ferrite and pearlite. The proportion of ferrite and pearlite can in particular be at least 92%, preferably at least 94%, preferably at least 96%, more preferably at least 98%. Martensite, bainite, retained austenite and/or cementite are not desirable for the desired mechanical-technological properties and should be largely excluded using the aforementioned conditions, but can be individually or in total at a maximum of 10%, in particular a maximum of 8%, preferably a maximum of 6%, preferably at most 4%, particularly preferably at most 2%.

According to a further teaching, the invention proposes a hot-rolled flat steel product for use in pipe production with a ferritic-pearlitic microstructure having a proportion of at least 90% ferrite and pearlite, which has been produced in particular by the method according to the invention, containing in % by weight or consisting of in% by weight: C: 0.040 to 0.090%, in particular 0.045 to 0.085%, preferably 0.050 to 0.080%, Si: 0.25 to 0.50%, in particular 0.26 to 0.47%, preferably 0.27 to 0.45%, Mn: 1.40 to 2.10%, in particular 1.50 to 2.0%, preferably 1.60 to 1.90%, P: <0.050%, in particular 0.0001 to 0.020%, preferably 0.0010 to 0.015%, S: <0.010%, in particular 0.0001 to 0.0080%, preferably 0.0002 to 0.0050%, N: <0.010%, in particular 0.0001 to 0.0090%, preferably 0.0005 to 0.0075%, CR: <0.20%, in particular 0.0001 to 0.15%, preferably 0.0002 to 0.10%, Cu: <0.20%, in particular 0.0010 to 0.15%, preferably 0.0020 to 0.12%, Ni: <0.20%, in particular 0.0010 to 0.10%, preferably 0.0020 to 0.080%, Mon: <0.20%, in particular 0.0010 to 0.10%, preferably 0.0020 to 0.080%, Al: <0.10%, in particular 0.0010 to 0.090%, preferably 0.0050 to 0.080%, Nb: <0.15%, in particular 0.0001 to 0.10%, preferably 0.0010 to 0.090%, Ti: <0.10%, in particular 0.0001 to 0.090%, preferably 0.0002 to 0.080% Approx.: <0.010%, in particular 0.0002 to 0.0070%, preferably 0.0003 to 0.0050%, optionally one or more of the elements (B, V, Sn): B: <0.0050%, in particular up to 0.0010%, preferably up to 0.0009%, V: <0.010%, in particular up to 0.0080%, preferably up to 0.0060%, Sn: <0.020%, in particular up to 0.010%, preferably up to 0.0080%, Rest Fe and unavoidable impurities.

On the other hand, the hot-rolled flat steel product can have a bainitic microstructure with a proportion of at least 90% bainite. The proportion of bainite can in particular be at least 92%, preferably at least 94%, preferably at least 96%, more preferably at least 98%. Ferrite, pearlite, martensite, retained austenite and/or cementite are not desirable for the desired mechanical-technological properties and should be largely excluded using the aforementioned conditions, but can be individually or in total at a maximum of 10%, in particular a maximum of 8%, preferably a maximum of 6 %, preferably at most 4%, particularly preferably at most 2%.

According to a further alternative teaching, the invention proposes a hot-rolled flat steel product for use in pipe production with a bainitic microstructure having a proportion of at least 90% bainite, which has been produced in particular by the method according to the invention, containing in% by weight or consisting of in wt.%: C: 0.030 to 0.072%, in particular 0.035 to 0.071%, preferably 0.040 to 0.070%, Si: 0.31 to 0.60%, in particular 0.32 to 0.57%, preferably 0.34 to 0.55%, Mn: 1.40 to 1.90%, in particular 1.42 to 1.85%, preferably 1.45 to 1.80%, P: <0.050%, in particular 0.0001 to 0.020%, preferably 0.0010 to 0.015%, S: <0.010%, in particular 0.0001 to 0.0080%, preferably 0.0002 to 0.0050%, N: <0.010%, in particular 0.0001 to 0.0090%, preferably 0.0005 to 0.0075%, CR: <0.50%, in particular 0.010 to 0.45%, preferably 0.050 to 0.40%, Cu: <0.30%, in particular 0.0010 to 0.20%, preferably 0.0020 to 0.18%, Ni: <0.20%, in particular 0.0010 to 0.15%, preferably 0.0020 to 0.12%, Mon: <0.30%, in particular 0.0010 to 0.25%, preferably 0.0020 to 0.22%, AI: <0.10%, in particular 0.0010 to 0.090%, preferably 0.0050 to 0.080%, Nb: <0.15%, in particular 0.0001 to 0.10%, preferably 0.0010 to 0.090%, Ti: <0.10%, in particular 0.0001 to 0.090%, preferably 0.0002 to 0.080%, Approx.: <0.010%, in particular 0.0004 to 0.0070%, preferably 0.0007 to 0.0050%, optionally one or more of the elements (B, V, Sn): B: <0.0050%, in particular up to 0.0010%, preferably up to 0.0009%, V: <0.010%, in particular up to 0.0080%, preferably up to 0.0060%, Sn: <0.020%, in particular up to 0.015%, preferably up to 0.012%, Rest Fe and unavoidable impurities.

Furthermore, the invention also relates to a use of a hot-rolled flat steel product with one of the aforementioned composition and microstructure for the production of pipes as well as a spiral seam or longitudinal seam welded pipe made from a hot-rolled flat steel product with one of the aforementioned composition and microstructure.

The components of the microstructure can be determined using light optical microscopy (LOM) at a magnification of 200 to 2000 times.

The hot-rolled flat steel product has a tensile strength R _m of at least 570 MPa, in particular at least 600 MPa, preferably at least 620 MPa. The maximum tensile strength R _m can, for example, be at most 800 MPa, in particular at most 780 MPa, preferably at most 760 MPa.

The hot-rolled flat steel product has a yield strength R _t0.5 of at least 485 MPa, in particular at least 500 MPa, preferably at least 520 MPa. The maximum yield strength R _t0.5 can, for example, be at most 700 MPa, in particular at most 680 MPa.

The elongation at break A ₅₀ in the hot-rolled flat steel product is at least 10%, in particular at least 15%, preferably at least 20%.

The tensile strength R _m , the yield strength R _t0.5 and the elongation at break A ₅₀ are determined in the tensile test DIN EN ISO 6892-1:2017 determinable.

The invention is explained in more detail below using exemplary embodiments.

A first melt (see Examples 1 and 2) with the elements in wt.%, C=0.075%, Si=0.354%, Mn=1.8%, P=0.01%, S=0.001%, N =0.005%, Cr=0.058%, Cu=0.09%, Ni=0.055%, Mo=0.040%, AI=0.03%, B=0.0003%, Nb=0.063%, Ti=0.025%, V=0.002%, Sn=0.002%, Ca=0.001%, balance Fe and unavoidable impurities and a second melt (cf. Examples 3 to 5) with the elements in wt.%, C=0.050%, Si=0, 38%, Mn=1.6%, P=0.006%, S=0.001%, N=0.005%, Cr=0.29%, Cu=0.06%, Ni=0.09%, Mo=0.145% , AI=0.035%, B=0.0003%, Nb=0.069%, Ti=0.024%, V=0.002%, Sn=0.002%, Ca=0.001%, balance Fe and unavoidable impurities were each turned into slabs (previous product) cast in a continuous casting plant. The slabs produced were pre-heated/through-heated in a preheating furnace at a temperature of approx. 1240 °C. The preheated slabs were then pre-rolled in a two-stand roughing stand to a thickness of 60 mm and then hot-rolled in a five-stand finishing rolling mill to form a hot-rolled flat steel product (hot strip). The hot-rolled steel strip obtained in each case entered the finishing rolling mill with a pre-product temperature (“VT”) and left the finishing rolling mill with a final hot rolling temperature (“WET”) and, after exiting the finishing rolling mill, with a first average cooling rate (“KR1”) Temperature of 600 ° C, which is below A _r3 , and then cooled with a second average cooling rate (“KR2) to a coiling temperature (“HT”), at which it was coiled into a coil. The degrees of recrystallization (“I”) to (“IV”) after the first four rolling stands were also determined. The production specifications related to hot rolling are given in Table 1. Table 2 shows the mechanical-technological properties as well as the structural characteristics of the exemplary embodiments. Table 1: Examples - production specifications Example Thickness [mm] VT [°C] I [%] II [%] III [%] IV [%] WET [°C] KR1 [K/s] KR2 [K/s] HT [°C] invention 1 12.7 920 37 42 40 33 850 >20 < 5 560 no 2 12.7 1020 88 49 44 27 820 >30 < 5 500 Yes 3 17.5 900 22 30 30 21 800 >20 < 5 550 no 4 17.5 1050 93 65 58 27 870 >30 < 5 550 Yes 5 17.5 1050 93 65 58 27 870 >35 < 5 480 Yes Table 2: Examples - mechanical-technological properties and structural characteristics Example Microstructure R _m [MPa] R _t0.5 [MPa] A ₅₀ [%] 1 M+RA+B<2%, remainder F+P 732 635 >20 2 M+RA+B<3%, remainder F+P 722 639 >20 3 F+P+M+RA<2%, remainder B 713 639 > 18 4 F+P+M+RA<3%, remainder B 720 644 > 18 5 F+P+M+RA<4%, remainder B 729 672 > 18

It can be seen that the differences between the exemplary embodiments 2, 4 and 5 according to the invention compared to the reference examples 1 and 3 are not significantly reflected in the mechanical properties, but due to the higher temperatures of the preliminary products in the finish rolling mill, an increase in performance of up to 40% can be achieved. Due to the high temperatures and degrees of recrystallization, a gentle driving style of at least the first three rolling stands could also be reduced by at least 20% by reducing the rolling forces.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• DE 102013107010 A1 [0003]
• US 2011253267 A1 [0004]

Non-patent literature cited

• DIN EN ISO 6892-1:2017 [0041]

Claims (10)

Process for producing a hot-rolled flat steel product for use in pipe production, comprising the steps: a) melting a steel containing in wt.%: C: < 0.18%, Yes: < 1.50%, Mn: < 2.50%, P: < 0.10%, S: < 0.030%, N: < 0.020%, Cr: < 0.50%, Cu: < 0.50%, No: < 0.50%, Mon: < 0.50%, Al: < 2.0%, N.b.: < 0.15%, T: < 0.10%, Approx: < 0.010% up to a total of 0.2% of one or more of the elements B, V, Sn, the remainder Fe and unavoidable impurities; b) pouring the melt into a preliminary product; c) preheating the preliminary product to a temperature and/or holding the preliminary product at a temperature between 1100 and 1350 °C; d) hot rolling the preliminary product to a hot-rolled flat steel product in a finishing rolling mill comprising at least four and a maximum of nine rolling stands with a preliminary product temperature entering the finishing rolling mill of between 1000 and 1100 °C and a hot-rolled flat steel temperature leaving the finishing rolling mill of between 750 and 950 °C; e) cooling the resulting hot-rolled flat steel product to a coiling temperature of between 360 and 600 °C. Procedure according to Claim 1 , wherein at least in the first three roll stands of a finishing rolling mill, the hot rolling is carried out in the recrystallizing temperature range with a degree of recrystallization of the austenitic microstructure of at least 40%. Method according to one of the preceding claims, wherein in the first roll stand of a finishing rolling mill the hot rolling is carried out in the recrystallizing temperature range with a degree of recrystallization of the austenitic microstructure of at least 50%. Method according to one of the preceding claims, wherein the hot-rolled flat steel product after exiting the finishing rolling mill is cooled at a first average cooling rate to a temperature below A _r3 and optionally subsequently to the coiling temperature at a second average cooling rate which is lower than the first average cooling rate, is cooled. Procedure according to Claim 4 , wherein the first average cooling rate is at least 10 K/s and the optional second average cooling rate is at most 10 K/s. A method according to any one of the preceding claims, wherein the hot-rolled flat steel product is pickled after step e). Hot-rolled flat steel product for use in pipe production, in particular produced using a method according to one of the preceding claims, containing in% by weight or consisting of in% by weight: C: 0.040 to 0.090%, Si: 0.25 to 0.50%, Mn: 1.40 to 2.10%, P: <0.050%, S: <0.010%, N: <0.010%, CR: <0.20%, Cu: <0.20%, Ni: <0.20%, Mon: <0.20%, Al: <0.10%, Nb: <0.15%, Ti: <0.10%, Approx.: <0.010%, optionally one or more of the elements (Bi, V, Sn): B: <0.0050%, V: <0.010%, Sn: <0.020%, Rest Fe and unavoidable impurities, with a ferritic-pearlitic microstructure containing at least 90% ferrite and pearlite. Hot-rolled flat steel product for use in pipe production, in particular produced using a method according to one of the Claims 1 until 6 , containing in% by weight or consisting of in% by weight: C: 0.030 to 0.072%, Si: 0.31 to 0.60%, Mn: 1.40 to 1.90%, P: <0.050%, S: <0.010%, N: <0.010%, CR: <0.50%, Cu: <0.30%, Ni: <0.20%, Mon: <0.30%, Al: <0.10%, Nb: <0.15%, Ti: <0.10%, Approx.: <0.010%, optionally one or more of the elements (B, V, Sn): B: <0.0050%, V: <0.010%, Sn: <0.020%, Rest Fe and unavoidable impurities, with a bainitic microstructure containing at least 90% bainite. Using a hot-rolled flat steel product Claim 7 or 8th for the production of pipes. Spiral seam or longitudinal seam welded pipe made from a hot-rolled flat steel product Claim 7 or 8th .