Hot-rolled flat steel product and its production method

Abstract

The present invention relates to a hot-rolled flat steel product with a thickness > 1.5 mm, which has optimized mechanical properties and is particularly suitable for applying a zinc-based corrosion protection layer by hot-dip galvanizing. For this purpose, the flat steel product has the following composition by mass %, C: 0.04 - 0.23%, Si: 0.04 - 0.54%, Mn: 1.4 - 2.9%, Ti + V, where the sum of the Ti and V contents %Ti + %V applies 0.005% < %Ti + %V < 0.15%, and one or more elements from the group of "Al, Cr, Mo, B" are selectively present respectively, and if present, their contents are determined as follows: Al: 0.01 - 1.5%, the sum of the Cr and M contents %Cr + %Mo: 0.02 < %Mo + %Cr < 1.4%, B: 0.0005 - 0.005%, and the remainder consists of iron and inevitable impurities, where the inevitable impurities include < 0.02% P, < 0.005% S, < 0.01% N and < 0.005% Nb. The microstructure of the flat steel product consists of ferrite and bainitic ferrite with a total of 50 - 90% by area, martensite of 5 - 50%, retained austenite of 2 - 15% and other microstructure components of < 10%. At the same time, the flat steel product has a yield limit Rp0.2 > 290 MPa, a tensile strength Rm > 490 MPa and an elongation at break A80, which is calculated according to the following formula (1): A80 [%] = B - Rm / 37, where 31 < B < 51. A Zn coating is applied on at least one surface of the flat steel product by hot-dip coating. The present invention also relates to a method for producing such a flat steel product.

Description

TECHNICAL FIELD

[0001] The present invention relates to a hot-rolled flat steel product, which comprises a steel substrate and a zinc-based corrosion protection layer applied thereto by hot-dip coating.

[0002] Furthermore, the present invention also relates to a method for producing such a flat steel product.

[0003] In this context, a "flat steel product" is understood to mean a rolled product whose length and width are each significantly greater than its thickness. These products include, in particular, steel strips and steel plates.

[0004] In this context, unless otherwise expressly stated, information on the content of alloying components is always given in mass %.

[0005] Unless otherwise stated, the proportion of a specific component of the microstructure of the steel substrate of the flat steel product is expressed in area %.

[0006] In this context, "impurities" in steel, zinc or other alloys mean technically unavoidable steel concomitants that enter the steel during production or cannot be completely removed from the steel, but whose content is in any case very small and has no influence on the properties of the steel.

[0007] Image analysis is carried out by optical microscopy ("LOM") at magnifications from 200 to 2000 times and scanning electron microscopy ("REM") at magnifications from 2000 to 20000 times in an optical photographic manner to quantitatively determine the microstructure.

[0008] The distribution of manganese (Mn) in the microstructure of the steel substrate of the flat steel product according to the invention is determined by wavelength dispersive X-ray microanalysis (WDX) of the microstructure, for example as described by Reimer L. (1998) in "Elemental Analysis and

[0009] Imaging with X-Rays", published in Scanning Electron Microscopy, Springer Series in Optical Sciences, Volume 45, Springer, Berlin, Heidelberg.

[0010] The strength and elongation properties mentioned here, such as the tensile strength Rm, yield limit Rp0.2, uniform elongation Ag, elongation A50 and elongation A80 of the flat steel product, are determined in a tensile test in accordance with DIN-EN

[0011] 6892-1:2017, unless otherwise stated. BACKGROUND ART

[0012] High-load passenger vehicle and truck components, such as the crash structures and chassis of automobile bodies, require hot-dip galvanized steel sheets with a thickness exceeding 1.5 mm and a tensile strength exceeding 590 MPa.

[0013] Such components are typically produced using hot-rolled flat steel products composed of complex phase steel (CP-W), whose microstructure mainly consists of bainite. However, the CP-W steel has relatively low formability, which hinders the design of geometrically complex components.

[0014] Dual-phase steel (DP) consists of a combination of hard phases (such as martensite or bainite) and soft phases (such as ferrite). Due to its combination of high strength and good formability, it is suitable for producing complex components. However, cold-rolled dual-phase steel (DP-K) with a thickness greater than 1.5 mm is highly sensitive to surface defects, such as ungalvanized positions. Therefore, the maximum sheet thickness of hot-dip galvanized DP-K steel is generally limited to 2 mm.

[0015] Direct galvanizing of hot-rolled dual-phase steel (DP-W) is also not feasible. For galvanizing, the sheet must be heated to a temperature greater than 460 °C (the zinc bath temperature). However, at this temperature, the hard components in the microstructure, especially martensite, will be tempered and lose their DP characteristics.

[0016] One possibility is to anneal the hot-rolled strip in a hot-dip galvanizing plant through a typical DP-K annealing cycle and then galvanize it (i.e., partial austenitization in the critical temperature range, i.e., in the temperature range between the Ac1 and Ac3 temperatures of the corresponding steel, where the equilibrium of α-Fe and γ-Fe is produced). This method is similar to the manufacturing process of DP-K steel, except for the cold rolling step. However, there is a risk that omitting the cold rolling step will result in poorer mechanical properties compared to DP-K steel.

[0017] High-strength multiphase steel with a minimum tensile strength of 580 MPa is known from DE 10 2012 013 113 A1. This steel preferably has a dual-phase structure and allows the production of cold-rolled or hot-rolled steel strips with improved formability. In particular, components for lightweight vehicle structures can be produced therefrom. For this purpose, the known multiphase steel has the following composition by mass %: 0.075% ≤ C ≤ 0.105%, 0.600%

[0018] ≤ Si ≤ 0.800%, 1.000% ≤ Mn ≤ 2.250%, 0.280% ≤ Cr ≤ 0.480%, 0.010%

[0019] ≤ Al ≤ 0.060%, ≤ 0.020% P, ≤ 0.0100% N, ≤ 0.0150% S, and the remainder is iron and impurities.

[0020] Another high-strength multiphase steel with a minimum tensile strength of 580 MPa is a steel known from DE 10 2012

[0021] 006 017 A1. This steel preferably also has a duplex structure and is suitable for producing cold-rolled or hot-rolled steel strips with good formability. Components for light vehicle structures are particularly to be formed from such steel strips. For this purpose, the composition of the known steel by mass % is: 0.075%

[0022] ≤C≤0.105%, 0.200%≤Si≤0.300%, 1.000%≤Mn≤2.000%, 0.280%≤Cr≤0.480%, 0.010%≤Al≤0.060%, at most 0.020% P, 0.005%≤Nb≤0.025%, at most 0.0100% N, at most 0.0050% S, with the remainder consisting of iron and technically unavoidable impurities.

[0023] The steel known from DE 10 2013 013 067 A1 also belongs to the known multiphase steels explained above. This steel preferably has a duplex structure and is suitable for cold-rolled or hot-rolled steel strips with improved formability. This known steel should have a yield limit ratio of not more than 73%, and its composition by mass % is: 0.075%≤C≤0.105%, 0.600%≤Si≤0.800%, 1.000%≤Mn≤1.900%, 0.100%≤Cr≤0.700%, 0.010%≤Al≤0.060%, 0.0020%≤N≤0.0120%, ≤0.0030% S, 0.005%≤Nb≤0.050%, 0.005%≤Ti≤0.050%, 0.0005%≤B≤0.0040%, ≤0.200% Mo,

[0024] ≤0.040% Cu, ≤0.040% Ni, with the remainder being iron and unavoidable impurities. Summary of the Invention

[0025] Against the background of the above prior art, the object of the present invention is to develop a flat steel product which not only has optimized mechanical properties but is also particularly suitable for applying a zinc-based corrosion protection layer by hot-dip galvanizing.

[0026] The present invention achieves this object by means of a flat steel product.

[0027] In addition, the present invention should provide a method by which the production of the flat steel product obtained according to the present invention can be reliably achieved.

[0028] To achieve this object, the present invention proposes a method. It goes without saying that when implementing the method according to the present invention, those skilled in the art not only have to implement the method steps mentioned in the claims and explained herein, but also, if necessary, perform all other steps and activities that are often carried out in the prior art during the actual implementation of such a method.

[0029] Advantageous design solutions of the present invention are given below and are explained in detail below together with the general inventive concept.

[0030] Accordingly, the present invention provides a hot-rolled flat steel product, which comprises a steel substrate and a zinc (Zn)-based corrosion protection layer coated thereon by hot-dip coating.

[0031] Herein, the steel of the steel substrate of the flat steel product according to the present invention comprises by mass %:

[0032] C: 0.04 - 0.23%,

[0033] Si: 0.04 - 0.54%,

[0034] Mn; 1.4 - 2.9%,

[0035] Ti + V, wherein the sum of the contents of Ti and V %Ti + %V is subject to the following provisions:

[0036] 0.005% ≤ %Ti + %V ≤ 0.15%,

[0037] And one or more elements from the group of "Al, Cr, Mo, B" selectively, if present, the content thereof is determined as follows:

[0038] Al: 0.01 - 1.5%

[0039] The sum of the contents of Cr + Mo: 0.02 - 1.4%

[0040] B: 0.0005 - 0.005

[0041] The balance consists of iron and inevitable impurities, wherein the inevitable impurities include P less than 0.02%, S less than 0.005%, N less than 0.01% and Nb less than 0.005%.

[0042] Herein, the steel substrate of the flat steel product according to the present invention is at least 1.5 mm thick, and its microstructure consists, by area %, of 50 - 90% ferrite and bainitic ferrite in total, 5 - 50% martensite, 2 - 15% retained austenite and at most 10% of other microstructure components inevitable due to production.

[0043] At the same time, the yield limit Rp0.2 of the flat steel product according to the present invention is at least 290 MPa, the tensile strength Rm is at least 490 MPa, and the elongation at break A80 is determined by the following formula (1):

[0044] A80 [%] = B - Rm / 37, where 3 ≤ B ≤ 51.

[0045] The flat steel product according to the present invention can be produced by at least the following working steps:

[0046] A) Produce a hot-rolled steel substrate in the form of a steel strip by at least the following sub-steps:

[0047] A.1) Melt the steel composed according to the specifications of the present invention;

[0048] A.2) Cast the steel melt into a pre-product, which is a slab or thin slab;

[0049] A.3) Preheat the pre-product at a preheating temperature of at least 1150 °C and at most 1350 °C;

[0050] A.4) Hot-roll the pre-product into a hot-rolled steel strip, where the finishing temperature of the hot rolling is at least 840 - 980 °C, and the thickness of the hot-rolled steel strip is 1.5 - 10 mm;

[0051] A.5) Cool the hot-rolled steel strip to a coiling temperature of 510 - 640 °C;

[0052] A.6) Coil the hot-rolled steel strip cooled to the coiling temperature.

[0053] B) Apply a zinc-based corrosion protection coating to the steel substrate in the form of a hot-rolled steel strip in at least the following continuously performed sub-steps:

[0054] B.1) Optionally pickling the hot-rolled steel strip;

[0055] B.2) Heat the hot-rolled steel strip to an annealing temperature of 750 - 950 °C at a heating rate of 0.5 - 100 °C / s, and hold the hot-rolled steel strip at this annealing temperature for an annealing duration of 10 - 1000 s;

[0056] B.3) Cool the hot-rolled steel strip to the bath entry temperature BET at a cooling rate of 0.5 - 100 °C / s, where BT ≤ BET ≤ (BT + 20 °C), and the temperature of the zinc melt bath is called BT and is 450 - 480 °C;

[0057] B.4) Pass the hot-rolled steel strip cooled to the bath entry temperature BET through a zinc melt bath, which consists of at most 5% by mass of Mg, at most 10% by mass of Al, the rest Zn and unavoidable impurities;

[0058] B.5) A flat steel product cooled at a cooling rate of 0.5 - 100 °C / s;

[0059] B.6) Selectively temper rolling the flat steel product with a temper rolling rate of 0.3 - 2.0%.

[0060] In working step A.1, a preheating temperature of at least 1150 °C is required to fully homogenize the microstructure of the pre-product. At lower temperatures, the microstructure of the pre-product will be inherited by the subsequently produced hot-rolled strip, so the Mn segregation required according to the present invention cannot be formed. Similarly, at lower preheating temperatures, alloying elements will be trapped in precipitates and thus cannot affect the mechanical properties of the flat steel product according to the present invention.

[0061] In order to be able to roll the pre-product alloyed according to the present invention into a hot-rolled steel strip in a reliable operation mode, a hot rolling end temperature of at least 840 °C is required. At lower hot rolling end temperatures, the rolling force will be too high, and thus the risk of damage to the rolls of the rolling mill for hot rolling will increase disproportionately. To minimize this risk, a hot rolling end temperature of at least 880 °C can be set. The hot rolling end temperature should not exceed 980 °C because a hot rolling end temperature above this upper limit cannot be achieved in practice.

[0062] The hot-rolled steel strip according to the present invention must have a thickness of at least 1.5 mm in order to form the Mn segregation required according to the present invention in the microstructure after hot rolling. At smaller strip thicknesses, the hot-rolled steel strip will undergo excessive deformation during hot rolling, which in turn will lead to an undesirable homogenization of the Mn distribution in the structure of the hot-rolled steel strip. Steel strips with a thickness exceeding 10 mm cannot be used for the intended purpose. Therefore, the maximum strip thickness is limited to 10 mm.

[0063] The coiling temperature of the hot-rolled steel strip forming the steel substrate of the flat steel product according to the present invention is at least 510 °C to ensure the formation of Mn segregation during the cooling of the hot-rolled steel strip in coil form. Higher coiling temperatures can promote this process, and thus a coiling temperature of at least 530 °C, especially at least 550 °C, is particularly advantageous. At too low coiling temperatures, an undesirable uniform Mn distribution will occur, and thus the mechanical properties pursued by the present invention cannot be achieved. Too high coiling temperatures will pose a risk of significant grain boundary oxidation. To prevent this, the coiling temperature is limited to 640 °C, preferably 620 °C.

[0064] After the hot-rolled steel strip in coil form is cooled, if necessary, it can be pickled in a conventional manner to remove the scale on the steel strip or prepare the surface of the steel strip for subsequent working steps.

[0065] For hot-dip galvanizing, the hot-rolled steel strip is first heated to the annealing temperature at a heating rate of 0.5 - 100 °C per second in the preheating stage. The heating rate must be within this window to ensure sufficient transformation of the microstructure, especially its complete recrystallization. For the same reason, an annealing temperature of 750 - 950 °C and a holding time of 10 - 1000 seconds are necessary. At too low an annealing temperature or too short a holding time, the microstructure will not fully crystallize, resulting in insufficient austenite to form the required proportion of martensite for the microstructure during subsequent cooling. The uncrystallized steel substrate will also cause significant anisotropy in the mechanical properties of the flat steel products according to the present invention.

[0066] Similarly, it is cooled from the annealing temperature to the zinc bath entry temperature BET at a cooling rate of 0.5 - 100 °C per second. Here, the bath entry temperature BET is at least equal to the melt bath temperature and at most 20 °C higher than the melt bath temperature to prevent significant changes in the melt bath temperature due to the entry of the hot-rolled steel strip.

[0067] Optionally, a heat treatment ("galvannealing") can be carried out again after hot-dip galvanizing, in which the hot-dip galvanized flat steel product is heated to a maximum of 550 °C to burn off the previously applied corrosion protection layer.

[0068] Whether directly after leaving the zinc bath or after an additional heat treatment, the resulting flat steel product is cooled to room temperature at a cooling rate of 0.5 - 100 °C / s.

[0069] The flat steel products produced in this way can also be optionally subjected to conventional skin pass rolling to optimize their dimensional accuracy and surface properties. The skin pass rate set here is usually at least 0.3% and at most 2.0%, and a skin pass rate of at least 0.5% has proven to be particularly practical. When the skin pass rate is less than 0.3%, it will cause a reduction in the surface roughness of the corrosion protection layer, which will have a negative impact on the formability of the flat steel product. When the skin pass rate exceeds 2.0%, the yield limit Rp0.2 will increase and the elongation at fracture A80 will decrease, so the elongation at fracture meeting Formula 1 cannot be achieved.

[0070] Surprisingly, it has been found that flat steel products including a steel substrate alloyed according to the present invention and having a microstructure according to the present invention achieve high elongation at fracture values in the hot-rolled state, which are comparable to the elongation at fracture A80 of conventional cold-rolled flat steel products ("DP-K steel") of the type mentioned at the beginning and having similar strength. Therefore, in practice, elongation at fracture values A80 can usually be achieved, for which the parameter B in Formula (1) is at least in the range of 31 - 51, preferably 36 - 46.

[0071] The combination of high strength and high fracture elongation values is achieved by the presence of a retained austenite proportion of 2 - 15 area% in the steel substrate of the flat steel product according to the invention, where at least 5 area% of the retained austenite proportion is generally present in the microstructure of the steel substrate of the flat steel product according to the invention and has a positive effect on the mechanical properties of the flat steel product. Therefore, the retained austenite content determinable in the flat steel product according to the invention is significantly higher than that of cold-rolled flat steel products with a comparable alloy composition.

[0072] According to the knowledge of the present invention, the presence of a relatively large proportion of retained austenite in the microstructure is the result of inheriting the Mn segregation present in the hot-rolled steel substrate of the flat steel product according to the invention, and is maintained by the annealing treatment carried out on the flat steel product for its hot-dip galvanizing. Therefore, it can be demonstrated that in the production method of the flat steel product according to the invention, after coiling (sub-step A.6 of the method according to the invention) and before hot-dip galvanizing (working step B of the method according to the invention), the hot-rolled steel substrate has a highly anisotropic and inhomogeneous microstructure, which has a high pearlite content present in strips. The results of wavelength-dispersive X-ray microanalysis (WDX) of this microstructure show that Mn is segregated in the pearlite bands, and the Mn segregation exists in a highly anisotropic and inhomogeneous distribution after coiling and before hot-dip galvanizing.

[0073] In continuous hot-dip galvanizing, the steel substrate of the flat steel product according to the invention is annealed (sub-step B.2 of the method according to the invention) before entering the melt bath, during which it is held at the annealing temperature for a certain duration. Here, according to the invention, the annealing temperature and the annealing duration are coordinated with each other such that no redistribution of Mn segregation occurs. Therefore, in the flat steel product according to the invention after completion of hot-dip galvanizing, despite the annealing treatment required for preparing the zinc corrosion protection layer, there is still an anisotropic and inhomogeneous Mn distribution in the steel substrate, which is "inherited" from the final microstructure present after coiling of the hot-rolled steel substrate of the flat steel product.

[0074] Because Mn contributes very significantly to the stability of austenite during annealing in the intercritical region, compared with hot-rolled flat steel products coiled at a lower temperature deviating from the specifications of the present invention, both the transformation temperature and the retained austenite content after cooling are distributed in a more inhomogeneous manner. In the flat steel product produced according to the invention, compared with the microstructure regions of the steel substrate with a lower Mn concentration, the microstructure regions of the steel substrate with a higher Mn concentration are more likely to transform, and thus retain more austenite after cooling. It transforms at a higher temperature or does not transform at all, and thus maintains a higher proportion of the original ferrite here.

[0075] The non-uniformity of manganese distribution in the steel substrate of the fully processed flat steel product according to the present invention can be quantified by the total area ratio in the steel substrate microstructure where its manganese concentration (by mass %) is more than 15% higher than the average value of the manganese concentration in the entire microstructure of the flat steel product. The sum of the area ratios in the steel substrate microstructure of the flat steel product according to the present invention where its manganese concentration is more than 15% higher than the average value of the manganese concentration in the entire microstructure is denoted as "X". In the flat steel product according to the present invention, X accounts for at least 10%, especially at least 12%, and advantageously at least 15% of the total microstructure. The area ratios forming the sum X can be evaluated by WDX measurement, where the Mn concentration is typically determined on a measurement surface of at least 200 x 200 μm with a step size of 0.5 μm.

[0076] The steel composition of the steel substrate of the flat steel product according to the present invention, which exists as a hot-rolled steel strip during the production process according to the present invention, is as follows.

[0077] Carbon (C) is present in the steel substrate of the flat steel product according to the present invention in an amount of 0.04 - 0.23% by mass. C is a basic element for forming martensite and austenite, and in order to achieve the required strength properties of the flat steel product according to the present invention, it is necessary to form martensite and austenite. To achieve this effect to a sufficient degree, the steel according to the present invention contains at least 0.04% by mass, and when the C content is at least 0.07% by mass, the expected effect is particularly reliably achieved. An excessive C content will have an adverse effect on the welding behavior of the flat steel product. Generally, the weldability of steel decreases as its C content increases. Therefore, in order to avoid the negative impact of the C content on its processing performance, the C content of the steel according to the present invention is limited to a maximum of 0.23% by mass, especially a maximum of 0.20% by mass, and the negative impact of the presence of C can be particularly reliably avoided at a content of at most 0.17% by mass.

[0078] Silicon (Si) is present in the steel substrate of the flat steel product according to the present invention in an amount of 0.04 - 0.54% by mass. Si is required to suppress the formation of pearlite in the microstructure during annealing, which will have a negative impact on the mechanical properties of the final product. For this purpose, a minimum Si content of 0.04% by mass is required. Additionally, an excessive Si content will prevent the formation of pearlite during the coiling process, thereby preventing the segregation of Mn in the steel substrate microstructure. Significant segregation of Mn during the coiling process is a prerequisite for achieving a high sum X and the required mechanical properties. An excessive Si content will also damage the surface quality of the flat steel product according to the present invention. For these reasons, the upper limit of the Si content is limited to 0.54% by mass.

[0079] Aluminum (Al) can be selectively added to the steel substrate of the flat steel product according to the invention in an amount of 0.01 - 1.5% by mass in order to contribute to suppressing the formation of pearlite. Even when deoxidizing the melt with Al in the usual way, an Al content of at least 0.01% by mass is produced. However, too high an Al content has a negative impact on the castability of the steel and deteriorates the coating behavior during the hot-dip galvanizing process. This negative impact of the presence of Al in the steel of the substrate of the flat steel product according to the invention can be particularly reliably avoided by limiting the Al content to at most 1.0% by mass, in particular at most 0.5% by mass.

[0080] Manganese (Mn) is present in the steel substrate of the flat steel product according to the invention in an amount of 1.4 - 2.9% by mass. Manganese is a mixed crystal element that helps to increase the strength of the material. The presence of Mn in the steel of the substrate of the flat steel product according to the invention also makes the austenite in the substrate microstructure more stable. The particularity of the alloy concept according to the invention in combination with the production according to the invention of the flat steel product according to the invention is that the flat steel product according to the invention is an optimal combination of high tensile strength and high elongation at break, which is the result of the segregation of Mn in the pearlite bands of the steel substrate after coiling and which remains the case even when the flat steel product has been annealed for hot-dip galvanizing and passed through the hot-dip bath. In order for Mn to be enriched in the pearlite bands by segregation to a sufficient extent, the Mn content must be at least 1.4% by mass, where it is advantageous in terms of the reliability of the positive effect of Mn on the properties of the flat steel product according to the invention when the Mn content is at least 1.5% by mass. However, too high an Mn concentration also has an adverse effect on weldability. Therefore, the upper limit of the Mn content of the steel substrate of the flat steel product according to the invention is limited to 2.9% by mass, preferably 2.5% by mass, where the contribution of Mn to the properties of the flat steel product according to the invention can be particularly effectively utilized when the Mn content is at most 2.2% by mass.

[0081] Chromium (Cr) and molybdenum (Mo) can be added as selective elements for increasing strength to the steel of the steel substrate of the flat steel product according to the invention. In addition, the presence of Cr and / or Mo increases the formation of martensite relative to pearlite during the cooling of the flat steel product from the trans-critical region in a continuous coating apparatus. If these effects are to be utilized, the total content of chromium and molybdenum needs to reach at least 0.02% by mass, in particular at least 0.05% by mass. However, when the Cr content is too high, the risk of significant grain boundary oxidation also increases. For cost reasons, too high a Mo content should also be avoided. Therefore, in order to be able to effectively utilize the effects of Cr and Mo in the steel of the steel substrate of the flat steel product according to the invention, the upper limit of the total content of Cr and Mo is set at 1.4% by mass, preferably 1.0% by mass. Here, Cr and Mo do not necessarily have to be used in combination, and can also be added separately to the steel in amounts of 0.02 - 1.4% by mass, in particular 0.05 - 1.0% by mass as specified by the invention, to achieve the described effects. However, when Cr and Mo are present simultaneously in effective amounts, particularly advantageous effects are produced, provided that the sum of these amounts is within the range specified by the invention.

[0082] At least one element of titanium (Ti) and vanadium (V) is present as an essential component in the steel of the steel substrate of the flat steel product according to the invention in an amount of 0.005 - 0.15% by mass, where, here, the best effects of these elements occur when Ti and V are present simultaneously in effective amounts. Ti and V are microalloying elements which form fine precipitates in the steel. Such precipitates prevent the coarsening of austenite grains at temperatures above the Ar1 temperature of the steel and, in this way, lead to a refinement of the microstructure. A finer microstructure is favorable for the segregation of Mn during the coiling process which is carried out during the production of the flat steel product according to the invention, since the diffusion distance of Mn is reduced due to the presence of Ti and / or V. The Ti- and V-containing precipitates also contribute to the strength of the flat steel product according to the invention by dispersion hardening. In order to achieve these effects of Ti and V, the total content of Ti and / or V needs to reach at least 0.005% by mass. When the content exceeds 0.15% by mass, the presence of Ti and / or V no longer results in any particular increase in the properties required according to the invention. On the contrary, if the sum of the contents of Ti and V is at most 0.1% by mass, they can be utilized particularly effectively.

[0083] According to the invention, the content of niobium (Nb) is limited to less than 0.005% by mass, so that if niobium is present, it also belongs to technically ineffective impurities. Higher Nb contents lead to the formation of fine Nb precipitates which can easily form cracks during continuous casting or during the cooling or reheating of the slab. Therefore, the Nb content is preferably limited to less than 0.003% by mass, in particular less than 0.002% by mass.

[0084] Boron (B) can also be selectively added to the steel of the steel substrate of the flat steel product according to the invention in an amount of 0.0005 - 0.005% by mass to prevent the formation of ferrite during the cooling process from the trans-critical region in the production of the flat steel product. In this way, B promotes the formation of bainite, resulting in an increase in strength. For this purpose, the minimum content of B is required to be 0.0005% by mass. However, too high a B content will lead to undesirable embrittlement. Therefore, according to the invention, if B is added, the upper limit of the B content is set to not exceed 0.005% by mass, especially 0.002% by mass.

[0085] Phosphorus (P) is an impurity that is not desired but generally inevitable technically in the steel of the steel substrate of the flat steel product according to the invention, and thus should be as low as possible. P has been proven to be particularly disadvantageous in terms of weldability. To reliably avoid its adverse effects, according to the invention, the content of P is limited to less than 0.02% by mass, preferably less than 0.01% by mass, especially less than 0.005% by mass.

[0086] Sulfur (S) is also an impurity that is not desired but generally inevitable technically in the steel of the steel substrate of the flat steel product according to the invention, and thus should be as low as possible. At higher concentrations, S will cause the formation of MnS or (Mn, Fe)S, which will have an adverse effect on the elongation behavior of the flat steel product according to the invention. To avoid this adverse effect, the S content according to the invention is limited to less than 0.005% by mass, preferably less than 0.002% by mass.

[0087] Nitrogen (N) is also an impurity that is not desired but generally inevitable technically in the steel of the steel substrate of the flat steel product according to the invention, and thus should be as low as possible. For example, N will form nitrides with aluminum or titanium. In the case of a high N content, this will lead to coarse precipitates, which may be harmful to the formability of the flat steel product. Therefore, according to the invention, the content of N is limited to less than 0.01% by mass, preferably less than 0.005% by mass.

[0088] In traditional steel production, calcium (Ca) also enters the steel because calcium is added for deoxidation and desulfurization and to improve castability. Too high a Ca concentration will lead to the formation of undesirable inclusions, which have a negative impact on mechanical and rolling properties. Therefore, the upper limit of the Ca content is limited to at most 0.005% by mass, preferably at most 0.002% by mass.

[0089] Copper (Cu), nickel (Ni), tin (Sn), arsenic (As), cobalt (Co), zirconium (Zr), lanthanum (La) and / or cerium (Ce) are also alloying elements that are impurities in the steel of the steel substrate of the flat steel product according to the present invention, and their presence per se is undesirable. In order to reliably prevent the influence of these elements on the properties of the flat steel product according to the present invention, in the steel of the steel substrate of the flat steel product according to the present invention, the Cu content is limited to at most 0.2% by mass, the Ni content is limited to at most 0.1% by mass, the Sn content is limited to at most 0.05% by mass, the As content is limited to at most 0.02% by mass, the Co content is limited to at most 0.02% by mass, the Zr content is limited to at most 0.0002% by mass, the La content is limited to at most 0.0002% by mass, and the Ce content is limited to at most 0.0002% by mass.

[0090] Oxygen (O) is also an undesirable impurity because when there is more O, oxide inclusions will be formed, which has a negative impact on both the mechanical properties of the flat steel product and the castability and rollability of the steel of its steel substrate. Therefore, the oxygen content is limited to at most 0.005% by mass, preferably 0.002% by mass.

[0091] Hydrogen (H) is also one of the undesirable impurities in the steel of the steel substrate of the flat steel product according to the present invention. As the smallest atom, H has strong mobility at the interstitial sites in steel and can cause core cracking during the cooling process after hot rolling, especially in ultra-high strength steels. Therefore, the H content in the steel of the steel substrate of the flat steel product according to the present invention is reduced to a maximum of 0.001% by mass, preferably a maximum of 0.0006% by mass, more preferably a maximum of 0.0004% by mass, and most preferably a maximum of 0.0002% by mass.

[0092] There are no special requirements for the composition of the corrosion protection coating and thus for the composition of the relevant melt bath through which the flat steel product passes during the hot-dip galvanizing process. Therefore, the corrosion protection coating of the flat steel product according to the present invention mainly consists of zinc (Zn), and the rest can be composed in a conventional manner.

[0093] Correspondingly, in addition to Zn and inevitable impurities, the corrosion protection layer can also contain at most 20% by mass of Fe, at most 5% by mass of Mg and at most 10% by mass of Al. Generally, if they are present respectively, at least 5% by mass of Fe, at least 1% by mass of Mg and / or at least 1% by mass of Al are set to achieve the best corrosion protection performance. Detailed Embodiments

[0094] The present invention will be explained in more detail below with the aid of examples.

[0095] To test the present invention, steel A-I was melted and cast into slabs, the composition of which is given in Table 1. Contents of alloying elements that are so small as to be "0" in the technical sense, i.e., so small as to have no effect on the properties of the steel, are indicated by the entry "-" in Table 1.

[0096] The slabs were fully heated in a preheating furnace, where the preheating temperature was VT.

[0097] Subsequently, the preheated slabs were hot-rolled in a conventional manner to form hot-rolled steel strips W1-W35, where the hot rolling was ended at the rolling end temperature ET.

[0098] The hot-rolled steel strips W1-W35 obtained in this way were coiled in the same conventional manner starting from the coiling temperature HT to form coils respectively. If necessary, they were cooled to the coiling temperature HT in a conventional manner before coiling.

[0099] To demonstrate the effect of the present invention, when producing hot-rolled steel strips W1-W35 each composed of steel A-I, one of the combinations I-VIII of the preheating furnace temperature VT, the hot rolling end temperature ET, and the coiling temperature HT given in Table 2 was selected respectively. The preheating furnace temperature VT, the hot rolling end temperature ET, and the coiling temperature HT belonging to each of the combinations I-VIII are given in Table 2. Here, those preheating furnace temperatures VT, hot rolling end temperatures ET, and coiling temperatures HT that do not conform to the specifications of the present invention respectively are emphasized with underlines.

[0100] After the hot-rolled steel strips W1-W35 were cooled in coil form, they were coated with a zinc-based corrosion protection layer by hot-dip galvanizing. For this purpose, these strips were each subjected to one of six variants of annealing treatment and melt application, in which they were heated to the annealing temperature GT at a heating rate HR in the preheating stage and then held at this annealing temperature for an annealing duration of 40 s to 100 s respectively. Subsequently, the hot-rolled steel strips W1-W35 were cooled to the bath entry temperature BET at a cooling rate KR1, which was equal to the bath temperature of the melt bath respectively, through which the hot-rolled steel strips passed after their respective annealing treatments a-f. This melt bath consisted of at least 99 mass% zinc here. The finished flat steel products now based on the hot-rolled steel strips W1-W35 that emerged from the melt bath were then cooled to room temperature at a cooling rate KR2. The parameters heating rate HR, annealing temperature GT, cooling rate KR1, bath entry temperature BET, and cooling rate KR2 belonging to the variants a-f of the annealing treatment and melt application are recorded in Table 3.

[0101] The mechanical properties and the constituents of the microstructure are determined on the flat steel products obtained in the above-described manner. The results of these studies, the yield limit Rp0.2, the tensile strength Rm, the elongation at break A80, the parameter "B" in formula (1), the ferrite proportion F in the microstructure, the martensite proportion M in the microstructure, the austenite proportion A in the microstructure, the proportion SO of other constituents in the microstructure, and the sum X of the area proportions in the steel substrate microstructure in which the Mn concentration is more than 15% higher than the average value of the Mn concentration in the microstructure, are summarized in Table 4, where, for the flat steel products manufactured from the hot-rolled steel strips W1 - W35, it is also given which of the steels A - I the steel substrate of each respective flat steel product consists of, which combinations I - VIII of the hot-rolled steel strip manufacture ("WEZ" column) and which of the variants a - f of the annealing treatment and the melt application ("GS" column) the respective steel substrates were subjected to.

[0102] The flat steel products produced from the hot-rolled steel strips W1, W3, W6, W7, W8 and W27 were not manufactured in accordance with the method according to the invention:

[0103] In the flat steel product produced from the hot-rolled steel strip W1, the slab was heated at too low a preheating temperature VT, so that the slab was not fully annealed. Consequently, the alloying elements and the production method had no influence on the mechanical properties.

[0104] The hot-rolled steel strip W3 contains too little Mn, so that the Mn in the pearlite bands of the hot-rolled steel strip structure does not segregate to a sufficient extent. This results in a lower retained austenite content and thus in a relatively low elongation at break A80 of the flat steel product produced from the hot-rolled steel strip W3. Consequently, the parameter B is below 31.

[0105] When producing the hot-rolled steel strips W6, W7 and W8, too low a coiling temperature was set. This results in a similar effect on the Mn segregation and thus in insufficient mechanical properties, just like in the flat steel products produced from the hot-rolled steel strip W3.

[0106] During the annealing treatment of the hot-rolled steel strip W27, too low a GT was set, so that the microstructure was not fully recrystallized. This results in a lower austenite content in the steel substrate microstructure of the obtained flat steel product and thus in a lower elongation at break A80.

[0107]

[0108]

[0109]

[0110] *) Parameters not in accordance with the invention are underlined

[0111]

Claims

1. Coated hot-rolled strip, comprising - a steel substrate with a thickness of at least 1.5 mm, The composition of the steel substrate by mass % is as follows: C:0.04-0.23％， Si: 0.04 - 0.54%, Mn; 1.4 - 2.9%, Ti + V, where the sum of the contents of Ti and V %Ti + %V is subject to the following regulations: 0.005% ≤ %Ti + %V ≤ 0.15%, Cr and Mo, where the sum of the contents of Cr and Mo %Cr + %Mo applies: 0.02 ≤ %Mo + %Cr ≤ 1.4%, And respectively selectively one or more elements in the "Al and B" group, if present, and their contents are determined as follows: Al：0.01-1.5％ B:0.0005-0.005％ The remainder consists of iron and unavoidable impurities, where the unavoidable impurities include P less than 0.02%, S less than 0.005%, N less than 0.01% and Nb less than 0.005%, - The microstructure consists of 50 - 90% ferrite and bainitic ferrite, 5 - 50% martensite, 2 - 15% retained austenite and at most 10% of other microstructure components unavoidable due to production by area %, And - The yield limit Rp0.2 is at least 290 MPa, the tensile strength Rm is at least 490 MPa, and the elongation at fracture A80 is determined by the following formula (1): A80 [%] = A - Rm / 37, where 31 ≤ A ≤ 51, And - Includes a zinc-based corrosion protection layer applied by hot-dip coating on at least one of its surfaces.

2. The coated hot-rolled strip according to claim 1, Characterized in that The microstructure of the steel substrate contains at least 5 area % - 15 area % of retained austenite.

3. The coated hot-rolled strip according to any one of the preceding claims, Characterized in that The parameter A of formula (1) applies: 36 ≤ A ≤ 46.

4. The coated hot-rolled strip according to claim 1 or 2, Characterized in that The sum of the area ratios X of the manganese concentration in the microstructure of the steel substrate being more than 15% higher than the average value of the manganese concentration in the total microstructure of the coated hot-rolled strip accounts for at least 10% of the total microstructure of the steel substrate.

5. The coated hot-rolled strip according to claim 4, Characterized in that And X is at least 12%.

6. The coated hot-rolled strip according to claim 5, Characterized in that And X is at least 15%.

7. The coated hot-rolled strip according to claim 1 or 2, Characterized in that The corrosion protection layer consists of at least 75 mass % Zn.

8. A method for producing the coated hot-rolled strip obtained according to any one of claims 1 to 7, in which at least the following working steps are carried out: A) Produce a hot-rolled strip in the form of a steel strip at least through the following sub-steps: A.1) Melting a steel melt, the composition of which in mass % is: C: 0.04 - 0.23%, Si: 0.04 - 0.54%, Mn: 1.4 - 2.9%, Ti + V, where the sum of the Ti and V contents %Ti + %V applies: 0.005% ≤ %Ti + %V ≤ 0.15%, Cr and Mo, where the sum of the Cr and Mo contents %Cr + %Mo applies: 0.02 ≤ %Mo + %Cr ≤ 1.4%, and one or more elements from the group "Al and B" selectively, if present, the contents of which are determined as follows: Al: 0.01 - 1.5%, B: 0.0005 - 0.005%, with the remainder consisting of iron and inevitable impurities, where the inevitable impurities include less than 0.02% of P, less than 0.005% of S, less than 0.01% of N and less than 0.005% of Nb; A.2) Casting the steel melt into a pre - product, the pre - product being a slab or a thin slab; A.3) Pre - heating the pre - product at a pre - heating temperature of at least 1150 °C and at most 1350 °C; A.4) Hot - rolling the pre - product into a hot - rolled strip, where the finishing temperature of the hot - rolling is at least 840 - 980 °C and the thickness of the hot - rolled steel strip is 1.5 - 10 mm; A.5) Cooling the hot - rolled strip to a coiling temperature of 510 - 640 °C; A.6) Coiling the hot - rolled strip cooled to the coiling temperature, B) Applying a zinc - based corrosion protection layer to the steel substrate in the form of a hot - rolled strip in at least the following successively - carried - out sub - steps: B.1) Optionally pickling the hot - rolled strip; B.2) Heating the hot - rolled strip at a heating rate of 0.5 - 100 °C / s to an annealing temperature of 750 - 950 °C and holding the hot - rolled strip at the annealing temperature for an annealing duration of 10 - 1000 s; B.3) Cooling the hot - rolled strip at a cooling rate of 0.5 - 100 °C / s to the bath entry temperature BET, where BT ≤ BET ≤ (BT + 20 °C), where the temperature of the zinc melt bath is called BT and is 450 - 480 °C; B.4) Passing the hot - rolled strip cooled to the bath entry temperature BET through a zinc melt bath, the zinc melt bath consisting of at most 5 mass % of Mg, at most 10 mass % of Al, the remainder Zn and inevitable impurities; B.5) Cooling the resulting coated hot - rolled strip at a cooling rate of 0.5 - 100 °C / s; B.6) Optionally skin - passing the coated hot - rolled strip with a skin - pass rate of 0.3 - 2.0%.

9. The method according to claim 8, characterized in that, the coiling temperature is at least 530 °C.

10. The method according to claim 9, characterized in that, the coiling temperature is at least 550 °C.

11. The method according to any one of claims 8 to 9, characterized in that, the coiling temperature is at most 620 °C.

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