CN110959049B

CN110959049B - Flat steel product with good aging resistance and method for the production thereof

Info

Publication number: CN110959049B
Application number: CN201880048391.0A
Authority: CN
Inventors: 贝恩德·林克; 马里亚·科耶尔; 曼努埃拉·鲁滕贝里
Original assignee: ThyssenKrupp Steel Europe AG; ThyssenKrupp AG
Current assignee: ThyssenKrupp Steel Europe AG; ThyssenKrupp AG
Priority date: 2017-07-21
Filing date: 2018-07-11
Publication date: 2022-04-08
Anticipated expiration: 2038-07-11
Also published as: EP3974554A1; CN109280861A; CN114686777A; ES2899657T3; CN110959049A; WO2019016041A1; EP3655560A1; EP3655560B1

Abstract

The invention relates to a coated flat steel product suitable for press hardening, which has particularly good aging resistance, and to a method for the production thereof. The steel of the flat steel product, apart from iron and unavoidable impurities, consists of (in weight%): 0.10-0.4% C, 0.05-0.5% Si, 0.5-3.0% Mn, 0.01-0.2% Al, 0.005-1.0% Cr, 0.001-0.2% V, < 0.1% P, < 0.05% S, < 0.02% N and optionally one or more elements B in the following amounts "B, Ti, Nb, Ni, Cu, Mo, W": 0.0005 to 0.01%, Ti: 0.001-0.1%, Nb: 0.001-0.1%, Ni: 0.01-0.4%, Cu: 0.01-0.8%, Mo: 0.002-1.0%, W: 0.001-1.0%. The flat steel product has a continuously extending yield strength or a yield strength difference between an upper and a lower yield strength value of at most 45Mpa, a uniform elongation Ag of at least 11.5%.

Description

Flat steel product with good aging resistance and method for the production thereof

Technical Field

The invention relates to a coated flat steel product suitable for press hardening, which has particularly good aging resistance, and to a method for the production thereof.

Background

When the invention refers to a "flat steel product", this means a steel strip, a steel sheet or a slab obtained therefrom, etc. A slab is generally understood to have a more complex profile than the steel strip or sheet from which it is produced.

High demands are made on the mechanical properties of the steels used in the body construction and also on their processing behavior. Flat steel products intended to be deformed into steel components are subjected to different manufacturing steps. In addition it will be cold deformed. This can be done by, for example, straightening, cutting or shaping. Good cold forming performance additionally shows good dimensional accuracy, quality of the cut edges and a well-balanced surface of the cold formed part. Good cold forming performance is promoted by steels with low yield strength and high uniform elongation. In the processing, it has proven particularly advantageous here for the yield strength to run ideally continuously or to exhibit only a weak yield strength. The continuously progressing yield strength is also often referred to as the elongation limit.

The aging of the steel is caused by free carbon in the ferrite. At temperatures above 300 ℃, the solubility of carbon in ferrite is significantly greater than at room temperature, resulting in a certain free carbon content. Temperatures of more than 300 ℃ are usually reached during coating, for example in hot dip coating. Thus, in the temperature-time curve typical for the coating process, carbon may diffuse in the steel. The free carbon content is then significantly greater than the equilibrium content at room temperature, since approaching thermodynamic equilibrium requires a longer period of time than is available during cooling to room temperature after coating. Thus, ferrite is very strongly supersaturated with carbon at room temperature. However, as an interstitial alloy element, carbon can diffuse very slowly even at room temperature and accumulate at defects, such as, in addition, dislocations. This phenomenon is also known as aging, and the interstitial dissolved atoms deposited at the defects are known as Cottrell clouds. Dislocations are blocked by carbon, resulting in significant yield strength, which is highly undesirable for cold forming. Moreover, straightening of the flat steel product is made difficult by the discontinuous deformation behaviour. The increased deformation resistance leads to increased tool wear during the cutting of the blank and the possible subsequent deep-drawing cold-forming leads to an uneven, asymmetrical surface. In this respect, the aging of the steel due to free carbon should be prevented or at least reduced as much as possible.

A flat steel product is known from EP 2848709 a1, which is formed from a steel containing 0.2 to 0.5 wt.% C, 0.5 to 3.0 wt.% Mn, 0.002 to 0.004 wt.% B and the following contents of optionally one or more of the elements "Si, Cr, Al, Ti": 0.1-0.3 wt% Si, 0.1-0.5 wt% Cr, 0.02-0.05 wt% Al, 0.025-0.04 wt% Ti. The flat steel product is coated with a corrosion protection coating formed of an aluminium-zinc alloy. The coated flat steel product is provided for producing a component by means of press hardening. The flat steel products obtained in this way are resistant to ageing only to a small extent and have a strongly pronounced yield strength after coating and ageing.

Disclosure of Invention

The object of the invention is to provide a coated flat steel product suitable for press hardening, having good aging resistance, and a method for the production thereof.

The object with respect to a flat steel product is achieved by a flat steel product having the features specified in claim 1. Advantageous and preferred embodiments of the inventive flat steel product are given in the claims dependent on claim 1. The object on the method is achieved by a method having the features stated in claim 10. Advantageous and preferred embodiments of the method according to the invention are given in the claims dependent on claim 10.

The steel of the flat steel product of the invention consists, apart from iron and unavoidable impurities, of (in wt%): 0.10-0.4% C, 0.05-0.5% Si, 0.5-3.0% Mn, 0.01-0.2% Al, 0.005-1.0% Cr, 0.001-0.2% V, < 0.1% P, < 0.05% S, < 0.02% N and optionally one or more of "B, Ti, Nb, Ni, Cu, Mo, W" in the following amounts: b: 0.0005 to 0.01%, Ti: 0.001-0.1%, Nb: 0.001-0.1%, Ni: 0.01-0.4%, Cu: 0.01-0.8%, Mo: 0.002-1.0%, W: 0.001-1.0%.

When the specifications of the alloy content and composition are made herein, the specifications relate to weight or mass as long as there is no other description.

The carbon in the flat steel product according to the invention acts with a delay on the formation of ferrite and bainite. At the same time, the austenite is stabilized and the Ac3 temperature is lowered. The carbon content of the steel of the inventive flat steel product is limited to 0.10 to 0.4 wt.%. A carbon content of at least 0.10 wt.% is necessary to ensure quenchability of the flat steel product and a tensile strength of the press-quenched product of at least 1000 Mpa. If a higher strength level is to be pursued, the C content is preferably set to at least 0.15 wt.%. If the C content is increased further to a value of at least 0.19% by weight, in particular at least 0.205% by weight, the quenchability is further improved, so that the flat steel product has a very good combination of quenchability and strength. However, a carbon content of more than 0.4 wt.% has a negative effect on the mechanical properties of the flat steel product, since a C content of more than 0.4 wt.% promotes the formation of brittle martensite during press-quenching. Furthermore, weldability can be adversely affected by a high C content. In order to improve weldability, the carbon content may preferably be set to 0.3 wt% at the maximum. At C contents of up to 0.25 wt.%, in particular up to 0.235 wt.%, the weldability can again be significantly improved and, in addition, in the bending test according to VDA238-100, a good ratio of force absorption and maximum bending angle is obtained in the press-quenched state.

Silicon is used to further improve the quenchability of the flat steel product and the strength of the press-quenched product by solid solution strengthening. Furthermore, silicon makes it possible to use iron-silicon-manganese as alloying agent, which has a favourable effect on production costs. From an Si content of 0.05 wt.%, a hardening effect already occurs. A significantly increased strength occurs from an Si content of at least 0.15 wt.%, in particular at least 0.20 wt.%. Si contents above 0.5 wt.% adversely affect coating performance, especially in the case of Al-based coatings. An Si content of up to 0.4 wt.%, in particular up to 0.30%, is preferably set in order to improve the surface quality of the coated flat steel product.

Manganese acts as a hardening element by strongly retarding the formation of ferrite and bainite. In the case where the manganese content is less than 0.5 wt%, ferrite and bainite are formed during press quenching even at a very fast cooling rate, which should be avoided. If a martensitic structure should be ensured, in particular in the region of greater deformation, a Mn content of at least 0.9 wt.%, in particular at least 1.10 wt.%, is preferred. Manganese contents above 3.0 wt.% have an adverse effect on the processing properties, so that the Mn content of the flat steel product according to the invention is limited to a maximum of 3.0 wt.%. Firstly, the weldability is strongly limited, so that the Mn content is preferably limited to a maximum of 1.6 wt.%, and in particular to 1.30 wt.%. In addition, for economic reasons, the manganese content is preferably less than or equal to 1.6% by weight.

Aluminum is used as a deoxidizer to bind oxygen. In addition, aluminum prevents the formation of cementite. For reliable oxygen binding, at least 0.01 wt.%, in particular at least 0.02 wt.%, of aluminum is required in the steel. However, since the Ac3 temperature also moves significantly upward with increasing Al alloy content, the Al content is limited to 0.2 wt.%. From a content of 0.2 wt.%, Al too strongly impedes the conversion to austenite prior to the press-quenching, so that austenitization can no longer be carried out efficiently in terms of time and energy. For a common furnace temperature of 850-.

Chromium is added to the steel of the flat steel product according to the invention in a content of 0.005-1.0% by weight. Chromium affects the quenchability of the flat steel product by slowing down the diffusion conversion during press quenching. Chromium in the steel flat product according to the invention advantageously acts on the hardenability from a content of 0.005 wt.%, wherein a Cr content of at least 0.1 wt.%, in particular at least 0.18 wt.%, is preferred for reliable process control, in particular for preventing bainite formation. If the steel contains more than 1.0 wt.% chromium, the coating performance will deteriorate. In order to obtain good surface quality, the Cr content may preferably be limited to at most 0.4 wt.%, in particular at most 0.28 wt.%.

Chromium is a carbide former and in itself reduces the fraction of dissolved carbon present in the flat steel product. This occurs primarily when the flat steel product is cooled slowly at cooling rates of at most 25K/s or at most 20K/s, as is the case when the coated flat steel product is cooled to room temperature in a temperature range between 600 ℃ and 450 ℃ or in a temperature range between 400 ℃ and 220 ℃. The carbon atoms bonded by the chromium do not diffuse into and block the dislocations, so that the formation of a pronounced yield strength is reduced or completely suppressed. The Cr content is selected such that, when the coating process is carried out, only a small amount of carbon is bound by the chromium before coating and chromium carbides are formed mainly during cooling after coating. Chromium carbide is the preferred nucleus for other precipitates (e.g. vanadium carbide) and vice versa. This results in the still free carbon binding more rapidly, thereby further reducing or completely inhibiting the formation of significant yield strength.

Vanadium (V) is of particular interest in the steel of the flat steel product according to the invention. Vanadium is an element with high carbon affinity. When vanadium is free, i.e. present in an unbound or dissolved state, it may bind supersaturated dissolved carbon in the form of carbides or clusters, or at least reduce its diffusion rate. It is important here that V is present in dissolved form. Surprisingly, particularly very low V contents have proven to be particularly advantageous for the aging resistance. At higher V contents, larger vanadium carbides may be formed at higher temperatures and then no longer dissolve at the 650-900 ℃ temperature that is typical for continuous annealing in hot dip plating facilities. A minimum amount of vanadium of 0.001 wt.% can already hinder free carbon when dislocations accumulate. Starting from a V content of 0.2 wt.%, the improvement in aging resistance by vanadium no longer occurs. The aging inhibiting effect of vanadium is particularly pronounced at contents up to 0.009 wt.%, with the greatest effect being provided starting from the preferred content of 0.002 wt.%. In order to make particularly reliable use of the aging-inhibiting effect of vanadium, the vanadium content may be limited in a preferred embodiment to a maximum of 0.004 wt.%, in particular to a maximum of 0.003 wt.%. At contents greater than 0.009 wt.%, the formation of vanadium carbides increases. From a vanadium content of 0.009 wt.% in steel, vanadium carbides may not dissolve at temperatures of 700 to 900 ℃, such as the annealing temperatures typically used in hot dip plating facilities. With increasing vanadium content, more free vanadium is inevitably provided, since the precipitation kinetics of vanadium carbides are constantly accelerated, so that, although the vanadium carbides become larger and more stable, the fraction of dissolved vanadium no longer increases. This effect occurs particularly when the content exceeds 0.030% by weight, and therefore the content is preferably set to a value of at most 0.030% by weight. Since vanadium contributes to the improvement of strength by precipitation strengthening in addition to the reduction of the aging effect, a higher content set up to 0.2 wt% may be preferable for improving strength. The vanadium content of the steel of the flat steel product according to the invention is limited on the one hand to a maximum of 0.2 wt.% for cost reasons. On the other hand, higher contents do not lead to a significant improvement in mechanical properties.

Phosphorus (P) and sulfur (S) are elements entrained in steel as impurities by iron ore and cannot be completely removed in an industrial steel manufacturing process. The P content and S content should be kept as small as possible, since mechanical properties such as impact energy (kerbschlagagareit) deteriorate with increasing P content or S content. Furthermore, the increased embrittlement of the martensite occurs from a P content of 0.1 wt.%, so that the P content of the flat steel product according to the invention is limited to a maximum of 0.1 wt.%, in particular a maximum of 0.02 wt.%. The S content of the flat steel product according to the invention is limited to a maximum of 0.05 wt.%, in particular a maximum of 0.003 wt.%.

Nitrogen (N) is present in small amounts in steel due to the steel manufacturing method. The N content is kept as small as possible and should be at most 0.02% by weight. Especially in boron-containing alloys, nitrogen is detrimental, since nitrogen prevents the conversion retarding effect of boron by forming boron nitride, and therefore in this case the nitrogen content should preferably be at most 0.01 wt.%, especially at most 0.007 wt.%.

Boron, titanium, niobium, nickel, copper, molybdenum and tungsten may optionally be alloyed to the steel of the flat steel product according to the invention, each alone or in combination with one another.

Boron may optionally be alloyed to improve the quenchability of the flat steel product in such a way that the boron atoms or boron precipitates accumulated on the austenite grain boundaries reduce the grain boundary energy, thereby inhibiting the nucleation of ferrite during press quenching. At contents of at least 0.0005 wt.%, in particular at least 0.0020 wt.%, a significant influence on the quenchability occurs. In contrast, when the content exceeds 0.01 wt%, increased boron carbide, boron nitride or boron carbonitride is formed, which also becomes a preferable nucleation point for ferrite nucleation and again reduces the quenching effect. For this reason, the boron content is limited to a maximum of 0.01% by weight, in particular a maximum of 0.0035% by weight. When boron is alloyed, titanium is preferably also alloyed to bind nitrogen. In this case, the Ti content should preferably be at least 3.42 times the nitrogen content.

Titanium (Ti) is a microalloying element that may optionally be alloyed to assist in grain refinement. In addition, titanium forms coarse titanium nitride with nitrogen, and therefore the Ti content should be kept relatively low. Titanium combines with nitrogen, enabling boron to exert its strong ferrite inhibiting effect. For a sufficient binding of nitrogen, at least 3.42 times the nitrogen content is required, wherein for a sufficient availability at least 0.001 wt% Ti should be added, preferably at least 0.023 wt% Ti. From 0.1 wt% Ti, cold rollability and recrystallizability are significantly deteriorated, and thus a larger Ti content should be avoided. In order to improve cold rollability, the Ti content may be preferably limited to 0.038 wt%.

Niobium (Nb) may optionally be alloyed to contribute to grain refinement from a content of 0.001 wt%. However, niobium deteriorates the recrystallizability of the steel. At Nb contents above 0.1 wt.%, the steel can no longer be recrystallized in a conventional through-furnace before hot coating. In order to reduce the risk of deterioration of the recrystallizability, the Nb content may be preferably limited to 0.003 wt%.

Copper (Cu) may optionally be alloyed to improve quenchability when added at least 0.01 wt.%. In addition, copper improves the resistance of uncoated sheet or trim edges to atmospheric corrosion. From the content of 0.8 wt%, the hot rollability is remarkably deteriorated due to the low melting point Cu phase on the surface, so that the Cu content is limited to at most 0.8 wt%, preferably at most 0.10 wt%.

Nickel (Ni) stabilizes the austenite phase and may optionally be alloyed to lower the Ac3 temperature and inhibit the formation of ferrite and bainite. Furthermore, nickel has a positive influence on the hot-rollability, especially when the steel contains copper. Copper deteriorates hot rolling property. To counter the negative effect of copper on hot rollability, 0.01 wt.% nickel may be alloyed into the steel. For economic reasons, the nickel content should remain limited to a maximum of 0.4% by weight, in particular a maximum of 0.10% by weight.

To improve process stability, molybdenum (Mo) may optionally be alloyed, as it significantly slows the formation of ferrite. From a content of 0.002 wt.%, molybdenum-carbon clusters are formed dynamically on grain boundaries up to ultra-fine molybdenum carbides, which significantly slows the mobility of grain boundaries and thus the phase transformation of the diffusion. In addition, the grain boundary energy is reduced by molybdenum, which reduces the nucleation rate of ferrite. Due to the high cost associated with molybdenum alloys, the content should be up to 1.0 wt.%, preferably up to 0.1 wt.%.

In order to slow down the formation of ferrite, tungsten (W) may optionally be alloyed in a content of 0.001-1.0 wt.%. A positive effect on the quenchability is already obtained at a W content of at least 0.001 wt.%. For cost reasons, up to 1.0 wt% of tungsten plus alloy.

The flat steel product according to the invention has a high uniform elongation Ag of at least 11.5% after coating. The yield strength of the flat steel product according to the invention has a continuous course or only a small apparent (

). In the sense of the present invention, a continuous course means that no significant yield strength is present. The yield strength with a continuous course may also be referred to as the elongation limit rp 0.2. A lower apparent yield strength is understood here as an apparent yield strength, wherein the difference Δ Re between the upper and lower yield strength values ReH, ReL is at most 45 MPa. The method is applicable to the following steps:

Δ Re ≦ 45MPa (ReH-ReL), where ReH ≦ upper yield strength in MPa, and ReL ≦ lower yield strength in MPa.

Particularly good ageing resistance can be achieved in flat steel products for which the difference Δ Re is at most 25 Mpa.

The method according to the invention for producing a coated flat steel product suitable for press hardening, having particularly good aging resistance, comprises the following working steps:

a) providing a slab or thin slab consisting of (in weight%): 0.10-0.4% C, 0.05-0.5% Si, 0.5-3.0% Mn, 0.01-0.2% Al, 0.005-1.0% Cr, 0.001-0.2% V, ≦ 0.1% P, ≦ 0.05% S, ≦ 0.02% N and optionally one or more of "B, Ti, Nb, Ni, Cu, Mo, W" in the following amounts, B: 0.0005 to 0.01%, Ti: 0.001-0.1%, Nb: 0.001-0.1%, Ni: 0.01-0.4%, Cu: 0.01-0.8%, Mo: 0.002-1.0%, W: 0.001-1.0%, the balance being iron and unavoidable impurities;

b) heating the slab or slab sufficiently at a temperature of 1100-;

c) optionally pre-rolling the substantially heated slab or slab to an intermediate product having an intermediate product temperature (T2) of 1000-1200 ℃;

d) hot rolling to a hot rolled flat steel product, wherein the end point rolling temperature (T3) is 750-1000 ℃;

e) optionally winding the hot-rolled flat steel product, wherein the winding temperature (T4) is at most 700 ℃;

f) descaling the hot-rolled flat steel product;

g) optionally cold rolling the flat steel product, wherein the cold rolling degree is at least 30%;

h) annealing the flat steel product at an annealing temperature (T5) of 650-;

i) cooling the flat steel product to a pre-cooling temperature (T6), which is 600-:

j) coating the flat steel product with a corrosion protection coating;

k) cooling the coated flat steel product to room temperature, wherein the cooling takes place at a maximum average cooling rate of 25K/s (CR1) in a temperature range between 600 ℃ and 450 ℃ and at a maximum average cooling rate of 20K/s (CR2) in a temperature range between 400 ℃ and 220 ℃;

l) optionally flattening (Dressieren) the coated flat steel product.

In working step a), a semifinished product is provided which is formed in accordance with the invention and corresponds to the alloy specified for the flat steel product. This may be a slab produced in conventional slab continuous casting or thin slab continuous casting.

In working step b), the semifinished product is heated sufficiently at a temperature (T1) of 1100-1400 ℃. If the semifinished product is cooled after casting, the semifinished product is first reheated to 1100-1400 ℃ in order to be sufficiently heated. The sufficient heating temperature should be at least 1100 ℃ to ensure good deformability of the subsequent rolling process. The heating temperature should not be greater than 1400 ℃ enough to avoid a partially molten phase in the semifinished product.

In an optional working step c), the semifinished product is pre-rolled to form an intermediate product. The thin slabs are generally not subjected to pre-rolling. Thick slabs which should be rolled into hot strip can be subjected to pre-rolling when required. In this case, the temperature of the intermediate product (T2) at the end of the pre-rolling should be at least 1000 ℃, so that the intermediate product contains sufficient heat for the subsequent working steps of the final rolling. However, high rolling temperatures can also promote grain growth during the rolling process, which adversely affects the mechanical properties of the flat steel product. In order to keep the grain growth small during the rolling process, the temperature of the intermediate product at the end of the pre-rolling should be no more than 1200 ℃.

In working step d), the slab or thin slab is rolled to a hot-rolled flat product, or if working step c) is carried out, the intermediate product is rolled to a hot-rolled flat product. If working step c) is carried out, the intermediate product is subjected to final rolling directly after the pre-rolling. Typically, rolling is continued at the latest 90s after the end of the pre-rolling. The slab, the thin slab or the intermediate product, if the work step c) is carried out, is rolled at the end-rolling temperature (T3). The end rolling temperature, i.e. the temperature of the flat steel product that is finished hot rolled at the end of the hot rolling process, is 750-. When the finish rolling temperature is less than 750 ℃, the amount of free vanadium is reduced because a larger amount of vanadium carbide is precipitated. The vanadium carbide precipitated in the final rolling is very large. They typically have an average particle size of 30nm or more and are no longer soluble during subsequent annealing, for example, before hot dip plating is performed. The end-point rolling temperature is limited to a value of at most 1000 ℃ to prevent the coarsening of austenite grains. Furthermore, an end rolling temperature of up to 1000 ℃ is crucial for setting a winding temperature (T4) of less than 700 ℃.

The hot rolling of the flat steel product can be carried out as continuous hot strip rolling or as reverse rolling. Working step e) is provided for the case of continuous hot strip rolling with optional winding of the hot rolled flat steel product. For this purpose, the hot strip is cooled to the winding temperature after hot rolling in less than 50 seconds (T4). For this purpose, for example, water, air or a combination of both can be used as cooling medium. The winding temperature (T4) should be up to 700 ℃ to avoid the formation of large vanadium carbides. The winding temperature has in principle no lower limit. However, a winding temperature of at least 500 ℃ has proven to be advantageous for cold rollability. The wound hot strip is then air cooled to room temperature in the conventional manner.

In working step f), the hot-rolled flat steel product is descaled in a conventional manner by pickling or by other suitable treatment.

The hot-rolled flat steel product cleaned of scale can be subjected to an optional cold rolling prior to the annealing treatment in working step g), for example to meet the high requirements of the flat steel product with regard to thickness tolerances. The cold reduction (KWG) should be at least 30% in order to introduce sufficient deformation energy into the flat steel product for rapid recrystallization. Here, the cold rolling degree KWG is understood as the quotient of the reduction in thickness Δ dKW in the cold rolling divided by the hot strip thickness d:

KWG＝ΔdKW/d

where Δ dKW is the reduction in thickness in mm during cold rolling and d is the hot strip thickness in mm, where the reduction in thickness Δ dKW is the difference between the thickness of the flat steel product before cold rolling and the thickness of the flat steel product after cold rolling. The flat steel product before cold rolling is usually a hot strip of hot strip thickness d. The flat steel product after cold rolling is often also referred to as cold strip. The degree of cold rolling can in principle take very high values of over 90%. However, a cold reduction of up to 80% has proven advantageous for avoiding strip cracking.

In the working step h), the flat steel product is subjected to an annealing treatment at an annealing temperature (T5) of 650-900 ℃. For this purpose, the flat steel product is first heated to the annealing temperature in 10 to 120s and then held at the annealing temperature for 30 to 600 s. The annealing temperature is at least 650 c, preferably at least 720 c, to keep the vanadium in solution. Thermodynamically, it is observed that vanadium carbides are precipitated at a V content of 0.002 wt.% and at temperatures above 650 ℃, or that already formed vanadium carbides are no longer soluble. However, very fine vanadium carbide is thermodynamically unstable due to its high surface energy. This effect is utilized in the present invention to dissolve vanadium or to keep already dissolved vanadium in solution at temperatures of 650-. At annealing temperatures above 900 ℃, the aging stability is not improved, so that the annealing temperature is also limited to 900 ℃ for economic reasons.

In working step i), after annealing, the flat steel product is cooled to a pre-cooling temperature (T6) to prepare it for a subsequent coating process. The pre-cooling temperature is less than the annealing temperature and is matched to the temperature of the coating bath. The pre-cooling temperature is 600 ℃ and 800 ℃, preferably at least 640 ℃, particularly preferably at most 700 ℃. The duration of the cooling of the annealed flat steel product from the annealing temperature T5 to the pre-cooling temperature T6 is preferably 10 to 180 s.

The flat steel product is subjected to a coating treatment in work step j). The coating treatment is preferably carried out by means of continuous hot dip coating. The coating can be applied to only one, both or all sides of the flat steel product. The coating treatment is preferably carried out as a hot dip coating process, in particular as a continuous process. Here, the flat steel product is usually contacted on all sides with the molten bath, so that all sides are coated. The molten bath containing the alloy to be applied to the flat steel product in liquid form typically has a temperature (T7) of 640-720 ℃. Aluminium-based alloys have proven to be particularly suitable for coating ageing-resistant flat steel products with corrosion-protective coatings. The molten bath containing the corrosion protection coating in liquid form to be applied to the flat steel product typically contains, in addition to aluminum, 3 to 15% by weight of silicon, preferably 9 to 12% by weight of silicon, up to 5% by weight of iron and up to 0.5% by weight of unavoidable impurities, the sum of the above components amounting to 100% by weight. The unavoidable impurities are here, for example, unavoidable fractions of chromium, manganese, calcium or tin.

After the coating process, the coated flat steel product is cooled to room temperature in work step k). The cooling rate is set in such a way that as large a proportion as possible of the supersaturated dissolved carbon can be bound by the vanadium. The average cooling rate (CR1) should therefore lie in a temperature range which is optimal for the precipitation kinetics of vanadium, which temperature range lies between 600 ℃ and 450 ℃ for a flat steel product having the composition according to the invention, and which average cooling rate (CR1) is at most 25K/s, preferably at most 18K/s, particularly preferably at most 12K/s.

The scale of free carbon binding by vanadium increases when cooled at a lower cooling rate in the temperature range of 400 ℃ to 220 ℃ than in the temperature range of 600 ℃ to 450 ℃. Therefore, the average cooling rate (CR2) between 400 ℃ and 220 ℃ should be at most 20K/s, preferably 14K/s, particularly preferably at most 9.5K/s. In the temperature range between 400 ℃ and 220 ℃, the free carbon of the flat steel product also has a diffusion rate sufficient to recombine with vanadium, which favours the recombination of the free carbon. Furthermore, in this temperature range, the driving force for the growth of vanadium carbides is particularly high, whereby free carbon is also incorporated. This applies in particular to V contents of 0.002 to 0.009 wt.%.

Furthermore, the driving force for the formation of iron carbide, which is preferably produced on already existing carbides of microalloying elements, such as vanadium, niobium or titanium, is particularly high in the temperature range between 400 ℃ and 220 ℃. Free carbon is also bound by the formation of iron carbide, which advantageously contributes to the aging behavior. In the temperature ranges between the annealing temperature and 600 ℃, between 450 ℃ and 400 ℃ and between 220 ℃ and room temperature, the cooling rate has no significant effect on the aging resistance. For process-technical reasons, an average cooling rate of at most 25K/s is preferably set between the annealing temperature and 600 ℃ and between 450 ℃ and 400 ℃, and an average cooling rate of at most 20K/s is set between 220 ℃ and room temperature. For economic reasons it is preferred that the average cooling rates in the respective temperature ranges are each at least 0.1K/s. The average cooling rate is understood here to mean a calculated average cooling rate, which is mathematically derived from the quotient of the temperature difference of the observed cooling temperature range divided by the time required for cooling in this temperature range. This is for example for cooling from temperature TX to temperature TY: where TX is the temperature at the beginning of cooling (in K), TY is the temperature at the end of cooling (in K), and Δ t is the duration of cooling from TX to TY (in seconds).

In principle, the cooling can be carried out at any rate, since the proportion of free carbon decreases continuously, which improves the aging tendency. For technical and economic reasons, the cooling rate of the entire cooling process, i.e. the cooling of the coated flat steel product after it emerges from the coating bath until room temperature is reached, can be limited downward to a typical value of at least 0.1K/s.

The corrosion protective coating on the steel substrate after cooling typically contains 3 to 15 wt.% silicon, preferably 9 to 12 wt.% silicon, particularly preferably 9 to 10 wt.% silicon, up to 5 wt.% iron, up to 0.5 wt.% unavoidable impurities and the balance aluminum. The unavoidable impurities can be, for example, unavoidable chromium, manganese, calcium or tin fractions. The composition of the coating can be determined, for example, by means of glow discharge spectroscopy (GDOES).

Alternatively, the coated flat steel product may be subjected to a flattening of up to 2% to improve the surface roughness of the flat steel product.

The flat steel product produced according to the invention is suitable for press hardening and has a corrosion protection coating, a uniform elongation Ag of at least 11.5% and a continuous yield strength or a pronounced yield strength in which the difference between the upper yield strength and the lower yield strength is at most 45 MPa.

In a preferred embodiment, the continuous or lower yield strength is at least 380MPa, preferably at least 400MPa, in particular more than 400MPa, particularly preferably at least 410MPa and very particularly preferably at least 420 MPa.

In a further preferred embodiment, the flat steel product has a tensile strength of at least 540MPa, particularly preferably at least 550MPa and very particularly preferably at least 560 MPa.

Detailed Description

The invention is further illustrated by the following examples.

In order to confirm the effect of the present invention, various experiments were performed. For this purpose, slabs with the compositions given in table 1, having a thickness of 200-. The manufacturing parameters are given in table 2. The slabs were removed from the furnace at the respective full heating temperature T1 and subjected to hot rolling. The test was carried out as continuous hot strip rolling. For this purpose, the slab is first pre-rolled to form an intermediate product having a thickness of 40mm, wherein the intermediate product has a respective intermediate product temperature T2 at the end of the pre-rolling phase, which intermediate product can also be referred to as a pre-strip in the hot strip rolling. The pre-strip is passed directly to the final rolling after the pre-rolling, so that the intermediate product temperature T2 corresponds to the rolling start temperature for the final rolling stage. The pre-strip was rolled into hot strip with a final thickness of 3-7mm and the respective end rolling temperature T3 given in table 2, cooled to the respective winding temperature and wound into a coil at the respective winding temperature T4 and then cooled in still air. In the hot strip having the characteristics in Table 2Before the cold rolling of the given cold rolling degree, the hot strip is descaled in a conventional manner by means of pickling. The cold-rolled flat steel products were heated in the continuous annealing furnace to the respective annealing temperature T5 and were each held at the annealing temperature 100s before they were cooled to their respective pre-cooling temperature T6 at a cooling rate of 1K/s. The cold strips with their respective pre-cooling temperatures T6 were delivered through a molten coating bath at a temperature T7. The composition of the coating bath was as follows: 9% by weight of Si, 2.9% by weight of Fe, 87.8% by weight of aluminum and the balance unavoidable impurities. After coating, the coated strip was blown in a conventional manner, thereby producing 150g/m²The coated overlay of (1). The strip was first cooled to 600 ℃ at an average cooling rate of 10-15K/s. During the further cooling between 600 ℃ and 450 ℃ and between 400 ℃ and 220 ℃, the strip was cooled with the respective cooling rates CR1 and CR2 given in table 2. Cooling the strip at a cooling rate of 5-15K/s each between 450 ℃ and 400 ℃ and below 220 ℃.

After cooling to room temperature, the reaction mixture was cooled to room temperature according to DIN EN ISO 6892-1: 2009-12, samples are taken from the cooled steel strip transversely to the rolling direction. According to DIN EN ISO 6892-1: 2009-12 tensile testing was performed on the samples. The results of the tensile test are given in table 3. The following material characteristic values were determined in the range of the tensile test: yield strength classes, which are indicated with Re as apparent yield strength, with Rp as continuous yield strength, and with the value of the elongation limit Rp0.2 in continuous yield strength, the values of the lower yield strength ReL in apparent yield strength, the upper yield strength ReH and the difference Δ Re between upper and lower yield strength, tensile strength Rm, uniform elongation Ag and elongation at break a 80. All samples have a continuous yield strength Rp or only a slight apparent yield strength with a difference Δ Re between the upper and lower yield strengths of up to 41MPa and a uniform elongation Ag of at least 11.5%. Here, there is a continuous yield strength Rp for samples 8, 12-17, 19, 21, 22, and 24 and a significant yield strength Re for samples 1-7, 9-11, 18, 20, and 23. The yield strength values given in table 3 for samples 1-7, 9-11, 18, 20 and 23 with significant yield strengths are the values for the lower yield strength ReL determined over the range of tensile tests. The values given in table 3 for samples 8, 12 to 17, 19, 21, 22 and 24 with continuous yield strength are the values of the elongation limit rp0.2 determined in the tensile test range.

Claims

1. A flat steel product coated with an aluminium-based alloy suitable for press-quenching,

wherein the steel of the steel substrate of the flat steel product consists of, in weight-%:

C：0.10-0.4％，

Si：0.05-0.5％，

Mn：0.5-3.0％，

Al：0.01-0.2％，

Cr：0.18-1.0％，

V：0.002-0.009％，

P：≤0.1％，

S：≤0.05％，

N：≤0.02％，

and one or more optional elements selected from B, Ti, Nb, Ni, Cu, Mo and W

B：0.0005-0.01％，

Ti：0.001-0.1％，

Nb：0.001-0.1％，

Ni：0.01-0.4％，

Cu：0.01-0.8％，

Mo：0.002-1.0％，

W：0.001-1.0％，

The balance being iron and unavoidable impurities, and wherein,

the flat steel product has a continuously extending yield strength (Rp0.2), or the yield strength has a difference (Delta Re) between an upper yield strength value (ReH) and a lower yield strength value (ReL) of at most 45 MPa.

2. A flat steel product according to claim 1, characterised in that it has a uniform elongation Ag of at least 11.5%.

3. A flat steel product according to claim 1, characterised in that the steel of the steel substrate of the flat steel product has a carbon content of at most 0.3% by weight.

4. Flat steel product according to any one of the preceding claims, characterized in that the corrosion protection coating on the steel substrate consists of 3-15 wt.% silicon, up to 5 wt.% iron, up to 0.5 wt.% unavoidable impurities and the balance aluminium.

5. A method for manufacturing a flat steel product suitable for hot forming, comprising the steps of:

a. providing a slab or thin slab consisting of (in weight%): 0.10-0.4% C, 0.05-0.5% Si, 0.5-3.0% Mn, 0.01-0.2% Al, 0.18-1.0% Cr, 0.002-0.009% V, ≦ 0.1% P, ≦ 0.05% S, ≦ 0.02% N, and optionally one or more elements "B, Ti, Nb, Ni, Cu, Mo, W" in the following amounts, B: 0.0005 to 0.01%, Ti: 0.001-0.1%, Nb: 0.001-0.1%, Ni: 0.01-0.4%, Cu: 0.01-0.8%, Mo: 0.002-1.0%, W: 0.001-1.0%, the balance being iron and unavoidable impurities;

b. heating the slab or slab sufficiently at a temperature of 1100-;

c. optionally pre-rolling the substantially heated slab or slab to an intermediate product having an intermediate product temperature (T2) of 1000-;

d. hot rolling to a hot rolled flat steel product, wherein the end point rolling temperature (T3) is 750-1000 ℃;

e. optionally winding the hot-rolled flat steel product, wherein the winding temperature (T4) is at most 700 ℃;

f. descaling the hot-rolled flat steel product;

g. optionally cold rolling the flat steel product, wherein the cold rolling degree is at least 30%;

h. annealing the flat steel product at an annealing temperature (T5) of 650-;

i. cooling the flat steel product to a pre-cooling temperature (T6), wherein the pre-cooling temperature is 600-;

j. coating the flat steel product with a corrosion protection coating by means of continuous hot dip coating with an aluminium-based alloy;

k. cooling the coated flat steel product to room temperature, wherein cooling is carried out at an average cooling rate of at most 25K/s (CR1) in a temperature range between 600 ℃ and 450 ℃ and at an average cooling rate of at most 20K/s (CR2) in a temperature range between 400 ℃ and 220 ℃ and at a lower cooling rate in the temperature range between 400 ℃ and 220 ℃ than in the temperature range between 600 ℃ and 450 ℃;

optionally flattening the coated flat steel product.

6. The method according to claim 5, characterized in that the annealing temperature (T5) in working step h is at least 720 ℃.

7. The method according to claim 5, characterized in that the average cooling rate (CR1) between 600 ℃ and 450 ℃ is at most 18K/s.

8. The method according to claim 5, characterized in that the average cooling rate (CR2) between 400 ℃ and 220 ℃ is at most 14K/s.

9. The method according to claim 8, characterized in that the average cooling rate (CR2) between 400 ℃ and 220 ℃ is at most 9.5K/s.

10. Method according to any one of claims 5 to 9, characterized in that the molten bath containing the corrosion protection coating in liquid form to be applied to the flat steel product contains, in addition to aluminium, 3-15% by weight of silicon, up to 5% by weight of iron and up to 0.5% by weight of unavoidable impurities, wherein the sum of the above components is 100% by weight.