EP4314386A1

EP4314386A1 - Coated flat steel product and method for the production thereof

Info

Publication number: EP4314386A1
Application number: EP22713421.0A
Authority: EP
Inventors: Daniel SCHWANKE; Stefan BIENHOLZ
Original assignee: ThyssenKrupp Steel Europe AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2021-03-24
Filing date: 2022-03-14
Publication date: 2024-02-07
Also published as: US20240200199A1; WO2022200099A1; CN117157429A; DE102021107330A1; JP2024516505A

Abstract

The invention relates to a flat steel product having a tensile strength R_m of at least 800 MPa, which is coated with a metal cover, the metal cover consisting of a system comprising the elements zinc and manganese, which has been deposited from the gas phase. The invention also relates to a method for the production thereof.

Description

Coated flat steel product and method for its manufacture

Technical Field

The invention relates to a flat steel product with a tensile strength R _m of at least 800 MPa, which is coated with a metal coating, and a method for its production.

Technical background

Steels with metal coatings that are deposited from the gas phase are generally known from the prior art, for example, they are described in German Offenlegungsschrift DE 1 621 376 A. Comparable metal coatings deposited from the gas phase, which are intended for hot forming, in which the steels or the components produced by means of hot forming and (press) hardening have a final tensile strength of 1500 MPa and more, are from German laid-open specifications DE 10 2014 004 652 A1 and DE 10 2018 128 131 A1.

In the case of high-strength multi-phase steels, especially so-called quench and partitioning steels (Q&P), zinc-containing metal coatings increasingly crack formation due to liquid metal embrittlement, also known as liguid metal embrittlement (LME), during resistance joint welding (WPS). There are countless publications on LME-induced crack formation during the WPS, including the international disclosure documents WO 2017/234839 A1 and WO 2018/234938 A1 dealing with the LME in connection with WPS, which describe high-strength Q&P steels with zinc coatings. From these publications it can also be seen that the alloying element silicon, chromium and molybdenum with increasing content in the steel sheet favors the LME, so that it is proposed here to carry out annealing to adjust the microstructure Q&P in an atmosphere in which a targeted te Dew point control adjusts the partial pressure of the oxygen in such a way that the aim is to diffuse oxygen into the steel sheet during the annealing phase, thereby binding silicon to form silicon dioxide in the area close to the surface of the steel sheet, and thereby reducing the elemental silicon content under the zinc coating in the steel sheet and thereby in turn, the resistance to LME can be increased. The teaching of these publications is aimed at a defined setting of the alloying elements silicon, chromium and molybdenum and thus the corresponding structure in the range from 0 to 100 μm of the steel sheet under the zinc coating in order to provide a high resistance to LME on the zinc-coated Q&P steel be able. It is only mentioned in passing that, according to an alternative, the zinc coating can also be deposited from the gas phase. It is also known from the prior art that the LME sensitivity of steels increases with increasing strength in connection with zinc-based coatings and thus causes problems with the WPS as a result of cracking due to the fact that zinc in the coating liquefies during the WPS, into the Substrate penetrates and can deposit at the grain boundaries of the steel, making it more susceptible to brittle fracture and leading to premature failure when stressed in later use.

Other approaches are known for suppressing the LME problem by increasing the contact surfaces of the welding electrode or by modifying the combination of material and sheet metal thickness, see US Pat. No. 9,333,588 B2.

Summary of the Invention

The invention is therefore based on the object of providing a flat steel product with a tensile strength R _m of at least 800 MPa in conjunction with a metal coating and specifying a corresponding method for its production, with which an LME-induced cracking tendency during the WPS can be reduced without corresponding Having to take measures and/or adjustments in ongoing (standard) processes, as described in the prior art.

According to a first aspect of the invention, this object is achieved by a flat steel product with the features of patent claim 1.

According to the invention, a steel flat product with a tensile strength R _m of at least 800 MPa is provided, which is coated with a metal coating, the metal coating consisting of a system with the elements zinc and manganese, which is (was) separated from the gas phase.

It is essential for the invention that the metal coating consists of a system with the elements zinc and manganese, whereby zinc contributes to cathodic corrosion protection and manganese has a positive influence on the LME cracking tendency of the steel (substrate), since the presence of manganese in the System of the metal coating, the melting temperature of the system he can be increased, which in turn can lead to a reduced and / or delayed melting of the system in the WPS. The susceptibility to brittle fracture can be reduced as a result. Furthermore, it has surprisingly been found that the metal coating deposited from the gas phase typically does not provide any hydrogen due to the process, which can arise due to the process in other coating processes, in particular in the case of an electrolytic coating, and can be stored in the metal grid. In the case of steels with tensile strengths of at least 800 MPa and higher, the stored hydrogen can lead to hydrogen-induced brittle fractures.

The principle of deposition from the gas phase, for example CVD (chemical vapor deposition) or PVD (physical vapor deposition) processes, is prior art. The PVD process is preferred. This technology is not to be confused with the application of coatings by electrolytic plating, nor with the application of coatings by hot dip coating.

The flat steel product according to the invention has a tensile strength R _m of at least 800 MPa, in particular at least 850 MPa, preferably at least 910 MPa, preferably at least 950 MPa. The tensile strength R _m of the flat steel product according to the invention is a maximum of 1700 MPa, in particular a maximum of 1600 MPa, preferably a maximum of 1520 MPa, preferably a maximum of 1490 MPa. The tensile strength R _m can be determined in a tensile test according to DIN EN ISO 6892-1:2017. The flat steel product according to the invention is used exclusively for cold forming applications and not for hot forming applications (including hardening), so that the corresponding properties are already present in the flat steel product before cold forming.

According to one configuration, the system has a layer made from a zinc-manganese alloy. The system is thus deposited from the gas phase in one step and a layer of a zinc-manganese alloy is produced on the flat steel product. The metal coating on the steel flat product thus consists of a single-layer alloy of zinc and manganese, deposited from the gas phase. In particular, the deposition is controlled in such a way that a zinc content of between 10 and 90% by weight and a manganese content of between 90 and 10% by weight are set in the zinc-manganese alloy. A manganese content of at least 10% by weight is required to ensure a reduced LME cracking tendency during the WPS, the content being in particular at least 20% by weight, preferably at least 30% by weight, preferably at least 40% by weight. Conversely, the manganese content in the alloy (layer) is limited to a maximum of 90% by weight, so that the metal coating tive the system can ensure adequate cathodic corrosion protection with at least 10% by weight, in particular at least 20% by weight, preferably at least 30% by weight, preferably at least 40% by weight zinc, since the metal coating or the system consists of of the gas phase with a thickness of between 0.5 and a maximum of 20 μm, in particular a maximum of 15 μm, preferably a maximum of 10 μm, on the flat steel product. The higher the zinc content in the system or in the (relatively thin) metal coating, the higher the cathodic protection against corrosion.

According to an alternative embodiment, the system has a manganese layer and a zinc layer. The metal coating is therefore two-layered and consists of a zinc and a manganese layer, each deposited from the gas phase. The system is deposited in two steps by first depositing a manganese layer on the steel flat product and then a zinc layer on the manganese layer. Thus, the layer of manganese is arranged on the flat steel product and the layer of zinc is arranged on the layer of manganese. Both layers can each be deposited with a thickness between 0.5 and a maximum of 20 μm, in particular a maximum of 15 μm, preferably a maximum of 10 μm, preferably a maximum of 7 μm.

Compared to the single-layer system, the two-layer system has the advantage that the zinc outer layer provides complete and full cathodic protection and the manganese inner layer provides a complete barrier during WPS. The disadvantage compared to the single-layer system is that two separate gas-phase stages have to be run through in order to deposit the layers successively.

According to one embodiment, the flat steel product can either be hot-rolled or cold-rolled. It depends on the purpose of use. The hot-rolled flat steel product (hot strip) can have a thickness of between 1.5 and 10 mm. The cold-rolled flat steel product (cold strip) can have a thickness of between 0.5 and 4 mm. The process, starting with the casting of a melt, in particular with a chemical composition which is listed below as preferred, into a preliminary product, and heating the preliminary product to a temperature so that it can be hot-rolled into a flat steel product, is current of the technique. If the required minimum tensile strengths are already set in the hot strip, a person skilled in the art is familiar with a corresponding procedure. If a cold-rolled steel flat product with a minimum tensile strength of 800 MPa is to be produced from the hot strip, this is also state of the art, the hot strip in particular to be subjected to pickling before being cat-rolled into a cold-rolled strip. The desired properties are set in a subsequent annealing process. The essence of the invention is not the manufacture of the steel flat products with a tensile strength of at least 800 MPa, but rather a suitable coating concept which, in the case of steels in the specified tensile strength class of 800 MPa and higher, can counteract the special LME susceptibility of these tensile strength classes in WPS.

The steel flat product contains at least two different phases in the microstructure. The microstructure thus contains at least two components of ferrite, pearlite, martensite, bainite, austenite, residual austenite and/or cementite, as well as microstructure components that are unavoidable during production. These include, for example, dual-phase steels (DP steels), which have a microstructure of a mixture of hard phases, such as martensite, and soft phases, such as ferrite. Complex-phase steels (CP steels) mainly contain phases with a medium hardness, such as bainite and/or (tempered) martensite, optionally in connection with precipitation hardening. Quench&Partitioning (QP steels) mainly contain martensite (including tempered martensite) and retained austenite. Alternatively or additionally, precipitations can be present in the structure.

According to one embodiment, the flat steel product contains, in addition to Fe and unavoidable production-related impurities in % by weight

C: 0.001 to 0.50%,

Mn: 0.10 to 3.0%,

Si: 0.01 to 2.0%,

AI: 0.002 to 1.5%,

P: up to 0.020%,

S: up to 0.020%,

N: up to 0.020%, optionally with one or more alloying elements from the group (Ti, Nb, V, Cr, Mo, W, Ca, B, Cu, Ni, Sn, As, Co, 0, H).

Ti: up to 0.20%,

Nb: up to 0.20%,

V: up to 0.20%, Cr: up to 2.0%,

Mon: up to 2.0%,

W: up to 1.0%,

Approx: up to 0.050%,

B: up to 0.10%,

Cu: up to 1.0%,

Ni: up to 1.0%,

Sn: up to 0.050%,

As: up to 0.020%,

Co: up to 0.50%,

0: up to 0.0050%,

H: up to 0.0010%.

According to a second aspect of the invention, the object is achieved by a method having the features of patent claim 8.

The process for producing a metal-coated steel flat product with a tensile strength R _m of at least 800 MPa comprises the steps:

- providing a hot-rolled or cold-rolled steel flat product;

- Coating of the steel flat product with a metal coating.

According to the invention, the metal coating consists of a system with the elements zinc and manganese and is deposited on the flat steel product from the gas phase.

Description of the Preferred Embodiments

An assessment of the extent to which the cracks are detrimental to the function of the component cannot be made precisely when considering a WPS joint point. The avoidance or at least a significant reduction in cracks in the WPS is therefore of great importance for the application.

From a melt consisting of Fe and unavoidable production-related impurities in % by weight of C = 0.25%, Si = 1.5%, Mn = 2.2%, Al = 0.03%, Cr = 0.7 %, P = 0.005%, a preliminary product was cast, which was first hot and rolled into a steel flat product subsequently cold rolled to a thickness of 1.5 mm. The cold-rolled flat steel product was subjected to a Q&P process, in which a structure consisting essentially of martensite (including tempered martensite) / bainite and 9% retained austenite (RA) as well as structural components that were unavoidable due to production was set. Samples were taken from the flat steel product produced in this way,

- which a) have been left uncoated,

- which b) were hot-dip coated on both sides with a zinc coating (Z) with 7 μm each, whereby the RA dropped to 7%, of which some of the samples bl) an additional heat treatment (ZF) at approx. 630 °C for approx 15 s and due to heat treatment/diffusion the RA further decreased to 3%,

- which c) have been electrolytically coated on both sides with a zinc coating (ZE) with 6 μm,

- which d) was deposited with a zinc-manganese alloy (ZnMn-PVD) via the gas phase simultaneously with zinc and manganese on both sides with 6 pm, the deposition being controlled in such a way that a single-layer system with 60% by weight zinc and 40% by weight manganese gave

- Which e) was first deposited with a layer of manganese (Mn-PVD) via the gas phase on both sides with 2 μm and then on the layer of manganese with a layer of zinc (Zn-PVD) via the gas phase on both sides with 4 μm was deposited, resulting in a two-layer system with a Mn-Zn PVD coating on the samples.

Another sample was taken from the steel flat product and submitted to the tensile test according to DIN EN ISO 6892-1:2017. A tensile strength R _m of 1183 MPa was determined. The steel flat products or samples were coated on a laboratory scale but with the parameters of the large series with the metal coatings b) to e) described in each case.

Due to the naturally occurring scattering in WPS investigations of LME-induced cracking, large amounts of material would normally have to be used in many series of measurements. Due to the poor quantification of the LME associated measurement variables, Within WPS investigations, only qualitative statements on the LME sensitivity of steels can be determined. The high material requirement would disqualify the current status of the test for use in the laboratory. Therefore, a laboratory-scale test and optimization concept was developed in the form of an "LME Gleeble hot tensile test". The test was carried out on a commercially available Gleeble3500 test device. The process control variables used corresponded to the thermomechanical loads acting on the WPS in the area of crack formation. The pulling speed used was 3 mm/s for a measuring length of 10 mm. In order to determine the real strain values in the measurement area of the samples, the strain was measured without contact using a laser. The heating rate was 1000 K/s. The liguidus phase of zinc between 500 and 900 °C in 100 °C steps was used as the temperature interval.

Hot tensile tests were carried out for all samples a) to e). After mounting in the test fixture, the test chamber was closed and a pre-programmed script was executed as follows. The measurement frequency during the hot tensile tests was at least 5,000 Hz. The specimen was heated conductively and after the specimen had reached the test temperature in the aforementioned temperature window of between 500 and 900 °C, the specimen was stretched at the specified tensile rate until it failed. The quality of the measurement data collected was then checked using the Origin analysis software. The evaluation routine of the hot tensile tests was based on the standard for tensile tests [DIN EN ISO 6892-1:2017]. The raw data from successfully conducted hot tensile tests were converted into a cubic function with the aid of a computer. The necessary support points and the technical failure time of the samples were entered into an evaluation module by the system operator.

From the individual measurement data, the changes in mechanical properties and fracture behavior were recorded as a function of temperature. For better comparability of the influence of the different metal coatings, so-called relative change curves were generated from the absolute measured values. The reference variables of the change curves were basically the measurement results of the uncoated samples a). The size of the changes caused by the respective metal coatings were determined as a function of temperature and used as a measure of the intensity of the LME effect.

For samples b) with Z, a strong reduction in the technical breaking point was determined for all test temperatures. In particular, the elongation value of the technical breaking point in the Reduced by >85% compared to samples a). The samples bl) with ZF showed no significant changes in the technical breaking point at the test temperatures of 500 and 600 °C. This reduction was very pronounced at the remaining test temperatures (700-900 °C). The samples c) with ZE showed comparable behavior to samples b). For samples d) with Zn/Mn alloy PVD, no significant elongation value of the technical breaking point was determined for all test temperatures, with the technical breaking point being approx. <10% lower compared to samples a). The samples e) with Mn-Zn-PVD were the result in the order of magnitude of the samples d).

The change in plastic energy absorption showed similar results as the change in technical breaking point. The results confirmed the negative influences of Z of samples b). Similarly, ZF of samples bl) at the test temperatures of 500 and 600° C. showed no limitations in the plastic energy absorption capacity. This reduction was also very pronounced here at the remaining test temperatures between 700 and 900 °C. The samples c) showed comparable behavior as the samples b). In the case of samples d), a slight reduction in the plastic energy absorption capacity was discernible at the test temperatures between 600 and 800 °C. The remaining test temperatures showed hardly any influence from the metal coating. Samples e) were also in the order of magnitude of samples d).

The change in reduction of area at break also showed similar results, as did the change in technical breaking point and plastic energy absorption capacity. When considering the area of reduction in fracture, it should be noted that, in contrast to technical specimen failure and the plastic energy absorption capacity, it is a local measurement variable.

The results confirm the negative influence of Z of the specimens b) on the reduction in area due to brittle fracture surfaces throughout for all test temperatures. In the ZF of samples bl) there were no reductions in reduction of area at the test temperatures of 500 and 600 °C. However, from a test temperature of 700 °C, strong brittle fracture behavior was detected at the fracture surface. Here, too, samples c) with ZE behaved similarly to samples b). In samples d) with Zn/Mn-PVD, a slight reduction in reduction of area at fracture was evident at a test temperature of 800 °C. For the test temperatures of 600, 800 and 900 °C, cracks were detected behind the actual fracture surface. Samples e) with Mn-Zn-PVD showed comparable results to samples d). ok

The investigation of the influence of the different metal coatings on the technical breaking point, the plastic energy absorption capacity and the reduction in area of fracture show that LME-induced cracking in metal coatings containing zinc cannot be ruled out across the board.

In the hot tensile test, samples b) to e) with a wide variety of metal coatings were examined for their "LME sensitivity". The uncoated samples a) served as a reference. Apart from this, other coatings not mentioned here as well as other steel concepts can also be examined without having to go through complex and quantitative WPS examinations. In particular, all LME-sensitive steel materials with a tensile strength R _m of at least 800 MPa can be examined here.

An avoidance or reduction of crack frequency, crack depth and crack length is predicted if the change in the technical breaking point over the temperature range from 500 to 900 °C applies: f(x)=0.1375-x-58.75 or if in the test range from 500 to 900 °C when using a uniform temperature increment in the test interval, the following applies: f(x)=7.25-Vx-155 or if for the sum of all measured values in the temperature range from 500 to 900 °C when using a uniform temperature increment in Check interval:

Sf(x)/n < 40.

In the experimental WPS investigations, process parameters and material-thickness combinations are used, which lead to LME cracks with a high probability and good reproducibility in the zinc-coated sample. The thermomechanical loads applied using the Gleeble method represent the mean effective thermomechanical loads of the WPS experiments. The validation is considered successful if the "LME sensitivity" in the Gleeble method is comparatively low and the WPS investigations show significantly reduced crack frequencies and lower crack depths (or no cracks at all). Experimental WPS investigations were carried out on samples a) to e). The parameters of the WPS investigations are listed in Table 1. The sample series for the WPS investigations were produced immediately after the current intensity required to achieve the target point diameter had been determined. The welding electrodes were then milled inside the welding machine using a mobile cap milling device and conditioned with three welds. Samples showing weld spatter were discarded. The welding results were judged to be easily comparable due to the uniform point diameters, current levels and process control parameters. Table 1

The results from the WPS investigations agreed well with the predictions of the Gleeble hot tensile tests, but could not be fully quantitatively correlated due to the WPS process-related scattering of the investigation results. To improve the accuracy of the crack detection, all welded specimens were decoated prior to crack characterization. Crack characterization was carried out for all specimens on the upper side of the LME specimen using a macroscope. The frequency of cracks in a sample series was determined using the binary classification (crack/no crack). To analyze the crack morphology, the LME cracks were digitally measured and counted using three selected samples per sample series. At least three metallographic sections were made to determine the depth of the crack. The cut position was marked on the sample and led through the center of the longest crack on the surface of the joint. Furthermore, it was also necessary to determine the mean crack depth in order to confirm a successful qualitative transfer of the Gleeble findings.

The highest crack frequency and the deepest and longest cracks were expected for the Z-coated samples b). Due to the high loads acting in the defined reference welding tasks, no improvement in crack frequency was predicted by the samples bl) coated with ZF, since the determined critical temperature of 700 °C can be exceeded during the welding process at many points on the surface of the joint. The highest frequency of cracks and the longest cracks were found in the welded samples b) and bl), with the frequency of cracks being highest in bl) of series 2, but the average crack length was smaller than in samples bl) of series 1 and the samples b) series 1 and 2. These results agreed well with the strong LME effect demonstrated in the Gleeble method. The result for samples c) was also similar to that for samples b). In the welding results of both samples d) and e), no cracks were detected in series 1. In series 2, small cracks in the area of the electrode indentation were only found in samples e). One explanation for this would be that the penetrating welding electrode broke up the manganese layer in these areas and allowed the liquid zinc to penetrate. In the case of samples d), which had a small proportion of liquid zinc phases in the system of the metal coating, no cracks occurred in series 2 either. Thus, based on the results for samples d) and e), a significantly reduced crack frequency and severity in the WPS welding tests was predicted. The results from the WPS trials agree well with the predictions from the Gleeble trials, but can due to the WPS process-related scattering of the test results cannot be completely quantitatively correlated.

The WPS results, in particular of samples e) of series 2, showed that LME crack formation cannot be ruled out when welding an LME-sensitive substrate material with a zinc-containing coating, but the number and severity of cracks can be significantly reduced compared to pure zinc layers.

Claims

patent claims

1. Flat steel product with a tensile strength R _m of at least 800 MPa, determined according to DIN EN ISO 6892-1:2017, which is coated with a metal coating, the flat steel product containing at least two different phases in the structure, characterized in that the metal coating consists of a system with the elements zinc and manganese, which has been deposited from the gas phase.

2. Steel flat product according to claim 1, wherein the system comprises a layer of a zinc-manganese alloy.

3. Flat steel product according to claim 2, wherein the zinc-manganese alloy has a zinc content of between 10 and 90% by weight and a manganese content of between 90 and 10% by weight.

4. Flat steel product according to claim 1, wherein the system has a layer of manganese and a layer of zinc.

5. Steel flat product according to claim 4, wherein the layer of manganese is arranged on the steel flat product and the layer of zinc is arranged on the layer of manganese.

6. Steel flat product according to one of the preceding claims, wherein the steel flat product is hot-rolled or cold-rolled.

7. Flat steel product according to one of the preceding claims, wherein the flat steel product consists of Fe and impurities in % by weight that are unavoidable due to production

C: 0.001 to 0.50%,

Mn: 0.10 to 3.00%,

Si: 0.01 to 2.0%,

AI: 0.002 to 1.5%,

P: up to 0.020%,

S: up to 0.020%,

N: up to 0.020%, optionally one or more alloying elements from the group (Ti, Nb, V, Cr, Mo, W, Ca, B, Cu, Ni, Sn, As, Co, 0, H) with

Ti: up to 0.20%,

Nb: up to 0.20%,

V: up to 0.20%,

Cr: up to 2.0%,

Mon: up to 2.0%,

W: up to 1.0%,

Approx: up to 0.050%,

B: up to 0.10%,

Cu: up to 1.0%,

Ni: up to 1.0%,

Sn: up to 0.050%,

As: up to 0.020%,

Co: up to 0.50%,

0: up to 0.0050%,

H: up to 0.0010%.

8. Process for the production of a flat steel product coated with a metal coating with a tensile strength R _m of at least 800 MPa, determined according to DIN EN ISO 6892-1:2017, the flat steel product containing at least two different phases in the structure, comprising the steps:

- providing a hot-rolled or cold-rolled steel flat product;

- coating the steel flat product with a metal coating; characterized in that the metal coating consists of a system with the elements zinc and manganese and is deposited on the flat steel product from the gas phase.

9. The method according to claim 8, wherein the system is deposited in one step and a layer of a zinc-manganese alloy is produced on the steel flat product.

10. The method according to claim 9, wherein the deposition is controlled in such a way that a zinc content of between 10 and 90% by weight and a manganese content of between 90 and 10% by weight are set in the zinc-manganese alloy.

11. The method according to claim 8, wherein the system is deposited in two steps, in which first a layer of manganese is sequentially deposited on the steel flat product and then a layer of zinc is deposited on the layer of manganese.