Patent Application
20210301364
This application is a national stage of, and claims priority to, Patent Cooperation Treaty Application No. PCT/EP2019/068581, filed on Jul. 10, 2019, which application claims priority to DE Application No. 10 2018 118 015.2, filed on Jul. 25, 2018, which applications are hereby incorporated herein by reference in their entireties.
It is well known that metallic components can be coated for corrosion protection and formed into parts by hot forming. In practice, aluminum-silicon coated high-strength and ultra-high-strength quenched and tempered steels, in particular manganese-boron containing quenched and tempered steels such as 22MnB5 or 34MnB5, are used for safety-relevant vehicle body components.
For example, a method for producing a hot stamped coated steel section is known from WO 2009/090555 A1, comprising the steps of: precoating a steel strip with aluminum or aluminum alloy by melt dipping, wherein the thickness of the precoating is 20 to 33 micrometers on each side, cutting the precoated steel strip into a steel blank, heating the steel blank in a furnace, transferring the heated blank to a die, hot stamping the steel blank in the die, and cooling the steel blank.
From DE 10 2007 019 196 A1 a process for producing flexibly rolled strip material with a cathodic corrosion protection coating is known. The coating of the strip material is carried out at elevated strip temperature in a zinc pot (hot dipped galvanized steel). The zinc coating is rolled with the same ratio as the actual strip thickness, wherein a final coating thickness after flexible rolling of greater than or equal to 7.5 micrometers is aimed for.
A process for producing a sheet metal component is known from WO 2008/113426 A2, wherein a hot strip or cold strip is hot-dip coated or electrolytically coated and then subjected to a flexible rolling process. In the flexible rolling process, different sheet thicknesses of the flexibly rolled steel strip are produced by different rolling pressures. Depending on the sheet thickness after flexible rolling, the coating is formed to different thicknesses during the coating process, wherein the coating thickness is formed greater with increasing expected rolling pressure.
From WO 2006 097 237 A1 a process and an installation for hot-dip coating of hot-rolled steel strip is known. The steel strip passes through a pickling station, a rinsing station, a drying station, a heating furnace and then a melting bath. The finished thickness and the thickness tolerance of the hot-dip coated steel strip are achieved by a controlled thickness reduction in a rolling stand in the process line by monitoring the finish thickness at the outlet of the rolling stand by a thickness gauge and feeding back deviations from the target thickness as an actuating signal to the adjustment of the rolling stand.
From WO 2016/1981 86 A1 a process for hot forming a steel component is known. The steel component is provided with a corrosion-resistant scale protection layer and, prior to hot forming, a surface oxidation is carried out in which a corrosion-resistant oxidation layer is formed on the scale protection layer.
The present disclosure encompasses a process for producing coated hardened steel products, in particular for use as a structural component of a motor vehicle. The method for producing a coated and hardened component, in particular as a structural component for a motor vehicle, has good corrosion protection resistance in areas with different thicknesses.
A method for producing a hardened steel product comprises: providing a steel substrate with a base material of hardenable steel; coating the steel substrate with a precoating comprising aluminum to produce a pre-coated steel substrate, the coating of the pre-coated steel substrate having a thickness (d1) of at least 34 micrometers (μm); flexible rolling the precoated steel substrate such that successive portions of the precoated steel substrate are rolled out differently to produce a varying thickness along the length of the precoated steel substrate, wherein after the flexible rolling the precoating has in thinner first portions a reduced first thickness (d2a) of less than 33 micrometers and in thicker second portions a reduced second thickness (d2b) which is thicker than the reduced first thickness (d2a); working a blank out of the flexibly rolled strip material; heating the blank such that the base material of the blank is at least partially austenitized, wherein diffusion processes take place between the base material and the precoating through the heating; hot forming the heated blank, wherein the heated blank is formed and cooled rapidly such that a hardened steel product with coating is produced.
An advantage is that the substrate has a sufficiently thick coating even after flexible rolling. It has been found that during heating for subsequent hot forming the coating grows due to the diffusion processes, so that the final thickness of the coating after hot forming is greater than the respective coating thickness after flexible rolling and before heating for hot forming. Because the precoating has a thickness of at least 34 micrometers, the coating is sufficiently thick to achieve good corrosion protection even in the thinner first portions, despite the reduction in thickness that occurs during flexible rolling, due to the subsequent heating for hot forming. In the thicker portions of the finished component, which usually have to withstand higher loads, the coating is also correspondingly thicker so that these portions are particularly well protected. Overall, this results in a load-optimized and/or weight-reduced component with excellent coating protection in all thickness regions.
The steel substrate can, for example, be a hardenable and/or heat-treatable steel material, in particular a material containing manganese. It can contain other microalloying elements In addition to manganese. The steel material may, for example, contain the following proportions of alloying elements, in percent by weight respectively:
carbon (C) with at least 0.15% and at most 0.5%, in particular at most 0.4%;
manganese (Mn) with at least 0.5% and at most 5.0%, in particular at least 0.8% and at most 2.5%;
aluminum (Al) with a maximum of 0.1%;
silicon (Si) with at least 0.1% and at most 0.9%, in particular at most 0.5%;
chromium (Cr) with a minimum of 0.01% and a maximum of 1.0%;
tital (Ti) with at most 0.02%, in particular at most 0.01%;
boron (B) with at least 0.0005% and at most 0.080%, in particular at least 0.002% and at most 0.006%;
phosphorus (P) with at most 0.1%, in particular at most 0.01%;
sulfur (S) with at most 0.05%, in particular at most 0.01;
optionally other alloying elements with a content of up to 1.55% (1550 ppm);
the rest iron (Fe) and unavoidable impurities.
As optional further alloying elements, the substrate may contain, respectively in percent by weight, in particular at least one of:
copper (Cu) with at most of 0.1%;
nickel (Ni) with at most of 0.1%;
niobium (Nb) with at most of 0.1%;
molybdenum (Mo) with at most of 1.0%;
vanadium (V) with at most of 0.25%;
without being limited thereto. The mass fraction of the optional alloying elements can also be lower, for example molybdenum can also be included with at most 0.8%, 0.5% or 0.25%. The mass fraction of the optional alloying elements in total amounts to maximum 1.55%, in particular maximum 1.0%, in particular maximum 0.8%. The alloying element niobium advantageously produces a fine-grained structure of a component hot-formed from the alloy. In particular in combination with molybdenum, which can inhibit grain growth, a particularly fine-grained structure is obtained, which in turn has a favorable effect on the strength of the component made therefrom.
Examples of usable boron-manganese-containing steel materials are 17MnB5, 20MnB5, 20MnB8, 22MnB5, 26MnB5 or 34MnB5. The starting material (strip material) may have a tensile strength of at least 450 MPa, for example. A formed part made from the coated steel substrate may have a final tensile strength of, for example, at least 1100 MPa, in particular at least 1500 MPa. It is also possible for certain portions of the formed part to be configured, where necessary, to a lower tensile strength of less than 1100 MPa and higher ductility in return. The steel substrate may have an initial thickness of, for example, between 1.0 and 4.0 mm.
Preferably, the coating contains at least 85 weight percent aluminum, which includes the possibility of using a pure aluminum coating (100 weight percent Al), as well as the use of an alloy containing as main alloying component aluminum with at least 85 weight percent and optionally further alloying components, for example silicon with for example between 5 and 15 weight percent and/or iron with up to 5 weight percent and/or one or more other alloying elements in smaller proportions. The proportion of the other alloying elements, for example at least one of the group of Mn, Cr, Ti, B, P, S, Cu, Ni, Nb, Mo, V, may together be, for example, up to 1.5 weight percent. In the context of the present disclosure, due to the main component aluminum, the terms aluminum coating or aluminum-based coating are also used, and are intended to include the mentioned possibilities of other alloy compositions. The aluminum coating can be applied to the steel substrate, for example, by hot-dip coating in a molten bath containing at least 85 percent by weight of aluminum and, as the case may be, other alloying components, or by other conventional coating processes. An exemplary composition of the molten bath or the applied coating may contain up to 3 weight percent iron, 9 to 12 weight percent silicon, optionally one or more further alloying elements with a total of up to 1.5 weight percent, and remainder aluminum. It is understood that unavoidable impurities may also be present.
According to an example, the precoating is applied to the steel substrate with a thickness (d1) of at least 36 micrometers, in particular at least 40 micrometers. The steel substrate precoated in this way forms the basis for a hardened component with varying thicknesses to be produced therefrom. The coating of the steel substrate may be applied, for example, by means of hot-dip coating, wherein the steel substrate is immersed in a pool of molten coating material. It is to be understood that other known coating processes may be used as well.
The precoated steel substrate is flexibly rolled after the precoating, wherein it is to be understood that further steps, such as heating, winding on or unwinding from the coil, straightening, cleaning or the like can be interposed. Optionally, it can be provided in particular that the steel substrate is heated in the coating device after application of the precoating in order to achieve pre-diffusion between the precoating and the steel substrate. Heating for pre-diffusion is carried out at temperatures below the melting temperature of the coating material, for example in a temperature window between 0.5 and 0.9 times the melting temperature of the coating material. Due to the pre-diffusion, a thicker interdiffusion zone is already formed between the base material of the steel substrate and the coating material during the coating process. This makes it possible to carry out heating more quickly in the course of hot forming, which has an overall favorable effect on the cycle times in hot forming.
In flexible rolling, strip material with substantially uniform sheet thickness is rolled out into strip material with varying sheet thickness along its length by changing the rolling gap during the process. The portions of varying thickness produced by flexible rolling extend transversely to the longitudinal direction and rolling direction of the strip material, respectively. After flexible rolling, the strip material can be easily wound again into a coil and fed for further processing elsewhere, or it can be processed further directly, for example by cutting the strip material to length to form individual sheet elements.
Flexible rolling can be carried out with rolling degrees of at least 1% and/or a maximum of 60% starting from the initial thickness (d1) of the precoated steel substrate, in particular with rolling degrees between 3% and 55%. Flexible rolling also reduces the thickness of the precoating along with the steel substrate. Herein, after flexible rolling the precoating can comprise in thinner first portions in particular a reduced first thickness (d2a) of less than 20 micrometers. Alternatively or in addition, the flexible rolling is carried out such that after the flexible rolling the precoating comprises in thicker second portions a reduced second thickness (d2b) of more than 33 micrometers, in particular of more than 36 micrometers. It is understood that between thinnest portions and thickest portions of the strip material, depending on the desired component geometry, there may be any other thickness regions or transition regions in between.
In a process step downstream of flexible rolling, blanks are produced from the flexibly rolled strip material. This process step is also referred to as separating. The separation can be carried out by mechanical cutting or by laser cutting. In the context of the present disclosure, the term blanks is intended to encompass both rectangular metal sheets that have been cut out of the strip material and shaped cuts. Shape cuts are sheet metal elements cut out of the strip material, the outer contour of which is already adapted to the shape of the end product.
After separation, the sheet blanks are hot formed, wherein further process steps may be interposed, as required. For hot forming, the blank is heated to austenitizing temperature at least in a partial region; then placed into a hot forming tool and formed in the hot forming tool and rapidly cooled so that a hardened formed part is produced. Heating is performed in a suitable heating device, such as a continuous furnace. Heating to austenitizing temperature means a temperature range at which at least partial austenitizing takes place and/or is present, i.e. a microstructure in the two-phase region ferrite and austenite. For this, the blank is heated to a temperature above Ac1, i.e., the temperature at which the formation of austenite begins. For example, the blank may be heated to a temperature above 880° C. and/or up to 960° C. According to an example, the blank is heated at a heating rate greater than 12 K/sec for austenitizing at least until a temperature of 700° C. is reached. Rapid heating reduces the manufacturing time. After heating to austenitizing temperature and insertion into the hot forming tool, the blank is formed and rapidly cooled. Rapid cooling of the formed part in the forming tool produces a hardened, at least partially martensitic microstructure in the part. This process of hot forming and rapid cooling in a forming tool is also known as press hardening.
Through the heating and hot forming, the precoating and the underlying steel substrate form the coating, which increases compared to the thickness of the precoating due to diffusion processes. Herein, the first final thickness (d3a) in the thinner first portions of the finished component is preferably formed with more than 15 micrometers, in particular more than 20 micrometers, and less than 50 micrometers, in particular less than 40 micrometers. Alternatively, or in addition, the coating may have in the thicker second portions a second final thickness (d3b) of less than 60 micrometers, in particular less than 50 micrometers, and/or more than 30 micrometers, in particular more than 35 micrometers, after the hot forming.
Surprisingly, it has been found that the coating in the thinner regions grows more than in the thicker regions during hot forming. In particular, the coating is formed with a final thickness ratio (d3a/d3b) of the first final thickness (d3a) to the second final thickness (d3b) that is greater than an intermediate thickness ratio (d2a/d2b) of the reduced first thickness (d2a) to the reduced second thickness (d2b). In this way, the different coating thicknesses overall converge in an advantageous manner, so that good overall corrosion protection is achieved in all portions of the component.
According to a first possibility, hot forming can be carried out as an indirect process comprising the sub-steps of cold preforming, subsequent heating of the cold preformed component to austenitizing temperature, and subsequent hot forming to produce the final contour of the product. According to a second possibility, hot forming can also be carried out as a direct process, characterized by heating the component directly to austenitizing temperature and then hot forming it to the desired final contour in one step. No prior (cold) preforming takes place in this case.
According to an example, prior to forming in the forming tool, the coating can be produced such that a metal oxide layer is formed on the surface. A metal oxide layer is corrosion-resistant and inert, so that tool wear during forming is reduced. In the case that a metal oxide layer is formed on the coating surface, the layer thicknesses specified in the present disclosure for the condition after hot forming refer to the total coating thickness, i.e. including the oxide layer.
Example are explained below with reference to the drawing figures, which are as follows.
In the context of the present disclosure, the steel substrate 2 may include a hardenable steel flat product which may contain, for example, the following proportions of alloying elements, each in percent by weight:
carbon (C) with at least 0.15% and at most 0.5%, in particular at most 0.4%;
manganese (Mn) with at least 0.5% and at most 5.0%, in particular at least 0.8% and at most 2.5%;
aluminum (Al) with a maximum of 0.1%;
silicon (Si) with at least 0.1% and at most 0.9%, in particular at most 0.5%;
chromium (Cr) with a minimum of 0.01% and a maximum of 1.0%;
tital (Ti) with at most 0.02%, in particular at most 0.01%;
boron (B) with at least 0.0005% and at most 0.080%, in particular at least 0.002% and at most 0.006%;
phosphorus (P) with at most 0.1%, in particular at most 0.01%;
sulfur (S) with at most 0.05%, in particular at most 0.01%;
optionally other alloying elements with a content of up to 1.55% (1550 ppm); the rest iron (Fe) and unavoidable impurities.
This alloy composition includes, for example, steel materials containing boron-manganese, such as 17MnB5, 20MnB5, 20MnB8, 22MnB5, 26MnB5 and 34MnB5. The steel material may have an initial yield strength of, for example, 150 to 1100 MPa and/or a tensile strength of at least 450 MPa. The optional further alloying elements may be selected from the group:
copper (Cu) with a maximum of 0.1%;
nickel (Ni) with a maximum of 0.1%;
niobium (Nb) with a maximum of 0.1%;
molybdenum (Mo) with a maximum of 1.0%;
vanadium (V) with a maximum of 0.25%;
without being limited thereto, said percentages referring in each case to percent by mass of the steel substrate. One or more of the optional alloying elements mentioned may be used. The mass fraction of the optional alloying elements in total amounts to a maximum of 1.55%, in particular a maximum of 1.0%, preferably a maximum of 0.8%.
In method step S1, the steel substrate 2, which in the initial state may be wound to a coil 3, is provided with a precoating 4. In the state applied to the steel substrate, the precoating 4 contains aluminum with at least 85 percent by weight and silicon with up to 15 percent by weight. It is understood that other alloying elements may be included at the expense of the silicon content, for example iron and/or other alloying elements totaling up to 5 weight percent. The precoating 4 may be applied to the steel substrate 2 by generally known methods. A possibility is the application by a hot-dip process. In this process, the steel substrate 2 passes through a molten bath 5 of liquid coating material 4 in a coating device 6, which adheres to the surface of the substrate 2, so that a pre-coated steel substrate is produced. The melt of the coating material may contain, for example, 8 to 15% by weight of silicon, 2 to 4% by weight of iron, optionally one or more further alloying elements, such as, for example, at least one from the group of Mn, Cr, Ti, B, P, S, Cu, Ni, Nb, Mo, V, of together up to 1.5% by weight, and as the remainder aluminum, as well as unavoidable impurities.
The precoating 4 is applied to the steel substrate 2 with a thickness d1 of at least 36 micrometers, in particular at least 40 micrometers. The coating thickness d1 can have a maximum thickness of 60 micrometers, in particular up to 50 micrometers.
After application of the first coating 4, the coated steel substrate 2′ is flexibly rolled (S2). For this, the coated steel strip 2′, which before flexible rolling has a substantially constant sheet thickness D1 along its length, is rolled by rolls 7, 8 such that it acquires a varying sheet thickness D2a, D2b, D2c along the rolling direction. The coated and flexibly rolled steel substrate is marked with reference 12.
During rolling, the process is monitored and controlled, with the data determined by a sheet thickness measurement 9 being used as an input signal for controlling the rolls 7, 8. Flexible rolling is carried out in accordance with the desired target thickness profile of a blank to be cut from the strip material 12 and/or a component to be produced therefrom. Flexible rolling can be carried out with rolling degrees of at least 1% and/or at most 60% starting from the initial thickness D1 of the precoated steel substrate 2′, in particular with rolling degrees between 3% and 55%.
After flexible rolling, the strip material 12 can be rewound to a coil 3 so that it can be transported to a subsequent processing station. After the rolling process, the steel strip 12 can be straightened in a subsequent process step, which takes place in a strip straightening device. The process step of straightening is optional and can also be omitted.
After flexible rolling (S2) and straightening respectively (if provided), the coated and flexibly rolled steel strip 12 is separated in method step S3. Individual sheet blanks 22 are worked out of the steel strip 12, for example by a punching and/or cutting device 10. Depending on the shape of the sheet blanks 22 to be produced, these can be punched out of the strip material 12 as a shaped cut, with an edge which is not used further being discarded as scrap, or the strip material 12 can simply be cut to length into sections.
The blanks 22 are hot-formed in a subsequent step S4, which can also be referred to as press hardening. During hot forming, respectively press hardening, the blanks 22 are heated to a temperature that is usually above the AC1 and/or AC3 temperature of the material, for example between 750° C. to 1000° C. Heating may be accomplished by suitable methods, such as by inductive heating, conductive heating, roller hearth furnace heating, hot plate contact heating, infrared, or other known methods. After heating to austenitizing temperature, the blank 22 is then placed in a hot forming tool 11 and formed therein and cooled and/or quenched so rapidly that at least a partial martensitic hardness structure is formed in the formed part so produced.
According to a first possibility, hot forming (S4) can be carried out as a direct process. In this case, the blank 22 is heated directly to austenitizing temperature and then hot formed to the desired final contour in one step. No prior (cold) preforming takes place here. According to a second possibility, hot forming can also be carried out as an indirect process comprising the sub-steps of cold preforming, subsequent heating of the cold preformed component to austenitizing temperature and subsequent hot forming to produce the final contour of the formed part.
Due to the heating of the blank 22 carried out in the course of hot forming, diffusion processes take place between the base material of the steel substrate 2 and the coating material 4. In this process, iron diffuses from the steel substrate 2 into the coating material 4 so that the thickness d3 of the coating 4 increases overall compared to the thickness d2 present after the flexible rolling, that is, the coating thicknesses d3a, d3b of the hot-formed component 32 are each thicker than the corresponding coating thicknesses d2a, d2b before the hot forming. The holding time for austenitizing the coated blank 22 depends on the selected temperature and can range from 4 to 10 minutes. POssibly, in the thinner first portions a, the coating 4 of the hot-formed product 32 has a final coating thickness d3a of more than 15 micrometers, in particular more than 20 micrometers. In the thicker second portions b, the coating 4 may have after heating and/or hot forming a second final thickness d3b, in particular of more than 30 micrometers, preferably more than 35 micrometers. For good weldability of the produced component, it is favorable if the final thickness d3a of the coating 4 in the thin portions a is less than 50 micrometers, in particular less than 40 micrometers, and in the thicker portions b is less than 60 micrometers, in particular less than 50 micrometers.
Optionally, surface oxidation of the coated and flexibly rolled substrate 2 can be carried out before hot forming (S4). In this case, an oxidation layer is formed on the coating 4. This leads to higher heat absorption, so that heating times can be shortened. In an example, in the course of hot forming, the blank can be heated at a heating rate of more than 12 K/sec at least until a temperature of 700° C. is reached.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102018118015.2 | Jul 2018 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/068581 | 7/10/2019 | WO | 00 |
