Patent Application
20190085434
The invention relates to a method for producing a component by hot-forming a pre-product of steel. Pre-products for hot-forming are understood hereinafter to be e.g. sheets cut from the coil or plate blanks or seamless or welded pipes which occasionally can additionally be cold-drawn. The invention also relates to a hot-formed component produced from a steel.
Such components produced by hot-forming are used mainly in the automotive and commercial vehicle industries, but there are also possible applications in engineering for producing white goods or in the construction industry.
The fiercely competitive market means that automobile producers are constantly forced to find solutions for reducing fleet consumption whilst maintaining the highest possible level of comfort and occupant protection. On the one hand, the weight saving of all of the vehicle components plays a decisive role as does, on the other hand, the most favourable possible behaviour of the individual components in the event of high static and dynamic loading during operation and also in the event of a crash.
Furthermore, the reduction in CO2 emissions along the entire manufacturing chain represents a particular challenge which is met by innovative solutions in terms of process technology. In particular, the focal point is on process steps which are based directly or indirectly upon the combustion of fossil fuels.
The suppliers of pre-materials attempt to take this requirement into consideration in that by providing high-strength and super high-strength steels the wall thicknesses can be reduced whilst at the same time achieving improved component behaviour during manufacture and operation.
Therefore, these steels must satisfy comparatively stringent requirements in terms of strength, extensibility, toughness, energy consumption and corrosion-resistance and their processability e.g. during cold-forming and welding.
Amongst the aforementioned aspects, the production of components of hot-formable steels is acquiring increasing significance because they are ideal for meeting the increased requirements upon component properties, with material outlay being reduced.
The production of components by means of quenching of pre-products of press-hardenable steels by hot-forming in a forming tool is known from patent document IDE 601 19 826 T2. In this case, a sheet plate previously heated above the austenitization temperature to 800-1200° C. and possibly provided with a metallic coat of zinc or on the basis of zinc is formed in an occasionally cooled tool by hot-forming to produce a component, wherein during forming or after hot-forming, by reason of rapid heat extraction the sheet or component in the forming tool undergoes quench-hardening (press-hardening) and thereby obtains the required microstructure properties and strength properties.
The metallic coat is applied as corrosion protection, typically in the continuous hot-dipping method, onto a hot strip or cold strip or onto the pre-product produced therefrom, e.g. as hot-dip galvanising or aluminium coating.
Subsequently, the plate is suitably cut to size for the forming tool of the hot-forming procedure It is also possible to provide the workpiece to be formed in each case, or the blank, with a hot-dip coat.
The application of a metallic coat onto the pre-product, to be formed, prior to hot-forming is advantageous in the case of this method because during press-hardening a disadvantageous change in the surface of the steel substrate caused by scaling of the basic material can be effectively avoided by reason of the coat and excessive tool wear can be effectively avoided by reason of an additional lubricating effect.
Known steels for this application which are suitable for press-hardening are e.g. manganese-boron steel “22MnB5”.
In order to obtain components having very high strengths of more than 980 MPa whilst maintaining a sufficiently high level of toughness, it is known from laid-open document EP 2 546 375 A1 to correspondingly form a steel having a microstructure, which is predominantly ferritic in the initial state and has perlite proportions, by means of press-form hardening, and to adjust, by means of stepwise process control, a microstructure of bainite, tempered martensite and residival austenite on the finished component. In this case, the sheet to be formed is heated initially to a temperature of 750 to 1000° C. and is maintained at this temperature for 5 to 1000 seconds, then said sheet is formed at 350 to 900° C. and cooled to 50 to 350° C. Finally, said sheet is reheated to a temperature of 350 to 490° C. and this temperature is maintained for a period of 5 to 1000 seconds. The microstructure on the finished component has 10 to 85% martensite, 5 to 40% residual austenite and at least 5% bainite.
However, the production of a component by hot-forming by means of press-hardening has several disadvantages.
On the one hand, this method requires a large amount of energy on account of the heating of the pre-product to austenitization temperature per se and additionally for the conversion of ferrite into austenite, which makes the method expensive and results in considerable CO2 emissions.
Moreover, in order to avoid excessive scaling of the sheet surface, as described above, an additional metallic protective layer or an additional lacquer-based protective layer is required or considerable amount of reworking of the surface which has become scaled by heating and forming is required.
Moreover, since forming is performed at temperatures above the Ac3 temperature, in general significantly above 800° C., extremely stringent requirements in terms of temperature stability are applied to these protective layers and therefore cathodic corrosion protection of the pre-product on a basis of zinc can be used in this case only to a limited extent and with increased process outlay because the zinc vaporises at these high temperatures. Consequently, the process of press-hardening uses predominantly steel sheets which have an AlSi coating but which do not offer any cathodic corrosion protection of the formed component.
A further disadvantage is that the formed component must be cooled in an accelerated manner in the forming tool itself, a further tool outside the forming press or using gaseous or liquid media in order to achieve the desired level of strength. The duration of this cooling procedure considerably reduces the throughput of components per unit of time, thus reducing economic feasibility.
In summary, it can be stated that the known method for producing components of steel by hot-forming by means of press-hardening above the austenitization temperature Ac3 results in high manufacturing and energy costs and therefore high component costs on account of the required large heating furnaces associated with long heating times and the cooling of the component in the tool as required at the end of the process. Moreover, it is not possible to ensure any cathodic corrosion protection by the application of a coating prior to heating and forming.
Laid-open document DE 10 2011 108 162 A1 discloses a method for producing a component by semi-hot-forming a pre-product of steel below the Ac1 conversion temperature, in which the required increase in strength in the component is achieved by cold-forming the pre-product prior to heating to forming temperature. Optionally, an additional increase in strength in the component can be achieved by using higher-strength materials, such as bainitic, martensitic, micro-alloyed and dual-phase or multi-phase steels. The disadvantage in this case is the additional outlay caused by the necessary cold-forming prior to heating to forming temperature. During hot-forming, dual-phase steels also have the disadvantage of sensitivity to edge crack-induced failure during forming. References to alloy compositions to be specifically observed or specifications for the microstructure of the pre-product for specific adjustment of the mechanical properties of the component after semi-hot-forming when using higher-strength steels are not disclosed.
Laid-open document DE 10 2013 009 232 A1 discloses a method for producing a component by semi-hot-forming a pre-product of steel, in which the pre-product is heated to forming temperature and is then formed, wherein, after forming, the component has a bainitic microstructure having a minimum tensile strength of 800 MPa. Heating is performed to a temperature below the Ac1 conversion temperature, wherein the pre-product already is made of a steel having a microstructure of at least 50% bainite, and wherein the pre-product has the following alloy composition in wt.%: C: 0.02 to 0.3; Si: 0.01 to 0.5; Mn: 1.0 to 3.0; P: max. 0.02; S: max. 0.01; N: max. 0.01; Al: to 0.1; Cu: to 0.2; Cr: to 3.0; Ni: to 0.2; Mo: to 0.2; Ti: to 0.2; V: to 0.; Nb: to 0.1 and B: to 0.01.
Although this alloying concept can already be used to produce components having a very high tensile strength of over 800 MPa and an expansion of over 10% and to produce cathodic corrosion protection of zinc, the forming capability of this material still does not meet the most stringent requirements for the production of complex component geometries. In particular, the elongation at fracture achieved and the strength are still too low for many requirements.
The object of the invention is to provide a method for producing a component by hot-forming a pre-product of steel at temperatures below the Ac3 conversion temperature, which is cost-effective and by means of which a once again improved forming capability of the steel material is achieved in the component with minimum tensile strengths of 700 MPa to be achieved. A corresponding component which is produced by semi-hot-forming is also to be provided.
A method in accordance with the invention is described in claims 1 to 33 and a hot-formed component in accordance with the invention is described in claims 34 to 38.
According to the teaching of the invention, this object is achieved by a method for producing a component by hot-forming a pre-product of steel, in which the pre-product is heated to a temperature above 60° C. and below the Ac3 conversion temperature and is then formed, wherein the component has a minimum tensile strength of 700 MPa with an elongation at fracture of over 22%, wherein the pre-product has the following alloy composition in wt. %:
C: 0.0005 to 0.9
Mn: more than 3.0 to 12
with the remainder being iron including unavoidable steel-associated elements, with optional addition by alloying of one or more of the following elements (in wt. %): Al: to 10, Si: to 6, Cr: to 6, Nb: to 1.5, V: to 1.5, Ti: to 1.5, Mo: to 3, Cu: to 3, Sn: to 0.5, W to 5, Co: to 8, Zr: to 0.5, Ta: to 0.5, Te: to 0.5, B: to 0.15, P: max. 0.1, S: max. 0.1, N: max. 0.1, Ca to 0.1.
The steel used for the method in accordance with the invention has a multi-phase microstructure, comprised of ferrite and/or martensite and/or bainite and residual austenite. The residual austenite proportion can be 5% to 80%. The residual austenite can partially or completely convert into martensite by the TRIP effect when mechanical stresses are present. The alloy in accordance with the invention has a TRIP and/or TWIP effect when subjected to mechanical stress accordingly. Owing to the strong solidification (similar to cold solidification) induced by the TRIP and/or TWIP effect and by the increase in the dislocation density, the steel achieves very high values in terms of elongation at fracture, in particular uniform elongation, and tensile strength. In an advantageous manner, this property is achieved by the residual austenite present only with manganese contents of over 3 wt. %.
The use of the term “to” in the definition of the content ranges, such as e.g. 0.01 to 1 wt. %, means that the limit values −0.01 and 1 in the example—are also included.
The steel in accordance with the invention is suitable in particular for producing complexly formed components by semi-hot-forming which have not only a very good forming capability during forming but also have high strength and elongation at fracture in the operating state and advantageously are provided with zinc-based cathodic corrosion protection.
Advantageously, the steel in the initial state has a tensile strength Rm of >700 to 2000 MPa with an elongation at fracture A80 in dependence upon the achieved tensile strength of at least 3 to 40%.
In contrast to the method for producing a component by means of press-hardening which is known from DE 601 19 826 T2 or EP 2 546 375 A1, the method in accordance with the invention has the advantage that with a considerably lower energy requirement for the heating procedure the use of a multi-phase steel in the initial state having residual austenite serves to provide a component having mechanical characteristic values of tensile strength and elongation at fracture which are considerably better than the mechanical properties of the components of known steels for semi-hot-forming. In addition, in comparison with press-hardening, energy costs are saved by reason of lower heating temperatures.
The steel containing medium manganese and comprising a manganese content of over 3 wt. % is provided as a flat product (hot strip or cold strip) or as a seamless or welded pipe having a corrosion protection layer (Zn, Zn alloys, inorganic or organic coatings with Zn, AlSi or other inorganic or organic coatings) and is subsequently warm-formed (HWU). Warm is defined in this case as forming after heating of the pre-product to a temperature <700° C., preferably <450° C., more preferably <350° C. to 60° C., wherein the austenite proportion in the pre-product is retained completely or partially during forming and the possible onset of a TRIP effect is suppressed completely or partially. Heating to <450° C., preferably <350° C., facilitates the use of cathodic, zinc-based corrosion protection.
Furthermore, semi-hot-forming improves the forming properties in comparison with forming at RT and advantageously increases the resistance to hydrogen embrittlement and delayed crack formation. Cooling is performed in still air, i.e. compared to press-hardening it does not require any accelerated and/or regulated cooling.
The component can be cooled technically in an accelerated manner, optionally after semi-hot-forming, by means of airflow, oil, water or other active media.
The pre-product and the hot-formed component manufactured therefrom have, before and after semi-hot-forming, a tensile strength of 700 to 2000 MPa, preferably 850 to 1800 MPa, particularly preferably >1000 to 1.800 MPa at expansions A80 of >3 to 40%, preferably >6 to 30%. Higher required expansions tend to produce lower strengths and vice versa.
Therefore, the product of required tensile strength and achieved elongation at fracture (Rm×A80) can be considered to be decisive for characterising these component properties.
Tests on the finished component, i.e. after hot-forming, advantageously demonstrate the following excellent results for the product of Rm×A80 in MPa %:
Rm of 700 to 800 MPa: Rm×A80≥15400 up to 50000
Rm of over 800 to 900 MPa: Rm×A80≥14400 up to 50000
Rm of over 900 to 1100 MPa: Rm×A80≥13500 up to 45000
Rm of over 1100 to 1200 MPa: Rm×A80≥13200 up to 45000
Rm of over 1200 to 1350 MPa: Rm×A80≥11200 up to 45000
Rm of over 1350 to 1800 MPa: Rm×A80≥8000 up to 45000
Rm of over 1800 MPa: Rm×A80≥4000 up to 30000
Heating of the material which is to be warm-formed is preferably effected inductively or alternatively by radiation or conductively. Optionally, heating of the material is effected prior to semi-hot-forming directly in the forming tool, thus making it possible to save on an additional furnace unit and to omit a process step. This is considered in particular at heating temperatures of <450° C., preferably <350° C.
The customer requirement for cathodic corrosion protection in conjunction with high-strength steels having required tensile strengths Rm of up to 1500 MPa can thus be met in an advantageous manner by semi-hot-forming of the inventive comprising steels containing more than 3 to 12 wt. % manganese. Furthermore, the heating temperature below Ac3 advantageously brings about only a small decrease in the strength of the pre-product whilst at the same time offering improved forming properties compared with forming at RT, in which the onset of the TRIP/TWIP effect would occur. By reason of the fact there is no conversion or else only a partial conversion of the microstructure after semi-hot-forming, the components undergo only slight distortion during cooling.
Furthermore, it is possible to achieve energy saving potentials and a reduction in CO2 emissions during semi-hot-forming compared with press-hardening at temperatures above Ac3.
Particularly uniform and homogeneous material properties can be achieved if the steel of the pre-product has the following alloy composition in wt. %:
C: 0.05 to 0.42
Mn: >5 to <10
with the remainder being iron including unavoidable, steel-associated elements, with optional
addition by alloying of one or more of the following elements (in wt. %):
Al: 0.1 to 5, in particular >0.5 to 3
Si: 0.05 to 3, in particular >0.1 to 1.5
Cr: 0.1 to 4, in particular >0.5 to 2.5
Nb: 0.005 to 0.4, in particular 0.01 to 0.1
B: 0.001 to 0.08, in particular 0.002 to 0.01
Ti: 0.005 to 0.6, in particular 0.01 to 0.3
Mo: 0.005 to 1.5, in particular 0.01 to 0.6
Sn: <0.2, in particular <0.05
Cu: <0.5, in particular <0.1
W: 0.01 to 3, in particular 0.2 to 1.5
Co: 0.01 to 5, in particular 0.3 to 2
Zr: 0.005 to 0.3, in particular 0.01 to 0.2
Ta: 0.005 to 0.3, in particular 0.01 to 0.1
Te: 0.005 to 0.3, in particular 0.01 to 0.1
V: 0.005 to 0.6, in particular 0.01 to 0.3
Ca 0.005 to 0.1
Alloy elements are generally added to the steel in order to influence specific properties in a targeted manner. An alloy element can thereby influence different properties in different steels. The effect and interaction generally depend greatly upon the quantity, presence of further alloy elements and the solution state in the material. The correlations are varied and complex. The effect of the alloy elements in the alloy in accordance with the invention will be discussed in greater detail hereinafter. The positive effects of the alloy elements used in accordance with the invention will be described hereinafter:
Carbon C: is required to form carbides, stabilises the austenite and increases the strength. Higher contents of C impair the welding properties and result in the impairment of the expansion and toughness properties, for which reason a maximum content of 0.9 wt. % is set. The minimum content is set at 0.0005 wt. %. Preferably, a content of 0.05 to 0.42 wt. % is set because in this range the ratio of residual austenite to other phase proportions can be adjusted in a particularly advantageous manner.
Manganese Mn: stabilises the austenite, increases the strength and the toughness and permits a deformation-induced martensite formation and/or twinning in the alloy in accordance with the invention. Contents of less than or equal to 3 wt. % are not sufficient to stabilise the austenite and thus impair the expansion properties whereas with contents of 12 wt. % and more the austenite is stabilised too much and as a result the strength properties, in particular the yield strength, are reduced. For the manganese steel in accordance with the invention having average manganese contents, a range of over 5 to <10 wt. % is preferred because in this range the ratio of the phase proportions to each other and the conversion mechanisms can be advantageously influenced during semi-hot-forming and cold-forming.
Aluminium Al: improves the strength and expansion properties, decreases the specific density and influences the conversion behaviour of the alloy in accordance with the invention. Contents of Al of more than 10 wt. % impair the expansion properties and cause predominantly brittle fracture behaviour. For the manganese steel in accordance with the invention having average manganese contents, an Al content of 0.1 to 5 wt. % is preferred in order to increase the strength and at the same time maintain effective expansion. In particular, contents of >0.5 to 3 wt. % permit a particularly large product of strength and elongation at fracture.
Silicon Si: impedes the diffusion of carbon, reduces the relative density and increases the strength and expansion properties and toughness properties. Contents of more than 6 wt. % prevent further processing by cold-rolling by reason of embrittlement of the material. Therefore, a maximum content of 6 wt. % is set. Optionally, a content of 0.05 to 3 wt. % is set because contents in this range positively influence the forming properties. Si contents of >0.1 to 1.5 wt. % have turned out to be particularly advantageous for forming and conversion properties.
Chromium Cr: improves the strength and reduces the rate of corrosion, delays the formation of ferrite and perlite and forms carbides. The maximum content is set to 6 wt. % since higher contents result in an impairment of the expansion properties and substantially higher costs. For the manganese steel in accordance with the invention having average manganese contents, a Cr content of 0.1 to 4 wt. % is preferred in order to reduce the precipitation of coarse Cr carbides. In particular, contents of >0.5 to 2.5 wt. % have proven to be advantageous for stabilising the austenite and precipitating fine Cr carbides.
Molybdenum Mo: acts as a carbide forming agent, increases the strength and increases the resistance to delayed crack formation and hydrogen embrittlement. Contents of Mo of more than 3 wt. % impair the expansion properties, for which reason a maximum content of 3 wt. % is set. For the manganese steel in accordance with the invention having average manganese contents, an Mo content of 0.005 to 1.5 wt. % is preferred in order to avoid the precipitation of excessively large Mo carbides. In particular, contents of 0.01 wt. % to 0.6 wt. % bring about the precipitation of desired Mo carbides with at the same time reduced alloy costs.
Phosphorus P: is a trace element from the iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorous increases the hardness by means of mixed crystal solidification and improves the hardenability. However, attempts are generally made to lower the phosphorous content as much as possible because inter alia it exhibits a strong tendency towards segregation owing to its low diffusion rate and greatly reduces the level of toughness. The attachment of phosphorous to the grain boundaries can cause cracks along the grain boundaries during hot-rolling. Moreover, phosphorous increases the transition temperature from tough to brittle behaviour by up to 300° C. For the aforementioned reasons, the phosphorous content is limited to a maximum of 0.1 wt. %, wherein contents <0.04 wt. % are advantageously sought for the aforementioned reasons.
Sulphur S: like phosphorous, is bound as a trace element in the iron ore. It is generally not desirable in steel because it exhibits a strong tendency towards segregation and has a greatly embrittling effect, whereby the expansion and toughness properties are impaired. An attempt is therefore made to achieve amounts of sulphur in the melt which are as low as possible (e.g. by deep vacuum treatment). For the aforementioned reasons, the sulphur content is limited to a maximum of 0.1 wt. %. It is particularly advantageous to limit the S content to <0.2 wt. % in order to reduce the precipitation of MnS.
Nitrogen N: N is likewise an associated element from steel production. In the dissolved state, it improves the strength and toughness properties in steels containing a high content of manganese of greater than or equal to 4 wt. % Mn. Lower Mn-alloyed steels of <4 wt. % with free nitrogen tend to have a strong ageing effect. The nitrogen even diffuses at low temperatures to dislocations and blocks the same. It thus produces an increase in strength associated with a rapid loss of toughness. Binding of the nitrogen in the form of nitrides is possible e.g. by alloying aluminium, vanadium, niobium or titanium. For the aforementioned reasons, the nitrogen content is limited to a maximum of 0.1 wt. %, wherein contents <0.05 wt. % are preferably sought to substantially avoid the formation of AlN.
Microalloy elements are generally added only in very small amounts (<0.1 wt. % per element). In contrast to the alloy elements, they mainly act by precipitation formation but can also influence the properties in the dissolved state. Despite the small amounts added, microalloy elements greatly influence the production conditions and the processing properties and final properties.
Typical microalloy elements are vanadium, niobium and titanium. These elements can be dissolved in the iron lattice and form carbides, nitrides and carbonitrides with carbon and nitrogen.
Vanadium V and niobium Nb: these act in a grain-refining manner in particular by forming carbides, whereby at the same time the strength, toughness and expansion properties are improved. Contents of more than 1.5 wt. % do not provide any further advantages. Optionally, for vanadium and niobium, a minimum content of greater than or equal to 0.005 wt. % and a maximum content of 0.6 (V) or 0.4 (Nb) wt. % is preferably provided, in which the alloy elements advantageously provide grain refinement. Furthermore, in order to improve the economic feasibility whilst at the same time achieving optimum grain refinement, the contents of V can be restricted to 0.01 wt. % to 0.3 wt. % and the contents of Nb can be restricted to 0.01 to 0.1 wt. %.
Tantalum Ta: tantalum acts in a similar manner to niobium as a carbide forming agent in a grain-refining manner and thereby improves the strength, toughness and expansion properties at the same time. Contents over 0.5 wt. % do not provide any further improvement in the properties. Thus, a maximum content is optionally set to 0.5 wt. %. Preferably, a minimum content of 0.005 and a maximum content of 0.3 wt. % are set, in which the grain refinement can advantageously be produced. In order to improve economic feasibility and to optimise grain refinement, a content of 0.01 wt. % to 0.1 wt. % is particularly preferably sought.
Titanium Ti: acts in a grain-refining manner as a carbide forming agent, whereby at the same time the strength, toughness and expansion properties are improved and the inter-crystalline corrosion is reduced. Contents of Ti of more than 1.5 wt. % impair the expansion properties, for which reason a maximum content of Ti of 1.5 wt. % is set. Optionally, a minimum content of 0.005 and a maximum content of 0.6 wt. % are set, in which Ti is advantageously precipitated. Preferably, a minimum content of 0.01 wt. % and a maximum content of 0.3 wt. % are provided which ensure optimum precipitation behaviour with low alloy costs.
Tin Sn: tin increases the strength but, similar to copper, accumulates beneath the scale layer and at the grain boundaries at higher temperatures. This results, owing to the penetration into the grain boundaries, in the formation of low melting point phases and, associated therewith, in cracks in the microstructure and in solder brittleness, for which reason a maximum content of less than or equal to 0.5 wt. % is optionally provided. For the aforementioned reasons, contents of less than 0.2 wt. % are preferably adjusted. Contents of <0.05 wt. % are particularly advantageously preferred in order to avoid low melting point phases and cracks in the microstructure.
Copper Cu: reduces the rate of corrosion and increases the strength. Contents of 3 wt. % and more impair the producibility by forming low melting point phases during casting and hot-rolling, for which reason a maximum content of 3 wt. % is set. Optionally, a maximum content of less than 0.5 wt. % is set, in which the occurrence of cracks during casting and hot-rolling can be advantageously prevented. Cu contents of <0.1 wt. % have turned out to be particularly advantageous in order to avoid low melting point phases and to avoid cracks.
Tungsten W: acts as a carbide forming agent and increases the strength and heat resistance. Contents of W of more than 5 wt. % impair the expansion properties, for which reason a maximum content of 5 wt. % W is set. Optionally, a maximum content of 3 wt. % and a minimum content of 0.01 wt. % are set, in which the precipitation of carbides advantageously takes place. In particular, a minimum content of 0.2 wt. % and a maximum content of 1.5 wt. % are preferably provided which permits optimum precipitation behaviour with low alloy costs.
Cobalt Co: increases the strength of the steel, stabilises the austenite and improves the heat resistance. Contents of more than 8 wt. % impair the expansion properties, for which reason a maximum content of 8 wt. % is set. Optionally, a maximum content of less than or equal to 5 wt. % and a minimum content of 0.01 wt. % are set which advantageously improve the strength and heat resistance. Preferably, a minimum content of 0.3 wt. % and a maximum content of 2 wt. % are provided which advantageously influences the austenite stability along with the strength properties.
Zirconium Zr: acts as a carbide forming agent and improves the strength. Contents of Zr of more than 0.5 wt. % impair the expansion properties for which reason a maximum content of 0.5 wt. % is set. Optionally, a maximum content of 0.3 wt. % and a minimum content of 0.005 wt. % are set, in which carbides are advantageously precipitated. Preferably, a minimum content of 0.01 wt. % and a maximum content of 0.2 wt. % are provided which advantageously permit optimum carbide precipitation with low alloy costs.
Boron B: delays the austenite conversion, improves the hot-forming properties of steels and increases the strength at room temperature. It achieves its effect even with very low alloy contents. Contents above 0.15 wt. % greatly impair the expansion and toughness properties, for which reason the maximum content is set to 0.15 wt. %. Optionally, a minimum content of 0.001 and a maximum content of 0.08 wt. % are set, in which the strength-increasing effect of boron is advantageously used. Furthermore, a minimum content of 0.002 wt. % and a maximum content of 0.01 wt. % are preferred which permit optimum use for increasing strength whilst at the same time improving the conversion behaviour.
Tellurium Te: improves the corrosion resistance and the mechanical properties as well as the machining capability. Furthermore, Te increases the strength of MnS which as a result is lengthened to a lesser extent in the rolling direction during hot-rolling and cold-rolling. Contents above 0.5 wt. % impair the expansion and toughness properties, for which reason a maximum content of 0.5 wt. % is set. Optionally, a minimum content of 0.005 wt. % and a maximum content of 0.3 wt. % are set which advantageously improve mechanical properties and increase the strength of MnS present. Furthermore, a minimum content of 0.01 wt. % and a maximum content of 0.1 wt. % are preferred which permit optimisation of the mechanical properties whilst at the same time reducing the alloy costs.
Calcium Ca: is used for modifying non-metallic oxidic inclusions which could otherwise result in the undesired failure of the alloy as a result of inclusions in the microstructure which act as stress concentration points and weaken the metal composite. Furthermore, Ca improves the homogeneity of the alloy in accordance with the invention. In order to achieve a corresponding effect, a minimum content of 0.0005 wt. % is optionally necessary. Contents of above 0.1 wt. % Ca do not provide any further advantage in the modification of inclusions, impair producibility and should be avoided by reason of the high vapour pressure of Ca in steel melts. Therefore, a maximum content of 0.1 wt. % is provided
Typical applications of components which are hot-formed in accordance with the invention and are produced from metal sheets or pipes as pre-products concern in particular automotive engineering but e.g. also mobile crane construction and longitudinal and transverse beams in commercial vehicles and trailers or safety and chassis parts in passenger cars and wagon construction.
For example, a sheet metal plate or a pipe can be used as the pre-product. The sheet metal plate can be manufactured from a hot strip or cold strip and the pipe can be a seamlessly hot-rolled pipe or a welded pipe produced from a hot strip or cold strip.
The hot-rolled or welded pipe can be warm-formed after production once again with one or multiple drawing and/or annealing processes or in a hydraulic expanding process, e.g. by means of internal high pressure forming (IHU).
Moreover, in accordance with the invention it is advantageously possible to perform the individual forming steps at different speeds and at different temperatures within the temperature range in accordance with the invention. For instance, it is possible e.g. to advantageously prevent the martensitic formation in the first steps in order to improve the forming properties and facilitate further forming, and in the last forming step to select a temperature range which permits partial martensitic conversion of the microstructure with the aim of increasing strength. Furthermore, it is advantageously possible to perform several forming procedures with fewer intermediate heating procedures and thus in one extended temperature range, whereby the number of intermediate heating procedures can advantageously be reduced. Different forming speeds similarly permit targeted influencing of the martensitic conversion and stress distribution in the component.
Furthermore, in accordance with the invention a multi-stage method can advantageously also be performed, in which the semi-hot-forming process is followed by a final cold-forming procedure (e.g. rolling, pressing, deep-drawing, incremental forming), whereby an overall higher forming capacity can be achieved in comparison with cold-forming alone.
The pre-product and the component produced therefrom are characterised by very high tensile strength with sufficiently high expansion. Moreover, an effective welding capability is provided by reason of the chemical composition.
Furthermore, the pre-product can be provided in a known manner with a lacquer-based scaling-inhibiting or corrosion-inhibiting layer or with a metallic coat. The metallic coat can contain zinc and/or magnesium and/or aluminium and/or silicon. A pipe as a pre-product can be coated both on the inner side and outer side,
In contrast to well-established manufacturing routes, even a surface-coated hot strip or cold strip or pipe can be used for forming following on from heating because semi-hot-forming sustains adhesion and ductility. The metallic coat is resistant to short-term reheating of the substrate/coating (steel strip/coating) combination below the Ac3 temperature of the substrate in order to withstand the reheating prior to semi-hot-forming and the actual semi-hot-forming.
By reason of the comparatively small amount of heat, large-scale reheating units, such as e.g. on tunnel furnaces or chamber furnaces, can be dispensed with in favour of rapid-acting and direct-acting systems (inductive, conductive, direct in the tool and in particular radiation).
Moreover, for the described novel method considerably less heat energy is required, or the energy efficiency is higher than in the case of press-hardening. As a result, the process costs are lower and the CO2 emission is reduced. In contrast to press-hardenable steels, technical accelerated cooling in the tool can advantageously be dispensed with depending upon the application, thus significantly increasing the throughput of semi-finished products per forming tool. Any technically accelerated cooling which is possibly required can be performed outside the tool.
Preferably, reheating is performed before semi-hot-forming by means of induction because in this case the energy efficiency is high and heating duration is short. Furthermore, heating can be advantageously performed by means of radiation because in this case the efficiency is similarly considerably higher than heating in a furnace or with conductive heating and energy is input into the material more rapidly and effectively depending upon the surface characteristics.
The material is also very suitable for partial heating. By using e.g. radiators, individual regions of the pre-product to be formed can be heated in a targeted manner in order to obtain formability-optimised zones and to adjust the strength locally by the proportion of martensite converted by the TRIP effect. This advantageously permits the use of conventional presses for cold-forming so that a complex hot-forming installation, as required for press-hardening, can be dispensed with.
A steel strip for producing a pre-product (strip, sheet, pipe) can be produced from the inventive steel according to the following method steps:
smelting a steel melt containing (in wt. %): C: 0.0005 to 0.9 Mn: more than 3.0 to 12, with the remainder being iron including unavoidable steel-associated elements, with optional addition by alloying of one or more of the following elements (in wt. %): Al: to 10, Si: to 6, Cr: to 6, Nb: to 1.5, V: to 1.5, Ti: to 1.5, Mo: to 3, Cu: to 3, Sn: to 0.5, W to 5, Co: to 8, Zr: to 0.5, Ta: to 0.5, Te: to 0.5, B: to 0.15, P: max. 0.1, 5: max. 0.1, N: max. 0.1, Ca to 0.1.
casting the steel melt to form a pre-strip by means of a horizontal or vertical strip casting process approximating the final dimensions or casting the steel melt to form a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,
re-heating the slab or thin slab to 1050° C. to 1250° C. and then hot-rolling the slab or thin slab to form a hot strip or thick plate, or re-heating the produced pre-strip which approximates the final dimensions, in particular with a thickness greater than 3 mm, to 1000° C. to 1.200° C. and then hot-rolling the pre-strip to form a hot strip or thick plate, or hot-rolling the pre-strip without re-heating from the casting heat to form a hot strip or thick plate with optional intermediate heating between individual rolling passes of the hot-rolling,
reeling the hot strip and optionally the thick plate at a reeling temperature between 780° C. and room temperature,
optionally annealing the hot strip or thick plate with the following parameters: annealing temperature: 450 to 900° C., annealing duration: 1 minute to 48 hours,
optionally cold-rolling the hot strip or the produced pre-strip which approximates the final dimensions, with a thickness of less than 5 mm to form a cold strip,
optionally annealing the cold strip with the following parameters: annealing temperature: 450 to 900° C., annealing duration: 1 minute to 48 hours, a flat steel product having a good combination of strength, expansion and deformation properties, and an increased resistance to delayed crack formation and hydrogen embrittlement which has a TRIP and/or TWIP effect during mechanical loading owing to its residual austenite content in the microstructure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 104 800.3 | Mar 2016 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/055623 | 3/10/2017 | WO | 00 |
