Patent Application
20190185951
The invention relates to a method for producing an ultra high strength steel strip with improved properties during further processing, and to a corresponding steel strip.
In particular, the invention relates to the production of a steel strip consisting of a manganese-containing TRIP (Transformation Induced Plasticity) and/or TWIP (Twinning Induced Plasticity) steel having excellent cold-formability and warm-formability, increased resistance to hydrogen-induced delayed crack formation (delayed fracture), to hydrogen embrittlement and to liquid metal embrittlement during welding.
European patent application EP 2 383 353 A2 discloses a manganese-containing steel, a flat steel product formed from this steel and a method for producing this flat steel product. The steel has a tensile strength of 900 to 1500 MPa and an elongation at fracture A80 of at least 4%. The highest described elongation at fracture A80 is 8%. Furthermore, the steel consists of the elements (contents are in weight percent and relate to the steel melt): C: to 0.5; Mn: 4 to 12.0; Si: up to 1.0; Al: up to 3.0; Cr: 0.1 to 4.0; Cu: up to 4.0; Ni: up to 2.0; N: up to 0.05; P: up to 0.05; S: up to 0.01, with the remainder being iron and unavoidable impurities. Optionally, one or more elements from the group “V, Nb, Ti” are provided, wherein the sum of the contents of these elements is at most equal to 0.5. For an Mn content of 5 and an Al content of 2, the total is 7. The microstructure of this flat steel product consists of 30 to 100% martensite, tempered martensite or bainite, with the remainder being austenite. This steel is said to be characterised in that it can be produced in a more cost-effective manner than steels containing a high content of manganese and at the same time has high elongation at fracture values and, associated therewith, a considerably improved deformability. A method for producing a flat steel product from the high-strength, manganese-containing steel described above comprises the following working steps: —melting the previously described steel melt, —producing a starting product for subsequent hot rolling, in that the steel melt is cast into a string, from which at least one slab or thin slab is separated off as a starting product for the hot rolling, or into a cast strip which is supplied to the hot rolling process as a starting product, —heat-treating the starting product in order to bring the starting product to a hot-rolling starting temperature of 1150 to 1000° C., —hot rolling the starting product to form a hot strip having a thickness of at most 2.5 mm, wherein the hot rolling is terminated at a hot-rolling end temperature of 1050 to 800° C., —reeling the hot strip to form a coil at a reeling temperature of s 700° C. Optionally, the hot strip can be annealed at 250 to 950° C., subsequently cold-rolled and then annealed at 450 to 950° C. Also, following on from the cold rolling or hot rolling of the flat steel product, said product is provided with a metallic corrosion-protection coat or an organic coat.
Furthermore, German laid-open document DE 10 2012 013 113 A1 already describes so-called TRIP steels which have a predominantly ferritic basic microstructure having incorporated residual austenite which can convert into martensite during deformation (TRIP effect). The manganese content of the steel strip is 1.00 to 2.25 weight percent. The steel strip is coated and temper-rolled in a melting bath. Owing to its intense cold-hardening, the TRIP steel achieves high values for uniform elongation and tensile strength. TRIP steels are used inter alia in structural components, chassis components and crash-relevant components of vehicles, as sheet metal blanks and as welded blanks.
European patent EP 1 067 203 B1 discloses a method for producing a steel strip. In this case, a thin strip having a thickness of 1.5 mm to 10 mm is cast from a steel melt consisting at least of the following elements (contents in weight percent) C: 0.001 to 1.6; Mn: 6 to 30; Al: to 6; P: to 0.2; S: to 0.5; N: to 0.3 and the remainder being iron and unavoidable impurities. The thin strip is hot-rolled with a reduction degree between 10% and 60%, acid-cleaned, cold-rolled with a reduction degree between 10% and 90% and recrystallisation-annealed for 1 to 2 min at 800 to 850° C.
Japanese patent JP 3 317 303 B2 discloses a high strength steel strip having the following composition in weight percent: C: 0.05-0.3 Si: <0.2, Mn: 0.5-4.0 P: ≤0.1; S: ≤0.1; Ni: 0-5.0; Al: 0.1-2.0 and N≤0.01. In this case, the following equations are satisfied: Si+Al=0.5; Mn+⅓ Ni≥1.0. The microstructure contains ≥5 vol. % residual austenite. In a vacuum laboratory furnace, a melt of the previously described steel is melted. By means of hot-forging, a test block having a thickness of 25 mm is produced. This is then heated to 1250° C. in an electric furnace for one hour. Subsequently, hot rolling is performed at 930 to 1150° C. in order to achieve a steel strip thickness of 5 mm. For reeling simulation, the steel strip is cooled immediately to 500° C. and is annealed in an electric furnace at this temperature for one hour.
Proceeding from this, the object of the present invention is to provide a method for producing an ultra high strength steel strip consisting of a manganese-containing TRIP and/or TWIP steel having strengths between 1100 and 2200 MPa, which is cost-effective and wherein the steel strip has improved properties during further processing, in particular a good combination of strength and forming properties, increased resistance to hydrogen-induced delayed crack formation, to hydrogen embrittlement and to liquid metal embrittlement. Furthermore, an ultra high strength and cost-effective steel strip is to be provided having improved properties during further processing.
This object is achieved by a method for producing a flat steel product, in particular using the aforementioned steel, having the features of claim 1 and by an ultra high strength steel strip having the features of claim 10. Advantageous embodiments of the invention are described in the dependent claims.
In accordance with the invention, a method for producing an ultra high strength steel strip, comprising the steps of: —melting a steel melt containing (in wt. %): C: 0.1 to <0.3; Mn: 4 to <8; Al: >1 to 2.9; P: <0.05; S: <0.05; N: <0.02; with the remainder being iron including unavoidable steel-associated elements, with optional adding by alloying of one or more of the following elements (in wt. %): Si: 0.05 to 0.7; Cr: 0.1 to 3; Mo: 0.01 to 0.9; Ti: 0.005 to 0.3; B: 0.0005 to 0.01 via the process route of blast furnace-steel plant or the electric arc furnace process each with optional vacuum treatment of the melt; —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, —heating to a rolling temperature of 1050 to 1250° C. or in-line rolling out of the casting heat, —hot rolling the pre-strip or the slab or the thin slab to form a hot strip having a thickness of 12 to 0.8 mm, at a final rolling temperature of 1050 to 800° C., —reeling the hot strip at a temperature of more than 200 to 800° C., —pickling the hot strip, —annealing the hot strip in a continuous or discontinuous annealing installation for an annealing time of 1 min to 48 h and at temperatures of 540° C. to 840° C., —cold rolling the hot strip at room temperature or elevated temperature in one or a plurality of rolling passes, —optionally electrolytically galvanising or hot-dip galvanising the steel strip, provides a cost-effectively produced steel strip having a strength of 1100 to 2200 MPa, a good combination of strength, elongation and forming properties and an increased resistance to delayed crack formation, to hydrogen embrittlement and to liquid metal embrittlement, which additionally has a TRIP and/or TWIP effect during mechanical loading.
Typical thickness ranges for the pre-strip are 1 mm to 35 mm and for slabs and thin slabs they are 35 mm to 450 mm. Provision is preferably made that the slab or thin slab is hot-rolled to form a hot strip having a thickness of 12 mm to 0.8 mm or the pre-strip, cast to approximately the final dimensions, is hot-rolled to form a hot strip having a thickness of 8 mm to 0.8 mm. The cold strip in accordance with the invention has a thickness of at most 3 mm, preferably 0.1 to 1.4 mm.
In the context of the above method in accordance with the invention, a pre-strip produced with the two-roller casting process and approximating the final dimensions and having a thickness of less than or equal to 3 mm, preferably 1 mm to 3 mm is already understood to be a hot strip. The pre-strip thus produced as a hot strip does not have a 100% cast structure owing to the introduced deformation of the two rollers running in opposite directions. Hot rolling thus already takes place in-line during the two-roller casting process which means that separate heating and hot rolling is not necessary.
The cold rolling of the hot strip can take place at room temperature or advantageously at elevated temperature prior to the first rolling pass in one or a plurality of rolling passes.
The cold rolling at elevated temperature is advantageous in order to reduce the rolling forces and to aid the formation of deformation twins (TWIP effect). Advantageous temperatures of the material being rolled prior to the first rolling pass are 60 to 450° C.
If the cold rolling is performed in a plurality of rolling passes, it is advantageous to intermediately heat or cool down the steel strip between the rolling passes to a temperature of 60 to 450° C. because the TWIP effect is brought to bear in a particularly advantageous manner in this region. Depending upon the rolling speed and degree of deformation, intermediate heating, e.g. at very low degrees of deformation and rolling speeds, and also additional cooling, caused by heating the material with rapid rolling and high degrees of deformation, can be performed.
After cold rolling of the hot strip at room temperature, the steel strip is to be advantageously annealed in particular in a continuous annealing installation, advantageously for an annealing time of 1 to 15 min and at temperatures of 720° C. to 840° C., in order to restore sufficient forming properties. Optionally, annealing can be performed by means of a discontinuous annealing installation at a temperature of 550° C. to 820° C. and an annealing time of 30 min to 48 h. If required in order to achieve specific material properties, this annealing procedure can also be performed with the steel strip rolled at elevated temperature.
After the annealing treatment, the steel strip is advantageously cooled to a temperature of 250° C. to room temperature and subsequently, if required, in order to adjust the required mechanical properties, in the course of ageing treatment, is reheated to a temperature of 300 to 450° C., is maintained at this temperature for up to 5 min and subsequently is cooled to room temperature. The ageing treatment can be performed advantageously in a continuous annealing installation.
If required, the steel strip can be temper-rolled after the cold rolling, as a result of which the surface structure required for the final application is adjusted. The temper rolling can be performed e.g. by means of the Pretex®-method.
In one advantageous development, the steel strip produced in this manner acquires a further coating on an organic or inorganic basis instead of or after the electrolytic galvanising or hot-dip galvanising. They can be e.g. organic coatings, synthetic material coatings or lacquers or other inorganic coatings, such as e.g. iron oxide layers.
The steel strip produced in accordance with the invention can be used both as a metal sheet, metal sheet portion or blank or can be further processed to form a longitudinal or helical seam-welded pipe.
Furthermore, the steel sheet or steel strip is suitable in a particularly advantageous manner for further processing to form a component by means of cold forming or warm forming, e.g. In the automotive industry, in infrastructure construction and engineering.
The steel strip having improved properties during further processing has a TRIP/TWIP effect, having a microstructure (in vol. %) consisting of 10 to 80% austenite, 10 to 90% martensite, with the remaining being ferrite and bainite having a proportion together of less than 20%. In this case, a proportion of at least 20% of the martensite is present as annealed martensite and optionally a proportion of >10% of the austenite is present in the form of annealing or deformation twins.
By reason of the annealing treatments in accordance with the invention, the steel strip has a particularly fine grain with an average grain size of the phase components:
austenite: less than 500 nm
martensite, ferrite, bainite: less than 650 nm.
By reason of the final annealing of the cold strip which is produced at room temperature or at elevated temperatures, the austenite is present in a metastable state and optionally with deformation twins, as a result of which it converts partially into martensite when a mechanical force is applied (e.g. forming) per TRIP effect.
The austenite proportion of the steel in accordance with the invention can convert partially or completely into martensite when mechanical stresses are applied (TRIP effect).
The alloy in accordance with the invention, when subjected to a corresponding mechanical load, also has twinning during plastic deformation (TWIP effect). Owing to the intense cold-hardening induced by the TRIP and/or TWIP effect, the steel achieves high values in terms of elongation at fracture, in particular uniform elongation, and tensile strength.
The steel in accordance with the invention can then be formed in a particularly advantageous manner by means of warm forming at 60 to 450° C. because the austenite stability at these temperatures at least partially suppresses conversion of austenite into martensite (TRIP effect), wherein 50 to 100% of the starting austenite is retained and optionally converts partially into deformation twins (TWIP effect). The deformation twins can convert into martensite at room temperature with further energy being expended (TRIP effect, increased energy absorption capacity e.g. in the event of a crash). The residual elongation which has remained until the component fails is considerably increased during warm forming in comparison with cold forming. Furthermore, the prevention of the TRIP effect during warm forming brings about a considerable improvement with respect to undesired hydrogen-induced influences (delayed crack formation, hydrogen embrittlement). Also, the warm forming advantageously serves to raise the 0.2% elasticity limit of the formed material, whereby e.g. the sheet thickness could be advantageously reduced.
The method in accordance with the invention can be used to produce a very cost-effective steel strip having an alloy concept, in which, in addition to iron, only the elements carbon, manganese and aluminium are required. The required annealing treatment can be performed advantageously by means of continuous annealing, which is considerably more economical than batch-type annealing.
A steel strip produced according to the method in accordance with the invention advantageously has an elasticity limit Rp0.2 of 300 to 1550 MPa, a tensile strength Rm of 1100 to 2200 MPa and an elongation at fracture A80 of more than 4 to 41%, wherein high strengths tend to be associated with lower elongations at fracture and vice versa:
Rm of over 1100 to 1200 MPa: Rm×A80≥25000 up to 45000 MPa %
Rm of over 1200 to 1400 MPa: Rm×A80≥20000 up to 42000 MPa %
Rm of over 1400 to 1800 MPa: Rm×A80≥10000 up to 40000 MPa %
Rm of over 1800 MPa: Rm×A80≥7200 up to 20000 MPa %
A test piece body A80 was used for the elongation at fracture tests as per DIN 50 125.
The elongation and toughness properties are advantageously improved by the onset of the TRIP and/or TWIP effect of the alloy in accordance with the invention.
The steel strip produced in accordance with the invention offers a good combination of strength, elongation and deformation properties. Moreover, the production of this manganese steel in accordance with the invention having a medium manganese content (medium manganese steel) on the basis of the alloy elements C, Mn, Al is very cost-effective.
Owing to the increased Al content, the steel has a lower relative density compared with other manganese steels alloyed with a small amount of Al and having medium manganese contents. The manganese steel in accordance with the invention is also characterised by an increased resistance to delayed crack formation (delayed fracture) and to hydrogen embrittlement and liquid metal embrittlement during welding.
The use of the term “to” in the definitions 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.
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 elongation and toughness properties, for which reason a maximum content of less than 0.3 wt. % is set. In order to achieve a sufficient strength for the material, a minimum addition of 0.1 wt. % Is required.
Manganese Mn: stabilises the austenite, increases the strength and the toughness and renders possible a deformation-induced martensite formation and/or twinning in the alloy in accordance with the invention. Contents of less than 4 wt. % are not sufficient to stabilise the austenite and thus impair the elongation properties, whereas with contents of 8 wt. % and more the austenite is stabilised too much and as a result the strength properties, in particular the 0.2% elasticity limit, are reduced. For the manganese steel in accordance with the invention having medium manganese contents, a range of 4 to <8 wt. % is preferred.
Aluminium Al: an Al content of greater than 1 wt. % improves the strength and elongation properties, decreases the relative density and influences the conversion behaviour of the alloy in accordance with the invention. Contents of Al of more than 2.9 wt. % impair the elongation properties. Higher Al contents also considerably impair the casting behaviour in the continuous casting process. This produces increased outlay when casting. Al Contents of more than 1 wt. % delay the precipitation of carbides in the alloy in accordance with the invention. Therefore, a maximum content of 2.9 wt. % and a minimum content of more than 1 wt. % are set.
Furthermore, for the sum of Mn and Al a minimum content (in wt. %) of more than 6.5 and less than 10 should be maintained in order to be able to ensure the desired conversion behaviour. A content of Mn+Al of 10 wt. % and more impairs the castability, thus reducing output and thus increasing costs. In the case of contents of Mn+Al of 6.5 wt. % or less, it is not possible to ensure sufficient austenite stability for the desired conversion behaviour.
Silicon Si: the optional addition of Si in contents of more than 0.05 wt. % impedes the diffusion of carbon, reduces the relative density and increases the strength and elongation properties and toughness properties. Furthermore, an improvement in the cold-rollability could be seen by adding Si by alloying. Contents of more than 0.7 wt. % result in embrittlement of the material and negatively influence the hot- and cold-rollability and the coatability e.g. by galvanising. Therefore, a maximum content of 0.7 wt. % and a minimum content of 0.05 wt. % are set.
Chromium Cr: the optional addition of 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 3 wt. % since higher contents result in an impairment of the elongation properties. A minimum Cr content for efficacy is set to 0.1 wt. %.
Molybdenum Mo: The optional addition of 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 0.9 wt. % impair the elongation properties, for which reason a maximum content of 0.9 wt. % and a minimum content of 0.01 wt. % required for sufficient efficacy are set.
Phosphorus P: is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorous increases the hardness by means of solid solution hardening 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 phosphorus content is limited to values of less than 0.05 wt. %.
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 tendency towards extensive segregation and has a greatly embrittling effect, whereby the elongation 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 desulphurisation). For the aforementioned reasons, the sulphur content is limited to values of less than 0.05 wt. %.
Nitrogen N: is likewise an associated element from steel production. In the dissolved state, it improves the strength and toughness properties in steels containing a higher content of manganese of greater than or equal to 4 wt. % Mn. Lower Mn-alloyed steels of less than 4 wt. % with free nitrogen tend to have a strong ageing effect. The nitrogen diffuses even at low temperatures to dislocations and blocks 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 adding aluminium or titanium by alloying, wherein in particular aluminium nitrides have a negative effect upon the forming properties of the alloy in accordance with the invention. For the aforementioned reasons, the nitrogen content is limited to less than 0.02 wt. %.
Titanium Ti: acts in a grain-refining manner as a carbide-forming agent, whereby at the same time the strength, toughness and elongation properties are improved, and reduces the inter-crystalline corrosion. Ti contents of more than 0.3 wt. % impair the elongation properties, for which reason a maximum Ti content of 0.3 wt. % Is set. Optionally, a minimum content of 0.005 is set in order to bind nitrogen and advantageously precipitate Ti.
Boron B: delays the austenite conversion, improves the hot-forming properties of steels and increases the strength at ambient temperature. It achieves its effect even with very low alloy contents. Contents above 0.01 wt. % greatly impair the elongation and toughness properties, for which reason the maximum content is set to 0.01 wt. %. Optionally, a minimum content of 0.0005 wt. % is set in order to advantageously use the strength-increasing effect of boron.
Tests were performed in order to examine the mechanical properties of steel strips produced in accordance with the invention and consisting of an exemplary alloy 1. The alloy 1 contains, in addition to iron and melting-induced impurities, extracts of the following elements in the stated contents in wt. %:
For the purposes of comparison, the steel strips produced from the above-mentioned alloy 1 were cold-rolled, i.e. at room temperature and therefore below 50° C., and also rolled in accordance with the invention at 250° C. The measured rolling forces are given as follows:
Cumulative rolling force is understood to be the adding up of the rolling forces of the individual passes in order to obtain a comparable measure for the expenditure of force. The rolling force was standardised to a band width of 1000 mm. The degree of deformation e is defined as the quotient of the change in thickness Δd of the steel strip under investigation and the initial thickness d0 of the steel strip under investigation. The reduction in rolling force is the calculated decrease in the rolling force at 250° C. compared with the rolling force during cold rolling.
The elongation at fracture A80 was also determined:
The elongation characteristic values represent the elongation in the rolling direction. It is apparent that there is a considerable increase in the elasticity limit whilst the elongation at fracture remains the same.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 115 618.3 | Aug 2016 | DE | national |
| 10 2016 121 002.1 | Nov 2016 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/070913 | 8/18/2017 | WO | 00 |
