ALLOYED STEEL

Abstract

A vanadium alloyed steel that includes carbon and vanadium and has a steel microstructure with coherent interphase precipitates that have vanadium. A method for producing a vanadium alloyed steel that includes providing a starting steel; heating the starting steel to at least partially austenitise the starting steel; and holding the temperature at 650 C.200 C. for about 25 minutes or less.

Claims

1. A vanadium alloyed steel comprising carbon and vanadium, wherein the steel microstructure comprises coherent interphase precipitates comprising vanadium.

2. A vanadium alloyed steel according to claim 1, comprising (in wt %): carbon in the range of from about 0.06 to about 1.1; and vanadium in the range of from about 0.1 to about 1.5.

3. A vanadium alloyed steel according to claim 1 or 2, wherein the steel is a modulus-enhanced steel comprising an average elastic modulus of greater than about 210 GPa, optionally wherein the steel comprises an average elastic modulus of up to about 300 GPa.

4. A vanadium alloyed steel according to claim 1, 2 or 3, wherein the coherent interphase precipitates have a particle size of about 9 nm or less.

5. A vanadium alloyed steel according to claim 4, wherein the coherent interphase precipitates have a particle size of in the range of from about 5 nm to about 9 nm.

6. A vanadium alloyed steel according to any preceding claim, wherein the coherent interphase precipitates comprise vanadium carbides.

7. A vanadium alloyed steel according to claim 6, wherein the coherent interphase precipitates comprise precipitates having enhanced levels of vanadium with the chemical formula V.sub.yC.sub.y, where x>y (e.g. the coherent interphase precipitates comprise V.sub.4C.sub.3).

8. A vanadium alloyed steel according to claim 7, wherein the coherent interphase precipitates comprise V.sub.6C.sub.5, and/or V.sub.5C.sub.3.

9. A vanadium alloyed steel according to any preceding claim, wherein the steel composition comprises at least one or more of (in wt. %): about 0.015 or less nitrogen, about 1.6 or less molybdenum, about 1 or less copper, about 1.2 or less silicon, about 0.3 or less chromium, and/or about 1.6 or less manganese.

10. A vanadium alloyed steel according to any preceding claim, wherein the steel composition comprises: carbon in the range of from about 0.06 to about 1.1. vanadium in the range of from about 0.1 to about 1.5, about 0.015 or less Nitrogen, about 1.6 or less Molybdenum, about 1 or less Copper, about 1.2 or less Silicon, about 0.3 or less Chromium, and about 1.6 or less Manganese.

11. A vanadium alloyed steel according to claim 9 or 10, wherein the coherent Interphase precipitates comprise Mo, optionally wherein the coherent interphase precipitates comprise precipitates having the chemical formula (Mo,V).sub.xC.sub.y, where x>y, e.g. (Mo,V).sub.4C.sub.3/(Mo,V)C.

12. A vanadium alloyed steel according to claim 9, 10 or 11, wherein the steel microstructure comprises VN precipitates, optionally wherein the microstructure comprises intra-granular VN nucleated acicular ferrite.

13. A vanadium alloyed steel according to any preceding claim, in which the steel comprises a ferrite phase and the coherent interphase precipitates are formed in the ferrite phase to form nodular or knotted ferrite.

14. A vanadium alloyed steel according to claim 13, wherein the ferrite phase comprises grains having a mean size of less than about 20 m, for example in the range of from about 5 m to about 20 m.

15. A vanadium alloyed steel according any preceding claim, wherein the steel comprises a single phase ferritic steel e.g. HSLA, AHSS.

16. A vanadium alloyed steel according to any of claims 1 to 14, wherein the steel comprises a pearlite phase, optionally wherein the steel comprises vanadium enhanced cementite.

17. A vanadium alloyed steel according to claim 16, wherein the vanadium enhanced cementite comprises Vanadium dissolved in cementite e.g. to form Fe.sub.2VC and/or FeV.sub.2C.

18. A vanadium alloyed steel according to any preceding claim, wherein the steel comprises a tensile strength in the range of about 360 Mpa to about 2000 Mpa.

19. A method for preparing a vanadium alloyed steel in accordance with any preceding claim, the method comprising: a. providing a starting steel; b. heating the starting steel to at least partially austenitise the starting steel; c. holding the temperature at 650 C.200 C. for about 25 mins or less.

20. A method according to claim 18, wherein the starting steel has a composition comprising (in wt %): carbon in the range of from about 0.06 to about 1.1, and vanadium in the range of from about 0.1 to about 1.5; and optionally one or more of (in w (%): about 0.015 or less Nitrogen, about 1.6 or less Molybdenum, about 1 or less Copper, about 1.2 or less Silicon, about 0.3 or less Chromium, and/or about 1.6 or less Manganese.

21. A method according to claim 19 or 20, wherein, prior to the temperature being held at 650 C.200 C., the steel is cooled at a cooling rate of in the range of from about 2 C./s to about 80 C./s, optionally to a temperature of 650 C.200 C.

22. A method according to claim 19, 20 or 21, wherein, after heating the starting steel, the steel is hot rolled at temperatures above the recrystallisation stop temperature (RST), optionally by recrystallisation controlled rolling and/or V(C,N) precipitation controlled rolling.

23. A method according to claim 22, wherein hot rolling the steel comprises hot rolling at substantially the V(C,N) precipitation temperature time nose.

24. A method according to any of claims 19 to 23, wherein, during or after the temperature is held, a magnetic field is applied to the steel composition.

25. A method according to any of claims 19 to 24, wherein after the temperature has been held at 650 C.200 C. for about 25 mins or less, the steel is reheated to at least partially austenitise the steel.

26. A method according to claim 25, wherein after the steel has been reheated, again holding the temperature at 650 C.200 C. for about 25 mins or less.

27. A method according to claim 25 or 26, wherein after the steel has been reheated, it is cooled at a cooling rate of in the range of from about 2 C./s to about 80 C./s, optionally to a temperature of 650 C.200 C.

Description

FIGURES

[0191] FIG. 1 illustrates a schematic showing the interaction of a coherent precipitate and an incoherent precipitate with a surrounding ferrite lattice matrix;

[0192] FIG. 2 shows a comparison of lattice parameter misfits and limiting coherent precipitate sizes for precipitates including Ti, V, Zr or Nb;

[0193] FIG. 3 shows a relationship between strength and precipitate particle size;

[0194] FIGS. 4a and 4b are schematic illustrations of the capacity of coherent interphase precipitates to act as a hydrogen trap;

[0195] FIG. 5a illustrates the impact of a strain field in a ferrite matrix on the average lattice parameter in a ferrite steel;

[0196] FIG. 5b illustrates the impact of a strain field in a ferrite matrix on the average lattice parameter in a pearlite steel;

[0197] FIG. 6 illustrates three typical beam compositions (in accordance with Eurocode 3);

[0198] FIG. 7a shows a schematic illustration of a VN-nucleated acicular ferrite structure;

[0199] FIG. 7b shows a schematic illustration of a core-shell nano-particle;

[0200] FIG. 8 shows a process diagram representative of a process for manufacturing a steel in accordance with the present disclosure;

[0201] FIG. 9 shows a flow chart describing additional steps that may be applied to the process of FIG. 8;

[0202] FIG. 10 shows a process diagram representative of a process for manufacturing a modulus-enhanced hot-rolled high carbon wire steel in accordance with the present disclosure; and

[0203] FIG. 11 shows a flow chart describing additional steps that may be applied to the process of FIG. 10.

EXAMPLES

[0204] The vanadium alloyed steels and production processes disclosed herein will now be explained with reference to the following non-limiting examples.

Example 1

[0205] Four exemplary steel compositions according to the present disclosure are described in table 1. Each composition also includes a balance amount of iron.

TABLE-US-00001 TABLE 1 (all wt %) Steel C Mn V Cu Si N Cr Mo Type Modulus- Max 0.2 1.6 0.3 0.5 0.5 0.015 0.3 0.2 Ferritic; Enhanced Fire Min 0.06 0.4 0.1 0 0 0 0 0 HSLA Resistant Alloy HSLA Modulus- Max 0.3 0.8 1.5 1 0.5 0.015 0.3 0.5 Ferritic; Enhanced Min 0.06 0 0.3 0 0 0 0 0 AHSS Nano- structured AHSS (3rd Gen) Modulus- Max 0.3 1.6 1.5 1 0.5 0.015 0.3 0.5 Enhanced Min 0.06 0 0.1 0 0 0 0 0 Ferritic; Hydrogen HSLA Resistant Steel Modulus- Max 1.1 1.6 1 0.5 1.2 0.01 0.1 0.2 Pearlitic; Enhanced Min 0.6 0.6 0.1 0 0 0 0 0 High- Pearlitic Steel Carbon

[0206] It has been determined that the ferritic compositions set out in Table 1 will have the characteristics and properties set out in Table 2.

TABLE-US-00002 TABLE 2 Steel Characteristics and Properties Max Min Average Elastic Modulus 300 Gpa 210 Gpa Ultimate Tensile Strength 1800 Mpa 690 Mpa

Example 2

[0207] A typical composition for an enhanced-modulus AHSS steel is provided in Table 3. Such a composition also includes a balance amount of iron. In this case, the balance amount of iron is 97.6955 wt %.

TABLE-US-00003 TABLE 3 (all wt %) Steel C Mn V Cu Si N Cr Mo Type Modulus- Typical 0.2 0.6 0.6 0.3 0.3 0.0045 0.1 0.2 Ferritic; Enhanced AHSS Nano- structured AHSS (3rd Gen)

Example 3

[0208] A conventional steel of grade S690MC (In accordance with EN10051) was compared to a nano-structured enhanced modulus steel as disclosed herein. The conventional 5690MC steel is taken as having a nominal elastic modulus of 210 GPa (in accordance with European Standard EN 1993 Jan. 1: Eurocode 3: Design of steel structures, and European Standard EN 1993 Jan. 12: General-High strength steels). The modulus enhanced steel is taken to have an average elastic modulus of 230 GPa, in other words, an increase of 20 GPa.

[0209] FIG. 6 illustrates three typical beam compositions (In accordance with Eurocode 3). The consequences with respect to reduced deflections with an enhanced elastic modulus are set out in Table 4. These results were obtained via a computer simulation.

TABLE-US-00004 TABLE 4 Max Deflection - Max Deflection - Nano-structured Current Steel Higher Modulus Steel Section a) 16.2 mm 14.7 mm Section b) 19.3 mm 17.6 mm Section c) 16.7 mm 15.2 mm

Example 4

[0210] With reference to FIG. 8, an exemplary method for producing the enhanced modulus steel disclosed herein will now be described.

[0211] A slab (or alternatively a billet) of cast steel is heated to a reheating temperature in the range of from about 1100 to about 1300 C. (Treh). This is indicated by reference numeral 102 on FIG. 8. This temperature is maintained until the steel slab is fully heated through its thickness (i.e. at Teq as shown in FIG. 8). By heating the steel slab in this way, the steel is heated to form austenite (), having a grain microstructure as illustrated in the schematic indicated by reference numeral 104.

[0212] The heated steel is then hot rolled at temperatures in the range of from about 820 C. to about 1200 C., firstly in a roughing mill as indicated by reference numeral 106, followed by hot rolling in a finishing mill indicated by reference numeral 108. During hot rolling. the temperature of the steel cools under standard air cooling, for example at about 0.7 C./s.

[0213] Hot rolling in the finishing mill 108 can be carried out using conventional recrystallisation controlled rolling. Alternatively, in particular where the steel has a composition including a relatively high amount of nitrogen. V(C,N) precipitation controlled rolling can be carried out. This is carried out at the V(C,N) precipitation time temperature nose 110, at which there is optimal free energy for VN to precipitate. The V(C,N) precipitation time temperature nose varies depending to the particular steel composition, however is typically about 850 C.

[0214] As the steel is hot rolled, in particular in the finishing mill, VN precipitates in austenite as intra-granular nucleants for acicular ferrite formation. At this stage, the steel has a microstructure as illustrated schematically at reference numeral 112.

[0215] Prior to the recrystallization stop temperature (RST), and after hot rolling has been completed, the steel is rapidly cooled 114 at a cooling rate in the range of from about 2 C./s to about 80 C./s until a temperature of 650100 C. is reached. This may be by forced air cooling, lamellar cooling or any other suitable cooling means. Rapidly cooling the steel in this way prevents or impedes the formation of undesirable phases and/or promotes the formation of acicular ferrite, illustrated schematically at reference numeral 116.

[0216] When the temperature of 650100 C. is reached, the steel is formed into a coll. The temperature of the steel is then held 118 at 650100 C. for a time in the range of from about 10 minutes to about 20 minutes, after which the steel is air cooled. As described above, isothermal holding in this manner promotes the generation of interphase precipitates.

[0217] In this way, a modulus enhanced nanostructured steel is produced.

[0218] Optionally, the method may further include the steps set out in FIG. 9 to increase the number density of interphase precipitates.

[0219] After the isothermal holding step 118, the coiled steel is cooled and then a blank is stamped or cut 120 from the decoiled strip. Optionally this step may be omitted.

[0220] The steel may then be austenitised 122 a second time by heating to a temperature in the range of from about 920 C. to about 1150 C., for example using a roller hearth furnace and/or inductive heating equipment. The reheated steel is then cooled 124 at a rate in the range of from about 2 C./s to about 50 C./s until a temperature of 65040 C. is reached. Cooling can be carried out using forced air cooling, lamellar cooling, or any other suitable method.

[0221] A static magnetic field in the range of from about 0.2T to about 16T can then be applied 26 to the steel. Whilst the magnetic field is applied, the temperature is held at 65040 C. for about 15 minutes or less. For example, this may be carried out using an insulated thermal box.

[0222] Alternatively, after a blank has been stamped or cut from the decoiled strip 120, a variable magnetic field is applied 128 to the steel. For example, this may be applied using an induction heating system. Whilst the variable magnetic field is applied, the temperature is held at 650100 C. for 15 minutes or less.

[0223] In alternative embodiments, a magnetic field is applied to the steel during the step of isothermal holding 118, either by using a static magnetic field or a variable magnetic field.

[0224] The resulting modulus-enhanced nanostructured steel is then air cooled 130 and ready for cold stamping and/or forming as required.

[0225] Alternatively, instead of decoiling and stamping a blank from the steel, a full coil can be reheated (i.e. austenitised) 122, cooled 124 then held at 650100 C. for the required time. This can be carried out as a batch annealing process using an insulated thermal box, e.g. a bell furnace suitable for applying heat treatment to full coils.

[0226] This method is applicable to both ferritic and pearlitic steels.

Example 5

[0227] FIG. 10 shows a method of making a modulus-enhanced hot-rolled high carbon wire steel. A billet of cast steel is heated 202 to a reheating temperature, as described in relation to example 4.

[0228] The heated steel is then hot rolled 206 at temperatures in the range of from about 840 C. to about 1200 C. using a roughing mill, intermediate mill, finishing mill and/or no-twist v-block mill. Hot rolling is carried out above the recrystallization stop temperature, such that grain deformation is kept to a minimum.

[0229] After hot rolling has been completed, the steel is rapidly cooled 214 at a cooling rate of in the range of from about 2 C./s to about 80 C./s until a temperature of 650 100 C. is reached.

[0230] The temperature is then held 218 at 650100 C., for example, using an in-line induction heating coil. With reference to FIG. 11, the steel is then air cooled 230.

[0231] Optionally, the steel may then be cold drawn 132. In this process, the steel wire is induction heated and austenised, fan cooled to 60050 C., then the temperature is held at 650200 C. for less than 5 minutes. These steps are applied in a continuous, slow moving fashion. Optionally, a static magnetic field may also be applied at this stage in order to refine the interlamellar spacing of the pearlite, which can further contribute to lattice strain and modulus enhancement.

[0232] Unless otherwise stated, each of the integers described herein may be used in combination with any other integer as would be understood by the person skilled in the art. Further, although all aspects of the invention preferably comprise the features described in relation to that aspect, it is specifically envisaged that they may consist or consist essentially of those features outlined in the claims. In addition, all terms. unless specifically defined herein, are intended to be given their commonly understood meaning in the art.

[0233] Further, in the discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, is to be construed as an implied statement that each intermediate value of said parameter, lying between the smaller and greater of the alternatives, is itself also disclosed as a possible value for the parameter.

[0234] In addition, unless otherwise stated, all numerical values appearing in this application are to be understood as being modified by the term about.