Steel sheet suitable for enamelling and method for producing same

Abstract

A process for producing an enamelable, cold-rolled, and finally annealed steel sheet is described. The steel sheet consists of the following elements (in wt.%): C: 0.05–0.09%, Mn: 1.0–2.0%, V: 0.02–0.1%, Nb: 0–0.3%, Ti: 0–0.3%, Si: <0.3%, Al: <0.1%, Ni: <0.35%, Co: <0.2%, N: <0.04%, S: <0.04%, P: <0.1%, Mo: <0.3%, Ca: <0.2%, the remainder being iron and unavoidable impurities.

Description

[0001] The invention relates to an enamelable, cold-rolled, and finally annealed steel sheet, as well as to a method for producing an enamelable, cold-rolled, and finally annealed steel sheet. Furthermore, the invention relates to a cold-rolled and finally annealed steel sheet that is enameled.

[0002] Enamelable steels are used, for example, in container construction (boilers, tanks, etc.) because the weight and / or material usage can be reduced through enameling.

[0003] However, steels suitable for enamelling usually have rather low strengths and elongation limits (yield strengths), which are further reduced during enamel firing.

[0004] Higher-strength, enamel-suitable steels are characterized by a lower loss of strength during enamel firing. A minimum value for the yield strength should not be exceeded to ensure sufficient formability of the steel.

[0005] Another important property of enamelable steels is their fishscale resistance. After the enameling process, fishscale-like defects can occur in the enamel layer, caused by hydrogen outgassing. These defects should be avoided. However, common alloy concepts using titanium and / or niobium for cold-rolled, enamelable steels often do not exhibit good fishscale resistance. Furthermore, these concepts often do not allow for a higher yield strength after enameling due to carbide coarsening and the associated grain coarsening during enamel firing.

[0006] One objective underlying the invention can be seen as creating an enamelable, cold-rolled, and finally annealed steel sheet that, after the enameling process, exhibits high strength and, at the same time, a sufficiently high yield strength for formability. Furthermore, the steel sheet should be resistant to fish scales.

[0007] The object underlying the invention is described by a method for producing an enamelable, cold-rolled, and finally annealed steel sheet. The steel sheet contains (in wt. %): C: 0.05–0.09%, Mn: 1.0–2.0%, V: 0.02–0.1%, Nb: 0–0.3%, Ti: 0–0.3%, Si: <0.3%, Al: <0.1%, Ni: <0.35%, Co: <0.2%, N: <0.04%, S: <0.04%, P: <0.1%, Mo: <0.3%, Ca: <0.2%, the remainder being iron and unavoidable impurities.

[0008] By adding vanadium, the strength increase of the enamelable steel achieved by the manganese microalloy is not lost during the enameling process, or is lost only to a lesser extent than with conventional microalloys. This means that the vanadium prevents the strength gained through microalloying from decreasing too much during the enameling process. This is achieved by vanadium dissolving during firing (enamelling) and, upon cooling (from the enameling process), precipitating out to refine the grain. A finer grain results in greater strength (i.e., counteracts the loss of strength that always occurs during enameling and thus maintains greater strength). At the same time, a high yield strength can be achieved and maintained even after enameling.

[0009] Another aspect is that the combination of manganese and vanadium allows for partial or even complete omission of titanium and / or niobium, which are typically used as microalloys to increase the strength of enamelable steel. This increases the steel's resistance to fish scales. For example, niobium contents of less than 0.1%, 0.05%, 0.04%, 0.03%, 0.02%, or 0.01% can be provided (here and below, all percentages related to alloying elements are in wt.%). Alternatively, or in combination, titanium contents of less than 0.1%, 0.05%, 0.04%, 0.03%, 0.02%, or 0.01% can also be provided.

[0010] Phosphorus increases strength and can be added in comparatively high concentrations, e.g., equal to, greater than, or less than 0.05% or 0.01%. Low-phosphorus steel sheets (with P < 0.005%) are also possible.

[0011] All elements for which no lower limit is specified are optional elements that cannot be included (ie with a content of 0%) in the steel composition.

[0012] The enamelled, cold-rolled and finally annealed steel sheet can have a yield strength Rp0.2 > 300 MPa (in the longitudinal direction of the steel sheet) after a single or two-fold enamel baking process.

[0013] As already mentioned, vanadium prevents excessive strength loss of the enameled steel sheet (by redissolving in the steel structure during enameling). For example, a vanadium content between 0.025% and 0.07%, especially between 0.03% and 0.05% or 0.06%, can be advantageous.

[0014] The manganese content can, for example, be between 1.1% and 1.8%, in particular between 1.2% and 1.7%. Further preferred ranges can be found in Table 1. This ensures high strength of the steel sheet. For the values in Table 1, the steel sheet consists of the alloying elements listed, the remainder being iron and unavoidable impurities.

[0015] In particular, the process described in more detail below allows the production of steel sheets with a high thickness. In addition to the usual thinner thicknesses, comparatively thicker sheets in the range of 1.6 mm to 4 mm, and especially 2 mm to 3 mm, can also be achieved.

[0016] An enameled, cold-rolled, and finally annealed steel sheet can have the composition of an enamelable steel sheet and a yield strength Rp0.2 > 300 MPa. Furthermore, it contains an enamel layer (because it is enameled).

[0017] A method for producing an enamelable, cold-rolled, and finally annealed steel sheet comprises melting a steel melt having the above-mentioned composition of the enamelable steel sheet. The method further comprises pouring the steel melt into a precursor, hot-rolling the precursor into a hot strip, cold-rolling the hot strip into a cold-rolled steel sheet, and finally annealing the cold-rolled steel sheet.

[0018] A final hot rolling temperature can, for example, be between 890°C and 950°C. In particular, a final hot rolling temperature can be between 920°C and 940°C, for example.

[0019] A high final hot rolling temperature can be advantageous because it promotes the formation of coarse (globular) cementite / pearlite. This is beneficial because it is "broken up" during cold rolling, thereby forming pores for hydrogen absorption. The more pores there are for hydrogen absorption, the lower the risk of fish scale formation during enameling.

[0020] The higher the final hot rolling temperature, the higher the coiling temperature can be set if coiling of the hot strip is planned.

[0021] Cold rolling of the hot strip to form cold-rolled steel sheet can be carried out in one or more stages. For example, a total cold rolling degree of at least 50%, 60%, or 65% can be achieved. Higher total cold rolling degrees above 75% and even 80% are also possible. The higher the total cold rolling degree, the more effective the creation of pores for hydrogen absorption (i.e., fish scale resistance can be increased).

[0022] The final annealing of cold-rolled steel sheet can be carried out, for example, in the range between 550°C and 700°C, in particular between 550°C and 650°C, or between 550°C and 620°C, or between 550°C and 600°C. The lower the final annealing temperature, the lower the strength loss of the cold-rolled steel sheet. However, the final annealing temperature must be above the recrystallization temperature of the steel sheet to ensure the desired formability.

[0023] While continuous annealing plants are typically used for the final annealing of enamelable steels, the final annealing of cold-rolled steel sheet can be carried out using a batch annealing process. With batch annealing, the annealing atmosphere can be precisely controlled, and relatively long annealing times and a uniform temperature distribution can be achieved cost-effectively. This allows for lower final annealing temperatures and thus lower strength losses.

[0024] For example, the batch annealing of cold-rolled steel sheet can be carried out in a batch annealing furnace with a total annealing time whose lower limit is, for example, 20, 25, or 30 hours and whose upper limit is, for example, 40 hours. Longer total annealing times can lead to strength losses.

[0025] As an alternative to batch annealing, the final annealing can also be carried out as a continuous annealing in a continuous annealing furnace. The annealing time in the continuous annealing furnace can be between 500 and 1000 seconds, depending on the thickness and width of the steel sheet. The holding time at the set maximum annealing temperature of, for example, 650–750°C, preferably 690–725°C, can be between 20 and 500 seconds, in particular between 50 and 300 seconds. Longer holding times of, for example, more than 200, 300, or 400 seconds allow for targeted hardening within the desired range.

[0026] The process may further comprise coiling the hot-rolled steel sheet. Particularly at high hot-rolling final temperatures, high coiling temperatures of 600°C - 750°C, in particular 650°C - 750°C, can advantageously be achieved. A high coiling temperature can promote the fish scale resistance of the cold-rolled steel sheet.

[0027] Examples and possible embodiments of the invention are explained in more detail below with reference to the drawing.

[0028] In this shows Figure 1 in schematic representation a process sequence for the production of an enamelable, cold-rolled steel sheet according to the present disclosure.

[0029] The following based on Figure 1 The process steps explained are merely examples and may be replaced or supplemented by other or similar process steps. In particular, additional processes may be provided between the process steps described below, which are not discussed in detail in this description.

[0030] The starting point of steel production is a blast furnace process 1 in which a steel melt is melted.

[0031] After a Figure 1After the steel has been subjected to post-treatment (secondary metallurgy) not shown, the steel melt has a composition within the ranges specified above.

[0032] Subsequently, the steel is cast 2, with which pre-products, for example so-called rolling ingots, are manufactured.

[0033] The precursors produced during the casting 2 of the molten steel (e.g., continuous casting) are then hot-rolled in a rolling station 3. Hot rolling takes place at a final rolling temperature between 890 and 950°C, preferably 920 and 950°C, to enable high coiling temperatures.

[0034] After hot rolling, the hot strip is optionally coiled into a coil in station 4. The coiling temperature can vary over a wide range, for example, from approximately 600°C to approximately 750°C. Coiling temperatures above 650°C, e.g., equal to or greater than 675°C, 700°C, or 725°C, are preferred. Since coiling temperatures above approximately 650°C increase fish scale resistance, high coiling temperatures can be advantageous.

[0035] In the further process, the hot strip is cold rolled in a rolling station 5. The total cold rolling degree can be at least 50% or higher, e.g., equal to or greater than 55%, 60%, 65%, or even 70%, 75%, or 80%. The higher the cold rolling degree, the more coarse cementite / pearlite is crushed during cold rolling, generating more pores for hydrogen absorption. These increase fish scale resistance.

[0036] After cold rolling, the cold-rolled steel sheet is annealed at a final annealing temperature (e.g., the temperature of an annealing furnace chamber) between 550°C and 700°C, in particular between 550°C and 650°C, or between 550°C and 620°C, or between 550°C and 600°C. The final annealing is carried out in a final annealing station 6, for example, a continuous annealing furnace or a bell-type annealing furnace. The annealing time (total annealing time) in a bell-type annealing furnace can, in particular, be equal to or greater than 20, 25, or 30 hours, and can, for example, have an upper limit of 40 hours. The final annealing, also referred to as recrystallization annealing (since the final annealing step causes recrystallization of the steel sheet), guarantees the formability of the steel and can, in all cases, enable a yield strength of Rp0.2 > 300MPa.Since strength losses increase with increasing final annealing temperature, low final annealing temperatures (which must, however, be above the recrystallization temperature) are preferred.

[0037] The enameling process can, for example, be carried out at the customer's site. It has been shown that during the subsequent enameling process, which can be carried out at 800–850°C in a firing station, a large portion of the vanadium dissolves again. Upon cooling, it has a grain-refining effect and thus increases strength. At the same time, high fish scale resistance can be achieved, for example, through a high coiling temperature and / or a high degree of cold rolling, as well as through microalloy-induced precipitation.

[0038] An enamelled sheet steel product can, for example, comprise a container (boiler, tank, silo, etc.).

[0039] The limit values of the alloying elements and their preferred ranges are summarized in a table below (UL: upper limit; PUL: preferred upper limit; SPUL: specifically preferred upper limit; SPLL: specifically preferred lower limit; PLL: preferred lower limit; LL: lower limit). Table 1 (Limit values and preferred ranges of alloying elements) C Mn V Nb Ti Si Al Ni Co N S P UL 0,09 2,0 0,1 0,3 0,3 0,3 0,1 0,35 0,2 0,04 0,03 0,1 PUL 0,08 1,7 0,07 0,1 0,1 0,1 0,05 0,2 0,1 0,02 0,027 0,07 SPOOL 0,075 1,5 0,05 0,045 0,08 0,02 0,035 0,15 0,08 0,015 0,025 0,07 SPLL 0,065 1,25 0,03 0,02 0,004 0,01 0,025 0,05 0,01 0,005 0,005 0,02 PLL 0,06 1,15 0,025 0,015 0,002 0,01 0,02 0,01 0,005 0,003 0,001 0,015 LL 0,05 1,0 0,02 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 All values are given in wt%. Table 1 (continued) Mon Ca UL 0,3 0,3 PUL 0,2 0,2 SPOOL 0,1 0,15 SPLL 0,08 0,07 PLL 0,05 0,04 LL 0,0 0,0 All values are given in wt%. Examples

[0040] Table 2 shows steel compositions (alloys: LG) Nos. 1 to 9. Alloys Nos. 3 to 9 are alloys according to the invention, while alloys Nos. 1 and 2 are not according to the invention due to their low V and Mn contents (relative to alloy No. 1). The residual content of all alloys consists of iron and unavoidable impurities. Furthermore, the table lists the necessary additional properties (fish scale resistance, adhesion) with respect to the enamel layer. Table 2 (Examples of steel compositions) LG C Mn V Nb Ti Si Al Ni Co N S P 1 0,060 0,55 0,002 0,002 0,0023 0,010 0,043 0,010 0,002 0,0042 0,0067 0,0740 2 0,076 1,42 0,002 0,049 0,0020 0,054 0,041 0,015 0,020 0,0040 0,0012 0,0058 3 0,077 1,46 0,027 0,045 0,0670 0,024 0,042 0,011 0,021 0,0056 0,0050 0,0085 4 0,061 1,03 0,054 0,030 0,0020 0,018 0,039 0,040 0,019 0,0063 0,0015 0,0093 5 0,054 1,23 0,036 0,023 0,0330 0,030 0,048 0,100 0,080 0,0051 0,0041 0,0076 6 0,072 1,10 0,021 0,002 0,0500 0,028 0,051 0,030 0,090 0,0063 0,0070 0,0096 7 0,068 1,38 0,032 0,017 0,0020 0,023 0,043 0,010 0,100 0,0050 0,0053 0,0100 8 0,073 1,31 0,036 0,015 0,0230 0,028 0,046 0,016 0,030 0,0060 0,0048 0,0068 9 0,071 1,28 0,025 0,035 0,0480 0,029 0,045 0,120 0,090 0,0057 0,0062 0,0072 All values are given in wt%. Table 2 (continued) LG Mon Ca Fish scale resistance Liability 1 0,11 0,01 good good 2 0,21 0,01 bad bad 3 0,09 0,12 good good 4 0,12 0,05 very good good 5 0,01 0,09 good good 6 0,03 0,07 good good 7 0,04 0,14 good very good 8 0,22 0,11 good good 9 0,15 0,02 good good

[0041] It is evident that alloy No. 2, which has a high manganese content but too low a vanadium content, does not allow for a sufficient quality of the enamel layer.

[0042] Table 3 shows test results for alloys Nos. 1 to 9 at different final bell annealing temperatures. The annealing time (total annealing time) was set so that the final annealing temperature was reached for all areas of the steel strip (holding time, for example, approximately 0.5–1 hour). The total annealing time for the coil was approximately 35 hours (longer total annealing times can also be used). The enamel firing always took place at 830°C. For steel specimens of alloys Nos. 1 to 5, the mechanical properties were also determined for double enamel firing. The mechanical properties of tensile tests on the single- and double-enamelled steel specimens were determined as the yield strength at 0.2% plastic deformation (Rp0.2) in MPa and the elongation at break (A30) in percent. The product of the yield strength and the elongation at break is also given. Table 3 (mechanical characteristics for single and double enamel firing processes) LG Hood annealing temp. Enamel baking process (830°C) Rp0.2 (MPa) A30 (%) Rp0.2 × A30 (MPa%) Enamel firing processes (830°C) Rp0.2 (MPa) A30 (%) Rp0.2 × A30 (MPa%) 1 600 1x 287 34 9758 2× 263 40 10520 1 650 1x 277 36 9972 2× 256 40 10240 1 700 1x 243 41 9963 2x 240 41 9840 2 600 1x 356 31 11036 2x 289 36 10883 2 650 1x 341 33 11253 2x 276 33 10277 2 700 1x 335 39 12931 2x 264 35 9307 3 600 1x 389 29 11370 2x 367 31 11423 3 650 1x 344 29 10010 2x 328 28 9247 3 700 1x 305 33 10072 2x 304 29 8749 4 600 1x 357 29 10353 2x 348 32 11136 4 650 1x 336 30 10080 2x 331 33 10923 4 700 1x 298 34 10132 2× 309 36 11124 5 600 1x 373 28 10444 2x 351 33 11583 5 650 1x 358 31 11098 2× 329 32 10528 5 700 1x 309 36 11124 2x 305 34 10370 6 600 1x 379 30 11370 6 650 1x 360 29 10440 6 700 1x 311 34 10574 7 600 1x 385 30 11550 7 650 1x 351 32 11232 7 700 1x 316 37 11692 8 600 1x 381 29 11049 8 650 1x 349 30 10470 8 700 1x 310 35 10850 9 600 1x 381 27 10287 9 650 1x 345 29 10005 9 700 1x 303 35 10605

[0043] Alloy No. 2 without vanadium shows a significant loss of yield strength (Rp0.2 value) after penetration. The Rp0.2 values are already below 300 MPa after a single penetration, and an even greater decrease occurs after double penetration. The steel samples according to the invention consistently showed an Rp0.2 value above 300 MPa even after double penetration, and in some cases significantly higher (e.g., above 320 MPa, 330 MPa, 340 MPa, and occasionally above 350 MPa (Alloy No. 5)).

[0044] This means that the compositions Nos. 3 to 9 according to the invention with higher vanadium contents show a significant improvement in the Rp0.2 values, so that even after two firings, values above 300 MPa can still be guaranteed.

[0045] Table 4 shows test results for alloy No. 3 at different final bell annealing temperatures. The mechanical data were determined before enamel firing (0 firings), after a single enamel firing (1 firing), and after two enamel firings (2 firings). The tensile tests were conducted in the longitudinal direction (L) and in the transverse direction (Q). In addition to the mechanical data already mentioned, the tensile strength (Rm) in MPa, the uniform elongation (Ag) in percent, the elastic modulus (EMODUL) in kN / mm², and the Vickers hardness (HV5: test force 5 kp = 49.03 N) in HV (Vickers hardness) were determined. The enamel firing was always carried out at 800°C. Table 4 (Alloy No. 3 - mechanical data) Burn-in processes Hood annealing temperature Tensile tests hardness Direction Rp0.2 (MPa) Rm (MPa) Ag (%) A30 (%) EMODULE (kN / mm 2< ) HV5 (HV) 0 600 L 566 636 10,1 18,8 205 157 Q 638 688 8,3 14,7 223 650 L 463 533 15,0 25,7 221 149 Q 490 547 10,8 20,6 213 700 L 403 485 15,5 30,5 203 153 Q 431 499 14,7 28,9 206 1 600 L 389 489 15,4 29,2 207 165 Q 426 506 14,8 27,2 219 650 L 340 460 15,4 29,1 200 161 Q 354 467 13,6 24,6 203 700 L 305 468 16,8 30,9 200 170 Q 307 491 15,2 30,3 198 2 600 L 367 480 16,3 31,1 181 149 Q 389 499 15,4 29,8 220 650 L 328 456 15,6 28,8 195 167 Q 337 464 13,8 27,0 211 700 L 304 464 16,8 28,8 193 155 Q 323 477 16,5 32,0 202

[0046] Table 4 shows that enameling could be carried out with virtually no loss of hardness. Furthermore, a high tensile strength Rm of the steel sheet could be maintained even after one or two enameling cycles.

[0047] By using Mn-V in combination, the strength loss between single and double firing was limited to < 15 MPa. Depending on the selected final annealing temperature, the strength loss between the final annealed material and the single-fired material was limited to less than 150 MPa. The higher the final annealing temperature, the lower the strength loss after a single enamel firing.

Claims

1. Enamelable, cold-rolled and finally annealed steel sheet, the steel sheet consisting of the following elements (in wt.%): C: 0.05 - 0.09%, Mn: 0.5 - 2.0%, V: 0.02 - 0.1%, Nb: 0 - 0.3%, Ti: 0 - 0.3%, Si: < 0.3%, Al: < 0.1%, Ni: < 0.35%, Co: < 0.2%, N: < 0.04%, S: < 0.04%, P: < 0.1%, Mo: < 0.3%, Ca: < 0.2%, the remainder being iron and unavoidable impurities.

2. Enamelable, cold-rolled and finally annealed steel sheet according to claim 1, wherein the steel sheet has a yield strength Rp0.2 > 300 MPa after a single enameling process.

3. Enamelable, cold-rolled and finally annealed steel sheet according to claim 1 or 2, wherein the steel sheet has a yield strength Rp0.2 > 300 MPa after a two-stage enameling process.

4. Enamelable, cold-rolled and finally annealed steel sheet according to one of the preceding claims, wherein V: 0.025 - 0.07% or 0.03 - 0.06%.

5. Enamelable, cold-rolled and finally annealed steel sheet according to one of the preceding claims, wherein Mn: 1.1 - 1.8% or 1.2 - 1.7%.

6. Enamelable, cold-rolled and finally annealed steel sheet according to one of the preceding claims, wherein the steel sheet has a thickness in the range of 1.6 mm to 4 mm, or between 2 mm and 3 mm.

7. Enamelled, cold-rolled and finally annealed steel sheet, the steel sheet consisting of the following elements (in wt.%): C: 0.05 - 0.09%, Mn: 1.0 - 2.0%, V: 0.02 - 0.1%, Nb: 0 - 0.3%, Ti: 0 - 0.3%, Si: < 0.3%, Al: < 0.1%, Ni: < 0.35%, Co: < 0.2%, N: < 0.04%, S: < 0.04%, P: < 0.1%, Mo: < 0.3%, Ca: < 0.2%, the balance being iron and unavoidable impurities, and having a yield strength Rp0.2 > 300 MPa.

8. A process for producing an enamelable, cold-rolled and finally annealed steel sheet, comprising: melting a steel melt consisting of the following elements (in wt. %): C: 0.05 - 0.09%, Mn: 1.0 - 2.0%, V: 0.02 - 0.1%, Nb: 0 - 0.3%, Ti: 0 - 0.3%, Si: <0.3%, Al: <0.1%, Ni: <0.35%, Co: <0.2%, N: <0.04%, S: <0.04%, P: <0.1%, Mo: <0.3%, Ca: <0.2%, the remainder being iron and unavoidable impurities; casting the steel melt to form a precursor; hot-rolling the precursor to form a hot strip; Cold rolling of the hot strip into a cold-rolled steel sheet; and final annealing of the cold-rolled steel sheet.

9. The method according to claim 8, wherein a hot rolling final temperature is 890 - 950°C.

10. The method according to claim 8 or 9, wherein the cold rolling of the hot strip into a cold-rolled steel sheet is carried out in one or more stages with a total cold rolling degree of at least 50% or 60% or 65%.

11. The method according to any one of claims 8 to 10, wherein the final annealing of the cold-rolled steel sheet is carried out at a final annealing temperature in the range between 550°C and 700°C, in particular 550°C and 650°C or 550°C and 620°C or 550°C and 600°C.

12. The method according to any one of claims 8 to 11, wherein the final annealing of the cold-rolled steel sheet comprises a batch annealing.

13. The method according to claim 12, wherein the batch annealing of the cold-rolled steel sheet is carried out with a total annealing time whose lower limit is 20 or 25 or 30 hours and whose upper limit is 40 hours.

14. A method according to any one of claims 8 to 10, wherein the final annealing of the cold-rolled steel sheet is carried out as a continuous annealing in a continuous annealing furnace.

15. The method according to claim 14, wherein an annealing temperature of 650 - 750°C, preferably 690 - 725°C, is set in the continuous annealing furnace.

16. The method according to any one of claims 8 to 15, further comprising: coiling the hot-rolled steel sheet, wherein a coiling temperature is 600 - 750°C.

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