METHOD FOR PREPARING TITANIUM-CONTAINING ULTRA-LOW-CARBON STEEL

Abstract

Disclosed is a method for preparing a titanium-containing ultra-low-carbon steel, comprising molten iron pretreatment, converter primary smelting, vacuum refining, continuous casting, hot rolling, pickling, and cold rolling. After vacuum refining decarbonization is finished, the content of free oxygen in molten steel is 100-350 ppm; after Al is then added for deoxidation treatment, the circulation time of the molten steel is greater than or equal to 3 min; after other alloys and rare earth elements are then added to the molten steel to adjust the components of the molten steel to the specifications of a finished product, the circulation time of the molten steel is greater than or equal to 2 min; and finally, an oxide Re.sub.2O.sub.3.Math.Al.sub.2O.sub.3 is generated in the molten steel, and the vacuum refining is finished. The method can effectively improve the properties of a deoxidation inclusion in steel, solve the problem of smooth running of casting of the molten steel, reduce the incidence of cold rolling defects caused by Al.sub.2O.sub.3, and improve the product quality of the titanium-containing ultra-low-carbon steel.

Claims

1. A method for preparing a titanium-containing ultra-low-carbon steel, comprising hot metal pretreatment, primary converter refining, vacuum refining, continuous casting, hot rolling, pickling and cold rolling; wherein in the vacuum refining, after decarburization is completed, a free oxygen content in molten steel is 100 to 350 ppm; then Al is added for deoxidization treatment, after which a circulation time of the molten steel is ?3 min; then additional alloying and rare earth components are added to the molten steel, after which a circulation time of the molten steel is ?2 min, wherein an oxide of Re.sub.2O.sub.3.Math. Al.sub.2O.sub.3 is finally generated in the molten steel; and the vacuum refining is completed.

2. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 1, wherein in the vacuum refining, before the decarburization treatment, the free oxygen content in the molten steel is adjusted to satisfy a mass ratio O/C=1.25 to 2.15; and/or the rare earth component comprises Ce or La, and is added in an amount based on a mass ratio REM/T.O=0.7 to 3.0, wherein REM represents a mass of the rare earth component added in kg, and T.O represents a total oxygen amount in the steel in ppm; and/or a content of impurities other than rare earth element(s) in the rare earth component is <0.1 wt %, wherein a total oxygen amount T.O is <100 ppm, and a N content is ?30 ppm.

3. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 2, wherein the oxide of Re.sub.2O.sub.3.Math.Al.sub.2O.sub.3 is Ce.sub.2O.sub.3.Math.Al.sub.2O.sub.3 or La.sub.2O.sub.3.Math.Al.sub.2O.sub.3.

4. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 2, wherein a vacuum refining device used for the vacuum refining is an RH furnace or a VD furnace or a VOD furnace.

5. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 1, wherein in the hot metal pretreatment, KR desulfurization is adopted, after which 70% to 80%, of a hot metal ladle top slag is removed; and/or a S content in the hot metal after the desulfurization is ?20 ppm.

6. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 1, wherein in the primary converter refining, a top-bottom combined blowing process is adopted, wherein when blowing is stopped, the free oxygen content in the molten steel is ?600 ppm; and/or during tapping, when a tapping amount reaches 1/6 to 1/4, lime is added to a steel ladle in an amount of 1.6 to 3 kg/t steel; and when the tapping amount reaches 4/5 or more, aluminum slag is added to the steel ladle in an amount of 1.0 to 1.4 kg/t steel; and/or after the tapping is completed, a composition of a steel ladle top slag is adjusted to: CaO=40 to 50 wt %, FeO+MnO?7.0 wt %.

7. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 1, wherein the titanium-containing ultra-low-carbon steel comprises the following components in mass percentage: C?0.005%, Si?0.05%, Mn: 0.05 to 0.3%, Al: 0.04 to 0.15%, Ti: 0.04 to 0.1%, P?0.05%, S?0.02%, N?0.003%, and a balance of Fe and unavoidable impurities, wherein an Al content is greater than a Ti content.

8. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 1, wherein the titanium-containing ultra-low-carbon steel comprises the following components in mass percentage: C<0.0018%, Si?0.03%, Mn: 0.07 to 0.15%, Al: 0.04 to 0.07%, Ti: 0.04 to 0.06%, P?0.015%, S?0.005%, N?0.003%, and a balance of Fe and unavoidable impurities, wherein an Al content is greater than a Ti content.

9. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 1, wherein after Al is added for deoxidization treatment, the circulation time of the molten steel is 3 to 10 min; and/or after the additional alloying and rare earth components are added to the molten steel, the circulation time of the molten steel is 2 to 10 min.

10. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 1, wherein during the continuous casting, a pass rate of the mold liquid level fluctuation within ?5 mm is >92%; and/or a pass rate of the mold liquid level fluctuation within ?3 mm is >32%.

11. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 1, wherein during the cold rolling, a cold rolling defect rate caused by Al.sub.2O.sub.3 is less than 0.05%.

12. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 1, wherein a titanium consumption in said method is less than 0.7 kg/t steel.

13. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 2, wherein in the vacuum refining, before the decarburization treatment, the free oxygen content in the molten steel is adjusted to satisfy a mass ratio O/C=1.3 to 2.0.

14. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 5, wherein in the hot metal pretreatment, KR desulfurization is adopted, after which 3/4 of a hot metal ladle top slag is removed.

15. The method for preparing a titanium-containing ultra-low-carbon steel according to claim 6, wherein in the primary converter refining, during tapping, when a tapping amount reaches 1/5, lime is added to a steel ladle in an amount of 1.6 to 3 kg/t steel; and when the tapping amount reaches 9/10, aluminum slag is added to the steel ladle in an amount of 1.0 to 1.4 kg/t steel.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0052] Other features, objects and advantages of the present disclosure will become more apparent by reading the detailed description of the non-limiting embodiments with reference to the following drawings:

[0053] FIG. 1 is a schematic flow chart of the method for preparing titanium-containing ultra-low-carbon steel according to some embodiments of the present disclosure;

[0054] FIG. 2 is a schematic view of the typical inclusions in the cold-rolled finished steel obtained according to a conventional process;

[0055] FIG. 3 is a schematic view of the typical inclusions in the cold-rolled finished steel obtained according to the present disclosure;

[0056] FIG. 4 is a schematic diagram of the pass rate of the liquid level fluctuation in the mold;

[0057] FIG. 5 is a schematic diagram of the undergrade ratio of steel slab.

DETAILED DESCRIPTIONS

[0058] In order to better understand the above technical solutions of the present disclosure, the technical solutions of the present disclosure will be further described below with reference to the Examples.

[0059] As shown in FIG. 1, the method for preparing a titanium-containing ultra-low-carbon steel provided by the present disclosure includes hot metal pretreatment, primary converter refining, vacuum refining, continuous casting, hot rolling, pickling and cold rolling. After decarburization in the vacuum refining process is completed, the free oxygen content in the molten steel is 100 to 350 ppm. Then, after Al is added for deoxidization treatment, the circulation time of the molten steel is ?3 minutes. Then, additional alloying and rare earth components are added to the molten steel, and the circulation time of the molten steel is ?2 minutes. Oxide Re.sub.2O.sub.3.Math.Al.sub.2O.sub.3 is generated in the final molten steel. The vacuum refining is completed. The additional alloying component added during the vacuum refining is determined according to the specific composition of the finished steel. For example, the additional alloying component may include but is not limited to one or more of alloying elements Mn, Nb, V, and B, etc.

[0060] In the hot metal pretreatment, the hot metal is desulfurized by the KR process. After desulfurization, 3/4 of the top slag in the hot metal ladle is removed, wherein the S content in the hot metal after desulfurization is ?20 ppm.

[0061] During the primary converter refining process, a top-bottom combined blowing process is used for the converter to ensure the strength of the bottom blowing. When the blowing is stopped, the free oxygen content in the molten steel is ?600 ppm. During tapping from the converter, when the tapping amount reaches 1/5, lime is added to the steel ladle in an amount of 1.6 to 3 kg/t steel. When the tapping amount reaches 9/10, aluminum slag is added to the steel ladle in an amount of 1.0 to 1.4 kg/t steel. In some embodiments, after the tapping in the primary converter refining process is completed, the top slag in the steel ladle is modified, and the composition of the top slag in the steel ladle is adjusted to: CaO=40 to 50 wt %, FeO+MnO?7.0 wt %. After the modification to the top slag in the steel ladle is completed, vacuum refining is performed.

[0062] During the vacuum refining process, in the early stage of the vacuum refining process, the free oxygen content in the molten steel is adjusted to satisfy the mass ratio O/C=1.25 to 2.15, preferably O/C=1.27 to 2.1, in some embodiments, O/C=1.3 to 2.0. Then, when the decarburization treatment is completed, the free oxygen O in the molten steel is between 100 and 350 ppm. In some embodiments, the free oxygen O is between 100 and 300 ppm. After Al is added for deoxidization treatment, the molten steel continues to circulate for a time ?3 min. In the later stage of the vacuum refining process, additional alloying element(s) and rare earth(s) (including rare earth element Ce or La) are added. The composition and temperature of the molten steel are adjusted to the specified ranges, and the circulation time of the molten steel is ?2 min. Finally, oxide Re.sub.2O.sub.3.Math.Al.sub.2O.sub.3 (such as Ce.sub.2O.sub.3.Math.Al.sub.2O.sub.3 or La.sub.2O.sub.3.Math.Al.sub.2O.sub.3) is generated in the molten steel. The vacuum refining is completed. The amount of the rare earth added is based on the mass ratio REM/T.O=0.7 to 3.0, wherein REM represents the mass of the rare earth in kg, and T.O represents the total oxygen in the steel in ppm. In the rare earth added, the content of the impurities other than the rare earth element(s) is <0.1 wt %, wherein the total oxygen T.O is <100 ppm, and the N content is ?30 ppm.

[0063] The steel types suitable for the above method for preparing a titanium-containing ultra-low-carbon steel are titanium-containing ultra-low-carbon steel products. Such titanium-containing ultra-low-carbon steel comprises the following components in mass percentage: C?0.005%, Si?0.05%, Mn: 0.05 to 0.3%, Al: 0.04 to 0.15%, Ti: 0.04 to 0.1%, P?0.05%, S?0.02%, N?0.003%, and a balance of Fe and unavoidable impurities, wherein the Al content is greater than the Ti content to ensure that the final deoxidization of the molten steel before the rare earth is added is controlled by Al in the molten steel. During the casting process, the pass rate of the mold liquid level fluctuation within ?5 mm is >92%; and the pass rate of the mold liquid level fluctuation within ?3 mm is >32%.

[0064] The method for preparing the titanium-containing ultra-low-carbon steel of the present disclosure is further demonstrated below with reference to the specific Examples. In the Examples, the titanium-containing ultra-low-carbon steel comprises the following components in mass percentage: C?0.0018%, Si?0.03%, Mn: 0.07 to 0.15%, Al: 0.04 to 0.07%, Ti: 0.04 to 0.06%, P?0.015%, S?0.005%, N?0.003%, and a balance of Fe and unavoidable impurities, wherein the Al content is greater than the Ti content.

EXAMPLE 1

[0065] The process path used in this Example was hot metal pretreatment (hot metal desulfurization and dephosphorization).fwdarw.primary converter refining (top-bottom combined converter blowing, tapping).fwdarw.modification to the top slag in the steel ladle.fwdarw.vacuum refining (decarburization, deoxidization, alloying and rare earth treatment).fwdarw.continuous casting.fwdarw.hot rolling.fwdarw.pickling.fwdarw.cold rolling.

[0066] This Example demonstrates a typical smelting scheme according to the present disclosure. The KR process was used for desulfurization. After the desulfurization, 3/4 of the top slag of the hot metal ladle was removed. The S content in the desulfurized hot metal was 15 ppm. During the primary converter refining process, top-bottom combined blowing was employed. When the converter blowing was completed, C=220 ppm, O=580 ppm in the molten steel. Slag cutoff tapping was performed. In the early stage of tapping (when the tapping amount reached 1/5), lime was added in an amount of 2.2 kg/t steel. In the final stage (when the tapping amount reached 9/10), aluminum slag was added in an amount of 1.1 kg/t steel. Before the vacuum refining treatment, the composition of the top slag in the steel ladle was FeO+MnO=6.50 wt %, CaO: 42 wt %, and the slag thickness was 110 mm. In the early stage of the vacuum refining treatment (before the decarburization treatment), the free oxygen content in the molten steel was adjusted, so that the mass ratio O/C in the molten steel=1.27. When the decarburization in the vacuum refining process was completed, the free oxygen O in the molten steel: 320 ppm. Then, after Al was added for decarburization, the molten steel continued to circulate for 4.5 min. In the later stage of the vacuum refining process, additional alloying and rare earth components were added. The rare earth component was CeLa alloy (Ce:La mass ratio: 2:1). The content of the impurities other than the rare earth elements in the rare earth component was <0.1 wt %, wherein the total oxygen T.O was <100 ppm, and the N content was ?30 ppm. The composition of the molten steel was adjusted to the specified range. After adding the rare earth, the molten steel was circulated for 5 minutes. After the refining was completed, continuous casting was performed, followed by hot rolling, pickling and cold rolling, where REM/T.O=1.2.

[0067] Process effects: during the continuous casting process in this Example, the pass rate of the mold liquid level fluctuation within ?5 mm was 94.2%, and the pass rate of the mold liquid level fluctuation within ?5 mm was 36%. For the cold-rolled steel in this Example, the undergrade ratio of steel slab was 40%, and the steel defect rate caused by Al.sub.2O.sub.3 was 0.02%.

[0068] Tables 1 and 2 show some other embodiments of the technical solution of the present disclosure used in actual production, as well as Comparative Group I using titanium pre-deoxidization and rare earth treatment, and Comparative Group II using a conventional process without rare earth treatment, for comparison. The process for Comparative Group I (Comparative Examples 1 to 6): hot metal pretreatment (desulfurization, dephosphorization).fwdarw.primary refining (top-bottom combined converter blowing, tapping).fwdarw.modification to the top slag in the steel ladle.fwdarw.vacuum refining (decarburization, titanium pre-deoxidization, Al deoxidization, alloying and rare earth treatment).fwdarw.continuous casting.fwdarw.hot rolling.fwdarw.pickling.fwdarw.cold rolling. The process for Comparative Group II (Comparative Examples 7 to 12): hot metal pretreatment (desulfurization, dephosphorization).fwdarw.primary refining (top-bottom combined converter blowing, tapping).fwdarw.modification to the top slag in the steel ladle.fwdarw.vacuum refining (decarburization, Al deoxidization, alloying).fwdarw.continuous casting.fwdarw.hot rolling.fwdarw.pickling.fwdarw.cold rolling. The differences of Examples 2 to 6, Comparative Group I and Comparative Group II from Example 1 in process parameters are shown in Table 1.

[0069] As shown in Tables 1 and 2, as compared with the process including titanium pre-deoxidization and rare earth treatment and the process including the conventional process without rare earth treatment, during the continuous casting process of the method for preparing the titanium-containing ultra-low-carbon steel of the present disclosure, the pass rates of the mold liquid level fluctuation within +5 mm and +3 mm are >92% and >32% respectively, better than the conventional process without rare earth treatment. The composition of the oxide inclusions in the titanium-containing ultra-low-carbon steel of the present disclosure changes from sole Al.sub.2O.sub.3 to Re.sub.2O.sub.3.Math.Al.sub.2O.sub.3. The undergrade ratio of steel slab is about 35% for the titanium-containing ultra-low-carbon steel of the present disclosure, better than the conventional process without rare earth treatment (about 37% on average). The vacuum refining time is less than 27 minutes, comparable to the conventional process without rare earth treatment. The titanium consumption is comparable to the conventional process without rare earth treatment, and about 0.5 kg/t steel less than the process including first addition of titanium and rare earth treatment. With the use of the method for manufacturing a titanium-containing ultra-low-carbon steel according to the present disclosure, the vacuum refining time and the titanium consumption are comparable to the conventional process without rare earth treatment, and can ensure smooth working of continuous casting process, greatly reduce the cold rolling defect rate caused by Al.sub.2O.sub.3 (>90% lower), and significantly improve the product quality of the titanium-containing ultra-low-carbon steel.

[0070] Therefore, the method for preparing a titanium-containing ultra-low-carbon steel according to the present disclosure can effectively improve the characteristics of the deoxidization inclusions in the steel, solve the smooth working problem of the molten steel in continuous casting process, and reduce the rate of cold rolling defects caused by Al.sub.2O.sub.3 in the cold-rolled finished steel. It's suitable for improving the product quality of the titanium-containing ultra-low-carbon steel product, and valuable for promotion and application in steel mills.

TABLE-US-00001 TABLE 1 Free oxygen content in molten steel/ppm Steel ladle slag At the point composition before Oxygen/carbon ratio of stopping vacuum decarburization/% before vacuum converter At the end of CaO FeO + MnO decarburization blowing decarburization REM/T.O Ex. 1 42 6.5 1.27 580 320 1.20 2 44 6.2 1.35 550 310 1.50 3 47 5.8 1.5 500 300 1.45 4 45 5.5 1.45 540 315 2.35 5 48 5.7 1.7 570 320 1.25 6 45 5.0 2.1 545 305 2.22 Comparative 1 46 6.4 1.68 650 370 1.32 Group I 2 43 6.2 1.55 700 402 0.95 3 45 5.7 2.2 680 380 2.14 4 49 5.4 3.2 630 350 2.38 5 42 6.6 2.7 640 355 1.64 6 47 4.8 2.4 620 330 0.89 Comparative 7 49 5.5 2.3 650 390 0 Group II 8 42 5.4 2.5 530 315 0 9 52 6.3 3.1 680 370 0 10 44 6.6 2.4 580 330 0 11 46 5.7 2.1 625 348 0 12 47 5.2 1.8 644 360 0

TABLE-US-00002 TABLE 2 Cold RH Pass rate of mold rolling Titanium vacuum Oxide liquid level Undergrade defect rate consumption refining composition fluctuation/% rate of steel caused by (kg/t steel) time/min in steel ?5 mm ?3 mm slab/% Al.sub.2O.sub.3/% Ex. 1 0.67 26 Re.sub.2O.sub.3Al.sub.2O.sub.3 94.2 36 40 0.02 2 0.65 25 Re.sub.2O.sub.3Al.sub.2O.sub.3 96.2 37 30 0.01 3 0.66 25 Re.sub.2O.sub.3Al.sub.2O.sub.3 97 38 20 0.00 4 0.66 26 Re.sub.2O.sub.3Al.sub.2O.sub.3 97 37 30 0.01 5 0.68 24 Re.sub.2O.sub.3Al.sub.2O.sub.3 95.8 36.2 40 0.035 6 0.67 24.5 Re.sub.2O.sub.3Al.sub.2O.sub.3 96 36 30 0.012 Comparative 1 1.16 31 Re.sub.2O.sub.3Al.sub.2O.sub.3 87.1 29 30 0.04 Group I 2 1.15 32 Re.sub.2O.sub.3Al.sub.2O.sub.3 86.1 30 40 0.042 3 1.17 30 Re.sub.2O.sub.3Al.sub.2O.sub.3 89.5 28.5 40 0.01 4 1.18 28 Re.sub.2O.sub.3Al.sub.2O.sub.3 88.7 28 30 0.025 5 1.16 30 Re.sub.2O.sub.3Al.sub.2O.sub.3 88.1 29 40 0.03 6 1.17 29 Re.sub.2O.sub.3Al.sub.2O.sub.3 86.6 30 30 0.02 Comparative 7 0.66 25.5 Al.sub.2O.sub.3 89.1 30.5 40 1.48 Group II 8 0.67 24 Al.sub.2O.sub.3 87.1 27 30 1.55 9 0.67 26 Al.sub.2O.sub.3 90.2 29 40 1.95 10 0.65 28 Al.sub.2O.sub.3 88.6 27.8 30 1.75 11 0.66 25 Al.sub.2O.sub.3 90.1 30 40 1.37 12 0.68 27 Al.sub.2O.sub.3 86.2 29 30 1.35

[0071] One of ordinary skill in the art should recognize that the above Examples are only used to illustrate the present disclosure and are not used to limit the present disclosure. All changes and modifications made to the above Examples fall within the scope defined by the claims of the present disclosure without departing from the spirit and scope of the present disclosure.