V-N MICROALLOYED STEEL AND METHOD FOR PRODUCING V-N MICROALLOYED AND SURFACE-CRACK-FREE CONTINUOUS CASTING BLANK

Abstract

Disclosed are a V-N microalloyed steel and a method for producing a V-N microalloyed and surface-crack-free continuous casting blank. The V-N microalloyed steel is composed of the following chemical components by mass percentage: 0.09%-0.13% of C, 0.1%-0.4% of Si, 1.0%-3.0% of Mn, less than or equal to 0.05% of P, less than or equal to 0.05% of S, 0.1%-0.4% of V, 0.011%-0.2% of N and the balance of Fe and unavoidable impurity elements. A continuous casting blank is subjected to component control according to the chemical components of the V-N microalloyed steel; and the production method therefor comprises converter smelting, LF refining and continuous casting steps in sequence. According to the present invention, by means of reasonable component design and smelting and continuous casting processes, the thermoplasticity of the continuous casting blank is improved, so that a high-temperature brittle region is prevented in a casting blank straightening region of the continuous casting blank or the thermoplasticity is good enough such that no surface crack appears, the casting blank is good in terms of surface quality and does not need to be cleaned, and the production efficiency is improved.

Claims

1. A V-N microalloyed steel, characterized in that the V-N microalloyed steel consists of the following chemical components by mass percentage: C: 0.09?0.13%, Si: 0.1?0.4%, Mn: 1.0?3.0%, P: ?0.05%, S: ?0.05%, V: 0.1?0.4%, N: 0.011?0.2%, and the balance of Fe and unavoidable impurity elements.

2. A production method for V-N microalloyed surface-crack-free continuous casting blanks, characterized in that the casting blanks are produced according to the chemical components of the V-N microalloyed steel according to claim 1; the method includes the following processes in sequence: converter smelting, LF refining and continuous casting.

3. The production method according to claim 2, characterized in that a cross-sectional specification of the continuous casting blanks is (150-350) mm*(1250-2400) mm.

4. The production method according to claim 2, characterized in that in the converter smelting process, a molten iron proportion is controlled between 88.0wt %?91.0wt %, a top-bottom combined blowing mode is used throughout entire process, and nitrogen gas is blown first and then argon gas is blown later during blowing process, wherein an argon-blowing time shall not be less than 3 minutes; the blowing process adopts a one blow to end blowing mode without reblowing process; a C content at smelting endpoint is controlled between 0.09?0.13 wt %, and a tapping temperature is between 1625?1645? C.

5. The production method according to claim 4, characterized in that LF refining is performed after tapping, and the refining is without vacuum degassing.

6. The production method according to claim 5, characterized in that in the continuous casting process, a weak water cooling mode is adopted in continuous casting; in the continuous casting process, protective casting is adopted, water port is closed immediately while large ladle alarms at the end of casting, reopening the water port is strictly prohibited after the water port is closed, an intermediate ladle is covered with alkaline covering agent, and a crystallizer uses low-carbon steel protective slag; a specific amount of water in the continuous casting process is 0.7?1.25 L/kg; a superheat degree of molten steel is 10?25? C.; further preferably, a casting speed during continuous casting process is 1.0?1.3 m/minute.

7. The production method according to claim 4, characterized in that deoxidation alloying is carried out during tapping process in the converter smelting process, while slag filtrating is also carried out; a specific operation method of the deoxidation alloying is as follows: during tapping process, a silicon containing substance is selected for deoxidation with an addition amount of 3.5?4.0 kg/ton of steel; and silicon manganese and vanadium nitrogen alloys are selected for alloying, wherein an addition amount of the vanadium nitrogen alloy is 1?2 kg/ton of steel; the slag filtrating is carried out using a substance containing CaO with an addition amount of 3.5?4.0 kg/ton of steel, and the substance containing CaO needs to be added before molten steel reaches 3/4.

8. The production method according to claim 5, characterized in that in the LF refining process, in an early stage of LF refining, an argon-blowing amount for argon-blowing stirring is 400?1000 L/minute, an argon-blowing stirring time is 3?4 minutes, a silicon containing substance should be used for deoxidation, and the components are fine-tuned under the condition of argon-blowing stirring.

9. The production method according to claim 8, characterized in that in the LF refining process, at the end of LF refining, an argon-blowing amount for argon-blowing stirring is 100?200 L/minute, and a time of argon-blowing stirring is ?5 minutes; a total refining time is controlled within 40?50 minutes, and N content obtained is 100?2000 ppm.

10. The production method according to claim 6, characterized in that in the continuous casting process, a specific distribution of water volume in the continuous casting process is as follows: a water volume in inner and outer arcs of a wide-side foot-roller of the casting blank accounts for about 8.0?10.0% of the total water volume, a water volume in a narrow-side foot-roller accounts for 3.4?4.5% of the total water volume, and a water volume in inner and outer arcs of a second zone of a vertical bending section accounts for 11.0?15.9% of the total water volume, a water volume in inner and outer arcs of a third zone accounts for 13.0?15.9% of the total water volume, a water volume in inner and outer arcs of a fourth zone accounts for 12.0?13.0% of the total water volume, a water volume in inner and outer arcs of a fifth zone of arc section 1 accounts for 8.5?9.5% of the total water volume, a water volume in inner and outer arcs of a sixth zone corresponding to arc sections 2 and 3 accounts for 12.0?14.0% of the total water volume, a water volume in inner and outer arcs of a seventh zone of sections 4?5 accounts for 8.0?11.5% of the total water volume, a water volume in inner and outer arcs of a eighth zone of straightening section 6, section 7 and section 8 accounts for 8.0?11.5% of the total water volume, and a remaining water volume is allocated to a horizontal section.

Description

DETAILED DESCRIPTION OF THE EMBODYMENTS

[0036] The following will provide a detailed explanation of the technical solution of the present invention in conjunction with embodiments. Each example is provided through the interpretation of the present invention rather than limiting it. In fact, those skilled in the art will be aware that modifications and variations can be made in the present invention without departing from the scope or spirit of the present invention. For example, features shown or described as part of one embodiment may be used in another embodiment to generate another embodiment. Therefore, it is expected that the present invention includes such modifications and variations within the scope of the accompanying claims and their equivalents.

[0037] According to the embodiment of the present invention, it provides a surface-crack-free continuous casting blanks V-N microalloyed steel. The V-N microalloyed steel is composed of the following chemical components by mass percentage: C: 0.09?0.13%, Si: 0.1?0.4%, Mn: 1.0?3.0%, P: ?0.05%, S: ?0.05%, V: 0.1?0.4%, N: 0.011?0.2%, and the balance of Fe and a small amount of unavoidable impurity elements. The present invention also provides a production method for preparing surface-crack-free microalloyed steel for continuous casting blanks with the above-mentioned chemical components, the method sequentially includes the following processes: converter smelting, LF refining, and continuous casting.

[0038] The alkaline covering agent and low-carbon steel protective slag used in the production process of the continuous casting blank of the present invention are commonly used commercial products for producing microalloyed steel.

[0039] Due to the carbonitride precipitates during continuous casting process, the plasticity of steel is decreased, the occurrence rate of surface cracks of continuous casting blank is significantly higher than that of ordinary carbon steel blank, and problems such as transverse cracks, longitudinal cracks, and shape cracks occur. In the smelting process of the invention, the contents of C are controlled, S is reduced, the use of a deoxidant is controlled, the inclusion morphology is changed, and the purity of steel is improved. In combination with LF refining and reasonable continuous casting process, a microalloyed steel with fine grains and good plasticity is obtained, thereby reducing the occurrence of surface cracks in a continuous casting blank of microalloyed steel, improving the surface quality of a continuous casting blank.

Example 1

[0040] This example provides a V-N microalloyed continuous casting blank and its production method, and the chemical composition of the continuous casting blank is shown in Table 1 by mass percentage.

TABLE-US-00001 TABLE 1 The chemical component of the VN microalloyed continuous casting blank from Example 1 chemical component(wt %), the balance of Fe and unavoidable impurity elements Example C Si Mn P S V N 1 0.10 0.23 1.62 0.014 0.007 0.12 0.012

[0041] The production method sequentially includes the following processes: converter smelting, LF refining and continuous casting.

(1) Converter Smelting:

[0042] During a converter smelting process, a 120t converter is used for smelting, the smelting raw materials are molten iron and steel scrap, and the ratio of molten iron is controlled at 90%; nitrogen gas is used for blowing in the early stage of blowing and argon gas used for blowing in the later stage with 4 minutes of the argon gas blowing, and a gun-to-bottom type for blowing is carried out in the blowing process, absolutely eliminating the supplementary blowing process, thereby preventing the molten steel from being peroxided so as to improve the cleanliness of the molten steel.

[0043] The C content at the smelting endpoint is controlled at 0.10wt %, and the tapping temperature is 1632? C.; deoxidation alloying and slag filtrating are carried out during tapping, silicon containing material is selected as deoxidizing agent with the addition amount of about 3.6 kg/t steel; the alloyed raw material is vanadium nitrogen alloy (with a V content of 77 wt %) with an addition of about 1.6 kg/ton of steel; and a substance containing CaO is selected for slag filtrating (with a CaO content of 90 wt %) with an added amount of 3.8 kg/ton of steel.

(2) LF Refining:

[0044] The steel ladle is directly transferred to the LF refining station for argon blowing refining after tapping; in the early stage of refining, the argon blowing rate at 400 L/minute is selected for argon blowing stirring for 3 minutes, then silicon containing substances are added for deoxygenation, and a small amount of silicon manganese is added to adjust the Mn content; at the end stage of refining, the argon blowing rate is adjusted to 150 L/minute and stirred for 6 minutes; the total refining time is controlled at 45 minutes, the N content obtained is 120 ppm and the content of other components meets the requirements of molten steel composition in the composition control process.

(3) Continuous Casting:

[0045] In the protective casting process, the large ladle is protected with a long water port and a conical asbestos pad is added to ensure the air-tightness of the connection.

[0046] After the casting starts, a coal gas flame is firstly used to check air-tightness of the lower water port of the ladle and the bowl part of long water port. If the flame is sucked in, it indicates poor sealing at this location, and a new long water port is jointed. If the air-tightness meets the requirements, argon gas is introduced into the bowl part connection of the long water port to form an argon sealing state during the pouring process, further preventing oxygen and nitrogen increase in the molten steel caused by the loose connection between the bowl part of long water port and the lower water port of the ladle due to air intake.

[0047] Immediately close the water port when the large ladle alarmed at the end of pouring. It is strictly prohibited to reopen the water port after it is closed. The tundish is covered with alkaline covering agent, and the crystallizer is protected with low-carbon steel protective slag (special P0050 of Shinagawa 250).

[0048] This example is the third furnace for casting, that is, a third furnace steel which is cast after the start of continuous casting. The degree of superheat is 20? C., and the actual drawing speed is 1.1 m/minute.

[0049] During the continuous casting process, the specific distribution of water volume with a specific water flow (specific amount of water) of 1.2 L/kg is as follows: the amount of water in the inner and outer arcs of the wide-side foot-roller of the casting blank accounts for about 8.6% of the total amount of water, the amount of water in the narrow-side foot-roller accounts for 3.5% of the total amount of water, and the amount of water in the inner and outer arcs of the second zone of the vertical bending section accounts for 15.8% of the total amount of water, the water volume in the inner and outer arcs of the third zone accounts for 15.6% of the total water volume, the water volume in the inner and outer arcs of the fourth zone accounts for 12.9% of the total water volume, the water volume in the inner and outer arcs of the fifth zone of arc section 1 accounts for 9.3% of the total water volume, the water volume in the inner and outer arcs of the sixth zone corresponding to arc sections 2 and 3 accounts for 12.4% of the total water volume, the water volume in the inner and outer arcs of the seventh zone of the sections 4?5 accounts for 8.1% of the total water volume, the water volume in the inner and outer arcs of the eighth zone of the straightening sections 6, section 7 and section 8 accounts for 8.9% of the total water volume, while the remaining water volume is allocated to the horizontal section.

[0050] Implementation effect: The continuous casting blank obtained from the multi furnace steel cast by the above method has a cross-sectional area of 200 mm*2000 mm, good surface quality, no cracks detected at low magnification, and no cracks observed on the surface of the casting blank during hot state observation, without the need for casting blank cleaning.

Example 2

[0051] This example provides a V-N microalloyed continuous casting blank, and the chemical composition of the continuous casting blank is shown in Table 2 by mass percentage.

TABLE-US-00002 TABLE 2 The chemical component of the VN microalloyed continuous casting blank from Example 2 chemical component(wt %), the balance of Fe and unavoidable impurity elements Example C Si Mn P S V N 2 0.09 0.40 1.00 0.014 0.006 0.25 0.032

[0052] The production method sequentially includes the following processes: converter smelting, LF refining, and continuous casting.

(1) Converter Smelting:

[0053] During a converter smelting process, a 120t converter is used for smelting, the smelting raw materials are molten iron and steel scrap, and the ratio of molten iron is controlled at 89%; nitrogen gas is used for blowing in the early stage of blowing and argon gas used for blowing in the later stage with 3 minutes of the argon gas blowing, and a gun-to-bottom type for blowing is carried out in the blowing process, absolutely eliminating the supplementary blowing process.

[0054] The C content at the smelting endpoint is controlled at 0.09t %, and the tapping temperature is 1630? C.

[0055] Deoxidation alloying and slag filtrating are carried out during tapping, and silicon containing material is selected as deoxidizing agent (that is, silicon carbide) with the addition amount of about 3.8 kg/t steel; the alloyed raw material is vanadium nitrogen alloy (with a V content of 77 wt %) with an addition of about 1.3 kg/ton of steel; and a substance containing CaO is selected for slag filtrating (with a CaO content of 90 wt %) with an added amount of 3.6 kg/ton of steel.

(2) LF Refining:

[0056] The steel ladle is directly transferred to the LF refining station for argon blowing refining after tapping, in the early stage of refining, the argon blowing rate at 400 L/minute is selected for argon blowing stirring for 4 minutes, and then silicon containing substances are added for deoxygenation; at the end stage of refining, the argon blowing rate is adjusted to 150 L/minute and stirred for 7 minutes; the total refining time is controlled at 48 minutes, the N content obtained is 100 ppm, and the content of other components meets the requirements of molten steel composition in the composition control process.

(3) Continuous Casting:

[0057] In the protective casting process, the large ladle is protected with a long water port and a conical asbestos pad is added to ensure the air-tightness of the connection. After the casting starts, a coal gas flame is firstly used to check air-tightness of the lower water port and the bowl part of the long water port. If the flame is sucked in, it indicates poor sealing at this location, and a new long water port is jointed. If the air-tightness meets the requirements, argon gas is introduced into the bowl part connection of the long water port to form an argon sealing state during the pouring process, further preventing oxygen and nitrogen increase in the molten steel caused by the loose connection between the long water port bowl part and the lower water port of the ladle due to air intake. Immediately close the water port when the large ladle alarmed at the end of pouring. It is strictly prohibited to reopen the water port after it is closed. The tundish is covered with alkaline covering agent, and the crystallizer is protected with low-carbon steel protective slag (special P0027-3 of Shinagawa 250).

[0058] This example is the fourth furnace of steel poured after the continuous casting start. The degree of superheat is 15? C., and the actual drawing speed is 1.2 m/minute.

[0059] During the continuous casting process, the specific distribution of water volume with a specific water flow of 1.0 L/kg is as follows: the water volume in the inner and outer arcs of the wide-side foot-roller of the casting blank accounts for about 8.1% of the total water volume, the water volume in the narrow-side foot-roller accounts for 4.3% of the total water volume, and the water volume in the inner and outer arcs of the second zone of the vertical bending section accounts for 11.4% of the total water volume, the water volume in the inner and outer arcs of the third zone accounts for 13.1% of the total water volume, the water volume in the inner and outer arcs of the fourth zone accounts for 12% of the total water volume, the water volume in the inner and outer arcs of the fifth zone of arc section 1 accounts for 9.0% of the total water volume, the water volume in the inner and outer arcs of the sixth zone corresponding to arc sections 2 and 3 accounts for 13.6% of the total water volume, the water volume in the inner and outer arcs of the seventh zone of the sections 4?5 accounts for 11.2% of the total water volume, the water volume in the inner and outer arcs of the eighth zone of the straightening sections 6, section 7 and section 8 accounts for 11.3% of the total water volume, while the remaining water volume is allocated to the horizontal section.

[0060] Implementation effect: The continuous casting blank obtained from the multi furnace steel cast by the above method has a cross-sectional area of 250 mm*1800 mm, good surface quality, no cracks detected at low magnification, and no cracks observed on the surface of the casting blank during hot state observation, without the need for casting blank cleaning.

[0061] The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may undergo various modifications and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of this invention shall be included within the scope of protection of this invention.