Method for controlling surface crack defects of high-manganese steel continuous casting billet

Abstract

The invention belongs to the technical field of ferrous metallurgy, and discloses a high manganese steel continuous casting billet surface crack defect control method, which comprises the following key process steps: (1) steelmaking process: molten steel is subjected to LF furnace refining and VD furnace vacuum treatment, and then is subjected to protective casting to form a continuous casting billet with the section of 240mm * 240mm-430mm * 350mm, and casting powder in protective casting adopts self-grinding high-carbon casting powder; (2) the continuous casting pulling speed is 0.8-0.9 m / min, a casting blank with the continuous casting pulling speed of 0.70 m / min is picked out for waste judgment and does not flow into the next procedure, the secondary cooling water quantity is 1.65-1.75 L / kg, the electromagnetic stirring parameters of a crystallizer are that the current is 300-350 A, the frequency is 3-5 Hz, and the electromagnetic stirring parameters of the tail end are that the current is 350-360 A, and the frequency is 10-12 Hz; and (3) the casting blank is rapidly put into a cooling bed, the temperature of the casting blank put into the cooling bed is 900-1000 DEG C, stack cooling is conducted for 24 hours, then unstacking is conducted, and the unstacking temperature is 100-200 DEG C. According to the method, by deeply analyzing a crack generation mechanism and through a steelmaking process, casting powder components and a casting blank slow cooling process, the surface crack generation of the casting blank is improved, and the product percent of pass is increased.

Description

Technical Field

[0001] The present invention belongs to the field of iron and steel metallurgy, and particularly relates to a method for controlling surface crack defects of high manganese steel continuous casting billets. Background Art

[0002] As an important engineering material, high manganese steel is widely used in fields such as wear resistance and corrosion resistance. However, during the continuous casting process, due to its unique physical and chemical properties, such as low thermal conductivity, well-developed columnar crystals, large grain size, and easy oxidation, high manganese steel is prone to surface crack defects in the casting billet. These cracks not only affect the appearance quality of the casting billet, but may also lead to a decline in mechanical properties and a shortening of service life. At present, although there are some control methods for high manganese steel continuous casting crack defects, most of them lack in-depth mechanism research and data support, and the effects are limited.

[0003] Patent CN101412082B provides a production method for preventing cracks in medium-carbon high manganese steel. Although this invention patent has a certain effect in preventing cracks in medium-carbon high manganese steel square billets produced by the continuous casting process, it only optimizes the continuous casting pulling speed, cooling intensity, adjusts the Al content, and controls the Ti content.

[0004] Chinese invention patent CN102423795A provides a continuous casting method for high manganese steel. By optimizing the continuous casting process, it can avoid surface crack, double skin, and welding slag defects of the casting billet to a certain extent.

[0005] The existing methods have limited effects. Therefore, a method for controlling surface crack defects of high manganese steel continuous casting billets is needed to improve the generation of surface cracks in the casting billet and increase the product qualification rate. Summary of the Invention

[0006] To achieve the above object, the present invention proposes a method for controlling surface crack defects of high manganese steel continuous casting billets. By studying the mechanism of crack generation, the steelmaking process, mold powder composition, and casting billet slow cooling process are improved to reduce the generation of surface cracks in the casting billet and increase the product qualification rate.

[0007] The technical solution adopted by the present invention is: a method for controlling surface crack defects of high manganese steel continuous casting billets, characterized in that the percentage composition of high manganese steel is: C: 0.65% - 0.75%, Si: 0.17% - 0.37%, Mn: 0.95% - 1.65%, P < 0.015%, S < 0.015%. At the same time, the residual elements are controlled as As < 0.015%, Sn < 0.015%, Cr < 0.080%, Cu < 0.050%, and the rest are Fe and inevitable impurity elements. The cross-sectional size of the continuous casting billet is 240mm * 240mm to 430mm * 350mm, and its key process steps include:

[0008] (1) The steelmaking process includes: refining molten steel in an LF furnace, subjecting it to vacuum treatment in a VD furnace, and then casting it into continuous casting billets with a cross-section of 240mm*240mm to 430mm*350mm under protective casting. The protective slag used in the protective casting is a self-developed high-carbon protective slag;

[0009] (2) The continuous casting withdrawal speed is 0.8 - 0.9m / min. Among them, when the continuous casting withdrawal speed is 0.70m / min, the cast billets are picked out and judged as scrap and do not flow into the subsequent processes. The secondary cooling water volume is 1.65 - 1.75L / kg. The parameters of the mold electromagnetic stirring are a current of 300 - 350A and a frequency of 3 - 5Hz, and the parameters of the final electromagnetic stirring are a current of 350 - 360A and a frequency of 10 - 12Hz;

[0010] (3) The cast billets are quickly transferred to the cooling bed. The temperature of the cooling bed is 900 - 1000°C, and they are stacked and cooled for 24 hours and then unstacked. The unstacking temperature is 100 - 200°C.

[0011] Furthermore, the self-developed high-carbon protective slag has a protective slag alkalinity CaO / SiO2 of 1.4 - 1.5, a B2O3 / Li2O content of 4% - 5%, an MnO content of 3% - 4%, an MgO content of 2% - 3%, and a nano-level Al2O3 particle content of 1% - 3%.

[0012] Furthermore, the cooling rate of the stacking and cooling process is 5 - 10°C / min.

[0013] Preferably, the continuous casting withdrawal speed is 0.8m / min, the secondary cooling water volume is 1.65L / kg, the parameters of the mold electromagnetic stirring are a current of 300A and a frequency of 3Hz, and the parameters of the final electromagnetic stirring are a current of 350A and a frequency of 10Hz.

[0014] Preferably, the continuous casting withdrawal speed is 0.9m / min, the secondary cooling water volume is 1.75L / kg, the parameters of the mold electromagnetic stirring are a current of 350A and a frequency of 5Hz, and the parameters of the final electromagnetic stirring are a current of 360A and a frequency of 12Hz.

[0015] Further preferably, the self-developed high-carbon protective slag has a CaO / SiO2 alkalinity of 1.4, a B2O3 / Li2O content of 4%, an MnO content of 3%, an MgO content of 2%, and a nano-level Al2O3 particle content of 1%.

[0016] Further preferably, the self-developed high-carbon protective slag has a CaO / SiO2 alkalinity of 1.5, a B2O3 / Li2O content of 5%, an MnO content of 4%, an MgO content of 3%, and a nano-level Al2O3 particle content of 3%.

[0017] The principle of the present invention:

[0018] Due to the relatively high Mn content in high manganese steel continuous casting billets, brittle inclusions such as MnS and MnAl2O4 are easily formed, resulting in cracks on the surface. A slag basicity of CaO / SiO2 = 1.4 - 1.5 for the protective slag is beneficial to reducing crack generation; CaF2 will exacerbate the intergranular corrosion and hot crack sensitivity of high manganese steel, so B2O3 / Li2O composite fluxes are used to replace CaF2; a small amount of MnO and MgO can reduce the reactivity between the slag and the molten steel, improve the thermal stability of the slag film, relieve thermal stress cracks, and thus reduce the risk of crack generation. The continuous casting speed is controlled within the range of 0.8 - 0.9 m / min to avoid the increase in internal stress of the casting billet caused by too fast casting speed and the uneven growth of the thickness of the primary solidified shell caused by too slow casting speed. When the casting speed is lower than 0.70 m / min, the crack rate of the casting billet increases significantly by 200%, so the casting billets with a casting speed lower than 0.70 m / min are rejected. The present invention controls the temperature-time coupling of the stacking cooling process to synchronously reduce thermal stress and tissue stress. The stacking cooling process controls the cooling rate at 5 - 10 °C / min, uses the high-temperature creep effect to gradually release the residual stress, avoids surface transverse cracks or corner cracks caused by thermal stress concentration, and at the same time slowly cools to provide time for the diffusion of elements such as carbon and sulfur, reduces grain boundary segregation, and inhibits the precipitation of brittle second phases (such as MnS and MnAl2O4).

[0019] The beneficial effects of the present invention are as follows: Aiming at the surface cracks of high manganese steel continuous casting billets, the present invention effectively controls crack generation by optimizing the steelmaking process, self-developing high-carbon protective slag, accurately controlling continuous casting parameters (casting speed, secondary cooling water volume, electromagnetic stirring), and innovating the stacking cooling process. The self-developed protective slag strengthens desulfurization and reduces brittle phases, the precise process avoids stress increase, the stacking cooling process releases residual stress, reduces brittle precipitation, significantly improves the quality of casting billets, reduces the rejection rate (from 5% to 0.5%), and significantly improves production efficiency and economic benefits. Description of the Drawings

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0021] Figure 1 It is the metallographic diagram of product defects before optimization;

[0022] Figure 2 It is the low-matching diagram of the casting billets of the present invention;

[0023] Figure 3 It is the stacking diagram of the embodiments of the present invention. Detailed Embodiments

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0025] Analysis of surface cracks of round steel:

[0026] I. Crack investigation of 70Mn2 in Furnace 15203003-2

[0027] 1.1 Rolling and grinding conditions

[0028] For the 70Mn2 round steel with a diameter of Ф90mm (furnace number 15203003-2) rolled on May 14, open cracks were found on the surface of the round steel during subsequent grinding, and the crack depth was relatively deep. This led to a large amount of grinding and rejection. The detailed situation is as follows:

[0029] A total of 63.66 tons of billets were produced in this furnace, which were rolled in 2 batches. Among them, 22.41 tons were taken off the production line in Batch B1372, and 37.05 tons were taken off the production line in Batch B1373 (actually 51.186 tons of this batch were put on the rack). A total of 16.67 tons were ground due to steel quality reasons (cracks), and 58.95 tons were rejected.

[0030] 1.2 Smelting conditions:

[0031]

[0032]

[0033] 1.3 Conditions of other furnaces in this casting

[0034] All 7 furnaces of steel in the casting on April 22 where Furnace 15203003-2 of 70Mn2 steel is located have been rolled. The surface quality of the round steel in other furnaces is normal. The continuous casting process of these 7 furnaces of steel is as follows:

[0035]

[0036] Analysis of crack causes for the above situation:

[0037] Combined with the previous sampling analysis results, the reasons for the surface cracks of the round steel are:

[0038] 2.1. In the non-steady casting of high-Mn steel, Mn in the molten steel is oxidized to MnO and floats up to be absorbed and dissolved by the mold powder, which denatures the mold powder, resulting in poor lubrication on the surface of the primary solidified shell in the mold, and uneven growth of the thickness of the primary solidified shell.

[0039] 2.2. When the pouring temperature is low, poor melting of the protective slag in the crystallizer will result in a thin liquid slag layer, poor lubrication of the primary solidified shell, and uneven thickness growth.

[0040] 2.3 The thermal conductivity of high manganese steel is relatively low, only 1 / 4 to 1 / 6 of that of carbon steel. During the pouring process of the ingot, the internal temperature gradient is large and the thermal stress is large.

[0041] 2.4 Since Mn is an element that expands the austenite zone, high manganese steel has well-developed columnar crystals, coarse grains, and weak intercrystalline bonding.

[0042] The 15203001-2 and 15203003 furnaces are poured under non-steady-state conditions, and the molten steel is easily re-oxidized, which increases the possibility of causing the protection slag to be denatured when pouring high manganese steel. In addition, the pulling speed is low, and the pouring temperature of the 15203003-2 furnace is also low, which greatly reduces the effectiveness of the protection slag. Coupled with the characteristics of high manganese steel itself, the weak parts of the shell may cause intergranular cracking as long as the thermal stress and organizational stress are not too large, and surface cracks will occur during the rolling process.

[0043] Differences in surface cracks between 70Mn2 and 65Mn:

[0044] The situation is different. The surface quality of 70Mn2 in the casting on April 22 was normal overall, only the continuous casting process was abnormal and there were many cracks in the furnace. However, the two castings of 65Mn steel that had been produced had many cracks in all furnace numbers, and the grinding rate was very high.

[0045] The surface cracks of 70Mn2 are individual problems, while those of 65Mn are overall problems.

[0046] 1. The composition of the two steel grades is compared as follows:

[0047]

[0048] Considering that the surface cracks of 70Mn2 are individual problems, the following improvement measures are recommended:

[0049] 1. Use self-developed high-carbon protective slag to improve the surface cracks of the casting.

[0050] 2. The continuous casting speed is controlled at 0.8-0.9 m / min. The ingots with a speed lower than 0.70 m / min shall be picked out and scrapped, and the ingots with high crack incidence shall be isolated.

[0051] 3. The ingots are quickly put down from the cooling bed and densely stacked for cooling for 24 hours to prevent high Mn steel slag pits and indentation stress cracks.

[0052] 4. Use nitrogen reduction (≤60ppm) technology throughout the process and strictly prohibit the use of No. 1 converter smelting to prevent high nitrogen from deteriorating surface quality.

[0053] 5. The temperature of the two-bar preheating section ≤ 650 °C, the preheating time ≥ 80 min, and the total time in the furnace ≥ 200 min. Slowly heat to prevent stress cracks in high-Mn steel.

[0054] After taking the above measures, the specific experimental examples of the present invention are as follows:

[0055] Example 1:

[0056] A method for controlling surface crack defects of high-Mn steel continuous casting billets. The percentage composition of high-Mn steel is: C: 0.65%, Si: 0.17%, Mn: 0.95%, P < 0.015%, S < 0.015%. At the same time, the residual elements are controlled as As < 0.015%, Sn < 0.015%, Cr < 0.080%, Cu < 0.050%, and the rest are Fe and inevitable impurity elements. The cross-sectional size of the continuous casting billet is 240mm * 240mm. Its key process steps include:

[0057] (1) The steelmaking process includes: the molten steel is refined by the LF furnace and vacuum treated by the VD furnace, and then poured into a continuous casting billet with a cross-section of 240mm * 240mm under protective casting. The protective slag used in the protective casting is a self-developed high-carbon protective slag. Based on the traditional high-Mn steel protective slag, the alkalinity, viscosity, and crystallization performance are adjusted, and the melting speed of the protective slag is optimized to adapt to the casting conditions of high-Mn steel. The alkalinity of the protective slag is CaO / SiO2 = 1.4, the content of B2O3 / Li2O is 4%, the content of MnO is 3%, the content of MgO is 2%, and the content of nano-level Al2O3 particles is 1%.

[0058] (2) The continuous casting pulling speed is 0.8 m / min, the secondary cooling water volume is 1.65 L / kg, the parameters of the mold electromagnetic stirring are current 300 A and frequency 3 Hz, and the parameters of the final electromagnetic stirring are current 350 A and frequency 10 Hz. Among them, when the continuous casting pulling speed is 0.70 m / min, the cast billet is judged as defective and does not flow into the subsequent processes.

[0059] (3) The cast billet is quickly transferred to the cooling bed, the temperature of the cooling bed is 900 °C, and it is stacked and cooled for 24 hours and then unstacked. The unstacking temperature is 100 °C to further improve the surface quality of the cast billet.

[0060] Example 2:

[0061] A method for controlling surface crack defects of high-Mn steel continuous casting billets. The percentage composition of high-Mn steel is: C = 00.75%, Si = 0.37%, Mn = 1.65%, P < 0.015%, S < 0.015%. At the same time, the residual elements are controlled as As < 0.015%, Sn < 0.015%, Cr < 0.080%, Cu < 0.050%, and the rest are Fe and inevitable impurity elements. The cross-sectional size of the continuous casting billet is 430mm * 350mm. Its key process steps include:

[0062] (1) The steelmaking process includes: the molten steel is refined by an LF furnace and vacuum treated by a VD furnace, and then is poured into a continuous casting billet with a cross-section of 430 mm * 350 mm under protective casting. The protective slag used in the protective casting is a self-developed high-carbon protective slag. Based on the traditional high-manganese steel protective slag, the alkalinity, viscosity, and crystallization performance are adjusted, and the melting speed of the protective slag is optimized to adapt to the casting conditions of high-manganese steel. The alkalinity of the protective slag is CaO / SiO2 = 1.5, the content of B2O3 / Li2O is 5%, the content of MnO is 4%, the content of MgO is 3%, and the content of nano-level Al2O3 particles is 3%.

[0063] (2) The continuous casting speed is 0.9 m / min, the secondary cooling water volume is 1.75 L / kg, the parameters of the mold electromagnetic stirring are a current of 350 A and a frequency of 5 Hz, and the parameters of the final electromagnetic stirring are a current of 360 A and a frequency of 12 Hz. Among them, when the continuous casting speed is 0.70 m / min, the cast billet is picked out and judged as scrap and does not flow into the subsequent processes.

[0064] (3) The cast billet is quickly transferred to the cooling bed. The temperature of the cooling bed is 1000 °C, and it is stacked and cooled for 24 hours and then unstacked. The unstacking temperature is 100 °C, which further improves the surface quality of the cast billet.

[0065] In summary, before the technical solution of the present invention was adopted, as Figure 1 shown, the steel produced by all furnace numbers had many cracks and a very high grinding rate. After adopting the technical solution of the present invention, as Figure 2 and Figure 3 shown, the cracks were significantly improved.

[0066] The above are only the preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A method for controlling surface crack defects of high manganese steel continuous casting billets, characterized in that, The percentage composition of high manganese steel is: C: 0.65% - 0.75%, Si: 0.17% - 0.37%, Mn: 0.95% - 1.65%, P < 0.015%, S < 0.015%. At the same time, the residual elements are controlled as As < 0.015%, Sn < 0.015%, Cr < 0.080%, Cu < 0.050%. The rest are Fe and inevitable impurity elements. The cross-sectional size of the continuous casting billet is 240mm * 240mm - 430mm * 350mm. Its key process steps include: (1) The steelmaking process includes: the molten steel is refined by an LF furnace and vacuum treated by a VD furnace, and then poured into a continuous casting billet with a cross-section of 240mm * 240mm - 430mm * 350mm under protective casting. The protective slag used in the protective casting is a self-developed high-carbon protective slag; (2) The continuous casting drawing speed is 0.8 - 0.9m / min. Among them, when the continuous casting drawing speed is 0.70m / min, the cast billet is picked out and judged as defective and does not flow into the subsequent processes. The secondary cooling water volume is 1.65 - 1.75L / kg. The parameters of the mold electromagnetic stirring are current 300 - 350A and frequency 3 - 5Hz. The parameters of the final electromagnetic stirring are current 350 - 360A and frequency 10 - 12Hz; (3) The cast billet is quickly transferred to the cooling bed. The temperature of the cooling bed is 900 - 1000°C. After stacking and cooling for 24 hours, it is unstacked. The unstacking temperature is 100 - 200°C.

2. The method for controlling surface crack defects of a high manganese steel continuous casting billet according to claim 1, characterized in that, The self-developed high-carbon protective slag has a slag basicity of CaO / SiO2 of 1.4 - 1.5, a B2O3 / Li2O content of 4% - 5%, a MnO content of 3% - 4%, a MgO content of 2% - 3%, and a nano-Al2O3 particle content of 1% - 3%.

3. A method for controlling surface crack defects of a high manganese steel continuous casting billet according to claim 2, characterized in that, The cooling rate of the stacking and cooling process is 5 - 10°C / min.

4. A method for controlling surface crack defects of a high manganese steel continuous casting billet according to claim 3, characterized in that, The continuous casting drawing speed is 0.8m / min, the secondary cooling water volume is 1.65L / kg, the parameters of the mold electromagnetic stirring are current 300A and frequency 3Hz, and the parameters of the final electromagnetic stirring are current 350A and frequency 10Hz.

5. The method for controlling surface crack defects of a high manganese steel continuous casting billet according to claim 3, characterized in that, The continuous casting drawing speed is 0.9m / min, the secondary cooling water volume is 1.75L / kg, the parameters of the mold electromagnetic stirring are current 350A and frequency 5Hz, and the parameters of the final electromagnetic stirring are current 360A and frequency 12Hz.

6. A method for controlling surface crack defects of a high manganese steel continuous casting billet according to claim 4 or 5, characterized in that, The self-developed high-carbon protective slag has a slag basicity of CaO / SiO2 of 1.4, a B2O3 / Li2O content of 4%, a MnO content of 3%, a MgO content of 2%, and a nano-Al2O3 particle content of 1%.

7. A method for controlling surface crack defects of a high manganese steel continuous casting billet according to claim 4 or 5, characterized in that, The self-developed high-carbon protective slag has a slag basicity of CaO / SiO2 of 1.5, a B2O3 / Li2O content of 5%, a MnO content of 4%, a MgO content of 3%, and a nano-Al2O3 particle content of 3%.

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