CN111876678A - Process method for solving cracks of high-strength steel casting blank - Google Patents
Process method for solving cracks of high-strength steel casting blank Download PDFInfo
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- CN111876678A CN111876678A CN202010674689.1A CN202010674689A CN111876678A CN 111876678 A CN111876678 A CN 111876678A CN 202010674689 A CN202010674689 A CN 202010674689A CN 111876678 A CN111876678 A CN 111876678A
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/10—Supplying or treating molten metal
- B22D11/11—Treating the molten metal
- B22D11/111—Treating the molten metal by using protecting powders
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/16—Controlling or regulating processes or operations
- B22D11/22—Controlling or regulating processes or operations for cooling cast stock or mould
- B22D11/225—Controlling or regulating processes or operations for cooling cast stock or mould for secondary cooling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/0006—Adding metallic additives
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/0056—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00 using cored wires
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/0087—Treatment of slags covering the steel bath, e.g. for separating slag from the molten metal
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
- C21C7/06—Deoxidising, e.g. killing
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
- C21C7/064—Dephosphorising; Desulfurising
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/10—Handling in a vacuum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C33/00—Making ferrous alloys
- C22C33/04—Making ferrous alloys by melting
- C22C33/06—Making ferrous alloys by melting using master alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
Abstract
The invention discloses a method for solving the cracking problem of a high-strength steel casting blank with medium carbon, high manganese, high aluminum and high silicon contents before the casting blank enters a heating furnace, aiming at the problem, the technical scheme of the invention is that on the basis of reducing the primary cracks of the casting blank by controlling sulfur, hydrogen, continuous casting dynamic secondary cooling water and low-alkalinity lithium oxide-containing protective slag, the casting blank is hot-charged into the furnace, so that the cracks caused by stress alternating concentration caused by the alternating change of the casting blank heat conductivity coefficient reduction → rise → reduction → rise in the process that the casting blank is cooled to room temperature (within the range of 1000-50 ℃) are avoided, and the cracking problem of the high-strength steel casting blank before the casting blank enters the heating furnace is solved; the product performance meets the requirements, and the quality meets the use requirements of users.
Description
Technical Field
The invention belongs to the technical field of metallurgical industrial production, and relates to high-strength steel with tensile strength of more than 950MPa and a casting blank production process thereof. More particularly, the present invention relates to a method for controlling cracks in a cast slab.
Background
The high-strength steel for the mechanical stirring tank belongs to niobium-titanium microalloyed steel in the medium-carbon, high-manganese, high-aluminum and high-silicon content range, and the casting blank crack sensitivity is strong.
As shown in FIG. 1, there was a problem that transverse cracks occurred in the corner portions of the cast slab at the beginning of the production trial, and transverse breakage occurred in the cast slab when the cracks were severe. As shown in FIG. 2, metallographic examination revealed that the matrix structure of the ingot consisted mainly of ferrite and pearlite, and no significant abnormality was observed in the structure at the crack. Cracks are formed along ferrite grain boundaries, and are partly transgranular cracks, and iron oxide beads and a remarkable decarburization phenomenon are not observed at the cracks, so that it is concluded that the cracks are formed at a non-high temperature.
Due to the characteristics of the components of the casting blank steel, the high-strength steel casting blank generates surface microcracks before the continuous casting blank exits a fan-shaped section; in addition, in the process of cooling the casting blank to room temperature after the casting blank is taken out of the continuous casting machine, the heat conductivity coefficient of the casting blank is greatly changed within the temperature range of 50-1000 ℃, so that the transverse crack of the primary corner is expanded to cause brittle fracture along the crystal. The cracking problem of the casting blank before entering the heating furnace cannot be effectively solved by adjusting the continuous casting secondary cooling system, the process parameters of the electromagnetic stirring roller, the slow cooling of the casting blank in a slab warehouse in a stacking way or the slow cooling of a heat preservation pit and the like. Therefore, research on a casting blank production process and a casting blank crack control method of the high-strength steel is needed to solve the problem of cracking of a casting blank before the casting blank is fed into a heating furnace.
However, in patent documents of similar or similar steel grades, steel grade components, rolling processes and mechanical properties are mostly disclosed, and few technical solutions for solving the problem of billet cracking are clearly disclosed. For example, chinese patent document CN106636925B discloses a cold-rolled TRIP steel with high strength-elongation product and a preparation method thereof; the Chinese patent document CN107012398B discloses a niobium microalloyed TRIP steel and a preparation method thereof, which implies that a method for alleviating casting blank cracks by 'heating and then preserving heat after casting blank forging' and 'hot rolling after billet pre-heat preservation' is adopted. However, none of these prior arts completely solves the problem of cracking of the cast slab before it is introduced into the furnace.
Disclosure of Invention
The invention provides a process method for solving cracks of a high-strength steel casting blank, and aims to solve the problem of cracking of the casting blank before the casting blank is fed into a heating furnace.
In order to achieve the purpose, the invention adopts the technical scheme that:
the invention relates to a process method for solving casting blank cracks of high-strength steel, wherein the high-strength steel is niobium-titanium microalloyed steel with medium-carbon, high-manganese, high-aluminum and high-silicon contents, and the contents of main chemical elements of the niobium-titanium microalloyed steel in percentage by mass are respectively as follows: c: 0.10-0.20%; si: 1.10-1.50%; mn: 1.60-1.90%; and Als: 0.40-0.70%;
the process method comprises the following steps: KR molten iron pre-desulfurization → top and bottom combined blown converter smelting → top slag pre-reduction of an alloy fine tuning station → LF refining → slab continuous casting → hot charging and rolling of casting blanks.
The requirements of the process method are as follows:
1. the content of S, H in the steel is controlled below 0.0020 percent and 0.0002 percent respectively according to the mass percentage;
2. performing calcium treatment modification on inclusions in steel in an LF (ladle furnace) process;
3. the slab continuous casting adopts lithium oxide-containing crystallizer casting powder with the alkalinity of 1.25-1.35; the continuous casting secondary cooling adopts a weak cooling system, and the specific water amount is 0.55-0.65L/kg; electromagnetic stirring rollers are used for the segment D1 and the segment D2;
4. after being cut to length, the casting blank is directly sent into a heating furnace for hot charging and rolling, and the surface temperature of the casting blank is ensured to be higher than 450 ℃.
The control process for controlling the S content in the steel to be less than or equal to 0.0020 percent by the process method comprises the following steps:
1. mechanically stirring and desulfurizing CaO-based fluorine-free desulfurizer to control the S content of molten iron to be below 0.0050%, and slagging off; the slag skimming bright surface is more than 80 percent;
2. deoxidizing and alloying in the process of primary molten steel tapping, and adding lime; after the molten steel reaches an alloy fine-tuning station, bottom blowing argon is started to carry out strong stirring to reduce the top slag of the steel ladle, the color of the top slag is changed from black to yellow brown, and the desulfurization rate in the process is required to be more than 20%; wherein the flow rate of bottom-blown argon is 200-500 NL/min, and the retention time is 5-8 min.
3. Heating and raising temperature of an LF refining electrode, desulfurizing white slag, adjusting components and temperature to target values, and treating calcium, wherein the S content in steel is required to be less than or equal to 0.0020 percent before an LF calcium feeding line.
The content of H in the steel is less than or equal to 0.0002 percent by maintaining RH at the vacuum degree of below 67Pa for 15-20 min and carrying out vacuum degassing.
The mode for carrying out calcium treatment modification on inclusions in steel in the LF refining process is as follows: after the components are qualified, feeding a pure calcium core-spun yarn to carry out calcium treatment on the impurities, ensuring that the yarn feeding is carried out by blowing argon gas at the bottom for weak stirring for 3-6 min, and continuing to weakly stir for 3-5 min after the yarn feeding, wherein the flow of the weak stirring argon gas is 10-30 NL/min.
The main control method of RH refining is as follows: keeping the vacuum degree below 67Pa for 15-20 min for vacuum dehydrogenation, wherein the end point is that H is less than or equal to 1.5 ppm; during the period, the ferrotitanium alloy is added, and the Ti content in the steel is adjusted to a target value.
The slab continuous casting process comprises the following steps:
1. the method comprises the following steps of (1) adopting lithium oxide-containing crystallizer casting powder with alkalinity of 1.20-1.50, wherein the main components of the casting powder are CaO: 38.3-45.8% of SiO2：20.0～30.0％、Li2O：1.85～4.24％；
2. The secondary cooling adopts a weak cooling system, the specific water amount is 0.50-0.65L/kg, and the automatic dynamic adjustment of the secondary cooling water is realized according to the steel type components, the casting pulling speed, the temperature of the tundish molten steel and the temperature requirements of the casting blank at 11 loop cooling points of the fan-shaped section;
3. the casting blank is dynamically and lightly reduced, and the reduction position and the reduction amount are automatically and dynamically adjusted according to the factors of steel components, casting pulling speed and molten steel casting temperature;
4. electromagnetic stirring rollers are used in the segment D1 and the segment D2. The electromagnetic stirring frequency is 4-8 Hz, the upper coil current is 250A +/-10A, and the lower coil current is 450A +/-20A.
The casting blank hot charging method comprises the following steps: directly feeding the casting blank into a heating furnace after the casting blank is cut to length, wherein the temperature of the casting blank reaches above 650 ℃; when the number of continuous casting furnaces reaches more than 4, in order to buffer the feeding rhythm of casting blanks, the subsequent casting blanks need to be fed into a heat preservation pit with a gas burner for heating for heat preservation; it must be ensured that the slab surface temperature is above 450 ℃ and is charged into the furnace to reduce the risk of slab cracking below this temperature.
The technical scheme is used for the production process of the niobium-titanium microalloyed steel casting blank for the mechanical stirring tank with the tensile strength of more than 950Mpa in the medium-carbon, high-manganese, high-aluminum and high-silicon content range, the cracking problem of the casting blank before the casting blank is put into a heating furnace can be effectively solved, the product performance meets the requirement, and the quality meets the use requirement of a user; experimental data show that before the scheme of the invention is implemented, 14 cracked and 2 cracked in 16 casting blanks of about 600 tons produced by two furnaces in two times; after the scheme of the invention is implemented, the 48 furnaces of 15000 tons of casting blanks are produced in batches, and the crack occurrence rate is zero.
Drawings
FIG. 1 is a view showing an actual state of a corner transverse crack and a transverse fracture of a cast slab according to the prior art;
FIG. 2 is a microscopic metallographic image relating to a cast slab according to the prior art;
FIG. 3 is a graph of the actual measured temperature profile of one of the test pieces under test according to the present invention;
FIG. 4 is a graph of another actual measured temperature profile of a test of the present invention;
FIG. 5 is a graph showing tensile strength and reduction of area of a cast slab obtained by actual measurement in example 4.
Detailed Description
The following detailed description of the embodiments of the present invention will be given in order to provide those skilled in the art with a more complete, accurate and thorough understanding of the inventive concept and technical solutions of the present invention.
The main chemical elements of the high-strength steel comprise C, Si, Mn and Als. In particular to a high-strength steel casting blank production process with tensile strength of more than 950MPa and a casting blank crack control method, which solve the cracking problem before the casting blank enters a heating furnace.
In order to overcome the defects of the prior art and achieve the purpose of solving the cracking problem of the casting blank before the casting blank enters a heating furnace, the invention adopts the technical scheme that:
the invention relates to a process method for solving casting blank cracks of high-strength steel, which aims at influencing the casting blank cracks of steel types, wherein the high-strength steel is niobium-titanium microalloyed steel with medium-carbon, high-manganese, high-aluminum and high-silicon contents, and the contents of main chemical elements of the niobium-titanium microalloyed steel in percentage by mass are respectively as follows: c: 0.10-0.20%; si: 1.10-1.50%; mn: 1.60-1.90%; and Als: 0.40-0.70%; (the percentages of the components in the present invention are by mass unless otherwise specified.)
The process flow adopted by the invention for solving the cracking problem before the casting blank is put into the heating furnace is as follows:
KR molten iron pre-desulfurization → 300 tons of top-bottom combined blown converter smelting → top slag pre-reduction of an alloy fine adjustment station → LF refining → slab continuous casting → hot charging and rolling of casting blanks.
The invention solves the cracking problem of high-strength steel casting blank before feeding into heating furnace, and adopts the technical principle that the casting blank hot charging into furnace is adopted on the basis of reducing the primary crack of casting blank by controlling sulfur, hydrogen, continuous casting dynamic secondary cooling water and low alkalinity lithium oxide containing protective slag, so as to avoid the crack caused by stress alternating concentration caused by the alternating change of casting blank heat conductivity coefficient reduction → rise → reduction → rise in the process of cooling the casting blank to room temperature (within the range of 1000-50 ℃).
The requirements of the process method are as follows:
1. the content of S, H in the steel is controlled below 0.0020 percent and 0.0002 percent respectively according to the mass percentage;
2. calcium treatment denaturation of inclusions in steel in the LF process (but not in the RH process);
3. alkalinity (CaO/SiO) for slab continuous casting2) 1.25-1.35 of lithium oxide-containing crystallizer casting powder; the continuous casting secondary cooling adopts a weak cooling system, and the specific water amount is 0.55-0.65L/kg; electromagnetic stirring rollers are used for the segment D1 and the segment D2;
4. after being cut to length, the casting blank is directly sent into a heating furnace for hot charging and rolling, and the surface temperature of the casting blank is ensured to be higher than 450 ℃.
The control process for controlling the S content in the steel to be less than or equal to 0.0020 percent by the process method comprises the following steps:
1. mechanically stirring and desulfurizing CaO-based fluorine-free desulfurizer to control the sulfur content of molten iron to be below 0.0050%, and slagging off; the slag skimming bright surface is more than 80 percent;
2. deoxidizing and alloying in the process of primary molten steel tapping, and adding lime; after the molten steel reaches an alloy fine-tuning station, bottom blowing argon is started to carry out strong stirring to reduce the top slag of the steel ladle, the color of the top slag is changed from black to yellow brown, and the desulfurization rate in the process is required to be more than 20%; wherein the flow rate of bottom-blown argon is 200-500 NL/min, and the retention time is 5-8 min.
3. Heating and raising temperature of an LF refining electrode, desulfurizing white slag, adjusting components and temperature to target values, and treating calcium, wherein the S content in steel is required to be less than or equal to 0.0020 percent before an LF calcium feeding line.
Molten iron is added into a converter to be smelted by the converter, deoxidized and alloyed, argon is blown from the bottom of an alloy fine-tuning station to be strongly stirred for 3-5 min, the top slag of the steel ladle is preliminarily reduced, and the desulfurization rate is more than 20%.
The KR molten iron pre-desulfurization method comprises the following steps: the molten iron is subjected to pre-slagging, mechanical stirring desulfurization and post-slagging, and S is less than or equal to 0.0050% after the molten iron is desulfurized.
The H content in the steel is less than or equal to 0.0002 percent by maintaining RH for 15-20 min under the vacuum degree of 67Pa for vacuum degassing, and the aim is to avoid hydrogen-induced cracks in the using process of the product.
The LF refining method comprises the following steps: after the molten steel reaches LF refining, lowering an electrode, raising the temperature, slagging, desulfurizing, alloying step by step, feeding a calcium wire for calcium treatment after the components are qualified, wherein the S content in the steel is less than or equal to 0.0020 percent; and after the calcium treatment is finished, blowing argon at the bottom for weak stirring for 6-9 min to remove impurities.
The mode for carrying out calcium treatment modification on inclusions in steel in the LF refining process is as follows: after the components are qualified, feeding a pure calcium core-spun yarn to carry out calcium treatment on the impurities, ensuring that the yarn feeding is carried out by blowing argon gas at the bottom for weak stirring for 3-6 min, and continuing to weakly stir for 3-5 min after the yarn feeding, wherein the flow of the weak stirring argon gas is 10-30 NL/min.
The technical principle of calcium-treatment denaturation of inclusions in steel in the LF process (not in the RH process) is as follows: in the LF calcium treatment in advance, the generated calcium aluminate and CaS sulfur oxides can be effectively removed in a circulation process of RH for more than 20 minutes, and the oxidation loss of [ Ti ] in steel and the generation of new titanium oxide inclusions caused by the oxidation loss can be avoided in the RH calcium treatment.
The main control method of RH refining is as follows: keeping the vacuum degree below 67Pa for 15-20 min for vacuum dehydrogenation, wherein the end point is that H is less than or equal to 1.5 ppm; during the period, the ferrotitanium alloy is added to adjust the Ti content in the steel to a target value, so as to improve the yield of the titanium.
The slab continuous casting process comprises the following steps:
1. by using alkalinity (CaO/SiO)2) The lithium oxide-containing crystallizer casting powder is 1.20-1.50, and the main components of the casting powder are CaO: 38.3-45.8% of SiO2：20.0～30.0％、Li2O: 1.85-4.24 percent, and aims to reduce SiO in slag under the condition of keeping proper alkalinity2Activity of, increase Al2O3Activity of inhibiting SiO in aluminum reduction mold flux in steel2To prevent deterioration of the mold flux. Meanwhile, a certain amount of lithium oxide is added to stabilize the viscosity of the molten slag, keep the normal consumption of the covering slag and finally enable the performance of the covering slag to meet the requirement of high-aluminum steel continuous casting;
2. the secondary cooling adopts a weak cooling system, the specific water amount is 0.50-0.65L/kg, and the automatic dynamic adjustment of the secondary cooling water is realized according to the steel type components, the pouring pulling speed, the temperature of the tundish molten steel and the temperature requirements of the casting blank at 11 loop cooling points of the fan-shaped section.
3. The casting blank is dynamically and lightly reduced, and the reduction position and the reduction amount are automatically and dynamically adjusted according to factors such as steel composition, casting pulling speed, molten steel casting temperature and the like;
4. electromagnetic stirring rollers are used in the segment D1 and the segment D2. The electromagnetic stirring frequency is 4-8 Hz, the upper coil current is 250A +/-10A, and the lower coil current is 450A +/-20A.
The casting blank hot charging rolling comprises the following steps: the casting blank is directly sent into a heating furnace after being cut to length, and needs to be stacked in a slab warehouse for waiting for buffering the furnace entering rhythm when the number of continuous casting furnaces reaches more than 4, but the surface temperature of the casting blank is guaranteed to be higher than 450 ℃ for hot charging and rolling.
The casting blank hot charging method comprises the following steps: directly feeding the casting blank into a heating furnace after the casting blank is cut to length, wherein the temperature of the casting blank reaches above 650 ℃; when the number of continuous casting furnaces reaches more than 4, in order to buffer the feeding rhythm of casting blanks, the subsequent casting blanks need to be fed into a heat preservation pit with a gas burner for heating for heat preservation; it must be ensured that the slab surface temperature is above 450 ℃ and is charged into the furnace to reduce the risk of slab cracking below this temperature.
And for the first 3 furnaces of the casting times, directly sending the casting blank into a heating furnace for hot charging and rolling after the casting blank is cut to length, wherein the surface temperature of the casting blank when the casting blank is fed into the furnace is more than 600 ℃. When the number of continuous casting furnaces reaches more than 4, stacking and waiting are needed in a slab warehouse to buffer the furnace feeding rhythm, but the surface temperature of the casting blank is guaranteed to be higher than 450 ℃ for hot charging and rolling.
Example 1, measured temperature profile:
after the casting blank is cut by the continuous casting machine → stacking is waited → the actual temperature measured in the middle of the upper surface, the corner part, the middle of the end surface and the middle of the narrow surface of the casting blank before the casting blank is fed into the heating furnace is shown in fig. 3 and 4. Fig. 3 and 4 show the results of two tests with different billet numbers. The results of both tests were essentially identical.
Example 2, thermal conductivity of cast slab:
the continuous casting slab produced by the process adopts an LFA 427 laser thermal conductivity instrument to measure the thermal conductivity coefficient of the high-strength steel casting blank, and the result shows that: when the casting blank is cooled from 1000 ℃ to 50 ℃, the fluctuation range of the thermal conductivity of the high-strength steel is 26.69-14.28W/(m.K) (the unit of the thermal conductivity is W/m.K). With the reduction of the temperature, the heat conductivity of the casting blank shows an alternating rule of reduction → rise → reduction → rise, and at the temperature of 735 ℃ and 398 ℃, the heat conductivity is respectively 14.29W/(m.K) and 17.18W/(m.K), the heat conductivity is obviously reduced, the stress release is weakened, and the temperature point is a temperature point at which cracks are generated. Therefore, the hot charging rolling can avoid the stress alternation and reduce the cracks.
Example 3, main chemical components and casting blank cracking ratio of the casting blank of the example:
the main chemical components of the continuous casting slab produced by the process meet the control requirements, and the casting slab has no cracks or cracking defects. See the following table (before and after optimization in the table, before and after the practice of the invention):
| examples | C％ | Si％ | Mn％ | P％ | S％ | Als％ | H/ppm | The percentage of cracking of the casting blank is% |
| 1 after optimization | 0.142 | 1.11 | 1.73 | 0.010 | 0.0005 | 0.49 | 1.0 | 0 |
| 2 after optimization | 0.139 | 1.14 | 1.71 | 0.013 | 0.0007 | 0.47 | 1.2 | 0 |
| After 3 optimizing | 0.133 | 1.24 | 1.80 | 0.012 | 0.0007 | 0.49 | 0.8 | 0 |
| After 4 optimization | 0.136 | 1.26 | 1.81 | 0.0084 | 0.0006 | 0.51 | 0.9 | 0 |
| 5 before optimization | 0.151 | 1.25 | 1.83 | 0.013 | 0.0054 | 0.53 | 1.8 | 87.5％ |
| 6 before optimization | 0.14 | 1.17 | 1.72 | 0.012 | 0.0031 | 0.45 | 2.1 | 100％ |
Example 4 and example actually measured tensile strength and reduction of area of a casting blank:
the continuous casting slab produced by the process is sampled and processed into a sample with phi 120mm multiplied by 10mm, and the tensile strength and the reduction of area are measured on a Gleeble2000D thermal simulation experiment machine. As shown in fig. 5, the results show that: the plastic wave trough of the high-strength steel casting blank occurs between 650 ℃ and 900 ℃, and the corresponding section shrinkage rates are all lower than 30%; the reduction of area Z is only 12% at around 800 ℃. Therefore, the casting blank hot charging rolling is required to avoid the low plasticity temperature range.
The invention has been described above with reference to the accompanying drawings, it is obvious that the invention is not limited to the specific implementation in the above-described manner, and it is within the scope of the invention to apply the inventive concept and solution to other applications without substantial modification.
Claims (8)
1. A process method for solving the problem of cracks of a high-strength steel casting blank is characterized by comprising the following steps: the high-strength steel is niobium-titanium microalloyed steel with medium-carbon, high-manganese, high-aluminum and high-silicon contents, and the contents of main chemical elements in percentage by mass are respectively as follows: c: 0.10-0.20%; si: 1.10-1.50%; mn: 1.60-1.90%; and Als: 0.40-0.70%; the process method comprises the following steps: KR molten iron pre-desulfurization → top and bottom combined blown converter smelting → top slag pre-reduction of an alloy fine tuning station → LF refining → slab continuous casting → hot charging of casting blank into a furnace.
2. The process method for solving the problem of the cracks of the high-strength steel casting blank according to claim 1, which is characterized by comprising the following steps of: the requirements of the process method are as follows:
1) the content of S, H in the steel is controlled below 0.0020 percent and 0.0002 percent respectively according to the mass percentage;
2) calcium treatment denaturation is carried out on inclusions in the steel in the LF process;
3) the slab continuous casting adopts the lithium oxide-containing crystallizer casting powder with the alkalinity of 1.25-1.35; the continuous casting secondary cooling adopts a weak cooling system, and the specific water amount is 0.55-0.65L/kg; electromagnetic stirring rollers are used for the segment D1 and the segment D2;
4) and directly sending the casting blank into a heating furnace for hot charging and rolling after the casting blank is cut to length, wherein the surface temperature of the casting blank is required to be higher than 450 ℃.
3. The process method for solving the problem of the cracks of the high-strength steel casting blank according to claim 2, which is characterized in that: the control process for controlling the S content in the steel to be less than or equal to 0.0020 percent by the process method comprises the following steps:
1) mechanically stirring and desulfurizing by adopting a CaO-based fluorine-free desulfurizing agent, controlling the content of S in molten iron to be below 0.0050%, and slagging off; the slag skimming bright surface is more than 80 percent;
2) deoxidizing and alloying in the process of primary molten steel tapping, and adding lime; after the molten steel reaches an alloy fine-tuning station, bottom blowing argon is started to carry out strong stirring to reduce the top slag of the steel ladle, the color of the top slag is changed from black to yellow brown, and the desulfurization rate in the process is required to be more than 20%; wherein the flow rate of bottom-blown argon is 200-500 NL/min, and the holding time is 5-8 min;
3) heating and raising the temperature of an LF refining electrode, desulfurizing white slag, adjusting the components and the temperature to target values, and treating calcium, wherein the S content in the steel is required to be less than or equal to 0.0020 percent before an LF calcium feeding line.
4. The process method for solving the problem of the cracks of the high-strength steel casting blank according to claim 1, which is characterized by comprising the following steps of: the content of H in the steel is less than or equal to 0.0002 percent by maintaining RH at the vacuum degree of below 67Pa for 15-20 min and carrying out vacuum degassing.
5. The process method for solving the problem of the cracks of the high-strength steel casting blank according to claim 1, which is characterized by comprising the following steps of: the mode for carrying out calcium treatment modification on inclusions in steel in the LF refining process is as follows: after the components are qualified, feeding a pure calcium core-spun yarn to carry out calcium treatment on the impurities, and ensuring that the feeding yarn is opened at the bottom and argon is blown to weakly stir for 3-6 min; and after feeding the wires, continuing to weakly stir for 3-5 min, wherein the flow of weakly-stirred argon is 10-30 NL/min.
6. The process method for solving the problem of the cracks of the high-strength steel casting blank according to claim 5, which is characterized by comprising the following steps of: the main control method of RH refining is as follows: keeping the vacuum degree below 67Pa for 15-20 min for vacuum dehydrogenation, wherein the end point is that H is less than or equal to 1.5 ppm; during the period, the ferrotitanium alloy is added, and the Ti content in the steel is adjusted to a target value.
7. The process method for solving the problem of the cracks of the high-strength steel casting blank according to claim 1, which is characterized by comprising the following steps of: the slab continuous casting process comprises the following steps:
1) the method comprises the following steps of adopting lithium oxide-containing crystallizer covering slag with alkalinity of 1.20-1.50, wherein the main components of the covering slag are CaO: 38.3-45.8% of SiO2：20.0～30.0％、Li2O：1.85～4.24％。
2) The secondary cooling adopts a weak cooling system, the specific water amount is 0.50-0.65L/kg, and the automatic dynamic adjustment of the secondary cooling is realized according to the steel type components, the casting pulling speed, the temperature of the tundish molten steel and the temperature requirements of the casting blank at 11 loop cooling points of the fan-shaped section;
3) the casting blank is subjected to dynamic soft reduction, and the reduction position and the reduction amount are automatically and dynamically adjusted according to parameters of steel components, casting pulling speed and molten steel casting temperature;
4) the segment D1 and the segment D2 are thrown with electromagnetic stirring rollers; the electromagnetic stirring frequency is 4-8 Hz, the upper coil current is 250A +/-10A, and the lower coil current is 450A +/-20A.
8. The process method for solving the problem of the cracks of the high-strength steel casting blank according to claim 1, which is characterized by comprising the following steps of: the casting blank hot charging method comprises the following steps: directly feeding the casting blank into a heating furnace after the casting blank is cut to length, wherein the temperature of the casting blank reaches above 650 ℃; when the number of continuous casting furnaces reaches more than 4, in order to buffer the feeding rhythm of casting blanks, the subsequent casting blanks need to be fed into a heat preservation pit with a gas burner for heating for heat preservation; it must be ensured that the slab surface temperature is above 450 ℃ and is charged into the furnace to reduce the risk of slab cracking below this temperature.
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