Method for improving flaw detection qualification rate of thick slab pipeline steel

Abstract

The invention discloses a method for improving the flaw detection qualification rate of thick slab pipeline steel. The method comprises the following steps: (1) refining and slagging: controlling the alkalinity of steel ladle top slag to be 4-6.5 after refining; (2) calcium treatment: according to the oxygen content and the sulfur content of tapping, refining and carrying out dynamic calcium treatment, and controlling a proper calcium-sulfur ratio to 2.2-2.8; (3) deslagging: carrying out deslagging of a large ladle of slag; (4) production organization: reducing the residence time of molten steel to be less than 30 minutes, and trying high-temperature stuffing sand for pipeline steel; (5) casting and ending blank optimization: surrounding slag entrapment at the last stage of casting, deepening the insertion depth of a water gap of a tail furnace to 120-175mm; the steel remaining amount of the tundish is increased to 12 tons; (6) dividing head and tail blanks: reasonably dividing the weights and lengths of the head and tail blanks according to flaw detection maps and continuous casting sections of the head and tail rolled steel plates; and (7) controlling the hydrogen content in the whole process. The method has the advantages that the qualified rate of energy standard special-grade flaw detection is increased, the edge defects of the steel plate are avoided, and the non-planned judgment change rate and related cost loss are reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of iron and steel metallurgy, and in particular relates to a method for improving the flaw detection pass rate of thick slab pipeline steel. Background Art

[0002] The edge defect problem of pipeline steel plates is a difficult problem for steel companies to control at present. Due to the low-carbon and low-phosphorus design of thick slab pipeline steel, the over-oxidation at the end of the converter, the short floating time of continuous casting inclusions, the insignificant cooling effect of the billet, and the influence of factors such as the rolling compression ratio, width ratio and steel plate cooling, the flaw detection pass rate is always lower than that of thin slabs. With the stricter use of energy standard special grade flaw detection for pipeline steel, the flaw detection pass rate of wide and thick plate pipeline steel is even lower, resulting in a large number of revisions and unplanned changes, affecting the orders of pipeline steel and the company's brand and benefits. In addition, with the increasing demand of customers for thick plate quality and the gradual promotion of national energy standard flaw detection standards, most steel grades need to be inspected and shipped according to energy standard special grade standards, and it is urgent to improve the energy standard special grade flaw detection pass rate of thick slabs. Summary of the invention

[0003] The purpose of the present invention is to solve the problems of low flaw detection pass rate and high incidence rate of steel plate edge defects in existing wide and thick plate pipeline steels, and to provide a method for improving the flaw detection pass rate of thick plate billet pipeline steels, which can improve the energy standard special grade flaw detection pass rate, avoid steel plate edge defects and form relevant smelting specifications, greatly reduce the unplanned re-judgment rate and related cost losses, and improve contract fulfillment and product quality levels.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions: A method for improving the pass rate of thick slab pipeline steel flaw detection, specifically comprising: (1) Refining and slag making: Control the slag basicity. After refining, the basicity of the ladle top slag is controlled at 4~6.5. After adjustment, the top slag is treated with a small electrode and a small flow for more than 3 minutes to ensure that the top slag is in a molten state at the steelmaking temperature. The slag promotes the uniformity of the top slag and the adsorption of inclusions, thereby improving the adsorption effect of the ladle top slag. (2) Calcium treatment: According to the oxygen and sulfur content of the steel, dynamic calcium treatment is carried out during refining, and the appropriate calcium-sulfur ratio is controlled to 2.2-2.8 to stabilize the calcium treatment effect, improve the calcium treatment efficiency, improve the inclusions at the edge of the pipeline steel weld, and stabilize the inclusion modification effect; (3) Slag discharge: Carry out slag discharge from large tundishes to improve the casting tonnage of tundishes; optimize the operating standards for replacement of protective sleeves, addition of covering agents, and setting of slag detection alarm values ​​to reduce secondary contamination of molten steel; (4) Production organization: Reduce the retention time of molten steel to less than 30 minutes, try to use high-temperature drainage sand for pipeline steel, and reduce the oxygen burning caused by the molten steel not flowing due to long steel filling time or sintering of drainage sand in the ladle; (5) Optimization of pouring and finishing: To prevent slag rolling at the end of casting, the water inlet insertion depth of the tail furnace is deepened to 120-175 mm to slow down the liquid surface activity in the non-steady state at the end of casting; the steel retention in the tundish is increased to 12 tons, the timing of reducing the casting speed in the later stage of casting is postponed, and the non-steady-state casting time is reduced; as the mold slag is easily rolled in when it is non-steady, the rolling conditions of different mold slags in different flow fields are compared and the best one is used; (6) Division of head and tail billets: According to the flaw detection map of the head and tail rolled steel plates and the continuous casting section, the weight and length of the head and tail billets are reasonably divided. The head billet is marked not less than 25t, and the tail billet is marked not less than 30t and not less than 6.5m. The use principles of head and tail billets are standardized to improve the utilization rate of head and tail billets and the flaw detection pass rate of sub-head and tail billets; (7) Control of hydrogen content throughout the entire process.

[0005] Furthermore, in the step (2), seamless calcium silicon wire is used to replace pure calcium wire to reduce secondary oxidation caused by splashing during the calcium treatment process.

[0006] Furthermore, in step (7), the advantages of the new KR desulfurization equipment are fully utilized to effectively control the S content of the converter steel, reduce the LF desulfurization pressure, and avoid high initial hydrogen caused by deep desulfurization of a large amount of slag.

[0007] Furthermore, in step (7), a refined slag test is carried out, and part of the refined slag is used to replace lime to reduce the amount of lime added.

[0008] Furthermore, in the step (7), combined with the experience of controlling hydrogen content in non-vacuum steel, the LF deoxidation slag making and bottom blowing control are standardized to reduce the increase of hydrogen in molten steel caused by exposed molten steel.

[0009] Furthermore, in step (7), the effects of different lifting gas flow rates, bottom blowing flow rates, and immersion tube insertion depths on degassing are compared to improve the RH dehydrogenation efficiency and develop the optimal degassing parameters, i.e., the gas flow rate is increased to 100 Nm 3 / h, bottom blowing flow rate 100NL / min, immersion pipe insertion depth 500~700mm.

[0010] Furthermore, in step (7), for hydrogen-sensitive steel grades, ordinary steel grades are arranged for mixed pouring in the first two furnaces to avoid the influence of hydrogenation in the ladle during the opening pouring or the butt joint on the quality of the molten steel. By mixing other steel grades, the influence of unstable hydrogenation in the ladle in the first two furnaces on the final hydrogen content of the pipeline steel is avoided, and the hydrogen content in the ladle of the continuous pouring furnace is ensured to be within 2ppm.

[0011] Furthermore, in the step (7), based on the hydrogen diffusion law of thick slab billets and steel plates, a stack cooling comparison test is carried out on the billets from the first two furnaces and a 22 mm thick steel plate, and the stack cooling time is ≥10 h.

[0012] In the technical solution of the present invention, a set of methods for improving the qualified rate of energy-standard super-grade flaw detection and solving the problem of edge defect control of steel plates is adopted, and relevant smelting specifications are formed, which are gradually extended from pipeline steel grades to other orders with energy-standard super-grade flaw detection requirements such as pipe piles, hydropower, etc. The qualified rate of energy-standard super-grade flaw detection of steel plates smelted with billets using this specification is currently stable at more than 98%, which is 3% higher than the previous conventional process, greatly reducing the unplanned re-judgment rate and related cost losses of such products, and improving contract fulfillment and product quality levels. DETAILED DESCRIPTION Example

[0013] To make the present invention more clearly understood, a method for improving the qualified rate of thick slab pipeline steel flaw detection according to the present invention is further described below. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0014] In this example, (S1) first aims to reduce the number of inclusions, and work is carried out through appropriate inclusion modification, inclusion adsorption by low-basicity slag, and prevention of secondary oxidation, including: S11: Refining and slag making process. After refining, the basicity of the ladle top slag is controlled at 4.5~6.5. After adjusting the basicity, the ladle top slag is treated with a small electrode and a small flow rate for more than 3 minutes to make the top slag uniform and the inclusions float.

[0015] S12: Calcium treatment process. According to the oxygen and sulfur content of the steel, dynamic calcium treatment is carried out in the refining process, and the appropriate calcium-sulfur ratio is controlled to 2.2~2.8 to stabilize the calcium treatment effect, improve the calcium treatment efficiency, and improve the inclusions at the edge of the pipeline steel weld. Try to use seamless silicon calcium wire instead of pure calcium wire to reduce the secondary oxidation of splashing during the calcium treatment process.

[0016] S13: Carry out slag discharge from large ladles to improve the casting tonnage of the tundish; standardize the replacement of protective sleeves, addition of covering agents, setting of slag detection alarm values ​​and other operating standards to reduce secondary contamination of molten steel.

[0017] S14: Production organization. Reasonable production organization can reduce the retention time of molten steel to less than 30 minutes; high-temperature drainage sand can be used for pipeline steel to reduce the oxygen burning caused by the molten steel not flowing due to long steel filling time or sintering of drainage sand.

[0018] (S2) Optimize the pouring and finishing operations of thick slab pipeline steel to improve the internal quality of the billets in the head and tail furnaces, including: S21: Around the slag roll at the end of casting, the tail furnace deepens the nozzle insertion depth to 120~175mm to slow down the liquid level activity in the non-steady state at the end of casting; S22: Increase the steel retention in the tundish from 8t to 12t, postpone the timing of reducing the casting speed in the later stage of casting, and reduce the non-steady-state casting time; S23: As the protective slag is easily entangled when it is in a non-steady state, compare the entanglement of different protective slags in different flow fields and use the best one.

[0019] (S3) Rationally evaluate and utilize the internal quality of the head and tail billets of thick slab pipeline steel. By comparing and analyzing the big data of the flaw detection patterns of the second head and tail rolled steel plates, evaluate the impact of the head and tail billets on quality, formulate reasonable head and tail billet classification standards, and improve the utilization rate of the head and tail billets of thick slabs.

[0020] (S4) Hydrogen content control in the whole process: S41: Give full play to the advantages of the new KR desulfurization equipment, effectively control the S content of converter steel, reduce the LF desulfurization pressure, and avoid high initial hydrogen caused by deep desulfurization of large slag volumes.

[0021] S42: Conduct refined slag tests, use part of the refined slag to replace lime, and reduce the amount of lime added. Reduce the contradiction between slag making and hydrogenation during the rainy season, ensure that the slag is made well, and control the hydrogenation of molten steel within a reasonable range, reduce the initial hydrogen content of molten steel, and improve the internal quality of thick plate billets.

[0022] S43: Standardize the LF deoxidation slag making and process bottom blowing control to reduce the increase of hydrogen in molten steel caused by exposed molten steel. Based on the experience of controlling hydrogen content in non-vacuum steel, formulate the LF deoxidation slag making and process bottom blowing control operation specifications to reduce the increase of hydrogen in molten steel caused by exposed molten steel.

[0023] S44: Carry out research on RH degassing efficiency, compare the effects of different boost gas flow rates, bottom blowing flow rates, and immersion tube insertion depths on degassing, and improve RH dehydrogenation efficiency. Develop optimal degassing parameters: gas flow rate increased to 100Nm 3 / h, bottom blowing flow rate 100NL / min, immersion pipe insertion depth 500~700mm.

[0024] S45: For hydrogen-sensitive steels such as pipelines and hydropower, ordinary steels are mixed in the first two furnaces to avoid the impact of hydrogenation in the opening or butt-joint ladle on the quality of molten steel. By mixing other steels, the impact of unstable hydrogenation in the first two furnaces on the final hydrogen content of pipeline steel is avoided. After the implementation of this process, the hydrogen content of the ladle in the continuous casting furnaces is within 2ppm, with an average of 1.69ppm, and the number of unqualified flaw detection caused by hydrogen-induced cracks has been greatly reduced.

[0025] S46: Study the hydrogen diffusion law of thick slab billets and steel plates, and carry out comparative tests on stack cooling of billets from the first two furnaces and 22mm thick steel plates.

[0026] Steel plate stack cooling test was carried out on 22, 26.2 and 30.4mm thick steel plates. The stack cooling time was ≥10h. After stack cooling, 3644 tons of thick plates were inspected, and 33.77 tons failed. The qualified rate of flaw detection of stack-cooled steel plates was 99.1%. The delayed cracking problem of pipeline steel plates was effectively solved by controlling hydrogen content and stack cooling of steel plates.

[0027] (S5) Conduct comparative test on pulling speed and quality comparison of pipeline steel: Three 0.75m / min pulling speed tests were carried out on the 3# machine pipeline steel, with a total of 9 furnaces tested, 1,448 tons of billets were produced, 1,070 tons were inspected, and the flaw detection pass rate was 98.7%; 63 samples were analyzed for inclusions, with a 1.5-level pass rate of 98.4%, and a 1.0-level inclusion pass rate of 93.6%; 3 steel plates were randomly inspected by edge oblique probing, and all were qualified; during the same period, 6,175 tons of pipeline steel was produced at a pulling speed of 0.7m / min and flaw detection was carried out, with a flaw detection pass rate of 99.4%; 475 samples were analyzed for inclusions, with a 1.5-level pass rate of 99.2%, and a 1.0-level inclusion pass rate of 93.3%. Tests at different pulling speeds provide technical support for subsequent billet crack control and efficient production.

[0028] In the present invention, the benefits of improving the pass rate of thick slab pipeline steel flaw detection are as follows: Before improvement: From August 2019 to January 2020, the pass rate of wide and thick plate pipeline steel flaw detection was only 95.80%; After improvement: From February to October 2020, the pipeline steel smelting process was improved, with a cumulative flaw detection of 130,625.039 tons, an average of 14,514 tons per month, and an estimated output of 29,028 tons from November to December, with a flaw detection pass rate of 99.0%; The loss due to downgrade and re-judgment caused by unqualified NDT is calculated based on the business unit cost of 1,000 yuan / ton, of which X70 is 4,850 yuan / ton, X80 is 5,500 yuan / ton, and the contract product is 3,850 yuan / ton. The NDT volume of X80 is 53,034 tons. The benefit that can be generated in 2020 = (130,625.039 + 29,028-53,034) tons * (99.00%-95.80%) * (4,850-3,850) yuan / ton + 53,034 tons * (99.00%-95.80%) * (5,500-3,850) yuan / ton = 6.212 million yuan.

[0029] The benefits of extending the thick slab pipeline steel smelting process to other steel grades and improving the qualified rate of special grade flaw detection are as follows: In May, the specification was extended to other steel types such as pipe piles and hydropower, and the total number of flaw detection orders was 287,791.33 tons from May to October, with a flaw detection pass rate of 98.5%, an increase of 3.1% compared with the previous pass rate of 95.4% for ordinary processes. Among them, there are 164041.06 tons of steel with internal control S upper limit ≥ 0.005%, accounting for 57%. Compared with before optimization, this part of steel has increased molten iron pretreatment, and the steelmaking cost has increased by 23 yuan / ton. The above steel is calculated based on the lowest cost Q345B, that is, the genuine Q345B is 4300 yuan, the agreement plate is 3850 yuan, and the minimum price difference is changed to 450 yuan / ton. The benefit generated by this part of steel = (287791.33-164041.06)*(98.5%-95.4%)*450+164041.06*(98.5%-95.4%)*(450-23)=3.8977 million.

[0030] To sum up, the total benefit = 6.212 million yuan + 3.8977 million yuan = 10.1097 million yuan.

[0031] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A method for improving the pass rate of thick slab pipeline steel flaw detection, characterized in that: (1) Refining and slag making: Control the slag basicity. After refining, the basicity of the ladle top slag is controlled at 4~6.

5. After adjustment, the top slag is treated with a small electrode and a small flow rate for more than 3 minutes to ensure that the top slag is in a molten state at the steelmaking temperature, promote the uniformity of the top slag and the adsorption of inclusions, and improve the adsorption effect of the ladle top slag; (2) Calcium treatment: According to the oxygen and sulfur content of the steel, dynamic calcium treatment is carried out during refining, and the appropriate calcium-sulfur ratio is controlled to 2.2-2.8 to stabilize the calcium treatment effect, improve the calcium treatment efficiency, improve the inclusions at the edge of the pipeline steel weld, and stabilize the inclusion modification effect; (3) Slag discharge: Carry out slag discharge from large tundishes to improve the casting tonnage of tundishes; optimize the operating standards for replacement of protective sleeves, addition of covering agents, and setting of slag detection alarm values ​​to reduce secondary contamination of molten steel; (4) Production organization: Reduce the retention time of molten steel to less than 30 minutes, try to use high-temperature drainage sand for pipeline steel, and reduce the oxygen burning caused by the molten steel not flowing due to long steel filling time or sintering of drainage sand in the ladle; (5) Optimization of pouring and finishing: To prevent slag rolling at the end of casting, the water inlet insertion depth of the tail furnace is deepened to 120-175 mm to slow down the liquid surface activity in the non-steady state at the end of casting; the steel retention in the tundish is increased to 12 tons, the timing of reducing the casting speed in the later stage of casting is postponed, and the non-steady-state casting time is reduced; as the mold slag is easily rolled in when it is non-steady, the rolling conditions of different mold slags in different flow fields are compared and the best one is used; (6) Division of head and tail billets: According to the flaw detection map of the head and tail rolled steel plates and the continuous casting section, the weight and length of the head and tail billets are reasonably divided. The head billet is marked not less than 25t, and the tail billet is marked not less than 30t and not less than 6.5m. The use principles of head and tail billets are standardized to improve the utilization rate of head and tail billets and the flaw detection pass rate of sub-head and tail billets; (7) Control of hydrogen content throughout the entire process.

2. The method for improving the flaw detection pass rate of thick slab pipeline steel according to claim 1, characterized in that: In the step (2), a seamless calcium silicon wire is used to replace the pure calcium wire to reduce the secondary oxidation caused by splashing during the calcium treatment process.

3. The method for improving the flaw detection pass rate of thick slab pipeline steel according to claim 1 or 2, characterized in that: In the step (7), the advantages of the new KR desulfurization equipment are fully utilized to effectively control the S content of the converter steel, reduce the LF desulfurization pressure, and avoid high initial hydrogen caused by deep desulfurization of large slag volume.

4. The method for improving the flaw detection pass rate of thick slab pipeline steel according to claim 1 or 2, characterized in that: In the step (7), a refined slag test is carried out, and part of the refined slag is used to replace lime to reduce the amount of lime added.

5. The method for improving the flaw detection pass rate of thick slab pipeline steel according to claim 1 or 2, characterized in that: In the step (7), combined with the experience of controlling hydrogen content in non-vacuum steel, the LF deoxidation slag making and bottom blowing control are standardized to reduce the increase of hydrogen in molten steel caused by exposed molten steel.

6. The method for improving the flaw detection pass rate of thick slab pipeline steel according to claim 1 or 2, characterized in that: In step (7), the effects of different lifting gas flow rates, bottom blowing flow rates, and immersion tube insertion depths on degassing are compared to improve the RH dehydrogenation efficiency and develop the optimal degassing parameters, i.e., the gas flow rate is increased to 100 Nm 3 / h, bottom blowing flow rate 100NL / min, immersion pipe insertion depth 500~700mm.

7. The method for improving the pass rate of thick slab pipeline steel flaw detection according to claim 1 or 2, characterized in that: In the step (7), for hydrogen-sensitive steel grades, ordinary steel grades are arranged for mixed pouring in the first two furnaces to avoid the influence of hydrogenation in the middle package during the opening pouring or the butt pouring on the quality of the molten steel. By mixing other steel grades, the influence of unstable hydrogenation in the middle package of the first two furnaces on the final hydrogen content of the pipeline steel is avoided, and the hydrogen content of the middle package in the continuous pouring furnace is ensured to be within 2ppm.

8. The method for improving the pass rate of thick slab pipeline steel flaw detection according to claim 1 or 2, characterized in that: In the step (7), based on the hydrogen diffusion law of thick slab billets and steel plates, a stack cooling comparison test is carried out on the billets from the first two furnaces and 22 mm thick steel plates, and the stack cooling time is ≥10 h.