A method for improving the flaw detection qualification rate of thick slab pipeline steel

Abstract

The application discloses a method for improving the flaw detection qualified rate of thick slab pipeline steel, which comprises the following steps: (1) refining and slagging: the basicity of the ladle top slag at the end of refining is controlled to be 4-6.5; (2) calcium treatment: dynamic calcium treatment is carried out according to the oxygen and sulfur content of the tapping, and the suitable calcium-sulfur ratio is controlled to be 2.2-2.8; (3) slagging: the ladle slagging is carried out; (4) production organization: the molten steel residence time is reduced to below 30 min, and the high-temperature drainage sand is used for pipeline steel; (5) opening pouring and optimization of tailing slab: the end of the pouring is rolled around the slag, the depth of the submerged entry nozzle in the tail furnace is increased to 120-175 mm, and the tundish steel retention amount is increased to 12 tons; (6) head and tail slab division: the head and tail slab weight and length are reasonably divided according to the head and tail rolling steel plate flaw detection spectrum and the continuous casting section; and (7) hydrogen content control in the whole process. The method has the advantages of improving the qualified rate of the energy standard special flaw detection, avoiding the edge defects of the steel plate, reducing the non-planned judgment rate and related cost loss.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel metallurgy, and particularly relates to a method for improving the flaw detection qualified rate of thick slab pipeline steel. BACKGROUND

[0002] The problem of edge defects of pipeline steel plates is a difficult problem to control for steel enterprises. Due to the low-carbon and low-phosphorus design of thick slab pipeline steel, the over-oxygen at the end point of the converter, the short floating time of inclusions during continuous casting, the unclear effect of billet stacking, and the influence of rolling compression ratio, spreading ratio and plate stacking, the flaw detection qualified rate is always lower than that of thin slab. With the adoption of more stringent special level flaw detection for pipeline steel, the flaw detection qualified rate of wide and thick plate pipeline steel is even lower, resulting in a large number of rejudgments and unplanned events, affecting the order receiving of pipeline steel and the brand and benefit of the company. In addition, with the increasing demand of customers for thick plate quality and the gradual popularization of national standard flaw detection standards, most steel grades need to be inspected according to the special level standard before being put into the market, and it is urgent to improve the special level flaw detection qualified rate of thick slab. SUMMARY

[0003] The purpose of the present application is to solve the problems of low flaw detection qualified rate of wide and thick plate pipeline steel and high edge defect rate of steel plates, and a method for improving the flaw detection qualified rate of thick slab pipeline steel is provided, which can improve the special level flaw detection qualified rate, avoid edge defects of steel plates, form relevant smelting specifications, greatly reduce the non-planned rejudgment rate and related cost loss, and improve the contract performance and product quality level.

[0004] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0005] A method for improving the flaw detection qualified rate of thick slab pipeline steel, specifically comprising:

[0006] (1) Refining slagging: control the slag basicity, control the top slag basicity of the ladle at 4-6.5 at the end of refining, adjust the small electrode and small flow for more than 3 minutes, ensure that the top slag is in a molten state at the steelmaking temperature, and the slag promotes the uniformity and adsorption of inclusions of the top slag, and improves the adsorption effect of the top slag of the ladle;

[0007] (2) Calcium treatment: according to the tapping oxygen and sulfur content, dynamic calcium treatment is carried out during refining, the calcium to sulfur ratio is controlled to 2.2-2.8, the calcium treatment effect is stabilized, the calcium treatment efficiency is improved, the edge inclusion morphology of pipeline steel is improved, and the inclusion modification effect is stabilized;

[0008] (3) Slagging: carrying out ladle slagging, improving the casting tonnage of tundish, optimizing the operation standards of protective sleeve replacement, covering agent addition and detection alarm value setting of slag, and reducing the secondary pollution of molten steel;

[0009] (4) Production organization: Reduce the molten steel residence time to 30 min or less, use high-temperature drainage sand for pipeline steel, and reduce the need for oxygen burning due to long molten steel holding time or drainage sand sintering in the ladle;

[0010] (5) Optimization of casting start and end billets: Around the end of casting, the depth of the submerged nozzle is inserted to 120-175 mm to slow down the liquid surface activity under non-steady state at the end of casting. Increase the tundish steel holding capacity to 12 tons, delay the timing of reducing the casting speed at the later stage, and reduce the non-steady state casting time. For the non-steady state of the protective slag, compare the rolling of different protective slags under different flow fields, and select the best one;

[0011] (6) Head and tail billet division: According to the head and tail rolling steel plate flaw detection spectrum and continuous casting section, reasonably divide the head and tail billet weight and length, the head billet weight is not less than 25t, the tail billet weight is not less than 30t and the length is not less than 6.5m, standardize the use principle of head and tail billets, improve the utilization rate of head and tail billets and the flaw detection qualified rate of secondary head and tail billets;

[0012] (7) Hydrogen content control in the whole process.

[0013] Further, in step (2), seamless silicon-calcium wire is used instead of pure calcium wire to reduce secondary oxidation during calcium treatment process.

[0014] Further, in step (7), KR desulfurization equipment is used to control the S content of the converter tapping, reduce the LF desulfurization pressure, and avoid high initial hydrogen caused by large amount of slag and deep desulfurization.

[0015] Further, in step (7), refine the slag test, replace part of the lime with part of the lime, and reduce the lime addition amount.

[0016] Further, in step (7), combined with the experience of non-vacuum steel hydrogen content control, standardize the LF deoxidizing slag making and process bottom blowing control, and reduce the hydrogen increase of molten steel caused by exposure.

[0017] Further, in step (7), the influence of different lifting gas flow, bottom blowing flow and immersion tube insertion depth on degassing is compared, the RH dehydrogenation efficiency is improved, and the optimal degassing parameters are determined, i.e. gas flow is increased to 100 Nm 3 / h, bottom blowing flow is 100 NL / min, and immersion tube insertion depth is 500-700 mm.

[0018] Further, in step (7), for hydrogen-sensitive steel, the first two furnaces are arranged for mixed casting of ordinary steel to avoid the influence of hydrogen increase in the tundish on the quality of molten steel during casting start or butt joint. By mixing other steel, the influence of unstable hydrogen increase in the tundish on the hydrogen content of pipeline steel at the end of the first two furnaces is avoided, and the tundish hydrogen content is controlled within 2 ppm in continuous casting.

[0019] Further, based on the hydrogen diffusion law of the thick slab blank and the steel plate, the blank of the first two furnaces and the 22mm-thick steel plate are subjected to the stack cooling comparative test, and the stack cooling time is greater than or equal to 10h.

[0020] In the technical solution of the present application, a method for improving the UT qualified rate of the special level and solving the control of the edge defects of the steel plate is provided, and a related smelting specification is formed. The method is gradually promoted from the pipeline steel to the pipe pile, water and electricity and other orders with UT requirements, and the UT qualified rate of the steel plate smelted by the specification is currently stabilized at more than 98%, which is improved by 3% compared with the previous conventional process. The non-planned judgment rate and the related cost loss of such products are greatly reduced, and the contract performance and the product quality level are improved. DETAILED DESCRIPTION EMBODIMENT

[0021] In order to make the present application clearer, a method for improving the UT qualified rate of the thick slab pipeline steel is further described below. The specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0022] In the present example, (S1) first, the number of inclusions is reduced, and work is carried out through suitable inclusion modification, low-alkalinity slag adsorption of inclusions, prevention of secondary oxidation, etc., including:

[0023] S11: Refining slagging process. The top slag alkalinity of the ladle at the end of refining is controlled at 4.5-6.5, and after adjusting the alkalinity, the small electrode and small flow are treated for more than 3min to promote the uniformity of the top slag and the floating of inclusions.

[0024] S12: Calcium treatment process. According to the oxygen and sulfur content of tapping, dynamic calcium treatment is carried out during refining, the calcium to sulfur ratio is controlled to 2.2-2.8, the calcium treatment effect is stabilized, the calcium treatment efficiency is improved, and the inclusions at the edge of the pipeline steel weld are improved. Seamless calcium-silicon wire is used instead of pure calcium wire to reduce the secondary oxidation of spatter during calcium treatment.

[0025] S13: Develop tundish slag discharge, increase tundish casting tonnage; standardize the operation standards of protective sleeve replacement, covering agent addition and slag detection alarm value setting, etc., to reduce the secondary pollution of molten steel.

[0026] S14: Production organization. Reasonable production organization, reduce the molten steel residence time to less than 30min; pipeline steel uses high-temperature drainage sand to reduce the need for oxygen burning due to long molten steel holding time or drainage sand sintering.

[0027] (S2) Optimize the thick slab pipeline steel opening and end blank operation to improve the internal quality of the head and tail furnace blank, including:

[0028] S21: Around the end of casting slag, the tail furnace deepens the nozzle insertion depth to 120~175mm, slows down the liquid surface activity under non-steady state at the end of casting;

[0029] S22: Increase the tundish steel holding capacity from 8t to 12t, delay the timing of reducing the casting speed in the later stage, and reduce the non-steady state casting time;

[0030] S23: For the non-steady state of protective slag, it is easy to roll in, and the rolling in of different protective slag in different flow fields is compared, and the best one is used.

[0031] (S3) Reasonable evaluation of the internal quality of thick slab pipeline steel head and tail billet. Through the comparison and analysis of the flaw detection map of the head and tail rolled steel plate, the influence of the head and tail billet quality is evaluated, the reasonable head and tail billet division standard is formulated, and the utilization rate of thick slab head and tail billet is improved.

[0032] (S4) Hydrogen content control in the whole process:

[0033] S41: Give full play to the advantages of KR desulfurization equipment, effectively control the S content of converter tapping, reduce the pressure of LF desulfurization, and avoid high initial hydrogen caused by large amount of slag and deep desulfurization.

[0034] S42: Develop refining slag test, change part of refining slag to replace part of lime, reduce lime addition. Reduce the contradiction between Meiyu season slagging and hydrogen increase, ensure good slag while steel hydrogen increase is controlled within a reasonable range, reduce the initial hydrogen content of variety steel, and improve the internal quality of thick slab billet.

[0035] S43: Standardize LF deoxidizing slag making and process bottom blowing control to reduce hydrogen increase caused by steel exposure. Combined with the experience of non-vacuum steel hydrogen content control, formulate LF deoxidizing slag making and process bottom blowing control operation specification to reduce hydrogen increase caused by steel exposure.

[0036] S44: RH degassing efficiency research, compare the influence of different lifting gas flow, bottom blowing flow, and immersion tube insertion depth on degassing, and improve the RH dehydrogenation efficiency. The optimal degassing parameters are: gas flow is increased to 100Nm 3 / h, bottom blowing flow is 100NL / min, and immersion tube insertion depth is 500~700mm.

[0037] S45: For hydrogen sensitive steel such as pipeline, water and electricity, arrange ordinary steel mixed casting for the first 2 furnaces to avoid the influence of ladle hydrogen increase on steel quality during start casting or butt joint. Through mixed casting with other steel, the influence of unstable ladle hydrogen increase on pipeline steel end point hydrogen content is avoided. After the implementation of this process, the ladle hydrogen content in continuous casting furnace is controlled within 2ppm, with an average of 1.69ppm, and the number of unqualified flaw detection caused by hydrogen induced cracking is greatly reduced.

[0038] S46: Study the hydrogen diffusion law of thick slab billets and steel plates, and carry out the first 2 furnace billets and 22mm thick steel plate stack cooling comparison test.

[0039] Carry out steel plate stack cooling test for 22, 26.2, 30.4mm thick steel plates, stack cooling time≥10h, 3644 tons of thick plate stack cooling after flaw detection, 33.77 tons of unqualified, stack cooling steel plate flaw detection qualified rate 99.1%, the problem of delayed cracking of pipeline steel plate is effectively solved through hydrogen content control and steel plate stack cooling.

[0040] (S5) Carry out pipeline steel speed comparison test and quality comparison tracking:

[0041] 3# machine pipeline steel carries out three times of 0.75m / min speed test, a total of 9 furnaces, 1448 tons of billets are produced, 1070 tons of flaw detection, the flaw detection qualified rate is 98.7%; 63 samples of inclusions are analyzed, the qualified rate of 1.5 level is 98.4%, and the qualified rate of 1.0 level inclusion is 93.6%; 3 steel plates are selected for edge oblique exploration, all of which are qualified; 6175 tons of pipeline steel are produced at the same period of 0.7m / min speed, the flaw detection qualified rate is 99.4%; 475 samples of inclusions are analyzed, the qualified rate of 1.5 level is 99.2%, and the qualified rate of 1.0 level inclusion is 93.3%, different speed test provides technical support for subsequent billet crack control and efficient production.

[0042] In the present application, the benefits of improving the flaw detection qualified rate of thick slab pipeline steel are as follows:

[0043] Before improvement: from August 2019 to January 2020, the flaw detection qualified rate of wide and thick plate pipeline steel was only 95.80%;

[0044] After improvement: from February to October 2020, the cumulative flaw detection of pipeline steel smelting process after improvement was 130625.039 tons, with an average of 14514 tons per month, and the estimated output of November and December was 29028 tons, with a flaw detection qualified rate of 99.0%;

[0045] The degradation loss caused by unqualified flaw detection is calculated as 1000 yuan per calculation, among which X70 is 4850 yuan / ton, X80 is 5500 yuan / ton, and protocol product is 3850 yuan / ton, among which X80 flaw detection is 53034 tons, so the benefits in 2020 can be generated= (130625.039+29028-53034) tons* (99.00%-95.80%) * (4850-3850) yuan / ton+53034 tons* (99.00%-95.80%) * (5500-3850) yuan / ton=621.20 million yuan.

[0046] The benefits of improving the qualified rate of special flaw detection of other steel grades by promoting the thick slab pipeline steel smelting process are as follows:

[0047] The specification is adopted to promote to pipe pile, water and electricity and other steel types of special level flaw detection orders since May, and the cumulative flaw detection is 287791.33 tons from May to October, the flaw detection qualified rate is 98.5%, which is 3.1% higher than the previous ordinary process qualified rate 95.4%; wherein the internal control S upper limit is greater than or equal to 0.005% steel 164041.06 tons, accounting for 57%, the part of steel increases hot metal pretreatment compared with the optimization, and the steelmaking cost increases by 23 yuan / ton, the above steel is calculated according to the cost of Q345B, that is, Q345B normal product 4300, protocol plate 3850 yuan, the minimum difference of 450 yuan / ton, then the part of steel generates benefit = (287791.33-164041.06) * (98.5%-95.4%) * 450 + 164041.06 * (98.5%-95.4%) * (450-23) = 3.8977 million yuan.

[0048] In summary, the total benefit = 6.212 million yuan + 3.8977 million yuan = 10.1097 million yuan.

[0049] In addition to the above embodiments, the present application can have other implementation manners. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present application.

Claims

1. A method for improving the flaw detection qualification rate of thick slab pipeline steel, characterized in that, It comprises the following steps: (1) Refining slagging: control the slag basicity, control the top slag basicity of ladle at 4~6.5 at the end of refining, adjust the small electrode and small flow for more than 3 minutes, ensure the top slag in molten state at the steelmaking temperature, promote the uniformity of top slag and the adsorption of inclusions, and improve the adsorption effect of top slag of ladle; (2) Calcium treatment: according to the oxygen and sulfur content of tapping, dynamic calcium treatment is carried out in refining, the calcium to sulfur ratio is controlled to 2.2~2.8, the calcium treatment effect is stabilized, the calcium treatment efficiency is improved, the inclusion morphology of pipeline steel edge is improved, and the inclusion modification effect is stabilized; (3) Slag discharge: carry out ladle slag discharge, improve the tundish casting tonnage; Optimize the operation standard of protective sleeve replacement, covering agent addition and slag discharge detection alarm value setting, reduce the secondary pollution of molten steel; (4) Production organization: reduce the molten steel residence time to 30 minutes or less, use high temperature drainage sand for pipeline steel, reduce the need for oxygen burning due to long molten steel holding time or drainage sand sintering of ladle; (5) Optimization of starting casting and tailing billet: around the end of casting, the depth of submerged entry nozzle is increased to 120~175mm, the liquid surface activity under non-steady state at the end of casting is slowed down; Increase the tundish steel retention amount to 12 tons, delay the timing of reducing the casting speed in the later stage of casting, reduce the non-steady state casting time; According to the non-steady state of protective slag, compare the rolling of different protective slag under different flow field, and select the best one; (6) Head and tail billet division: according to the head and tail plate flaw detection spectrum and continuous casting section, reasonably divide the weight and length of head and tail billet, the weight of head billet is not less than 25t, the weight of tail billet is not less than 30t and the length is not less than 6.5m, standardize the use principle of head and tail billet, improve the utilization rate of head and tail billet and the flaw detection qualified rate of secondary head and tail billet; (7) The whole process of hydrogen content control: comparison of different gas flow, bottom blowing flow and immersion tube insertion depth on the influence of degassing, improve the RH dehydrogenation efficiency, and develop the optimal degassing parameters, that is: gas flow is increased to 100 Nm 3 / h, bottom blowing flow 100 NL / min, immersion tube insertion depth 500~700mm; For hydrogen sensitive steel, the first two furnaces are arranged for mixed casting of ordinary steel to avoid the influence of hydrogen increase in starting casting or butt joint tundish on the quality of molten steel, and through mixed casting of other steel, the influence of unstable hydrogen increase in the first two furnaces on the hydrogen content of pipeline steel at the end is avoided, and the hydrogen content in tundish in continuous casting furnace is controlled within 2ppm.

2. The method for improving the flaw detection qualified rate of thick slab pipeline steel according to claim 1, wherein: in the step (2), seamless calcium-silicon wire is used instead of pure calcium wire to reduce the secondary oxidation of calcium treatment process spatter.

3. The method for improving the flaw detection qualified rate of thick slab pipeline steel according to claim 1 or 2, wherein: in the step (7), KR desulfurization equipment is used to control the S content of converter tapping to avoid high initial hydrogen caused by large amount of slag and deep desulfurization.

4. The method for improving the flaw detection qualified rate of thick slab pipeline steel according to claim 1 or 2, wherein: in the step (7), refining slag test is carried out, part of the composition of refining slag is replaced by part of lime, and the addition amount of lime is reduced.

5. The method for improving the flaw detection qualified rate of thick slab pipeline steel according to claim 1 or 2, wherein: in the step (7), LF deoxidization slagging and process bottom blowing control are standardized to reduce the hydrogen increase of molten steel caused by exposure.

6. The method for improving the flaw detection qualified rate of thick slab pipeline steel according to claim 1 or 2, wherein: ​ ​ ​ ​ In step (7), based on the hydrogen diffusion law of the thick slab blank and the steel plate, the blank of the first two furnaces and the 22 mm thick steel plate are compared by stack cooling, and the stack cooling time is ≥10 h.