A method for producing a seamless steel pipe for fire-resistant structures of Q345 grade

Abstract

The application discloses a production method of Q345 steel grade seamless steel pipe for fire-resistant structure, and a production process thereof is briefly described as follows: hot metal pretreatment, top and bottom combined blowing converter smelting, LF furnace refining, VD vacuum treatment, round billet continuous casting, pipe billet heating, piercing, continuous rolling pipe, fixed tension reducing, heat treatment, cold bed, saw cutting and straightening; the chemical components of the steel pipe are as follows in percentage by weight: C: 0.14-0.17%; Si: 0.20-0.30%; Mn: 1.30-1.40%; P: ≤0.020%; S: ≤0.010%; Al: 0.015-0.040%; Ti: 0.008-0.020%; Mo: 0.20-0.30%, and the rest is iron and impurities. The application adopts carbon manganese steel plus titanium micro-alloying treatment, and adds molybdenum elements to improve the mechanical properties under high temperature, so as to meet the use requirements of fire resistance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of black metallurgy, and particularly relates to a production method of Q345 steel grade seamless steel pipe for fire-resistant structure. BACKGROUND

[0002] Steel structure is a structural form of target structure realized by advanced steel material, and is a comprehensive embodiment of structural design, material application and function realization of structure. The seamless steel pipe for steel structure is mainly made by seamless pipe manufacturing process, and is commonly used for connection and support part of steel structure.

[0003] When fire occurs, the strength of steel material is rapidly reduced under the influence of high temperature, and the original strength of building structure cannot be maintained. When the temperature of the steel structure of such steel material exceeds 550 DEG C, the yield strength of the steel material is even reduced by more than 50%. When the temperature is above 600 DEG C for a certain time, the steel material will completely lose the original structure bearing capacity.

[0004] Because the steel material is mainly used as a bearing material at room temperature, it is required to maintain a high yield strength under high temperature conditions for a short time (usually 1-3 hours) when fire occurs. 2 / 3 of the yield strength of steel material at room temperature is equivalent to the long-term allowable stress value of the material. When fire occurs, if the yield point of the fire-resistant steel can still maintain this value or above, the building will not collapse. Therefore, it is required that the yield strength of the fire-resistant steel at a certain high temperature is not less than 2 / 3 of the yield strength at room temperature.

[0005] At the same time, as a building structure material, it is always desired to improve the ability of steel material to absorb seismic energy as much as possible. If the yield strength of the steel material is low, it is beneficial to absorb energy during an earthquake. Generally, the yield strength of the fire-resistant steel is required to be not more than 80%.

[0006] In addition, it is also very important to control the yield strength fluctuation range of the building steel. When the yield strength fluctuation is small, the steel structure is a whole failure mechanism, and the plastic deformation capacity of the whole is high, and the seismic performance is excellent. Therefore, it is necessary to use steel material with narrow yield interval for seismic design. At the same time, the building fire-resistant material also needs to have good welding performance. According to the general requirements of building steel, the performance indicators of the building fire-resistant steel pipe are determined as follows:

[0007] (1) fire resistance: Rel 600℃ ≥(2 / 3) Rel 20℃ ;

[0008] (2) room temperature mechanical properties and other quality indicators meet the requirements of ordinary building steel standards;

[0009] (3) seismic resistance is less than 80% at room temperature. SUMMARY

[0010] The application aims to provide a production method of Q345 steel grade seamless steel pipe for fire-resistant structure, which adopts carbon manganese steel with titanium micro-alloying treatment, and adds molybdenum element to improve the mechanical properties at high temperature, so as to meet the use requirements of fire resistance and have excellent performance.

[0011] To solve the above technical problems, the application adopts the following technical scheme:

[0012] The production method of the Q345 steel grade seamless steel pipe for fire-resistant structure has the following production process: hot metal pretreatment, top and bottom combined blowing converter smelting, LF furnace refining, VD vacuum treatment, round billet continuous casting

[0013] pipe blank heating, piercing, continuous rolling pipe, fixed tension reducing, heat treatment, cold bed, sawing, straightening; wherein:

[0014] 1. The blast furnace hot metal is pretreated to reduce the weight percentage of S in the hot metal to below 0.050%;

[0015] 2. The pretreated hot metal is mixed into the top and bottom combined blowing converter, 10% scrap steel is added, single slag process is adopted, the final slag basicity and endpoint target are controlled: [C] tapping ≥0.10%; deoxidation and alloying are carried out during tapping, the final deoxidation adopts aluminum deoxidation process, and the tapping process must be blocked or slagged, and lime block is added after the alloying is completed during the tapping process;

[0016] 3. Refining is carried out in the LF furnace: normal argon blowing is required during the whole refining process, the heating temperature is gradually increased from low to high; white slag operation is adopted according to the composition and temperature of the converter molten steel for desulfurization, composition adjustment and temperature rising operation;

[0017] 4. The bottom soft argon blowing is maintained for more than 10 minutes after the LF furnace refining is completed;

[0018] 5. VD vacuum treatment is carried out: vacuum degree ≤0.10KPa, deep vacuum time ≥10 minutes;

[0019] 6. The length of the silicon calcium wire is fed, and the argon blowing is more than 15 minutes after the wire feeding; the molten steel after VD vacuum treatment is round billet continuous casting, and the low speed, constant speed control and electromagnetic stirring process are adopted; the molten steel superheat degree is controlled; the billet is straightened and cut into round pipe blank;

[0020] The round pipe blank that passes the inspection and has a sulfur print of not more than 2.0 level is used for pipe making, and the pipe making process is as follows;

[0021] The temperature of each section of the preheating section, the heating section and the soaking section is continuously checked, and the temperature control of each section of the ring furnace is shown in the following table;

[0022]

[0023] The hot-rolled seamless steel pipe is heat treated to obtain qualified steel pipe meeting the standard requirements in performance;

[0024] The chemical components of the steel pipe are as follows in percentage by weight: C: 0.14-0.17%; Si: 0.20-0.30%; Mn: 1.30-1.40%; P: ≤0.020%; S: ≤0.010%; Al: 0.015-0.040%; Ti: 0.008-0.020%; Mo: 0.20-0.30%, and the rest is iron and inevitable impurities.

[0025] Further, the heat treatment is normalizing treatment: normalizing temperature 910±10℃, normalizing holding time 70min, and cooling mode is air cooling.

[0026] Further, the computer control system records the heating temperature of each section on line, and the temperature of the preheating section and the soaking section is controlled well to ensure thorough and uniform heating without overburning.

[0027] Further, the hot tool must be measured before use, and the handling roller is checked before rolling to avoid scratching.

[0028] Further, the heated pipe blank is rolled into a seamless steel pipe, and the geometric size is checked at least once for each batch.

[0029] Further, the yield strength ratio of the seamless steel pipe after heat treatment is less than 0.8.

[0030] Further, the grain size of the seamless steel pipe after heat treatment reaches grade 8, and the structure is ferrite plus pearlite plus a small amount of bainite.

[0031] Compared with the prior art, the beneficial technical effects of the present application are:

[0032] The present application adopts 16MnMoTi steel, the material component is reasonably designed, and the cost is low, and the performance of the steel pipe prepared by cooperating with the optimized process meets the fire-resistant use requirements. DETAILED DESCRIPTION

[0033] The present application is further described in detail below in combination with examples.

[0034] A production method of a Q345 steel grade seamless steel pipe for fire-resistant structure, specifically comprising:

[0035] 1. Steel smelting

[0036] 1.1 Actual smelting finished product components

[0037] The components of the trial-manufactured pipe blank are strictly controlled within the required range, and the specific data are shown in Table 2.

[0038] Table 2 Product composition (%)

[0039]

[0040] 1.2 Sulfur print inspection results

[0041] The results of the sulfur print inspection of the cast billets are shown in Table 3, in which the center cracks and inclusions are rated as first class, the intermediate cracks and subsurface cracks are rated as zero class, and the quality of the cast billets is good.

[0042] Table 3 Results of sulfur print inspection of cast billets

[0043]

[0044] 1.3 Inclusion control

[0045] As can be seen from the data in Table 4, the inclusions are well controlled.

[0046] Table 4 Results of metallographic inspection

[0047]

[0048] 2. Seamless steel pipe specifications

[0049] The seamless steel pipe specifications are φ426 x 20 mm.

[0050] 3. Steel heat treatment

[0051] 3.1 Heat treatment schedule

[0052] The as-rolled steel pipe was normalized in the heat treatment furnace operating line, and the heat treatment schedule is shown in Table 5:

[0053] Table 5 Heat treatment schedule

[0054]

[0055] 3.2 Mechanical properties

[0056] After heat treatment, the samples were inspected, and the tensile test results are shown in Table 6. The mechanical properties after treatment meet the standard requirements, and the yield ratio is 0.67, which is less than the required 0.8.

[0057] Table 6 Tensile test results

[0058]

[0059] 3.3 Impact energy

[0060] The impact test results are shown in Table 7. As can be seen from the data in the table, the impact energy fully meets the standard requirements:

[0061] Table 7 Normalized impact test results

[0062]

[0063] 3.4 Grain size after heat treatment

[0064] Grain size reached 8 level after heat treatment, the structure is ferrite plus pearlite plus a small amount of bainite.

[0065] 4. High temperature tensile strength

[0066] High temperature tensile strength is greater than two-thirds of room temperature at 600℃, which meets the requirements.

[0067] Table 8 High temperature mechanical properties

[0068]

[0069] The above-described embodiments are only to describe the preferred modes of the present application, and not to limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A method for producing seamless steel pipes for refractory structures of Q345 grade, characterized in that, The production process can be summarized as follows: Hot metal pretreatment → Top and bottom blowing converter smelting → LF furnace refining → VD vacuum treatment → Round billet continuous casting → Tube billet heating → Piercing → Continuous tube rolling → Fixed-tension reduction → Heat treatment → Cooling bed → Sawing → Straightening; Wherein: 1) Pre-treat the blast furnace molten iron to reduce the weight percentage of sulfur content in the molten iron to below 0.050%; 2) The pretreated molten iron is added to the top and bottom blown converter, and 10% scrap steel is added by weight. The single slag process is used for smelting. The final slag basicity and the endpoint target are controlled: [C] ≥ 0.10% at tapping. Deoxidation and alloying are carried out during tapping. The final deoxidation adopts the aluminum deoxidation process. Slag blocking or slag removal must be carried out during tapping. After the alloy is added during tapping, quicklime blocks are added. 3) Refining in the LF furnace: Argon is blown normally throughout the refining process as required, and the heating rate is gradually increased from low to high charge. Slag formation, desulfurization, composition adjustment, and heating operations are carried out according to the composition and temperature of the converter steel. White slag formation is adopted. 4) After the LF furnace refining process is completed, maintain bottom soft argon blowing for more than 10 minutes; 5) Perform VD vacuum treatment: vacuum degree ≤ 0.10 kPa, deep vacuum time ≥ 10 minutes; 6) Feed in a fixed length of silicon-calcium wire, and blow argon for more than 15 minutes after feeding; continuously cast the molten steel after VD vacuum treatment into round billets, using low casting speed, constant casting speed control and electromagnetic stirring process; control the superheat of the molten steel; after straightening, cut the billet into round tube billets; The round tube blanks that pass inspection and have a sulfur mark of no more than 2.0 are then processed into tubes. The tube processing procedure is as follows: Continuously check the temperature of each section of the preheating section, heating section, and soaking section. The temperature control of each section of the ring furnace is as follows: Rolling mill: Φ460mm mill; Preheating section I 800-1000℃; Preheating section II 800-1150℃; Heating section I 1220-1290℃; Heating section II 1240-1290℃; Soaking section I 1260-1290℃; Soaking section II 1260-1290℃; The rolled seamless steel pipe is heat-treated to obtain a qualified steel pipe whose performance meets the standard requirements; wherein the heat treatment is normalizing treatment: normalizing temperature 910±10℃, normalizing holding time 70min, and cooling method is air cooling. The chemical composition of the steel pipe by weight percentage is as follows: C: 0.14-0.17%; Si: 0.20-0.30%; Mn: 1.30-1.40%; P: ≤0.020%; S: ≤0.010%; Al: 0.015-0.040%; Ti: 0.008-0.020%; Mo: 0.20-0.30%, with the remainder being iron and unavoidable impurities.

2. The method for producing seamless steel pipes for Q345 grade fire-resistant structures according to claim 1, characterized in that, The computer control system records the heating temperature of each section online, and is required to control the temperature of the preheating section and the soaking section to ensure thorough and uniform heating without overheating.

3. The method for producing seamless steel pipes for Q345 grade fire-resistant structures according to claim 1, characterized in that, Hot tools must be measured before use, and the roller table must be inspected and treated before rolling to avoid scratches.

4. The method for producing seamless steel pipes for Q345 grade fire-resistant structures according to claim 1, characterized in that, The heated billet is rolled into seamless steel pipe, and the geometric dimensions are checked at least once per batch using hot sampling.

5. The method for producing seamless steel pipes for Q345 grade fire-resistant structures according to claim 1, characterized in that, The yield strength ratio of the heat-treated seamless steel pipe is less than 0.8.

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