[0015] The Q345B steel plate of the low-alloy series structural steel Q345B with a thickness of less than 50mm with a thickness below 50mm contains the following mass percentages (unit, wt%):
[0016] C: ≤0.18, Si: ≤0.45, Mn: ≤1.40, P: ≤0.018, S: ≤0.005, Ti: ≤0.020, Als: ≤0.015, the others are Fe and residual elements.
[0017] Carbon equivalent [Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15]: ≤0.42.
[0018] The production method adopted by the present invention includes: hot metal pretreatment, converter smelting, LF refining, continuous steel casting, hot billet delivery and rolling mill, casting billet heating, rolling, controlled cooling, stacking slow cooling; in the refining step, molten steel Refining adopts large amount of slag for slagging, and the holding time of white slag is controlled above 10 minutes; during the refining process, Ti is used for microalloying; after the continuous casting and pouring step, the cast slab is hot sent to the rolling mill for rolling.
[0019] 1. Process route
[0020] The invention adopts a continuous casting section of 250mm×1600mm for production. The main process route is: high-quality molten iron → KR molten iron pretreatment → top and bottom combined blowing converter → argon blowing treatment → LF furnace refining → continuous casting pouring → casting billet hot delivery to the rolling mill →Walking heating furnace→3800mm rolling mill→ACC laminar flow cooling→11-roll hot straightening machine→steel stack cooling→finishing→testing→warehousing.
[0021] 2. Design of smelting chemical composition
[0022] Composition design: In order to ensure good welding performance and good plastic toughness, the carbon equivalent Ceq should be controlled within 0.43%. Carbon and manganese are mainly used to increase the strength of the steel plate. The Ti element is controlled between 0.005-0.020%, mainly for Refine the grain to improve the plasticity and toughness of the steel plate.
[0023] Three, production process control points
[0024] 1. KR hot metal pretreatment process: the molten iron at the station must be desulfurized to ensure that the molten iron S ≤ 0.005% after desulfurization, and the temperature is greater than 1280 ℃.
[0025] 2. Converter smelting process: the Si content of the molten iron entering the furnace is below 0.60%, the temperature of the molten iron is ≥1270℃, the scrap steel strictly adopts high-quality leftover materials, the process gun position is controlled between 1.0 and 1.6m, and the lime is added strictly according to the lime addition model to ensure The basicity of slag is between 3.2 and 3.5, the end point C content is controlled above 0.06%, and the P content is controlled below 0.015%. During the tapping process, silicon-aluminum-barium-calcium is used for pre-deoxidation. In the early stage of tapping, a slag stopper is used to stop the slag, and a slag stopper is used to stop the slag in the later stage. After the molten steel arrives at the argon station, add the aluminum wire at one time to ensure that the Als content after argon is controlled between 0.040-0.060%, and leave the station after 3 minutes of forced blowing, and the temperature of the departure station is controlled to be above 1580℃.
[0026] 3. LF refining process: molten steel refining adopts large amount of slag for slagging, and the holding time of white slag is controlled above 10 minutes; the deoxidizer is mainly calcium carbide, aluminum particles, ferrosilicon powder, aluminum wire, etc.; ensure soft blowing time after refining ≥20min, to prevent the phenomenon of violent blowing; control the temperature of the upper steel according to the actual situation to ensure that the superheat of the molten steel in the tundish is controlled within 15±5℃.
[0027] 4. Continuous casting pouring: Before pouring steel, ensure that the casting machine equipment is in good condition, the superheat of the tundish is 15±5℃, the drawing speed: 0.7m/min, the specific water volume: 0.80L/㎏, the continuous casting steel requires full protection casting, The protective tube must be sleeved within 1 min after the large ladle is poured, and the molten steel must not see red during the pouring of the tundish. In the process of pouring steel, the amount of argon blowing from the stopper rod is reasonably controlled to ensure slight fluctuation of the mold liquid level.
[0028] 5. Heating process point: casting billet heating Group III steel heating process is carried out. Considering the actual temperature of the heating furnace and the casting billet there is a certain temperature difference, the highest heating temperature is 1240℃.
[0029] 6. Rolling process: Strictly adhere to the rolling requirements of "high temperature, low speed and large reduction", the opening temperature of 1050℃~1150℃; adopt TMCP rolling, the thickness of cold steel is 80~100mm, in order to ensure deformation and penetration, The rolling linear speed of the work roll is controlled by ≤1.5m/s, and the final rolling temperature of the first stage is 950℃~1000℃. The start-rolling temperature of the second stage is below 920℃, and the second stage adopts low-reduction rolling to ensure the original shape, and the final rolling temperature is ≤860℃.
[0030] 7. Controlled cooling process: the water inlet temperature>760℃, the redness temperature control is 550~600℃, the cooling speed is controlled at 10~15℃/s, and then sent to the strong straightening machine for hot straightening.
[0031] 8. Test results
[0032] 8.1 Testing of mechanical properties of steel plates
[0033] The composition and mechanical properties are implemented in accordance with GB/T1591-2008, see Table 1 for details.
[0034] Table 1 Mechanical properties of Q345B steel plate
[0035]
[0036] A total of 135 batches of Q345B with a thickness of 20-50mm were produced in this trial, of which 50 batches were rolled with specifications of 20-40 (including 40) mm, and the yield strength was controlled at 350-445 MPa, reaching an average of 380 MPa, which is better than the standard Wealthy 45MPa; The tensile strength is controlled at 490~590 MPa, with an average of 545MPa, which is 75 MPa richer than the standard; the elongation is controlled at 26%-35%, with an average of 29%, which is 9% richer than the standard; 20℃ V type The impact energy is controlled at 182~290 J, with an average of 242J. Among them, a total of 85 batches of rolling with a specification of 40-50 mm, the yield strength is controlled at 340-435 MPa, with an average of 380 MPa, which is 55 MPa richer than the standard; the tensile strength is controlled at 495-585 MPa, with an average of 535 MPa, It is 65 MPa richer than the standard; elongation is controlled at 26%-31%, with an average of 28%, and 9% richer than the standard; the 20℃ V-type impact energy is controlled at 164~259 J, with an average of 223J. The steel plate inspection performance indicators in the two specification intervals fully meet the requirements of Q345B.
[0037] 8.2 External inspection and flaw detection
[0038] The surface quality requirements are strictly implemented in accordance with GB/T709-2008;
[0039] Flaw detection implements GB/T2970-2004 Ⅲ level flaw detection standard;
[0040] The external inspection of the steel plate developed has a genuine quality rate of 100%; a flaw detection rate of 100%, of which the first-level rate is 89% and the third-level pass rate is 11%, which has achieved the expected results.
