A method for producing Q550D steel plate with low delayed crack sensitivity by online quenching

Abstract

In view of the problem that "the cooling intensity of the traditional online quenching process is relatively large, the plate shape changes dramatically, the head and tail ends are highly sensitive to cracks due to steel defects, and the crack generation and expansion have a certain delay, which is easy to miss detection and cause quality disputes", the present invention provides a method for producing Q550D steel plates with low delayed crack sensitivity by online quenching. The present invention makes full use of thickness grading, adopts low Mn, trace Nb, and composite addition of Cr, Mo, and V components. The delayed crack sensitivity of Q550D online quenching is reduced in the whole process of smelting, rolling, cooling, cutting and heat treatment, which can effectively reduce stress concentration at the head and tail ends, and reduce and delay the probability of cracking. The present invention adopts corresponding control measures to realize the batch production of Q550D steel plates with low delayed crack sensitivity and online quenching with a maximum thickness of 55mm under hot charging process, and adopts hot charging and online quenching and rolling processes at the same time, which has the advantages of low energy consumption and short production cycle.

Description

Technical Field

[0001] The invention relates to the technical field of wide and thick plate surface quality control, and in particular to a method for producing a low delayed crack sensitivity online quenching Q550D steel plate. Background Art

[0002] Quenched and tempered steel plate is a kind of steel plate that obtains the required high strength and high toughness through quenching and tempering heat treatment. Q550D is a widely used quenched and tempered steel plate, mainly used in mining, machinery and other engineering machinery fields. At present, the quenching and tempering process is mainly divided into two processes: offline quenching + tempering and online quenching + tempering. The online quenching process can greatly shorten the production cycle and reduce production costs because it omits the offline reheating process. However, due to the high cooling intensity, the plate shape changes drastically, and the head and tail ends are more sensitive to cracks due to steel defects (see Figure 1 ), and the crack generation and expansion has a certain delay, which can easily lead to missed detection and quality disputes.

[0003] At present, there are few control technologies related to delayed cracks in online quenching processes. CN104988299B discloses a control method for avoiding delayed cracks in cutting of thick medium and high carbon steel in winter. This patent aims to solve the problem of martensitic brittle structure generated by excessively fast cooling of the cut surface in winter during the normalizing process of medium and high carbon steel, and takes corresponding measures on the composition of high carbon steel plates and the slow cooling process of cutting. This technology is applied to steels with higher carbon content and is not suitable for the prevention and control of delayed cracks in low carbon steel. It only takes preventive measures for the cutting and slow cooling processes, and does not control key processes such as steelmaking and rolling, which has certain limitations.

[0004] CN115433814A discloses a low-cost Q550D steel plate production method, which requires the addition of a large amount of Mn, Nb, and V, has high alloy cost and high crack sensitivity, and can only produce steel plates with a thickness of 14 to 22 mm.

[0005] CN113604737B discloses a Q550D high-strength steel plate and its preparation method. The preparation method adopts VN microalloying strengthening mechanism. Since RH vacuum degassing is not required, a large amount of nitrides are precipitated at the grain boundaries, which can easily cause more serious cracks, and the low-temperature impact toughness is unstable. In terms of cost, compared with the hot-loading online quenching process, the overall difference is small, and only the thick specifications above 50mm have advantages. Summary of the invention

[0006] In view of the above problems, the present invention provides a method for producing Q550D steel plate with low delayed crack sensitivity by online quenching. The method reduces the delayed crack sensitivity of Q550D steel plate by online quenching from the whole process of key processes such as chemical composition design, steelmaking, rolling, heat treatment and cutting, and can effectively reduce stress concentration at the head and tail ends, and reduce and delay the probability of cracking.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is as follows: a method for producing a low delayed crack sensitivity online quenching Q550D steel plate, characterized in that, in terms of weight percentage, the chemical composition of the Q550D steel plate is: C: 0.14-0.18%, Si: 0.20-0.50%, Mn: 1.0-1.4%, S≤0.01%, P≤0.02%, Nb≤0.02%, Cr≤0.4%, Ti≤0.03%, V≤0.04%, Mo≤0.4%, Als≥0.020%, and the rest is Fe and trace impurities.

[0008] The thickness specification of the above-mentioned steel plate is: 20~55mm, and the width specification is: 1500~3000mm.

[0009] Preferably, the chemical composition of the steel plate with a thickness of 20 to 30 mm is, in weight percentage, as follows: C: 0.14 to 0.18%, Si: 0.20 to 0.50%, Mn: 1.0 to 1.3%, S≤0.01%, P≤0.02%, Nb≤0.02%, Cr≤0.3%, Ti≤0.02%, Als≥0.020%, and the rest is Fe and trace impurities.

[0010] Preferably, the chemical composition of the steel plate with a thickness of 30 to 55 mm is, in weight percentage, as follows: C: 0.15 to 0.18%, Si: 0.20 to 0.50%, Mn: 1.1 to 1.4%, S≤0.01%, P≤0.02%, Nb≤0.02%, Cr≤0.3%, Ti≤0.02%, V≤0.04%, Mo≤0.4%, Als≥0.020%, and the rest is Fe and trace impurities.

[0011] The chemical composition design principles are as follows:

[0012] Carbon (C): It can significantly improve the strength of steel plates, but as the content increases, the plasticity and low-temperature impact toughness will decrease significantly, so its content should be reasonably controlled.

[0013] Niobium (Nb): It can significantly increase the recrystallization temperature of steel, expand the rolling range of the non-recrystallization zone, form a large number of dispersed carbonitrides with C or N elements in the steel, pin it to the original austenite grain boundary, fully refine the grains, and enhance the crack arrest ability.

[0014] Vanadium (V) and titanium (Ti): give full play to the precipitation strengthening effect, refine the grains and improve the strength of the steel plate.

[0015] Manganese (Mn), chromium (Cr), and molybdenum (Mo): play a role in solid solution strengthening, improving the hardenability and strength of steel plates.

[0016] The present invention adds a trace amount of Nb element to fully refine the grains and enhance the crack arresting ability. At the same time, under the premise of ensuring the strength, the thickness grading is used to minimize the addition of Mn, Cr, Mo, V and Ti, thereby reducing the crack sensitivity and the alloy cost.

[0017] The present invention also provides a production method of Q550D steel plate, comprising: 1) smelting and continuous casting; 2) heating; 3) rolling; 4) water cooling; 5) heat treatment; 6) cutting, wherein:

[0018] The heating in step 2) is as follows: the steel billet is heated to the tapping temperature in a heating furnace by hot charging or cold charging; preferably, hot charging is adopted, the surface temperature of the slab when charging the furnace is ≤550°C, the tapping temperature is 1150-1220°C, the heating rate is 8-10 min / cm, and the soaking period is not less than 40 min, so as to ensure that the steel plate is evenly and thoroughly burned, and the carbonitrides and alloy elements are fully dissolved back and austenitized.

[0019] The step 3) rolling is as follows: a 4300mm rolling mill is used for rough rolling and finishing rolling with a double stand, the rough rolling is performed to form an intermediate billet, and the intermediate billet is finished by finishing rolling; the reduction rate of the last pass in the rough rolling stage is not less than 20%, the core rolling pressure is increased, and the rolling capacity of various defects such as looseness and cracks in the core of the casting is improved; the thickness of the intermediate billet is 2 to 4 times the thickness of the finished product; the start rolling temperature in the finishing rolling stage is 900 to 1000° C., the number of loaded rolling passes is not more than 8, and the head and tail are kept slightly buckled by adjusting the sled SKI value, so as to make the head and tail ends of the steel plate in a compressive stress state, and avoid overcooling and warping, excessive residual stress, and increased crack sensitivity in the subsequent water cooling process.

[0020] The step 4) water cooling is as follows: after rolling, water cooling is performed using a MULPIC water cooling device, the water cooling start temperature is 820-850°C, the final cooling temperature is 100-150°C, the cooling rate is 15-35°C / s, and in order to prevent the head and tail of the steel plate from being overcooled, head and tail shielding is required, the shielding distance is set to 0.5-1.5m, the upper and lower water ratio is 0.8-1, the water-cooled plate shape is kept straight as a whole, the head and tail are slightly buckled, so that the steel plate is in a compressive stress state after cooling.

[0021] The step 5) heat treatment is as follows: the steel plate is subjected to online quenching, fire cutting into sections, and tempering after shot blasting; wherein the head and tail of the steel plate are not cut off before entering the heat treatment furnace, and the steel plate is tempered in the furnace within 48 hours after online quenching, with a tempering temperature of 580-650°C and a holding time of 10-20 minutes. Not cutting off the head and tail before heat treatment puts the head and tail of the steel plate in a compressive stress state to prevent the steel plate from heating up too quickly during the heat treatment of the cross section after cutting, resulting in large thermal stress and cracking at the defect. Cutting after tempering is completed and the stress is fully released can greatly reduce the residual stress at the end of the steel plate, and the probability of cracks is significantly reduced.

[0022] The step 6) cutting is as follows: after tempering, the steel plate is sampled and cut to size, wherein mechanical cutting is adopted for 30 mm and below, and flame cutting is adopted for 30-55 mm thickness. When flame cutting is adopted for thick specifications, if the temperature is too low, preheating is performed before cutting to reduce the cutting thermal stress, and the cutting temperature is ensured to be ≥150°C. After cutting in winter, asbestos or other heat-insulating materials are used to insulate and slowly cool the cut section to prevent the core of the cut section from generating brittle structure and initiating cracks due to the segregation of some elements and excessive cooling rate.

[0023] The above step 1) smelting and continuous casting is as follows: the molten iron is desulfurized by KR and then smelted in a converter, refined by LF, and refined by RH; full-process protection casting is adopted to reasonably control parameters such as molten steel superheat and casting speed.

[0024] Furthermore, its LF refining time is ≥50min, RH refining vacuum degree is ≤133Pa, and pure degassing time is greater than 5min.

[0025] Furthermore, the superheat of the molten steel is ≤20°C, and the drawing speed is controlled according to the following requirements based on the different thickness of the billet: 200mm: 1.1-1.4m / min, 250mm: 1.1-1.3m / min, 300mm: 0.85m / min, and full-process protective pouring is adopted.

[0026] Compared with the prior art, the advantages of the present invention are:

[0027] 1. In terms of composition, it adopts low Mn and trace Nb composition design, makes full use of thickness grading, has small alloy addition, low cost, low crack sensitivity, and reduces the proportion of delayed cracking from 5% of the traditional method to less than 0.5%;

[0028] 2. In terms of technology, the hot charging and online quenching and rolling process is adopted, which has the characteristics of low energy consumption and short production cycle, and can save more than 50% of the production cycle. The delayed crack prevention effect of this method is significant. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the actual object after the traditional online quenching Q550D steel plate is cooled ( Figure 1 -a) and tissue morphology (1-b);

[0030] Figure 2 This is the microstructure of the tempered Q550D steel plate quenched online in Example 1 of the present invention;

[0031] Figure 3 This is the microstructure of the tempered Q550D steel plate quenched online in Example 2 of the present invention; DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with specific examples.

[0033] Example 1

[0034] According to the present invention, a 25 mm thick steel plate is produced and prepared. The chemical composition of the steel plate is, by weight percentage, C: 0.17%, Si: 0.35%, Mn: 1.1%, S: 0.005%, P: 0.015%, Nb: 0.012%, Cr: 0.25%, Ti: 0.018%, Als ≥ 0.020%, and the rest is Fe and trace impurities.

[0035] The production method of Q550D steel plate is:

[0036] 1) Smelting: After KR pretreatment and desulfurization, the molten iron is smelted in a 120-ton top and bottom combined blowing converter, refined in a 120-ton LF ladle furnace, the refining time is 51 minutes, and refined by vacuum degassing in a 120-ton RH, the vacuum degree is ≤133Pa, the pure degassing time is greater than 5 minutes, and the casting is fully protected, the superheat is 18°C, the pulling speed is 1.2m / min, and the cross-sectional size is 250mm (thickness) × 2200mm (width);

[0037] 2) Heating: The continuous casting billet is hot charged, the charging temperature is 440°C, and it is heated by a walking beam heating furnace. The steel tapping temperature is 1190°C, the heating rate is 9min / cm, and the soaking time is 43min.

[0038] 3) Rolling: The 4300mm rolling mill adopts double-stand rolling for rough rolling and finishing rolling, the rough rolling start temperature is 1160℃, the large reduction mode is adopted, 5 passes are rolled, the 5th pass reduction rate is 23%, the rough rolling final rolling temperature is 1100℃, the rolling intermediate billet thickness is 75mm, the finishing rolling start temperature is 980℃, 6 passes are rolled with load, and the head and tail are kept slightly buckled by adjusting the sled SKI value;

[0039] 4) Water cooling: After rolling, water cooling is carried out at a temperature of 850°C. The steel plate is leveled by a pre-straightening machine and enters the MULPIC water cooling. The MULPIC water cooling is shielded at the head and tail, with a shielding distance of 1m, an upper and lower water ratio of 0.9, and a cooling rate of 34°C / s. After water cooling, the whole plate is straight with a slight buckle at the head and tail.

[0040] 5) Heat treatment: The steel plate is quenched online, then fire cut into sections, the edges are cut off, and the head and tail are not cut off. Within 48 hours after online quenching, it is put into the furnace for tempering. Before tempering, the upper and lower surfaces of the steel plate are shot blasted. The tempering temperature is 598℃ and the temperature is kept for 19 minutes. The microstructure after tempering is as follows Figure 2 As shown;

[0041] 6) Cutting: After tempering, mechanical cutting is used.

[0042] Embodiment 2:

[0043] According to the present invention, a 50 mm thick steel plate is produced and prepared. The chemical composition of the wide and thick steel plate is, by weight percentage, C: 0.17%, Si: 0.35%, Mn: 1.25%, S: 0.008%, P: 0.013%, Nb: 0.018%, Cr: 0.2%, Ti: 0.016%, V: 0.02%, Mo: 0.1%, Als ≥ 0.020%, and the rest is iron and unavoidable impurities.

[0044] The production method of Q550D steel plate is:

[0045] 1) Smelting: After KR pretreatment and desulfurization, the molten iron is smelted in a 120-ton top and bottom combined blowing converter, refined in a 120-ton LF ladle furnace, with a refining time of 55 minutes, and vacuum degassing and refining in a 120-ton RH, with a vacuum degree of ≤133Pa and a pure degassing time of more than 5 minutes. The whole process is cast with protection, the superheat is 19°C, the pulling speed is 0.85m / min, and the cross-sectional size is 300mm (thickness) × 2400mm (width), and the steel is slowly cooled to room temperature after it is offline;

[0046] 2) Heating: The continuous casting billet is charged to a furnace at a temperature of 20-50°C and heated by a walking beam heating furnace. The tapping temperature is 1200°C, the heating rate is 9 min / cm, and the soaking time is 50 min.

[0047] 3) Rolling: The 4300mm rolling mill adopts double-stand rolling for rough rolling and finishing rolling, the rough rolling start temperature is 1170℃, the large reduction mode is adopted, 7 passes are rolled, the 7th pass reduction rate is 24%, the rough rolling final rolling temperature is 1105℃, the rolling intermediate billet thickness is 100mm, the finishing rolling start temperature is 960℃, 8 passes are rolled with load, and the head and tail are kept slightly buckled by adjusting the sled SKI value;

[0048] 4) Water cooling: After rolling, water cooling is carried out at a temperature of 850°C. The steel plate is leveled by a pre-straightening machine and enters the MULPIC water cooling. The MULPIC water cooling uses head and tail shielding, with a shielding distance of 0.8m, an upper and lower water ratio of 0.95, and a cooling rate of 14°C / s. After water cooling, the whole plate is straight with a slight buckle at the head and tail.

[0049] 5) Heat treatment: The steel plate is quenched online, then fire cut into sections, the edges are cut off, and the head and tail are not cut off. Within 48 hours after online quenching, it is put into the furnace for tempering. Before tempering, the upper and lower surfaces of the steel plate are shot blasted. The tempering temperature is 610℃ and the temperature is kept for 20 minutes. The microstructure after tempering is as follows Figure 3 As shown;

[0050] 6) Cutting: Use flame cutting, preheat the steel plate before cutting, the cutting temperature is ≥150℃, and use thermal insulation asbestos or other materials to insulate the section and slow down cooling after cutting in winter.

[0051] In this embodiment, step 2) uses a cold charging heating method, which can also be replaced by a hot charging heating method. If the continuous casting slab is heated by hot charging, then step 1) does not require slow cooling, and step 2) the surface temperature of the slab when charged into the furnace is ≤550°C. The hot charging method can shorten the production cycle and reduce energy consumption.

[0052] In summary, the traditional online quenching process has a high cooling intensity and a dramatic change in plate shape. In particular, the cooling at the head and tail ends of the steel plate is more uneven, stress is more likely to be concentrated, steel segregation and inclusions at the ends of the steel plate are more serious, and crack sensitivity is high. In the subsequent cutting process, the defective parts are prone to cracking under the action of stress (see Figure 1 ), and it takes a while for cracks to germinate and expand, which can easily lead to missed inspections and quality objections to the outflowing products. The method for producing low-delay crack sensitivity online quenching Q550D steel plates of the present invention makes full use of thickness grading, adopts low Mn, trace Nb, and composite addition of Cr, Mo, and V components, and subsequently takes corresponding control measures for the entire process of smelting, rolling, cooling, cutting, and heat treatment. It can realize batch production of online quenching low-delay crack sensitivity Q550D steel plates with a maximum thickness of 55mm under hot charging process. The production cycle of this method can be shortened by more than 50% compared with the cold charging and offline quenching and tempering process, the alloy cost is low, and the crack sensitivity is greatly reduced (see Figure 2 and Figure 3 ), the delayed crack generation rate was controlled from 5% to less than 0.5%.

Claims

1. A method for producing a low delayed crack sensitivity online quenching Q550D steel plate, comprising: 1) Chemical composition design, smelting and continuous casting; 2) Heating; 3) Rolling; 4) Water cooling; 5) Heat treatment; 6) Cutting, characterized by In terms of weight percentage, the chemical composition of the Q550D steel plate is: C: 0.14-0.18%, Si: 0.35-0.50%, Mn: 1.0-1.25%, S≤0.01%, P≤0.02%, Nb: 0.012-0.02%, Cr≤0.4%, Ti≤0.03%, V≤0.04%, Mo≤0.4%, Als≥0.020%, and the rest is Fe and trace impurities; The thickness specification of the Q550D steel plate is 20-55 mm, and the width specification is 1500-3000 mm; The heating in step 2) is as follows: the steel billet is hot charged, and the surface temperature of the steel billet is ≤550°C; the steel billet is heated in the heating furnace to the tapping temperature, the tapping temperature is 1150-1220°C, the heating rate is 8-10min / cm, and the soaking time is not less than 40min; The step 3) rolling comprises: using a rough rolling and a finishing rolling double-frame rolling in a rolling mill, rolling the rough rolling into an intermediate billet, and the intermediate billet into a finished product through finishing rolling; the reduction rate of the last pass in the rough rolling stage is not less than 20%, the thickness of the intermediate billet is 2 to 4 times the thickness of the finished product, the starting rolling temperature in the finishing rolling stage is 900 to 1000° C., the number of rolling passes with load is not more than 8, and the head and tail are kept slightly buckled by adjusting the sled SKI value; The step 4) water cooling is as follows: after rolling, the steel plate is leveled by a pre-straightening machine, and then water-cooled by a MULPIC water cooling device, with a water cooling start temperature of 820-850°C, a final cooling temperature of 100-150°C, and a cooling rate of 15-35°C / s. To prevent the head and tail of the steel plate from being overcooled, head and tail shielding is required, with a shielding distance of 0.5-1.5m and an upper and lower water ratio of 0.8-1, so as to keep the overall water-cooled plate straight and slightly buckled at the head and tail; The step 5) heat treatment comprises: the steel plate undergoes online quenching, fire cutting, segmentation, shot blasting and then tempering; The steel plate is not cut off before entering the heat treatment furnace, and is tempered in the furnace within 48 hours after online quenching. The tempering temperature is 580~650℃ and the holding time is 10~20min. The step 6) cutting comprises: after the steel plate is tempered, sampling is performed and the length is determined, wherein 30 mm and below are cut mechanically, and 30-55 mm thick are cut by flame; Through the above method, the delayed crack generation rate is reduced from 5% in the traditional method to less than 0.5%.

2. The production method according to claim 1, characterized in that: The Q550D steel plate has a thickness of 20 to 30 mm, and its chemical composition, in weight percentage, is: C: 0.14 to 0.18%, Si: 0.35 to 0.50%, Mn: 1.0 to 1.1%, S≤0.01%, P≤0.02%, Nb: 0.012 to 0.02%, Cr≤0.3%, Ti≤0.02%, Als≥0.020%, and the rest is Fe and trace impurities.

3. The production method according to claim 1, characterized in that: The Q550D steel plate has a thickness of 30 to 55 mm, and its chemical composition, by weight percentage, is: C: 0.15 to 0.18%, Si: 0.35 to 0.50%, Mn: 1.1 to 1.25%, S≤0.01%, P≤0.02%, Nb: 0.018 to 0.02%, Cr≤0.3%, Ti≤0.02%, V≤0.04%, Mo≤0.4%, Als≥0.020%, and the rest is Fe and trace impurities.

4. The production method according to claim 1, characterized in that: When flame cutting is adopted in step 6), the cutting temperature is ≥150° C. and the cut section is kept warm and slowly cooled after cutting in winter.

5. The production method according to any one of claims 1 to 4, characterized in that: The step 1) smelting and continuous casting comprises: the molten iron is desulfurized by KR and then smelted in a converter, refined by LF, and refined by RH; the whole process protection casting is adopted to reasonably control the superheat degree and casting speed of the molten steel.

6. The production method according to claim 5, characterized in that: The LF refining time is ≥50 min, the RH refining vacuum degree is ≤133 Pa, and the pure degassing time is greater than 5 min.

7. The production method according to claim 5, characterized in that: The superheat degree of the molten steel is ≤20°C, and the drawing speed is controlled according to the following requirements based on the different thickness of the billet: 200mm: 1.1-1.4m / min, 250mm: 1.1-1.3m / min, 300mm: 0.85m / min, and full-process protective casting is adopted.

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