Low-compression-ratio, low-alloy and heat-treatment-free EH40 extra-thick plate and manufacturing method thereof

Abstract

The invention discloses a low-compression-ratio, low-alloy and heat-treatment-free EH40 extra-thick plate and a manufacturing method thereof, and belongs to the field of extra-thick plate production. The extra-thick plate comprises the following components in percentage by mass: 0.04 to 0.06 percent of C, 0.2 to 0.3 percent of Si, 1.5 to 1.7 percent of Mn, less than or equal to 0.012 percent of P, less than or equal to 0.002 percent of S, 0.2 to 0.3 percent of Cr, 0.05 to 0.07 percent of Nb, 0.01 to 0.02 percent of Ti, 0.02 to 0.04 percent of Al and the balance of Fe and impurity elements. The proportion of the components meets the matching relation that C + P + S is smaller than 0.07, C + Mn / 6 + Cr / 5 is larger than or equal to 0.35 and smaller than or equal to 0.38, and C + Nb is larger than or equal to 0.1 and smaller than or equal to 0.12. Under the conditions that the thickness of a plate blank is only 230 mm and the compression ratio is only 2.5-3.8, through reasonable component matching and on the premise that impact toughness improving elements such as Cu and Ni are not added, the super-thick EH40 steel plate with the thickness of 60-90 mm meeting the impact requirement of the core part at the temperature of-40 DEG C can be produced without a heat treatment process, and the steel plate has the advantages of being low in production cost and short in manufacturing process, has lower requirements for a continuous casting machine, and is suitable for large-scale production. The method can be widely applied to domestic and overseas medium-thickness plate manufacturers.

Description

Technical Field

[0001] The invention relates to the production of extra-thick plates, in particular to an EH40 extra-thick plate with low compression ratio, low alloy and no heat treatment and a manufacturing method thereof. Background Art

[0002] With the upgrading of national steel grades, the use of steel plates with a large thickness of more than 60mm and the requirements of -40℃ or even lower temperature impact resistance has gradually increased. EH40 is a weldable fine-grained structural steel, which is widely used in wind turbine tower structures in extremely cold service environments. The thick EH40 steel plate is affected by the segregation of harmful elements in the continuous casting billet and the relatively small effective compression ratio. Its strength-toughness matching performance, especially the impact toughness, is often difficult to achieve. Steel mills have to use large-thickness continuous casting billets or even die-cast billets, and obtain the required strength-toughness matching performance by adding impact toughness-improving elements such as Cu and Ni, and adding heat treatment processes to the steel plates after rolling. There are precedents in China for producing -40℃ low-temperature impact steel with EH40 strength grade of more than 80mm, but they are all produced with continuous casting billets or die-cast billets of more than 300mm, and usually require the addition of impact toughness-improving elements such as Cu and Ni or the refinement of grains through heat treatment and other means to achieve the purpose of improving the toughness of the steel plate. For example, patent CN201010159178.2 discloses a 100mm thick Q390E grade extra thick steel plate and its manufacturing method, which requires the addition of 0.1-0.4 Ni; patent CN201510760947.7 discloses a method for producing high toughness extra thick plates from continuous casting billets, which requires the use of normalizing process to improve toughness. At present, there are no reports of using 230mm continuous casting billets to produce EH40 or steel with the same strength and toughness requirements without adding Cu, Ni and other elements and without heat treatment. Summary of the invention

[0003] Purpose of the invention: The purpose of the present invention is to provide a low compression ratio, low alloy, heat treatment-free EH40 extra thick plate; another purpose of the present invention is to provide a method for manufacturing a low compression ratio, low alloy, heat treatment-free EH40 extra thick plate.

[0004] Technical solution: A low compression ratio, low alloy, heat treatment-free EH40 extra-thick plate, characterized in that it includes the following components in mass percentage: C: 0.04-0.06, Si: 0.2-0.3, Mn: 1.5-1.7, P≤0.012, S≤0.002, Cr: 0.2-0.3, Nb: 0.05-0.07, Ti: 0.01-0.02, Al: 0.02-0.04, and the rest are Fe and impurity elements; and the above component ratios meet the ratio relationship of C+P+S<0.07, 0.35≤C+Mn / 6+Cr / 5≤0.38 and 0.1≤C+Nb≤0.12.

[0005] A method for manufacturing a low compression ratio, low alloy, heat treatment-free EH40 extra-thick plate, characterized by comprising the following steps:

[0006] S1. Composition design: According to the above chemical composition and corresponding ratio, low C, low harmful elements P and S are used to avoid element segregation and the formation of pearlite structure in room temperature structure to deteriorate impact toughness; appropriate CEQ design to avoid insufficient strength; trace strengthening elements Nb and Ti are used to refine grains and improve strength;

[0007] S2. Steelmaking and continuous casting: Use converter smelting, reasonable slag making and slag removal to control low P during smelting, use LF refining to remove S deeply to ensure low P and S composition; then use low superheat casting, and use light pressure to break dendrite bridge at the end of solidification to avoid segregation of C, Mn and harmful elements P, S, etc., which will deteriorate the impact toughness of the steel plate;

[0008] S3. Slab heating: The slab heating temperature is controlled between 1130℃ and 1150℃, and then the slab is kept in the furnace for 220min to 260min. During the heating process, Nb is dissolved into austenite as much as possible, but the grain size of the slab after heating should be avoided to be too large.

[0009] S4. Controlled rolling: The steel plate rolling adopts two-stage controlled rolling. In the first stage, 3 to 5 passes are rolled in the austenite recrystallization zone above 1030℃. After each rolling pass, intermediate cooling equipment is used to cool 1 to 3 passes to ensure that the surface temperature is lower than the core temperature to achieve differential temperature rolling; the last 3 passes are subjected to large reduction (more than 13%, including more than 18% in the last pass). When the slab is thin, large reduction is achieved to ensure that the reduction penetrates into the core and is rolled into an intermediate slab with a thickness of 1.4 to 2.0 times that of the finished product. The second stage is controlled rolling in the critical temperature zone of austenite to ferrite transformation, with the starting rolling temperature of 760℃~740℃ and the final rolling of about 720℃, with a reduction of 6%~13% in each pass. During the rolling process, some deformed austenite grains are induced to precipitate fine ferrite grains embedded in the austenite grains through deformation induction. The austenite grains undergo phase transformation and transform into granular bainite dispersed around the ferrite during the subsequent accelerated cooling process of the MULPIC equipment.

[0010] S5. Controlled cooling: Use MULPIC accelerated cooling, with a cooling rate between 8℃ / s and 12℃ / s, and a final cooling temperature between 400℃ and 500℃, avoiding too low or too high temperatures, so that the steel plate after rolling can produce a self-tempering effect. The final structure of the steel plate is a mixed structure of ferrite and granular bainite.

[0011] Preferably, in step S1, the designed composition includes, by mass percentage, C: 0.05, Si: 0.25, Mn: 1.65, P≤0.01, S≤0.001, Cr: 0.25, Nb: 0.06, Ti: 0.015, Al: 0.03, and the rest is Fe and impurity elements.

[0012] Preferably, in step S3, during the heating process, the core temperature of the slab is first heated to about 1150°C in the third heating section of the heating furnace, and then cooled to about 1100°C in the equalizing section. The purpose is to make the core temperature of the slab higher than the surface temperature, laying a good foundation for subsequent differential temperature rolling and ensuring that the pressure penetrates into the core.

[0013] The above special restrictions on the chemical composition of steel are due to:

[0014] Carbon is one of the main strengthening elements in extra thick plates. If the carbon addition is too low, more alloying elements need to be added, which is not conducive to reducing the cost of the steel plate. However, carbon is also an unfavorable element for impact toughness. When the carbon content is above 0.06%, the impact toughness at -40°C deteriorates under TMCP process conditions, so the carbon content is limited to 0.04~0.06%.

[0015] Chromium is also one of the main strengthening elements in extra thick plates, and can also improve the hardenability of steel plates. However, too much chromium is not good for impact toughness, so the chromium content is limited to 0.2-0.3%.

[0016] Manganese is a weak deoxidizer in the smelting process. At the same time, manganese can effectively improve the strength of steel. However, when the manganese content is too high, it is easy to form a serious segregation zone in the extra-thick plate, reducing the impact toughness of the steel. It combines with sulfur to form manganese sulfide, which easily forms hydrogen traps and causes the internal quality of the steel plate to deteriorate. At the same time, too high manganese is not conducive to the welding performance of the steel plate. Therefore, the manganese content is limited to 1.5-1.7%. At the same time, taking into account the strength requirements of the steel plate, the limit is 0.35≤C+Mn / 6+Cr / 5≤0.38.

[0017] Phosphorus and sulfur are harmful elements, especially for large thickness and -40℃ impact toughness. It is difficult to fully refine the grains through rolling and cooling, and they are strong segregation elements, which can easily cause the impact toughness of the steel plate to be unacceptable. Therefore, they are limited to less than 0.012% and 0.002% respectively, and must meet the requirement of C+P+S<0.07.

[0018] Both niobium and titanium can hinder the growth of austenite grains through pinning during heating, thus refining the grains. At the same time, they hinder the recrystallization of austenite through solute drag during rolling, and hinder the growth of austenite grains through pinning, so that the steel plate can accumulate deformation at a higher temperature during rolling. In addition, in the final structure of the steel plate, the yield strength of the steel plate can be significantly improved through the precipitation strengthening effect of niobium and titanium. Excessive niobium and titanium are not conducive to alloy cost and slab corner crack control, nor are they conducive to the impact toughness of the steel plate. Therefore, niobium and titanium are limited to 0.05-0.07% and 0.01-0.02% respectively, while taking into account the requirements of strength and toughness, and the ratio relationship of 0.1≤C+Nb≤0.12 must be met.

[0019] Beneficial effect: With a slab thickness of only 230 mm and a compression ratio of only 2.5 to 3.8, the present invention can produce 60 to 90 mm extra-thick EH40 steel plates that meet the core -40°C impact requirement through reasonable component matching without adding impact toughness improving elements such as Cu and Ni, and without going through a heat treatment process. The invention has the advantages of low production cost and short manufacturing process, and has lower requirements for continuous casting machines, and can be widely used in medium and thick plate manufacturers at home and abroad. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 1 is a metallographic photograph of a finished steel plate made according to an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] Example

[0023] This embodiment provides a method for manufacturing low compression ratio, low alloy, heat treatment-free EH40 extra thick plate, the manufacturing process includes smelting, continuous casting, heating, rolling, cooling, stack slow cooling, flaw detection, fire cutting to product, etc., and the following control points are controlled: smelting composition design, slab heating, steel plate controlled rolling, steel plate controlled cooling after rolling. The specific control is as follows:

[0024] (1) Melting composition design (wt%): C: 0.05, Si: 0.25, Mn: 1.65, P≤0.01, S≤0.001, Cr: 0.25, Nb: 0.06, Ti: 0.015, Al: 0.03, the rest are Fe and impurity elements. During molten steel smelting, the molten iron is pre-treated by KR to remove sulfur, and then smelted in a converter to remove phosphorus. It is then sent to the LF furnace for further refining to remove sulfur to less than 0.001%, and then sent to the RH refining for degassing.

[0025] (2) Steelmaking and continuous casting: Steelmaking adopts converter smelting. During the smelting process, reasonable slag formation and slag removal are controlled to ensure that P ≤ 0.12%. LF refining is used to deeply remove S to below 0.001%. The continuous casting overheat is controlled at about 20°C. At the end of solidification, light pressure is used to break up dendrites for bridging to avoid segregation of C, Mn and harmful elements P, S, etc., which deteriorates the impact toughness of the steel plate.

[0026] (3) Slab heating: Considering the solid solution temperature of Nb and the requirement to avoid excessive coarsening of austenite grains after heating, the furnace temperature of the slab is controlled at about 1140°C and the furnace time is 240 minutes. The slab is heated at 1160°C in the third heating stage and cooled to about 1100°C in the soaking stage, so that the surface temperature of the slab is lower than the center temperature, producing a differential heating effect.

[0027] (4) Controlled rolling: The thickness of the finished product is 80 mm, and the process is controlled in two stages. In the first stage, four passes of rough rolling with large reduction are performed in the austenite recrystallization zone. The reduction rates of the first three passes are 13%, 14%, and 14%, respectively, and the reduction rate of the last pass is 19%. After each rolling pass, the product is cooled three times in the intermediate cooling device to achieve differential temperature rolling. The waiting thickness is 120 mm. In the second stage, the target temperature for the start rolling is 750°C, the final rolling is 720°C, and the cumulative reduction rate is 50%.

[0028] (5) Controlled cooling: MULPIC accelerated cooling is used, the cooling target rate is 10℃ / s, and the final cooling target temperature is 450℃.

[0029] (6) Slow cooling in stacking: After the steel plate is rolled, it is taken off the production line to the stacking position for slow cooling within 30 minutes. The stacking temperature is above 350°C. The final steel plate structure is a mixed structure of ferrite and granular bainite, such as Figure 1 shown.

[0030] (7) NDT: After slow cooling, the steel plate is tested offline.

[0031] (8) Flame cutting: After flame cutting, the finished products are put into the finished product warehouse.

[0032] The heating, rolling and cooling processes of the steel plate of this embodiment are shown in Table 1, the performance of the embodiment is shown in Table 2, and the structure of the steel plate of the embodiment is shown in Table 2. Figure 1 shown.

[0033] Table 1 Example Heating, rolling and cooling process

[0034]

[0035] Table 2 Performance of the embodiments

[0036]

[0037]

[0038] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A low compression ratio, low alloy, heat treatment-free EH40 extra thick plate, characterized in that: The invention comprises the following components in percentage by mass: C: 0.04-0.06, Si: 0.2-0.3, Mn: 1.5-1.7, P≤0.012, S≤0.002, Cr: 0.2-0.3, Nb: 0.05-0.07, Ti: 0.01-0.02, Al: 0.02-0.04, and the rest are Fe and impurity elements; and the above component ratios meet the ratio relationship of C+P+S<0.07, 0.35≤C+Mn / 6+Cr / 5≤0.38 and 0.1≤C+Nb≤0.

12.

2. A method for manufacturing a low compression ratio, low alloy, heat treatment-free EH40 extra-thick plate, characterized in that: The following steps are involved: S1. Composition design: Based on the above chemical composition and corresponding ratio, the design adopts low C, low harmful elements P and S, and trace strengthening elements Nb and Ti; S2, Steelmaking and Continuous Casting: Use converter smelting, reasonable slag making and slag removal to control low P during smelting, use LF refining to remove S deeply; then use low superheat casting, and use light pressure to break dendrite bridge at the end of solidification; 、 S3, slab heating: the slab heating temperature is controlled between 1130℃~1150℃, and then the slab is kept in the furnace for 220min~260min; S4, controlled rolling: The steel plate rolling adopts two-stage controlled rolling. In the first stage, 3 to 5 passes are rolled in the austenite recrystallization zone above 1030℃. After each rolling pass, the intermediate cooling equipment is used for cooling for 1 to 3 passes to ensure that the surface temperature is lower than the core temperature to achieve differential temperature rolling; the last 3 passes are large reduction. When the slab is thin, large reduction is achieved to ensure that the reduction penetrates into the core and is rolled into an intermediate slab with a thickness of 1.4 to 2.0 times that of the finished product; the second stage is controlled rolling in the critical temperature zone of the transformation from austenite to ferrite, with a starting rolling temperature of 760℃ to 740℃, a final rolling of 720℃, and a reduction of 6% to 13% per pass; S5. Controlled cooling: Use MULPIC accelerated cooling, the cooling rate is between 8℃ / s and 12℃ / s, and the final cooling temperature is between 400℃ and 500℃.

3. The method for manufacturing the low compression ratio, low alloy, heat treatment-free EH40 extra thick plate according to claim 2, characterized in that: In step S1, the designed composition includes, by mass percentage, C: 0.05, Si: 0.25, Mn: 1.65, P≤0.01, S≤0.001, Cr: 0.25, Nb: 0.06, Ti: 0.015, Al: 0.03, and the rest are Fe and impurity elements.

4. The method for manufacturing the low compression ratio, low alloy, heat treatment-free EH40 extra thick plate according to claim 2, characterized in that: In step S3, during the heating process, the core temperature of the slab is first heated to about 1150°C in the third heating section of the heating furnace, and then cooled to about 1100°C in the soaking section.

Images