A 785MPa grade ultra-low carbon bainitic steel and its preparation method

Abstract

The present invention relates to the field of metal materials technology, and in particular to a 785MPa-grade ultra-low carbon bainitic steel and a preparation method thereof. The 785MPa-grade ultra-low carbon bainitic steel comprises the following chemical compositions by weight: C: 0.01% to 0.03%, Si: 0.5% to 0.8%, Mn: 1.0% to 2.0%, Mo: 1.5% to 2.0%, Nb: 0.03% to 0.07%, Ni: 4.0% to 6.0%, Cr: 0.3% to 0.7%, Ti: 0.01% to 0.03%, Al: 0.01% to 0.03%, with the remainder being iron and unavoidable impurities. The steel prepared by the present invention has a yield strength of ≥785MPa, a weld joint impact energy of ≥50J at -40°C, a weld joint CTOD of ≥0.17mm at -20°C, and a yield strength ratio of 0.77-0.80. The problem of poor matching between strength and toughness and weldability and high yield strength ratio of existing high-strength hull steel is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and in particular to a 785MPa grade ultra-low carbon bainite steel and a preparation method thereof. Background Art

[0002] In recent years, ship structural steel has been gradually developing towards higher strength and easier welding. Typically, this is achieved by increasing the carbon content and adding large amounts of alloying elements such as Ni, Cr, Mo, and V to enhance the strength of the steel. However, the high carbon content and carbon equivalent make welding of the steel difficult. In actual welding, preheating and post-heating processes are required, significantly increasing the difficulty of welding. Furthermore, residual stress after welding can easily lead to cracks, posing a safety hazard.

[0003] In addition, due to the high strength level, high-strength hull structure steel usually adopts tempering heat treatment, and the structure is tempered martensite or tempered martensite + bainite. The yield strength ratio of the steel plate after tempering heat treatment is relatively high (≥0.93). The high yield strength ratio will reduce the service safety margin of the steel and increase the service risk.

[0004] Ultra-low carbon bainitic steel is a new class of high-strength, high-toughness, and versatile steels developed internationally over the past two decades. By significantly reducing carbon content, the effects of carbon on weldability are eliminated. Furthermore, due to the optimal distribution of soft and hard phases in bainite, bainitic steel has a low yield strength ratio. Therefore, developing ultra-low carbon bainitic steel with high strength, a low yield strength ratio, and ease of welding is a key area of ​​research in the shipbuilding steel industry. Summary of the Invention

[0005] In view of the above analysis, the embodiments of the present invention aim to provide a 785MPa grade ultra-low carbon bainitic steel and a preparation method thereof, so as to solve the problems of poor matching between strength and toughness and weldability and high yield strength ratio of existing high-strength hull structure steel.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] On the one hand, an embodiment of the present invention provides a 785MPa grade ultra-low carbon bainitic steel, whose chemical composition comprises, by weight: C: 0.01% to 0.03%, Si: 0.5% to 0.8%, Mn: 1.0% to 2.0%, Mo: 1.5% to 2.0%, Nb: 0.03% to 0.07%, Ni: 4.0% to 6.0%, Cr: 0.3% to 0.7%, Ti: 0.01% to 0.03%, Al: 0.01% to 0.03%, and the rest is iron and unavoidable impurities.

[0008] Furthermore, the microstructure of the steel is granular bainite, and the content of granular bainite is 100%.

[0009] On the other hand, an embodiment of the present invention provides a method for preparing 785 MPa grade ultra-low carbon bainite steel, which is used to prepare the above-mentioned 785 MPa grade ultra-low carbon bainite steel, comprising the following steps:

[0010] Step 1: heating and forging the steel ingot to obtain a steel billet;

[0011] Step 2: Place the steel billet obtained in step 1 into a heating furnace for heating;

[0012] Step 3: Rough rolling and finish rolling the heated billet;

[0013] Step 4: Cool the steel billet after finish rolling to obtain a finished plate.

[0014] Furthermore, in step 1, the forging heating temperature of the steel ingot is 1190°C-1210°C, the start forging temperature is 1150°C-1160°C, and the final forging temperature is greater than 850°C.

[0015] Furthermore, in step 2, the heating temperature is 1190° C.-1210° C., and the heating time is 1.8 h-2.2 h.

[0016] Furthermore, in step 3, the rough rolling is performed in three passes, the deformation amount in the first pass is 15% to 17%, the deformation amount in the second pass is 23% to 25%, and the deformation amount in the third pass is 20% to 22%.

[0017] Furthermore, in step 3, the starting rolling temperature of the rough rolling is 1150-1180°C, and the finishing rolling temperature is 960-980°C.

[0018] Furthermore, in step 3, the finishing rolling is performed in three passes, the deformation amount in the first pass is 19% to 21%, the deformation amount in the second pass is 24% to 26%, and the deformation amount in the third pass is 33% to 34%.

[0019] Furthermore, in step 3, the start temperature of finishing rolling is 950-980°C, and the final temperature of finishing rolling is 750-850°C.

[0020] Furthermore, in step 4, the cooling rate is 1 to 5°C / s.

[0021] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0022] 1. The 785MPa grade ultra-low carbon bainitic steel and preparation method of the present invention adopt a low-carbon design. The strength of the steel no longer depends on the carbon content and the total amount of alloying elements, but relies on dislocation strengthening and fine grain strengthening in the bainite structure. By controlling the low carbon content, cementite in the bainitic ferrite matrix is ​​basically eliminated, and the toughness, weldability and yield strength ratio of the steel are further improved.

[0023] 2. The 785MPa grade ultra-low carbon bainitic steel and preparation method provided by the present invention have good stability, a yield strength ≥785Mpa (such as 805-831MPa), and a tensile strength ≥1030MPa (such as 1031-1072MPa). The strength and toughness of the ultra-low carbon bainitic steel obtained within this process window are at the same level. In the preparation method of the 785MPa grade ultra-low carbon bainitic steel provided by the present invention, the preparation process is simple, no post-rolling heat treatment is required, and the structure obtained after controlled rolling and controlled cooling is uniform and the performance is stable.

[0024] 3. The present invention prepares ultra-low carbon bainite structure through controlled rolling and controlled cooling technology. Relying on the combination of soft and hard phases in the bainite structure, 785MPa grade ultra-low carbon bainite steel with a low yield ratio is prepared, and the yield ratio is 0.77-0.80.

[0025] 4. The present invention substantially eliminates cementite in the bainitic ferrite matrix by reducing the carbon content, and further improves the toughness and weldability of the steel through the control of other elements and the synergistic effect between the elements. The impact energy of the weld center at -40°C is ≥50J, and the CTOD of the weld joint at -20°C is ≥0.17mm.

[0026] In the present invention, the above-mentioned technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of the present invention will be described in the following description, and some advantages will become apparent from the description or be learned through practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like parts throughout the drawings.

[0028] Figure 1 The microstructure of the 785MPa grade ultra-low carbon bainite steel in Example 1;

[0029] Figure 2 The microstructure of the 785MPa grade ultra-low carbon bainite steel in Example 4;

[0030] Figure 3This is the structure of the 785MPa grade ultra-low carbon bainite steel in Example 6. DETAILED DESCRIPTION

[0031] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, and are not used to limit the scope of the present invention.

[0032] The present invention provides a 785MPa grade ultra-low carbon bainite steel. The chemical composition of the 785MPa grade ultra-low carbon bainite steel plate comprises, by weight, C: 0.01%-0.03%, Si: 0.5%-0.8%, Mn: 1.0%-2.0%, Mo: 1.5%-2.0%, Nb: 0.03%-0.07%, Ni: 4.0%-6.0%, Cr: 0.3%-0.7%, Ti: 0.01%-0.03%, Al: 0.01%-0.03%, and the remainder is iron and unavoidable impurities.

[0033] At present, my country's high-strength hull structure steel is often prepared by quenching and tempering heat treatment, and the structure is tempered martensite or tempered martensite + bainite. The steel plate after quenching and tempering heat treatment has a high yield strength ratio, and the matching between strength, toughness and weldability is poor. The 785MPa grade ultra-low carbon bainitic steel of the present invention basically eliminates the cementite in the bainitic ferrite matrix by reducing the carbon content, and through the control of other elements and the synergistic effect between the elements, the toughness, weldability and yield strength ratio of the steel are improved. Yield strength ≥785 MPa (e.g., 805-831 MPa), tensile strength ≥1030 MPa (e.g., 1031-1072 MPa), elongation ≥16.0% (e.g., 16.5-18.0%), reduction of area ≥74% (e.g., 74-80%), impact energy ≥140 J (e.g., 140 J-202 J) at -40°C, impact energy ≥50 J at -40°C for welded joints, and CTOD ≥0.17 mm at -20°C for welded joints. This steel can address the problems of poor matching between strength and toughness and weldability, and high yield-to-strength ratio, in existing high-strength hull structural steels.

[0034] The reasons for limiting the composition of the slab of the 785 MPa grade ultra-low carbon bainite steel and the method for producing the same in the present invention will be explained. Hereinafter, only mass percentages in the composition are expressed in %.

[0035] Carbon (C): The carbon element ensures the formation of full bainite structure. C should be less than 0.05%. Reducing the carbon content can significantly improve the welding performance of steel. Studies have shown that when the carbon content is 0.01% to 0.03%, weldability is guaranteed while obtaining a steel with good strength and toughness matching.

[0036] Silicon (Si): Silicon acts as a deoxidizing element and a solid-solution strengthening element, increasing the strength of steel. Excessive silicon content can reduce the steel's low-temperature toughness and impair weldability. Therefore, the Si content is controlled between 0.50% and 0.80%.

[0037] Manganese (Mn): Manganese is an essential element for bainite formation. It causes a distinct bend in the supercooled austenite transformation curve and significantly delays high-temperature transformation, separating the upper and lower C curves of the steel. Mn easily reacts with sulfur in the steel to form MnS impurities, impairing the steel's properties. Therefore, Mn addition is typically between 1.0% and 2.0%.

[0038] Molybdenum (Mo): Mo can lower the bainite transformation temperature, promote bainite phase transformation, and shorten the bainite transformation time. The addition amount of Mo is 1.5% to 2.0%.

[0039] Niobium (Nb): Nb inhibits the deformation and recrystallization of high-temperature austenite, increases the recrystallization temperature, expands the non-recrystallized zone, increases the deformation accumulation in the non-recrystallized zone during rolling, introduces high-density dislocations, and promotes microstructure refinement. The Nb addition level is 0.03% to 0.07%.

[0040] Nickel (Ni): Ni lowers the ductile-brittle transition temperature of steel, improving its strength and toughness. The Ni addition level is 4.0% to 6.0%.

[0041] Chromium (Cr): Cr can significantly lower the bainite transformation temperature, affect the C curve of bainite transformation, and increase the strength of bainitic steel. The addition amount of Cr is 0.3% to 0.7%.

[0042] Titanium (Ti): Titanium precipitates can inhibit austenite grain growth during the heating process of the ingot and the welding thermal cycle, thereby improving the toughness of the steel plate and the weld heat-affected zone. When the Ti content is too low, the effect is weakened; when the Ti content exceeds 0.04%, large TiN particles are easily formed, which loses the effect. Therefore, the Ti content is controlled within 0.01-0.03%.

[0043] Aluminum (Als): Acid-soluble aluminum is an excellent deoxidizer for steel and an effective element for grain refinement. When the Als content is too low, it becomes difficult to control the oxygen content in the steel. When the Als content exceeds 0.08%, hot cracking of the ingot is likely to occur, while the toughness of the steel is also reduced. Therefore, the Als content is controlled between 0.01% and 0.03%.

[0044] The present invention also provides a method for preparing the above-mentioned 785MPa grade ultra-low carbon bainite steel, which is used to prepare the above-mentioned 785MPa grade ultra-low carbon bainite steel, comprising the following steps:

[0045] Step 1: heating and forging the steel ingot to obtain a steel billet;

[0046] Step 2: Place the steel billet obtained in step 1 into a heating furnace for heating;

[0047] Step 3: Rough rolling and finish rolling the heated billet;

[0048] Step 4: Cool the steel billet after finish rolling to obtain a finished plate.

[0049] Specifically, in step 1, the forging heating temperature of the steel ingot is 1190°C-1210°C, the start forging temperature is 1150°C-1160°C, and the final forging temperature is greater than 850°C to obtain a steel billet. If the start forging temperature is too high, decarburization is likely to occur, burning the steel ingot; if the start forging temperature is too low, cracks are likely to occur during forging, and forging is difficult. If the final forging temperature is greater than 850°C, if the final forging temperature is too low, recrystallization cannot proceed, the cold deformation strengthening phenomenon cannot be eliminated, the deformation resistance is large, the plasticity is reduced, and even cracks are generated on the forging and damage to equipment and tools. If the final forging temperature is too high, the grains of the billet grow after deformation, forming a coarse structure, which reduces the mechanical properties of the forging.

[0050] Specifically, in step 2, the heating temperature is 1190° C.-1210° C., the heating time is 1.8 hours-2.2 hours, and rolling is performed after heating. Heating causes plastic deformation of the steel, and the starting rolling temperature is determined based on 80% of the solidus temperature in the alloy phase.

[0051] Specifically, in step 3, rough rolling is divided into 3 rolling passes, the deformation of the first pass is 15% to 17%, the deformation of the second pass is 23% to 25%, and the deformation of the third pass is 20% to 22%; the rough rolling start temperature is 1150 to 1180°C, and the rough rolling finish temperature is 960 to 980°C; the finishing rolling is divided into 3 rolling passes, the deformation of the first pass is 19% to 21%, the deformation of the second pass is 24% to 26%, and the deformation of the third pass is 33% to 34%, the finishing rolling start temperature is 950 to 980°C, and the finishing rolling finish temperature is 750 to 850°C.

[0052] It should be noted that the rough rolling in step 3 is recrystallization zone rolling: the steel billet is heated to the austenitizing temperature and plastically deformed. Dynamic or static recrystallization occurs during each deformation pass or between passes, and the recrystallization process is completed. After repeated rolling and recrystallization, the austenite grains are refined. Finishing rolling is non-recrystallization zone rolling: plastic deformation occurs below the austenite recrystallization temperature. No dynamic or static recrystallization occurs after the austenite is deformed. The deformed austenite grains are elongated, with a large number of deformation bands within the grains. There are many nucleation points during the phase transformation, further refining the ferrite grains.

[0053] For example, rough rolling is performed in three passes, with the first pass deforming 16.6%, the second pass deforming 24%, and the third pass deforming 21%. The start and finish temperatures for rough rolling are 1150°C and 980°C, respectively. Finishing rolling is performed in three passes, with the first, second, and third passes deforming 20%, 25%, and 33.3%, respectively. The start and finish temperatures for finishing are 950°C and 850°C, respectively. After three roughing and finishing passes, the grain size of the steel slab is refined.

[0054] Specifically, in step 4, the steel slab after finish rolling is cooled to room temperature at a cooling rate of 1-5°C / s to produce a plate. Different cooling rates result in different microstructures. Excessively fast cooling rates result in lath bainite, which significantly increases the strength of the steel. To obtain granular bainite steel with a similar microstructure and strength level, the cooling rate is limited to 1-5°C / s.

[0055] It should be noted that the present invention adopts a low-carbon design. The strength of the steel no longer depends on the carbon content and the total amount of alloying elements, but relies on dislocation strengthening and fine grain strengthening in the bainite structure. By controlling the low carbon content, the cementite in the bainite ferrite matrix is ​​basically eliminated. By regulating the composition, the Mn-Mo-Nb content is increased, the hardenability of the steel is improved, and a uniform granular bainite structure is obtained by combined with controlled rolling and controlled cooling. The granular bainite structure consists of a matrix ferrite structure (soft phase) and MA islands (hard phase) distributed in the matrix. The yield strength is mainly determined by the strength of the soft phase, and the tensile strength is mainly determined by the hard phase. The soft and hard phases in the granular bainite are well matched, which ensures the strength of the prepared steel plate while reducing the yield strength ratio.

[0056] The preparation method of the present invention is highly practical and can obtain ultra-low carbon bainitic steel with consistent organizational type, stable performance, and well-matched strength and toughness under relatively wide production conditions. The bainite in the obtained ultra-low carbon bainitic steel is granular bainite with a content of 100%.

[0057] The ultra-low bainite steel prepared by the present invention has a yield strength of ≥785 MPa (such as 805-831 MPa), a tensile strength of ≥1030 MPa (such as 1031-1072 MPa), an elongation after fracture of ≥16.0% (such as 16.5-18.0%), a cross-sectional shrinkage of ≥74% (such as 74-80%), an impact energy at -40°C of ≥140 J (such as 140 J-202 J), an impact energy of a welded joint at -40°C of ≥50 J, a CTOD of a welded joint at -20°C of ≥0.17 mm, and a yield strength ratio of 0.77-0.80.

[0058] The advantages of the present invention in precisely controlling the chemical composition, content and preparation process parameters of the elements will be demonstrated below with specific examples and comparative examples.

[0059] Example 1

[0060] A 785MPa grade ultra-low carbon bainitic steel, whose chemical composition by weight comprises: C: 0.02%, Si: 0.5%, Mn: 1.5%, Mo: 1.8%, Nb: 0.05%, Ni: 4.2%, Cr: 0.5%, Ti: 0.01%, Al: 0.02%, and the remainder is iron and unavoidable impurities. The preparation method comprises the following steps:

[0061] Step 1: heating and forging the steel ingot with the above composition to obtain a steel billet;

[0062] The forging heating temperature is 1200°C, the start forging temperature is 1150°C, and the final forging temperature is 950°C to obtain a steel billet.

[0063] Step 2: Place the steel billet obtained in step 1 into a heating furnace for heating;

[0064] The heating temperature is 1190°C and the heating time is 2h.

[0065] Step 3: Rough rolling and finish rolling the heated billet;

[0066] The steel billet is rolled. During the rolling process, the rough rolling is performed in three passes, with the deformation of the first pass being 16.6%, the deformation of the second pass being 24%, and the deformation of the third pass being 21%. The rough rolling start temperature is 1150°C, and the rough rolling finish temperature is 980°C. The finish rolling is performed in three passes, with the deformation of the first pass being 20%, the deformation of the second pass being 25%, and the deformation of the third pass being 33.3%. The finish rolling start temperature is 950°C, and the finish rolling finish temperature is 850°C.

[0067] Step 4: Cooling the steel billet after finish rolling to obtain a steel plate;

[0068] The cooling rate after finishing rolling was 3.0°C / s.

[0069] The 785MPa grade ultra-low carbon bainite steel prepared in this embodiment has granular bainite morphology and mechanical properties: tensile strength of 1042MPa; yield strength of 813MPa; yield strength ratio of 0.78; elongation after fracture of 17.0%; reduction of area of ​​76%; impact energy at -40°C of 189J; impact energy at -40°C of the weld center of 58J; CTOD of the weld joint at -20°C of 0.18mm. The microstructure diagram is shown in the figure below. Figure 1 shown.

[0070] Example 2

[0071] A 785MPa grade ultra-low carbon bainitic steel, the chemical composition of which comprises, by weight, C: 0.01%, Si: 0.8%, Mn: 1.8%, Mo: 1.5%, Nb: 0.06%, Ni: 5.8%, Cr: 0.6%, Ti: 0.01%, Al: 0.03%, and the remainder is iron and unavoidable impurities. The preparation method comprises the following steps:

[0072] Step 1: heating and forging the steel ingot to obtain a steel billet;

[0073] The forging heating temperature is 1190°C, the start forging temperature is 1150°C, and the final forging temperature is 860°C to obtain a steel billet.

[0074] Step 2: Place the steel billet obtained in step 1 into a heating furnace for heating;

[0075] The heating temperature is 1200°C and the heating time is 1.8h.

[0076] Step 3: Rough rolling and finish rolling the heated billet;

[0077] The steel billet is rolled. During the rolling process, the rough rolling is performed in three passes, with the deformation of the first pass being 16%, the deformation of the second pass being 23%, and the deformation of the third pass being 22%. The rough rolling start temperature is 1180°C, and the rough rolling finish temperature is 960°C. The finish rolling is performed in three passes, with the deformation of the first pass being 21%, the deformation of the second pass being 26%, and the deformation of the third pass being 33%. The finish rolling start temperature is 950°C, and the finish rolling finish temperature is 750°C.

[0078] Step 4: Cooling the steel billet after finish rolling to obtain a steel plate;

[0079] The cooling rate of the steel plate after rolling is 5℃ / s.

[0080] The 785MPa grade ultra-low carbon bainitic steel prepared in this embodiment has a granular bainite morphology and the following mechanical properties: tensile strength of 1031MPa; yield strength of 825MPa; yield strength ratio of 0.78; elongation after fracture of 17.5%; cross-sectional reduction rate of 78%; impact energy at -40°C of 168J; impact energy at -40°C of the weld center of 61J, and CTOD of the weld joint at -20°C of 0.20mm.

[0081] Example 3

[0082] A 785MPa grade ultra-low carbon bainitic steel, comprising, by weight, 0.02% C, 0.8% Si, 1.0% Mn, 2.0% Mo, 0.03% Nb, 6.0% Ni, 0.3% Cr, 0.02% Ti, and 0.01% Al, with the remainder being iron and unavoidable impurities. A preparation method thereof comprises the following steps:

[0083] Step 1: heating and forging the steel ingot to obtain a steel billet;

[0084] The forging heating temperature is 1210°C, the start forging temperature is 1160°C, and the final forging temperature is 880°C to obtain a steel billet.

[0085] Step 2: Place the steel billet obtained in step 1 into a heating furnace for heating;

[0086] The heating temperature in the heating furnace is 1200°C and the heating time is 1.9h.

[0087] Step 3: Rough rolling and finish rolling the heated billet;

[0088] The steel billet is rolled. During the rolling process, the rough rolling is performed in three passes, with the deformation of the first pass being 15%, the deformation of the second pass being 25%, and the deformation of the third pass being 21.3%. The rough rolling start temperature is 1170°C, and the rough rolling finish temperature is 980°C. The finish rolling is performed in three passes, with the deformation of the first pass being 19%, the deformation of the second pass being 24%, and the deformation of the third pass being 34%. The finish rolling start temperature is 950°C, and the finish rolling finish temperature is 850°C.

[0089] Step 4: Cooling the steel billet after finish rolling to obtain a steel plate;

[0090] The cooling rate after rolling is 1℃ / s.

[0091] The 785MPa grade ultra-low carbon bainitic steel prepared in this embodiment has a granular bainite morphology and the following mechanical properties: tensile strength of 1065MPa; yield strength of 831MPa; yield strength ratio of 0.78; elongation after fracture of 17.0%; cross-sectional reduction rate of 78%; impact energy at -40°C of 173J; impact energy at -40°C of the weld center of 52J, and CTOD of the weld joint at -20°C of 0.20mm.

[0092] Example 4

[0093] A 785MPa grade ultra-low carbon bainitic steel, comprising, by weight, 0.03% C, 0.7% Si, 1.2% Mn, 1.6% Mo, 0.07% Nb, 4.0% Ni, 0.5% Cr, 0.03% Ti, and 0.02% Al, with the remainder being iron and unavoidable impurities. A preparation method thereof comprises the following steps:

[0094] Step 1: heating and forging the steel ingot to obtain a steel billet;

[0095] The forging heating temperature is 1200°C, the start forging temperature is 1160°C, and the final forging temperature is 950°C to obtain a rough steel billet.

[0096] Step 2: Place the steel billet obtained in step 1 into a heating furnace for heating;

[0097] The heating temperature of the heating furnace is 1210℃ and the heating time is 2h.

[0098] Step 3: Rough rolling and finish rolling the heated billet;

[0099] The steel billet is rolled. During the rolling process, the rough rolling is performed in three passes, with the deformation of the first pass being 17%, the deformation of the second pass being 24.3%, and the deformation of the third pass being 20.6%. The rough rolling start temperature is 1160°C, and the rough rolling finish temperature is 980°C. The finish rolling is performed in three passes, with the deformation of the first pass being 19%, the deformation of the second pass being 25%, and the deformation of the third pass being 33%. The finish rolling start temperature is 950°C, and the finish rolling finish temperature is 800°C.

[0100] Step 4: Cooling the steel billet after finish rolling to obtain a steel plate;

[0101] The cooling rate after rolling is 1℃ / s.

[0102] The 785MPa grade ultra-low carbon bainite steel prepared in this embodiment has granular bainite morphology and the following mechanical properties: tensile strength of 1072MPa; yield strength of 805MPa; yield strength ratio of 0.78; elongation after fracture of 16.5%; reduction of area of ​​74%; impact energy at -40°C of 140J; impact energy at -40°C of the weld center of 50J; CTOD of the weld joint at -20°C of 0.17mm. The microstructure diagram is shown in FIG. Figure 2 shown.

[0103] Example 5

[0104] A 785MPa grade ultra-low carbon bainitic steel, the chemical composition of which comprises, by weight, C: 0.02%, Si: 0.7%, Mn: 1.4%, Mo: 1.6%, Nb: 0.05%, Ni: 3.2%, Cr: 0.5%, Ti: 0.02%, Al: 0.02%, and the remainder is iron and unavoidable impurities. The preparation method comprises the following steps:

[0105] Step 1: heating and forging the steel ingot to obtain a steel billet;

[0106] The forging heating temperature is 1200°C, the start forging temperature is 1150°C, and the final forging temperature is 900°C to obtain a rough steel billet.

[0107] Step 2: Place the steel billet obtained in step 1 into a heating furnace for heating;

[0108] The heating temperature was 1190°C and the heating time was 2.1 h.

[0109] Step 3: Rough rolling and finish rolling the heated billet;

[0110] The steel billet is rolled. During the rolling process, the rough rolling is performed in three passes, with the deformation of the first pass being 15.3%, the deformation of the second pass being 24%, and the deformation of the third pass being 22%. The rough rolling start temperature is 1150°C, and the rough rolling finish temperature is 980°C. The finish rolling is performed in three passes, with the deformation of the first pass being 20%, the deformation of the second pass being 24.6%, and the deformation of the third pass being 34%. The finish rolling start temperature is 960°C, and the finish rolling finish temperature is 750°C.

[0111] Step 4: Cooling the steel billet after finish rolling to obtain a steel plate;

[0112] The cooling rate of the steel billet after finish rolling is 2°C / s.

[0113] The 785MPa grade ultra-low carbon bainitic steel prepared in this embodiment has a granular bainite morphology and the following mechanical properties: tensile strength of 1066MPa; yield strength of 821MPa; yield strength ratio of 0.77; elongation after fracture of 16.5%; cross-sectional reduction rate of 78%; impact energy at -40°C of 195J; impact energy at -40°C of the weld center of 57J, and CTOD of the weld joint at -20°C of 0.18mm.

[0114] Example 6

[0115] A 785MPa grade ultra-low carbon bainitic steel, whose chemical composition by weight comprises: C: 0.02%, Si: 0.5%, Mn: 2.0%, Mo: 1.7%, Nb: 0.05%, Ni: 3.8%, Cr: 0.4%, Ti: 0.02%, Al: 0.02%, and the remainder is iron and unavoidable impurities. The preparation method comprises the following steps:

[0116] Step 1: heating and forging the steel ingot to obtain a steel billet;

[0117] The forging heating temperature is 1190°C, the start forging temperature is 1160°C, and the final forging temperature is 850°C to obtain a crude steel billet.

[0118] Step 2: Place the steel billet obtained in step 1 into a heating furnace for heating;

[0119] The heating temperature is 1200°C and the heating time is 2.2h.

[0120] Step 3: Rough rolling and finish rolling the heated billet;

[0121] The steel billet is rolled. During the rolling process, the rough rolling is performed in three passes, with the deformation of the first pass being 17%, the deformation of the second pass being 23%, and the deformation of the third pass being 20%. The rough rolling start temperature is 1150°C, and the rough rolling finish temperature is 960°C. The finish rolling is performed in three passes, with the deformation of the first pass being 19%, the deformation of the second pass being 25%, and the deformation of the third pass being 33.6%. The finish rolling start temperature is 950°C, and the finish rolling finish temperature is 800°C.

[0122] Step 4: Cooling the steel billet after finish rolling to obtain a steel plate;

[0123] The cooling rate after rolling is 4℃ / s.

[0124] The 785MPa grade ultra-low carbon bainite steel prepared in this embodiment has granular bainite morphology and the following mechanical properties: tensile strength of 1047MPa; yield strength of 817MPa; yield strength ratio of 0.80; elongation after fracture of 18.0%; reduction of area of ​​80%; impact energy at -40°C of 202J; impact energy at -40°C of the weld center of 53J; CTOD of the weld joint at -20°C of 0.22mm. The microstructure diagram is shown in FIG. Figure 3 shown.

[0125] Table 1 shows the chemical composition and content of the example steel; Table 2 shows the process parameters of the preparation process of the example steel; Table 3 shows the mechanical properties of the example steel; Table 4 shows the welding properties of the example steel.

[0126] Table 1 Chemical composition of steel in Examples and Comparative Examples (wt%, %)

[0127] serial number C Si Mn Mo Nb Ni Cr Ti Al Example 1 0.02 0.5 1.5 1.8 0.05 4.2 0.5 0.01 0.02 Example 2 0.01 0.8 1.8 1.5 0.06 5.8 0.6 0.01 0.03 Example 3 0.02 0.8 1.0 2.0 0.03 6.0 0.3 0.02 0.01 Example 4 0.03 0.7 1.2 1.6 0.07 4.0 0.5 0.03 0.02 Example 5 0.02 0.7 1.4 1.6 0.05 3.2 0.5 0.02 0.02 Example 6 0.02 0.5 2.0 1.7 0.05 3.8 0.4 0.02 0.02

[0128] Table 2 Preparation process of Examples and Comparative Examples

[0129]

[0130] Table 3 Mechanical properties of the steels of the examples and comparative examples

[0131]

[0132] Table 4 Weldability of Examples and Comparative Examples Steel

[0133]

[0134] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing 785MPa grade ultra-low carbon bainitic steel, characterized in that: No tempering heating treatment is required and it is prepared by the following steps: Step 1: heating and forging the steel ingot to obtain a steel billet; Step 2: Place the steel billet obtained in step 1 into a heating furnace for heating; Step 3: Rough rolling and finish rolling the heated billet; Step 4: Cooling the steel billet after finish rolling to obtain a finished plate; In step 3, the rough rolling is performed in three passes, the starting rolling temperature of the rough rolling is 1160-1180° C., and the finish rolling is performed in three passes, the starting rolling temperature of the finish rolling is 960-980° C.; In step 4, the cooling rate is 1 to 5°C / s; The 785MPa grade ultra-low carbon bainitic steel comprises, by weight, the following chemical compositions: C: 0.01% to 0.03%, Si: 0.5 to 0.8%, Mn: 1.0% to 1.8%, Mo: 1.6% to 2.0%, Nb: 0.06% to 0.07%, Ni: 4.0% to 6.0%, Cr: 0.3% to 0.7%, Ti: 0.01% to 0.03%, Al: 0.01% to 0.03%, and the remainder being iron and unavoidable impurities; The yield strength ratio of the 785MPa grade ultra-low carbon bainitic steel is 0.77-0.80; The impact energy of the 785MPa grade ultra-low carbon bainitic steel at -40°C is ≥140J, the impact energy of the weld center at -40°C is ≥50J, and the CTOD of the weld joint at -20°C is ≥0.17mm.

2. The preparation method according to claim 1, characterized in that In the step 1, the forging heating temperature of the steel ingot is 1190° C.-1210° C., the start forging temperature is 1150° C.-1160° C., and the final forging temperature is greater than 850° C.

3. The preparation method according to claim 1, characterized in that In step 2, the heating temperature is 1190° C.-1210° C., and the heating time is 1.8 h-2.2 h.

4. The preparation method according to claim 1, characterized in that In step 3, the rough rolling is performed in three passes, the deformation amount of the first pass is 15% to 17%, the deformation amount of the second pass is 23% to 25%, and the deformation amount of the third pass is 20% to 22%.

5. The preparation method according to claim 4, characterized in that In the step 3, the starting rolling temperature of the rough rolling is 1170-1180° C., and the finishing rolling temperature is 960-980° C.

6. The preparation method according to claim 1, characterized in that In step 3, the finishing rolling is performed in three passes, with the deformation of the first pass being 19% to 21%, the deformation of the second pass being 24% to 26%, and the deformation of the third pass being 33% to 34%.

7. The preparation method according to claim 6, characterized in that In step 3, the finishing rolling temperature is 750-850°C.

8. The preparation method according to claim 1, characterized in that In the step 4, the cooling rate is 2-5°C / s.

9. A 785MPa grade ultra-low carbon bainitic steel, prepared according to the preparation method according to any one of claims 1 to 8, characterized in that: The chemical composition includes by weight: C: 0.01% to 0.03%, Si: 0.5 to 0.8%, Mn: 1.0% to 1.8%, Mo: 1.6% to 2.0%, Nb: 0.06% to 0.07%, Ni: 4.0% to 6.0%, Cr: 0.3% to 0.7%, Ti: 0.01% to 0.03%, Al: 0.01% to 0.03%, and the rest is iron and unavoidable impurities; The microstructure of the 785MPa grade ultra-low carbon bainite steel is granular bainite, and the content of granular bainite is 100%.

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