阅读说明：本技术 一种含铜超低碳贝氏体钢及其制备方法 (Copper-containing ultra-low carbon bainite steel and preparation method thereof ) 是由 王天琪 杨才福 柴希阳 柴锋 罗小兵 梁丰瑞 师仲然 于 2019-11-28 设计创作，主要内容包括：本发明涉及一种含铜超低碳贝氏体钢及其制备方法,属于金属材料技术领域,解决现有含铜超低碳贝氏体钢生产窗口较窄、宽厚板表面心部差异较大、性能波动较大的问题。本发明提供一种含铜超低碳贝氏体钢,化学成分按重量计包含：C：0.01％～0.03％、Mn：0.8％～1.5％、Mo：1.5％～2.0％、Nb：0.03％～0.07％、Ni：3.0％～4.0％、Cr：0.4％～0.6％、B：0.0010％～0.0018％、Cu：1.4％～1.6％,其余为铁和不可避免的杂质。本发明实现了超低碳贝氏体钢的生产适应性好、组织性能稳定。(The invention relates to copper-containing ultra-low carbon bainite steel and a preparation method thereof, belongs to the technical field of metal materials, and solves the problems of narrow production window, large surface center difference of wide and thick plates and large performance fluctuation of the conventional copper-containing ultra-low carbon bainite steel. The invention provides a copper-containing ultra-low carbon bainite steel, which comprises the following chemical components in percentage by weight: c: 0.01-0.03%, Mn: 0.8% -1.5%, Mo: 1.5% -2.0%, Nb: 0.03 to 0.07 percent, Ni: 3.0-4.0%, Cr: 0.4% -0.6%, B: 0.0010-0.0018%, Cu: 1.4 to 1.6 percent, and the balance of iron and inevitable impurities. The invention realizes good production adaptability and stable structure performance of the ultra-low carbon bainite steel.)

1. The copper-containing ultra-low carbon bainite steel is characterized by comprising the following chemical components in percentage by weight: c: 0.01-0.03%, Mn: 0.8% -1.5%, Mo: 1.5% -2.0%, Nb: 0.03 to 0.07 percent, Ni: 3.0-4.0%, Cr: 0.4% -0.6%, B: 0.0010-0.0018%, Cu: 1.4 to 1.6 percent, and the balance of iron and inevitable impurities.

2. The steel of claim 1, wherein the bainite is ultra low carbon bainite.

3. A method for manufacturing a copper-containing ultra low carbon bainite steel according to any one of claims 1 to 2, comprising the steps of:

step 1: heating and forging the steel ingot to obtain a steel billet;

step 2: putting the billet obtained in the step 1 into a heating furnace for heating;

and step 3: carrying out rough rolling and finish rolling on the heated steel billet;

and 4, step 4: cooling the steel billet after finish rolling to obtain a steel plate;

and 5: and (4) putting the steel plate obtained in the step (4) into a heating furnace and then carrying out tempering and heating treatment.

4. The method as claimed in claim 3, wherein the steel ingot obtained in the step 1 is heated and forged at 1180-1200 ℃, the open forging temperature 1150-1160 ℃, and the finish forging temperature > 850 ℃ to obtain the steel billet.

5. The method as claimed in claim 3, wherein the heating temperature in step 2 is 1180-1200 ℃, the heating time is 1.8-2.2 hours, and the rolling is performed after the heating.

6. The method for preparing the copper-containing ultra-low carbon bainite steel as claimed in claim 3, wherein the rough rolling in step 3 is performed in 2 passes, the first pass deformation is 25% -27%, the second pass deformation is 24% -25%, and the cumulative deformation is 43% -44%; the initial rolling temperature of rough rolling is 1100-1150 ℃, and the final rolling temperature of rough rolling is 950-1000 ℃.

7. The method for preparing the copper-containing ultra-low carbon bainite steel according to claim 3, wherein the finish rolling in step 3 is divided into 4 passes, the first pass deformation is 20-21%, the second pass deformation is 22-23%, the third pass deformation is 22-23%, the fourth pass deformation is 24-26%, the cumulative deformation is 63-65%, the start temperature of the finish rolling is 930-950 ℃, and the finish rolling temperature is 700-900 ℃.

8. The method as claimed in claim 3, wherein the cooling rate of the cooling in the step 4 is 0.5 ℃/s to 25 ℃/s, thereby obtaining a steel plate.

9. The method as claimed in claim 8, wherein the cooling in step 4 is sand burying, air cooling or laminar flow.

10. The method as claimed in claim 3, wherein the heating furnace in step 5 is used for tempering and heating the steel plate at 550-650 ℃, the holding time is 2 hours, and the steel plate is air-cooled after holding.

Technical Field

The invention relates to the technical field of metal materials, in particular to copper-containing ultra-low carbon bainite steel and a preparation method thereof.

Background

Ultra-low carbon bainitic steel is a new category of high-strength, high-toughness and multipurpose steel which is developed internationally for more than 20 years. The alloy design idea of the ultra-low carbon bainite steel is different from that of the original high-strength low-alloy steel. The content of carbon in the ultra-low carbon bainite steel is greatly reduced, and the influence of carbon elements on the toughness and the welding performance of the steel is eliminated. At present, the ultra-low carbon bainite steel is widely applied to the fields of petroleum pipelines, ships, large structural parts, marine facilities and the like.

Although the ultra-low carbon bainite steel has the advantages of good toughness matching property, excellent welding performance and the like, some disadvantages still exist. Bainite is found in studies on isothermal transformation of steel, and the structure and morphology of bainite produced vary depending on the production conditions. The bainite steel with different tissue forms has the defects of narrow production window, large surface center difference of the wide and thick plates, large performance fluctuation and the like due to large difference of obdurability of the bainite steel with different tissue forms. Therefore, the development of ultra-low carbon bainite steel with good production adaptability and stable structure property is imminent.

Disclosure of Invention

In view of the above analysis, embodiments of the present invention are directed to provide a copper-containing ultra-low carbon bainite steel and a preparation method thereof, so as to solve the problems of a narrow production window and large performance fluctuation in the prior art.

The invention is realized by the following technical scheme:

the invention provides a copper-containing ultra-low carbon bainite steel, which comprises the following chemical components in percentage by weight: c: 0.01-0.03%, Mn: 0.8% -1.5%, Mo: 1.5% -2.0%, Nb: 0.03 to 0.07 percent, Ni: 3.0-4.0%, Cr: 0.4% -0.6%, B: 0.0010-0.0018%, Cu: 1.4 to 1.6 percent, and the balance of iron and inevitable impurities.

Further, the form of bainite in the copper-containing ultra-low carbon bainite steel is ultra-low carbon bainite.

The invention provides a preparation method of copper-containing ultralow-carbon bainite steel, which is used for preparing the copper-containing ultralow-carbon bainite steel and comprises the following steps:

step 1: heating and forging the steel ingot to obtain a steel billet;

step 2: putting the billet obtained in the step 1 into a heating furnace for heating;

and step 3: carrying out rough rolling and finish rolling on the heated steel billet;

and 4, step 4: cooling the steel billet after finish rolling to obtain a steel plate;

and 5: and (4) putting the steel plate obtained in the step (4) into a heating furnace and then carrying out tempering and heating treatment.

Further, the heating forging temperature of the steel ingot in the step 1 is 1180-1200 ℃, the opening forging temperature is 1150-1160 ℃, and the finish forging temperature is more than 850 ℃ to obtain a steel billet.

Further, in the step 2, the heating temperature is 1180-1200 ℃, the heating time is 1.8-2.2 h, and the rolling is carried out after the heating.

Further, in the step 3, the rough rolling is divided into 2 passes of rolling, the deformation of the first pass is 25-27%, the deformation of the second pass is 24-25%, and the accumulated deformation is 43-44%; the initial rolling temperature of rough rolling is 1100-1150 ℃, and the final rolling temperature of rough rolling is 950-1000 ℃.

Furthermore, in the step 3, the finish rolling is divided into 4 passes of rolling, the first pass deformation is 20-21%, the second pass deformation is 22-23%, the third pass deformation is 22-23%, the fourth pass deformation is 24-26%, the accumulated deformation is 63-65%, the finish rolling temperature is 930-950 ℃, and the finish rolling temperature is 700-900 ℃.

Further, the cooling speed of the cooling in the step 4 is 0.5 ℃/s-25 ℃/s, and the steel plate is obtained.

Further, the cooling mode in the step 4 is sand burying, air cooling or laminar flow.

Further, in the step 5, the heating furnace is used for tempering and heating the steel plate, the heating temperature is 550-650 ℃, the heat preservation time is 2 hours, and the steel plate is air-cooled after heat preservation.

Compared with the prior art, the invention can realize at least one of the following beneficial effects:

1. the copper-containing ultra-low carbon bainite steel provided by the invention is simple in component, and only contains C, Mn, Mo, Nb, Ni, Cr, B and Cu, while in the ultra-low carbon bainite steel in the prior art, various alloy elements such as Si, Ti, V, La and the like are added for ensuring the performance of obtaining a full bainite structure and better strength, toughness and the like, and only a proper amount of Cu is added.

2. The invention provides a copper-containing ultra-low carbon bainite steel and a preparation method thereof, which adopt a low carbon design, and the strength of the steel does not depend on the content of carbon and the total amount of alloy elements, but depends on dislocation strengthening and fine grain strengthening in a bainite structure. After the carbon content is reduced, the ultra-low carbon bainite steel has better strength and toughness and excellent welding performance, and after the carbon content is greatly reduced, cementite in a bainitic ferrite matrix is basically eliminated, so that the toughness and the welding performance of the steel are further improved. Meanwhile, the Cu element is added, so that the strength of the ultra-low carbon bainite steel is improved while the toughness of the steel is not damaged, and the formation of pro-eutectoid ferrite before bainite transformation can be further inhibited by the compound addition of the Cu and the B. The addition amount of Cu selected by the invention is 1.4-1.6%.

3. According to the copper-containing ultralow-carbon bainite steel and the preparation method thereof, the prepared copper-containing ultralow-carbon bainite steel has good stability, the tensile strength is 963 MPa-1047 MPa, the yield strength is 840 MPa-934 MPa, and the strength and the toughness of the obtained ultralow-carbon bainite steel under the process window are at the same level; in the preparation method of the copper-containing ultra-low carbon bainite steel, the finish rolling temperature is 700-900 ℃, the cooling speed of cooling is 0.5-25 ℃/s, the preparation method has a wider process window, and the ultra-low carbon bainite steel with consistent performance and structure can be obtained in a wider range.

4. The invention does not contain V, Ti elements, does not adopt micro-alloying of Nb, V and Ti to improve the strength of the steel, and is completely different from the alloy components of the current mainstream ultra-low carbon bainite steel.

5. The invention basically eliminates cementite in the base body of the bainitic ferrite by reducing the carbon content, and further improves the toughness and the weldability of the steel by controlling copper and other elements and the synergistic action among the elements. In addition, by controlling the rolling process, the ultra-low carbon bainite steel is obtained, and the form of the obtained ultra-low carbon bainite is granular bainite; the ultra-low carbon bainite steel obtained by reducing the carbon content in the steel has excellent mechanical property, the tensile strength is 963MPa to 1047MPa, the yield strength is 840MPa to 934MPa, the elongation after fracture is 12.5 percent to 17.0 percent, the reduction of area is 72.0 percent to 80.0 percent, and the impact energy at minus 40 ℃ is 188J to 249J.

In the invention, the technical schemes can be combined with each other to realize more preferable combination schemes. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and drawings.

Drawings

The drawings are only for purposes of illustrating particular embodiments and are not to be construed as limiting the invention, wherein like reference numerals are used to designate like parts throughout.

FIG. 1 is CCT curve of copper-containing ultra-low carbon bainite steel;

FIG. 2 shows the rolled structure of the copper-containing ultra-low carbon bainitic steel of example 1;

FIG. 3 is a tempered structure of the copper-containing ultra low carbon bainitic steel of example 1;

FIG. 4 shows the rolled structure of the copper-containing ultra-low carbon bainitic steel of example 4;

FIG. 5 shows a tempered structure of the copper-containing ultra low carbon bainitic steel of example 4;

FIG. 6 shows the rolled structure of the copper-containing ultra low carbon bainitic steel of example 5;

FIG. 7 shows a tempered structure of a copper-containing ultra-low carbon bainitic steel in example 5.

Detailed Description

The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate preferred embodiments of the invention and together with the description, serve to explain the principles of the invention and not to limit the scope of the invention.

The invention provides a copper-containing ultra-low carbon bainite steel, which comprises the following chemical components in percentage by weight: c: 0.01-0.03%, Mn: 0.8% -1.5%, Mo: 1.5% -2.0%, Nb: 0.03 to 0.07 percent, Ni: 3.0-4.0%, Cr: 0.4% -0.6%, B: 0.0010-0.0018%, Cu: 1.4 to 1.6 percent, and the balance of iron and inevitable impurities.

The copper-containing ultra-low carbon bainite steel has tensile strength of 963MPa to 1047MPa, yield strength of 840MPa to 934MPa, elongation after fracture of 12.5 percent to 17.0 percent, reduction of area of 72.0 percent to 80.0 percent, and impact energy of 188J to 249J at minus 40 ℃.

As shown in FIGS. 2 to 7, the obtained ultra low carbon bainitic steels were all obtained. The chemical composition of the copper-containing ultra-low carbon bainite steel of the present invention is described in detail as follows:

carbon (C): the carbon element ensures that a full bainite structure is formed, C is less than 0.05%, the reduction of the carbon content can obviously improve the welding performance of the steel, and researches show that when the carbon content is 0.01-0.03%, the steel with better toughness matching is obtained while the weldability is ensured.

Manganese (Mn): manganese is a basic element for bainite formation, so that a remarkable river bay appears on a transformation curve of super-cooled austenite, high-temperature transformation is remarkably delayed, and upper and lower C curves of steel are separated. Mn is liable to form MnS impurities with S in the steel, impairing the properties of the steel. Therefore, the amount of Mn added is 0.8% to 1.5%.

Molybdenum (Mo): the Mo element can reduce the bainite transformation temperature, promote the bainite transformation and shorten the bainite transformation time. The addition amount of Mo selected by the invention is 1.5-2.0%.

Niobium (Nb): the Nb element can inhibit the deformation recrystallization behavior of high-temperature austenite, improve the recrystallization temperature, enlarge a non-recrystallization region, increase the deformation accumulation during rolling of the non-recrystallization region, introduce high-density dislocation and promote the tissue refinement. The addition amount of the selected Nb is 0.03 to 0.07 percent.

Nickel (Ni): the Ni element mainly reduces the ductile-brittle transition temperature and improves the toughness, and the Ni element is compounded with the Cu element to prevent the hot brittleness of the Cu. Therefore, the addition amount of Ni selected by the invention is 3.0-4.0%.

Chromium (Cr): the Cr element can obviously reduce the transformation temperature of bainite, influence the C curve of bainite transformation and improve the strength of bainite steel. Therefore, the addition amount of Cr selected by the invention is 3.0-4.0%.

Boron (B): boron can improve the tempering stability of the ultra-low carbon bainite steel; and the B and the Nb are added in a compounding way, so that the bainite transformation temperature can be reduced, the structure is further refined, and the dislocation density in the matrix is improved; the ferrite transformation can be remarkably delayed by the composite addition of B and Mo. However, the hot workability and toughness of the steel are adversely affected by an excessively high B content, so that the amount of B is selected to be 0.0010% to 0.0018% in the present invention.

Copper (Cu): the copper element mainly utilizes the precipitation strengthening effect of copper in the later period of aging, the strength of the ultra-low carbon bainite steel is improved on the premise of not damaging the toughness, and the formation of pre-eutectoid ferrite before bainite transformation can be further inhibited by the compound addition of Cu and B. The addition amount of Cu selected by the invention is 1.4-1.6%.

The invention provides a preparation method of copper-containing ultra-low carbon bainite steel, which adopts a heating furnace for heating and smelting, and adopts continuous casting or die casting for casting. As shown in figure 1, in a cooling speed range in a CCT curve, the bainite steel prepared by the method only passes through a bainite phase region and does not have a ferrite phase region, the ultra-low carbon bainite steel prepared by the method provided by the invention can obtain a pure bainite structure in a wider cooling speed range, and a bainite platform is wide. The preparation method comprises the following steps:

step 1: heating and forging the steel ingot to obtain a steel billet;

the forging heating temperature is 1180-1200 ℃, the forging starting temperature is 1150-1160 ℃, and the final forging temperature is more than 850 ℃ to obtain the billet. The forging temperature is too high, so that decarburization is easy, and steel ingots are burned out; the open forging temperature is too low, cracks are easy to appear in forging, and the forging is labored.

The finish forging temperature is higher than 850 ℃, the finish forging temperature is too low, recrystallization cannot be carried out, the cold deformation strengthening phenomenon cannot be eliminated, the deformation resistance is large, the plasticity is reduced, and even cracks are generated on the forged piece and equipment and tools are damaged; the final forging temperature is too high, the grains grow up after the blank deforms, a coarse structure is formed, and the mechanical property of the forge piece is reduced.

Step 2: putting the billet obtained in the step 1 into a heating furnace for heating;

the heating temperature is 1180-1200 ℃, the heating time is 1.8-2.2 h, and the rolling is carried out after the heating. The steel is plastically deformed by heating, and the initial rolling temperature is determined according to 80% of the solidus temperature of the alloy phase.

And step 3: carrying out rough rolling and finish rolling on the heated steel billet; the rough rolling is divided into 2 passes of rolling, the deformation of the first pass is 25-27%, the deformation of the second pass is 24-25%, and the accumulated deformation is 43-44%; the initial rolling temperature of rough rolling is 1100-1150 ℃, and the final rolling temperature of rough rolling is 950-1000 ℃;

the finish rolling is divided into 4 passes of rolling, the first pass deformation is 20-21%, the second pass deformation is 22-23%, the third pass deformation is 23-24%, the fourth pass deformation is 24-26%, the accumulated deformation is 63-65%, the finish rolling temperature is 930-950 ℃, and the finish rolling temperature is 700-900 ℃.

Preferably, the rough rolling is divided into 2 passes of rolling, the deformation of the first pass is 25 percent, the deformation of the second pass is 24.4 percent, and the accumulated deformation is 43.3 percent; the initial rolling temperature of rough rolling is 1100-1150 ℃, and the final rolling temperature of rough rolling is 950-1000 ℃; the finish rolling is divided into 4 passes of rolling, the first pass deformation is 20.6%, the second pass deformation is 22.2%, the third pass deformation is 23.8%, the fourth pass deformation is 25%, and the cumulative deformation is 64.7%. The pass deformation amounts are respectively 25% and 24.4%, and the accumulated deformation amount is 43.3%; after 2 passes of rough rolling and 4 passes of finish rolling, the crystal grains of the billet are well refined, as shown in figure 2, the obtained billet has a bainite structure and good strength, the mechanical property is that the tensile strength is 1047MPa, the yield strength is 934MPa, the elongation after fracture is 17.0 percent, the reduction of area is 80.0 percent, and the impact work at-40 ℃ is 249J.

And 4, step 4: cooling the steel billet after finish rolling to obtain a steel plate;

and cooling the steel billet subjected to finish rolling to room temperature at a cooling speed of 0.5-25 ℃/s to obtain the plate.

And 5: putting the steel plate obtained in the step (4) into a heating furnace and then carrying out tempering and heating treatment;

heating at 550-650 ℃, keeping the temperature for 2h, and then air-cooling to room temperature. The steel sheet obtained as described above is further subjected to heat treatment to precipitate copper in the steel.

The alloy composition system of the invention is simple and is easy to generate bainite structure, and CCT curve shows that ferrite transformation is remarkably delayed under the alloy system, and the bainite platform area is widened, so that the bainite structure can be obtained in a wider current cooling speed range (lower than 40.5 ℃/s).

The preparation method has strong practicability, and can obtain the ultra-low carbon bainite steel with consistent tissue type, stable performance and good toughness and toughness matching under wider production conditions, wherein the bainite in the ultra-low carbon bainite steel is granular bainite with the content of 100 percent.