CN111850413B

CN111850413B - Preparation method of easily-welded fatigue-resistant high-strength and high-toughness alloy steel

Info

Publication number: CN111850413B
Application number: CN202010782566.XA
Authority: CN
Inventors: 李红英; 陈博轩; 汤伟
Original assignee: Central South University
Current assignee: Central South University
Priority date: 2020-08-06
Filing date: 2020-08-06
Publication date: 2021-09-21
Anticipated expiration: 2040-08-06
Also published as: CN111850413A

Abstract

The invention provides a high-strength and high-toughness alloy steel easy to weld and resistant to fatigue and a preparation method thereof. The alloy steel comprises the following chemical components in percentage by mass: c: 0.30-0.40%, Si: 0.35-0.50%, Cr: 1.00-1.50%, Mn: 0.80-0.95%, Mo: 0.50-0.65%, W: 1.00-1.35%, V: 0.05-0.15%, Cu: 0.05 to 0.10%, Ni: 0.05-0.20%, Al: 0.01-0.02%, Ca: 0.0015-0.0050%, P < 0.01%, S < 0.003%, and the balance of Fe and unavoidable impurities. The preparation method comprises the following steps: steel making, casting, preheating treatment, rolling and heat treatment. The steel provided by the invention has the advantages of high strength, good toughness, easy welding and fatigue resistance, and can be applied to large-scale structural members and pipelines.

Description

Preparation method of easily-welded fatigue-resistant high-strength and high-toughness alloy steel

Technical Field

The invention relates to the field of alloy steel and preparation thereof, in particular to high-toughness alloy steel easy to weld and resistant to fatigue and a preparation method thereof.

Background

In the field of infrastructure, on one hand, the complexity of construction projects and environments thereof is continuously increased, on the other hand, higher requirements are provided for construction efficiency, and the higher requirements are bound to be provided for engineering machinery when complex infrastructure projects are completed in a shorter time. In the case of a crane, the structural members of the crane must be large in specification, light in weight and precise in order to have the capability of lifting a large heavy object to a high place. With the increasing functions and forces of engineering machinery, the requirements on steel used for structural members are more and more strict, and the steel has the advantages of better strength, toughness, easy welding and fatigue resistance, high hardenability, uniform mechanical properties and mechanical property stability in service in complex environments. Increasing the content of carbon and other alloying elements is an effective way to increase the strength of steel, but increasing the carbon equivalent has an adverse effect on the toughness, weldability and fatigue resistance of steel. The foundation construction and the engineering machinery manufacture in China are at the international leading level, the foundation construction scale and the construction speed are all the surprise of the world, various engineering machinery plays an important supporting role, however, high-quality steel required by the engineering machinery manufacture depends on import, how to improve the welding performance and the fatigue resistance of the steel on the basis of giving consideration to the obdurability is the key problem which needs to be solved urgently in the steel industry in China.

The patent publication No. CN108085617A discloses a steel production technical scheme, which adopts low-carbon component design and pre-hardening treatment, greatly increases the content of alloy elements such as Mn, Cr and the like in order to ensure the strength and hardenability of steel, and causes the increase of raw material cost.

The patent with publication number CN102650021A discloses a preparation method of bainite pre-hardened steel, which adopts a low-alloying chemical composition formula, ensures the strength of the alloy by improving the content of Mn elements, improves the hardenability of the alloy by reducing the content of Cr and Mo elements, and avoids hot brittleness caused by more MnS formation, the technical scheme of the invention requires strict control of the S content of molten steel, thereby increasing the smelting cost; in addition, the reduction of Cr and Mo elements easily causes high-temperature temper brittleness of the steel, and influences subsequent processing of the steel.

Patent publication No. CN105568173A discloses a technical proposal of high-toughness heat-resistant steel, which improves the toughness by refining crystal grains through double vacuum melting and large forging ratio forming, but the room temperature impact toughness of the product of the technical proposal is only about 60J.

Compared with the prior art, the invention adopts the component design scheme of the medium-low carbon low alloy, and the preparation process of preheating treatment, rolling, quenching and twice tempering is adopted, so that the strength, the toughness, the welding performance and the fatigue resistance of the steel are well matched, and the high-performance alloy steel is obtained.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides an easily-welded fatigue-resistant high-strength and high-toughness steel and a preparation method thereof, so that the alloy steel with lower carbon equivalent has good toughness, weldability and fatigue resistance on the premise of ensuring ultrahigh strength.

One purpose of the invention is to overcome the defects of the prior art and provide a component design scheme of a high-strength and high-toughness steel material which is easy to weld and fatigue resistant

The invention relates to an easy-welding fatigue-resistant high-strength and high-toughness steel, which comprises the following chemical components in percentage by mass: wherein the content of C is 0.30-0.40%, the content of Si is 0.35-0.50%, the content of Cr is 1.00-1.50%, the content of Mn is 0.80-0.95%, the content of Mo is 0.50-0.65%, the content of W is 1.00-1.35%, the content of V is 0.05-0.l 5%, the content of Cu is 0.05-0.10%, the content of Ni is 0.05-0.20%, the content of Al is 0.01-0.02%, the content of Ca is 0.0015-0.0050%, P is less than 0.01%, S is less than 0.003%, and the balance is Fe and other unavoidable impurities.

The content of the C element in the invention is within the range of 0.30-0.40 wt%, specifically, 0.30 wt%, 0.31 wt%, 0.32 wt%, 0.33 wt%, 0.34 wt%, 0.35 wt%, 0.36 wt%, 0.37 wt%, 0.38 wt%, 0.39 wt%, 0.40 wt%; when the content of C is 0.30 wt% or more, high hardenability and sufficient strength of the steel material can be secured, but when the content of C is more than 0.40 wt%, it causes significant deterioration in plasticity and toughness of the steel material, and also affects weldability and hydrogen crack resistance of the steel material.

The content of Si element in the invention is in the range of 0.35-0.50 wt%, specifically, 0.35 wt%, 0.36 wt%, 0.37 wt%, 0.38 wt%, 0.39 wt%, 0.40 wt%, 0.41 wt%, 0.42 wt%, 0.43 wt%, 0.44 wt%, 0.45 wt%, 0.46 wt%, 0.47 wt%, 0.48 wt%, 0.49 wt%, 0.50 wt%; when the Si content is 0.35 wt% or more, hardenability and tempering resistance of the steel can be ensured, and the strength of the steel can be improved, but when the Si content is more than 0.50 wt%, cold embrittlement is easily caused, and the welding quality of the steel is seriously affected.

The content of Mn element in the invention is in the range of 0.80-0.95 wt%, specifically, 0.80 wt%, 0.81 wt%, 0.82 wt%, 0.83 wt%, 0.84 wt%, 0.85 wt%, 0.86 wt%, 0.87 wt%, 0.88 wt%, 0.89 wt%, 0.90 wt%, 0.91 wt%; 0.92 wt%, 0.93 wt%, 0.94 wt%, 0.95 wt%, when the content of Mn is 0.80 wt% or more, the solid solution strengthening effect of Mn can be sufficiently exerted to increase the proportion of martensite in the quenched steel, but when the content of Mn is more than 0.95 wt%, both the carbon equivalent and the cold crack susceptibility index of the steel are increased, which is disadvantageous to the welding of the steel.

The content of Cr element in the invention is in the range of 1.00-1.50 wt%, specifically, 1.00 wt%, 1.10 wt%, 1.20 wt%, 1.30 wt%, 1.40 wt% and 1.50 wt%; when the content of Cr is 1.00 wt% or more, hardenability of steel can be secured and high-temperature oxidation resistance is facilitated, but when the content of Cr is more than 1.50 wt%, coarse carbides are easily precipitated on grain boundaries, and toughness and weldability of steel are significantly reduced.

The content of Mo element in the invention is within the range of 0.50-0.65 wt%, specifically, such as 0.50 wt%, 0.55 wt%, 0.60 wt%, 0.65 wt%; when the content of Mo is more than or equal to 0.50 wt%, the high-temperature tempering brittleness of the steel can be reduced or inhibited, and the steel can be tempered at a higher temperature, so that the residual stress is effectively eliminated, and the ductility and toughness are improved, but when the content of Mo is more than 0.65 wt%, the contribution to the improvement of the strength is small, and the resource waste is caused.

The content of the W element is within the range of 1.00-1.35 wt%, specifically, 1.00 wt%, 1.05 wt%, 1.10 wt%, 1.15 wt%, 1.20 wt%, 1.25 wt%, 1.30 wt%, 1.35 wt%; when the content of W is 1.00 wt% or more, bainite transformation is promoted and the heat resistance and creep resistance of the steel are enhanced, but when the content of W is more than 1.35 wt%, the toughness of the steel is remarkably decreased.

The content of the V element is within the range of 0.05-0.10 wt%, specifically, 0.05 wt%, 0.06 wt%, 0.07 wt%, 0.08 wt%, 0.09 wt%, 0.10 wt%; when the content of V is 0.05 wt% or more, there is a good precipitation strengthening effect, the tempering stability of the steel can be improved and there is a remarkable secondary hardening effect, but when the content of V exceeds 0.10 wt%, there is a negative influence on the welding property and toughness, and the impact toughness of the steel is remarkably reduced.

The content of the Ni element is within the range of 0.05-0.20 wt%, an austenite phase region can be enlarged, ferrite grains are refined, a matrix structure of the steel is easy to slide in a cross mode at a low temperature, and the toughness is greatly improved.

The content of the Cu element is within the range of 0.05-0.10 wt%, and the Cu can act with Ni in a synergistic manner, so that a passive film is formed on the surface of steel, the invasion of H is prevented, the corrosion resistance of alloy steel is improved, and the welding performance of steel is improved.

The content of Ca element is within the range of 0.0015-0.0050 wt%, and proper amount of Ca is added into molten steel for calcification treatment to spheroidize sulfide and improve the impact toughness of steel, but when the content of Ca is too high, nonmetallic inclusion harmful to the toughness of steel is easily generated.

The Al element content of the invention is in the range of 0.01-0.02 wt%, Al and O, N have strong affinity, and can play a positive deoxidation effect, and form AlN with N, so that the crystal grain can be refined, and the N fixation effect can be played.

In the invention, the content of S is less than 0.003 wt%, the content of P is less than 0.01 wt%, high toughness is obtained by strictly controlling impurity elements such as S, P and the like, the hot cracking and cold cracking tendency is reduced, and the fatigue-resistant welding joint is obtained.

The second purpose of the invention is to overcome the defects of the prior art and provide a preparation method of a high-strength and high-toughness steel which is easy to weld and fatigue resistant; the preparation method comprises the following steps: proportioning, smelting and refining according to the chemical components, carrying out rapid component analysis in front of the furnace, carrying out component adjustment according to the designed material component proportion, and then carrying out casting, preheating treatment, rolling and heat treatment.

The preheating treatment of the invention comprises incomplete austenitizing and isothermal annealing.

The incomplete austenitizing temperature is 810-850 ℃, the temperature is preferably 820-840 ℃, and the heat preservation time is 1-4h, preferably 2-3 h.

The isothermal annealing temperature is 710-750 ℃, preferably 720-740 ℃, and the heat preservation time is 2-5h, preferably 3-4 h; when the annealing temperature is lower than 710 ℃, pearlite which cannot be completely spheroidized presents a fine flake shape, so that the hardness of steel is higher, and the subsequent rolling treatment is not facilitated, when the annealing temperature is not higher than 750 ℃, fine-spherical cementite is uniformly distributed on ferrite, and the pearlite is in a point shape and a small sphere shape, so that the growth of crystal grains during subsequent quenching heating can be effectively inhibited.

The rolling is hot rolling, and comprises longitudinal rolling and transverse rolling; the initial rolling temperature of the longitudinal rolling is 1030-1120 ℃, the final rolling temperature is 980-1010 ℃, and the reduction rate is 30-40%; the initial rolling temperature of the transverse rolling is 940-. The heat treatment of the invention comprises quenching and twice tempering.

The quenching temperature is 860-890 ℃, the heat preservation time is 1-2 hours, when the austenitizing temperature is lower than 860 ℃, carbides are less dissolved, fine acicular and hidden acicular martensite structures are obtained after cooling, although the strength of steel is improved, the toughness and the welding performance are obviously reduced; the austenitizing temperature is not higher than 890 ℃, secondary cementite and alloy carbide in the annealed steel are dissolved in austenite, and a lath-shaped martensite structure is obtained after cooling, so that the toughness and the fatigue resistance of the steel can be greatly improved.

Furthermore, the quenching temperature is 870-890 ℃, the heat preservation time is 1-1.5h, the obtained structure consists of lath martensite and a small amount of retained austenite, and the toughness of the tempered steel are better matched.

The tempering temperature of the two-time tempering is 540-580 ℃, and the tempering time is 1-4 h; when the tempering temperature is lower than 540 ℃, the original martensite grain boundary is obvious, the tempered martensite is distributed in a needle shape, and the toughness of the steel still has a space for improving, but when the tempering temperature is higher than 580 ℃, the recrystallized alpha ferrite obviously grows, the carbide particles quickly gather and grow, the structure boundary is fuzzy, and the strength and the fatigue resistance of the steel can be obviously reduced.

Furthermore, the tempering temperature is 550-570 ℃, the tempering time is 2-3 hours, the tempered structure is tempered sorbite and fine and dispersedly distributed granular carbide, the orientation and fine lath shape of quenched martensite are kept to a great extent, an obvious cellular structure appears, and the steel has good strength and toughness and fatigue resistance.

The technical scheme of the invention has the following beneficial effects:

1. the invention can prevent the cracking of the rolled steel during rolling, promote the formation of a large number of crystal cores, greatly reduce the possible defects of chain carbides and the like after conventional annealing and ensure that the carbides become fine and uniform, thereby further improving the comprehensive performance of the steel.

2. Through the twice tempering, the toughness and the welding performance of the steel can be improved on the premise of low strength loss, and the best matching of the strength and the toughness is realized.

3. The invention improves the comprehensive performance of the ultrahigh-strength steel through the optimized component formula and the heat treatment process, saves energy, reduces consumption and cost, and thus has better economic benefit.

Drawings

FIG. 1 is a schematic view of a heat treatment process in example 13;

FIG. 2 is an SEM photograph of example 13 after preheating treatment and quenching at 880 ℃;

FIG. 3 is an SEM photograph of example 13 after two tempers.

Detailed Description

In order to make the technical problems, technical solutions and technical effects to be solved by the present invention clearer, the following detailed description is made with reference to the accompanying drawings and examples, which are only for the purpose of understanding the present invention and should not be construed as specifically limiting the present invention.

Aiming at the existing problems, the invention provides the easily-welded and fatigue-resistant high-strength and high-toughness alloy steel and the preparation method thereof, and the steel with matched strength and toughness, easy welding and fatigue resistance is obtained by adopting the chemical component mixture ratio with lower raw material cost and through preheating treatment and proper quenching and tempering process as shown in figure 1.

Table 1 shows the mass percentages of the main alloying elements in the examples and comparative examples of this embodiment, comparative example 12 has no Ni added, and the other elements in the examples and comparative examples have the same contents, wherein Ca is 0.0025 wt%, Cu is 0.07 wt%, Ni is 0.15 wt%, Al is 0.015 wt%, and the balance is Fe and inevitable impurities, where P is <0.01 wt%, and S is <0.003 wt%.

TABLE 1 Mass percents of main alloying elements of examples and comparative examples

The method comprises the steps of proportioning according to a determined chemical composition ratio, melting, dephosphorizing, performing external refining, stirring by argon gas, casting into a billet, performing preheating treatment, performing hot rolling, and performing heat treatment. Table 2 lists the heat treatment schemes of examples and comparative examples.

TABLE 2 Heat treatment protocol for examples and comparative examples

The yield strength, tensile strength and elongation were tested according to the standard GB/T228.1-2010 and the transverse impact work at room temperature was tested according to GB/T229-2007, the results are shown in Table 3.

TABLE 3 mechanical Properties of the steels of examples and comparative examples

As can be seen from Table 3, the yield strengths of the steel materials in the examples of the invention are all greater than 1050MPa, the tensile strengths are all greater than 1265MPa, the hardnesses are all greater than 37.4HRC, the elongations are all greater than 16.5%, and the room-temperature transverse impact powers are all greater than 183J. FIG. 2 is an SEM photograph of example 13 after preheating treatment and quenching at 880 ℃, and it can be seen that, after the preheating treatment of incomplete austenitizing, rolling and quenching are carried out, martensite laths are crossed with each other, and retained austenite is distributed between the martensite laths in block and strip shapes, so that the strength and hardness of the steel are improved; FIG. 3 is an SEM photograph of the steel of example 13 after two tempers, which shows that the long retained austenite is completely decomposed, a large amount of fine dispersed point-like and long-strip cementite is distributed at the austenite grain boundary and lath interface, and a large amount of fine dispersed carbide is distributed in the matrix structure, thereby improving the fatigue resistance of the steel.

The components which are not in the scope of the invention or the preheating treatment and the heat treatment process provided by the invention are not adopted, the mechanical property indexes of the steel are lower, the defects generated in the rolling and heat treatment processes of the steel are difficult to eliminate, and the fatigue resistance is poorer. For example, comparative example 1 has too high temperature for the preheating treatment, low strength and hardness of the steel, and poor fatigue resistance; in comparative example 3, the preheating temperature was too low, and the ductility and toughness were poor; in the comparative examples 10 and 11, only one tempering is adopted, the toughness of the steel is obviously lower, and the welding performance is poorer; comparative example 4, in which the content of C is 0.16 wt%, is lower than the lower limit of the content of C provided by the present invention, the strength is remarkably reduced; the comparative example 6, in which the content of Si is 0.58 wt%, exceeds the upper limit of the Si content provided by the present invention, the toughness is remarkably decreased.

While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. The preparation method of the high-strength and high-toughness alloy steel easy to weld and resistant to fatigue is characterized in that the alloy steel consists of the following chemical components in percentage by mass:

C：0.30-0.40％；

Si：0.35-0.50％；

Cr：1.00-1.50％；

Mn：0.80-0.95％；

Mo：0.50-0.65％；

W：1.00-1.35％；

V：0.05-0.15％；

Cu：0.05-0.10％；

Ni：0.05-0.20％；

Al：0.01-0.02％；

Ca：0.0015-0.0050％；

P＜0.01％，S＜0.003％；

the balance of Fe and inevitable impurities;

the preparation method comprises the following steps: batching, primary smelting and refining according to the chemical components, performing rapid stokehole component analysis and component adjustment, and then performing casting, preheating treatment, rolling and heat treatment;

the preheating treatment comprises incomplete austenitizing and isothermal annealing; the heating temperature for incomplete austenitizing is 810-850 ℃, and the heat preservation time is 1-4 h; the annealing temperature of the isothermal annealing is 710-750 ℃, and the heat preservation time is 2-5 h;

the heat treatment comprises quenching and twice tempering, wherein the quenching is heat preservation at 860-890 ℃ for 1-2h and then water quenching; the tempering temperature and the tempering time of the two tempering steps are the same, the temperature is kept for 1-4h at 580 ℃ in 540-fold mode, and the rolling step is hot rolling and comprises longitudinal rolling and transverse rolling.

2. The method as claimed in claim 1, wherein the non-complete austenitizing heating temperature is 820-840 ℃ and the holding time is 2-3 h.

3. The method as claimed in claim 1, wherein the annealing temperature of the isothermal annealing is 720-740 ℃ and the holding time is 3-4 h.

4. The method as claimed in claim 1, wherein the rolling temperature of the longitudinal rolling is 1030-1120 ℃, the finishing temperature is 980-1010 ℃ and the reduction rate is 30-40%.

5. The preparation method as claimed in claim 1, wherein the rolling temperature of the transverse rolling is 940-.

6. The method as claimed in claim 1, wherein the quenching is carried out by heat preservation at 870-890 ℃ for 1-1.5h and then water quenching.

7. The method as claimed in claim 1, wherein the two tempers are both maintained at 550-570 ℃ for 2-3 h.