CN112695241A - 12.9-grade anti-delayed fracture weather-resistant bolt steel and preparation method thereof - Google Patents

Abstract

The invention discloses 12.9-grade anti-delayed fracture weather-resistant bolt steel and a preparation method thereof, wherein the steel comprises the following components in percentage by mass: 0.35-0.45%, Si: 0.10 to 0.40%, Mn: 0.50-0.90%, P is less than or equal to 0.012%, S is less than or equal to 0.005%, Cr: 0.80-1.20%, Cu: 0.20 to 0.50%, Ni: 0.80-1.20%, V + Nb + Ti: 0.10 to 0.50%, Mo: 0.10 to 0.50%, Alt: 0.015 to 0.045%, and the balance of Fe and inevitable impurities. By designing the components and the preparation process parameters, the steel has the strength reaching level 12.9 and has excellent atmospheric corrosion resistance and delayed fracture resistance.

Description

12.9-grade anti-delayed fracture weather-resistant bolt steel and preparation method thereof

Technical Field

The invention relates to bolted steel and a preparation method thereof, in particular to 12.9-grade anti-delayed fracture weather-resistant bolted steel and a preparation method thereof.

Background

In the manufacturing and installation process of steel structure bridges and buildings, the high-strength bolts have the technical characteristics of simple construction process, convenience in mounting and dismounting, large bearing capacity, difficulty in loosening and the like due to the connection form, are widely applied to the bridges and the buildings, and the using amount of a single bridge or a single building reaches hundreds of thousands to millions of sets, so that the design of the high-strength bolts, the production of raw materials and the manufacturing of high-strength bolt products are greatly emphasized in the world.

The highway bridge at home and abroad adopts 8.8-grade bolts at most, and the railway bridge adopts 10.9-grade bolts at most. Class 11.9 and 13.9 bridge bolts have been used in japan in the last century but have been abandoned because of delayed fracture. Bolts of grade 12.9 and above are applied to engines at present, but reports of application to bridges are not found. Some weather-resistant bolts are reported in China, but all the weather-resistant bolts are below 10.9 grades. Along with the improvement of bridge load and span and the lightweight development of steel structure buildings, the requirements of higher design stress and lightweight are also provided for bolts, the most effective measure is further high reinforcement of the bolts, the application of 12.9-grade delayed fracture resistant and weather resistant bolts is the inevitable trend of the development of bridge engineering and building engineering, and the method has important significance for the development of the bridge and building industries.

The 12.9-level high-strength bolt is an extremely important stressed structure in the service process of a steel structure, but is in service in a natural environment for a long time, and serious corrosion phenomena including uniform corrosion, crevice corrosion and galvanic corrosion can occur, the shape and the size of the high-strength bolt can be changed, the stress is further changed, hidden dangers are brought to the service of a bridge, and therefore the problem of atmospheric corrosion resistance needs to be solved urgently. Another bottleneck in the application of 12.9 grade high strength bolts to steel structures is the delayed fracture problem. The delayed fracture of the high-strength bolt is mainly caused by hydrogen existing in raw materials, hydrogen entering in the bolt manufacturing process and hydrogen invading in the bolt service process, and meanwhile, the bolt is a notch part and has high notch sensitivity, so that sudden brittle fracture is easily generated at the root of a thread or the transition part of a screw and a head, and serious potential safety hazard is brought to safety. At present, because no industrialized dehydrogenation standard and detection standard exist, the development of the high-strength bolt for resisting delayed fracture at home and abroad is mainly concentrated in the field of material research, and researches show that a proper amount of microalloying elements V, Ti, Nb and the like are added to form fine carbonitride, the fine carbonitride can be used as a hydrogen trap to inhibit hydrogen diffusion and make hydrogen distribution uniform besides refining austenite grains before promotion, the composition is designed to be applied to 10.9-grade bolt steel for resisting delayed fracture, but along with the requirement of lightweight bridge, the bolt strength is improved, the delayed fracture risk is increased, and further series researches need to be carried out on the material aspect to improve the delayed fracture resistance of the high-strength bolt steel.

Disclosure of Invention

The purpose of the invention is as follows: in order to overcome the defects of the prior art, the invention provides 12.9-grade anti-delayed fracture weathering bolted steel which has better atmospheric corrosion resistance and delayed fracture resistance, can be applied to bridges and replaces the existing 10.9-grade bolted steel for bridges.

The invention also aims to provide a preparation method of the 12.9-grade delayed fracture resistant and weather resistant bolt steel.

The technical scheme is as follows: the invention relates to 12.9-grade delayed fracture resistant and weather resistant bolt steel which comprises the following components in percentage by mass: 0.35-0.45%, Si: 0.10 to 0.40%, Mn: 0.50-0.90%, P is less than or equal to 0.012%, S is less than or equal to 0.005%, Cr: 0.80-1.20%, Cu: 0.20 to 0.50%, Ni: 0.80-1.20%, V + Nb + Ti: 0.10 to 0.50%, Mo: 0.10 to 0.50%, Alt: 0.015 to 0.045%, and the balance of Fe and inevitable impurities.

Specifically, the corrosion resistance index I of the steel is more than or equal to 6.5, wherein:

I＝26.01(％Cu)+3.88(％Ni)+1.20(％Cr)+1.49(％Si)+17.28(％P)-7.29(％Cu)(％Ni)-9.10(％Ni)(％P)-33.39(％Cu)²。

the component design mechanism is as follows:

aiming at the atmospheric corrosion resistance, on the basis of common low alloy steel, a proper amount of elements such as Cr, Ni, Cu and the like are added, and a dense and stable protective rust layer is formed on the surface of the steel, so that the penetration of ions such as H +, O2-, Cl-and the like formed by oxygen, water, salt and acid liquor in the atmospheric environment is prevented, the rust-stopping effect is exerted, and the atmospheric corrosion resistance is obviously improved.

For resistance to delayed fracture, the steel prevents grain boundary embrittlement by reducing the P, S content as much as possible to reduce grain boundary segregation, while reducing the Mn content which promotes P, S co-segregation; adding Al, Ti, Nb, V and other elements to generate dispersed and precipitated carbonitride to refine austenite grains, so that the strength is improved and the toughness is improved; elements with strong tempering softening resistance such as Mo, V and the like are added, so that the tempering temperature can be increased to spheroidize the carbide under the condition of keeping the strength unchanged, a tempering temperature area which is easy to cause grain boundary catalysis is avoided, and the carbide is fine and uniform; adjusting the contents of alloy elements such as Ni and Mn to obtain higher notch toughness; adding an alloy element such as Mo for inhibiting the generation of corrosion pits to reduce the amount of hydrogen invading the steel surface as much as possible; adding proper amount of micro alloying elements V, Ti, Nb and the like to form fine carbonitride, and besides refining austenite grains before quenching, the fine carbonitride can also be used as a hydrogen trap to inhibit hydrogen diffusion and make hydrogen distributed uniformly so as to make the invaded hydrogen harmless.

Corresponding to the 12.9-grade anti-delayed fracture weather-resistant bolt steel, the technical scheme adopted by the preparation method provided by the invention comprises the following steps:

(1) smelting is sequentially carried out by an electric furnace, LF refining and vacuum smelting, wherein the hydrogen (H) content after vacuum treatment is less than or equal to 2.0 ppm;

(2) a rolling procedure: the heating temperature of the soaking section is 990-1100 ℃, the total heating time is more than or equal to 120min, and the bar mill rolls the bar into round steel with the specification of 20-36 mm;

(3) bolt quenching and tempering heat treatment process: 860 and 930 ℃ oil quenching and 580 and 650 ℃ tempering heat treatment.

Has the advantages that: compared with the prior art, the bolt steel realizes that the strength is improved to 12.9 grade through the design of components and preparation process parameters, the toughness performance is excellent, the V-port impact value at minus 20 ℃ reaches more than 32J, and the bolt steel has better low-temperature resistance. Meanwhile, excellent atmospheric corrosion resistance and delayed fracture resistance are obtained, the atmospheric corrosion resistance is improved by 21% compared with that of common 42CrMo bolt steel, and the excellent performance is also shown in a delayed fracture resistance test.

Drawings

FIG. 1 is a graph of corrosion weight loss rate versus time under simulated industrial atmospheric environmental conditions;

FIG. 2 is a constant load notched tensile specimen TDS;

FIG. 3 is a schematic diagram of a constant load notch tensile test.

Detailed Description

The present invention will be described in further detail with reference to examples.

The composition of each example is shown in table 1, with the balance being Fe and impurities.

Table 1 ingredients of examples (wt.%)

The preparation method of each embodiment is as follows according to the parameter requirements of the invention:

in the embodiment 1, an electric furnace is adopted for smelting, LF refining and vacuum smelting are carried out, hydrogen [ H ] is determined to be 1.8ppm after vacuum treatment, and 150mm square continuous casting billets are produced through continuous casting;

heating temperature 990 and 1050 ℃ of a soaking section of the continuous casting billet, total heating time 130min, rolling the continuous casting billet into round steel with the specification of 22mm by a bar mill, and performing the same heat treatment process as bolt processing on a round steel sample: quenching temperature of 860 deg.C, holding for 40min, oil quenching, tempering temperature of 550 deg.C, holding for 80min, and water cooling.

In the embodiment 2, an electric furnace is adopted for smelting, LF refining and vacuum smelting are carried out, hydrogen [ H ] is determined to be 1.6ppm after vacuum treatment, and 150mm square continuous casting billets are produced through continuous casting;

the heating temperature of the soaking section of the continuous casting billet is 1010-1060 ℃, the total heating time is 135min, a bar mill rolls the continuous casting billet into round steel with the specification of 25mm, and the round steel sample is subjected to the heat treatment process which is the same as the bolt processing: quenching temperature is 890 ℃, heat preservation is 40min, oil quenching, tempering temperature is 580 ℃, heat preservation is 80min, and water cooling is carried out.

Example 3 adopts electric furnace smelting, LF refining and vacuum smelting, the hydrogen [ H ] is determined to be 1.5ppm after vacuum treatment, and 150mm square continuous casting billet is produced by continuous casting;

the heating temperature of the soaking section of the continuous casting billet is 1020 ℃ and 1070 ℃, the total heating time is 140min, a bar mill rolls the continuous casting billet into round steel with the specification of 30mm, and the round steel sample is subjected to the heat treatment process which is the same as the bolt processing: quenching at 920 deg.C, maintaining for 40min, oil quenching, tempering at 620 deg.C, maintaining for 80min, and water cooling.

Example 4 adopts electric furnace smelting, LF refining and vacuum smelting, the hydrogen [ H ] is determined to be 1.3ppm after vacuum treatment, and 150mm square continuous casting billet is produced by continuous casting;

the heating temperature of the soaking section of the continuous casting billet is 1040-: quenching temperature 930 deg.C, holding for 40min, oil quenching, tempering temperature 650 deg.C, holding for 80min, and water cooling.

The mechanical properties after heat treatment of the example specimens are shown in Table 2:

TABLE 2 mechanical Property parameters

The detection results all meet the mechanical property requirements of 12.9-grade high-strength bolt steel, the V-port impact values at the temperature of-20 ℃ are all more than or equal to 27J, and the material disclosed by the invention has excellent strength and toughness and low-temperature resistance.

Samples are taken from examples to prepare hanging pieces, and compared with the matching weather-proof plate Q420qENH and the hanging pieces made of the common 12.9-grade high-strength bolt steel 42CrMo, the periodic infiltration accelerated corrosion test simulating the industrial atmospheric environment is carried out by referring to TB/T2375-1993 periodic infiltration corrosion test method for the weather-proof steel for the railway. The relevant experimental parameters were as follows: the solution is 0.01mol/LNaHSO3, and the pH value is 4.4-4.8; the test temperature is 45 +/-2 ℃; the test humidity is 70 + -5% RH, the cycle period is 60 + -3 min, wherein the infiltration time is 12 + -1.5 min; the test was divided into 4 cycles of 24h (1 day), 48h (2 days), 96h (4 days), 168h (7 days), 216h (9 days), 432h (18 days), respectively. The results of the cycle immersion accelerated corrosion test in a simulated industrial atmospheric environment are shown in table 3.

TABLE 3

Referring to fig. 1, the experimental results show that: the rust layers of the 12.9-grade weathering bolted steel and the 42CrMo bolted steel have a certain protection effect on a matrix in a corrosion period, so that both the two steels have certain corrosion resistance, and the corrosion weight loss rate of the 12.9-grade weathering bolted steel is obviously lower than that of 42CrMo in a later corrosion period, so that the atmospheric corrosion resistance of the 12.9-grade weathering bolted steel is more excellent than that of common 42CrMo bolted steel, and the atmospheric corrosion resistance is improved by 21%. The weather-resistant corrosion rate curves of the 12.9-grade weather-resistant bolt steel and the parent metal are approximately overlapped, the weather resistance of the weather-resistant bolt steel and the parent metal is equivalent, and the matching degree of the materials is higher.

And (3) detecting the delayed fracture resistance:

the experimental method comprises the following steps: the 12.9-grade delayed fracture resistant weathering bolt steel after heat treatment of examples 1-4 was processed into standard TDS (as shown in FIG. 2), electrochemically charged with 0.1mol/L NaOH solution, and then measured for hydrogen content and hydrogen evolution rate curve by means of temperature-rising dehydrogenation. And determining a hydrogen charging process when the hydrogen content reaches 3ppm through a temperature rise dehydrogenation test, performing electrochemical hydrogen charging on a constant load sample by using the hydrogen charging process, and after the hydrogen charging is finished, performing a test of loading 0.9 times of tensile strength for 100 hours in a constant load testing machine (as shown in figure 3).

The four samples were subjected to a notch tensile test of 0.9 times tensile strength loading under the conditions of hydrogen charging of 3.85ppm, 3.98ppm, 4.13ppm and 4.07ppm, respectively, and no fracture occurred within 100 hours. This indicates that the grade 12.9 delayed fracture resistant weathering bolted steel of the present invention is excellent in delayed fracture resistance.

Claims (6)

1. 12.9-grade delayed fracture resistant and weather resistant bolt steel is characterized by comprising the following components in percentage by mass: 0.35-0.45%, Si: 0.10 to 0.40%, Mn: 0.50-0.90%, P is less than or equal to 0.012%, S is less than or equal to 0.005%, Cr: 0.80-1.20%, Cu: 0.20 to 0.50%, Ni: 0.80-1.20%, V + Nb + Ti: 0.10 to 0.50%, Mo: 0.10 to 0.50%, Alt: 0.015 to 0.045%, and the balance of Fe and inevitable impurities.

2. The 12.9-grade delayed fracture resistant weathering bolted steel of claim 1, characterized by a corrosion resistance index I ≥ 6.5, wherein:

I＝26.01(％Cu)+3.88(％Ni)+1.20(％Cr)+1.49(％Si)+17.28(％P)-7.29(％Cu)(％Ni)-9.10(％Ni)(％P)-33.39(％Cu)²。

3. the 12.9-grade delayed fracture resistant weathering bolted steel of claim 2, characterized in that the composition comprises, in mass percent, C: 0.38-0.42%, Si: 0.20 to 0.30%, Mn: 0.62-0.79%, P is less than or equal to 0.012%, S is less than or equal to 0.005%, Cr: 0.88 to 1.10%, Cu: 0.30-0.40%, Ni: 0.92-1.10%, V + Nb + Ti: 0.25 to 0.36%, Mo: 0.25 to 0.42%, Alt: 0.025-0.035%, and the balance of Fe and inevitable impurities.

4. The 12.9-grade delayed fracture resistant weathering bolted steel of claim 2, characterized in that the composition comprises, in mass percent, C: 0.35-0.38%, Si: 0.30-0.40%, Mn: 0.79-0.90%, P is less than or equal to 0.012%, S is less than or equal to 0.005%, Cr: 0.88 to 1.20%, Cu: 0.40 to 0.50%, Ni: 0.80-0.92%, V + Nb + Ti: 0.36 to 0.50%, Mo: 0.10 to 0.25%, Alt: 0.035 to 0.045%, and the balance of Fe and inevitable impurities.

5. The 12.9-grade delayed fracture resistant weathering bolted steel of claim 2, characterized in that the composition comprises, in mass percent, C: 0.42 to 0.45%, Si: 0.10 to 0.20%, Mn: 0.50-0.62%, P is less than or equal to 0.012%, S is less than or equal to 0.005%, Cr: 0.80 to 0.88%, Cu: 0.20 to 0.30%, Ni: 1.10-1.20%, V + Nb + Ti: 0.10 to 0.25%, Mo: 0.42 to 0.50%, Alt: 0.015 to 0.025%, and the balance of Fe and inevitable impurities.

6. A method for preparing 12.9-grade delayed fracture resistant weathering bolted steel according to any of claims 1-5, characterized by comprising the following steps:

(1) smelting is sequentially carried out by an electric furnace, LF refining and vacuum smelting, wherein the hydrogen (H) content after vacuum treatment is less than or equal to 2.0 ppm;

(2) a rolling procedure: the heating temperature of the soaking section is 990-1100 ℃, the total heating time is more than or equal to 120min, and the bar mill rolls the bar into round steel with the specification of 20-36 mm;

(3) bolt quenching and tempering heat treatment process: 860 and 930 ℃ oil quenching and 580 and 650 ℃ tempering heat treatment.

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