CN111621714B - Round steel for bolt with excellent corrosion resistance and delayed fracture resistance and production method thereof - Google Patents

Round steel for bolt with excellent corrosion resistance and delayed fracture resistance and production method thereof Download PDF

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CN111621714B
CN111621714B CN202010604975.0A CN202010604975A CN111621714B CN 111621714 B CN111621714 B CN 111621714B CN 202010604975 A CN202010604975 A CN 202010604975A CN 111621714 B CN111621714 B CN 111621714B
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round steel
delayed fracture
corrosion resistance
temperature
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CN111621714A (en
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尹德福
汪开忠
张晓瑞
龚梦强
胡芳忠
于同仁
丁雷
姜婷
牟祖茂
孙凯
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Maanshan Iron and Steel Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/10—Handling in a vacuum
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum

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Abstract

The invention discloses a round steel for a bolt with excellent corrosion resistance and delayed fracture resistance and a production method thereof, belonging to the technical field of steel for fasteners. The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance comprises the following chemical components in percentage by weight: 0.55-0.60% of C, 1.80-2.00% of Si, 0.10-0.20% of V, 0.05-0.15% of Nb0.25-0.35% of Mn, 1.05-1.35% of Cr1, 1.50-1.80% of Mo1.10-0.30% of Ni0.10-0.30% of Cu, 0.20-0.35% of: 0.015 to 0.040 percent of AlsP, less than or equal to 0.010 percent of P, less than or equal to 0.010 percent of S, less than or equal to 0.00015 percent of H, less than or equal to 0.0010 percent of O, less than or equal to 0.005 percent of N, and the balance of Fe and other inevitable impurities. The technical scheme of the invention can effectively solve the problems, has excellent corrosion resistance and delayed fracture resistance, and can meet the performance requirement of high-stress fasteners of automobile engines.

Description

Round steel for bolt with excellent corrosion resistance and delayed fracture resistance and production method thereof
Technical Field
The invention belongs to the technical field of steel for fasteners, and particularly relates to round steel for a bolt with excellent corrosion resistance and delayed fracture resistance and a production method thereof.
Background
The fastener is a basic industry of equipment manufacturing industry in China and is widely applied to various fields of national economy. In recent years, with the rapid development of various industries such as automobiles, wind power, machinery, buildings and the like in China, the requirements of design stress and light weight are provided for materials used for manufacturing parts such as various fasteners (such as bolts, nuts and the like), and the most effective measure is to improve the strength of the fasteners. The light weight of the automobile body is to improve the fuel efficiency of the automobile and reduce CO2One of the effective methods for emissions is that fasteners are used in a higher tensile load environment than ever due to the demand for smaller and lighter engine and chassis parts, and therefore, it is necessary to solve the technical problems of reduced fatigue strength due to surface defects or non-metallic inclusions in steel, and delayed fracture associated with increased atmospheric corrosion and hydrogen sensitivity.
The ultrahigh strength fastener is used as a fastener of a connecting part and other cold heading forming parts, the requirements of high design stress and light weight are provided, in this respect, the requirements of the automobile manufacturing industry are the strongest, and the original automobile fastener, especially an engine bolt, cannot meet the requirement of high stress of an automobile engine easily. However, since the environment for operating the automobile is complicated, the bolt may be corroded when used in various atmospheric environments, and in order to prolong the service life of the bolt, the high-strength bolt used for connecting the automobile is subjected to corrosion prevention in a coating manner, such as electroplating or hot galvanizing. However, the corrosion prevention time limit of the methods such as surface spraying is generally 5-10 years, the service life of the bolt can be reduced due to aging and differentiation of the coating, and the corrosion prevention effect is not ideal; meanwhile, the H element introduced in the processes of electroplating and hot galvanizing of the ultrahigh-strength bolt can also increase the problem of hydrogen-induced delayed fracture in the using process of the bolt, so that the ultrahigh-strength fastener for the automobile must have excellent corrosion resistance and also needs to have excellent delayed fracture resistance so as to meet the requirements of light weight and high safety of the automobile.
Through search, the patent of the steel for the bolt has related disclosures. For example, the chinese patent application No. 01129512.0 discloses a delayed fracture-resistant high-strength bolt steel having a strength of 1400 to 1600MPa, which comprises the following specific components: 0.35-0.5% of C, 0.01-0.09% of Si, less than or equal to 0.30% of Mn, less than or equal to 0.010% of P, less than or equal to 0.008% of S, 0.5-1.5% of Cr0.7-1.5% of Mo0.20-0.50% of V, 0.01-0.08% of Nb0.002-0.04% of RE, 0.005-0.05% of Al, 0.006-0.015% of N, 0.01-0.15% of any one or the sum of two of Ti and Zr, and the balance of Fe and inevitable impurities. The maximum tensile strength of the bolt is only 1580MPa, the service performance requirement of the high-strength bolt cannot be met, RE and the like are added into chemical components, and the production cost is obviously increased.
For another example, chinese patent application No. 200810049411.4 discloses a method for making a 16.9-grade bolt resistant to delayed fracture, the chemical composition range of the steel ingot should be controlled to be C0.15-0.19%, Mn not more than 0.10%, Si not more than 0.10%, S not more than 0.005%, P not more than 0.008%, S + P not more than 0.010%, cr 1.80-2.00%, ni 9.50-10.50%, co 13.50-14.50%, mo 0.90-0.10%, Ti not more than 0.015%, O not more than 0.0020%, N not more than 0.0015%. The steel for the bolt not only adopts the more complex production processes of vacuum induction melting, vacuum consumable melting, forging forming, composite heat treatment and the like, but also has higher content of noble elements such as Ni, Co and the like, so that the production cost is higher, and the steel is not suitable for industrial production.
Disclosure of Invention
1. Problems to be solved
The invention aims to overcome the defects that the corrosion prevention effect is not ideal when the existing high-strength bolt is subjected to corrosion prevention by adopting a coating mode, and the problem of delayed fracture caused by hydrogen in the using process of the bolt is increased, and provides round steel for the bolt with excellent corrosion resistance and delayed fracture resistance and a production method thereof. The technical scheme of the invention can effectively solve the problems, has excellent corrosion resistance and delayed fracture resistance, and can meet the performance requirement of high-stress fasteners of automobile engines.
2. Technical scheme
In order to solve the problems, the technical scheme adopted by the invention is as follows:
the round steel for the bolt with excellent corrosion resistance and delayed fracture resistance comprises the following chemical components in percentage by weight: 0.55 to 0.60 percent of C, 1.80 to 2.00 percent of Si, 0.10 to 0.20 percent of V, 0.05 to 0.15 percent of Nb0.05 to 0.15 percent of Mn, 0.25 to 0.35 percent of Cr, 1.05 to 1.35 percent of Cr, 1.50 to 1.80 percent of Mol, 0.10 to 0.30 percent of Ni0.20 to 0.35 percent of Cu0.20 to 0.35 percent of: 0.015 to 0.040 percent of AlsP, less than or equal to 0.010 percent of P, less than or equal to 0.010 percent of S, less than or equal to 0.00015 percent of H, less than or equal to 0.0010 percent of O, less than or equal to 0.005 percent of N, and the balance of Fe and other inevitable impurities.
Furthermore, the weight percentage of each chemical component in the round steel for the grade 16.9 containing titanium bolt satisfies that 0.55 is less than or equal to 0.25 [% Mo ] +0.20 [% V ] +0.10 [% Nb ] +0.08 [% Si ] < 0.65.
Further, the weight percentage of C and Si also satisfies 3.2 ≦ ([ Si ]/[ C ]) ≦ 3.35.
The invention relates to a production method of round steel for a bolt with excellent corrosion resistance and delayed fracture resistance, which comprises the following chemical components: electric furnace smelting → LF + RH vacuum refining → large square billet continuous casting → heating → initial rolling and cogging → six continuous rolling → rolling square billet → flaw detection, coping → excellent rod heating furnace heating → controlled rolling and controlled cooling → round steel finished product → packaging and warehousing.
Furthermore, when the electric furnace is smelted, the steel tapping is deoxidized and alloyed, and the slag discharging in the steel tapping process is controlled.
Furthermore, the pure degassing time of the RH furnace during vacuum degassing is not less than 20 minutes, and the [ H ] is not more than 0.00015% after vacuum treatment.
Furthermore, the heating temperature of the excellent rod heating furnace is 950-1000 ℃ during heating, and the furnace time of the square billet is not more than 170 min.
Furthermore, the process parameters of rolling control and cooling control are as follows: the initial rolling temperature is 860-920 ℃, and the final rolling temperature is 780-840 ℃.
Furthermore, the round steel needs to be subjected to performance inspection, and the heat treatment process during inspection comprises quenching heating treatment and high-temperature tempering treatment, wherein the heating temperature during quenching heating is 900-960 ℃, and the tempering temperature during high-temperature tempering is 580-600 ℃.
3. Advantageous effects
Compared with the prior art, the invention has the beneficial effects that:
(1) according to the round steel for the bolt with excellent corrosion resistance and delayed fracture resistance, the chemical components and the content of the round steel are optimally designed, microalloying elements such as Nb, V and Mo are added, and the comprehensive coordination effect of the elements is utilized, so that the atmospheric corrosion resistance and delayed fracture resistance of the obtained round steel are effectively improved, the problem caused by the adoption of a coating for corrosion prevention of the existing bolt is avoided, the strength grade of the round steel is ensured, and the requirement of high stress fastener performance of an automobile engine can be met.
(2) According to the round steel for the bolt with excellent corrosion resistance and delayed fracture resistance, the contents of Nb, V, Mo and Si are further limited, and the compounding and coordination effects of all elements are utilized, so that the hydrogen embrittlement resistance is improved, and the delayed fracture resistance of the round steel is further improved; the added Mo also obviously improves the tempering softening resistance, and realizes the ultrahigh strength at high temperature; the added Mo can also reduce the amount of hydrogen invading the steel surface, inhibit the generation of corrosion pits and ensure the atmospheric corrosion resistance.
(3) According to the round steel for the bolt with excellent corrosion resistance and delayed fracture resistance, the content of C, Si elements is further limited on the basis of limiting the content of Nb, V, Mo and Si, the Si content is greatly increased, and the relative content of C, Si is controlled, so that the deterioration of delayed fracture resistance of the round steel can be avoided, and the delayed fracture resistance and the atmospheric corrosion resistance of the round steel are further ensured. And the corrosion resistance of the round steel is further ensured by adding elements such as Ni, Cu and the like.
(4) According to the production method of the round steel for the bolt with excellent corrosion resistance and delayed fracture resistance, disclosed by the invention, the chemical components, the content and the process parameters in the production process are optimally designed, the comprehensive action of each element is utilized, and the specific process parameters are controlled, so that the corrosion resistance and the delayed fracture resistance are further ensured, the mechanical properties such as the strength of the round steel can meet the requirements of light weight and high safety of an automobile, and the energy can be saved and the cost can be reduced by adopting the production method of the round steel for the ultrahigh-strength bolt.
Drawings
FIG. 1 shows the chemical composition and austenite grain size (wt%) of examples 1 to 5 and comparative examples 1 to 2 of the present invention.
FIG. 2 is a graph showing mechanical properties, delayed fracture resistance and corrosion resistance of examples 1 to 5 of the present invention and comparative examples 1 to 2.
FIG. 3 is a diagram of notched tensile delayed fracture specimens in accordance with the present invention.
Detailed Description
In order to solve the problems that when the high-strength bolt for the automobile is subjected to corrosion prevention in an electroplating or hot galvanizing coating mode, the corrosion prevention effect is not ideal, and hydrogen caused delayed fracture in the using process of the bolt can be increased by H elements introduced in the electroplating and hot galvanizing processes of the ultrahigh-strength bolt, the invention provides round steel for the bolt with excellent corrosion resistance and delayed fracture resistance, wherein the round steel for the bolt with excellent corrosion resistance and delayed fracture resistance comprises the following chemical components in percentage by weight: 0.55 to 0.60 percent of C, 1.80 to 2.00 percent of Si, 0.10 to 0.20 percent of V, 0.05 to 0.15 percent of Nb0.05 to 0.15 percent of Mn, 0.25 to 0.35 percent of Cr1.05 to 1.35 percent of Mo, 1.50 to 1.80 percent of Ni, 0.10 to 0.30 percent of Ni, 0.20 to 0.35 percent of Cu, 0.015 to 0.040 percent of Als0.015 to 0.040 percent of P, less than or equal to 0.010 percent of S, less than or equal to 0.00015 percent of H, less than or equal to 0.0010 percent of O, less than or equal to 0.005 percent of N, and the balance of Fe and other inevitable impurities. And the weight percentage satisfies:
①、0.55≤0.25[％Mo]+0.20[％V]+0.10[％Nb]+0.08[％Si]≤0.65；
②、3.2≤([Si]/[C])≤3.35。
the round steel for the bolt with excellent corrosion resistance and delayed fracture resistance does not need to be subjected to a cold heading process, so that the influence of high and low Si content on cold heading and cold extrusion is not needed to be considered. In order to ensure high strength performance, the main chemical components and contents of the round steel for the bolt are optimally designed, micro-alloying elements such as Nb, V, Mo and the like are added, and the common coordination action of the elements is utilized, so that the atmospheric corrosion resistance and delayed fracture resistance of the obtained round steel are effectively improved, the problem caused by the adoption of a coating for corrosion prevention of the existing bolt is avoided, and the use requirement of a high-strength fastener can be met.
The invention further limits the content ranges of Nb, V, Mo and Si by adopting a formula, thereby ensuring that fine carbides can be precipitated; when the value of the formula (i) is less than 0.55, although the contents of the individual elements of V, Mo, Nb and Si may be all within the above optimum range, the delayed fracture resistance of the fastener cannot be ensured; when the formula (i) is more than 0.65, the delayed fracture resistance tends to be saturated, the cost of the steel is increased, and the delayed fracture resistance of the round steel is further improved by utilizing the compounding and coordination effect of each element.
Meanwhile, the invention further limits the contents of Nb, V, Mo and Si by adopting a formula II, further limits the content of C, Si elements by adopting a formula II, slightly increases the content of C on the basis of SCM435, and greatly increases the content of Si so as to reduce the corrosion resistance deterioration caused by the increase of the content of C, thereby increasing the surface of steel and the amount of hydrogen invading into the steel and avoiding the deterioration of delayed fracture resistance; by limiting the weight percentage of C, Si elements, Si which has no influence on the corrosion resistance of steel is used for ensuring that the strength (namely [ Si ]/[ C ] in a formula) is between 3.2 and 3.35), and high strength is ensured, and meanwhile, excellent corrosion resistance and delayed fracture resistance are also obtained. When Nb and V are replaced with Ti and the amount of Ti added is controlled, the reduction of area and the austenite grain size can be more effectively increased and the mechanical properties of a part of the steel can be effectively improved, compared with the case where V, Nb or other elements are added.
In addition, the added Mo element can also reduce the amount of hydrogen invading into the steel surface and inhibit the generation of corrosion pits, and the atmospheric corrosion resistance can be ensured by adding proper Mo under the condition of meeting the formula I; and the added elements such as Ni, Cu and the like not only improve the corrosion resistance, but also inhibit the generation and absorption of hydrogen in the corrosion environment, and simultaneously obtain higher notch toughness. In order to ensure the corrosion resistance, the chemical components need to meet the requirement that the atmospheric corrosion resistance index is more than or equal to 9.0, so that the corrosion resistance is excellent.
It is to be noted that C is an element required for ensuring hardenability and strength, and as the content thereof increases, ductility and corrosion resistance thereof decrease. A relatively high C content is advantageous for the strength and the like of the steel, but is extremely disadvantageous for the cold heading property, plasticity and toughness of the steel, and also causes a reduction in yield ratio, an increase in decarburization sensitivity, and deterioration in fatigue resistance and workability of the steel. Si is a main deoxidizing element in steel, has a strong solid solution strengthening effect, can also improve the stability of a rust layer, and effectively improves the corrosion resistance. V can refine the crystal grains of the steel and improve the strength and delayed fracture resistance by carbides precipitated at the time of tempering; in addition, the V element can also improve the tempering resistance of the steel, vanadium carbonitride precipitated during tempering at higher temperature can generate secondary hardening to further improve the strength of the steel, and the vanadium carbonitride has stronger trap energy and can trap hydrogen to uniformly disperse the hydrogen in crystal, thereby inhibiting the diffusion of the hydrogen and the segregation of crystal boundary, and further improving the delayed fracture resistance of the steel. The Nb element can also finely refine grains, and the grain refinement not only can improve the obdurability of steel, but also can improve the low-temperature performance of the steel, and simultaneously improve the tempering resistance and can also improve the delayed fracture resistance of high-strength steel. The hydrogen trap binding energy of the carbide is lower than that of vanadium carbide and titanium carbide, and the trapping effect on hydrogen is slightly poor.
Mn is an element that improves hardenability and is useful for achieving high strength performance, but Mn and P tend to be strongly intergranular co-segregated during high-temperature tempering of quenched steel, thereby promoting temper brittleness, and excessively high Mn content deteriorates the weather resistance of steel, promotes segregation to grain boundaries, lowers grain boundary strength, and thus tends to cause delayed fracture. Cr is the most commonly used alloy element in high-strength bolt steel, and Cr canThe hardenability and the tempering resistance of the steel can be effectively improved to obtain the required high strength, meanwhile, the weather resistance of the steel can be obviously improved by the composite addition of Cr and Cu, the effect is difficult to achieve when the Cr content is small, but the toughness and the cold workability of the steel are deteriorated when the Cr content is large. Mo element is a more commonly used alloy element in high-strength bolt steel, so that the tempering resistance of the steel can be obviously improved, and carbide Mo formed by the Mo element2C has a trapping effect on hydrogen, and can improve the delayed fracture resistance of the steel; the segregation of Mo element in the prior austenite grain boundary can also improve the grain boundary bonding strength of steel; in addition, Mo also reduces the amount of hydrogen entering the steel surface, suppresses the formation of corrosion pits, and improves corrosion resistance. S, P, the delayed fracture resistance is greatly reduced, the P element can form micro segregation when the molten steel is solidified, and then the P element is deviated at the grain boundary when the austenitizing temperature is heated, so that the brittleness of the steel is obviously increased, and the delayed fracture sensitivity of the steel is increased; the S element forms MnS inclusion to be segregated in the grain boundary, thereby increasing the delayed fracture sensitivity of the steel, and the grain boundary strengthening can be realized by reducing Mn which promotes the grain boundary segregation while reducing the impurity P, S as much as possible.
The addition of the Ni element can improve the structure of a rust layer, improve the compactness and the cohesiveness to the surface of steel, improve the corrosion resistance of the steel, inhibit the adsorption of hydrogen and further be beneficial to improving the delayed fracture resistance. The addition of Cu is an element effective for improving corrosion resistance and inhibiting hydrogen intrusion, and further improving delayed fracture resistance, and can form a rust layer having good protection, and the copper element can also change the hygroscopicity of the rust layer, thereby increasing critical humidity, but too high Cu content can reduce high temperature plasticity of steel, easily cause cracks during hot working, and become a cause of cost increase. H is the most harmful element in steel, has extremely low solubility in solid steel, is melted into molten steel at high temperature, cannot escape in time during cooling, accumulates in tissues to form high-pressure fine pores, and is easy to generate hydrogen embrittlement and delay fracture in the later use process particularly for ultrahigh-strength fasteners. O forms various oxide inclusions in steel, and stress concentration is easily generated at the oxide inclusions under the action of stress, so that microcracks are causedThereby deteriorating the mechanical properties of the steel, particularly the toughness and fatigue resistance; therefore, in metallurgical production, appropriate measures are taken to reduce the content as much as possible. N precipitation of Fe in steel4N, the diffusion rate is slow, resulting in the steel being time-efficient, and N also reduces the cold workability of the steel.
The invention relates to a production method of round steel for a bolt with excellent corrosion resistance and delayed fracture resistance, which comprises the following chemical components: electric furnace smelting → LF + RH vacuum refining → large square billet continuous casting → heating → initial rolling and cogging → six continuous rolling → rolling square billet → flaw detection, coping → excellent rod heating furnace heating → controlled rolling and controlled cooling → round steel finished product → packaging and warehousing.
During electric furnace smelting, steel tapping is deoxidized and alloyed, and slag discharging in the steel tapping process is strictly controlled; during refining in the LF furnace, adjusting elements such as C, Si, Cr, Mn, Mo, V, Nb, Ni, Cu and the like to target values; when the RH furnace is used for vacuum degassing, the pure degassing time is more than or equal to 20 minutes, and H is less than or equal to 0.00015% after vacuum treatment; when the bloom is continuously cast, the target temperature of the molten steel of the tundish is stably controlled to be 10-30 ℃ above the liquidus temperature, and the bloom with the size of 380mm multiplied by 450mm is continuously cast.
Round steel rolling route: 380mm multiplied by 450mm large square blank → heating → rolling is 250mm multiplied by 250mm square blank → flaw detection, coping → excellent rod heating furnace heating → KOCKS controlled rolling and controlled cooling rolling → cooling bed → phi 18-30 mm hot rolled round steel finished product, preferably 20-24 mm hot rolled round steel finished product. Wherein the heating temperature of the excellent rod heating furnace is 950-1000 ℃ during heating, and is preferably 960-980 ℃; the time of the billet in the furnace is not more than 170min, preferably 140-160 min. During the rolling process of controlled rolling and controlled cooling of the round steel, the initial rolling temperature of the round steel is 860-920 ℃, and preferably 890-910 ℃; the finishing temperature is 780-840 ℃, and preferably 800-820 ℃. The round steel is required to be subjected to performance inspection, the heat treatment process during inspection comprises quenching heating treatment and high-temperature tempering treatment, and the heating temperature is 900-960 ℃ during quenching heating, preferably 910-930 ℃; the tempering temperature is 580-600 ℃ during high-temperature tempering.
The round steel can also be subjected to grain size inspection: quenching the steel at 840-880 ℃, then cooling with water, wherein the temperature of a quenching medium is 20-40 ℃, and cooling to obtain a metallographic sample with austenite grain size of more than or equal to 8.0 grade.
In the heat treatment process during mechanical property inspection, in order to fully dissolve alloy elements such as Nb, V, Mo and the like into austenite, the quenching heating temperature cannot be too low; but at the same time, the overhigh temperature easily causes the excessive growth of austenite grains and the easy decarburization, which causes the poor toughness and the poor delayed fracture resistance of the fastener, therefore, the heating temperature is set to 900-960 ℃, preferably 910-930 ℃ during quenching heating. The precipitation strengthening characteristics of carbides formed by Nb, V, Mo and other alloy elements can be shown at the temperature of more than 500 ℃, but the excessive high temperature can cause the excessive growth of the carbides, which is not beneficial to precipitation strengthening; meanwhile, the high-temperature tempering heat treatment can finely disperse carbides in prior austenite grains and improve the delayed fracture characteristic of the fastener, so the tempering temperature is preferably 580-600 ℃ during high-temperature tempering.
The invention optimally designs the chemical components, the content and the specific process parameters in the production process, utilizes the comprehensive action of each element to control the specific process parameters of the steps of heating, controlled rolling, controlled cooling and the like of the excellent rod heating furnace, and controls the quenching and high-temperature tempering temperatures during heat treatment in the inspection process, thereby further ensuring the corrosion resistance and the delayed fracture resistance of the round steel, and ensuring that the mechanical properties such as the strength and the like of the round steel can meet the requirements of light weight and high safety of automobiles. The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance, which is produced by the invention, has the tensile strength R after heat treatmentmMore than or equal to 1600MPa and the yield ratio RP0.2/RmMore than or equal to 0.9, the elongation A after fracture is more than or equal to 10 percent, the reduction of area Z is more than or equal to 45 percent, the structure is tempered sorbite and fine carbide which is evenly dispersed, the content of the tempered sorbite is more than or equal to 93 percent, the austenite grain size is more than or equal to 8.0 grade, the slow strain rate tensile test (SSRT) evaluation is adopted at room temperature, and the delayed fracture strength ratio R is R ═ Rch/Rch0Greater than 0.55 (notched tensile strength R of the hydrogen-charged specimen)chThe notched tensile strength of the non-hydrogenated sample is Rch0) The notch sensitivity was evaluated by using a notch intensity ratio, NSR ═ σc/RmGreater than 1 (tensile strength of smooth specimen R)mNotched tensile Strength of σc) The atmospheric corrosion resistance index I of the round steel is more than or equal to 9.0.
The round steel for the bolt produced by adopting the components and the process flow is subjected to performance test, and the specific test process and test results are as follows:
tensile, impact and notched tensile delayed fracture performance test: quenching the steel, wherein the temperature of the steel is 900-960 ℃ during quenching and heating, preferably 910-930 ℃, and then carrying out heat preservation for 30min and then carrying out oil cooling; and then tempering treatment is carried out, wherein the tempering temperature is 580-600 ℃, heat preservation is carried out for 120min, and the standard tensile sample with phi 10mm and the notch tensile delayed fracture sample with phi 5mm are processed through an air cooling heat treatment process, as shown in figure 3. The mechanical properties after quenching and tempering heat treatment are shown in fig. 2.
Atmospheric corrosion resistance test: the corrosion resistance was evaluated from the corrosion loss before and after immersion of the bolt in a 15% HCl aqueous solution for 40 minutes. The test results are shown in FIG. 2.
The invention is further described with reference to specific examples.
Example 1
The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance of the embodiment adopts the production process of the invention, and the process parameters are as follows: the pure degassing time is 20 minutes when the RH furnace is subjected to vacuum degassing; the target temperature of the tundish molten steel is stably controlled to be 15 ℃ above the liquidus temperature during bloom continuous casting; heating in a high-quality bar heating furnace at 950 ℃ for 140 min; the initial rolling temperature of the round steel is 910 ℃, and the final rolling temperature is 780 ℃. When the mechanical property of the round steel is tested, the heating temperature of the round steel is 900 ℃ when the round steel is quenched and heated in the heat treatment process, and the tempering temperature of the round steel is 580 ℃ when the round steel is tempered at high temperature.
The chemical compositions and contents used in this example are shown in FIG. 1, and the results of tensile, impact and notched tensile delayed fracture property tests and atmospheric corrosion resistance tests are shown in FIG. 2.
Example 2
The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance of the embodiment adopts the production process of the invention, and the process parameters are as follows: the pure degassing time is 20 minutes when the RH furnace is subjected to vacuum degassing; the target temperature of the tundish molten steel is stably controlled to be 20 ℃ above the liquidus temperature during bloom continuous casting; the heating temperature of the heating furnace is 1000 ℃ when the heating furnace is used for heating, and the time of the square billet in the furnace is 170 min; the initial rolling temperature of the round steel is 920 ℃, and the final rolling temperature is 840 ℃. When the mechanical property of the round steel is tested, the heating temperature of the round steel is 920 ℃ when the round steel is quenched and heated in the heat treatment process, and the tempering temperature of the round steel is 600 ℃ when the round steel is tempered at high temperature.
The chemical compositions and contents used in this example are shown in FIG. 1, and the results of tensile, impact and notched tensile delayed fracture property tests and atmospheric corrosion resistance tests are shown in FIG. 2.
Example 3
The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance of the embodiment adopts the production process of the invention, and the process parameters are as follows: the pure degassing time is 20 minutes when the RH furnace is subjected to vacuum degassing; the target temperature of the tundish molten steel is stably controlled to be 18 ℃ above the liquidus temperature during the continuous casting of the bloom; the heating temperature of the heating furnace is 960 ℃ when the heating furnace is used for heating, and the time of the square billet in the furnace is 150 min; the initial rolling temperature of the round steel is 890 ℃, and the final rolling temperature is 800 ℃. When the mechanical property of the round steel is tested, the heating temperature of the round steel is 910 ℃ when the round steel is quenched and heated in the heat treatment process, and the tempering temperature of the round steel is 600 ℃ when the round steel is tempered at high temperature.
The chemical compositions and contents used in this example are shown in FIG. 1, and the results of tensile, impact and notched tensile delayed fracture property tests and atmospheric corrosion resistance tests are shown in FIG. 2.
Example 4
The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance of the embodiment adopts the production process of the invention, and the process parameters are as follows: the pure degassing time is 20 minutes when the RH furnace is subjected to vacuum degassing; the target temperature of the tundish molten steel is stably controlled to be 18 ℃ above the liquidus temperature during the continuous casting of the bloom; the heating temperature of the heating furnace is 980 ℃ and the time of the square billet in the furnace is 150 min; the initial rolling temperature of the round steel is 910 ℃, and the final rolling temperature is 820 ℃. When the mechanical property of the round steel is tested, the heating temperature of the round steel is 915 ℃ during quenching and heating in the heat treatment process, and the tempering temperature of the round steel is 600 ℃ during high-temperature tempering.
The chemical compositions and contents used in this example are shown in FIG. 1, and the results of tensile, impact and notched tensile delayed fracture property tests and atmospheric corrosion resistance tests are shown in FIG. 2.
Example 5
The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance of the embodiment adopts the production process of the invention, and the process parameters are as follows: the pure degassing time is 20 minutes when the RH furnace is subjected to vacuum degassing; the target temperature of the tundish molten steel is stably controlled to be 20 ℃ above the liquidus temperature during bloom continuous casting; heating temperature of the heating furnace is 970 ℃ when heating, and the time of the square billet in the furnace is 150 min; the initial rolling temperature of the round steel is 900 ℃, and the final rolling temperature of the round steel is 810 ℃. When the mechanical property of the round steel is tested, the heating temperature of the round steel is 920 ℃ when the round steel is quenched and heated in the heat treatment process, and the tempering temperature of the round steel is 590 ℃ when the round steel is tempered at high temperature.
The chemical compositions and contents used in this example are shown in FIG. 1, and the test results of tensile, impact and notched tensile delayed fracture property test and atmospheric corrosion resistance test are shown in FIG. 2.
Example 6
The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance of the embodiment adopts the production process of the invention, and the process parameters are as follows: the pure degassing time is 20 minutes when the RH furnace is subjected to vacuum degassing; the target temperature of the tundish molten steel is stably controlled to be 15 ℃ above the liquidus temperature during bloom continuous casting; heating temperature of the heating furnace is 970 ℃ when heating, and the time of the square billet in the furnace is 150 min; the initial rolling temperature of the round steel is 900 ℃, and the final rolling temperature of the round steel is 810 ℃. When the mechanical property of the round steel is tested, the heating temperature of the round steel is 960 ℃ during quenching and heating in the heat treatment process, and the tempering temperature of the round steel is 600 ℃ during high-temperature tempering.
The chemical components and contents adopted in the embodiment are shown in figure 1, the weight percentage satisfies the formula I and the formula II, and the test results of the tensile, impact and notch tensile delayed fracture performance test and the atmospheric corrosion resistance test are shown in figure 2.
Example 7
The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance of the embodiment adopts the production process of the invention, and the process parameters in the production process are as follows: the pure degassing time is 20 minutes when the RH furnace is subjected to vacuum degassing; the target temperature of the tundish molten steel is stably controlled to be 15 ℃ above the liquidus temperature during bloom continuous casting; heating temperature of the heating furnace is 970 ℃ when heating, and the time of the square billet in the furnace is 150 min; the initial rolling temperature of the round steel is 860 ℃, and the final rolling temperature of the round steel is 790 ℃. When the mechanical property of the round steel is tested, the heating temperature of the round steel is 920 ℃ when the round steel is quenched and heated in the heat treatment process, and the tempering temperature of the round steel is 590 ℃ when the round steel is tempered at high temperature.
The chemical components and contents adopted in the embodiment are shown in figure 1, the weight percentage satisfies the formula I and the formula II, and the test results of the tensile, impact and notch tensile delayed fracture performance test and the atmospheric corrosion resistance test are shown in figure 2.
The invention also provides a comparative example 1 and a comparative example 2, the production process of the invention is adopted, the chemical components and contents are different from those of the examples 1-7, in particular, the figure 1 is shown, and the test results of the tensile, impact and notch tensile delayed fracture performance test and the atmospheric corrosion resistance test are shown in figure 2. As can be seen from FIG. 2, the strength of examples 1 to 7 all reached 1600MPa, the elongation reached 10% or more, the face shrinkage reached 45% or more, and the delayed fracture strength ratios R in the examples were all greater than 0.55, showing excellent delayed fracture resistance and ductility and toughness, and also having good corrosion resistance.
Example 8
The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance of the embodiment adopts the production process of the invention, and the process parameters are as follows: the pure degassing time is 20 minutes when the RH furnace is subjected to vacuum degassing; the target temperature of the tundish molten steel is stably controlled to be 17 ℃ above the liquidus temperature during bloom continuous casting; heating temperature of the heating furnace is 970 ℃ when heating, and the time of the square billet in the furnace is 150 min; the initial rolling temperature of the round steel is 900 ℃, and the final rolling temperature of the round steel is 810 ℃. When the mechanical property of the round steel is tested, the heating temperature of the round steel is 920 ℃ when the round steel is quenched and heated in the heat treatment process, and the tempering temperature of the round steel is 590 ℃ when the round steel is tempered at high temperature.
The chemical components and contents adopted in the embodiment are shown in fig. 1, the chemical components of the embodiment meet the formula I, but do not meet the formula II, and the test results of the tensile, impact and notch tensile delayed fracture performance test and the atmospheric corrosion resistance test are shown in fig. 1 and fig. 2. As can be seen from fig. 1 and 2, in this example, the mechanical properties such as strength of the round steel are improved as compared with comparative examples 1 and 2, and the corrosion resistance and delayed fracture resistance are also improved, but still need to be further improved as compared with examples 1 to 7 (each chemical element satisfies the formulas (i), (ii), and (iii)).

Claims (6)

1. The round steel for the bolt with excellent corrosion resistance and delayed fracture resistance is characterized by comprising the following chemical components in percentage by weight: 0.55 to 0.60 percent of C, 1.80 to 2.00 percent of Si, 0.10 to 0.20 percent of V, 0.05 to 0.15 percent of Nb0.05 to 0.15 percent of Mn, 0.25 to 0.35 percent of Cr, 1.05 to 1.35 percent of Cr, 1.50 to 1.80 percent of Mo1, 0.10 to 0.30 percent of Ni0.20 to 0.35 percent of Cu0.20, 0.015 to 0.040 percent of Als0.015 to 0.040 percent of P, less than or equal to 0.010 percent of S, less than or equal to 0.00015 percent of H, less than or equal to 0.0010 percent of O, less than or equal to 0.005 percent of N, and the balance of Fe and other inevitable impurities; the steel comprises the following chemical components in percentage by weight: 0.55 is less than or equal to 0.25 [% Mo ] +0.20 [% V ] +0.10 [% Nb ] +0.08 [% Si is less than or equal to 0.65; the technological parameters of controlled rolling and controlled cooling in the production process of the round steel for the bolt are as follows: the initial rolling temperature is 860-920 ℃, the final rolling temperature is 780-840 ℃, and the excellent rod heating furnace is used for heating before controlled rolling and controlled cooling, the heating temperature is 950-1000 ℃, and the furnace time of the square billet is not more than 170 min.
2. The round steel for a bolt excellent in corrosion resistance and delayed fracture resistance according to claim 1, characterized in that: the weight percentage of C and Si also satisfies that ([ Si ]/[ C ]) is less than or equal to 3.2 and less than or equal to 3.35.
3. A method for producing a round steel for a bolt excellent in corrosion resistance and delayed fracture resistance, characterized in that the round steel comprises the chemical composition according to claim 1 or 2, and the production method specifically comprises the steps of: electric furnace smelting → LF + RH vacuum refining → large square billet continuous casting → heating → initial rolling and cogging → six continuous rolling → rolling square billet → flaw detection, coping → excellent rod heating furnace heating → controlled rolling and controlled cooling → round steel finished product → packaging and warehousing.
4. The method for producing a round steel for bolts excellent in corrosion resistance and delayed fracture resistance according to claim 3, characterized in that: and during electric furnace smelting, tapping is performed for deoxidation alloying, and slag is discharged in the tapping process.
5. The method for producing a round steel for bolts excellent in corrosion resistance and delayed fracture resistance according to claim 4, characterized in that: the pure degassing time is not less than 20 minutes during the vacuum degassing of the RH furnace, and the [ H ] is not more than 0.00015 percent after the vacuum treatment.
6. The production method of round steel for bolts excellent in corrosion resistance and delayed fracture resistance according to any one of claims 3 to 5, characterized in that: the round steel is required to be subjected to performance inspection, the heat treatment process during inspection comprises quenching heating treatment and high-temperature tempering treatment, the heating temperature during quenching heating is 900-960 ℃, and the tempering temperature during high-temperature tempering is 580-600 ℃.
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