CN115216700A - 1700 MPa-level steel for fasteners and production method and heat treatment process thereof - Google Patents

1700 MPa-level steel for fasteners and production method and heat treatment process thereof Download PDF

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CN115216700A
CN115216700A CN202210916410.5A CN202210916410A CN115216700A CN 115216700 A CN115216700 A CN 115216700A CN 202210916410 A CN202210916410 A CN 202210916410A CN 115216700 A CN115216700 A CN 115216700A
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steel
percent
temperature
1700mpa
grade
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CN115216700B (en
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姜婷
汪开忠
于同仁
张晓瑞
尹德福
郭湛
丁雷
余良其
杨少朋
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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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/16—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling wire rods, bars, merchant bars, rounds wire or material of like small cross-section
    • 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
    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
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Abstract

The invention provides a 1700 MPa-level steel for fasteners, a production method and a heat treatment process thereof, and the steel comprises the following components: 0.40-0.60% of C, 0.35-0.55% of Si, 1.20-1.50% of Mn, 1.20-1.40% of Cr1.20, 0.020-0.050% of Nb0.80-1.00% of Co0.30-0.50% of Mo0.015-0.035% of Zr0.015-0.020% of La0.010-0.020%, alt0.040-0.060% of N, 0.010-0.020% of T.O, less than or equal to 0.0015% of T.O and the balance of Fe and other inevitable impurities. The comprehensive performance of the steel is improved through the formula design and the production process design, the strength level can reach 1700MPa level after the heat treatment, and the yield strength reduction degree at the high temperature of 600 ℃ is less than 20 percent.

Description

1700 MPa-level steel for fasteners and production method and heat treatment process thereof
Technical Field
The invention belongs to the technical field of steel for fasteners, and particularly relates to 1700 MPa-level steel for fasteners, a production method and a heat treatment process thereof.
Background
The fastener has the advantages of strong bearing capacity, high reliability, simple and convenient construction, good economy and the like, and is developed at high speed and widely applied. Due to the shortage of world resources, the trend of fasteners to achieve structural lightweightness by reducing the size, weight and number of fasteners with increasing fastener strength has been a trend of fasteners, and high-strength bolted connections have reached more than 90%. A large number of fastener enterprises in industrially developed countries turn to the production of high-strength, high-precision, special and special fasteners.
With the development of various production departments such as automobiles, machinery, buildings, light industry and the like, increasingly high requirements are put on materials used for manufacturing various fasteners (such as bolts, screws, nuts and the like), such as high performance and light weight of automobiles, high-rise building structures, super-long bridges and the like, and requirements of higher design stress and light weight are put on bolts used as connecting parts. In contrast, the most effective measure is to increase the strength of the bolt steel, and the bolt used in automobiles is rated at 9.8 or more in the U.S. At present, bolts for automobiles and construction machines even require strength of 1500MPa or more.
However, the strength grade of the steel for the fastener in China is low, the high temperature resistance is poor, and the development requirement of the fastener in the future cannot be met.
Disclosure of Invention
The invention aims to provide 1700 MPa-grade steel for fasteners and a production method thereof, and the steel with excellent comprehensive performance is obtained through component design and process production.
The invention also aims to provide a heat treatment process of the steel for the 1700 MPa-grade fastener, after heat treatment, the tensile strength is more than or equal to 1700MPa, the yield ratio is more than or equal to 0.90, the elongation is more than or equal to 9 percent, the face shrinkage is more than or equal to 45 percent, and the steel has excellent delayed fracture resistance: the delayed fracture strength ratio is more than or equal to 0.83, and the notch sensitivity NSR value is more than or equal to 1.65; in addition, the high-temperature performance is excellent: the reduction degree of the yield strength at the high temperature of 600 ℃ is less than 20 percent. The method is suitable for manufacturing 1700 MPa-level fasteners with high-temperature use scenes.
The specific technical scheme of the invention is as follows:
the 1700 MPa-grade steel for the fasteners comprises the following components in percentage by mass:
0.40-0.60% of C, 0.35-0.55% of Si, 1.20-1.50% of Mn, 1.20-1.40% of Cr, 0.020-0.050% of Nb, 0.80-1.00% of Co, 0.30-0.50% of Mo, 0.015-0.035% of Zr, 0.010-0.020% of La, 0.040-0.060% of Al, 0.010-0.020% of N, less than or equal to 0.0015% of O and the balance of Fe and other inevitable impurities.
Preferably, the 1700 MPa-grade steel for fasteners comprises the following components in percentage by mass:
0.43 to 0.55 percent of C, 0.38 to 0.51 percent of Si, 1.23 to 1.45 percent of Mn, 1.26 to 1.36 percent of Cr, 0.027 to 0.044 percent of Nb, 0.83 to 0.95 percent of Co0, 0.34 to 0.46 percent of Mo, 0.018 to 0.032 percent of Zr, 0.012 to 0.018 percent of La, 0.044 to 0.057 percent of Al, 0.010 to 0.019 percent of N, less than or equal to 0.0015 percent of O, and the balance of Fe and other inevitable impurities.
The composition of the 1700 MPa-grade steel for fasteners meets the following requirements: al/N is more than or equal to 3.0;
the 1700 MPa-grade steel for fasteners comprises the following components: r value ≧ 0.17, R =0.054 × (% Cr) +0.063 × (% Mo) +0.093 × (% Co) +0.125 × (% Zr). The R value is an index for evaluating the degree of influence of Cr, mo, co, and Zr on the heat resistance of steel and the degree of influence of each element by weighting and adding, and Cr, mo, co, and Zr are elements that mainly improve the heat resistance of the steel of the present invention.
The following experiments were performed: 0.40 to 0.60 percent of C, 0.35 to 0.55 percent of Si, 1.20 to 1.50 percent of Mn, 1.20 to 1.40 percent of Cr, 0.020 to 0.050 percent of Nb, 0.80 to 1.00 percent of Co, 0.30 to 0.50 percent of Mo, 0.015 to 0.035 percent of Zr, 0.010 to 0.020 percent of La, 0.040 to 0.060 percent of Al, 0.010 to 0.020 percent of N, less than or equal to 0.0015 percent of T.O and the balance of Fe and other inevitable impurities. And various steels meeting the requirement that Al/N is more than or equal to 3.0 are subjected to steel making and round bar forging through pilot-scale vacuum smelting, then are subjected to heat treatment of 880 ℃ quenching and oil cooling plus 550 ℃ tempering, and are subjected to mechanical properties at normal temperature and high temperature of 600 ℃ after heat treatment, and the high-temperature mechanical stretching is carried out according to GB/T4338 'Metal material high-temperature tensile test method'. The ratio of the yield strength at 600 ℃ to the yield strength at normal temperature is calculated, the ratio is more than or equal to 0.80, which indicates that the heat-resistant performance is excellent, the relation graph of the ratio and the R value is shown in figure 1, and it can be seen that the sufficient heat-resistant performance can be ensured only if the R value is more than or equal to 0.17.
The invention provides a production method of steel for a 1700 MPa-level fastener, which comprises the following process flows of:
electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → 380mm-500mm bloom continuous casting → bloom heating → 140mm x 140mm-250mm x 250mm small bloom cogging → small bloom heating → high speed wire rod low temperature rolling → stelmor cooling line cooling → phi 5.5-35mm finished wire rod.
And (3) continuously casting the 380mm-500mm bloom: during continuous castingElectromagnetic stirring is adopted, la lines are added into a crystallizer to adjust the La content, protective casting is adopted in the whole process, and the primary cooling water flow is 100-120m 3 The water amount of the secondary cooling is 1.0-1.2l/kg. If the maximum limit is exceeded, the casting blank cracks may occur, and the columnar crystal grows to cause coarse crystals, and if the minimum limit is exceeded, the pulling speed is low, and the production efficiency is insufficient.
The heating of the bloom is specifically as follows: the soaking temperature of the continuous casting bloom is controlled to be 1230-1300 ℃, if the soaking temperature is lower than 1230 ℃, the interior of the bloom can not be fully heated, and Cr, mo, co and Zr alloy elements can not be uniformly diffused, so that the equipment load is large during cogging, and the performance of steel is not uniform due to segregation; if it is higher than 1300 ℃, austenite grains start to become coarse, and the decarburization tendency is greatly increased;
the heating of the small square billet is specifically as follows: controlling the heating soaking temperature of the rolled billet to be 1050-1150 ℃, and if the soaking temperature is lower than 1050 ℃, cr, mo, co and Zr alloy elements cannot be uniformly diffused, so that the steel has composition segregation and is brittle; if it is higher than 1150 deg.C, full decarburization will occur;
the high-speed wire rod low-temperature rolling specifically comprises the following steps: controlling the finish rolling temperature at 770-810 ℃, the steel A of the invention C3 Point is 750 ℃, the temperature range is A C3 At the temperature of 20-60 ℃, the phase change of ferrite is induced by utilizing the severe thermal deformation of the austenite region under the large pressure, and the nucleation points are increased by utilizing the nail rolling action of enough AlN and Nb carbonitride on the grain boundary to obtain an ultra-fine grain structure, the grain size is less than or equal to 6 mu m, thereby obtaining high strength and plasticity.
The diameter reducing temperature is controlled to be 750-770 ℃, and if the diameter reducing temperature is lower than 750 ℃, the burden of subsequent rolling equipment is overlarge; if the temperature is higher than 770 ℃, the subsequent spinning temperature is possibly increased, the phase transition temperature is not reached before entering the heat-insulating cover, the complete phase transition on the stelmor line is difficult to complete, and a large amount of phase transition is martensite structure during coil collection, so that the brittle fracture of the wire rod is caused;
controlling the spinning temperature to be 755-775 ℃, and if the spinning temperature is lower than 755 ℃, entering a phase change stage before entering a heat-preserving cover; if the temperature is higher than 775 ℃, the phase transition temperature is not reached before the coil enters the heat-insulating cover, the complete phase transition on the stelmor line is difficult to complete, and a large amount of phase transition is martensite structure during coil collection, so that the brittle fracture of the wire rod is caused;
the stelmor cooling line cooling specifically comprises the following steps: the cover of the heat preservation section is controlled to be closed completely, the cooling speed of the wire rod is controlled to be not higher than 0.2 ℃/s, and an ideal structure of pearlite and ferrite with the area of 70% -85% and a small amount of bainite with the area of 15% -30% is obtained through slow cooling.
The design idea of the invention is as follows:
c: c is the most basic effective strengthening and hardenability element in steel. But as its content increases, ductility decreases and the risk of delayed fracture of the bolt increases. The content of C is controlled between 0.40 percent and 0.60 percent. Further preferably from 0.43% to 0.55% by weight of C.
Si: si is an important element for strengthening in steel, and the strength and hardness of the steel are improved through solid solution. However, the increase of the Si element increases the diffusion of carbon in the steel, and thus the decarburization of the steel is promoted. The content of Si is controlled between 0.35 percent and 0.55 percent. Further preferably from 0.38% to 0.51% Si.
Mn: mn and Fe form a solid solution, so that the hardness and strength of ferrite and austenite in the steel are improved, and meanwhile, mn is used for improving the stability of an austenite structure and remarkably improving the hardenability of the steel. However, excessive Mn reduces the plasticity of the steel, increases notch sensitivity of the material, and increases segregation of grain boundaries, resulting in a decrease in grain boundary strength and an increase in the risk of delayed fracture. The Mn content is controlled to be 1.20-1.50%. Further preferably 1.23 to 1.45% of Mn.
Cr: cr element remarkably improves the obdurability in steel, is precipitated in the form of carbide, increases hydrogen capture points and improves delayed fracture resistance. Cr has high melting point, improves the heat resistance and creep resistance of steel, and also provides good high-temperature oxidation resistance and oxidation corrosion resistance to the steel, but excessive Cr increases the temper brittleness tendency of the steel. The Cr content is controlled to be 1.20-1.40%. Further preferably Cr is 1.26 to 1.36 percent.
Nb: nb is a micro-alloying element which is very effective in refining grains, and the Nb in steel is characterized in that the recrystallization temperature of austenite is increased, so that the purpose of refining austenite grains is achieved, and the strong plasticity of the steel is improved. The steel of the invention also utilizes the effect of more stable carbide of Nb, and can fix carbon to promote alloy elements such as chromium, molybdenum and the like to be dissolved into solid solution more and promote solid solution strengthening at high temperature. However, the strengthening effect of excess Nb is no longer significant and increases the crack sensitivity of the steel. The content of Nb is controlled between 0.020% and 0.050%. Further preferably from Nb0.027% to 0.044%.
Co: co is a non-carbide forming element and strengthens ferrite in the steel. Meanwhile, co has oxidation resistance, and can remarkably improve the thermal stability and heat resistance of the steel. Excessive Co addition results in a decrease in material toughness and an increase in decarburization sensitivity of the steel. The content of Co is controlled between 0.80 percent and 1.00 percent. Further preferably from 0.83% to 0.95% of Co.
Mo: mo is generally dispersed in a matrix material as second phase particles or inclusions after steel is subjected to heat treatment, and atoms have a large adsorption effect on hydrogen, namely, the Mo has good hydrogen corrosion resistance and is an element which effectively delays delayed fracture of a fastener. Meanwhile, mo is an element for increasing hardenability and precipitation hardening, and is precipitated at a crystal boundary to strengthen the crystal boundary and effectively improve the condition of crystal boundary weakening at high temperature. However, excessive Mo content deteriorates cold workability of the steel. The content of Mo is controlled to be 0.30-0.50%. Further preferably 0.34 to 0.46% of Mo.
Zr: zr is a strong carbide forming element, produces fine dispersed carbide, and is coherent with the matrix to pin dislocation, so as to block dendritic crystal growth and grain boundary migration, generate crystal whisker and refined crystal grain. In addition, zr can be dissolved in austenite in a solid mode, and has a remarkable effect on the increase of hardenability. In addition, like Mo, zr is easy to precipitate at grain boundaries, and has a remarkable grain boundary strengthening effect at high temperature. However, too much Zr element causes brittle inclusions to be formed. The Zr content is controlled to be 0.015-0.035%. Further preferably Zr 0.018% -0.032%.
La: adding an appropriate amount of La element to the steel to make MnS and A1 2 O 3 The rare earth impurities are changed into impurities, and the effects of deoxidation and desulfurization are good. The tiny solid particles of the La element provide heterogeneous crystal nuclei or are eccentrically polymerized on a crystallization interface, so that the growth of crystal cells is hindered, and the normal-temperature mechanical property of the steel is improved. Excess La effect was no longer evident. La containsThe content is controlled to be 0.010-0.020%. Further preferably La0.012% -0.018%.
Al and N: al is a strong deoxidizing element, and simultaneously improves the oxidation resistance of steel, and the deoxidizing capacity of aluminum is much stronger than that of silicon and manganese. In the invention, aluminum neutralizes N to form a compound in steel, the compound is nailed and rolled in a grain boundary, grains are obviously refined, and the mechanical property of the steel is improved through fine grains. The Al content is controlled to be 0.040% -0.060%. Further preferably 0.044-0.057% of Al. The content of N is controlled to be 0.010-0.020%. Further preferably from 0.010% to 0.019% of N. Meanwhile, in order to obtain the best fine grain effect, al/N is controlled to be more than or equal to 3.0.
O: oxygen forms oxide inclusions in steel, and T.O is controlled to be less than or equal to 0.0015 percent.
The invention provides a heat treatment process of 1700 MPa-level fastener steel, which comprises quenching and tempering;
the quenching comprises the following steps: quenching at 880-910 deg.C for 2-3 min with diameter (mm) of the product; oil cooling after quenching;
the tempering is as follows: tempering at 530-580 deg.C for 5-7 min with diameter (mm); air cooling is carried out after tempering.
After the heat treatment, the mechanical property of the product reaches 1700Mpa, R m ≥1700MPa，R p0.2 The tensile strength is more than or equal to 1500MPa, the elongation A is more than or equal to 9 percent, the face shrinkage Z is more than or equal to 45 percent, the yield ratio is more than or equal to 0.90, and the tensile strength has excellent delayed fracture resistance: the delayed fracture resistance strength ratio is more than or equal to 0.83, and the notch sensitivity NSR value is more than or equal to 1.65; in addition, the high-temperature performance is excellent: the reduction degree of yield strength at the high temperature of 600 ℃ is less than 20 percent. The method is suitable for manufacturing 1700 MPa-level fasteners with high-temperature use scenes.
Since the material is more likely to absorb hydrogen as the internal stress increases with the increase in the strength grade of the steel, resulting in an increase in the hydrogen-induced delayed fracture sensitivity, it is necessary to pay particular attention to the delayed fracture resistance in developing a steel for fasteners of higher strength grade. The steel for the ultra-high strength and toughness fastener is designed by a formula design, a production process design and a heat treatment process, and the main design concept is as follows: (1) Because the crystal boundary of the steel is weakened at high temperature and the obdurability of the steel is seriously deteriorated, the steel of the invention is added with Mo and Zr elements, which are easy to precipitate at the crystal boundary and play a role in strengthening the crystal boundary at high temperature; (2) In addition, the high-temperature strength of the steel depends on solid solution strengthening and grain boundary strengthening, in order to ensure that more Cr, mo, co and Zr are solid-dissolved in a matrix, the steel is added with strong carbide to form an element Nb, and the carbon can be fixed by combining a proper heat treatment process to promote enough alloy elements such as Cr, mo and the like to be dissolved in a solid solution; (3) Because the steel has higher normal temperature strength level and obviously increases the delayed fracture risk, mo, nb and Cr elements and a proper production process and a heat treatment process are formed in the steel to be coherent with a matrix, a large number of hydrogen traps are formed, and the delayed fracture risk caused by hydrogen resistance of the steel is reduced.
Compared with the prior art, the 1700 MPa-grade steel for the fastener is designed by a formula design and a production process, the comprehensive performance of the steel is improved, after the heat treatment, the strength grade can reach 1700MPa grade, the tensile strength of the product is more than or equal to 1700MPa, the yield ratio is more than or equal to 0.90, the elongation is more than or equal to 9%, the face shrinkage is more than or equal to 45%, and the steel has excellent delayed fracture resistance: the delayed fracture strength ratio is more than or equal to 0.83, and the notch sensitivity NSR value is more than or equal to 1.65; in addition, the high-temperature performance is excellent: the reduction degree of the yield strength at the high temperature of 600 ℃ is less than 20 percent. The method is suitable for manufacturing 1700 MPa-level fasteners with high-temperature use scenes.
Drawings
FIG. 1 is a graph showing the relationship between the R value and the ratio of the yield strength at 600 ℃ to the yield strength at normal temperature;
FIG. 2 shows the structure of example 1 after heat treatment; the tempered sorbite matrix is internally provided with a large amount of solid solution alloy, and more fine carbides are precipitated on grain boundaries.
Detailed Description
Example 1 to example 7
The 1700 MPa-grade steel for the fasteners comprises the following components in percentage by mass: as shown in table 1, the balance not shown in table 1 is Fe and inevitable impurities.
Comparative examples 1 to 3
The steel for the fastener comprises the following components in percentage by mass: as shown in table 1, the balance not shown in table 1 is Fe and inevitable impurities.
TABLE 1 composition of steel for fastener of examples and comparative examples (unit: wt%)
Case(s) C Si Mn Cr Nb Zr Co
Example 1 0.4 0.35 1.5 1.31 0.02 0.035 0.8
Example 2 0.6 0.55 1.2 1.2 0.033 0.015 1
Example 3 0.45 0.43 1.37 1.4 0.05 0.023 0.85
Example 4 0.51 0.39 1.23 1.25 0.041 0.019 0.92
Example 5 0.49 0.48 1.45 1.37 0.024 0.032 0.89
Example 6 0.56 0.51 1.29 1.34 0.038 0.026 0.95
Example 7 0.52 0.46 1.35 1.24 0.046 0.031 0.83
Comparative example 1 0.51 0.43 1.42 1.31 0.033 0.033 0.89
Comparative example 2 0.49 0.41 1.29 1.23 0.038 0.016 0.81
Comparative example 3 0.41 0.23 1.41 1.05 / / /
Case(s) Mo La Al N O Al/N R value
Example 1 0.5 0.015 0.04 0.012 0.0011 3.33 0.181
Example 2 0.3 0.01 0.06 0.02 0.0009 3 0.179
Example 3 0.37 0.02 0.045 0.014 0.0008 3.21 0.181
Example 4 0.36 0.017 0.058 0.013 0.0014 4.46 0.178
Example 5 0.41 0.013 0.049 0.015 0.0009 3.27 0.187
Example 6 0.34 0.014 0.051 0.016 0.0008 3.19 0.185
Example 7 0.48 0.016 0.055 0.018 0.001 3.06 0.178
Comparative example 1 0.41 0.015 0.045 0.019 0.0009 2.37 0.183
Comparative example 2 0.35 0.017 0.052 0.017 0.001 3.06 0.166
Comparative example 3 0.21 / 0.032 0.0052 0.0011 6.15 0.07
The steel production of each example and comparative example comprises the following process flow:
electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → 380mm to 500mm bloom continuous casting → bloom heating → 140mm by 140mm to 250mm by 250mm billet cogging → billet heating → high speed wire rod low temperature rolling → stelmor cooling line cooling → phi 5.5 to 35mm wire rod finished product.
Wherein the content of the first and second substances,
adopting electromagnetic stirring during continuous casting, adding La line into a crystallizer to adjust La content, adopting protective casting in the whole process, and cooling onceWater flow 100-120m 3 The water amount of the secondary cooling is 1.0-1.2l/kg. If the maximum limit is exceeded, the casting blank cracks may occur, and the columnar crystal grows to cause coarse crystals, and if the minimum limit is exceeded, the pulling speed is low, and the production efficiency is insufficient.
Controlling the soaking temperature of heating of the continuous casting bloom to be 1230-1300 ℃, if the soaking temperature is lower than 1230 ℃, the interior of the bloom can not be fully heated, and Cr, mo, co and Zr alloy elements can not be uniformly diffused, so that the equipment load is large during cogging, and the performance of the steel is uneven due to segregation; if it is higher than 1300 ℃, austenite grains start to become coarse, and the decarburization tendency is greatly increased;
controlling the heating soaking temperature of the rolled billet to be 1050-1150 ℃, and if the soaking temperature is lower than 1050 ℃, cr, mo, co and Zr alloy elements cannot be uniformly diffused, so that the steel has composition segregation and is brittle; if higher than 1150 deg.C, full decarburization will occur;
controlling the finish rolling temperature at 770-810 ℃, the steel A of the invention C3 Point is 750 ℃ and the temperature range is A C3 At the temperature of 20-60 ℃, the phase change of ferrite is induced by utilizing the severe thermal deformation of the austenite region under the large pressure, and the nucleation points are increased by the nail rolling action of enough AlN and Nb carbonitrides on the grain boundary to obtain an ultra-fine grain structure, wherein the grain size is less than or equal to 6 mu m, thereby obtaining high strength and plasticity.
The diameter reducing temperature is controlled to be 750-770 ℃, and if the diameter reducing temperature is lower than 750 ℃, the burden of subsequent rolling equipment is overlarge; if the temperature is higher than 770 ℃, the subsequent spinning temperature is possibly increased, the phase transition temperature is not reached before entering the heat-insulating cover, the complete phase transition on the stelmor line is difficult to complete, and a large amount of phase transition is martensite structure during coil collection, so that the brittle fracture of the wire rod is caused;
controlling the spinning temperature to be 755-775 ℃, and if the spinning temperature is lower than 755 ℃, entering a phase change stage before entering a heat-preserving cover; if the temperature is higher than 775 ℃, the phase transition temperature is not reached before the coil enters the heat-insulating cover, the complete phase transition on the stelmor line is difficult to complete, and a large amount of phase transition is martensite structure during coil collection, so that the brittle fracture of the wire rod is caused;
the cover of the heat preservation section is controlled to be completely closed, the cooling speed of the wire rod is controlled to be below 0.2 ℃/s, and an ideal structure of pearlite, ferrite and a small amount of bainite is obtained through slow cooling.
The specific process parameters of the inventive examples and comparative examples are shown in Table 2.
TABLE 2 production Process parameters and product dimensions for the examples and comparative examples
Figure BDA0003775715070000101
The steel is smelted by pilot vacuum smelting, round bar forging is carried out, then heat treatment of 880 ℃ quenching (oil cooling) +550 ℃ tempering is carried out, mechanical properties at normal temperature and high temperature of 600 ℃ are carried out after the heat treatment, and the high-temperature mechanical stretching is carried out according to GB/T4338 'Metal material high-temperature tensile test method'. The ratio of the yield strength at 600 ℃ to the yield strength at normal temperature is calculated, the ratio is more than or equal to 0.80, which indicates that the heat-resistant performance is excellent, the relation graph of the ratio and the R value is shown in figure 1, and it can be seen that the sufficient heat-resistant performance can be ensured only if the R value is more than or equal to 1.70.
The performance detection method comprises the following steps:
hot rolling structure: taking a test sample with the length of 15mm from a hot-rolled wire rod, polishing the cross section, corroding by using 4% nitric acid alcohol, and performing tissue evaluation according to GB/T13298 metal microstructure inspection method; whether the wire rod has excellent use performances such as drawing and cold heading during the processing of the fastener can be judged through the structure.
Cold heading: the wire rod is taken out for cold upsetting according to the following requirements: x = h 1 H =1/3; (wherein h is the height of the sample before cold heading (twice the diameter of the wire rod); h 1 The height of the test piece after cold heading. ) After the cold upsetting test, the defects of cracks, fissures, cracks and hairlines which can be seen by naked eyes are not found on the surface of the sample. And (5) performing 30 groups of cold heading tests on each number, and counting the cracking rate. The cold heading performance of the fastener during processing can be judged through the cold heading cracking rate.
Stretching at normal temperature after heat treatment: the wire rod adopts the following quenching and tempering heat treatment process: 890 ℃ quenching, oil cooling, 550 ℃ tempering and air cooling. Straightening after heat treatment, carrying out a tensile test,test R m 、R p0.2 A, Z, and calculating the yield ratio. Judging whether the steel meets the 1700 MPa-level requirement through the tensile property after heat treatment.
Delayed fracture resistance test: subjecting the steel to a quenching and tempering heat treatment (heat treatment process same as above), and working the delayed fracture specimen, immersing the specimen in an aqueous acidic solution of 15% HCl for 30 minutes, washing with water and drying, then applying a constant load, comparing the load at which no fracture occurs for 100 hours or more. In this case, the value obtained by dividing the load at which no fracture occurred for 100 hours or more after the acid impregnation by the maximum load at the time of the tensile test without the acid impregnation was defined as the delayed fracture strength ratio. The delayed fracture strength ratio is 0.70 or more, and is judged to be acceptable, and 0.8 or more, is judged to be excellent.
Notch sensitivity test: the steel is subjected to quenching and tempering heat treatment (the heat treatment process is the same as the above), notch sensitivity test is carried out by adopting HB5214-1996 metal room temperature notch tensile test method, the ratio of the tensile strength of a notched sample to the tensile strength of an unnotched sample, namely the notch sensitivity is measured by the NSR value, the larger the NSR value is, the lower the notch sensitivity is, as the embodiment and the comparative example are both plastic samples and generate notch strengthening effect, the NSR value is larger than 1, the NSR value of the embodiment is larger than or equal to 1.65, the notch sensitivity is obviously superior to that of the comparative sample, and the notch sensitivity test method has good notch sensitivity and is suitable for manufacturing bolts.
High-temperature mechanical stretching: the steel is quenched and tempered, and then is subjected to a 600 ℃ high-temperature mechanical tensile test according to GB/T4338 'Metal Material high-temperature tensile test method'. The high-temperature tensile property is obtained, and the ratio of the high-temperature yield strength to the normal-temperature yield strength is calculated, and the ratio is more than 0.80, so that the steel has excellent high-temperature property.
These results are listed in table 3 along with the production process parameters.
Table 3 shows the structure and properties of the products of each example and comparative example.
TABLE 3 List of specific process parameters and performance measurements for inventive and comparative examples
Figure BDA0003775715070000121
The chemical composition and the production method of the steel in the examples 1 to 7 are properly controlled, and the chemical composition of the steel ensures that the relation (1) Al/N is more than or equal to 3.0; the heat resistance index R value of the relational expression (2) is more than or equal to 0.17, the steel has the strength grade of more than 1700MPa grade, the delayed fracture risk is low, and the steel has excellent high-temperature performance, and the production method also ensures that the coil rod structure is pearlite, ferrite and a small amount of bainite, so that the excellent service performance of the downstream fastener during processing is realized, the annealing can be simplified, and the cost is saved.
Although the chemical composition range of the comparative example 1 is in the required range, the Al/N ratio is not properly controlled, so that the fine grain effect of the steel is insufficient, the plasticity of the steel is low, the surface shrinkage and the elongation are poor, and the delayed fracture risk is high; the high temperature resistance index R value of the comparative example 2 is lower than 0.17, the high temperature performance is poor, the spinning temperature and the cooling speed of the wire rod in the production process are not controlled properly, a large amount of martensite and bainite exist in a wire rod microstructure, the cold heading performance is poor, the annealing and heat preservation time is increased when a user uses the wire rod, and the processing cost of the user is obviously increased; comparative example 3 is a commercially available high strength grade steel SCM440, which has a low strength grade and a high strength drop at high temperature.

Claims (12)

1. The steel for the 1700 MPa-grade fastener is characterized by comprising the following components in percentage by mass:
0.40-0.60% of C, 0.35-0.55% of Si, 1.20-1.50% of Mn, 1.20-1.40% of Cr, 0.020-0.050% of Nb0.020%, 0.80-1.00% of Co, 0.30-0.50% of Mo, 0.015-0.035% of Zr, 0.010-0.020% of La0.040-0.060% of Al, 0.010-0.020% of N, less than or equal to 0.0015% of O and the balance of Fe and other inevitable impurities.
2. The steel for a 1700MPa grade fastener of claim 1 comprising, in mass percent:
0.43 to 0.55 percent of C, 0.38 to 0.51 percent of Si, 1.23 to 1.45 percent of Mn, 1.26 to 1.36 percent of Cr, 0.027 to 0.044 percent of Nb0.83 to 0.95 percent of Co0.34 to 0.46 percent of Mo, 0.018 to 0.032 percent of Zr, 0.012 to 0.018 percent of La0.018, 0.044 to 0.057 percent of Al, 0.010 to 0.019 percent of N, less than or equal to 0.0015 percent of O, and the balance of Fe and other inevitable impurities.
3. The steel for a 1700MPa grade fastener of claim 1 or 2 wherein the composition of the steel for a 1700MPa grade fastener satisfies the following: al/N is more than or equal to 3.0.
4. The steel for a 1700MPa grade fastener of claim 1 or 2 wherein the composition of the steel for a 1700MPa grade fastener satisfies the following: r value ≧ 0.17, R =0.054 × (% Cr) +0.063 × (% Mo) +0.093 × (% Co) +0.125 × (% Zr).
5. A method of producing a steel for a grade 1700MPa fastener according to any one of claims 1 to 4, characterised in that it comprises the following process flow: electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → bloom continuous casting → bloom heating → small bloom cogging → small bloom heating → high-speed wire low-temperature rolling → stelmor cooling line cooling → finished wire rod.
6. The production method according to claim 5, wherein the bloom continuous casting: electromagnetic stirring is adopted during continuous casting, la lines are added into a crystallizer to adjust the La content, protective casting is adopted in the whole process, and the primary cooling water flow is 100-120m 3 The secondary cooling specific water amount is 1.0-1.2l/kg.
7. The production method according to claim 5, characterized in that the bloom heating is in particular: controlling the soaking temperature of the continuous casting bloom to be 1230-1300 ℃.
8. The production method according to claim 5, characterized in that the billet heating is in particular: the soaking temperature for heating the rolled small square billets is controlled to be 1050-1150 ℃.
9. The production method according to claim 5, wherein the high-speed wire rod low-temperature rolling is specifically: the finish rolling temperature is controlled at 770-810 ℃, the reducing diameter temperature is controlled at 750-770 ℃, and the spinning temperature is controlled at 755-775 ℃.
10. The production method according to claim 5, wherein the stelmor cooling line is specifically: the cover of the heat preservation section is controlled to be totally closed, and the cooling speed of the wire rod is controlled to be not higher than 0.2 ℃/s.
11. A heat treatment process of 1700 MPa-grade fastener steel is characterized by comprising 880-910 ℃ quenching and 530-580 ℃ tempering.
12. The heat treatment process of claim 11, wherein after the heat treatment, the mechanical properties of the product reach 1700Mpa grade, R m ≥1700MPa，R p0.2 1500MPa or more, the elongation A is 9% or more, the face shrinkage Z is 45% or more, the yield ratio is 0.90 or more, the delayed fracture resistance strength ratio is 0.83 or more, and the notch sensitivity NSR value is 1.65 or more; the reduction degree of the yield strength at the high temperature of 600 ℃ is less than 20 percent.
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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH06306543A (en) * 1993-04-15 1994-11-01 Nippon Steel Corp High strength pc wire rod excellent in delayed fracture resistance and its production
JPH11199926A (en) * 1998-01-21 1999-07-27 Toa Steel Co Ltd Production of bar steel for high strength bolt, excellent in cold workability and delayed fracture resistance
CN1900343A (en) * 2005-07-22 2007-01-24 新日本制铁株式会社 Steel with excellent delayed fracture resistance and tensile strength of 1600 mpa class or more, its shaped articles, and methods of production of the same
CN103334052A (en) * 2013-06-18 2013-10-02 上海大学 High-thermal conductivity high-abrasion resistance hot stamping die steel and preparation method thereof
CN107312970A (en) * 2017-06-24 2017-11-03 武汉钢铁有限公司 A kind of high-strength steel for fastener of super large and production method
WO2022131626A1 (en) * 2020-12-17 2022-06-23 주식회사 포스코 High strength steel sheet having excellent workability, and method for manufacturing same

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH06306543A (en) * 1993-04-15 1994-11-01 Nippon Steel Corp High strength pc wire rod excellent in delayed fracture resistance and its production
JPH11199926A (en) * 1998-01-21 1999-07-27 Toa Steel Co Ltd Production of bar steel for high strength bolt, excellent in cold workability and delayed fracture resistance
CN1900343A (en) * 2005-07-22 2007-01-24 新日本制铁株式会社 Steel with excellent delayed fracture resistance and tensile strength of 1600 mpa class or more, its shaped articles, and methods of production of the same
CN103334052A (en) * 2013-06-18 2013-10-02 上海大学 High-thermal conductivity high-abrasion resistance hot stamping die steel and preparation method thereof
CN107312970A (en) * 2017-06-24 2017-11-03 武汉钢铁有限公司 A kind of high-strength steel for fastener of super large and production method
WO2022131626A1 (en) * 2020-12-17 2022-06-23 주식회사 포스코 High strength steel sheet having excellent workability, and method for manufacturing same

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