CN106480372A - Non-quenched and tempered cold-heading steel wire rod for 8.8-grade fastener and production method thereof - Google Patents

Abstract

The invention discloses a non-quenched and tempered cold heading steel wire rod for grade 8.8 fasteners, which is characterized in that the components are: [C]: 0.25% to 0.30%, [Si]: 0.40% to 0.60%, [ Mn]: 1.10% to 1.30%, [P]≤0.020%, [S]≤0.015%, [Cr]: 0.10% to 0.15%, [V]: 0.06% to 0.10%, [Ti]: 0.01% to 0.03%, unavoidable impurities not higher than 0.1%, and the rest is iron.

Description

A non-quenched and tempered cold heading steel wire rod for 8.8 grade fasteners and its production method

technical field

The invention belongs to the technical field of iron and steel metallurgy, and relates to a non-quenched and tempered cold heading steel wire rod for 8.8-grade fasteners and a production method thereof.

Background technique

Usually high-strength fasteners (grade 8.8 and above) are produced by medium-carbon steel or medium-low-carbon alloy steel wire rods. Due to the good toughness of medium-carbon steel and low-alloy steel, steel wires that are lightly drawn after spheroidizing annealing have Low deformation resistance and good cold heading forming characteristics, but the tensile strength of steel wire after cold forming generally does not exceed 700MPa, and it needs to be quenched and tempered (quenching-high temperature tempering) to obtain tempered sorbite structure in order to achieve high-strength bolts required mechanical properties.

Using medium carbon steel or medium and low carbon alloy steel wire rods to produce high-strength fasteners requires time-consuming and energy-consuming spheroidizing annealing before cold drawing and cold heading, and after cold drawing and forming. Quenching and tempering treatment, the production process is complicated, time-consuming and energy-consuming. Therefore, the use of cold-strengthened non-quenched and tempered steel wire rods to manufacture high-strength bolt products can save the spheroidizing annealing treatment before cold drawing and the quenching and tempering treatment after forming, which greatly simplifies the production process and shortens the production process. cycle, reduces energy consumption, and avoids problems such as surface oxidation, decarburization, and workpiece deformation caused by heat treatment, so it has significant economic and social benefits.

Non-quenched and tempered cold heading steel is a new product developed in recent years. There is no spheroidizing annealing treatment before the wire rod is cold drawn, and no quenching and tempering (quenching and high temperature tempering) treatment is required for the fastener after cold heading. Low-temperature aging treatment can achieve excellent strength and toughness. It is mainly used to replace medium-carbon steel and medium-low carbon alloy steel to manufacture high-strength fasteners with strength levels of 8.8, 9.8, 10.9 or even 12.9. At present, non-quenched and tempered steels for cold heading are mainly divided into ferrite-pearlite type, bainite type and martensitic type. Among them, 8.8 and 9.8 grade fasteners are mainly made of ferrite + pearlite type non-quenched and tempered cold heading steel. Bainite or martensitic non-quenched and tempered cold heading steel is basically used for fasteners of grade 10.9 and above.

Technical solutions

The technical problem to be solved by the present invention is to provide a non-quenched and tempered cold heading steel wire rod for 8.8 grade fasteners, through precise design of alloy composition, coordinated smelting, continuous casting, continuous rolling and controlled rolling and controlled cooling of wire rods, Accurate control of the structure and performance of wire rods is achieved. The non-quenched and tempered cold heading steel wire rods produced for grade 8.8 fasteners are uniform in composition, ferrite + pearlite in structure, excellent in strength and plasticity, and meet the requirements of non-quenched and tempered production. Grade 8.8 high-strength fasteners.

The feature of the present invention is to produce non-quenched and tempered cold heading steel wire rods for high-strength fasteners meeting grade 8.8 through composition design combined with production technology. The composition of the wire rod involved is: [C]: 0.25% to 0.30%, Si]: 0.40%～0.60%, [Mn]: 1.10%～1.30%, [P]≤0.020%, [S]≤0.015%, [Cr]: 0.10%～0.15%, [V]: 0.06%～ 0.10%, [Ti]: 0.01% ~ 0.03%, unavoidable impurities not higher than 0.1%, the rest is iron.

The scoping rationale for these components is as follows:

As the C content increases, the strength and hardness of the steel increase, while the plasticity and toughness decrease. On the basis of ensuring the strength, reducing the carbon content is conducive to improving the toughness and cold workability of the steel. Therefore, the carbon C is controlled at 0.25% to 0.30%.

Si is a ferrite-forming element, which can significantly strengthen ferrite, increase the volume fraction of ferrite in steel, and refine ferrite grains. However, although Si content is too high, it is beneficial to the improvement of strength, but it will reduce the toughness of steel. , making cold heading more difficult, so under the condition of ensuring the tensile strength, its content should be reduced, so Si is controlled at 0.40% to 0.60%.

Mn is an important alloying element to increase the strength of non-quenched and tempered steel and improve the toughness of non-quenched and tempered steel. The strengthening effect of Mn is mainly manifested in the decrease of pearlite transformation temperature with the increase of Mn content. The decrease of transformation temperature means that the pearlite obtained by air cooling The clusters are finer, the spacing of pearlite sheets is smaller, the carburized sheets in pearlite are correspondingly thinner, and the strength and toughness of steel are improved; at the same time, the volume fraction of pearlite in steel also increases, and the cold workability of steel decreases. Mn is controlled at 1.10% to 1.30%.

Phosphorus and sulfur are harmful impurity elements in steel. Phosphorus will precipitate Fe ₃ P with high brittleness in steel, which will increase the strength and brittleness of steel at room temperature, that is, increase the cold brittleness of steel. Sulfur hardly dissolves in iron, but forms FeS with iron, and FeS and Fe form eutectic with low melting point. During hot working, the eutectic distributed in the grain boundary is easy to melt and cause cracking, which increases the hot brittleness of steel. In order to ensure the good performance of the wire rod, it is required that [P]≤0.020% and [S]≤0.015% in the steel, the lower the better without causing other influences.

Cr strengthens ferrite and pearlite by means of solid solution strengthening, which has the effect of refining the spacing of pearlite sheets and increasing the strength and toughness of steel; Cr strongly reduces the bainite transformation temperature, promotes the C curve of steel to shift to the right, and decomposes There are two parts of pearlite transformation and bainite transformation, which can effectively improve the hardenability of steel, so Cr is controlled at 0.10% to 0.15%.

The role of V in steel is mainly to refine grains. Through the dispersion and precipitation of its carbonitride material points and the solid solution of V, the strength and toughness of steel are greatly improved. In addition to grain refinement and strengthening, V in steel also It has a strong precipitation strengthening effect, and the formation temperature range of carbides, nitrides and carbonitrides of V is relatively low, the solubility is high, and the precipitation range is wide. V has little effect on inhibiting austenite grain growth at high temperature, but it is dispersed and precipitated in ferrite at low temperature, and the contribution of precipitation strengthening is the largest, so V is controlled at 0.06% to 0.10%.

The role of Ti in steel is mainly to refine grains. Through the dispersion and precipitation of its carbonitride material points and the solid solution of Ti, the strength and toughness of steel are greatly improved. In addition to grain refinement and strengthening, Ti can also It has a strong precipitation strengthening effect. In Ti-V composite strengthening steel, Ti(CN) is preferentially precipitated, and V is subsequently precipitated on the compound of Ti to form stable (TiV)(CN) particles, which can obtain good strength and toughness. non-quenched and tempered steel. Since Ti(CN) is preferentially precipitated, it will occupy part of C and N, which will deplete N in the subsequently formed V(CN) and reduce the effect of precipitation strengthening. Therefore, the Ti content should not be too high. Excessive Ti content will also reduce the toughness of the steel. Therefore, Ti is controlled at 0.01% to 0.03%.

The production process of non-quenched and tempered cold heading steel wire rod for high-strength fasteners of grade 8.8: hot metal pretreatment-converter smelting-LF furnace refining-bloom continuous casting-continuous rolling billet-wire heating, rolling, controlled cooling. Converter adopts double slag method for smelting, slag-retaining and steel tapping, control P≤0.010% in LF furnace; LF furnace makes high-alkalinity slag deep desulfurization, submerged arc heating, argon blowing, alloying; continuous casting billet size 280×380mm, The superheat of molten steel in the tundish is 25°C-30°C, the electromagnetic stirring current is 400-450A, and the casting speed is 0.6-0.7m/min; Rolling at 800°C-820°C, spinning temperature at 750°C-780°C, roller table speed at 6-9m/min, opening the first 2 to 4 insulation covers, closing the rest of the insulation covers, and turning off all the fans. Process to obtain fine ferrite and degenerated pearlite structure, which has good cold workability and toughness, and meets the production requirements of 8.8 high-strength non-quenched and tempered fasteners.

The non-quenched and tempered cold heading steel wire rod for 8.8 fasteners of the present invention can save the annealing treatment before cold drawing and the quenching and tempering treatment after forming the fasteners, simplify the production process, shorten the production cycle, and reduce the Energy consumption is reduced, and problems such as surface oxidation, decarburization, and workpiece deformation caused by heat treatment are avoided, so it has significant economic and social benefits.

detailed description

The present invention is further illustrated by some examples below.

Converter adopts double slag method to smelt slag and tap steel, control P≤0.010% in LF furnace; LF furnace high alkalinity slag deep desulfurization, submerged arc heating, argon blowing, alloying; continuous casting billet size 280×380mm, tundish The superheating degree of molten steel is 25°C-30°C, the electromagnetic stirring current is 400-450A, and the casting speed is 0.6-0.7m/min; ℃-780℃, roller table speed 6-9m/min, 1 ^# -2 ^# heat preservation covers open, other heat preservation covers closed, fans all closed, rolling wire rod diameter Φ5.5-Φ25mm.

Claims (2)

1. a kind of 8.8 grades of securing member non-quenched cold heading steel wire rods are it is characterised in that composition is by weight percentage： [C]:0.25%～0.30%, [Si]:0.40%～0.60%, [Mn]:1.10%～1.30%, [P]≤0.020%, [S]≤0.015%, [Cr]:0.10%～0.15%, [V]:0.06%～0.10%, [Ti]:0.01%～0.03%, Inevitably impurity is not higher than 0.1%, and remaining is ferrum.

2. a kind of 8.8 grades of securing member non-quenched cold heading steel Production method for wire rod according to claim 1, It is characterized in that：Add including molten iron pretreatment-converter smelting-LF stove refine-bloom continuous casting-tandem rolling cogging-wire rod Heat, rolling, control are cold；Converter adopts double slag process to smelt, slag-stopping tapping, controls into LF stove P≤0.010%； LF stove makes high basicity slag depth desulfurization, submerged arc heating, Argon, alloying；25 DEG C -30 of the Metal in Tundish degree of superheat DEG C, electromagnetic agitation electric current 400-450A, pulling rate 0.6-0.7m/min；Wire rod rolling adopts thermo-mechanical rolling technique, Wire rod heating-up temperature 1100-1200 DEG C, rolls into 800 DEG C -820 DEG C of finish rolling, 750 DEG C -780 DEG C of laying temperature, Roller table speed 6-9m/min, front 2～4 thermal insulation covers are opened, and remaining thermal insulation cover is closed, blower fan Close All.