Study on Softening Annealing Process of Low Carbon Steel Wire

Low carbon steel wire nails are usually cold drawn with coarse-diameter wire rod many times to reach the required wire diameter.Due to the work hardening phenomenon, the strength of cold-drawn steel wire is increased, so a softening annealing is needed before cold-drawing low carbon steel wire nails.Processing process of low carbon steel wire nail – generally as:Φ6.5mm wire by 16 cold-drawn Φ2.0mm steel wire – softening annealing – pickling – cleaning – a full diameter cold drawing – — – cold drawing into nails – softening annealing quality directly affects the quality of low carbon steel wire nail, also does not affect the service life of cold stamping die, so how to get high low carbon steel wire softening annealing quality is worth studying.

At present, many domestic factories use well annealing furnace for softening annealing, furnace temperature from 850C to 950C, through complete recrystallization to achieve the purpose of softening.This paper studies the softening annealing process of low carbon steel wire in order to find the best process system.

1. Test method

1.1  Test materials

The test material is low carbon steel wire.From φ6. 5mm wire rod by drawing to Φ 1. 95mm steel wire.

In this experiment, two processes of partial recrystallization annealing and recrystallization annealing are used for cold drawn low carbon steel wire.The heating equipment is RBH-30 type box type resistance furnace.The heating temperature is 780℃, 730℃, 700℃, 650℃ and 500℃ respectively.Heated and kept warm, then cooled with the furnace to 200℃.The loading capacity is 350kg.

1.2  Metallographic structure observation and mechanical properties test

The tensile samples were made of Ø1.95 ~ 200mm wire with a standard distance of 60mm, and the test was carried out on the basis of WE-30A type material test.Vickers hardness measurement for low carbon steel wire. Along the cross section), using HV-120 Vickers hardness tester, load 5kg.

The microstructure of low carbon steel wire was observed along the longitudinal direction, and the microhardness of iron cable was measured by 71 microhardness tester.

2. Test results and analysis

2.1 Influence of heating temperature on steel wire properties

Tensile strength value of cold drawn low carbon steel wire annealed at different temperatures.It can be seen that the tensile strength of steel wire is not a simple linear relationship as the heating temperature changes.The strength decreases with the increase of heating temperature between 500C and 700℃.However, the strength increases after annealing at 730℃ and decreases obviously at 780C.When cold drawn steel wire is heated at 500~700 m, only recrystallization and recrystallized grain growth occur in the heating and holding process because the heating temperature is lower than AC1.By tempering the recrystallization surface, the strength and hardness of the work hardened cold-drawn wire decreased and the plasticity increased.The higher the temperature, the more rapid and thorough the tempering recrystallization process is, and the more the strength and hardness decrease.When heated at 730℃, partial recrystallization process occurs due to heating temperature exceeding AC1.The original microstructure was partially austenitized and transformed into ferrite and pearlite with fine grains upon cooling.Therefore, increase the strength and hardness.When annealed at 780℃, the grain strength and hardness decrease obviously due to the large amount of recrystallization process.

2.2 Metallographic structure observation

The low carbon steel wire used in the test has a very low carbon halo, so the annealing metallographic structure is ferrite and a very small amount of pearlite (a small amount of carbide is distributed in the ferrite crystal grain).In the process of cold drawing, the grain is elongated due to plastic deformation.In the softening annealing process, recrystallization can occur and equiaxed grains can be recovered.Carbon steel in the general low 450℃ or so begins to recompaction, so the annealing temperature used in this test, the deformed grains have been recrystallized to form equiaxed grains.After annealing at 650℃, the recrystallized equiaxed grains are mostly recrystallized at 780℃, with coarse grain size.The microhardness measurement of ferrite matrix also reflects the change of the structure energy.The steel wire after softening annealing is drawn in the whole diameter before cold drawing.Due to the drawing deformation, especially in cold punching, the steel wire and plastic deformation, in the low carbon steel wire surface and work hardening.The microstructure of low carbon steel wire is lengthwise cut, and the edge is the elongated deformed grain with plasticity deformation, and the core is the equiaxed grain after softening annealing.For the low carbon steel wire made by cold drawing after softening annealing 650C, the microhardness measurement shows that the microhardness of the ferrite phase at the edge is Hm175.4 (KGF /mm2), and the microhardness of the ferrite phase near the core is Hm140.8 (KGF /mm2).The hardness of low carbon steel wire is slightly higher than that of original annealed steel wire, which is beneficial to the use of low carbon steel wire because of work hardening of soft annealed steel wire in the whole diameter drawing and cold stamping process.

3. Selection of softening annealing process and production effect

There is no uniform and exact requirement for the mechanical properties of low carbon steel wire.Generally want to have a certain hardness, but do not want too high hardness.Therefore, low carbon steel wire is expected to be hard outside and soft inside.As noted earlier, surface work hardening due to full diameter drawing and cold blanking.

Many foreign factories of low carbon steel wire softening annealing using higher temperature recrystallization annealing or partial recrystallization annealing. Hangzhou Ruiqin Mechanical Equipment Co., Ltd. originally used 750~780C for softening annealing. Due to high heating temperature and serious oxidation and decarbonization, it is easy to cause insufficient diameter size of steel wire, and sometimes scrap in batches.And often appear when the spike spike spike even skin, nail bending and other defects, reduce the qualified rate of products.By analyzing the microstructure and properties of steel wire, we conclude that the softening annealing of low carbon steel wire can be carried out by recrystallization annealing at 650℃ and 10℃.More than one year of production practice has proved that: annealing at 650℃, wire oxidation is very slight, does not affect the wire diameter.

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