CN1632138A - Process method for obtaining ultra-fine grain structure with bimodal grain size distribution in hypoeutectoid steel - Google Patents

Abstract

The present invention discloses a technological process used for obtaining double peak grain size distributed superfine grain structure from hypoeutectoid steel, said process is used for obtaining a superfine grain structure through a control technological process, said superfine grain structure is characterized in that the size distribution of the crystal grain structure is of double-peak characteristic, meaning that the superfine grain substrates having diameter less than 0.4 mum contain finite fractional number of relatively larger grains with size of 1-3 mum therein; the component of the steel used for engineering is characterized in than the carbonaceous mass fraction of the hypoeutectoid steel is smaller than 0.4%. In combination with control warm-rolling technology, the present invention can economically product superfine grain steel plate materials and strip materials with high ductibility.

Description

Process method for obtaining ultra-fine grain structure with bimodal grain size distribution in hypoeutectoid steel

technical field

The invention relates to the technical field of structure control technology of a class of engineering steel.

technical background

Due to the low work-hardening ability of ultrafine-grained or nano-crystalline materials, the uniform elongation and total elongation at room temperature are much lower than those of coarse-grained materials. Wang Shenggang et al. (Hardware Science and Technology, 2003, (12): 36) had only 6% elongation of the nano industrial pure iron plate prepared by deep rolling+400°C annealing. Y.T.Zhu (J.Mater.Res., 2003, 18: 1908) reported in the study of nanostructured Ti and H.Jin (Scripta Materialia, 2004, 50: 1319) in ultrafine-grained Al-Mg alloys that they are prone to plastic instability. This greatly limits the processing and practical application of ultrafine or nanocrystalline materials as structural materials. Therefore, it is an urgent problem to explore a method of bulk ultrafine/nanocrystalline steel that is more suitable for industrial production and improve its uniform elongation. Studies in recent years have shown that introducing an appropriate amount of relatively coarse grains into the ultrafine grain structure, that is, a grain structure that causes a bimodal grain size distribution, can greatly increase the elongation with little loss of strength. This organizational feature may become a method for obtaining uniform tensile deformation of nanostructured metals (Y.M.Wang, E.Ma.Acta Mater, 2004, 52:1699), and in pure Cu (Y.Wang, M.Chen, F .Zhou, E.Ma.Nature, 2002, 419:912) and Al alloys (D.Witkins, Z.Lee, R.Rodrigues, S.Nutt, E.J.Lavernia.Scripta Materialia, 2003, 49:297) obtained confirmed. So far, there is no report on the process of obtaining ultra-fine grain and nano-grain structure with bimodal grain size distribution in engineering steel.

Contents of the invention

In order to greatly improve the ductility of ultra-fine-grained steel with a small loss of strength and broaden the application range of ultra-fine-grained steel structures, the present invention provides a method for obtaining ultra-fine-grained steel with bimodal grain size distribution in hypoeutectoid steel. The process method of microstructure, the hypoeutectoid steel processed by this process method has the characteristics of high strength and good ductility. The process method combined with the controlled warm rolling technology can be economically used in the production of ultra-fine grain steel sheets and strips with excellent comprehensive mechanical properties.

The process method provided by the present invention for obtaining ultra-fine grain structure with bimodal grain size distribution in hypoeutectoid steel is: before warm deformation and cold deformation, the steel is heated to a temperature range of A _c1 ~ A _c3 + 20 ° C to keep warm and penetrate Burning, quenching immediately after 30-80% deformation to obtain a martensite + ferrite two-phase preparatory structure; single-pass or multi-pass deformation at a temperature of 700-350°C, with a total deformation of more than 50%; or After deforming by 50% at room temperature, recrystallization annealing is carried out at 350-700° C., and the annealing time is 0.2-18 hours. After the above treatment, the hypoeutectoid steel can obtain a relatively large grain structure with a certain fraction size of 1-3 μm in the ultra-fine grain matrix with a diameter of 0.4 μm or less, that is, obtain a bimodal grain size distribution ultra-fine grain structure, Therefore, the ductility of ultra-fine grain steel is greatly improved with little loss of strength.

Features of the present invention are as follows:

1. In the ultra-fine grain steel, the uneven or gradient structure of the grain size is caused, while the conventional control structure requires the grain size to be small and uniform.

2. Deformation quenching in the temperature range of A _c1 ~A _c3 +20°C can obtain a refined ferrite + martensite dual-phase preparatory structure. During the recrystallization annealing process after deformation, due to the difference in the chemical composition of the two phases The difference, resulting in the difference in the recrystallization kinetics, leads to the difference in the grain size of the two phases after recrystallization. The carbon content in the original martensite is high, the kinetic process of recrystallization after deformation is slow, and carbides will be precipitated to hinder the recrystallization. Finally, the original martensite forms an ultra-fine grain structure and the original ferrite forms a relatively coarse grain. A microstructure with a bimodal grain size distribution was obtained.

The invention can be combined with the controlled warm rolling technology to economically produce ultra-fine grain steel plates and strips with high ductility.

Detailed ways

Example: A 20CrMnTi steel cylindrical sample with a size of φ12×16mm was subjected to 80% compression deformation at 770°C with a Gleeble-3500 thermomechanical simulation test machine, quenched immediately, and then subjected to 40% room temperature compression deformation on a hydraulic press, and then Anneal at 550°C for 30min. After treatment, observe the microstructure with a transmission electron microscope, measure and count the grain size distribution, and obtain a bimodal grain size distribution structure, in which the volume fraction of grains below 200-50nm accounts for 50%, and the volume fraction of grains below 1-2μm accounted for 30%, and the volume fraction of grains in other size ranges accounted for 20%.

In this way, 20CrMnTi steel is subjected to 770°C compression 80% deformation quenching + 40% room temperature compression deformation + 550°C × 30min recrystallization treatment, and a double peak mainly composed of grains with a diameter of 50-200nm and 1-2μm is obtained. The organization of the grain size distribution.

Claims (1)

1. A method for hypoeutectoid steel to obtain bimodal grain size distribution ultra-fine grain structure, characterized in that: a. Before warm deformation and cold deformation, heat the steel to the temperature range of A _c1 ~ A _c3 + 20 ℃ and heat it through the fire. After 30 ~ 80% deformation, it is quenched immediately to obtain the preparation of martensite + ferrite two phases. organize; b. Under the temperature of 700 ~ 350 ℃ single pass or multi pass deformation, the total deformation is more than 50%. After the above treatment, the hypoeutectoid steel can obtain a relatively large grain structure with a certain fraction size of 1-3 μm in the ultra-fine grain matrix with a diameter of 0.4 μm or less, that is, obtain a bimodal grain size distribution ultra-fine grain structure; c. After deforming by 50% at room temperature, perform recrystallization annealing at 350-700° C., and the annealing time is 0.2-18 hours. After the above treatment, the hypoeutectoid steel can obtain a relatively large grain structure with a certain fraction size of 1-3 μm in the ultrafine grain matrix with a diameter of 0.4 μm or less, that is, obtain a bimodal grain size distribution ultrafine grain structure.