As-Cast Spot Segregation of Gear Steel and Its Evolution in the Rolled Products

doi:10.11900/0412.1961.2020.00482

Acta Metall Sin

2021, Vol. 57

Issue (10): 1281-1290 DOI: 10.11900/0412.1961.2020.00482

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As-Cast Spot Segregation of Gear Steel and Its Evolution in the Rolled Products

ZHANG Zhuang¹, LI Haiyang², ZHOU Lei², LIU Huasong¹, TANG Haiyan¹, ZHANG Jiaquan¹(

)

^1.School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China
^2.Special Steel Department, Nanjing Iron ＆ Steel Co. , Ltd. , Nanjing 210035, China

Cite this article:

ZHANG Zhuang, LI Haiyang, ZHOU Lei, LIU Huasong, TANG Haiyan, ZHANG Jiaquan. As-Cast Spot Segregation of Gear Steel and Its Evolution in the Rolled Products. Acta Metall Sin, 2021, 57(10): 1281-1290.

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Abstract

As a crucial transmission component and basic component in mechanical equipment, gears have extremely high requirements for homogeneity of elements. However, the banded segregation defects in a rolled bar will reduce the hardenability of the finished gear and cause uneven deformation in heat treatment. Generally, the formation of banded defects is believed to inseparable from semimacro segregation in the bloom castings. Spot segregation in gear steel bloom and banded segregation defects in a rolled bar were experimentally studied to explore its evolution. The correlation between the spot segregation and banded defects was clarified from the aspects of morphology characteristics, position distribution, element segregation, and spacing statistics using hot acid etching, dendritic etching, electron probe microanalysis (EPMA), and statistical calculations. The evolution law of spot segregation and the possibility of elimination of banded defects in the heat treatment stage were also discussed. Experimental results show that spot segregation mainly exists in the central equiaxed crystal area of the castings, and solute elements such as C, Mn, and Cr in the segregated spot show positive segregation. The wide banded defects are consistent with the spot segregation in their geometrical location and element distribution pattern. Morphologically, there are two types of banded defects in which the dendrite microsegregation and the independently existing spot segregation are changed into separate band structures during the hot rolling process, while the interconnected spot segregations convert into a type of converged band defect through the hot deformation. Thus, more fine and coarse banded defects have been observed in the final rolled bar products than the numbers of as-cast spot segregation. Heat treatment through heating and insulation at 1200^oC can reduce the degree of solute segregation in the banded defects with a width less than 40 μm, but it shows little beneficial effect on the more coarse-sized banded defects, which suggests that they should be controlled from the original as-cast dendrite morphology during the casting process.

Key words: gear steel solidification structure spot segregation banded defect evolution

Received: 02 December 2020

ZTFLH:

TG142.1

Fund: National Natural Science Foundation of China(U1860111、51874033);Project to Strengthen Industrial Development at the Grass-Roots Level(TC180A3Y1/14)

About author: ZHANG Jiaquan, professor, Tel: (010)62334222, E-mail: jqzhang@metall.ustb.edu.cn

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	Zhuang ZHANG
	Haiyang LI
	Lei ZHOU
	Huasong LIU
	Haiyan TANG
	Jiaquan ZHANG

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https://www.ams.org.cn/EN/10.11900/0412.1961.2020.00482 OR https://www.ams.org.cn/EN/Y2021/V57/I10/1281

Fig.1 Schematics of bloom casting hot acid etching sample (a) and its dendritic etching samples (b) (unit: mm)

Fig.2 Schematics of rolled bar samples for banded defects etching (a), longitudinal section (b), and transverse section (c) (unit: mm)

Fig.3 Morphology of hot acid etching A sample of bloom (a) and OM image of dendritic etching sample for the selected spot-like area (b) (I-IV represent the chilled layer, the columnar crystal zone, the columnar to equiaxed transition (CET) zone, and the equiaxed crystal zone, respectively)

Fig.4 OM images of dendritic etching sample at different areas (schematic shown in Fig.1b)

Fig.5 SEM image and map scanning of solute elements in the spot segregation area by EPMA (a) and corresponding line scanning along the line in Fig.5a (b)

Fig.6 Low (a) and high (b) magnified OM images of the banded defects in the middle of the longitudinal section sample

Fig.7 OM image and map scanning of solute elements in the banded defects areas by EPMA (a) and corresponding line scanning along the line in Fig.7a (b)

Fig.8 OM images of banded defects in the bar longitudinal section sample (a), spot defect in the bar transverse section sample (b), and hot acid etching morphology of the bloom casting sample B (c)

Fig.9 Frequency distribution histogram and normal distribution curve of band spacing (a) and spot spacing (b)

Fig.10 Schematics of morphological evolution of the complex spot segregation through rolling

Fig.11 Element diffusion distribution curves of banded segregation in bars of C (a) and Mn (b) during insulation process

Fig.12 Time for uniform diffusion of elements C (a) and Mn (b) during insulation process

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