Influence of cooling rate and boron content on the microstructure and mechanical properties of hot-rolled high strength interstitial-free steels

The effect of different fast cooling rates on the microstructure and mechanical properties of the V and Ti microalloyed high strength cold-rolled sheet was studied under laboratory conditions. Five different fast cooling rates were set up as 20°C/s, 50°C/s, 200°C/s, 500°C/s and 1000°C/s, respectively. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the microstructure, and the mechanical properties were also tested. The results showed that with the increase of fast cooling rate from 20°C/s to 1000°C/s, the grains of martensite and ferrite were finer, and the average grain size of both martensite and ferrite decreased from 7.7μm to 3.9μm. The proportion of ferrite in the two phases decreased while that of the martensite increased from 25.7% to 62.1%. The morphology of martensite tended to be lath, and the density of dislocation in the ferrite grains nearby the martensite gradually increased. With cooling rate rising from 20°C/s to 1000°C/s, the yield strength of the experimental steel increased from 381MPa to 1074MPa, and the tensile strength increased from 887MPa to 1199MPa. And the elongation decreased from 14.2% to 7.2%, and the product of strength and elongation decreased from 12.6GPa·% to 8.6GPa·%.

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Influence of Boron on Mechanical Properties and Microstructures of Hot-Rolled Interstitial Free Steel

Materials Science Forum ◽

10.4028/www.scientific.net/msf.495-497.537 ◽

2005 ◽

Vol 495-497 ◽

pp. 537-542 ◽

Cited By ~ 3

Author(s):

S.I. Kim ◽

Shi Hoon Choi ◽

Yeon Chul Yoo

Keyword(s):

Mechanical Properties ◽

High Strength ◽

Recrystallization Temperature ◽

Interstitial Free ◽

If Steel ◽

Softening Behavior ◽

Axial Tensile ◽

Dynamic Softening ◽

Work Hardening Rate ◽

Hot Rolled

This paper examines an effect of boron (B) on dynamic softening behavior, mechanical properties and microstructures for Nb-Ti added high strength interstitial free (IF) steel. For this purpose, IF steels containing 0ppm B, 5ppm B and 30ppm B were chosen. Continuous cooling compression test was performed to investigate dynamic softening behavior. Mechanical properties and microstructures of pilot hot-rolled IF steel sheet were analyzed by uni-axial tensile test and electron back-scattered diffraction (EBSD). It was found that no-dynamic recrystallization temperature (Tndrx) which can be determined from the relationship between flow stress and temperature is a constant of 955oC for all IF steels. However, an addition of B into IF steels increases work hardening rate at the temperature below Tndrx. It was also verified that B retards phase transformation of austenite into ferrite. EBSD analysis revealed that absence of B induces fine ferrite grain size and many high angle grain boundaries.

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Effects of Thermo-Mechanical Process on the Microstructure and Mechanical Properties of Low Carbon HSLA Steels

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.15-17.786 ◽

2006 ◽

Vol 15-17 ◽

pp. 786-791 ◽

Cited By ~ 1

Author(s):

J.S. Kang ◽

Y. Huang ◽

C.W. Lee ◽

Chan Gyung Park

Keyword(s):

Mechanical Properties ◽

Grain Size ◽

Cooling Rate ◽

High Strength ◽

Nucleation Site ◽

Polygonal Ferrite ◽

Low Alloy Steels ◽

Low Carbon ◽

Microstructure And Mechanical Properties ◽

Average Grain Size

Effects of deformation at austenite region and cooling rate on the microstructure and mechanical properties of low carbon (0.06 wt. % C) high strength low alloy steels have been investigated. Average grain size decreased and polygonal ferrite transformation promoted with increasing deformation amount at austenite region due to increase of ferrite nucleation site. Microstructure was also influenced by cooling rate resulting in the development of a mixture of fine polygonal ferrite and acicular ferrite at 10°C/s cooling rate. Discontinuous yielding occurred in highly deformed specimen due to the formation of polygonal ferrite. However, small grain size of highly deformed specimen caused lower ductile-to-brittle transition temperature than slightly deformed specimen.

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