On the Spheroidizing Annealing Behavior in Cr/Nb Microalloyed Medium Carbon Steels

Abstract

The industrial process for the production of fine‐blanking steels, which includes hot‐rolling, cold‐rolling, and spheroidizing annealing, is simulated at a laboratory scale using two medium carbon steels, respectively, microalloyed with Cr and Nb. Two types of hot‐rolled microstructures, that is, “ferrite + pearlite” and bainite are obtained by varying the interrupted cooling temperature (ICT). The microstructure and property evolution during spheroidizing annealing are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back scattered diffraction (EBSD), and micro‐hardness tester. In Cr microalloyed steel, the dual partitioning of Cr and Mn and slowed growth kinetics of cementite particle are correlated to explain the suppression of ferrite recrystallization. Contrarily, a fast spheroidizing annealing behavior, which led to uniform distribution of cementite particle and fully recrystallized ferritic matrix, is observed in Nb microalloyed steel especially from the hot‐rolled bainite microstructure.

Subcritical Spheroidization Annealing of Cold‐Rolled Cr–Mo Microalloyed Medium‐Carbon Steel with Different Initial Microstructure

“…In the early stage of spheroidizing annealing, carbide preferentially precipitates at GBs and grows directly on the basis of original carbide granules, and the growth rate at GBs is also significantly higher than that within grains. [12,21] The sharp corners are dissolved due to the difference in curvature radius between the carbide sharp corners and the carbide plane parts, which will destroy the surface tension balance of the phase boundary. To maintain the balance, the carbide continues to melt until it becomes spherical carbide with similar curvature radius everywhere.…”

“…During spheroidizing annealing, the original carbide continuously dissolves into spheres and grows up driven by the reduction of the free energy of the ferrite/carbide surface and the curvature difference between large and small particles, which is also accompanied by the precipitation and growth of new carbide. [8,12] As a result, the change process of the average grain size of carbide in cold-rolled steel with annealing time is quite intricate, and the change rule of the average grain size of the particles with annealing time is difficult to describe. For cold-rolled F þ P steel, the distribution of the aspect ratio of carbide particles at various annealing times is shown in Figure 9b.…”

“…Figure 14 shows TEM photos of local GBs of F þ B samples annealed for 8 h. Most carbides located on the GB of ferrite have pinning force, which hinders the GB migration. [2,12] The larger the number of small-sized carbide particles, the greater the pinning force on GBs, which can play a good role in inhibiting the growth of ferrite grains.…”

“…On the other hand, the effects of initial pearlite, bainite, and martensite microstructures on the spheroidizing process are studied. [11,12] However, few works have compared the microstructure evolution of the initial hot-rolled microstructure cold rolled at different reductions during subcritical annealing.…”

Failure Analysis of Boron Steel Components for Automotive Applications

“…Fine blanking maximizes service strength and minimizes peak contact pressure to extend tool life [7]. Usually, hot or cold rolled strips and sheets have to be spheroidized annealed to decrease hardness [8]. After fine blanking, components are quenched and tempered to reach the desired hardness, yield strength and ultimate tensile strength.…”