Design of carbide free bainitic steels for hot rolling practices

Figures & data

Figure 1. Examples of the type of theoretical calculations, mainly related with bainitic transformation, and the parameters used in the design process.

Table 1. Effects of increasing the chosen alloying elements on the different calculated design parameters at a RefT of 400°C.

	$B_s$	$M_s$	$B_s-M_s$	$C_{\gamma }$	$V_{\mathrm{\alpha b}}$	$V_{{\alpha }^{\mathrm{\prime }}}$	$V_{\mathrm{\gamma f}}$	$V_{\mathrm{\gamma b}}$	${\sigma }_y^{\gamma }$	$t_{{\alpha }_b}$	msp	Tdiff	tdiff	tadiff
Mn	↓↓	↓↓	↓	↓↓	↓↓	↑	↓	↔,↑	↓↓	↑	↓↓	↓↓	↑↑	↑↑
Cr	↓↓	↓↓	↔	↓	↓	↔↑	↔	↔,↑	↓	↑	↓	↓	↑	↑
Mo	↔,↓	↔,↓	↔	↔,↓	↔	↔	↔	↔	↑↑	↓	↑	↔	↔,↑	↔,↑
Si	↓	↔	↔,↓	↔,↑	↔	↔	↔	↔	↑↑	↓↓	↑↑	↔,↑	↓↓	↔
C	↓	↓↓	↑	↔	↓↓	↑↑	↓	↑	↑	↔,↓	↑	↓↓	↑	↔,↑

Note: Where $C_{\gamma }$ and ${\sigma }_y^{\gamma }$refers to the C content and the yield strength of the remaining austenite after bainitic transformation at RefT respectively. Double and single arrow means strong and moderate influence respectively, a horizontal double-side arrow indicates no influence and horizontal in combination with a single arrow indicates a slight effect. The upward and downward directions of the arrows indicate an increase or decrease of the corresponding design parameters.

Table 2. Selected chemical compositions (wt.%), and experimentally measured A_c3 and $M_s$ temperatures, all in °C.

Steel	C	Si	Mn	Cr	Mo	Ac3	$M_s$
F1	0.25	1.8	2	1.5	0.25	887 ± 4	346 ± 3
F2	0.3	1.5	1.25	1.75	0.25	858 ± 6	351 ± 3
F3		2.5	1.5	1.5	0.25	932 ± 4	346 ± 8
F4		2.5	1.75	1.25	0.25	935 ± 5	337 ± 4

Figure 2. SEM micrographs and results summary on the microstructural characterisation of the hot rolled and coiled samples. ${\alpha }_b,\gamma$,${\alpha }_T^{{}^{\mathrm{\prime }}}$ and MA stands for bainitic ferrite, retained austenite (blocks (b) and films (f)), tempered martensite and martensite-austenite constituent respectively. $V_i$ in vol.% and $t_{{\alpha }_b}$ in nm.

Figure 3. Strain stress and instantaneous work hardening. The straight line corresponds to the instability criterion, true strain = n. For each condition the specific values of YS, UTS, ${ϵ}_{\mathrm{u}}$ and ${ϵ}_{\mathrm{T}}$ are provided.

Figure 4. Average Charpy impact energies for the entire test temperature range, – 100–100°C. Charpy V-notch (CVN) specimens were extracted from the transverse to the rolling direction (ASTM-A370).