Enhancing the Tensile Properties and Ductile-Brittle Transition Behavior of the EN S355 Grade Rolled Steel via Cost-Saving Processing Routes

Abstract

Structural rolled steels are the primary products of modern ferrous metallurgy. Consequently, enhancing the mechanical properties of rolled steel using energy-saving processing routes without furnace heating for additional heat treatment is advisable. This study compared the effect on the mechanical properties of structural steel for different processing routes, like conventional hot rolling, normalizing rolling, thermo-mechanically controlled processing (TMCP), and TMCP with accelerating cooling (AC) to 550 °C or 460 °C. The material studied was a 20 mm-thick sheet of S355N grade (EN 10025) made of low-carbon (V+Nb+Al)-micro-alloyed steel. The research methodology included standard mechanical testing and microstructure characterization using optical microscopy, scanning and transmission electronic microscopies, energy dispersive X-ray spectrometry, and X-ray diffraction. It was found that using different processing routes could increase the mechanical properties of the steel sheets from S355N to S550QL1 grade without additional heat treatment costs. TMCP followed by AC to 550 °C ensured the best combination of strength and cold-temperature resistance due to formation of a quasi-polygonal/acicular ferrite structure with minor fractions of dispersed pearlite and martensite/austenite islands. The contribution of different structural factors to the yield tensile strength and ductile-brittle transition temperature of steel was analyzed using theoretical calculations. The calculated results complied well with the experimental data. The effectiveness of the cost-saving processing routes which may bring definite economic benefits is concluded.

Keywords: EN S355; TMCP; accelerated cooling; cost-saving; hot rolling; mechanical properties; microstructure; normalizing rolling; structural steel.

Figure 1

The schemes of the technological routes of S355N steel sheets processing.

Figure 2

The temperature-wise evolution of the absorbed impact energy for (a) longitudinal and (b) transverse specimens. (c) The temperature-wise ranging of the processing routes by the anisotropy index. (Red dotted lines in (a,b) show the minimum E level corresponding to the category “QL1”).

Figure 3

The fracture patterns of the V-notched specimens after the normalizing rolling (testing at −40 °C): (a) longitudinal, (b) transversal.

Figure 4

Microstructure of steel after the processing routes: (a) HR, (b) NR, (c) TMCP, (d,e) TMCP/AC₅₅₀, (f) TMCP/AC₄₆₀. ((a–d,f) are OM images, (e) is SEM image).

Figure 4

Microstructure of steel after the processing routes: (a) HR, (b) NR, (c) TMCP, (d,e) TMCP/AC₅₅₀, (f) TMCP/AC₄₆₀. ((a–d,f) are OM images, (e) is SEM image).

Figure 5

TEM images of steel after HR, NR, and TMCP treatments: (a,b,e) ferrite grains with a nearby pearlite colony, (c) pearlite colony with high dislocation density, (d) dislocation “walls” in ferrite grains (shown by the arrows), (f) cementite lamellae and selected area electron diffraction (SAED) showing the reflection on the [111] zone axis of ferrite and the reflection on the $[01\stackrel{\rightharpoonup }{2}]$ zone axis of cementite, (g) precipitates (Nb,V)C (shown by the arrows) and SAED showing the reflection on the [111] zone axis of carbide, (h) dislocation clots around the nano-precipitates inside ferrite grain. ((a)—HR, (b–d)—NR, (e–h)—TMCP).

Figure 5

TEM images of steel after HR, NR, and TMCP treatments: (a,b,e) ferrite grains with a nearby pearlite colony, (c) pearlite colony with high dislocation density, (d) dislocation “walls” in ferrite grains (shown by the arrows), (f) cementite lamellae and selected area electron diffraction (SAED) showing the reflection on the [111] zone axis of ferrite and the reflection on the $[01\stackrel{\rightharpoonup }{2}]$ zone axis of cementite, (g) precipitates (Nb,V)C (shown by the arrows) and SAED showing the reflection on the [111] zone axis of carbide, (h) dislocation clots around the nano-precipitates inside ferrite grain. ((a)—HR, (b–d)—NR, (e–h)—TMCP).

Figure 6

(a) EDS spectra and chemical composition of MC precipitate. (b) Size distribution of MC carbide.

Figure 7

Fine microstructure (TEM images) of steel after TMCP/AC₅₅₀ and TMCP/AC₄₆₀ treatments: (a) M/A “islands” (M/A), austenite films (A_f) in acicular ferrite grains and the SAED of M/A showing the reflection on the [111] zone axis of austenite, (b) cementite films and the SADEs of the reflections on the zone axes of [111] of ferrite and $[01\stackrel{\rightharpoonup }{2}]$ of cementite, (c) the bright-field and dark-field images of (Nb,V)C precipitates (shown by the arrows), (d) cementite lamellae (shown by the arrows) in bainite and the SAEDs of the reflections on the zone axes of [135] of ferrite and $[\overline{1}00]$ of cementite. ((a,b)—TMCP/AC₅₅₀; (c)—TMCP/AC₄₆₀).

Figure 8

Contribution (%) of structural strengthening factors to theoretic YTS.

Figure 9

Vector diagram of the change in T_DBT caused by strengthening effects of different structural factors depending on the processing scheme.

Figure 10

The temperature corresponding to the double reduction of the absorbed energy relative to testing at 0 °C.