Andrea Di Schino

Microstructure and Cleavage Resistance of High Strength Steels

The relationship between microstructure and cleavage resistance in quenched and tempered high strength bainitic and martensitic steels is investigated by means of Charpy-V three-point bending tests, uniaxial tensile test on unnotched specimens and EBSD. Steels under investigation are low/medium carbon (C=0.10%-0.40%) steels with yield strength in the range YS=500-1000 MPa. Results show that the tensile strength and the cleavage resistance of Q&T steels appear to be controlled by different structural parameters and not, as in the case of polygonal ferritic steels, by the same structural unit. In particular, yield strength is controlled by the mean subgrain size, whereas the structural unit controlling the critical cleavage stress is the covariant (bainitic or martensitic) packet, whose size is slightly lower than the average unit crack path (UCP). The critical stage in the fracture process appears to be the propagation of a Griffith crack from one packet to another, and the resistance offered by high-angle boundaries is approximately the same as that of low-C steels with bainitic or polygonal ferrite microstructure.

Metallurgical Design of High Strength/High Toughness Steels

The proper balance between yield strength, YS, and ductile to brittle transition temperature, DBTT, has been the main concern during development of high strength engineering steels and the effect of microstructure on impact toughness has attracted a great attention during the last decades. In this paper a review concerning the relationship between strength and toughness in steels will be presented and the effect of different microstructural parameters will be discussed, aiming to improve such properties in designing new high strength steels. Complex microstructures, obtained by quenching and tempering (Q&T) and thermo-mechanical (TM) processing are considered. The steels are low/medium carbon steels (C=0.04%-0.40%) with yield strength in the range YS=500-1000 MPa. Results show that the strength and the impact toughness behaviour are controlled by different microstructural parameters and not, as in the case of polygonal ferritic steels, by the same structural unit (the grain size) and that a “fine” microstructure is required in order to achieve high levels of both strength and toughness. The metallurgical design of high strength steels with toughness requirements is discussed using the same approach for both Q&T and TMCP processes.

Microstructure Evolution during Tempering of Martensite in a Medium-C Steel

To identify the characteristic microstructural length determining the mechanical properties of a quenched and tempered medium-C steel and its dependence on the prior austenite grain size, different tempering treatments have been carried out after a fully martensitic quenching. The resulting microstructures have been analyzed by Orientation Imaging Microscopy (OIM) and two kind of features have been taken into consideration: packets (i.e. domains delimited by high-angle boundaries) and cells (domains bounded by low-angle grain boundaries). The main results can be summarized as follows: 1. A very weak effect of austenite grain size on packet size was found. 2. A finer packet size was measured at mid-thickness with respect to surface after external and internal water quenching process. This phenomenon was attributed to the effect of the strain path on the phase transformation during quenching. 3. The through-thickness microstructural gradient remains substantially unchanged after tempering. 4. Grains with high-angle boundaries do not significantly grow after tempering; on the contrary, low-angle grain boundaries move, fully justifying the hardness evolution with the tempering temperature.

Boron Effect on Hardenability of High Thickness Forged Steel Materials

To fulfill the industrial demand of forged steels with high mechanical and microstructural requirements coupled with reduced cost, the possibility to decrease the content of Mo and other elements has been evaluated. In order to do that, the effect of boron addition (up to 30 ppm) on the steel hardenability has been investigated on two steels with different chemical composition at laboratory scale. In particular, the steel chemical composition has been designed in order to make effective the B addition in terms of hardenability. Two 80 kg ingots cast by a vacuum induction melting plant have been hot rolled by a pilot mill. The effect of B addition on hardenability has been evaluated and compared to that of steel for same application but without B. Results show an improvement of hardenability if 30 ppm B are added even if a Mo reduction is performed.

Microstructure Evolution during Quenching and Tempering of Martensite in a Medium C Steel

The microstructural evolution of a quenched medium-C steel during tempering was analyzed by means of Orientation Imaging Microscopy (OIM). The steel was heat treated in order to develop fully martensitic microstructures after quenching with a different prior austenite grain size (AGS). Main results can be summarized as below: A very poor effect of AGS on packet size was found in comparison to bainitic steels. A finer packet was measured at mid-thickness with respect to surface after the quenching process. This phenomenon was attributed to the effect of thermal strain path on phase transformation during quenching. The through-thickness microstructural gradient remains after tempering. High-angle boundary grains do not significantly grow after tempering; on the contrary, low-angle grain boundaries (cells) move, fully justifying the hardness evolution with tempering temperature.

Effect of quenching and partitioning process on a low carbon steel

Aim of this paper is to analyze the effect of manganese percentages on steel compositions. Laboratory as cast materials, in particular designed for Quenching and Partitioning process (Q&P), are here considered. The considered steel chemical composition was that of a 0.15C with 1.5Si, two different Mn contents and with no significant Al content. Two-Step Q&P heat treatments were carried out in laboratory by means of dilatometric tests. X-ray diffraction measurements have been carried out aimed to assess the retained austenite volume fraction. The tensile properties of the quenched and partitioned materials were analyzed. Results showed a marked dependence of strength, ductility and strain capacity values on heat treatment conditions. In the case of higher austenite contents, higher uniform elongation values were found. Higher tensile properties were found in the case of higher Mn steel with respect to the lower Mn one. The main novelty of this paper consists in applying Q&P to low carbon (low hardenability steel) showing the effect of such a process on mechanical properties of steels usually adopted for automotive applications.

Modelling Microstructure and Mechanical Properties of High Strength Steels During Hot Rolling in an ESP Plant

Production of hot rolled strips for oil and gas transport pipelines requires a fine and homogeneous microstructure and careful choice of chemical composition in order to meet strength, toughness, and weldability requirements. The current work is aimed to assess the feasibility of producing high strength hot rolled strips in a thin slab plant. The study was carried out using a metallurgical modelling system based on empirical equations describing austenite evolution, transformation and mechanical properties (UTS, 50% FATT). Using this approach, it is possible to study the effects of initial slab thickness, reduction ratio and chemical composition on the mechanical properties of strips in a thickness range of 10 to 20 mm. In this way, for each grade, the optimum rolling schedules, microstructures and chemical compositions could be obtained. Starting from this initial condition, the austenite evolution from the entrance of the first roughing stand, is simulated considering the following phenomena: • Recrystallization kinetics. • Recrystallized grain size. • Grain growth after recrystallization. • Strain induced precipitation of Nb(C, N).

Manganese Effect on Q&P CMnSi Steels

The present study is focused on analyzing the effect of Mn amount in two experimental steel compositions, specially designed for Quenching and Partitiong (Q&P), 0.15C-2.5Mn-1.5Si and 0.15C-3Mn-1.5Si without significant contribution of Al. Two-Step Q&P thermal treatments were performed at laboratory scale in a quenching dilatometer Bähr DIL805A/D. The fractions of retained austenite were evaluated by X-ray diffraction techniques. The mechanical properties of the Q&P samples were evaluated, a strong dependence of strength, uniform elongation and strain hardening values on process parameters has been found. Higher uniform elongation were related to higher residual austenite contents. The 0.15C-3Mn-1.5Si steel showed systematically the largest mechanical values with respect to the 0.15C-2.5Mn-1.5Si steel.

Improving Hardenability of High Thickness Forged Steel Materials by Boron Addiction

To fulfill the industrial demand of forged steels with high mechanical and microstructural requirements coupled with reduced cost, the possibility to decrease the content of Mo and other elements has been evaluated. In order to do that, the effect of boron addition (up to 30 ppm) on the steel hardenability has been investigated on two steels with different chemical composition at laboratory scale. In particular, the steel chemical composition has been designed in order to make effective the B addition in terms of hardenability. Two 80 kg ingots cast by a vacuum induction melting plant have been hot rolled by a pilot mill. The effect of B addition on hardenability has been evaluated and compared to that of steel for same application but without B. Results show an improvement of hardenability if 30 ppm B are added even if a Mo reduction is performed.

Micro-alloyed steels for forgings: effect of nano-precipitation on mechanical properties

The aim of the present paper deals with the production of heavy steel forgings of micro-alloyed steels. Improvements of mechanical properties of forgings up to level by sheets, strips and tubes have been achieved by mean of application of micro-alloying elements and thermo-mechanical treatments. The effect of vanadium addition has been assessed by means of metallurgical modelling step followed by a laboratory demonstration phase by means of ingot manufacturing. Heat treatment has been designed aimed to achieve the desired target tensile properties. Results show that ASTM A694 F70 grade requirements can be fulfilled by 0.15% V addition and a proper heat treatment in a ferrite-pearlite microstructure, representative of a forged component.