Pasi Suikkanen

Crystallographic Analysis of Martensite in 0.2C-2.0Mn-1.5Si-0.6Cr Steel using EBSD

The crystallography of martensite formed in 0.2C-2.0Mn-1.5Si-0.6Cr steel was studied using the electron backscattered diffraction (EBSD) technique. The results showed that the observed orientation relationship (OR) was closer to that of Nishiyama-Wassermann (N-W) than Kurdjumov-Sachs. The martensite consisted of parallel laths forming morphological packets. Typically, there were three different lath orientations in a morphological packet consisting of three specific N-W OR variants sharing the same {111} austenite plane. A packet of martensite laths with a common {111} austenite plane was termed a crystallographic packet. Generally, the crystallographic packet size corresponded to the morphological packet size, but occasionally the morphological packet was found to consist of two or more crystallographic packets. Therefore, the crystallographic packet size appeared to be finer than the morphological packet size. The relative orientation between the variants in crystallographic packets was found to be near 60°/ , which explains the strong peak observed near 60° in the grain boundary misorientation distribution. Martensite also contained a high fraction of boundaries with a misorientation in the range 2.5-8°. Typically these boundaries were found to be located inside the martensite laths forming sub-laths.

Effect of Austenite Pancaking on the Microstructure, Texture, and Bendability of an Ultrahigh-Strength Strip Steel

The effect of austenite pancaking in the non-recrystallization regime on microstructure and texture evolution and thereby on bendability was investigated in an ultrahigh-strength strip steel with a martensitic-bainitic microstructure. The results indicate that an increase in rolling reduction (Rtot) below the non-recrystallization temperature, which improves the strength and toughness properties, increases the intensities of the ~{554}〈225〉α and ~{112}〈110〉α texture components along the strip centerline and of the ~{112}〈111〉α component at the surface region. Even with the highest Rtot of 79 pct, the bendability along the rolling direction was good, but the preferred alignment of rod-shaped MA constituents along the rolling direction led to a dramatic decrease in the bendability transverse to the rolling direction, with severe cracking occurring even at small bending angles. The early cracking is attributed to localization of the strain in narrow shear bands. It is concluded that the Rtot value has to be limited to guarantee successful bendability.

Microstructure, Properties and Design of Direct Quenched Structural Steels

Direct quenching (DQ) is one of the latest process routes in production of ultra-high strength, high performance steels and Ruukki one of the pioneering companies in the utilization of direct quenching. Ruukki has applied direct quenching for the production of ultra-high-strength structural steels in the form of hot-rolled strip and plate. The paper briefly summarizes the physical metallurgy fundamental of direct steels and shows some selected examples of the microstructures and properties of steels produced by direct quenching. In addition, a brief review on the usability properties and design rules of ultra-high strength structural steels is made.

Processing Low and Ultra-Low Carbon Bainitic Steels with Excellent Property Combinations

Six experimental low and ultra-low carbon C-Mn-Mo-Nb-B and one conventional TMCP steel heats have been prepared to study the effects of chemical composition and hot deformation on the microstructure and the strength-toughness properties. In physical simulation tests, it was found that the deformation of austenite below the non-recrystallization temperature enhances the formation of higher-temperature bainitic morphologies and polygonal ferrite. On the other hand, hardness exhibits relatively low sensitivity to the degree of deformation below Tnr, whereas the deformation results in a distinct refinement in the microstructures, as determined by SEM-EBSD measurements, suggesting an improvement in the impact toughness. Simultaneous alloying with Mo-Nb-B seemed to be most efficient to provide high hardness and strength. Hot rolling trials indicated that the yield strength in the range 500-700 MPa with the excellent toughness down to –80 °C can be achieved in low carbon (≈ 0.03%) bainitic grades.

Applicability of the Master Curve Method to Ultra High Strength Steels

Although Ultra High Strength Steels (UHSS) with nominal strengths up to 1500 MPa have been available on the market for many years, the use of these steels in the civil engineering industry is still rather uncommon. One critical point limiting the use of UHSS steels lies in their rather poorly documented fracture properties in relation to more conventional steels covered by the codes. The major concept governing the assessment of steels is the Master Curve (MC) methodology. It provides a description for the fracture toughness scatter, size effect and temperature dependence in the ductile to brittle transition region. It enables a complete characterization of brittle fracture toughness of a material based on only a few small size specimens. The method combines a theoretical description of the scatter, a statistical size effect and an empirically found temperature dependence of fracture toughness. The fracture toughness in the brittle fracture regime is thus described with only one parameter, the transition temperature T0. At this temperature the mean fracture toughness for a 25.4 mm thick specimen is 100 MPa√m. The Master Curve method as defined in ASTM E1921-13a is applicable to ferritic structural steels with yield strength between 275 MPa and 825 MPa. Very few studies have been made with respect to the applicability of the Master Curve to Ultra High Strength Steels with yield strengths in the excess of 900 MPa. This is the topic of this work. Focusing on novel directly quenched high performance steels, the applicability of the Master Curve methodology with special emphasis on the temperature dependence will be investigated. Possible improvements to the Master Curve will be proposed for further consideration.Copyright

Innovation and Processing of Novel Tough Ductile Ultra-High Strength Steels through TMR-DQP Processing Route

Based on the recent concept of quenching and partitioning (Q&P), a novel TMR-DQP (thermomechanical rolling followed by direct quenching and partitioning) processing route has been established for the development of ultra-high strength structural steels with yield strengths ≈1100 MPa combined with good uniform and total elongations and impact toughness. Suitable compositions were designed based on high silicon and/or aluminium contents with or without small additions of Nb, Mo or Ni. The DQP parameters were established with the aid of physical simulation on a Gleeble simulator. Finally, the TMR-DQP processing route was designed for trials on a laboratory rolling mill. Metallographic studies showed that the desired martensite-austenite microstructures were achieved thus providing the targeted mechanical properties. The advantage of strained austenite in refining the martensite packets/blocks was clearly evident. No adverse effect of prolonged partitioning simulating the coiling stage has been noticed suggesting new possibilities for strip and plate products. Promising results in respect of microstructures and mechanical properties indicate that there are possibilities for developing tough ductile structural steels through the TMR-DQP route.

Hardness Profiles of Quenched Steel Heat Affected Zones

This paper reports the effects of chemical composition on the hardness of the heat affected zone of re-austenitized and water quenched steels. Heat affected zone peak temperatures in the range 300–1350 °C were simulated using a Gleeble 3800 simulator using thermal cycles appropriate to welds with cooling times between 800 and 500 °C of 12s. The maximum softening relative to the base material occurred in the intercritical and subcritical heat affected zones at the peak temperatures of 700 or 800 °C. Usually softening was greatest when the peak temperature was 700 °C. Linear regression analysis showed that carbon, and to some extent manganese and nickel, are detrimental at the peak temperature of 700 °C, but beneficial at the peak temperature of 800 °C in respect to softening relative to the base material, whereas niobium and especially molybdenum are beneficial at both temperatures. The beneficial effects of molybdenum alloying are seen down to peak temperatures of 400 °C whereas the effect of niobium microalloying is not statistically significant at peak temperatures lower than 700 °C. The softening in the intercritical, fine-grained and coarse grained heat affected zones are discussed and the effects of the alloying elements on the hardness of the subcritical heat affected zone are compared with their known effects on martensite hardness during conventional tempering.

Effect of Red Scale on the Bendability of Ultrahigh-Strength Strip Steel

The effect of red scale on the bendability of a thermomechanically rolled and direct quenched pilot-scale strip steel has been studied by comparing the bending behaviour of adjacent areas with and without red scale. The yield strength of the studied 8 mm thick strip was 960 MPa. The local microstructure and texture below the different scale surfaces were characterized using FESEM and FESEM-EBSD, chemical compositions were determined using GDOES, microhardness profiles were measured and bendability was determined using three-point brake press bending. Red scale was found to significantly affect bendability especially when the bend axis is transverse to the rolling direction. The minimum usable punch radius for defect-free bends in the absence of red scale was 12 mm (1.5 x thickness) while under red scale it was 30 mm (3.75 x thickness). Beneath the red scale the microstructure 50 to 400 μm below the surface was clearly different to that in the absence of red scale. Without the red scale the microstructure was mainly granular bainite with small fraction of upper bainite and polygonal ferrite. Below the red scale the microstructure was a mixture of upper bainite and granular bainite. As a result of the microstructural differences, the subsurface hardness changed substantially from 360 HV in the absence of red scale to 410 HV with red scale. The chemical composition did not change as a result of the presence or absence of red scale, which rules this factor out as possible cause of differences in bendability or final microstructure. Possible explanations for the observed effects of red scale on subsurface microstructure, and microstructure on bendability, are discussed in the paper.

Effect of re-austenitization on the transformation texture inheritance

Bainitic-martensitic microstructures produced by direct quenching austenite subjected to different degrees of pancaking have been re-austenitized and quenched to fully martensitic structures in order to investigate the effect of prior texture on the final martensite texture. Three different prior austenite pancaking states varying from convex-like to highly pancaked were investigated using an ultrahigh-strength strip steel hot rolled with various finish rolling temperatures followed by direct quenching. Microstructures were characterized using FESEM and transformation texture analysed using FESEM-EBSD at the strip surface, quarter- thickness and mid-thickness positions. The results show that an increase in rolling reduction below the non-recrystallization temperature increases the intensities of ~{554} α and ~{112} α texture components in the ferrite along the strip mid-thickness and of the ~{112} α component at the surface. The re-austenitization of the materials at 910°C for 30 min led to an inheritance of the same components from the parent specimens, but also increased the intensity of {001} α, {110} α and {011} α components.

Simulation of Line Annealing of Type 430 Ferritic Stainless Steel

The annealing behavior of cold rolled Type 430 ferritic stainless steel is the subject of this paper. The steel was cold rolled 79%, then heated at 6 °C/s to the soaking temperature of 841 °C, which is just below the Ae1 temperature. During heating, specimens were quenched from selected temperatures between 650 and 841 °C and after various times at 841 °C. These quenched samples underwent metallographic examination and micro-hardness determination. The results indicated that under the prevailing experimental conditions, the hardness appeared to correlate strongly with the extent of recrystallization. The kinetics of recrystallization appeared to originate in the cold worked state, where three kinds of grain were found: (i) smooth elongated, featureless of α-fiber orientation {001}<100>; (ii) irregular fishbone grains of the γ-fiber orientations {111}<112> plus {111}<110>; and (iii) twisted grains of the η-fiber orientation {001}<100>. It was found that the twisted grains of the η-fiber were the first to recrystallize, with the fishbone grains of the γ-fiber second, and the smooth elongated, featureless grains of the α-fiber last. It was found that the grains of the α-fiber orientation {001}<100> and the η-fiber orientation {001}<100> were replaced with grains of the γ-fiber orientations as recrystallization progressed. These results are discussed in terms of recrystallization and texture development.