Metallurgical Aspects of Casting High‐Manganese Steel Grades

Abstract

The possibility of using new, high-strength steels with an attractive forming behaviour (e.g. induced plasticity steels) in automotive manufacturing, offers a significant potential for improvement in the area of reducing the weight while simultaneously increasing crash safety. Induced plasticity steels not only possess high strength but also excellent elongation properties and high energy absorption capacities due to the TRIP and TWIP effects, Table 1. [1±4] This profile of mechanical properties is of great interest for crash-relevant automotive components.Common TWIP steel concepts are based on high Mn and Al contents, which are known to cause various problems during steelmaking. The search for suitable primary and secondary steelmaking processes, and the feasibility of casting these steel grades on conventional continuous casting machines, is the focus of ongoing research. The aim of the work presented here is to provide a basis for decisions on the subsequent development of the industrial production of high-Mn and -Al steels.At the very beginning, the following potential difficulties for the continuous casting of high-Mn, high-Al steels were identified: [5] ± high-temperature strength: is expected to be extremely high for these steels, and thus may be a problem for the strand guiding and soft reduction system in casting machines;± hot tearing: the wide brittle-temperature range during the solidification of high-Mn and -Al steels indicates a high crack susceptibility;± segregation at the micro-and macroscopic levels: possible formation of banded structures, resulting in difficulties during further processing;± non-metallic inclusions: the possible interaction between precipitates and mechanical properties means that steel cleanliness levels (secondary metallurgy) should be defined;± scale formation: partial oxidation plays a role in the formation of surface cracks in the casting and rolling process.This list does not consider necessary process adjustments, such as the development of suitable mold powders for highAl steels.The work presented here is concerned with the high-temperature mechanical properties of an high strength structural steel (HSSS) steel and the high-Mn, high-Al steel X50MnAl 25 1 in comparison to common ultra low carbon (ULC) and electrical sheet (ES) steels, the latter both known not to cause many problems during the continuous casting process. MethodologyThe high-temperature mechanical properties and the crack susceptibility were analyzed using the SSCT (submerged split-chill tensile) method. The principle of this testing method is described in detail elsewhere. [6±8] The SSCT test was developed to simulate the shell straining during the continuous casting process. A steel cylinder, split in two halves, is submerged in a liquid steel melt inside an induction fur- COMMUNICATIONS

Measuring Mechanical Behavior of Steel During Solidification: Modeling the SSCC Test

“…These models have been applied to predict crack formation during continuous casting. [18] Several computational models of the SSCT [7] and SSCC [1] tests have been performed to study phenomena such as shell strength, [74,75] grain size, [9] hot tearing, [1,10,37,76] and to evaluate proposed constitutive models. [12] Because of the complexity of the phenomena, many aspects of the test are still not fully understood.…”

“…[11] and [12]) are transformed into two integrated equations by invoking the backward Euler method and are solved by using a specialized Newton-Raphson method [10,13,84] The updated global equilibrium equations then are solved using the nonlinear procedures in the commercial software ABAQUS v. 6.9-1. [17] This approach has been implemented successfully [16,57] and has been validated by matching temperature and stress in the semianalytical solidification problem posed by Weiner and Boley.…”

“…After cooling to room temperature, the shell is detached, cut, and analyzed metallographically for hottear cracks. This test is highly repeatable [5] and has been used to investigate force-time [6] and stress-strain [2][3][4][5][6][7] relationships, shell strength [8] microstructure morphology, [7,9,10] and various defects for many different steel compositions, [1] other castable metal alloys, [11] and cooling rates. [12] Unfortunately, the SSCC experiment yields little information about its fundamental operation, including what force is being measured, and its relationship to the high-temperature thermal-mechanical behavior of the solidifying steel that is sought.…”

Thermophysical Properties and Solidification Defects of Fe-22Mn-0.7C TWIP Steel

“…In order to get a clear understanding of the solidification control, several investigations on thermophysical properties of TWIP steel were conducted. Scott et al, Gigacher et al, and Kang et al thermodynamically calculated the solidus and liquidus of Fe–(18–25)Mn–(0.5–0.6)C–(0‐1)Al TWIP steel within the range of 1283–1289 and 1407–1423 °C, respectively, with a mushy zone spacing more than 100 °C. However, the measured solidus of TWIP steels by DSC or DTA was much higher than the predicted .…”

Effect of V on Hot Deformation Characteristics of TWIP Steels

“…Even though these steels exhibit excellent mechanical properties, the production, and optimization of the mechanical properties of these steels poses challenges at several levels, i.e., continuous casting,10–11 hot and cold rolling,12–15 composition design,16 deformation mechanisms,17–20 etc. Regarding mechanical properties, it is important to explore the limits of strength improvement.…”