Microstructure and Mechanical Properties of an Austenitic CrMnNiMoN Spring Steel Strip with a Reduced Ni Content

Christina Schröder,Olena Volkova,Andreas Weiß,Marco Wendler

doi:10.3390/cryst10050392

Abstract

The article presents the mechanical properties of the austenitic stainless steel X5CrMnNiMoN16-4-4 after deformation by cold rolling and subsequent short-term tempering (deformation and partitioning (D&P) treatment). Tensile strengths of 1700–900 MPa and beyond were achieved both after work hardening and in the D&P-treated strip. The initial state of austenite in terms of grain size and pre-strengthening, as well as the selected cold rolling temperature significantly influenced the deformation-induced formation of α’ martensite and thus the flow and hardening behavior of the steel. The usage of two different rolling temperature regimes showed that the strength properties in the cold strip can be specifically adjusted. Lower deformation-induced martensite fractions enabled a larger thickness reduction of the strip without increasing the rolling force, while high deformation-induced martensite fractions led to strong hardening at low deformation levels. The D&P-treatment permits the strength of the cold-rolled strip with a predominantly austenitic microstructure to be increased to the required level. The total elongation of such a D&P strip was well over 2%. The D&P treatment of the spring steel strip is a cost-effective alternative to conventional tempering treatment.

Highlights

Spring steels are metastable austenitic steels with martensite start (Ms α’ ) temperatures near and below room temperature (RT) after solution annealing
Due to the induced TRIP/TWIP effect and an increased plasticity during rolling, a high deformability can be achieved during cold rolling without intermediate annealing
The tensile strength of the strips is mainly determined by the α’ martensite fraction and the work hardening of austenite

Summary

Introduction

Spring steels are metastable austenitic steels with martensite start (Ms α’ ) temperatures near and below room temperature (RT) after solution annealing. The required high strength is achieved by creating a mainly martensitic microstructure through appropriate cold forming and subsequent tempering. As a result of tempering, carbide precipitates are preferentially formed in the martensite and residual stresses are relieved [1,2,3]. The tolerance of the δ-ferrite fraction is 2 vol%. Stainless spring steels are classified as corrosion-resistant steels due to their chromium content exceeding 12 wt%. Additions of molybdenum and nitrogen improve the corrosion properties and increase the resistance to pitting corrosion [4]

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