Effect of Manganese on Microstructure and Mechanical Properties of Cast High Alloyed CrMnNi‐N Steels

Abstract

Within the Collaborative Research Center 799 novel composite materials which consist of a highly alloyed TRansformation-Induced Plasticity/Twinning-Induced Plasticity (TRIP/TWIP) CrMnNi cast steel matrix and a partially stabilized zirconium dioxide (Mg-PSZ) ceramic, referred to as TRIP-Matrix-Composites, are developed. [1] By applying an external load larger than the yield strength, the TRIP steel matrix shows a strain-induced phase transformation from metastable austenite to a 0 -martensite which leads to a concurrent increase of strength and ductility. By contrast, stress-induced formation of a 0 -martensite occurs at stresses below the yield strength, i.e., during elastic deformation. [2] The stress-induced transformation of the partially stabilized ZrO 2 from tetragonal phase to monoclinic modification can result in a further increase of the strength. Currently the interstitial-free austenitic CrMnNi cast stainless steels with TRIP/TWIP effect are in use. Representative alloys are 16Cr-7Mn-3Ni, 16Cr-7Mn-6Ni, and 16Cr-7Mn-9Ni. [3,4] These austenitic steel grades typically show low yield strengths of the order of 180-200 MPa. As a result of the low stress levels, only small fractions of the ceramic phase can transform to the monoclinic structure. In order to assist the phase transformation of the ceramic phase from tetragonal to monoclinic, the current steel research in the CRC 799 is focused on the increase of strength, especially the yield strength of the austenitic TRIP/TWIP CrMnNi cast steels by solid solution strengthening by nitrogen. Furthermore, the reduction of delta ferrite volume fractions, the adjustment of a pronounced TRIP/TWIP effect at operating temperatures and improved resistance against the intercrystalline corrosion by the addition of nitrogen is desired. In recent years, highly alloyed TRIP and TWIP steels have received much attention in both academia and industry because of their superior mechanical properties. [5][6][7]12] The high-manganese TWIP steels can be divided in the alloying systems FeMnC and CrMnC with additions of the main alloying elements such as N, Al, Si. In these steels g ! e transformation and deformation-induced twinning can appear in dependence of the SFE of the austenite. It is commonly believed that stacking fault energies below 18-20 mJ m À2 favor the g ! e phase transformation and higher values favor the twinning of austenite. [8][9][10] The longest known FeMnC steels are characterized by manganese contents between 15 and 30% and an austeniticThe effect of the manganese content (0-11%) on the transformation temperatures, the mechanical properties and microstructure development of five highly alloyed 14Cr-XMn-6Ni cast stainless steels with 0.1% nitrogen was studied. The examinations reveal that the M s , A s , and A f temperatures decrease with increasing manganese contents. As a result of low austenite stability, room temperature austenitic-martensitic as-cast microstructure was formed at manganese contents between 0 and 3%. At manganese levels of 6% and higher a fu...

Considerations in the Design of Formable Austenitic Stainless Steels Based on Deformation-Induced Processes

“…The schematic in Figure 9 generalizes the temperature dependence of tensile elongation in austenitic stainless steels [10,15,[38][39][40][41][42][43]. The schematic elongation curve consists of three regions marked I-III.…”

“…Therefore, the M d γ → α′ temperature may be used to predict the influence of alloying elements on the elongation curve. Tensile tests at various temperatures of austenitic stainless steels with varied amounts of Ni [39,42], Mn [43], Al [15], Cr [23,25,27], C [23,24], and N [25] have confirmed that they shift the peak elongation temperature to lower temperatures. All of the preceding alloying elements except Al are unanimously known to decrease both M d γ → α′ and M s temperatures [54,55].…”

Segregation-Induced Enhancement of Low-Temperature Tensile Ductility in a Cast High-Nitrogen Austenitic Stainless Steel Exhibiting Deformation-Induced α′ Martensite Formation

“…The observed behavior was explained by the presence of microstructural regions with different stabilities with respect to deformationinduced a¢ martensite formation caused by the segregation of alloying elements. Tensile elongation near room temperature of low stacking fault energy (SFE) austenitic steels including stainless, [1][2][3][4][5][6][7][8][9] high Mn, [10][11][12][13] and high Ni [14] steels varies in three temperature regimes as follows. At the highest temperature range (regime I), tensile elongation remains more or less constant or exhibits a weak temperature dependence.…”

“…The loss of ductility caused by the deformation-induced formation of a¢ martensite initiates the regime III of elongation, characterized by reduced elongations at lower temperatures. The large number of investigations in support of the detrimental effect of a¢ martensite formation on the tensile elongation of metastable austenitic stainless steels [1][2][3][4][5][6][7][10][11][12] indicates that any possible contribution to the ductility of a TRIP effect would not be large enough to prevent the loss of ductility below the M d cfia¢ temperature. This has been also suggested by the modeling of the a¢ TRIP effect contribution to the ductility.…”

Formability of strong metastable Fe–15Cr–3Mn–3Ni–0.2C–0.1N austenitic TRIP/(TWIP) steel – A comparison of different base materials

“…To ensure a structure free of carbides, an additional solution heat treatment (5) was carried out at 1150 C for 30 min before quenching in water.…”