The main goal of this contribution is to evaluate the mechanical properties, strengthening contributions and microstructure development of austenitic stainless steel AISI 316 LN with high nitrogen content in the states characterized as the initial state and the states after rolling with different thickness deformations. The initial state was represented by solution annealing (777 K/60 min). The deformation state was characterized by rolling thickness reductions carried out at ambient temperature (T_A = 295 K) with deformations in the range ε ∈ (0; 50> [%]. Studies of microstructures, mechanical properties and strengthening contributions before and after rolling were carried out. The initial state after solution annealing was as follows: offset yield strength R_p0.2 = 325 MPa, elongation A5 = 49% and diameter of grain size d = 214 μm. The state after ambient rolling with thickness deformation ε = 50% was as follows: R_p0.2 = 994 MPa, A5 = 4% and d = 64 μm. The maximum contribution to strengthening after rolling processing with 50% thickness deformation was dislocations (∆ R _{P0.2_DS} = 560 MPa) followed by twins (∆ R _{P0.2_DT} =140 MPa).

The article deals with increasing the mechanical properties of stainless steel 316 Ln-IG, which is intended for work in cryogenic temperatures (liquid nitrogen and liquid helium), such as conductor conduits for the ITER magnet system. The strength and plastic properties were increased by a combination of cold and cryo-rolling and heat treatment. The mechanical properties of rolled material were investigated at 293 K, 77 K, and 4.2 K. The work-hardening rate of the steel increased continuously with a lowering of the temperature. The maximum yield strength and ultimate tensile strength were achieved by the cryo-rolling process with a total thickness deformation of 50%. The material properties tested at ambient temperature were 0.2YS = 1050 MPa, UTS = 1200 MPa, and at 4.2 K, the values were 0.2YS = 1804 MPa and UTS = 2081 MPa. Two types of long-term heat treatment were applied after experimental rolling (823 K and 1093 K for 10 h). The highest precipitation hardening of steel was achieved at a temperature of 823 K after 50% deformation. The resulting grain size decreased from the initial 216 μm (before the rolling process) to 70 μm after ambient rolling and 72 μm after cryo-rolling.

In the present investigation, stress–strain curves and strain hardening rates on samples rolled at ambient temperature with thickness reductions of 0%, 10%, 30%, and 50% were studied. On the processed samples, static tensile tests at ambient temperature were performed. Transformation of the engineering stress–strain curves to true stress–strain curves and their numerical processing by first derivation (θ = dσ/dε) was carried out. Dependencies θ = f(ε_T) characterizing the strain hardening rates were derived. From the curves and the true stress–strain and strain hardening rates, the three stages describing different rates of strain hardening were identified. A rapid increase in true stress and a rapid decrease in the strain hardening rate in Stage I were observed. Quasi-linear dependencies with an increase in true stress but with a slow, gradual decline in the strain hardening rate in Stage II were obtained. Slowly increasing true strains, accompanied by a decrease in strain hardening rates and their transition to softening, led to the formation of plastic instability and necking in Stage III. The endpoints of the strain hardening rate depending on the cold rolling deformations lie in the following intervals: θ_{Stage I} ∈ MPa, θ_{Stage II} ∈ MPa, θ_{Stage III} ∈ <−144;−11,979> MPa. While in Stage I and Stage II, the plastic deformation mechanism is predominantly dislocation slip, in Stage III, the plastic deformation mechanism is twinning accompanied by dislocation slip.

The primary objective of this contribution is to numerically and graphically evaluate engineering stress–strain curves, transform them into true stress–strain curves, and de-scribe the key points of material processed by cold rolling with strains of ε_Roll = 0%, 10%, 30%, and 50%. The initial and final conditions for uniform plastic deformations have been described. The initial point of uniform deformation lies above the onset yield strength value (σ_T,S > R_P0,2). The necking point, as the final point of uniform deformation, was determined as the intersection point of the curves of the true stress–strain and strain hardening rate. The strain hardening coefficient and the recovery rate, as a function of cold rolling deformations, were derived. Convex polyhedra were derived which describe the dependencies of the development of maximal strain hardening rate values (θ_Max) and initial strain hardening rates (θ₀) as a function of cold rolling deformations and the diameter of grain. The decisive point at which the curves showed a local maximum was a cold rolling deformation ε_Roll = 30%. The saturation stress required to initiate dynamic recovery of the microstructure is significantly higher than the stress necessary for necking (σ_T,Sat > σ_T,Neck). The saturation strain required to initiate dynamic recovery of the microstructure is significantly higher than the strain needed for necking formation (ε_T,Sat > ε_T,Neck).