Innovative Methodology for Physical Modelling of Multi-Pass Wire Rod Rolling with the Use of a Variable Strain Scheme

Abstract

This paper presents the results of physical modelling of the process of multi-pass rolling of a wire rod with controlled, multi-stage cooling. The main goal of this study was to verify the possibility of using a torsion plastometer, which allows conducting tests on multi-sequence torsion, tensile, compression and in the so-called complex strain state to physically replicate the actual technological process. The advantage of the research methodology proposed in this paper in relation to work published so far, is its ability to replicate the entire deformation cycle while precisely preserving the temperature of the deformed material during individual stages of the reproduced technological process and its ability to quickly and accurately determine selected mechanical properties during a static tensile test. Changes in the most important parameters of the process (strain, strain rate, temperature, and yield stress) were analyzed for each variant. After physical modelling, the material was subjected to metallographic and hardness tests. Then, on the basis of mathematical models and using measurements of the average grain size, chemical composition, and hardness, the yield strength, ultimate tensile strength, and plasticity reserve were determined. The scope of the tests also included determining selected mechanical properties during a static tensile test. The obtained results were verified by comparing to results obtained under industrial conditions. The best variant was a variant consisting of physically replicating the rolling process in a bar rolling mill as multi-sequence non-free torsion; the rolling process in an NTM block (no twist mill) as non-free continuous torsion, with the total strain equal to the actual strain occurring at this stage of the technological process; and the rolling process in an RSM block (reducing and sizing mill) as tension, while maintaining the total strain value in this block. The differences between the most important mechanical parameters determined during a static tensile test of a wire rod under industrial conditions and the material after physical modelling were 1.5% for yield strength, approximately 6.1% for ultimate tensile strength, and approximately 4.1% for the relative reduction of the area in the fracture and plasticity reserve.

Keywords: cold upsetting steel; hot torsion test; mechanical properties; metallographic tests; physical modelling; variable strain state; wire rod rolling.

Figure 1

General scheme of the analyzed combined-type rolling mill [17]. In order to obtain a finished product with an even fine-grained ferritic–perlithic microstructure without a clear band structure, the final stage of deformation should take place in the austenitic range, when its temperature is 30–80 °C higher than the initial temperature of the austenite transformation, Ar₃ [31,32,33,34,35]. For 20MnB4 steel, the Ar₃ temperature is 780 °C. In the case of low-carbon and low-alloy steels, which are intended for further cold plastic processing, the most advantageous temperature for forming coils is a temperature of about 850–900 °C. Such a method of laying the coils provides an increased plasticity of the metal, beneficial for the cold drawing process, and allows a decrease in the recrystallizing annealing time after the drawing process [32].

Figure 2

Samples for physical modelling tests: (a) technical specification and (b) general view of 20MnB4 steel samples before and after physical modelling.

Figure 3

STD 812 torsion plastometer: (a) device chamber: 1—specimen, 2—holders, 3—thermocouples type S, 4—induction solenoid, 5—cooling system jets, 6—pyrometer, 7—sensors for laser measurement of specimen diameter and (b) basic specification [2,27].

Figure 4

General model of thermo-mechanical treatment representing the entire rolling process of a 20MnB4 steel wire rod with a diameter of 5.5 mm.

Figure 5

Method of sampling for metallographic tests from material after physical modelling: (a) general view and (b) sample metallographic microsections.

Figure 6

Cross-section of the material after physical modelling of the wire rod rolling process including marked measuring points: (a) general view and (b) distance table.

Figure 7

General course of temperature (nominal and obtained) and yield stress during physical modelling of rolling round 20MnB4 steel bars in a medium continuous rolling mill, for rolling a 5.5 mm diameter wire rod (test variants: V1, V2, and V3).

Figure 8

Example temperature distribution (nominal and obtained) during the initial stage of the deformation process in an NTM block: (a) accelerated cooling time before the NTM block (8.5 s) and (b) accelerated cooling time before the NTM block (55 s: accelerated cooling for 40 s and holding for 15 s).

Figure 9

General course of temperature (nominal and obtained) and yield stress during physical modelling of rolling a 20MnB4 steel wire rod with a diameter of 5.5 mm (test variant V1): (a) in an NTM block and (b) in an RSM block.

Figure 10

General course of temperature (nominal and obtained) and yield stress during physical modelling of rolling a 20MnB4 steel wire rod with a diameter of 5.5 mm (test variant V2): (a) in an NTM block and (b) in an RSM block.

Figure 11

General course of temperature (nominal and obtained) and yield stress during physical modelling of rolling a 20MnB4 steel wire rod with a diameter of 5.5 mm (test variant V3): (a) in an NTM block (torsion) and (b) in the RSM block (tension).

Figure 12

General course of yield stress during physical modelling of rolling a 20MnB4 steel wire rod with a diameter of 5.5 mm (test variant V1, test variant V2, test variant V3.

Figure 13

Example microstructures of 20MnB4 steel after physical modelling of a wire rod rolling process for test variant V1 (marking of points in accordance with Figure 5): (a) point P. 1, (b) point P. 1-2, (c) point P. 1-3, (d) point P. 1-4 and (e) point P. 1-5.

Figure 14

Example microstructures of 20MnB4 steel after physical modelling of a wire rod rolling process for test variant V2 (marking of points in accordance with Figure 5): (a) point P. 1, (b) point P. 1-2, (c) point P. 1-3, (d) point P. 1-4, and (e) point P. 1-5.

Figure 15

Example microstructures of 20MnB4 steel after physical modelling of the wire rod rolling process for test variant V3 (marking of points in accordance with Figure 5): (a) point P. 1, (b) point P. 1-2, (c) point P. 1-3, (d) point P. 1-4, and (e) point P. 1-5.

Figure 16

Distribution of the ferrite grain size and hardness of 20MnB4 steel after physical modelling of the rolling process of a wire rod with a diameter of 5.5 mm: (a) test variant V1, (b) test variant V2, and (c) test variant V3.

Figure 17

Sample results from testing the mechanical properties of the material after physical modelling of rolling a wire rod: (a) general view of a 20MnB4 steel sample after physical modelling, during determination of mechanical properties in a static tensile test, (b) general view of samples before and after the test, and (c) sample tensile curves of the tested material after physical modelling.

Figure 18

The view of the 20MnB4 steel sample after the deformation process in a complex deformation state (simultaneous torsion and tension) and location of the deformation outside the test area.