Analysis of the Uniformity of Mechanical Properties along the Length of Wire Rod Designed for Further Cold Plastic Working Processes for Selected Parameters of Thermoplastic Processing

Abstract

This study presents the results of research, the aim of which was to analyze the uniformity of the distribution of selected mechanical properties along the length of a 5.5 mm diameter wire rod of 20MnB4 steel for specific thermoplastic processing parameters. The scope of the study included, inter alia, metallographic analyses, microhardness tests, thermovision investigations, and tests of the wire rod mechanical properties (yield strength, ultimate tensile strength, elongation, relative reduction in area at fracture), along with their statistical analysis, for three technological variants of the rolling process differing by rolling temperature in the final stage of the rolling process (Reducing Sizing Mill rolling block [RSM]) and by cooling rate using STELMOR^® cooling process. The obtained results led to the conclusion that the analyzed rolling process is characterized by a certain disparity of the analyzed mechanical properties along the length of the wire rod, which, however, retains a certain stability. This disparateness is caused by a number of factors. One of them, which ultimately determines the properties of the finished wire rod, is the process of controlled cooling in the STELMOR^® line. Despite technological advances concerning technical solutions (among them, increasing the roller track speed in particular sections), it is currently not possible to completely eliminate the temperature difference along the length of the wire rod caused by the contact of individual coils with each other. From this point of view, for the analyzed thermoplastic processing parameters, there is no significant impact by the production process parameters on the quality of the finished steel product. Whereas, while comparing the mechanical properties and microstructure of the wire rod produced in the different technological combinations, it was found that the wire rod rolled in an RSM block at 850 °C and cooled after the rolling process on a roller conveyor at 10 °C/s had the best set of mechanical properties and the smallest microstructure variations. The wire rod produced in this way had the required level of plasticity reserve, which enables further deformation of the given type of steel in compression tests with a relative plastic strain of 75%. The uniformity of mechanical properties along the length of wire rods designed for further cold plastic working processes is an important problem. This is an important issue, given that wire rods made from 20MnB4 steel are an input material for further cold plastic working processes, e.g., for the drawing processes or the production of nails.

Keywords: mechanical properties; metallographic analysis; steel for cold upsetting; thermovision investigation; wire rod rolling.

Figure 1

Part of the DTTT graph depicting investigated thermoplastic processing conditions for a 5.5 mm diameter wire rod of 20MnB4 steel.

Figure 2

Scheme of physical modeling of rolling of 5.5 mm diameter wire rod of 20MnB4 steel grade.

Figure 3

Example microstructure of 20MnB4 steel after physical modeling of wire rod rolling process: (a) technological variant—V1, (b) technological variant—V2, V3.

Figure 4

Example thermogram of temperature distribution on the surface of 20MnB4 steel grade band before rolling stand No. 1.

Figure 5

Diagram of surface temperature distribution along the length of the rolled material at various places in the rolling mill.

Figure 6

Example of temperature distribution on the surface of 20MnB4 steel wire rod at the entry to the STELMOR^® roller conveyor: (a) thermogram, (b) temperature value at measuring points.

Figure 7

Mechanical properties tests along the length of 5.5 mm diameter wire rod of 20MnB4 steel grade: (a) wire rod coils, (b) samples for mechanical properties testing, (c) example sample before tensile test, (d) example sample after tensile test.

Figure 8

Examples of tensile curves of 20MnB4 steel grade wire rod samples: (a) technological variant—V1, (b) technological variant—V2, (c) technological variant—V3.

Figure 9

The course of changes in selected mechanical properties along the length of 5.5 mm diameter wire rod of 20MnB4 steel grade—technological variants V1–V3: (a) Yield Strength (YS), (b) Ultimate Tensile Strength (UTS), (c) Unit Elongation (A), (d) Relative Reduction in Area at Fracture (Z).

Figure 10

Normal distribution diagrams of selected mechanical properties along the length of a 5.5 mm diameter wire rod of 20MnB4 steel grade—technological variant V1: (a) Yield Strength (YS), (b) Ultimate Tensile Strength (UTS), (c) Unit Elongation (A), (d) Relative Reduction in Area at Fracture (Z).

Figure 11

Normal distribution diagrams of selected mechanical properties along the length of a 5.5 mm diameter wire rod of 20MnB4 steel grade—technological variant V2: (a) Yield Strength (YS), (b) Ultimate Tensile Strength (UTS), (c) Unit Elongation (A), (d) Relative Reduction in Area at Fracture (Z).

Figure 12

Normal distribution diagrams of selected mechanical properties along the length of a 5.5 mm diameter wire rod of 20MnB4 steel grade—technological variant V3: (a) Yield Strength (YS), (b) Ultimate Tensile Strength (UTS), (c) Unit Elongation (A), (d) Relative Reduction in Area at Fracture (Z).

Figure 13

General view of wire rod of 20MnB4 steel grade samples for cold upsetting tests: (a) method of samples preparation from the finished product for cold upsetting tests, (b) view of samples after cold upsetting test (wire rod after the rolling process according to technological variant V2) with relative plastic strain value: (b) 50%, (c) 67%, (d) 75%.

Figure 14

Scheme of a longitudinal section of wire rod with marked characteristic points.

Figure 15

Example of the microstructure of a 5.5 mm diameter wire rod of 20MnB4 steel grade—longitudinal section—technological variant V1: (a) magnification 500×, (b) magnification 200×.

Figure 16

Example of the microstructure of a 5.5 mm diameter wire rod of 20MnB4 steel grade—longitudinal section—technological variant V2: (a) magnification 500×, (b) magnification 200×.

Figure 17

Example of the microstructure of a 5.5 mm diameter wire rod of 20MnB4 steel grade—longitudinal section—technological variant V3: (a) magnification 500×, (b) magnification 200×.