Optimization of Metallurgical Factors for Production of High Strength, High Toughness Steel Plate by Controlled Rolling

Abstract

This pa/Jer describes a part of results of continued developmental activity to obtain optimum chemical composition and processing conditions in the production of high strmgth steel with high toughness by controlled rolling .I t was noted in laboratory experiment and mill test that niobium most e.ffectively improved the frac ture ap/Jearance transition temperature (v T s) when a strong controlled rolling was applied, while vanadium was less effective in this res/Ject. Steels containing both of niobium and vanadium also /Jossessed a v T s similar to th at of Nb-bearing steels . Addition of a small amount of nickel, chromium, and cop/Jer to Nbor V -bearing steels enhanced precipitation hardening presumably by lowering the r-+a transformation temperature withou t deteriorating the v T s. The improvement of v T , by controlled rolling was consistently described by the total rolling reduction given at below 900"C, and was less de/Jendent on rolling reduction of individual pass .L owering carbon content to below a level of 0 .07 0 0 in the Nb-bearing steel considerably improved impact shelf energy without arry loss in LYS. T ransverse shelf energy was also improved by decreasing sulfur content and application of cross rolling . H owever, an intensive controlled rolling slightly decreased shelf energy.A combination of the above knowledge has made the production q! tough high strength steels possible, and a few such examples qf line/Ji/Jf plate are shown.

Microalloyed steels

“…The transition temperature was shown to decrease in a linear manner with an increase in total reduction in ranges 2 and 3. 76 Kozasu et al 70 do not consider rolling below Ar 1 , or the changes in microstructure in terms of the role of dislocations. This has been addressed more recently Vervynckt et al 7 An important concept is the non-recrystallisation temperature T nr , which represents the start of the inhibition of complete static recrystallisation during cooling between rolling passes.…”

“…Furthermore, it soon became apparent that the number of rolling passes and their temperature needed to be controlled and hence the birth of 'controlled rolling'. [38][39][40] Within a short time, this led to the introduction of computer control in many other aspects of steelmaking, and later, the application of the results of academic computer modelling to aid the whole complex process of achieving a small, ,10 mm, homogeneously distributed ferrite grains. [41][42][43] Figure 2 shows schematically how the microstructure and properties of plate steels changed over time with advances in alloy design and processing.…”

Microstructural Characterizaton and Effects on Mechanical Properties of Boron Added Armor Steels

“…Parameters such as alloy design and the proper heat treatment (quenching and also tempering) are very important to determine the final properties of the product [3]. In this case the optimum mechanical properties, for example high strength, hardness and also toughness, can be obtained by performing a tempering heat treatment within a certain temperature and time range [4][5][6]. Figure 1 summarizes the mechanism of changes in an iron-carbon martensite structure during tempering at 100-700 ºC for 1 hour.…”

Microstructural and Fractographical Studies on Quenched and Tempered Armor Steels

“…In industrial applications, such steels are not convenient due to their low tough-ness after rapidly quenching. In these cases the optimum mechanical properties, for example high strength, hardness, and also toughness, can be obtained by performing a tempering heat treatment at proper temperature and time conditions [3][4][5][6]. Figure 1 summarizes the mechanism of hardness variations in a pure ironcarbon martensite structure during tempering heat treatment at 100-700 °C for 1 hour compared with steels where different carbide forming elements are alloyed.…”