August 2020 Volume 2

FORGING RESEARCH

1.0 Introduction The application of high strength low alloy steels (HSLA) has been the subject of extensive studies since the early 1970’s for many different applications; one particular collective example are the vanadium aluminum-nitrogen steels such as the VAN 80 HSLA steels developed by the former Jones and Laughlin Steel Corporation. [1] Prior to the development of HSLA steels, high strength forgings were achievable only through the application of a quenching and tempering (QT) process. This process was both uneconomical and detrimental to the environment through the necessity of extra processing steps. [2] However, 1974 through to 1980 saw the development of medium carbon steels which utilize the benefits of the microalloying elements niobium, titanium, and vanadium. The individuals who investigated these steels found that additions of these elements increased both the yield and ultimate strengths of the steels, with this effect being enhanced in the presence of accelerated cooling. [2] As can be seen in Figure 1, the processing of these steels is much simpler than the QT steels, and thus the economic viability of these steels is much higher. [2] Recrystallization controlled rolling or forging (RCR or RCF) combined with interrupted direct quenching (IDQ)/ indirect accelerated cooling (IAC) is a technology which has been in development since the early 1980’s. The attractive qualities of this technology are its uncomplicated nature, its elimination of normalizing steps, and its capability of application on unconventional forging plants. [3] The grain refinement achieved by RCR processing is central to increasing the toughness of these steels. Though this technology has been generally well accepted, the implementation of the RCR process in forging applications to create a new recrystallization controlled forging (RCF) process is relatively new, with a low-carbon, Nb microalloyed multiphase steel being designed for hot deformation under RCR conditions. [4]

strength and toughness combinations for automotive industry applications. To accomplish this goal, laboratory characterization experiments were conducted. The first of these experiments, the grain coarsening experiments, determined the T GC for each steel. Deformation experiments occurred next, determining both the post deformation austenite grain sizes, as well as the T 95 for each steel. The final characterization experiments, the cooling experiments, determined cooling paths to achieve ferrite, bainite and martensite constituent microstructures. Upon completion of these experiments, hot water quench trials were conducted in order to generate a cooling path capable of forming bainite in the full forgings in the industrial scale trials. The results of the characterization and hot water quench trials were used to design the full-scale forging trials, during which the MFC standard processing schedule was conducted. Following this processing schedule, the steels were cooled according to five differing cooling paths: ACRT, fan ACRT, WQRT, 27 second hot water quench, and 50 second hot water quench.The full forgings produced from these cooling paths were sectioned, and metallographic analysis occurred at five locations throughout the steel. Additionally, tensile and charpy v-notch samples were machined and tested from each of the full forging conditions. Finally, precipitation analysis was conducted in the steels, determining precipitate composition, shape and average size within the M3 fan ACRT condition. 2.0 Background 2.1 StrengtheningMechanisms 2.1.1 Strengthening Overview Plastic deformation in steels occurs due to the motion of dislocations throughout the structure. Strengthening methods refer to methods of changing the structure and/or the local chemistry of the steel, to make it more resistant to the motion of these dislocations, and thus requiring of higher stress levels to force the motion of the dislocations. In such a manner, the yield strength of a steel can be adjusted, and can be expressed according to a generalized form shown below: [5] σ y =σ 0 +σ SS +σ pptn +σ dis +σ texture +σ GB (2-1) Here σ 0 is the Peierl’s-Nabarro stress, which quantifies the intrinsic resistance to dislocation motion in the perfect BCC Iron lattice, σ SS is the solid solution strengthening contribution from solutes in solid solution in the ferrite, σ pptn is the precipitation strengthening contribution from the fine precipitates that may reside in the ferrite matrix, σ dis is the dislocation strengthening contribution from pre-existing dislocations present prior to the tensile test, σ texture is the texture hardening contribution resulting from the orientation dependent effects of the Schmidt factor Taylor factor and elastic constants, and σ GB is the grain boundary strengthening contribution or Hall-Petch hardening. (i.e. ∝ 1 √ where d is the grain size)

FIA MAGAZINE | AUGUST 2020 66 The principal goal of the research conducted herein was to implement various steel sitions designed for use in the recrystallization controlled forging system and implement l differing cooling conditions to provide a range of final microstructures with desirable Figure 1: Processing path of quenched and tempered steel vs the processing path of microalloyed medium-C steels [2] The principal goal of the research conductedhereinwas to implement various steel compositions designed for use in the recrystallization controlled forging system and implement several differing cooling conditions to provide a range of final microstructures with desirable 1: Processing path of quenched and tempered steel vs the processing path of microalloyed medium-C steels [2]