Microstructure-Properties Relationships of Thin Slab Direct Rolling (TSDR)-compatible Zn-Coated Press Hardened Steel (PHS)

Abstract

Thin slab casting and direct rolling (TSDR) technologies are one of the most promising steel making processing routes to maintain steel as a leading material in technological applications. TSDR industrial production begun at Nucor in Crawfordsville (USA) in 1989, as a low cost alternative for making hot rolled products.

The main difference between TSDR and conventional HSM technologies is that in TSDR the entire casting and rolling process is run continuously. In addition to the advantage of having cheaper capital investments, using less energy, having significantly lower operational costs and a smaller size, TSDR processes offer a more flexible processing route which can more easily adapt to market requirements. TSDR technologies have also ecological benefits because CO2 emissions are reduced due to the use of liquid metal produced from scrap metal in an Electric Arc Furnace (EAF) and energy consumption is reduced.

In the present study, the Continuous Endless Mill (CEM) processing developed by POSCO was used to explore new steel product design concepts. POSCO developed the CEM process over many years on the bases of the In-line Strip Process (ISP) TSDR route. Unlike the original CSP process, the ISP and CEM TSDR production lines do not have a long tunnel furnace after casting process.

Press hardening, also known as hot press forming (HPF), hot stamping, and die quenching, is an advanced technology to produce ultra-high strength steels (AHSS) for automotive applications. The HPF process was originally developed by Norrbottens Järnverks in 1973 to produce press hardened steel. It is now widely used to produce passenger safety-related anti-intrusion parts and structural reinforcements such as door beams, impact beams, bumpers, pillars, roof rails and tunnels. The most widely used PHS grade is the 22MnB5 steel grades. This steel grade has an ultra-high Ultimate Tensile Strength (UTS) (~ 1,500 MPa). The total elongation is rather limited, approximately 6 %, because of its fully martensitic microstructure. PHS should have enough hardenability to achieve fully martensitic microstructure in their as-produced final state. Therefore, boron is added as a hardening element to the 22MnB5 steel grade composition to retard the pro-eutectoid ferrite formation and the other austenite decomposition reactions during cooling after the HPF process.

In the present study, the development of a TSDR-specific PHS is presented. TSDR processing is not always stable in its operation and the cast thin slab may have to be cooled down and reheated before rolling. The cast thin slab directly hot-rolled has a larger initial as-cast austenite grain size as compared to the conventional processing during which the slab is cooled to room temperature and reheated to ~ 1,250 oC prior to rolling. The grain size and the morphology of the hot-rolled microstructure is different due to this different as-cast austenite grain size, and it impacts the mechanical properties of the hot-rolled steel. The press hardening of steel constitutes an additional HPF heat-treatment after hot or cold rolling. During the PH heat treatments, the microstructure of PHS is fully austenitized. It is transformed to a fully martensitic microstructure directly after the press forming by the cooling of the water-cooled dies. The final microstructures and mechanical properties of conventional PHS and a CEM-specific PHS after HPF heat-treatment are expected to be very similar. The TSDR-compatible PHS alloy design is based on the use of the higher N content, resulting from the Electric Arc Furnace (EAF) based steel production route. B and Ti are important alloying additions in conventional 22MnB5 PHS. B is added to increase the hardenability. Since solute B segregates to the austenite grain boundaries before quenching, it impedes the nucleation of pro-eutectoid ferrite and thus promotes the formation of harder phases such as bainite and martensite. B must be in solute form and allowed to segregate to the austenite grain boundaries in order to be effective as a hardenability agent. Ti is typically added to the steel composition to protect B as B is a strong nitride former. In the new alloy concept, N is utilized as an additional solute strengthening addition. Cr and Mn, rather than B, is added to the steel composition to achieve the required hardenability and obtain a fully martensitic microstructure after die-quenching. Although there are other hardenability agents such as C and Mo, only Cr and Mn were added to the TSDR-compatible steel composition because the C content needed to be maintained at a low level, comparable to the C content of conventional 22MnB5 steel for reasons of waldability.

22MnCrN5-X alloys with different amount of Cr and 3.5%Mn and 0.8%Cu-added PHS were chosen as candidate steels for the development of a TSDR-compatible PHS grade. In die-quenching simulation, 22MnCrN5-X alloys containing at least 0.8 wt-% Cr, 3.5%Mn and 0.8%Cu satisfied strength requirement (> 1,470 MPa of UTS) after transformation to an almost fully martensitic microstructure during die quench cooling. Retained austenite films were observed between martensitic laths in both 22MnCrN5-4 and 3.5%Mn_0.8%Cu-added samples.

The nature of the interaction between C in supersaturation and dislocations was studied by means of the IF technique. The IF spectrum obtained during the heating process for TSDR-compatible PHS with martensitic microstructure were decomposed into four broadened Debye shaped peaks (P2, P3, P4, and P5) and an exponential background from 200 K (-73 oC) to 800 K (527 oC). The IF experimental data was similar for the specimens before and after paint baking heat-treatment. The height of the P3 peak for, which is related to dislocation enhanced Snoek peak, decreased after the bake hardening simulation for both materials. Thermally activated C diffusion, kink formation on dislocations, and transition carbide precipitation can take place at the paint baking temperature. The experimental damping peak height was gradually reduced mainly due to a reduction of the free dislocation segment length as interstitial atoms move to the dislocations which act as sinks.

Zn-coating was applied to TSDR-compatible PHS by hot-dip galvanizing simulation. The Liquid Metal Induced Embrittlement (LMIE) behavior of the Zn-coated PHS was evaluated by Gleeble and TEM. In the present study, the formation of α-Fe(Zn) layer around Γ-Fe3Zn10 was observed in the crack tip region. This α-Fe(Zn) region did not participate in the austenite to martensite transformation during cooling. The formation of a thin layer of a new solid phase at the grain boundaries can bring about an intergranular fracture. The presence of a thin region of intergranular α-Fe(Zn) at the crack tip is very likely the cause of the crack opening. It is due to the rapid fracture of the thin ferrite layer at austenite grain boundaries since ferrite has a lower strength and a lower strain-hardening behavior as compared to austenite at the austenite to ferrite transformation temperature.