Impact of selective oxidation during inline annealing prior to hot-dip galvanizing on Zn wetting and hydrogen-induced delayed cracking of austenitic FeMnC steel

The work presented here demonstrated that changes to the near-surface microstructure could promote internal oxidation in a CMnSi advanced high strength steel (AHSS) grade. The starting material was industrially-supplied cold-rolled sheet. Samples with this microstructure were compared against two other conditions: annealed at 1 200°C and 850°C prior to oxidation. The surface regions of both annealed samples were decarburized. All samples were then oxidized at 850°C in an argon atmosphere with approximately 10 − 20 atm oxygen for 5-90 minutes. The oxidation treatments were performed in a high-temperature confocal scanning laser microscope (CSLM) setup that enabled in-situ visualization of surface oxide formation. Analysis of CSLM and scanning electron microscopy (SEM) images of sample surfaces indicated there were differences in oxide morphology and coverage between samples with different starting microstructures. Cold-rolled samples showed little internal oxidation but extensive external oxidation. Samples annealed prior to oxidation exhibited both internal and external oxidation. The differences in behavior were attributed to the formation of a decarburized surface layer during the pre-anneal, which led to ferritic iron near the surface. Another result of the pre-anneal was a reduction in dislocation density, which potentially led to fewer preferential nucleation sites for oxides very close to the sample surface.

Section: Introductionmentioning

confidence: 99%

Effect of Surface Microstructure on Oxidation of a CMnSi Advanced High Strength Steel

Story¹,

Webler²

2017

“…4,5) However, because of these additions, many grades of AHSS prove challenging to galvanize due to selective oxidation. [6][7][8] Traditional industrial annealing cycles run between 700°C and 900°C in an N 2 + H 2 atmosphere with a dewpoint of around − 30°C to − 20°C. [9][10][11] These conditions are sufficient to prevent iron from oxidizing but not silicon or manganese.…”

Section: Introductionmentioning

confidence: 99%

“…To enable prediction of oxidation behavior, all contributing factors need to be identified and accounted for. The effect of certain parameters such as atmospheric dewpoint [6][7][8]15,[17][18][19][20] and microstructure 10,14,[21][22][23] have been studied but many questions remain.…”

Section: Introductionmentioning

confidence: 99%

Effects of Surface Microstructure on Selective Oxidation Morphology and Kinetics in N<sub>2</sub> + 5%H<sub>2</sub> Atmosphere with Variable Dew Point Temperature

Story¹,

Webler²

2019

Starting surface microstructure has been shown to influence selective oxidation morphology and kinetics on cold-rolled grades of CMnSi advanced high strength steels (AHSSs). The research presented below examined this phenomenon after 120 and 1 800 s and under N 2 + 5%H 2 with 0°C and − 30°C dewpoint temperatures (DPTs). Starting microstructures were altered by pre-annealing, which led to decarburization and grain growth. All samples were oxidized in a high temperature confocal scanning laser microscope (CSLM) at 850°C. External oxides were subsequently examined using secondary electron scanning electron microscopy (SE SEM) and energy-dispersive spectroscopy (EDS). Cross-sections were produced and examined in a focused ion beam scanning electron microscope (FIB-SEM). Oxidizing atmosphere, surface microstructure, and time all influenced selective oxidation behavior. High and low DPT atmospheres had expected effects. Variation between microstructures was only apparent in 1 800 s samples oxidized in a high DPT atmosphere. Decarburized samples oxidized at higher DPTs exhibited surfaces covered with discrete iron nodules which may facilitate reactive zinc wetting. KEY WORDS: advanced high-strength steel (AHSS); selective oxidation; high-temperature confocal scanning laser microscope (HT-CSLM).

“…[3][4][5] Although Si and Mn are also added to cold-rolled steel to secure mechanical properties, it is well known that these elements deteriorate galvanizability due to selective surface oxidation (element segregation) during the recrystallization annealing process. [6][7][8][9][10][11][12][13] In the case of hot-rolled steel, the factors that affect the galvanizability of high tensile strength hot-rolled steel sheets had not been investigated in detail,…”

Section: Introductionmentioning

confidence: 99%

Influence of Selective Surface Oxidation of Si and Mn on Fe–Zn Alloying Reaction on Hot-rolled Steel Sheets

Koba¹,

Fushiwaki²,

Nagataki³

2019

The Fe-Zn alloying reaction and selective oxidation behavior of 0.7 mass% Si-1.15 mass% Mn added hot-rolled steel annealed at 600-800°C were investigated by comparison with those of cold-rolled steel. The Fe-Zn reactivity of the hot-rolled steel improved from 600°C to 700°C but deteriorated from 700°C to 800°C. Above 700°C, the amount of Fe-Si-Mn oxide on the steel surface increased with increasing temperature, and this oxide deteriorated Fe-Zn reactivity. Below 700°C, a thin layer of Fe oxide on the steel surface deteriorated Fe-Zn reactivity. This oxide layer was reduced by Si and Mn that diffused from the steel substrate. Therefore, as the temperature increased from 600°C to 700°C, Fe-Zn reactivity improved due to the formation of reduced iron on the steel surface. In the case of the cold-rolled steel, the same selective oxidation behavior and reduction mechanism of the Fe oxide were also confirmed, and as a result, the Fe-Zn reactivity of the cold-rolled steel showed behavior similar to that of the hot-rolled steel. However, the Fe-Zn reactivity of the cold-rolled steel improved at a lower temperature than that of the hot-rolled steel. This can be explained by the faster diffusion rates of Si and Mn in the cold-rolled steel than in the hot-rolled steel. That is, reduction of the surface Fe oxide layer by diffused Si and Mn proceeded at a lower temperature, and as a result, the Fe-Zn reactivity of the cold-rolled steel also improved at a lower temperature.