Effect of nano‐particulate sol–gel coatings on the oxidation resistance of high‐strength steel alloys during the press‐hardening process

Abstract

The need for lighter constructional materials in automotive industries has increased the use of high‐strength steel alloys. To enhance passenger's safety press hardening may be applied to steel parts. However, as the steel parts are heated up to 950 °C during this process they have to be protected by some kind of coating against the intense oxide formation usually taking place. As the coating systems used so far all have certain disadvantages in this work the ability of nano‐particulate thin coatings obtained by the sol–gel process to improve the oxidation resistance of 22MnB5 steel is investigated.The coatings obtained from three sols containing lithium aluminum silicate and potassium aluminum silicate showed the best performance against oxidation. The structural properties of the coating materials were characterized using different methods like XRD and differential thermal analysis. Comparison of the oxidation rate constants proved the ability of the coatings to protect against oxidation at temperatures up to 800 °C. Press‐hardening experiments in combination with investigations on the thermal shock resistance of the coated samples also showed the ability of the coatings to stay intact during press hardening with only slight spalling of the coatings in the bending areas. The absence of any secondary intermetallic phases and layer residues during laser beam welding experiments on coated samples proves the suitability of the nano‐particulate coatings for further industrial processing.

The Sacrificial Protection of Steel by Zinc-Containing Sol-Gel Coatings

“…Thin, sol-gel coatings have previously been applied to the boron-manganese steel alloy 22MnB5 with the aim of improving oxidation resistance. 33 The coatings were found to protect against oxidation up to 800°C and remained intact during press hardening. 33 The absence of secondary phase intermetallics and residues (which can form during the subsequent laser beam welding process) highlights the suitability of sol-gel coatings to further industrial processing.…”

“…To achieve the desired mechanical properties, steel parts are press hardened at temperatures of up to 950 °C. 33 The oxidation resistance of components is therefore of importance to avoid detrimental changes to both mechanical properties and surface appearance. Thin, sol-gel coatings have previously been applied to the boron-manganese steel alloy 22MnB5 with the aim of improving oxidation resistance.…”

“…33 The absence of secondary phase intermetallics and residues (which can form during the subsequent laser beam welding process) highlights the suitability of sol-gel coatings to further industrial processing. 33 The corrosion properties of 'higher abrasion resistant' sol-gel Zn-SiO 2 thin films have previously been characterized using electrochemical impedance spectroscopy (EIS). 34 The films were shown to behave differently to conventional sacrificial Zn based coatings (such as HDG), and any enhanced corrosion protection was proposed to be a result the tendency of Zn to react with Clions to form ZnCl 2 and meant that the coatings were deemed suitable for use in marine environments.…”

“…33 The coatings were found to protect against oxidation up to 800°C and remained intact during press hardening. 33 The absence of secondary phase intermetallics and residues (which can form during the subsequent laser beam welding process) highlights the suitability of sol-gel coatings to further industrial processing. 33 The corrosion properties of 'higher abrasion resistant' sol-gel Zn-SiO 2 thin films have previously been characterized using electrochemical impedance spectroscopy (EIS).…”