Abstract
The authors have investigated key technological parameters of stable production of IF steel sheet with BH-effect. The features of ultralow carbon IF steels and IF steels with BH effect are described. Scheme of IF-BH steel hardening during hot drying of the car body after painting is considered. Data on the chemical composition of IF and IF-BH steels produced at Russian and worldwide enterprises are presented. The authors have analyzed the reasons of appearance of such a defect in the steel sheet as slip bands that arise when the yield area appears on the tensile diagram of steel samples. The requirement on the “shelf life” presented to the IF-BH steel grades is given and disassembled. The article considers the formula for calculating effective carbon content in steel based on the total carbon content in steel, niobium, titanium and nitrogen. The range of carbon effective in steel is given to achieve the optimal value of the BH effect based on previously published works. The results of industrial IF-BH steel production have been analyzed for the conditions of Russian enterprise and recommendations were given on criteria such as the optimum carbon content range in solid solution, the recommended maximum total concentrations of carbon and nitrogen in steel, and the BH effect value guaranteeing a high yield of IF-BH steel sheet in the production. Calculations of various options of microalloying by titanium and niobium for IF-BH steel were done. Criteria are formulated that allow stably obtaining a given value of the BH effect in cold-rolled ultralow-carbon steels. A two-stage scheme for microalloying by titanium and niobium is proposed. The influence of grain size in sheet steel on the presence and value of the BH effect is described as well as on the presence/absence of a yield plateau on the tensile diagram in the sheet metal in the initial state.
Highlights
Scheme of IF-BH steel hardening during hot drying of the car body after painting is considered
The authors have analyzed the reasons of appearance of such a defect in the steel sheet as slip bands that arise when the yield area appears on the tensile diagram of steel samples
The article considers the formula for calculating effective carbon content in steel based on the total carbon content in steel, niobium, titanium and nitrogen
Summary
Григорович К.В.1, 2, академик РАН, д.т.н., профессор, заведующий лабораторией диагностики материалов. Описаны особенности сверхнизкоуглеродистых IF сталей и IF сталей с BH-эффектом, схема процесса упрочнения IF-BH стали при горячей сушке кузова автомобиля после покраски. Аббревиатура BH расшифровывается как bake hardening – эффект упрочнения стали при горячей сушке кузова автомобиля после покраски. Для проявления эффекта упрочнения стали при сушке химический состав регулируется таким образом, чтобы в стали оставалось 15 – 25 ppm углерода эффективного (Сэфф ) [5, 6]. В ряде работ показано, что содержание углерода эффективного в твердом растворе α-Fe, находящегося в диапазоне от 12 до 25 ppm, дает оптимальный BH-эффект при сушке и позволяет избежать процесса старения при комнатной температуре в течение «срока годности» [5, 6, 9]. Что общее содержание углерода и азота в сталях не превышает 40 ppm каждого для IF-BH и IF сталей
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