Mountain‐like surface cracks often appear on the thick plates of high‐strength low‐alloy (HSLA) steel after hot rolling, which have a serious impact on production efficiency. Herein, the formation cause of mountain‐like cracks is discussed using methods such as acid pickling and observations of microstructure and inclusions. The deformation behavior of microcracks is obtained by analyzing the rolling process of slab. The inclusions in the top of the crack are Type‐I inclusions with high elemental contents of K and P and Type‐II inclusions with high elemental contents of C. The inclusions in the sides of the crack are Al2O3. It is concluded that mountain‐like cracks are caused by surface microcracks containing Type‐I inclusions in the continuous casting slab. During rolling, the rapid deforming of side‐1 stretches the crack so that the unusual microstructure appears. The cracks extended in the rolling direction have Al2O3 inclusions. The mountain‐like crack with Type‐I inclusions in the top and Al2O3 in the sides eventually form. A single microcrack forms a single mountain‐like crack and multiple clustered microcracks form a continuous mountain‐like crack. To reduce mountain‐like cracks, reducing Type‐I inclusions to reduce surface defects of the slab is an effective means.

During the continuous casting and rolling process of gear steel, AlN precipitation often causes surface cracks on the cast slab. Rapid cooling of the slab surface is a new concept to inhibit AlN and improve the cracking resistance, but it requires further investigation for practical application. Here, the effects of the cooling rate and recovery temperature on AlN growth behavior in gear steel are investigated by designing five cooling conditions. A mathematical model to describe the growth characteristics is constructed based on Ostwald ripening theory. The experimental results show that the cooling rate significantly influences the size and quantity of AlN. When the cooling rate increases from 3.0 to 5.0 °C s−1, the average diameter of AlN decreases from 133.6 to 75.8 nm and the number density of AlN decreases from 6.8 × 105 to 2.1 × 105 cm−2. The calculated results show that AlN has maximum size, which significantly decreases with decreasing temperature. When AlN reaches the maximum size, it stops growing. AlN growth during the cooling process mainly occurs above 800 °C. Therefore, a high cooling rate and a recovery temperature lower than 800 °C are key to inhibiting AlN precipitation and improving the cracking resistance.

No abstract

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/srin.202300164.

In article http://doi.wiley.com/10.1002/srin.202200281, Cheng and co‐workers investigate the effect of cooling rate on AlN precipitates in gear steel. Steel samples at different cooling rates are obtained by water‐quenching. Size and density statistics of chain‐like AlN particles on grain boundaries are carried out. The growth behavior of AlN in gear steels during cooling is proposed with the growth model.