During solidification process, the surface temperature of liquid core is low and the core temperature is high, so it has good plasticity. The linear shrinkage is caused by phase transformation in the surface layer of slab, and the plasticity in the center is good, which can accommodate shrinkage without cracks. As the slab continues to cool, the surface layer loses its plasticity, the internal Y-A phase transformation occurs, the volume expands, and the surface layer of the slab cracks due to the low plasticity

从钢种的线收缩情况来看，800℃以上时高碳含量钢种的线收缩率低于低碳含量钢种，在800}600℃时线收缩率有一个转折，600℃以下时高碳含量钢种线收缩率高于低碳含量钢种，且温度越低差别越大，。在板坯出炉后冷 却到室温的过程中，中、高碳钢的冷 却速度显得更为重要。

According to the linear shrinkage of steel grades, the linear shrinkage of high carbon steel grades is lower than that of low carbon steel grades above 800 ℃, there is a turning point at 800} 600 ℃, and the linear shrinkage of high carbon steel grades is higher than that of low carbon steel grades below 600 ℃, and the lower the temperature, the greater the difference,. In the process of slab cooling to room temperature, the cooling rate of medium and high carbon steel is more important.

冷 却速度快会产生强烈的收缩应力，可能在含夹杂、孔洞、疏松等内部缺 陷的部位引起应力集中，从而形成裂纹源或产生裂纹。而一旦产生裂纹，内应力和搬运时所承受外界作用力的共同作用会使裂纹迅速扩大，在冷 却速度快的情况下很容易使裂纹扩展成为铸坯开裂。

High cooling rate will produce strong shrinkage stress, which may cause stress concentration in the parts containing inclusions, holes, porosity and other internal defects, thus forming crack sources or cracks. Once the cracks are formed, the internal stress and the external force during transportation will make the cracks expand rapidly. Under the condition of fast cooling speed, it is easy to make the cracks expand into slab cracks.

针对此钢种特性制定工艺优化措施:(1)二次冷 却采用缓冷模式。(2)铸坯以热送方式装入加热炉加热;如现场条件限 制不能实现热送，应将铸坯整齐码放缓冷坑中进行堆垛缓冷，堆垛缓冷时间>48h}堆垛缓冷后不得随意挪动。

According to the characteristics of this steel, the process optimization measures are made as follows: (1) slow cooling mode is adopted for secondary cooling（ 2) The billet is put into the heating furnace for heating by hot delivery; If the hot delivery can not be realized due to the limitation of site conditions, the billets should be stacked neatly in the slow cooling pit for slow cooling, and the slow cooling time should be more than 48 h} after the slow cooling, the billets should not be moved at will.