The influence of high heat input and inclusions control for rare earth on welding in low alloy high strength steel

In the current paper, it is analyzed for the influence of high heat input and inclusions control for rare earth on welding in low alloy high strength steel. It is observed for the structure for different heat input of the coarse-grained area. It is finest for the coarse grain with the high heat input of 200 kJ / cm and the coarse grain area with 400 kJ / cm is the largest. The performance with the heat input of 200 kJ / cm for -20 °C V-shaped notch oscillatory power is better than the heat input of 400 kJ / cm. The grain structure is the ferrite and bainite for different holding time. The grain structure for 5s holding time has a grain size of 82.9 μm with heat input of 200 kJ/cm and grain size of 97.9 μm for 10s holding time. For the inclusions for HSLA steel with adding rare earth, they are Al2O3-CaS inclusions in the Al2O3-CaS-CaO ternary phase diagram. At the same time, it can not be found for low melting calcium aluminate inclusions compared to the inclusions for the HSLA steel without rare earth. Most of the size for the inclusions is between 1 ~ 10μm. The overall grain structure is smaller and the welding performance is more excellent for adding rare earth.


Introduction
At present, high heat input welding has become a trend. But the high heat input welding makes toughness and strength reduced for the weld joint. The Rare Earth (Re) can refine the grain to improve strength and toughness and also can change the shape and distribution of the inclusions in steel. There are also some studies on the effect of Re in high heat input for welding and the induction of acicular ferrite behaviour for laboratory studies [1~5]. But there is little research for the base metal especially the control for the Re to improve the toughness in the industrial production in low alloy high strength (HSLA) steel. Re elements play an important role in the refinement of grains and the formation of intragranular ferrite. Also it can enhance the impact toughness at low temperature and welding performance for HSLA steel for refinement of heat affected zone grain size for high heat input welding. Therefore, it is important for the influence of inclusions for rare earth on welding in HSLA steel.

Steelmaking process for HSLA steel
In this paper, testing and analysis in steelmaking process were tracked and done in some manufacturers for HSLA steel plate production. The steelmaking process for HSLA steel is Pretreatment of hot metal → LD → LF refining → RH refining → Calcium treatment → CC as shown in Fig. 1. The Re content in alloy is more than 95% which the ratio of cerium and lanthanum is 2 for HSLA steel.

The thermal simulation test for high heat input
The structure and properties of 200kJ / cm heat input for welded coarse grain regions were simulated by Gleeble 2000D thermal simulation test machine. The peak heating temperature is 1350 ℃ and the holding time is 3S. The corresponding welding characteristic parameter t 8/5 is 120s, and the thermal cycling curve is shown in Fig. 2.

The structure for different high heat input
It is observed for the structure for different heat input of the coarse-grained area. We can draw a conclusion from Fig. 3 that it is finest for the coarse grain with the high heat input of 200 kJ / cm and the coarse grain area with 400 kJ / cm is the largest. The performance with the heat input of 200 kJ / cm for -20 ℃ V-shaped notch oscillatory power is better than the heat input of 400 kJ / cm. It is analyzed for the structure for different holding time of the coarse-grained area. We can see from Fig. 4 that it is finest for the coarse grain for 5s holding time with the high heat input of 200 kJ / cm and the coarse grain area with 10s holding time is the largest. From Fig. 4 we can also see that the grain structure is the ferrite and bainite. The grain structure for 5s holding time has a grain size of 82.9 μm with heat input of 200 kJ/cm and grain size of 97.9 μm for 10s holding time.

The grain structure for plate adding Re and without Re
It is observed for the structure of the welded joint with 200 kJ / cm heat input in the coarse-grained area. We can draw a conclusion from Fig. 9 that it is finer for the coarse grain with the high heat input of 200 kJ / cm for adding Re. It is a small amount of acicular ferrite and a large number of massive ferrite for the grain structure for plate without Re of the weld metal. We can also draw a conclusion that the deposited metal structure contains a large amount of acicular ferrite. The overall grain structure is smaller and the welding performance is more excellent.  Fig. 9 The grain structure for plate adding Re and without Re(a: inclusions with Re; b: without Re )

Conclusions
(1) It is observed for the structure for different heat input of the coarse-grained area. It is finest for the coarse grain with the high heat input of 200 kJ / cm and the coarse grain area with 400 kJ / cm is the largest. The performance with the heat input of 200 kJ / cm for -20 ℃ V-shaped notch oscillatory power is better than the heat input of 400 kJ / cm.
(2) The grain structure is the ferrite and bainite for different holding time. The grain structure for 5s holding time has a grain size of 82.9 μm with heat input of 200 kJ/cm and grain size of 97.9 μm for 10s holding time.
(3) For the inclusions for HSLA steel with adding Re, they are Al 2 O 3 -CaS inclusions in the Al 2 O 3 -CaS-CaO ternary phase diagram. At the same time, it can not be found for low melting calcium aluminate inclusions compared to the inclusions for the HSLA steel without Re. Most of the size for the inclusions is between 1 ~ 10μm (4) The overall grain structure is smaller and the welding performance is more excellent for adding Re.