Excellent series of strength, weldability, and processability of steel plates

Dongying steel plates for construction machinery are urgently required to improve their strength and wear resistance due to their ability to withstand wear from sand and ore, as well as to achieve structural lightweighting.

In recent years, in order to use this type of steel plate in cold regions, it is necessary to ensure its low-temperature toughness; To improve its workability, it is necessary to lower the preheating temperature during welding; In many situations, it is necessary to improve its bending processability. To meet these demands, JFE has developed two types of wear-resistant steel plates and one type of high-strength steel plate. Below is a brief introduction to the development and performance of this type of LE series steel plate.

2 Development Process

The thickness of wear-resistant steel plates 360LE and 500LE for construction machinery are divided into two categories: 6-19mm and 19.1-32mm, respectively; Their surface hardness ranges from 361 to 440 and 447 to 556, respectively; According to the Ceq requirements for different thickness ranges of two steel grades, the 360LE thin one should be ≤ 0.40% and the thick one should be ≤ 0.43%, the 500LE thin one should be ≤ 0.55%, and the thick one should be ≤ 0.58%.

The thickness of 780MPa high-strength steel plates is divided into two categories: 6-19mm and 19.1-32mm; The strength targets are YS ≥ 685MPa and TS=780-930MPa; Ceq is defined as ≤ 0.40% and ≤ 0.43% according to different thickness ranges.

Both series of steel plates mentioned above must ensure excellent low-temperature impact absorption at -40 ℃; Moreover, Ceq has been appropriately reduced to ensure welding performance.

The results of high hardness research closely related to wear resistance indicate that the hardness of steel plates is mainly determined by the C content (hereinafter referred to as [C]) in the steel and the M (martensite) content in the quenched steel. Therefore, in order to achieve the target hardness, it is important to ensure both [C] and complete M content in the composition design.

After quenching various steel plates with [C]=0.10% to 0.30% to produce a complete M structure, the surface hardness was measured. At the same time as determining the target hardness HBW to be ≥ 361 and ≥ 477, the lowest control values [C] were determined to be 0.14% and 0.26%, respectively.

In order to evaluate the hardenability of steel using DI, research was conducted on the premise that the development of a maximum target thickness of 32mm steel plate can achieve a complete M-structure. And investigated the critical cooling rate at which steels with different chemical compositions achieve complete M-structure. The results indicate that at a cooling rate of a maximum target plate thickness of 32mm, only by ensuring DI ≥ 45mm can complete M-structure be obtained after quenching. Moreover, in order to prevent the occurrence of delayed fracture and quenching cracking on high hardness steel in wear-resistant steel plates, the heat treatment strain should be minimized as much as possible, that is, the cooling rate and final cooling temperature should be appropriately controlled.

Micro alloying elements represented by Nb, Ti, and V promote the refinement of grain A through the dragging effect of solid solution atoms and the blocking effect of precipitates; Moreover, its solid solution into A also has the effect of improving the hardenability of steel. When designing alloys that can achieve these effects, we also studied how to control the heating temperature of continuous casting slabs, control rolling and heat treatment to refine A grains.

Due to the use of the aforementioned A-grain refinement technique, the original A-grains, which were originally coarse to 40-50 μ m, were refined to below approximately 20 μ m. Further observation of the microstructure of the fracture section during the Charpy impact test at -196 ℃ and the artificial occurrence of brittle fracture shows that compared to the original steel, the roughness of the developed steel has decreased, which is the result of the finer grain size that dominates toughness. Therefore, it can be confirmed that the size of the crystal package decreases with the refinement of the original A grains.

The study on the effect of original A-grain size on the Charpy impact absorption energy (VE-40 ℃) at -40 ℃ shows that the developed 0.15% C-M steel uses microalloying and TMCP (controlled cooling and rolling) technology to refine the original A-grain size of the steel to 20 μ m, resulting in a significant increase in VE-40 ℃ to 175-180J (even if refined to 30 μ m, there is still 90-98J); However, due to the original A-grain size of 30-50 μ m, the VE-40 ℃ of the original steel is only about 20-45J (only about 1/8-1/4 of the highest value and 1/4-1/2 of the lowest value of the developed steel). This indicates that as long as measures are taken to refine the original A grains of the developed steel to below 30 μ m or even 20 μ m, the low-temperature toughness can be significantly improved.

3. Develop steel plate performance

In order to investigate the welding performance of Dongying steel plate, Y-shaped groove welding cold cracking tests were conducted using 360LE steel plate with a thickness of 19mm and 500LE steel plate with a thickness of 20mm. The results indicate that regardless of the preheating temperature used for welding, both types of steel plates did not produce surface or internal welding cracks, demonstrating excellent welding performance.

The Brinell hardness and Charpy impact test results of 360LE and 500LE steel plates show that the hardness HBW10/3000 of the two steels are 361-440 and 477-556, respectively, which are equivalent to the original steel; However, the low-temperature impact absorption energy V E-40 ℃ of the two developed steel plates is as high as around 45-70J and 45J respectively, far higher than the original steel's around 20J. This indicates that the development of steel plates has high hardness and excellent low-temperature toughness; It is the world's first product to ensure low-temperature toughness of -40 ℃ and can be used in cold regions; Due to the improved impact damage resistance, the longevity of construction machinery component materials can be achieved.

Due to the use of microalloying technology mainly composed of Nb, V, and Ti, as well as deformation heat treatment represented by super online high-speed cooling, this steel plate has been developed The control of heat treatment technology has greatly improved the low-temperature toughness of 780MPa high-strength steel plate 780LE.

The tensile strength (TS) of 780LE steel plate is equivalent to that of the original steel, both of which are approximately 820-840MPa; Its YS (yield strength) is 750-810MPa, slightly higher than the original steel's 750-775MPa. The key is the quantitative index of low-temperature impact performance of 780LE steel plate VE-40 ℃=230-270J (average 250J), which is characterized by a small fluctuation range, indicating stable performance; Moreover, within the thickness range of 12-32mm, thicker steel plates have higher toughness. However, the VE-40 ℃ of the original steel plate was only 60-100J, which is only one-third of the average value of the developed steel plate; And within the thickness range of 18-40mm, its toughness value decreases with the increase of thickness.

4 Conclusion

In the field of wear-resistant steel plates and high-strength steel plates for construction machinery, JFE-EH360LE/500LE and JFE-HITEN780LE (LE series) developed steel plates, which first ensure low-temperature toughness of -40 ℃ in the world, not only can be used in cold regions, but also have excellent impact resistance and safety, and have excellent welding workability due to the moderate reduction of Ceq value.