Research on the Causes and Treatment of Weld Zone Fracture in Cold Rolled High Strength Steel Rolling Process

In response to the poor quality of laser welding of cold-rolled advanced high-strength steel and the problem of easy fracture at the weld seam during the rolling process, the reasons for the fracture at the weld seam were first analysed using scanning electron microscopy and other equipment, to clarify the fracture characteristics and organizational distribution. Combined with the characterization results of mechanical properties, the reasons for the fracture at the weld seam during the rolling process were elucidated; Then, the influence of welding process on weld quality is studied from four aspects: laser power, welding speed, pre heat power, and post heat power; Based on the above analysis, a typical cold rolled advanced high-strength steel laser welding technology has been developed. By optimizing welding parameters such as welding power, speed, and preheating before and after, the quality of the weld seam and the cup protrusion effect have been ensured. At the same time, a reasonable rolling process optimization plan has been proposed; After applying the research results to the site, the quality of the weld seam has been significantly improved, and the occurrence rate of band breakage at the high-strength steel weld seam has decreased from an average of 3.6% per month to 0.13%. The band breakage defect at the weld seam has been effectively treated.


Introduction
Cold rolled advanced high-strength steel, due to its inherent composition characteristics, is a difficult to weld steel in laser welding technology, with poor weld quality and cup protrusion effect [1][2][3].The coldrolled high-strength steel produced by a certain cold rolling unit of Ansteel is relatively difficult to coldrolled during the production process due to its high strength of the hot-rolled substrate, severe work hardening, high rolling compression ratio, and high rolling deformation resistance.Among them, unstable welding seam rolling has become one of the main bottlenecks in improving the production efficiency of cold-rolled high-strength steel [4][5][6][7].The main impact caused by the welding seam during the rolling process is: a) The welding seam is prone to fracture during the process of passing through the roller gap, causing the unit to shut down, produce waste products, shorten the roll replacement cycle, and damage to the relevant equipment of the unit; b) When the rolling mill passes through the weld seam, sudden changes in rolling parameters can easily cause rolling mill vibration, leading to poor thickness control at the outlet of the rolling mill and the occurrence of surface defects [8][9][10].
In previous literature related to weld seam breakage, typical examples include Zhang, Fan et al. [11] improving the welding quality of multiphase steel by changing laser welding power and welding speed; Tober, Gerhard et al. [12] found that appropriate heat treatment can make the strength and ductility of the welded joint more uniform, thereby improving the welding quality; Kumar, Nitin et al. [13] studied the effect of welding machine process parameters on the microstructure and mechanical properties of dual phase DP780 steel and CR340 steel dissimilar welding; Pawar, Sachin et al. [14] found that the presence of insulation impurities in water reduces the welding quality at the weld seam of cold rolled galvanized sheet; Fan, Jiafei et al. [15] studied the effect of different laser intensities on the mechanical properties of strip welds in cold continuous rolling mills.The above literature indicates that the process parameters of the welding machine have a significant impact on the quality of cold continuous rolling welds, but there is no specific solution.
Therefore, this article takes a cold rolling mill in Ansteel as the research object, analyses the weld seam of the on-site broken strip, and then studies the influence of welding process parameters on welding quality.Finally, corresponding solutions are proposed for the defect of the weld seam broken strip, improving the production efficiency and automation level of the unit.

Research on the Influence of Welding Process on Weld Quality
The main reasons that affect the quality of welding seams are: the hardness of the weld seam itself is high, the plasticity is poor, and the transition from the weld seam area to the base metal performance is poor.To avoid the occurrence of the above situation, experimental studies were conducted on the core parameters of laser welding of hot-rolled raw materials, including laser power, welding speed, and post heat power.

The Influence of Laser Power on Weld Quality
Laser power determines the heat input efficiency of the welding process.Low laser power leads to insufficient welding heat input, poor welding penetration, and significant differences between the front and back of the weld seam; If the laser power is too high, it will cause the expansion of the weld heat affected zone and energy waste.At the same time, the upper limit of laser power also depends on the design ability of the laser welding machine itself.The experimental design of laser power is shown in table 1.The weld quality data is shown in figure 1.Compared with figure 1 (a) and (b), when the laser power is low, there is a significant fluctuation in weld penetration and weld fullness, especially in the early stages of welding.Due to the quality of raw materials, the height difference on both sides of the weld fluctuates significantly in the middle; When the laser power is 11.8kW, the penetration and fullness of the weld seam are relatively stable, and the fluctuation of the height difference in the middle of the weld seam caused by the raw material is greatly improved.Therefore, in order to ensure the stability of weld penetration, fullness, and height difference, laser power should be maximized during process design.In practical applications, in order to ensure the use status of the laser light source, it is recommended not to use 100% of the design power for production.The upper limit of the power design for the laser welding machine used in the cold rolling plant is 12kW, and 11.8kW of laser power should be used for welding of cold rolled high-strength steel during laser welding.

The Influence of Welding Speed on Weld Quality
The joint influence of welding speed and laser power on heat input further affects the quality of the weld seam.If the welding speed is too low, the heat affected zone of the weld seam becomes larger, which is not conducive to the control of production rhythm; If the welding speed is too high, it will affect the penetration and performance transition of the weld seam.The experimental design of the welding speed is shown in table 2.  The weld quality data is shown in figure 2. Compared with figure 2 (a) and (b), when the welding speed is high, the height difference on both sides of the weld and the fullness of the weld fluctuates greatly, resulting in significant deviations in the early and late stages of welding, respectively; When the welding speed is 3.8m/min, the height difference, penetration, and fullness of the weld on both sides are relatively stable.At the same time, the change in welding speed will cause the heat treatment time of the front and rear induction heating devices.If the welding speed is too high, it will cause rapid cooling of the weld seam, thereby affecting the microstructure, properties, and transition of the weld seam.So in order to ensure the stability of weld penetration, fullness, and height difference, the welding speed should be ensured to be less than 5.0m/min during process design.

Influence of Post Heating Power on Weld Quality
A reasonable cooling rate of the weld seam can affect the microstructure at the weld seam, ultimately reducing the hardness and improving plasticity of the weld, while relaxing the stress concentration near the weld seam and improving the performance transition of the weld seam.The experimental design of post heat power is shown in table 3.
The metallographic structure of the weld is shown in figure 3. Compared with figure 3 (a) to (d), the transition zone from the center of 1 # weld to the heat affected zone is full of lath martensite with coarse grains.The center of 2 # and 3 # weld is a mixture of lath martensite and tempered martensite with small grains.The center structure of 4 # weld is still lath martensite.When the afterheat power is too low, the temperature cannot be maintained within the tempering temperature range of martensite within the effective heating time, so that a large of lath martensite formed by rapid cooling of liquid metal can be retained; When the afterheat power is appropriate, the lath martensite in the weld center can stay in the tempering temperature range for a long time, reducing the hardness of the weld center and improving the weld performance transition; When the afterheat power is too high, the weld temperature increases from rapid cooling to below Ms point and then to above the austenitizing temperature, and martensite with small Flat noodles length is formed in the subsequent rapid cooling.The hardness of 1 # -4 # welds is characterized as shown in figure 4. The hardness of the center of 1 # and 4 # welds is about 400HV, and there is no obvious transition, which is consistent with the characteristics of a large number of lath martensite with high hardness, and there is a risk of rolling fracture; The center hardness of welds 2 # and 3 # is about 330HV, and there is a clear transition zone.Tempered martensite as the transition structure plays a main role in reducing hardness, and the risk of rolling band breakage is relatively low.In the actual production process, in order to reduce the occurrence of welding seam rolling strip breakage, the post heat power should be set at 16kW-20kW during process design.

Research on Treatment Technology for Weld Zone Fracture
Through the above research, it can be known that the welding process has a significant impact on the quality of the weld seam, and the rolling process when passing through the weld seam can also have an impact on strip breakage.

Welding Process Optimization
Through the above experiments, the influence of four core welding process parameters, namely laser power, welding speed, pre heat power, and post heat power, on the quality of the weld seam and the reasonable adjustment direction, were obtained.Based on the steel quality grading based on carbon equivalent for different steel grades, the experimental method of welding process window was used to optimize the process of typical high-strength steel in the cold rolling plant.The proposed laser welding technology scheme for cold rolled high-strength steel is shown in table 4.

Improvement of Welding Process
Due to the poor surface quality of the weld seam, poor deformation ability of the weld seam, and the presence of a large number of microcracks, the situation of rolling broken bands at the weld seam can be classified through the cup protrusion test and surface morphology observation of the weld seam.However, rolling broken bands near the weld seam require multiple cup protrusion tests along the width direction of the weld seam and adjustment of the welding process, which is difficult to determine in actual production processes, it often occurs when producing difficult to weld varieties with higher strength levels and newly developed steel grades.This situation can be effectively judged before rolling.When multiple adjustments to the welding process still cannot meet the quality requirements of the weld seam, in order to ensure the production efficiency of the unit and reduce the risk of rolling strip breakage, the rolling mill can be used to open the roll gap and pass through the weld seam mode for production, as shown in figure 5.By manually controlling the lifting of the roller gap, no rolling is carried out near the weld seam, and strip steel with unqualified thickness near the weld seam is cut off after rolling.

Application
In order to further demonstrate the practicality of the research results, taking a cold continuous rolling mill of a certain steel plant as an example, typical specifications of steel grades and products were selected as the research objects.The research results obtained in Section 3 were applied to on-site production practice to verify their effectiveness.The main equipment parameters of the unit are shown in table 5, and the main types and composition of high-strength steel produced by the unit are shown in table 6.
Applying the laser welding process parameters of high-strength steel shown in table 5 to on-site production practice, two typical high-strength steel ASDP1180 welds with stable production and rolled strip breakage were selected for cup protrusion testing, as shown in figure 6.The qualified cup protrusion specimen cracked along the outer edge of the loaded base metal under load, and plastic deformation occurred in the center of the weld seam in a hemispherical shape.The center of the weld seam of the unqualified cup protrusion specimen cracked earlier than the base metal under upward load, Cracks propagate along the direction of the weld seam.The microstructure of the two welds was observed by optical microscope (OM).The lath martensite in the weld center area was formed by rapid cooling of liquid high temperature after laser melting.Compared with figure 7 (a) and (b), some tempered martensite appeared in the transition part from the weld center area of qualified samples to the heat affected zone, while the weld center area of unqualified samples was still dominated by a large number of lath martensite.Therefore, it can be seen that using the welding parameters shown in table 5 can improve the quality of the weld seam.
At the same time, the band breakage rate at the weld seam after the above research results were applied to the site was statistically analysed.Table 7 below shows the production data for a total of 6 months before and after the use of the research results mentioned in this article.The occurrence rate of band breakage at high-strength steel welds has decreased from an average of 3.6% per month to an average of 0.13% per month.The band breakage rate of high-strength steel welds has been effectively reduced, and the band breakage defects at welds have been effectively treated.

Conclusion
(1) The height difference near the broken weld seam fluctuates significantly, and there are protrusions at the upper and lower surface welds, which are prone to stress concentration during the rolling process and lead to weld seam fracture; The hardness of the central area of the weld seam with a broken strip is relatively high, and the hardness change span is large without obvious transition zone.Compared with normal welds, the yield and tensile strength of the weld seam with a broken strip are similar, but the plasticity is poor.
(2) To ensure the stability of weld penetration, fullness, and height difference, the welding speed should be ensured to be less than 5.0m/min during process design; Properly increasing the post heat

Figure 1 .
Figure 1.Welding quality under different laser powers.

Figure 2 .
Figure 2. Welding quality under different welding speeds.

Figure 3 .
Figure 3. Metallographic structure of welds with different post heat power.

Figure 4 .
Figure 4. Weld hardness of different post heat power.

Figure 5 .
Figure 5. Rolling mill opening roll gap passing weld pattern.

Table 1 .
Experimental Design of Laser Power.

Table 2 .
Experimental Design of Welding Speed.

Table 3 .
Experimental design of post thermal power.

Table 4 .
Laser Welding Process for Cold Rolled High Strength Steel.

Table 5 .
Main equipment and process parameters of parameters of cold continuous rolling unit.

Table 6 .
The type and composition of high-strength steel produced by the cold rolling mill.

Table 7 .
Statistics on the occurrence rate of band breakage defects at welds before and after process optimization.