In order to explore the application of laser cutting technology in the rescue and demolition of Q235 steel plates. In view of the demand for fast and efficient demolition tools in rescue operations, laser cutting shows great potential due to its non-contact, high-speed and high-precision characteristics. Taking 8mm and 15mm thick Q235 steel plates as the research objects, a high-power fiber laser was used to conduct long-distance cutting experiments. The influence of laser power and cutting speed on cutting quality was verified by theoretical analysis and experiments. The best cutting effect was obtained when the distance between the laser head and the workpiece was 10m, the cutting speed was 2.00mm/s, and the laser power was 10kW for 8mm thick steel plates; the best cutting effect was obtained when the distance between the laser head and the workpiece was 13m, the cutting speed was 2.25mm/s, and the laser power was 13kW for 15mm thick steel plates. In addition, the hardness test revealed the influence of laser power on the hardness of the material after cutting, indicating that laser cutting not only changes the geometry of the material, but also may affect its mechanical properties. This result is helpful for laser cutting technology in the field of rescue and demolition, and provides optimized process parameters for practical application.

In the face of emergencies such as collapsed houses, deformed vehicles and trapped people in sudden events such as earthquakes and traffic accidents, rescuers must quickly and effectively remove obstacles to implement rescue.
Therefore, continuously improving the performance of demolition tools and optimizing rescue strategies are of great significance to improving my country’s disaster rescue capabilities. As a non-contact demolition method, laser cutting has many advantages, such as fast cutting speed, small heat-affected zone, high precision and wide application range. Domestic and foreign researchers have carried out a series of studies in the field of laser cutting.
ZHONG et al. used a 20kW fiber laser to conduct oxygen and air laser cutting process tests on 25mm thick Q235B steel, and studied the effects of auxiliary gas type, auxiliary gas pressure, cutting speed and defocus on cutting quality. By optimizing the process parameters, the effects of smooth cut surface, fine stripes and almost no slag at the bottom can be achieved. JIANG et al. broke through the key technologies of 4kW high-energy fiber laser pump source, distributed heat dissipation cooling, multi-mode fusion safety management, studied the lightweight of superconducting thermal materials, performance of pump source energy density, and modularization of safety systems, and developed a non-contact fire rescue and demolition equipment with long non-contact demolition distance, high energy density, compact structure, light weight, safe and reliable management and use. LEI et al. took 1.5 mm thick galvanized steel plate as the research object, cut it with YAG laser, and used the single factor test method to analyze the effects of cutting speed, laser power, and defocus on the slit width and slag thickness. The results showed that: if the cutting speed is too high, the cutting will not be through; the increase of laser power will increase the slit width; the decrease of defocus will increase the slag thickness. ZHENG et al. developed a prototype of laser demolition equipment and carried out performance testing of laser demolition equipment. Studies have shown that equipment laser can achieve efficient cutting of demolition objects of different materials and thicknesses. Laser demolition equipment is a beneficial supplement to existing rescue and demolition equipment, and is particularly suitable for underwater accident cutting and demolition, traffic accident vehicle demolition, narrow space demolition and other rescue sites. The above research has great reference value for the comprehensive consideration of multiple factors such as process parameter optimization, material properties, and the synergistic effect of cutting speed and laser power. And it has an inspiring effect on the specific needs of the application field and technological progress and innovation. On this basis, in order to explore the influence of laser power and cutting speed on cutting quality, this experiment was carried out.
Laser cutting is based on the principles of optics, thermodynamics and materials science, and uses a high-brightness, monochromatic, directional laser beam generated by a laser. After the laser beam is transmitted and focused by the optical system, the energy is converted into heat energy, which quickly increases the surface temperature of the material to the melting point, achieves melting or evaporation, and then cuts. Laser power and material properties such as thermal conductivity, melting point and absorptivity affect the cutting process.
The experiment uses Q235 carbon steel plates with a thickness of 8mm and 15mm for long-distance high-power laser cutting experiments, mainly analyzing the influence of laser power and cutting speed on cutting quality, and finally obtaining optimized process parameters. At the same time, the hardness test of the cut section is carried out to analyze the effect of laser power on the hardness of the cut section, and the feasibility of applying laser cutting technology in demolition and rescue is comprehensively evaluated.

1 Experimental materials and equipment

The experiment uses Raycus RFL-C20000TZ continuous fiber laser for long-distance cutting. The experimental equipment is shown in Figure 1, including the self-developed laser output head, stepper motor slide, Kuka robot, screw air compressor, laser water cooler, auxiliary gas system and pulse dust collector. The Q235 steel plate is fixed on the stepper motor slide bracket. The system controls the laser through a computer program to adjust the laser power, cutting speed and other parameters to achieve precise cutting. The overall schematic diagram of the cutting system is shown in Figure 2. The experimental materials used in the experiment are 8mm and 15mm thick Q235 steel plates, whose chemical composition is shown in Table 1 and physical properties are shown in Table 2.

2 Experimental methods

The experimental method of this study includes the following steps:
(a) Experimental material preparation: 8mm and 15mm thick Q235 steel plates are selected as experimental materials.
(b) Equipment setting: Use Raycus RFL-C20000TZ continuous fiber laser, equipped with self-developed laser output head, connected to screw air compressor and laser water cooler to form cooling system.
(c) Cutting parameter setting: Through computer program control, adjust laser power, cutting speed and other parameters, and conduct single factor experimental design.
(d) Cutting process: The laser beam is transmitted to the surface of Q235 steel plate through the laser head, and the stepper motor slide drives the workpiece to move to achieve cutting, maintaining the consistency and repeatability of cutting speed.
(e) Data collection: Use vernier caliper to measure the slit width and slag height, and take the average value.
(f) Hardness test: Hardness test the cut section to analyze the effect of laser power on material hardness.
(g) Data processing: Statistical analysis method is used to process experimental data and evaluate the effect of different process parameters on cutting effect. The experimental method strictly follows scientific principles and ensures the repeatability and reliability of the results by precisely controlling experimental conditions and parameters. The schematic diagram of laser long-distance cutting of steel plate is shown in Figure 3.

3 Results and discussion

3.1 Single factor experiment of laser cutting Q235 carbon steel plate
In order to deeply analyze the influence of laser power and cutting speed on the cutting quality of Q235 steel plate, the influence mechanism of power on cutting effect is explored by comparing with existing literature.
3.1.1 Effect of laser power on cutting quality
In order to explore the influence of laser power on cutting quality, a single factor experiment was conducted. The experiment was conducted with steel plate thickness of 8mm and 15mm, the laser head was 10m, 13m, and 20m away from the workpiece surface, the laser power was set to 5kW, 6kW, 7kW, 8kW, 9kW, 10kW, 12kW, 13kW, 15kW, 16kW, and 17kW respectively, and the laser cutting speed was 2.00mm/s, 2.25mm/s, and 2.83mm/s. The experimental data are shown in Table 3. The experimental results are shown in Figure 4.

As can be seen from Figure 4, when the thickness of the carbon steel plate is 8mm, the distance between the laser head and the workpiece is 10m, and the cutting speed is constant at 2.00mm/s, when the laser power of 5kW, 6kW and 7kW is low, the slit width of the carbon steel plate is uneven and the slag is serious. When the power is increased to 8kW, the cutting quality is significantly improved, and the slag begins to be regularly distributed. The best effect is achieved until the power of 10kW.
When the thickness of the carbon steel plate is 15mm, the distance between the laser head and the workpiece is 13m, and the cutting speed is constant at 2.25mm/s, 10kW, 12kW, and 13kW are compared. It is found that the 10kW power does not penetrate the carbon steel plate when the cutting speed is too fast and the thickness is too thick. When the laser power is increased to 12kW, only part of the carbon steel plate can be penetrated. Until the laser power is increased to 13kW, the carbon steel plate is finally penetrated and the effect is very good. During the experiment, it was found that when the laser power increased to a certain intensity, the dross situation could be greatly improved.
When the thickness of the carbon steel plate was 15mm, the distance between the laser head and the workpiece was 20m, and the cutting speed was constant at 2.83mm/s, the laser powers of 15kW, 16kW, and 17kW were compared. It was found that the laser powers of 15kW and 16kW also had the situation of poor cutting effect due to too fast cutting speed and too thick carbon steel plate. When the power was increased to 17kW, the carbon steel plate was finally completely penetrated. Because the power
was high enough, the dross was still evenly distributed. The experimental results show that laser power is the key factor affecting cutting quality. With the increase of laser power, the slit width gradually increases, which is consistent with the research results in the reference. Parameters such as auxiliary gas type, auxiliary gas pressure, cutting speed and defocus have a significant effect on cutting quality. In this study, when the laser power increased from 5kW to 17kW, the kerf width of 8mm thick steel plate increased from 1.25mm to 1.98mm, showing the direct effect of laser power on cutting width. As shown in Figure 5. The increase in laser power means that more energy is input into the material, causing the material to melt faster, thereby speeding up the cutting speed. However, too high laser power may cause excessive melting and vaporization of the material, resulting in more dross and spatter. This phenomenon can be explained by the theory of heat conduction and melting dynamics. When the laser beam irradiates the surface of the material, the energy is absorbed and converted into heat energy, rapidly raising the surface temperature of the material, and the material begins to melt after exceeding the melting point. If the laser power is high enough, the material will evaporate directly from the solid state to the gaseous state, forming a cutting hole. The experimental results reveal the significant effect of laser power on cutting quality: low power may lead to incomplete cutting and dross, while high power may cause excessive melting and vaporization, also resulting in dross and spatter. The increase in laser power has a positive effect on the cutting width because the increased energy input leads to an expansion of the melting area. Therefore, the laser power is positively correlated with the cutting quality and negatively correlated with the slit width, indicating that at a fixed cutting speed, the laser power directly determines the melting rate and cutting quality of the material.

3.1.2 Effect of cutting speed on cutting quality
In order to explore the effect of cutting speed on cutting quality, a single factor experiment was conducted. The thickness of the steel plate was 8mm and 15mm, the distance between the laser head and the workpiece surface was 13m and 20m, the laser power was set to 6kW, 10kW, and 15kW, and the laser cutting speed was 1.0mm/s, 1.65mm/s, 2.25mm/s, 2.5mm/s, 2.83mm/s, and 3.13mm/s. The experimental data are shown in Table 4, and the experimental results are shown in Figure 6.

As can be seen from Figure 6, when the thickness of the carbon steel plate is 8mm, the distance between the laser head and the workpiece is 13m, the laser power is constant at 6kW, and the cutting speed is 1.0mm/s, although the effect of complete breakdown is achieved, the slag is too accumulated. When the cutting speed is increased to 1.65mm/s, it can also achieve complete breakdown and the slag situation is well improved. However, when the cutting speed is continuously increased to 2.25mm/s, the carbon steel plate is not penetrated and the slag situation is more serious than that at 1.00mm/s.
When the thickness of the carbon steel plate is 15mm, the distance between the laser head and the workpiece is 20m, the laser power is constant at 10kW, and the cutting speed is 1.00mm/s, the cutting quality is good and the slag is evenly distributed. When the cutting speed is increased to 1.65mm/s, the cutting effect is also good and the slag is evenly distributed. However, when the cutting speed was increased to 2.25mm/s, the carbon steel plate was not penetrated, because the power was high enough and the dross situation was still good.

When the thickness of the carbon steel plate was 15mm, the distance between the laser head and the workpiece was 20m, and the laser power was constant at 15kW, the cutting speed was 2.50mm/s, the cutting effect was very good and the dross distribution was uniform. However, when the cutting speed was increased to 2.83mm/s and 3.13mm/s, the carbon steel plate was not penetrated, because the power was sufficient and the dross situation was still good.
Studies have shown that slow cutting (such as 1.0mm/s) increases the kerf width and dross, while fast cutting (such as 3.13mm/s) may result in incomplete cutting. This finding is consistent with the research in the reference, which pointed out that excessive cutting speed will result in incomplete cutting. The adjustment of cutting speed needs to consider material melting and auxiliary gas efficiency to maintain cutting stability and quality. The cutting speed determines the laser thermal action time, affecting the width of the melting zone, the kerf width, the size of the heat-affected zone and the material removal efficiency. Too fast a cutting speed may lead to insufficient energy and discontinuous cutting. Therefore, a suitable cutting speed is crucial to ensure cutting quality and efficiency.

The cutting speed has a significant effect on the laser cutting quality of Q235 carbon steel plate. As shown in Figure 7, at a laser power of 6kW, the optimal cutting speed for an 8mm thick steel plate is 1.65mm/s, while the speeds of 1.0mm/s and 2.25mm/s result in excessive slag and failure to penetrate, respectively. For a 15mm thick steel plate, at a laser power of 10kW, the speeds of 1.00mm/s and 1.65mm/s
ensure the cutting quality, but 2.25mm/s fails to penetrate. At 15kW power, the cutting effect is uniform at 2.50mm/s, but 2.83mm/s and 3.13mm/s fail to penetrate.
Slow cutting results in wider kerfs and more dross, while fast cutting may result in incomplete cutting. Optimizing cutting speed is crucial to balancing material melting, auxiliary gas efficiency, cutting stability and quality. Cutting speed affects the laser thermal action time, which in turn affects the size of the melting area, kerf width, heat-affected zone and material removal efficiency. Therefore, choosing an appropriate cutting speed is critical to achieving efficient and high-quality laser cutting.

3.2 Hardness test
In order to further explore the specific effects of laser power and cutting speed on steel properties, hardness tests were also conducted. The hardness test results reveal the change law of material hardness during laser cutting. As shown in Figure 8, as the laser power increases, the hardness of the material after cutting also increases. This may be because the melting depth and remelting layer depth caused by high-power lasers increase, resulting in an increase in the surface hardness of the material. This finding is consistent with the reference, which studied the effect of high-power fiber laser cutting on the surface morphology and defect characteristics of SiC particle reinforced aluminum matrix composites.

4 Conclusions
This study, through a series of carefully designed experiments, deeply explored the application of laser cutting technology in the rescue and demolition of Q235 steel plates. The following are the key conclusions of this study:
(a) Through the single-factor experiment of laser cutting of Q235 carbon steel plate, the influence of laser power and cutting speed on cutting quality was obtained. With the increase of laser power, the slit width continued to increase; with the increase of cutting speed, the slit width continued to decrease.
(b) Laser power and cutting speed have a synergistic effect. If the laser power is increased without increasing the cutting speed, the cutting effect and quality are often unsatisfactory, and there will be too much slag. However, when the laser power is sufficient, increasing the cutting speed will lead to the inability to penetrate the carbon steel plate. Therefore, choosing the right laser power and cutting speed will make the cutting effect and quality twice as good with half the effort.
(c) The hardness test results reveal the change law of material hardness during laser cutting. With the increase of laser power, the hardness of the material after cutting also increases. This may be because the high-power laser causes an increase in the depth of melting and remelting layers, which leads to an increase in the surface hardness of the material.
The theoretical research and experimental analysis in this paper provide scientific guidance for the application of laser cutting technology in rescue and demolition, and it is expected to contribute to the technological progress and practical application of related fields.