Quenched steel plate surface pretreatment method

Abstract

According to the quenching-state steel plate surface pretreatment method, through shot blasting treatment before quenching and non-oxidation heating in the re-austenitizing process, the surface roughness of the steel plate has good primer adhesive force. The steel plate is controlled to be quenched and cooled within a certain temperature range below Bf or Mf, a certain residual temperature of the steel plate is reserved, water-based inorganic zinc silicate primer is immediately sprayed after the steel plate is quenched, the surface of the primer can be rapidly dried under the residual temperature action of the steel plate, and the steel plate is offline and stacked for more than 24 hours after being quenched and sprayed with paint, so that the paint surface is completely cured. The problems that a steel plate in a traditional technological process needs to be subjected to secondary shot blasting and primer drying, and the pretreatment technological process is complex are solved; in addition, quenching cooling parameters are accurately set, so that quenching water consumption is reduced, and meanwhile, the effects of energy conservation and emission reduction are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material preparation, and is a method for pretreating the surface of a quenched steel plate, in particular to quenching process control and surface treatment of the steel plate. Background Art

[0002] With the continuous development of social economy and engineering technology, the demand for high-performance steel materials continues to grow. Wear-resistant steel and ultra-high strength steel are usually delivered in a quenched state, and the parent phase structure is quenched bainite or martensite. At present, the common production process of steel enterprises is rolling and shot blasting to remove the surface rolled iron oxide scale-re-austenitization heating-high-pressure water quenching and cooling to room temperature. The steel plate is re-austenitized and heated in a heat treatment furnace heated by a radiation tube. The furnace is filled with nitrogen to protect the steel plate from oxidation during the heating process. However, re-oxidation is very likely to occur during the subsequent transportation and storage process, especially when long-distance sea transportation and long-term storage in highly corrosive environments such as offshore, the degree of rust on the steel plate increases, which has an adverse effect on the appearance quality and use of the steel plate. Therefore, in order to improve the problem of steel plate surface corrosion, the steel industry needs to use a special pretreatment production line to pretreat the surface of steel plates for some high-end demands with higher surface quality requirements. The specific process flow is quenching - secondary shot blasting to remove the surface oxide layer - spray workshop primer - hot air or far-infrared heating and drying - warehousing, which has the problems of complex process flow and high energy consumption. Summary of the invention

[0003] In view of the above-mentioned deficiencies of the prior art, the present invention aims to provide a method for surface pretreatment of quenched and tempered steel plates, which is suitable for steel plates with a thickness of 6 to 210 mm and delivered in a quenched state, and solves the problems of complex process flow and high energy consumption of surface pretreatment of quenched steel plates.

[0004] To achieve the above object, the present invention adopts the following technical solutions: A method for pretreating the surface of a quenched steel plate, the key process steps include: 1) Before quenching, the steel plate is shot blasted with a shot blasting machine to remove the iron oxide scale on the surface of the steel plate. Steel shots with a diameter of 0.8~1.4mm are used to remove the iron oxide scale on the surface of the steel plate. After shot blasting, the cleaning grade of the iron oxide scale on the surface of the steel plate reaches Sa 2.5, and the surface roughness is 30~70μm; 2) The re-austenitization heating before quenching adopts a radiant tube heating furnace, and the furnace is filled with nitrogen with a nitrogen purity of ≥99.9% to keep the furnace positive pressure and the residual oxygen content in the furnace below 500 ppm, so that the steel plate will not be oxidized during the heating process; 3) Quenching control: For martensitic steel plates, the final quenching temperature is controlled to be 50°C lower than the Mf phase transformation temperature, and the steel plate is kept at a residual temperature of 50~150°C; for bainitic steel plates, the final quenching temperature is controlled to be 50°C lower than the Bf point, and the steel plate is kept at a residual temperature of 50~150°C, where Mf is the martensitic transformation completion temperature and Bf is the bainitic transformation completion temperature.

[0005] Surface pretreatment after quenching: The paint sprayer is installed at the exit of the quenching machine and is connected to the conveyor roller. The paint sprayer is equipped with nozzles at the upper and lower positions, which can spray paint on the upper and lower surfaces of the steel plate at the same time. After the steel plate is quenched, the upper and lower surfaces of the steel plate are immediately sprayed with water-based inorganic zinc silicate primer, and the primer is dried by using the residual heat of the steel plate to make the primer dry quickly. After quenching and painting, the steel plate is stacked off the line for more than 24 hours to achieve complete curing of the paint surface.

[0006] Beneficial effects of the present invention: 1) By controlling the roughness of the steel plate before quenching and the non-oxidation protective heating during the re-austenitization process before quenching, the surface of the steel plate after quenching can be free of oxide layer, maintaining a certain roughness, and obtaining good primer adhesion.

[0007] 2) Through the precise measurement of the phase change temperature of the steel plate and the precise setting of the quenching and cooling parameters of the steel plate, the steel plate retains a certain residual temperature after quenching and cooling. The upper and lower surfaces of the steel plate are sprayed with paint by the pretreatment device arranged after the quenching machine. The residual temperature of the steel plate can make the paint film solidify quickly. After quenching and painting, the steel plate is stacked off the line for more than 24 hours to achieve complete curing of the paint surface.

[0008] 3) The secondary shot blasting process and subsequent primer drying process of the steel plate in the traditional process are omitted, and the steel plate pretreatment process is significantly simplified; in addition, the quenching cooling water volume is saved by accurately setting the quenching cooling parameters. The effect of energy saving and emission reduction can be achieved at the same time. DETAILED DESCRIPTION

[0009] The present invention is further described below with reference to the embodiments. Example 1

[0010] A surface pretreatment method for a 100 mm NM400 steel plate, the chemical composition of the steel plate is shown in Table 1.

[0011] Process steps: After smelting, continuous casting and rolling, the steel plate is shot blasted, and the surface cleaning level meets the Sa2.5 level. The surface roughness of the steel plate after shot blasting is 50~70μm. The CCT curve of the steel grade is determined by thermal simulation experiment, and the Mf point is 302℃. The re-austenitization heating of the quenching furnace is carried out according to the conventional heating process. The residual oxygen content in the furnace is 400 ppm. The steel plate is cooled in the quenching machine after leaving the quenching furnace. The quenching cooling parameters used are shown in Table 2. The surface moisture of the steel plate is blown clean by compressed air after leaving the quenching machine. Immediately after the steel plate leaves the quenching machine, the paint sprayer is started to spray water-based inorganic zinc silicate primer on the upper and lower surfaces of the steel plate. The steel plate is naturally cooled to room temperature on the roller, and the primer can be quickly dried within 10 minutes under the residual temperature of the steel plate. After quenching and painting, the steel plate is stacked off the line for more than 24 hours for sampling and inspection, and the paint surface quality is checked. Example 2

[0012] A surface pretreatment method for a 25 mm NM450 steel plate, the chemical composition of the steel plate is shown in Table 1.

[0013] Process steps: After smelting, continuous casting and rolling, the steel plate is shot blasted, and the surface cleaning level meets Sa2.5 level. The surface roughness of the steel plate after shot blasting is 30~60μm. The CCT curve of the steel grade is determined by thermal simulation experiment, and the Mf point is 258℃. The re-austenitization heating of the quenching furnace is carried out according to the conventional heating process. The residual oxygen content in the furnace is 400ppm. The steel plate is cooled in the quenching machine after leaving the quenching furnace. The quenching cooling parameters are shown in Table 2. The surface moisture of the steel plate is blown clean by compressed air after leaving the quenching machine. Immediately after the steel plate leaves the quenching machine, the paint sprayer is started to spray paint on the upper and lower surfaces of the steel plate. The steel plate is naturally cooled to room temperature on the roller, and the primer can be quickly dried within 10 minutes under the residual temperature of the steel plate. After quenching and painting, the steel plate is stacked off the line for more than 24 hours for sampling and inspection, and the paint surface quality is checked.

[0014] The mechanical properties of the steel plates of Example 1 and Example 2 and Comparative Example 1 and Comparative Example 2 processed by conventional processes are compared in Table 3. In the two embodiments, there is no oxide layer on the surface of the steel plates, the paint film is intact, and mechanical properties similar to those of the comparative example steel plates processed by conventional processes can be obtained.

[0015] Table 1 Chemical composition of example steel (wt%) .

[0016] Table 2 Quenching process parameters of the examples and comparative examples .

[0017] Table 3 Mechanical properties test results after quenching of the examples and comparative examples .

Claims

1. A method for pretreating the surface of a quenched steel plate, characterized in that The key process steps include: (1) Before quenching, the steel plate is shot blasted with a shot blasting machine to remove the iron oxide scale on the surface of the steel plate. Steel shots with a diameter of 0.8~1.4mm are used to remove the iron oxide scale on the surface of the steel plate. After shot blasting, the iron oxide scale cleaning level of the steel plate surface reaches Sa 2.5, and the surface roughness is 30~70μm; (2) The re-austenitizing heating before quenching adopts a radiant tube heating furnace, and the furnace is filled with nitrogen with a nitrogen purity of ≥99.9% to maintain a positive pressure in the furnace and the residual oxygen content in the furnace is less than 500 ppm, so that the steel plate does not oxidize during the heating process; (3) Quenching control: For martensitic steel plates, the final quenching temperature is controlled to be 50°C lower than the Mf phase transformation temperature, and the steel plate is kept at a residual temperature of 50~150°C; for bainitic steel plates, the final quenching temperature is controlled to be 50°C lower than the Bf point, and the steel plate is kept at a residual temperature of 50~150°C, where Mf is the martensitic transformation completion temperature and Bf is the bainitic transformation completion temperature.