Quenched and tempered steel plate surface pretreatment method

Abstract

According to the surface pretreatment method for the quenched and tempered steel plate, through shot blasting treatment before quenching and non-oxidation heating in the re-austenitizing process, the surface roughness of the steel plate has good paint adhesive force. The steel plate is controlled to be quenched and cooled within a certain temperature range below Bf or Mf, the steel plate keeps a certain residual temperature, high-temperature-resistant primer is immediately sprayed after the steel plate is quenched, paint can be rapidly cured under the residual temperature effect of the steel plate, the steel plate can be directly subjected to high-temperature tempering, the paint surface integrity after tempering is good, oxidation in the tempering process is prevented, and the service life of the steel plate is prolonged. And the delivery surface quality is met. The problems that a steel plate in a traditional technological process needs to be subjected to secondary shot blasting and paint drying, and the pretreatment technological process is complex are solved; in addition, through accurate setting of quenching cooling parameters, the amount of quenching cooling water is saved. And meanwhile, the effects of energy conservation and emission reduction can be achieved.

Description

Technical Field

[0001] The invention relates to the technical field of steel material preparation, in particular to a method for pre-treating the surface of a quenched and tempered steel plate, and in particular to quenching process control and surface treatment during the quenching and tempering process 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. The quenched and tempered steel plate adopts quenching and high-temperature tempering heat treatment, has good comprehensive mechanical properties, and is widely used in engineering machinery, pressure vessels, marine engineering, bridge construction and other fields. The production process of quenched and tempered steel plate includes quenching and high-temperature tempering process. At present, the common quenching and tempering production process of steel enterprises is rolling-shot blasting to remove the surface rolled iron oxide scale-reaustenitization heating-high-pressure water quenching cooling to room temperature-high-temperature tempering. The reaustenitization heating of the steel plate adopts 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. The tempering adopts a heat treatment furnace heated by an open flame burner. During the high-temperature tempering process, an oxide layer will be generated on the surface of the steel plate, which is very easy to cause oxidation and rust in the subsequent transportation and storage process. Especially in the case of long-distance sea transportation and long-term storage in a strong corrosive environment such as offshore, the degree of rust of the steel plate will increase, which will have an adverse effect on the appearance quality and use of the steel plate. Therefore, in order to improve the surface corrosion problem of quenched and tempered steel plates, the steel industry uses a special pretreatment production line to pretreat the surface of quenched and tempered steel plates for some high-end needs with higher surface quality requirements. The specific process is quenching and tempering - secondary shot blasting to remove the surface oxide layer - spraying workshop primer - hot air or far infrared heating and drying - storage. However, this existing production process has the problems of complex process 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 surface pretreatment method for quenched and tempered steel plates, which is suitable for quenched and tempered steel plates with a thickness of ≤40mm and a tempering temperature not higher than 670°C, so as to solve the problems of complex process flow and high energy consumption of the existing quenched and tempered steel plate surface pretreatment process.

[0004] To achieve the above object, the present invention adopts the following technical solutions: A method for pretreating the surface of a quenched and tempered steel plate comprises the following key process steps: (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, using steel shots with a diameter of 0.8~1.4mm; after shot blasting, the iron oxide scale cleaning level on the surface of the steel plate reaches Sa 2.5, and the surface roughness of the steel plate is 30~70μm; (2) A radiant tube heating furnace is used for heating before quenching, and the furnace is filled with nitrogen with a nitrogen purity of ≥99.9% to maintain a positive pressure in the furnace and keep the residual oxygen content in the furnace below 500 ppm; (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] A further solution is to perform further pretreatment on the surface of the quenched and tempered steel plate after quenching: the paint sprayer is installed at the exit of the quenching machine and is subsequently connected to a conveying roller; the paint sprayer is equipped with nozzles at the upper and lower positions, and can simultaneously spray paint on the upper and lower surfaces of the steel plate; after the steel plate is quenched, a high temperature resistant primer is immediately sprayed on the upper and lower surfaces of the steel plate, and the paint film thickness is controlled to be 15-30μm.

[0006] Beneficial effects of the present invention: The technical solution of the present invention can make the surface of the steel plate after quenching free of oxide layer and maintain a certain roughness by controlling the roughness of the steel plate before quenching and the non-oxidation protective heating during the re-austenitization process before quenching, so as to obtain good paint adhesion. At the same time, by accurately measuring the phase change temperature of the steel plate and accurately setting the quenching 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 rapidly, and the steel plate can be directly subjected to high-temperature tempering treatment, eliminating the secondary shot blasting process and subsequent paint drying process after the steel plate tempering in the traditional process, and the steel plate pretreatment process is significantly simplified; in addition, by accurately setting the quenching cooling parameters, the amount of quenching cooling water is saved. The effect of energy saving and emission reduction can be achieved at the same time. DETAILED DESCRIPTION

[0007] The following is further described with reference to the embodiments. Example 1

[0008] A method for surface pretreatment of a 30mm S690QL steel plate, the chemical composition of which is shown in Table 1. Key process steps: After smelting, continuous casting and rolling, the steel plate is shot blasted, and the surface cleaning level meets Sa 2.5 level. The surface roughness of the steel plate after shot blasting is 40~65μm. The CCT curve of the steel grade is determined by thermal simulation experiment, and the Mf point is 310℃. The re-austenitization heating in the quenching furnace is carried out according to the conventional heating process, and 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 in Table 2 are used. 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 paint can be quickly cured within 10 minutes under the residual temperature of the steel plate. The steel plate is tempered at 640℃ after quenching. After tempering, samples were taken to test mechanical properties and surface quality. The integrity of the paint film was good after tempering, and the actual film thickness was 20~22μm.

[0009] The mechanical properties of the steel plates in Example 1 and Conventional Process Comparative Example 1 are compared in Table 1. The mechanical properties of the steel plates in this example and Conventional Process Comparative Example 1 are similar. Example 2

[0010] A 20mm A514 Grade B steel plate, the chemical composition is shown in Table 1. Key process steps: After smelting, continuous casting and rolling, the steel plate is shot blasted, the surface cleaning level meets Sa 2.5 level, and the surface roughness of the steel plate after shot blasting is 40~70μm. The CCT curve of the steel grade is determined by thermal simulation experiment, and the Mf point is 313℃. The re-austenitization heating in the quenching furnace is carried out according to the conventional heating process, the residual oxygen content in the furnace is 420 ppm, the steel plate is cooled in the quenching machine after leaving the quenching furnace, and the quenching cooling parameters in Table 2 are used. 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 paint can be quickly solidified within 10 minutes under the residual temperature of the steel plate. The steel plate is tempered at 640℃ after quenching. After tempering, samples are taken to test the mechanical properties and check the surface quality. The paint film has good integrity after tempering, and the measured paint film thickness is 19~24μm.

[0011] Table 3 is a comparison of the mechanical properties of the steel plates of Example 2 and Comparative Example 2 treated with conventional processes. In Example 2, there is no oxide layer on the surface of the steel plate, the paint film is intact, and mechanical properties similar to those of the steel plate of Comparative Example 2 treated with conventional processes can be obtained.

[0012] Table 1 Chemical composition of steel plates in the examples (wt%) .

[0013] Table 2 Comparison of quenching process parameters between the embodiment and the comparative example .

[0014] Table 3 Mechanical properties test results after quenching and tempering of the examples and comparative examples .

Claims

1. A method for pretreating the surface of a quenched and tempered 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, using steel shots with a diameter of 0.8~1.4mm; after shot blasting, the iron oxide scale cleaning level on the surface of the steel plate reaches Sa 2.5, and the surface roughness of the steel plate is 30~70μm; (2) A radiant tube heating furnace is used for heating before quenching, and the furnace is filled with nitrogen with a nitrogen purity of ≥99.9% to maintain a positive pressure in the furnace and keep the residual oxygen content in the furnace below 500 ppm; (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.

2. A method for pretreating the surface of a quenched and tempered steel plate according to claim 1, characterized in that: Further pretreatment process of the surface of the quenched and tempered steel plate after quenching: the paint sprayer is installed at the exit of the quenching machine and is connected to the transmission roller afterwards; 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 high temperature resistant primer, and the paint film thickness is controlled to be 15~30μm.