Pretreatment method for evaluating austenite grain size of high-carbon tire cord steel

Abstract

The invention discloses a pretreatment method for evaluating the austenite grain size of high-carbon tire cord steel, and belongs to the technical field of metallographic detection. The method comprises the following specific steps: pretreating a high-carbon tire cord steel sample; placing the pretreated sample with the corrosion surface facing upwards in a cupel; placing the cupel on a crucible rack, quenching at 850-870 DEG C, preserving heat for 1 hour, and cooling with water; the sample is subjected to fine grinding, polishing, corrosion treatment and blow-drying after water cooling, and the corrosion treatment specifically comprises the steps of preparing a corrosive agent, stirring the corrosive agent at the temperature of 65-75 DEG C, standing, cooling to 50-60 DEG C, and immersing the sample for corrosion for 20-40 s. The pretreatment method provided by the invention can overcome the limitation of a traditional corrosive agent and realize clear and accurate detection of the austenite grain size.

Description

Technical Field

[0001] The invention relates to the technical field of metallographic detection, and in particular to a pretreatment method for evaluating austenite grain size of high carbon cord steel. Background Art

[0002] Austenite is an important organizational structure in steel materials, and the grain size of austenite has an important influence on the performance of steel. The grain size is directly related to the mechanical properties, heat treatment properties, welding properties, corrosion resistance and other aspects of steel.

[0003] In the production and processing of steel, the intrinsic grain size (i.e. the grain size inherent in steel under specific heat treatment conditions) has a particularly significant impact on process performance. It determines the deformation capacity and thermal stability of steel during heating, forging, rolling, etc. However, the effect of intrinsic grain size on the final performance of steel is often indirect and needs to be indirectly optimized by adjusting the heat treatment process.

[0004] In contrast, the actual grain size (i.e. the grain size of the steel in the final heat treatment state) has a more direct impact on the performance of the steel. The size and shape of the actual grain size directly determine the key indicators of the steel such as hardness, toughness, strength and fatigue resistance. Therefore, in the quality control of steel, it is very important to accurately measure and evaluate the actual grain size.

[0005] However, the detection of austenite grain size in high-carbon cord steel has always been a technical problem. Due to the particularity of the composition and heat treatment of high-carbon steel, traditional etchants often cannot fully and clearly present the austenite grain size. This not only affects the accurate evaluation of steel properties, but also brings challenges to steel production and quality control.

[0006] Therefore, developing a method that can effectively and efficiently display the austenite grain size in high carbon steel is of great significance for improving the quality of steel, optimizing the production process, and meeting the specific needs of customers. Summary of the invention

[0007] Technical problem to be solved: In view of the problem that the prior art cannot effectively display the austenite grain size in high-carbon cord steel, the present invention proposes a pretreatment method for evaluating the austenite grain size of high-carbon cord steel, which can overcome the limitations of traditional corrosive agents and achieve clear and accurate detection of austenite grain size.

[0008] Technical solution: A pretreatment method for evaluating the austenite grain size of high carbon cord steel, the specific steps are as follows:

[0009] Step 1, pre-treating the high carbon cord steel sample;

[0010] Step 2: Place the pre-treated sample with the corrosion surface facing upward in an ash dish;

[0011] Step 3: Place the ash dish on the crucible stand, quench at 850-870°C, keep warm for 1 hour, and cool with water;

[0012] Step 4: After water cooling, the sample is finely ground, polished, and corroded, and then blown dry. The specific corrosion treatment is as follows: prepare the corrosive agent, stir the corrosive agent at 65-75°C and let it stand, then cool it to 50-60°C and immerse the sample in the corrosion for 20-40s.

[0013] The technical solution of the present invention is mainly aimed at the actual grain size of austenite of high carbon cord steel, that is, the original austenite grain. The innovative pretreatment method of heat treatment combined with corrosion treatment can clearly show the austenite grain boundary, which is convenient for accurately evaluating the actual grain size of austenite of high carbon cord steel.

[0014] Preferably, the corrosive agent is a mixture of picric acid, distilled water and White Cat detergent.

[0015] Preferably, the ratio of picric acid, distilled water and White Cat detergent is (1-2) g: (90-110) mL: (10-20) mL.

[0016] Preferably, in the step 1, the wire diameter of the high carbon cord steel sample is 5 to 8 mm.

[0017] Preferably, the pretreatment in step 1 is polishing.

[0018] Preferably, the corroded surface of the sample in step 2 is a polished surface.

[0019] Preferably, the fine grinding in step 4 uses sandpaper with a mesh size of 1000# or above, such as 1000#, 1200# or 1500#.

[0020] Beneficial effects:

[0021] (1) The present invention adopts a heat treatment method in which a high carbon cord steel sample is quenched at 850-870°C, kept at this temperature for 1 hour, and then water-cooled, thereby avoiding the problems of sample surface decarburization, excessive microstructure growth, and incomplete quenching caused by the existing heat treatment method.

[0022] (2) The present invention can fully present the austenite grain size by fine grinding, polishing and etching the heat-treated sample in combination with a specific proportion of corrosive agent.

[0023] (3) The present invention achieves the purpose of stable corrosion of austenite grain boundaries by scientifically mixing picric acid, distilled water and white cat detergent and reasonably controlling the ratio within (1-2) g: (90-110) mL: (10-20) mL. White cat detergent, as a surfactant, breaks the free energy of the grain boundary surface and induces selective corrosion and coloring of picric acid; it also has a certain slow release effect, and the precise ratio can improve the uniformity and operability of corrosion.

[0024] (4) The present invention clarifies the optimal picric acid ratio. Since the solubility of picric acid in water under high temperature conditions is about 1.4 g / 100 mL, the present invention has determined through a large number of experiments that 1 g to 2 g of picric acid can effectively use the picric acid and ensure the corrosion effect. In addition, due to slight supersaturation, the picric acid will spontaneously dissolve after multiple heating reactions. It can be replenished in time and can be used repeatedly to maintain a good corrosion effect.

[0025] (5) The present invention determines that after quenching and tempering treatment, sandpaper with a mesh size of 1000# or above, such as 1000#, 1200#, 1500#, etc., is required to ensure that after the shallow surface layer is finely ground to remove the completely decarburized layer, the martensite layer area with a lower carbon concentration content is retained to the greatest extent. The needle-shaped cementite in this area is relatively regular, and the original austenite grain boundary is easier to display, thereby improving the reproducibility of the present invention.

[0026] In summary, the present invention can clearly show the austenite grain boundary through the pretreatment method of combining heat treatment with corrosion treatment, which is convenient for accurately evaluating the actual grain size of austenite in high carbon cord steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the austenite grain size corrosion effect diagram (100X) obtained in Example 1;

[0028] Figure 2 This is the austenite grain size corrosion effect diagram (200X) obtained in Example 1;

[0029] Figure 3 This is the austenite grain size corrosion effect diagram (500X) obtained in Example 1;

[0030] Figure 4 This is a graph showing the corrosion effect of austenite grain size obtained in Comparative Example 1;

[0031] Figure 5 This is the austenite grain size corrosion effect diagram obtained in Comparative Example 2;

[0032] Figure 6 This is the austenite grain size corrosion effect diagram obtained in Comparative Example 3. DETAILED DESCRIPTION

[0033] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0034] The main components of the high carbon cord steel LX82A in the examples of this specification are as follows, and the sample wire diameter is 5.5 mm.

[0035]

[0036] Example 1

[0037] The pretreatment method for evaluating the austenite grain size of high carbon cord steel described in this embodiment is as follows:

[0038] Step 1: Grind the sample coarsely, finely and polish it to ensure that the sample analysis surface is mirror-like and has no obvious scratches, meeting the requirements of the metallographic conventional sample preparation standard GB / T13298, and the sample is in a polished state;

[0039] Step 2: Place the sample with the polished surface facing upward in an ash dish;

[0040] Step 3: Place the ash dish on the crucible stand, quench at 860°C, keep warm for 1 hour, and cool with water;

[0041] Step 4: After fine grinding and polishing, use sandpaper with a mesh size of 1000# or above, such as 1000#, 1200#, 1500#, etc. (1500# sandpaper is selected in this embodiment) to lightly grind the surface until the black oxide layer is smoothly removed and polishing can be done to avoid excessive grinding; then corrode and blow dry. The corrosion method is: mix picric acid + distilled water + White Cat detergent in a ratio of 1g: 100mL: 20mL, heat to 70°C, stir and let stand, and when cooled to 55°C, immerse the sample in corrosion for 30s.

[0042] The obtained austenite grain size corrosion effect diagram can be found in Figure 1-3 ,As can be seen from the figure, the austenite grain boundaries of the high carbon cord steel are clearly displayed and the ,austenite grain size can be accurately evaluated.

[0043] Example 2

[0044] The same as Example 1, except that, in the corrosion method of step 4, picric acid + distilled water + White Cat dishwashing liquid are mixed in a ratio of 2g:110mL:20mL, heated to 75°C, stirred and allowed to stand, and when cooled to 60°C, the sample is immersed in corrosion for 40s.

[0045] Example 3

[0046] The same as Example 1, except that, in the corrosion method of step 4, picric acid + distilled water + White Cat detergent are mixed in a ratio of 1g:90mL:10mL, heated to 65°C, stirred and allowed to stand, and when cooled to 50°C, the sample is immersed in corrosion for 20s.

[0047] Comparative Example 1

[0048] In this comparative example, the sample is directly quenched at 860°C, kept warm for 1 hour, water-cooled, tempered at 550°C and kept warm for 1 hour, and air-cooled; directly ground after air-cooling: using conventional metallographic sample preparation mode, rough grinding (such as 100# sandpaper to 180# sandpaper, 180# sandpaper is used in this comparative example), fine grinding (800#, 1200#, 1500# sandpaper), and then polishing, etching, and drying. The etching method is: after mixing 1.5g picric acid + 100mL distilled water + 10mL detergent, the sample is directly immersed in the corrosion for 30s.

[0049] from Figure 4 It can be seen that the method provided in this comparative example has no obvious effect on displaying the austenite grain size. This is because the original analysis surface was ground off due to rough grinding, and the relatively better observation layer on the surface was ground off. What is presented under the optical microscope is the tempered martensite layer with supersaturated carbon content, and the grain boundary is very unclear.

[0050] Comparative Example 2

[0051] In this comparative example, the sample is directly quenched at 860℃ and kept warm for 1 hour, and then water-cooled. Then it is directly ground, rough ground (180# sandpaper) + fine ground (800#, 1200#, 1500# sandpaper) + polished, corroded, and blown dry. Corrosion method: 1.5g picric acid + 100mL distilled water + 10mL White Cat detergent are mixed, stirred at 70℃, and then the sample is corroded at a constant temperature of 70℃ for 90s.

[0052] from Figure 5 It can be seen that the method provided in this comparative example can only display martensite and retained austenite, and the grain boundary is not obvious. This is because, firstly, the observed analysis surface is all tempered martensite, and secondly, the corrosion temperature is too high, the corrosion effect and coloring effect of picric acid have been significantly enhanced, and the fine needle-shaped martensite has been displayed indistinguishably, which interferes with the identification of the grain boundary.

[0053] Comparative Example 3

[0054] In this comparative example, the sample is directly quenched at 860℃ and kept warm for 1 hour, and then water-cooled. Then it is directly ground, rough ground (180# sandpaper) + fine ground (800#, 1200#, 1500# sandpaper) + polished, corroded, and blown dry. Corrosion method: Mix the sodium picrate solution corrosive agent (13.3g wet picric acid + 166.7g sodium hydroxide + 500ml distilled water), put the sample into the solution and corrode for 30 minutes to show the effect.

[0055] from Figure 6It can be seen that the method provided in this comparative example has no obvious effect on displaying the austenite grain size. This is because the original austenite grain boundary is different from the network cementite grain boundary. Sodium picrate has priority in corroding the network cementite grain boundary. When the matrix is ​​not corroded, the cementite at the grain boundary can be corroded first; while the original austenite grain boundary has no obvious carbon potential gradient and the austenite crystal has an oversaturated carbon content. Therefore, sodium picrate has no priority for it at this time, and there is still no obvious corrosion and coloring for a long time.

[0056] In summary, the method of Example 1 has the best effect, and the austenite grain size is cleaned and reflected. Moreover, when processing multiple samples, the method has good stability and simple operation.

[0057] Finally, it should be noted that the above implementation modes are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned implementation modes, a person of ordinary skill in the art should understand that the technical solutions described in the aforementioned implementation modes can still be modified, or some or all of the technical features therein can be replaced by equivalents. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the implementation modes of the present invention.

Claims

1. A pretreatment method for evaluating the austenite grain size of high carbon cord steel, characterized in that: The specific steps are as follows: Step 1, pre-treating the high carbon cord steel sample; Step 2: Place the pre-treated sample with the corrosion surface facing upward in an ash dish; Step 3: Place the ash dish on the crucible stand, quench at 850-870°C, keep warm for 1 hour, and cool with water; Step 4: After water cooling, the sample is finely ground, polished, and corroded, and then blown dry. The specific corrosion treatment is as follows: prepare the corrosive agent, stir the corrosive agent at 65-75°C and let it stand, then cool it to 50-60°C and immerse the sample in the corrosion for 20-40s.

2. A pretreatment method for evaluating austenite grain size of high carbon cord steel according to claim 1, characterized in that: The corrosive agent is a mixture of picric acid, distilled water and white cat detergent.

3. A pretreatment method for evaluating austenite grain size of high carbon cord steel according to claim 2, characterized in that: The ratio of the picric acid, distilled water and white cat detergent is (1-2) g: (90-110) mL: (10-20) mL.

4. A pretreatment method for evaluating austenite grain size of high carbon cord steel according to claim 1, characterized in that: In the step 1, the wire diameter of the high carbon cord steel sample is 5 to 8 mm.

5. A pretreatment method for evaluating austenite grain size of high carbon cord steel according to claim 1, characterized in that: The pretreatment in step 1 is polishing.

6. A pretreatment method for evaluating austenite grain size of high carbon cord steel according to claim 5, characterized in that: In the step 2, the corroded surface of the sample is a polished surface.

7. A pretreatment method for evaluating austenite grain size of high carbon cord steel according to claim 1, characterized in that: In the step 4, the fine grinding is performed using sandpaper with a mesh number of 1000# or above.

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