Evaluation method of troostite-type banded structure

Abstract

The present invention relates to the technical field of metal material detection, and in particular to a method for evaluating a troostite type banded structure, which comprises: S1: heat treating a steel with a troostite content of ≥70%; S2: metallographic sample preparation, comprising: cutting the heat-treated steel to prepare a longitudinal metallographic sample, polishing, etching, and etching until the observation surface is gray-black; S3: banded structure rating, wherein the banded structure rating comprises weighted quantitative rating according to the results of microstructure rating and hardness rating. The method of the present invention can clearly and completely display the original banded structure of a troostite type wire rod product, so as to accurately evaluate its banded structure.

Description

Technical Field

[0001] The invention relates to the technical field of metal material detection, in particular to an evaluation method for a troostite-type banded structure. Background Art

[0002] The banded structure of steel is an internal defect. It is formed because during the solidification process of molten steel, due to the influence of component selection and crystallization, alloy elements such as carbon or residual harmful elements such as phosphorus and sulfur are segregated. In the subsequent rolling process of the steel billet, different structures are formed in which different structures are distributed alternately in layers along the direction of rolling deformation. For example, in medium and low carbon hypoeutectoid steels, it appears as a banded distribution of ferrite and pearlite, and in high carbon alloy steels, it may appear as a banded distribution of alloy carbides.

[0003] Banded structure makes steel anisotropic, significantly affecting the mechanical properties of steel, deteriorating the hot and cold processing properties of the material and the fatigue life of parts. Therefore, products with high quality requirements need to inspect and evaluate the banded structure of steel. Steel with serious banded structure will be restricted from leaving the factory or downgraded for use. Banded structure is essentially a metallurgical defect. Accurate identification and evaluation of banded structure can guide steel smelting production and the correct use of materials. The first prerequisite for accurately evaluating the banded structure in steel is to accurately, truly, clearly and completely display the banded structure in steel, and then quantitatively or qualitatively evaluate the banded structure in steel based on standard diagrams or intercept method and other methods.

[0004] However, in the existing related technologies, when evaluating steel grades with a troostite content ≥ 70%, its longitudinal banded structure cannot be clearly and completely displayed, and thus it is difficult to accurately evaluate its banded structure. Summary of the invention

[0005] The object of the present invention is to provide a method for evaluating a troostite-type banded structure, which can clearly and completely display the original banded structure of a troostite-type wire rod product, so as to accurately evaluate its banded structure.

[0006] The present invention is achieved in that:

[0007] The present invention provides a method for evaluating a troostite-type banded structure, comprising:

[0008] S1: Heat treatment of steel with sorbite content ≥ 70%;

[0009] S2: Metallographic sample preparation, including: cutting the heat-treated steel, preparing longitudinal metallographic specimens, polishing, etching, and etching until the observation surface is gray-black;

[0010] S3: Banded structure rating, wherein the banded structure rating includes weighted quantitative rating based on the results of microstructure rating and hardness rating.

[0011] In an optional embodiment, in step S1, the steel with a sorbite content of ≥70% includes spring steel and high carbon steel, wherein:

[0012] The composition of spring steel includes: C: 0.50%-0.65%, Si: 0.17%-2.0%, Mn: 0.25%-1.20%, Cr≤1.0%, V≤0.20%;

[0013] The composition of high carbon steel includes: C: 0.60%-0.90%, Si: 0.15%-0.40%, Mn: 0.50%-0.90%, Cr≤0.25%, V≤0.20%.

[0014] In an optional embodiment, in step S1, the spring steel and the high carbon steel are heat treated respectively, wherein:

[0015] The heat treatment process of spring steel includes: quenching temperature 900-920℃, holding time 30-35min; cooling medium: water; tempering temperature 420-440℃, holding time 85-95min, cooling medium: water;

[0016] The heat treatment process of high carbon steel includes: quenching temperature 880-900℃, holding time 30-35min; cooling medium: water; tempering temperature 400-420℃, holding time 85-95min, cooling medium: water.

[0017] In an optional embodiment, in step S2, the etching solution for etching the metallographic sample includes a mixed solution of nitric acid alcohol with a mass concentration of 8-12% and FeCl3 solution with a mass concentration of 2-4%.

[0018] In an optional embodiment, in step S1, the heat-treated sample is a full-section wire of 15-20 mm in length;

[0019] In step S2, metallographic sample preparation includes: cutting the full-section wire after heat treatment longitudinally along its central axis to prepare a longitudinal metallographic sample, polishing, etching, and etching until the observation surface is gray-black, rinsing with clean water, and drying with alcohol;

[0020] In step S3, the weighted quantitative rating L=L1×70%+L2×30%, wherein L1 is the rating result of the microstructure method, and L2 is the rating result obtained according to the hardness method.

[0021] In an optional embodiment, in step S3, the microstructure rating includes: rating the area at 1 / 4 diameter and the area at 1 / 2 diameter of the longitudinal section of the sample after metallographic sampling, and calculating the microstructure rating L1 according to the rating results, wherein L1 = the banding level L of the area at 1 / 2 diameter 1 / 2×80% + 1 / 4 diameter area band level L 1 / 4 ×20%.

[0022] In an optional embodiment, in step S3, the microstructure rating is performed at a magnification of 100 times by observing and counting the number of existing bands and the penetration of the bands, and rating is performed accordingly. The size of each microscopic observation field is 0.80 mm×1.20 mm.

[0023] In an optional embodiment, in step S3, the microstructure rating criteria include:

[0024] Banding level 0, standard: uniform tissue, no banding distribution;

[0025] Banding level 0.5, standard: 1-2 slight bands exist, and there are no bands that cross the field of view;

[0026] Banding level 1.0, standard: 1-2 slight bands, and 1 continuous band across the field of view; or, 3 slight bands, and no band across the field of view;

[0027] Banding level 1.5, standard: 2-3 obvious bands, and 1-2 continuous bands running through the field of view; or, 4 slight bands, and no bands running through the field of view;

[0028] Banding level 2, standard: 3-4 obvious bands, and 2-3 continuous bands across the field of view; or, 5 slight bands, and no bands across the field of view;

[0029] Banding level 2.5, standard: 4-5 obvious bands, and 3-4 bands that continuously cross the field of view; or, 6 slight bands, and no bands that cross the field of view;

[0030] Banding level 3.0, standard: 5-6 obvious bands, and more than one band that continuously runs through the field of view;

[0031] Banding level 3.5, standard: 5-6 obvious bands, and more than 2 bands that continuously run through the field of view;

[0032] Banding level 4.0, standard: There are 5-6 obvious bands, and there are more than 3 strips that continuously run through the field of view.

[0033] In an optional embodiment, in step S3, the hardness rating includes measuring the microhardness values ​​of the band-shaped and inter-band structures, and obtaining the hardness rating level L2 based on the difference between the two.

[0034] In an optional embodiment, in step S3, the grade standard of hardness rating includes:

[0035] Banding level 0, standard: microhardness difference ≤ 5;

[0036] Banding level 0.5, standard: 5<microhardness difference ≤10;

[0037] Banding level 1.0, standard: 10<microhardness difference ≤15;

[0038] Banding level 1.5, standard: 15<microhardness difference ≤20;

[0039] Banding grade 2.0, standard: 20<microhardness difference ≤25;

[0040] Banding grade 3.0, standard: 25<microhardness difference ≤30;

[0041] Band grade 4.0, standard: 30<microhardness difference ≤40.

[0042] The method for evaluating the troostite-type banded structure of the present invention has the following beneficial effects:

[0043] The evaluation method of the troostite type banded structure provided by the embodiment of the present invention comprises: firstly heat treating a steel with a troostite content ≥ 70%, then metallographically preparing a sample, and then grading the banded structure; wherein, the steel with a troostite content ≥ 70% is heat treated so that the original troostite + ferrite structure is transformed into a tempered troostite + tempered troostite structure after heat treatment due to the influence of component segregation, and there is a significant potential difference between the tempered troostite and the tempered troostite structure, resulting in a difference in corrosion, forming a color band difference of light and dark with different grayscales, and truly, clearly and amplifiedly displaying the original banded structure of the troostite type wire rod product, and there is an obvious hardness difference between the tempered troostite and the tempered troostite structure, and the two differences of hardness difference and corrosion are used to achieve accurate and reliable quantitative evaluation of the original banded structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0045] Figure 1 It is a diagram of the ferrite and pearlite layer distribution provided in the related art;

[0046] Figure 2 It is a diagram of a troostite-type longitudinal banded structure at 100x magnification in the related art;

[0047] Figure 3 This is a diagram showing the 1 / 4 area band rating of level 1.5 in Example 1 of the present invention;

[0048] Figure 4 This is a diagram showing the 1 / 4 area band rating of level 1.0 in Example 2 of the present invention;

[0049] Figure 5 This is a diagram of the core area band rating 3.0 level in Example 2 of the present invention;

[0050] Figure 6 This is a diagram showing the 1 / 4 area band rating of 0.5 level in Example 3 of the present invention;

[0051] Figure 7 This is a diagram showing the 1 / 4 area band rating level 0 of Example 4 of the present invention;

[0052] Figure 8 This is a rating diagram of the banded tissue in the core area of ​​Comparative Example 1 of the present invention;

[0053] Fig. 9 This is a graphic representation of the rating of the 1 / 4 region banded structure of Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0054] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0055] The standards for evaluating steel banding provided by existing related technologies mainly include GB / T13299-91, GB / T34474, GB / T18254, German standard SEP1520, American standard ASTME1268, etc. Among the above standards, GB / T13299-91 and GB / T34474 can be applied to the inspection of hypoeutectoid steel with a carbon content of less than 0.60%. The banded structure of such steel is clearly layered, showing the distribution of ferrite and pearlite in layers (such as Figure 1 As shown); GB / T18254 and German standard SEP1520 can be used to test the carbide band distribution in bearing steel; while American standard ASTME1268 is a standard that uses interception counting statistical method to test and evaluate the quantitative evaluation of the orientation of steel with obvious boundaries between two phases in steel.

[0056] For spring steels such as 55SiCr, 65Mn, and high carbon steel wire rods such as 60, 70, 80, SWRCH72A, and SWRCH82B, the structure is troostite + a small amount of small ferrite (such as Figure 2As shown in the figure, due to the characteristic of troostite structure, even if the wire has serious banding, the methods provided by the above standards (GB / T13299-91, GB / T34474, GB / T18254, ASTME1268) cannot clearly and completely display its longitudinal banded structure, let alone accurately evaluate its banded structure, which is not conducive to the evaluation of banded structure and internal quality grade sorting of high carbon steel wire rods such as 55SiCr, 65Mn and 60, 70, 80, SWRCH72A, SWRCH82B. For example, 55SiCr is suitable for manufacturing suspension springs of passenger cars with high strength grades. Suspension springs are safety parts with strength grades ranging from 1800MPa to 2000MPa. The higher the strength grade, the higher the metallurgical quality requirements for steel and the higher the uniformity of the structure. Severely banded 55SiCr steel wire will seriously affect the fatigue life of the suspension spring and even break prematurely, affecting safety. Therefore, 55SiCr steel wire with severe banding cannot be used to produce suspension spring products above 1900MPa. However, according to the above standards (GB / T13299-91, G The methods provided by B / T34474, GB / T18254, ASTME1268) cannot accurately evaluate the banded structure of this type of troostite-based steel, which to a certain extent affects the performance evaluation of this type of steel, thus bringing quality and safety risks; if it is a 55SiCr wire product, even if its composition and performance meet the quality requirements of 2000MPa-grade high-strength suspension spring products, if it has serious banded structure and is not graded, it will bring quality and safety risks if it is used to produce 2000MPa-grade high-strength suspension spring products by mistake, and the fatigue life will not meet the design requirements.

[0057] The invention provides a method for evaluating a troostite type banded structure, which can clearly and completely display the original banded structure of a troostite type wire rod product, so as to accurately evaluate the banded structure thereof.

[0058] The evaluation method of the troostite type banded structure of the present invention comprises:

[0059] S1: Heat treatment of steel with sorbite content ≥ 70%;

[0060] S2: Metallographic sample preparation, including: cutting the heat-treated sample, preparing a longitudinal metallographic specimen, polishing, etching, and etching until the observation surface is gray-black;

[0061] S3: Banded structure rating, wherein the banded structure rating includes weighted quantitative rating based on the results of microstructure rating and hardness rating.

[0062] Due to the influence of component segregation, the steel with troostite content ≥70% is transformed into tempered troostite + tempered troostite structure after heat treatment. There is a significant potential difference between the tempered troostite and the tempered troostite structure, which leads to a difference in corrosion, forming a color band difference of light and dark with different grayscales. It can truly, clearly and magnify the original banded structure of the troostite wire rod product, and there is an obvious hardness difference between the tempered troostite and the tempered troostite structure. The two differences of hardness difference and corrosion resistance are used to achieve accurate quantitative evaluation of the original banded structure.

[0063] In step S1, the steel types with sorbite content ≥ 70% include spring steel and high carbon steel, wherein:

[0064] The composition of spring steel (low alloy spring steel wire, troostite content ≥ 75%) includes: C: 0.50%-0.65% (for example: 0.50%, 0.55%, 0.65%, etc.), Si: 0.17%-2.0% (for example: 0.17%, 0.3%, 0.8%, 1.0%, 1.5%, 2.0%, etc.), Mn: 0.25%-1.20% (for example: 0.25%, 0.5%, 0.7%, 1.0%, 1.20%, etc.), Cr≤1.0% (for example: 1.0%, 0.9%, 0.8%, etc.), V≤0.20% (for example: 0.20%, 0.18%, 0.16%, etc.); the composition of high carbon steel (hot rolled wire of high carbon steel) includes: C: 0.60%-0.90% (for example: 0.60%, 0.75%, 0.90%, etc.), Si: 0.15%-0.40% (for example: 0.15%, 0.25%, 0.40%, etc.), Mn: 0.50%-0.90% (for example: 0.50%, 0.70%, 0.90%, etc.), Cr≤0.25% (for example: 0.25%, 0.20%, etc.), V≤0.20% (for example: 0.20%, 0.15%, etc.). The wire with the above composition range uses high wire controlled rolling and controlled cooling process to obtain a wire product with a microstructure dominated by troostite, and can configure a suitable sample heat treatment process according to the difference between spring steel and high carbon steel.

[0065] In the hot-rolled state, due to the longitudinal banded structure of the troostite, the original layered banded structure cannot be displayed, or the display is not obvious or complete, and cannot be quantitatively and accurately graded using the methods provided by the relevant technology. The inventors have found through a large number of studies that the original troostite + ferrite structure, due to the influence of component segregation, is transformed into tempered troostite + tempered troostite structure through a suitable heat treatment method. There is a significant potential difference between the tempered troostite and the tempered troostite structure, which leads to a difference in corrosion. The original banded structure of the troostite wire rod product can be truly, clearly and magnified, and there is an obvious hardness difference between the tempered troostite and the tempered troostite structure. The two differences of hardness difference and corrosion are used to achieve accurate quantitative evaluation of the original banded structure.

[0066] Moreover, the inventors have also found that by heat treating spring steel and high carbon steel with optimized processes respectively, the organizational differences can be maximized.

[0067] The heat treatment process of spring steel includes: quenching temperature 900-920℃, for example: 900℃, 910℃, 920℃, etc., holding time 30-35min, for example: 30min, 32min, 35min; cooling medium: water; tempering temperature 420-440℃, for example: 420℃, 430℃, 440℃, etc., holding time 85-95min, for example: 85min, 90min, 95min, cooling medium: water.

[0068] The quenching temperature is set at 900-920℃, and the holding time is about 35min, which can make the steel completely austenitized, and carbon and alloy will not diffuse significantly, the austenite grain size will not grow significantly, and the original size and morphology of the original austenite will be retained. If the heating temperature is lower than 900℃, abnormal structures such as ferrite and alloy carbides are likely to remain after subsequent quenching, affecting the true display of the original banded structure; if the heating temperature is higher than 920℃, the austenite grain size is easy to grow, the structure is coarse, and the original banded structure is affected.

[0069] Selecting water as the cooling medium can ensure that the sample is fully quenched, and residual ferrite is not easily formed after subsequent quenching. The original banded structure can be fully and truly displayed, thereby improving the accuracy and reproducibility of the rating.

[0070] The purpose of the tempering temperature of 420-440℃ and the holding time of about 90min is that after tempering at this temperature range, due to the segregation of components, tempered troostite and tempered troostite are formed in different component areas after tempering, so that the original banded structure is truly displayed, and the corrosion difference is maximized at this time, and the banded display is most intuitive. If the tempering temperature is higher than 440℃, it may all be transformed into tempered troostite; if it is lower than 420℃, the tempered troostite is not fully transformed, both of which will affect the corrosion difference and bring about rating errors.

[0071] The heat treatment process of high carbon steel includes: quenching temperature 880-900℃, for example: 880℃, 890℃, 900℃, etc., holding time 30-35min, for example: 30min, 32min, 35min; cooling medium: water; tempering temperature 400-420℃, for example: 400℃, 410℃, 420℃, etc., holding time 85-95min, for example: 85min, 90min, 95min, cooling medium: water.

[0072] The heat treatment process principle of high carbon steel is similar to that of spring steel, both of which are for better austenitization, sufficient hardening, and better formation of tempered troostite and tempered troostite, which will not be repeated here.

[0073] Furthermore, the heat-treated sample has a full-section wire length of 15-20 mm (eg, 15 mm, 18 mm, 20 mm, etc.).

[0074] The metallographic sample preparation of step S2 includes: cutting the full-section wire after heat treatment longitudinally along its central axis to prepare a longitudinal metallographic sample, polishing, etching, and etching until the observation surface is gray-black, rinsing with clean water, and drying with alcohol; the etching solution for etching the metallographic sample includes a mixed solution of nitric acid alcohol with a mass concentration of 8-12% (for example: 8%, 10%, 12%, etc.) and FeCl3 solution with a mass concentration of 2-4% (for example: 2%, 3%, 4%, etc.).

[0075] After the above treatment, the original banded structure of the sample can present the maximum contrast for a true and intuitive display; the prepared sample is placed under a microscope for observation, and there is a grayscale difference between tempered troostite and tempered troostite, which is displayed in a banded shape, achieving accurate rating.

[0076] In step S3, the weighted quantitative rating L=L1×70%+L2×30%, where L1 is the rating by the microstructure method and L2 is the hardness rating obtained by the hardness method. In this way, the evaluation can be performed more accurately.

[0077] Since the microstructure rating method and hardness rating method (hardness rating method) each have their own advantages and disadvantages, the advantage of the microstructure rating method is that the analysis speed is faster, the rating area is wider and more comprehensive; the disadvantage of the microstructure rating method is that the bands are distinguished and evaluated by color difference. Sometimes the bands are more obvious under a microscope, but the hardness difference is not large. This banded structure has little effect on subsequent use. On the contrary, the level of the banded microscopic rating is not high, but the hardness difference is obvious, which also affects the subsequent quality. The disadvantages of hardness testing are that it is slow, and production testing cannot measure hardness differences on a large scale. Moreover, if the banded structure of the sample is evaluated only based on the hardness difference between the most serious strips, it is not comprehensive and objective. Therefore, the present invention introduces the comprehensive evaluation of the rating results of the microstructure rating method and the hardness test method to fully combine the advantages of the two rating methods, perform weighted quantitative rating, and comprehensively calculate the sample banding level L. Moreover, based on the different contributions of the two methods to the banding evaluation, a weight coefficient is introduced into the formula to facilitate a more comprehensive and objective evaluation of the sample banding structure, improve the rating accuracy and reproducibility, and reduce the rating error.

[0078] Furthermore, in order to accurately and comprehensively evaluate the banded structure, the microstructure rating includes: rating the area at 1 / 4 diameter and the area at 1 / 2 diameter (core area) of the longitudinal section of the sample respectively, and calculating the microstructure rating L1 based on the rating results.

[0079] Since the wire needs to be processed by cold drawing, the banded structure in the area at 1 / 2 diameter (core area) has a greater impact on the cold drawing performance and the performance quality of the finished product than the banded structure in the area at 1 / 4 diameter. Therefore, the banded structure in the area at 1 / 2 diameter (core area) has a greater impact on the use of subsequent materials than the area at 1 / 4 diameter. In order to make the rating more objective and accurate, that is, to more objectively reflect the impact of the banded structure on the subsequent drawing process and the performance quality of the finished product, the banded structure in the area at 1 / 2 diameter (core area) has a greater performance risk, so the weight is increased. The banded structure in the 1 / 4 area has a relatively slight impact, so the weight is reduced. The banded level L1 is calculated according to the following formula: L1 = banded level L in the area at 1 / 2 diameter 1 / 2 ×80% + 1 / 4 diameter area band level L 1 / 4 ×20%.

[0080] The principles and methods of banded microstructure rating can be carried out according to the following requirements:

[0081] Observe and evaluate at 100 times magnification, and observe and count the number of bands and the degree of band penetration in a microscopic observation field. Consider the width of the band, the grayscale distinction between bands, and other factors to semi-quantitatively rate the level of banded tissue; the more bands there are and the greater the degree of penetration in a field of view, the higher the level, and vice versa. The level assessment principle of this method is to conduct a comprehensive assessment based on the impact of banded tissue on subsequent use and quality. The more bands there are and the greater the degree of band penetration, the greater the impact on subsequent use and quality, so the higher the rating level.

[0082] The size of each microscopic observation field is 0.80 mm × 1.20 mm, and the rating standards are shown in Table 1.

[0083] Table 1 Standards for microstructure rating

[0084]

[0085]

[0086] It should be noted that slight banding in Table 1 means: the grayscale distinction of the banding is not obvious, the bandwidth is small, and it is discontinuously distributed, and the degree of penetration of the field of view is ≤60%. Obvious banding means: the grayscale distinction of the banding is obvious, the bandwidth is obvious, and it is continuously distributed, and the degree of penetration of the field of view is not less than 60%.

[0087] It should also be noted that due to certain differences in the tissues of different samples, the rating principles in Table 1 cannot fully summarize all banded morphologies. In actual sample morphology, when the morphology is between the upper and lower levels in Table 1, it is rated according to the closest principle; for example: for samples between levels 2.0 and 2.5, if the morphology is closer to level 2.5, it is rated at level 2.5, and if it is closer to level 2.0, it is rated at level 2.0.

[0088] Banded structure has a significant impact on subsequent use and quality. Its essence is that there are performance differences and performance directions. There is a certain degree of hardness difference between banded and normal structures. It is this hardness difference that affects subsequent use and quality. Uneven hardness leads to uneven drawing stress and stress during service of finished products, which is prone to early failure at weak performance points. For example, during the drawing process, it is easy to induce tiny cracks at the uneven hardness of the core. The small cracks continue to expand during the subsequent drawing process, leading to drawing fractures. The greater the hardness difference between the banded area and the normal area, the greater the harm. On the contrary, the smaller the hardness difference, the smaller the harm. Therefore, the introduction of hardness difference to quantitatively evaluate banded structure is more objective and comprehensive.

[0089] To assess the banding level using the hardness rating method, a micro Vickers hardness tester (HV 0.2) can be used to measure the microhardness values ​​of the banded and inter-banded structures within the most severe banded structure of the specimen, so as to obtain a hardness rating level of L2 based on the difference between the two.

[0090] In order to reduce the measurement error, three points can be detected along the longitudinal direction of the band, and the average value can be taken to obtain the microhardness value of the banded structure; three points can be detected in the inter-band structure on one side of the band, and the average value can be taken to obtain the microhardness value of the inter-band component. The two average values ​​are used to calculate the difference in microhardness values ​​of the banded and inter-band structures. The larger the difference between the two average values, the higher the banding level.

[0091] The standards for hardness rating levels are shown in Table 2.

[0092] Table 2 Standards for hardness rating levels

[0093] Poor microhardness Band level Microhardness difference ≤5 Level 0 5<Microhardness difference≤10 Level 0.5 10<Microhardness difference≤15 Level 1.0 15<Microhardness difference≤20 Level 1.5 20<Microhardness difference≤25 Level 2.0 25<Microhardness difference≤30 Level 3.0 30<Microhardness difference≤40 Level 4.0

[0094] The method of the present invention uses microstructure rating to accurately, objectively and comprehensively assess its banding level; uses hardness rating to quantitatively assess banding by measuring the difference between the hardness within the band and the hardness between the bands; from the perspective of the risk of banding to the use of materials and subsequent finished products, the rating results of the microstructure rating and hardness rating methods are combined, and the banding level of the sample is quantitatively calculated using a comprehensive rating method. The rating results can also guide the improvement of smelting processes and reduce banding segregation, and the banding level can provide downstream manufacturers with guidance on the graded use of materials, maximize the utilization of materials, and effectively avoid the use of wire rods with serious banding levels in the production of high-strength, high-grade suspension springs and other high-performance materials, thereby reducing the quality risk of low-quality materials and high-quality use.

[0095] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0096] Example 1

[0097] A 12 mm diameter low alloy spring steel hot rolled wire was evaluated, the composition of which included: C: 0.50%, Si: 0.17%, Mn: 0.85%, Cr: 0.90%, V: 0.18%, and the microstructure in the hot rolled state was troostite + a small amount of ferrite, and the troostite content was 82%.

[0098] Take the full-section wire, the sample length is 20mm.

[0099] The process requirements for heat treatment of the sample are as follows: quenching temperature 900℃, holding time 35min; cooling medium: water; tempering temperature 420℃, holding time 90min, cooling medium: water.

[0100] Metallographic sample preparation: Take the full cross-section wire treated by the above heat treatment, cut it longitudinally along its axis, prepare the longitudinal metallographic sample, polish it into a mirror surface, use a mixture of 10% nitric acid alcohol and 3% FeCl3 by mass concentration, etch it until the observation surface is gray-black, rinse it with water, and blow it dry with alcohol. Place it under a microscope to observe the tempered troostite + tempered troostite banding rating.

[0101] Banding Rating:

[0102] Observe and evaluate at 100 times magnification, observe and count the number of bands in a microscopic observation field and the degree of band penetration. One observation field is 0.80mm×1.20mm; refer to Table 1 for the rating standards.

[0103] The banding structure rating was performed in the 1 / 4 diameter area and the core area of ​​the longitudinal section; the banding rating of the 1 / 4 area of ​​the sample was 1.5 (e.g. Figure 3As shown), the core area band rating is 2.5; the calculated band level L1 = 2.5×80% + 1.5×20% = 2.3.

[0104] Hardness test banding level:

[0105] Use a micro Vickers hardness tester (HV0.2) to measure the microhardness of the banded tissue and normal tissue in the most severe banded tissue of the specimen. Test 3 points along the longitudinal direction of the band and take the average value; test 3 points in the normal tissue on one side of the band and take the average value, and calculate the difference between the two average values. Refer to Table 2 for the rating standard of the banded tissue test by the hardness method.

[0106] The three longitudinal hardness values ​​of the sample's banded tissue are 278HV0.2, 279HV0.2, and 280HV0.2, with an average value of 279HV0.2; the hardness values ​​of normal tissue on either side of the banded tissue are 256HV0.2, 258HV0.2, and 253HV0.2, with an average value of 255.7HV0.2. The hardness difference between the bands = 279-255.7 = 23.3HV0.2. According to the hardness rating level standards in Table 2, L2 = 2.0 is determined.

[0107] Overall rating of banded tissue:

[0108] The final rating of the banded structure is a comprehensive assessment of the rating results of the microstructure rating method and the hardness test method. The weighted quantitative rating is L=2.3*70%+2.0*30%=2.2.

[0109] For comparison with Example 1, the method of GB / T13299-91 was used to grade the hot-rolled wire of low-alloy spring steel of the same specification. The banded structure was not obvious and the banded structure grade was 0.

[0110] Example 2

[0111] A high carbon steel hot-rolled wire with a diameter of 8.0 mm was evaluated, and its composition was as follows: C: 0.62%, Si: 0.25%, Mn: 0.65%, Cr: 0.05%; the microstructure in the hot-rolled state was troostite + a small amount of ferrite, and the troostite content was 70%.

[0112] Take the full cross-section wire, the sample length is 15mm.

[0113] The process requirements for heat treatment of the sample are as follows: quenching temperature 900℃, holding time 35min; cooling medium: water; tempering temperature 400℃, holding time 90min, cooling medium: water.

[0114] The preparation of metallographic samples, band rating of microstructure method, hardness method and comprehensive rating method are the same as those in Example 1 and will not be described here; the band rating of 1 / 4 area and the band rating of the core area are respectively as follows: Figure 4 and 5 As shown; the results of the microstructure method band rating are shown in Table 3, the results of the hardness method band rating are shown in Table 4, and the results of the comprehensive rating method are shown in Table 5.

[0115] For comparison with Example 2, the high carbon steel hot rolled wire rod of the same specification was rated using the method of GB / T34474. The banded structure was not obvious and the banded structure grade was 0.5.

[0116] Example 3

[0117] A 20 mm diameter spring steel hot-rolled wire was evaluated, whose composition was: C: 0.56%, Si: 1.45%, Mn: 0.65%, Cr: 0.65%, and the microstructure in the hot-rolled state was troostite + a small amount of ferrite, and the troostite content was 85%.

[0118] Take the full-section wire, the sample length is 20mm.

[0119] The process requirements for sample heat treatment are as follows: quenching temperature 920℃, holding time 35min; cooling medium: water; tempering temperature 440℃, holding time 90min, cooling medium: water.

[0120] The preparation of the sample metallographic sample, the microstructure banding rating, the hardness method, and the comprehensive rating method are the same as those in Example 1 and will not be described again. Figure 6 As shown; the results of the microstructure method band rating are shown in Table 3, the results of the hardness method band rating are shown in Table 4, and the results of the comprehensive rating method are shown in Table 5.

[0121] For comparison with Example 3, the hot-rolled spring steel wire rod of the same specification was rated using the method of GB / T34474. The banded structure was not obvious and the banded structure grade was 0.

[0122] Example 4

[0123] A 12 mm diameter high carbon steel hot rolled wire was evaluated, whose composition was: C: 0.82%, Si: 0.25%, Mn: 0.65%, Cr: 0.25%, and the microstructure in the hot rolled state was troostite + a small amount of ferrite, and the troostite content was 70%.

[0124] Take the full cross-section wire, the sample length is 15mm.

[0125] The heat treatment process requirements of the sample are as follows: quenching temperature 880℃, holding time 35min; cooling medium: water; tempering temperature 420℃, holding time 90min, cooling medium: water.

[0126] The preparation of the sample metallographic sample, the microstructure banding rating, the hardness method, and the comprehensive rating method are the same as those in Example 1 and will not be described again. Figure 7 As shown; the results of the microstructure method band rating are shown in Table 3, the results of the hardness method band rating are shown in Table 4, and the results of the comprehensive rating method are shown in Table 5.

[0127] In order to compare with Example 4, the high carbon steel hot rolled wire rod of the same specification was rated using the ASTM E1268 method. The banded structure was not obvious and the banded structure grade was 0.5.

[0128] Example 5

[0129] A 5.5 mm diameter spring steel hot-rolled wire was evaluated, whose composition was: C: 0.62%, Si: 1.75%, Mn: 0.85%, Cr: 0.20, and the microstructure in the hot-rolled state was troostite + a small amount of ferrite, and the troostite content was 90%.

[0130] Take the full-section wire, the sample length is 20mm.

[0131] The heat treatment process requirements of the sample are as follows: quenching temperature 900℃, holding time 35min; cooling medium: water; tempering temperature 430℃, holding time 90min, cooling medium: water.

[0132] The preparation of the sample metallographic sample, the band rating of the microstructure method, the hardness method, and the comprehensive rating method are the same as those in Example 1 and are not described here. The results of the band rating of the microstructure method are shown in Table 3, the results of the band rating of the hardness method are shown in Table 4, and the results of the comprehensive rating method are shown in Table 5.

[0133] For comparison with Example 5, the hot-rolled spring steel wire rod of the same specification was rated using the method of GB / T34474. The banded structure was not obvious and the banded structure grade was 0.5.

[0134] Table 3 Microstructure banding rating results

[0135] sample <![CDATA[1 / 4 Region L 1 / 4 > <![CDATA[Core region L 1 / 2 > Band Rating Example 1 1.5 2.5 Level 2.3 Example 2 1.0 3.0 Level 2.6 Example 3 0.5 2.0 Level 1.7 Example 4 0 2.5 Level 2.0 Example 5 2.0 3.0 2.8

[0136] Table 4 Hardness band rating results

[0137]

[0138] Table 5 Comprehensive method strip evaluation results

[0139]

[0140] Comparative Example 1

[0141] The difference between Comparative Example 1 and Example 1 is that the heat treatment process is different.

[0142] A 12 mm diameter low alloy spring steel hot rolled wire was evaluated, whose composition: C: 0.50%, Si: 0.17%, Mn: 0.85%, Cr: 0.90%, V: 0.18%, and the microstructure in the hot rolled state was troostite + a small amount of ferrite, and the troostite content was 82%.

[0143] Take the full-section wire, the sample length is 20mm.

[0144] Heat treatment of the sample, heat treatment process: quenching temperature 880℃, holding time 35min; cooling medium: water; tempering temperature 400℃, holding time 90min, cooling medium: water.

[0145] The preparation of the sample metallographic sample and the banding rating method of the microstructure method are the same as those in Example 1 and will not be described in detail. Figure 8 As shown in the figure, due to the quenching temperature of 880℃, there is a lot of residual ferrite in the structure (bright white strips), which interferes with the evaluation of the banded structure. At the same time, due to the tempering temperature of 400℃, the troostite is not fully transformed, and the tempered troostite in the steel is not fully transformed, and the distinction and identification from the tempered troostite is not great, which affects the microscopic rating of the banded structure.

[0146] Comparative Example 2

[0147] The difference between Comparative Example 2 and Example 2 is that the heat treatment process is different.

[0148] A high carbon steel hot-rolled wire with a diameter of 8.0 mm was evaluated, and its composition was as follows: C: 0.62%, Si: 0.25%, Mn: 0.65%, Cr: 0.05%, and its microstructure in the hot-rolled state was troostite + a small amount of ferrite, and the troostite content was 70%.

[0149] Take the full cross-section wire, the sample length is 15mm.

[0150] The heat treatment process requirements of the sample are as follows: quenching temperature 920℃, holding time 35min; cooling medium: water; tempering temperature 440℃, holding time 90min, cooling medium: water.

[0151] The preparation of the metallographic sample and the banding rating method of the microstructure method are the same as those in Example 2 and will not be described in detail. Fig. 9As shown; due to the quenching temperature of 920°C, the structure is slightly coarse, affecting the display of the original banded structure. At the same time, due to the tempering temperature of 440°C, the troostite is fully transformed, and a small amount of tempered troostite remains in the steel, which is not very distinguishable from the tempered troostite. The original banded structure cannot be truly displayed, let alone accurately graded, and is 1.0 level lower than that of Example 2.

[0152] In summary, the evaluation method of the troostite banded structure of the present invention can be applied to steels with a troostite content ≥ 70%, for example: spring steels such as 55SiCr and 65Mn with a troostite (troostite content ≥ 75%) as the main structure, and high carbon steel wire rods such as 60, 70, 80, SWRCH72A, and SWRCH82B with a carbon content between 0.60% and 0.90%; through the heat treatment process and the difference in corrosion caused by the potential difference between different structures, the original banded structure of the troostite wire rod product is truly and clearly displayed, and the microstructure rating method is used to accurately, objectively and comprehensively evaluate its banded level; the hardness rating method is used to quantitatively evaluate the banded structure by measuring the difference between the hardness within the band and the hardness between the bands; from the perspective of the risk of the banded structure to the use of materials and subsequent finished products, the rating results of the microstructure rating method and the hardness rating method are combined, and the comprehensive rating method is used to quantitatively calculate the banded structure level of the sample. The rating results can guide the improvement of smelting process, reduce banded segregation, provide users with banded structure levels, guide the graded use of materials, maximize material utilization, and effectively avoid the use of wire rods with serious banding levels in the production of high-performance materials such as high-strength and high-grade suspension springs, thereby reducing the quality risk of low-quality materials for high-end use.

[0153] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for evaluating a troostite-type banded structure, characterized in that: include: S1: Heat treatment of steel with sorbite content ≥ 70%; The steel with troostite content ≥70% includes spring steel and high carbon steel, wherein the composition of the spring steel includes: C: 0.50%-0.65%, Si: 0.17%-2.0%, Mn: 0.25%-1.20%, Cr≤1.0%, V≤0.20%; the composition of the high carbon steel includes: C: 0.60%-0.90%, Si: 0.15%-0.40%, Mn: 0.50%-0.90%, Cr≤0.25%, V≤0.20%; the heat treatment includes heat treatment of the spring steel and the high carbon steel respectively, wherein, The heat treatment process of the spring steel includes: quenching temperature 900-920°C, holding time 30-35min; cooling medium: water; tempering temperature 420-440°C, holding time 85-95min, cooling medium: water; the heat treatment process of the high carbon steel includes: quenching temperature 880-900°C, holding time 30-35min; cooling medium: water; tempering temperature 400-420°C, holding time 85-95min, cooling medium: water; S2: Metallographic sample preparation, including: cutting the heat-treated steel, preparing longitudinal metallographic specimens, polishing, etching, and etching until the observation surface is gray-black; S3: Banded structure rating, wherein the banded structure rating includes weighted quantitative rating according to the results of microstructure rating and hardness rating.

2. The method for evaluating the troostite-type banded structure according to claim 1, characterized in that: In step S2, the etching solution for etching the metallographic sample includes a mixed solution of nitric acid alcohol with a mass concentration of 8-12% and FeCl3 solution with a mass concentration of 2-4%.

3. The method for evaluating the troostite-type banded structure according to any one of claims 1 to 2, characterized in that: In step S1, the sample used for the heat treatment is a full-section wire with a length of 15-20 mm; In step S2, the metallographic sample preparation includes: cutting the full-section wire after heat treatment longitudinally along its central axis to prepare a longitudinal metallographic sample, polishing and etching, and etching until the observation surface is gray-black, rinsing with clean water, and drying with alcohol; In step S3, the weighted quantitative rating L=L1×70%+L2×30%, wherein L1 is the rating result of the microstructure method, and L2 is the rating result obtained according to the hardness method.

4. The method for evaluating the troostite-type banded structure according to claim 3, characterized in that: In step S3, the microstructure rating includes: rating the area at 1 / 4 diameter and the area at 1 / 2 diameter of the longitudinal section of the sample after metallographic sampling, and calculating the microstructure rating L1 according to the rating results, wherein L1 = the banding level L of the area at 1 / 2 diameter 1 / 2 ×80% + 1 / 4 diameter area band level L 1 / 4 ×20%.

5. The method for evaluating the troostite type banded structure according to claim 4, characterized in that: In step S3, the microstructure rating is performed by observing and counting the number of existing bands and the penetration of the bands in a field of view with a size of 0.80 mm×1.20 mm at a magnification of 100 times, and rating is performed accordingly.

6. The method for evaluating the troostite type banded structure according to claim 5, characterized in that: In step S3, the microstructure rating criteria include: Banding level 0, standard: uniform tissue, no banding distribution; Banding level 0.5, standard: 1-2 slight bands exist, and there are no bands that cross the field of view; Banding level 1.0, standard: 1-2 slight bands, and 1 continuous band across the field of view; or, 3 slight bands, and no band across the field of view; Banding level 1.5, standard: 2-3 obvious bands, and 1-2 continuous bands running through the field of view; or, 4 slight bands, and no bands running through the field of view; Banding level 2, standard: 3-4 obvious bands, and 2-3 continuous bands across the field of view; or, 5 slight bands, and no bands across the field of view; Banding level 2.5, standard: 4-5 obvious bands, and 3-4 bands that continuously cross the field of view; or, 6 slight bands, and no bands that cross the field of view; Banding level 3.0, standard: 5-6 obvious bands, and more than one band that continuously runs through the field of view; Banding level 3.5, standard: 5-6 obvious bands, and more than 2 bands that continuously run through the field of view; Banding level 4.0, standard: There are 5-6 obvious bands, and there are more than 3 strips that continuously run through the field of view.

7. The method for evaluating the troostite type banded structure according to claim 3, characterized in that: In step S3, the hardness rating includes measuring the microhardness values ​​of the banded and inter-banded structures, and obtaining the hardness rating level L2 based on the difference between the two.

8. The method for evaluating the troostite-type banded structure according to claim 7, characterized in that: In step S3, the grade standards of hardness rating include: Banding level 0, standard: microhardness difference ≤ 5; Banding level 0.5, standard: 5<microhardness difference ≤10; Banding level 1.0, standard: 10<microhardness difference ≤15; Banding level 1.5, standard: 15<microhardness difference ≤20; Banding grade 2.0, standard: 20<microhardness difference ≤25; Banding grade 3.0, standard: 25<microhardness difference ≤30; Band grade 4.0, standard: 30<microhardness difference ≤40.

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