Evaluation method for center segregation of medium-low carbon steel wire rod

Abstract

The invention specifically discloses a method for evaluating the center segregation of a medium and low carbon steel wire rod, the center segregation is divided into five grades of 0, 1.0, 2.0, 3.0 and 4.0, the 0 grade is no black core, the 1.0 grade is slight black core, core pearlite is dispersedly distributed, D is more than 0.00% and less than or equal to 0.04%, the 2.0 grade is medium black core, core pearlite is lightly gathered, D is more than 0.04% and less than or equal to 0.07%, the 3.0 grade is obvious black core, core pearlite is moderately gathered, and D is more than 0.07% and less than or equal to 0.12%. And the grade 4.0 is a serious black core, pearlite at the core part is seriously gathered, and D is greater than 0.12%. The center segregation is evaluated according to the color depth degree and the relative diameter ratio of the black core in the core area of the cross section of the wire rod, and meanwhile, an evaluation standard map is made, so that rapid detection is facilitated, the detection efficiency is improved, the blank of the center segregation detection standard of the medium-low carbon steel wire rod is filled up, and the center segregation detection of the medium-low carbon steel wire rod is standardized and normalized.

Description

Technical Field

[0001] The invention relates to the technical field of center segregation detection of medium and low carbon steel wire rods, and in particular to an evaluation method for center segregation of medium and low carbon steel wire rods. Background Art

[0002] The enrichment of elements (such as carbon, sulfur, and phosphorus) in the central area of the steel wire rod will form hard and brittle phases (such as pearlite and sulfide inclusions), resulting in a mismatch between the overall strength and toughness of the material, increased local brittleness, and easy to cause cracks. The difference in mechanical properties between the segregated area and the non-segregated area will form weak points, which can easily become a source of fracture when subjected to load, reducing the tensile strength and fatigue life of the material. During processing such as drawing and cold heading, the central segregated area is prone to microcracks or even fractures due to excessive hardness or insufficient plasticity, affecting the yield rate (such as steel wire, bolts, etc.). The difference in composition between the segregated area and the non-segregated area will form a micro-battery effect, accelerating local corrosion (such as pitting corrosion and intergranular corrosion), especially in humid or acidic environments. During welding, elements such as sulfur and phosphorus in the segregated area will migrate to the grain boundaries, resulting in increased brittleness in the heat-affected zone of the weld, which is prone to welding cracks or joint failure. During heat treatment (such as quenching and tempering), the segregation zone may cause abnormal local phase transformation due to composition differences, resulting in residual stress or deformation, which will affect the dimensional stability of the final product.

[0003] If centerline segregation is severe, high-stress components (such as springs, bearings, and fasteners) made from this type of steel may suddenly fracture during service, posing a safety hazard. Centerline segregation can significantly reduce the overall performance of the steel wire rod, impacting processing efficiency and the reliability of the final product. In high-end manufacturing fields (such as automotive and aerospace), strict control of segregation is critical to ensuring material quality.

[0004] Currently, the standards for testing centerline segregation include ISO16120-1 and YBT4413, but both require the carbon content of steel wire rod to be ≥0.4%. With product structure upgrades and rising market quality requirements, there is an urgent need to establish a segregation evaluation system for medium- and low-carbon steel (C < 0.4%) to support product quality upgrades and enhance market competitiveness. Existing literature, textbooks, and national or industry standards lack relevant provisions for testing centerline segregation in steel wire rods with a carbon content below 0.40%. This results in a lack of detection and characterization methods, which have little practical guidance for actual production and cannot meet quality improvement needs. Furthermore, inspection images according to the ISO16120-1 standard cannot detect centerline segregation. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for evaluating the center segregation of medium and low carbon steel wire rods, so as to solve the problem that there is no method for evaluating the segregation of medium and low carbon steel (C < 0.4%) in the industry.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for evaluating center segregation of medium- and low-carbon steel wire rods, the method comprising the following steps: cutting a sample, grinding a sample, mounting a sample, grinding and polishing a sample, hand-grinding a sample, polishing a sample, etching a sample, determining a test position, observing the sample, comparing the sample with a standard test pattern, and determining a center segregation grade of the wire rod. The center segregation grade is divided into five grades: 0, 1.0, 2.0, 3.0, and 4.0. The evaluation standard for center segregation of grade 0 is no black center, the evaluation standard for center segregation of grade 1.0 is slight black center, pearlite is dispersed in the center, and 0.00% < D≤ 0.04%, 2.0 level center segregation assessment standard is medium black heart, center pearlite slightly aggregated, 0.04%<D≤0.07%, 3.0 level center segregation assessment standard is obvious black heart, center pearlite moderately aggregated, 0.07%<D≤0.12%, 4.0 level center segregation assessment standard is severe black heart, center pearlite severely aggregated, D>0.12%. The "black heart" refers to the high carbon content in the center caused by center segregation, abnormal increase in pearlite or other non-ferrite organization aggregation, and the darker color area compared to the entire wire rod cross section.

[0008] D=D2 / D1*100%, D2=(R1+R2) / 2,

[0009] Where: D is the relative diameter ratio, D1 is the sample diameter, D2 is the average diameter of the pearlite aggregation area, R1 is the longest diameter of the black heart, R2 is the length of the black heart perpendicular to R1, and black hearts without obvious boundaries are not included.

[0010] Preferably, the standard test patterns include two categories, A and B. Category A is a carbon content of less than 0.30%, and the metallographic images of the corresponding levels of segregation at the five centers of the test position taken by an optical microscope at a magnification of 100X. Category B is a carbon content of 0.30% ≤ < 0.4%, and the metallographic images of the corresponding levels of segregation at the five centers of the test position taken by an optical microscope at a magnification of 100X.

[0011] Preferably, the method for cutting the sample is as follows: using a cutting machine to cut a sample of about 15 mm for preparing a metallographic sample to be observed and measured.

[0012] Preferably, the method for grinding the sample is to use a grinder to grind off the burrs on the cross sections of both ends of the sample perpendicular to the rolling direction to facilitate inlaying.

[0013] Preferably, the method for mounting the sample is as follows: the sample is placed in a mounting machine with the cross section perpendicular to the rolling direction facing downward for mounting.

[0014] Preferably, the method for grinding the sample is to grind the sample using a fully automatic plane pre-grinder.

[0015] Preferably, the method of grinding and polishing the sample is: using 400# and 800# water-abrasive sandpaper to coarsely grind the sample detection surface on a grinding and polishing machine; the method of hand-grinding the sample is: using 400# and 600# metallographic sandpaper to manually finely grind the sample detection surface.

[0016] Preferably, the method for polishing the sample includes polishing the test surface of the sample with a polishing machine.

[0017] Preferably, the method for etching the sample is: using 4% nitric acid alcohol solution, and the etching time is 20s.

[0018] Preferably, the method for determining the detection position is: observing the entire sample cross section under a 25X optical microscope lens, and finding the position where the black heart is most serious as the detection position; the method for observing the sample is: placing the determined detection position under an optical microscope at 100X for sample observation.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] ① The present invention can directly display the segregation area of the wire rod under an optical microscope realistically and clearly through the core microstructure. At the same time, it stipulates the evaluation principles and produces a detection standard atlas, making the detection process standardized, standardized, simple and convenient, and effectively solving the industry pain point of the lack of segregation detection standards for medium and low carbon steels.

[0021] ② A method for characterizing and evaluating the central segregation of medium and low carbon steel wire rods was proposed, which played an important role in providing data support for the optimization of medium and low carbon steel production processes and the formulation of optimal production processes, and played a positive role in product quality control.

[0022] ③ The method of the present invention can be widely applied to the evaluation and characterization of the center segregation of various medium and low carbon steel wire rods, and has good application effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Flow chart of the method for evaluating center segregation of low carbon steel wire rod of the present invention;

[0024] Figure 2 Schematic diagram of the relative diameter ratio of the wire rod of the present invention;

[0025] Figure 3 This is the standard diagram of segregation degree of Class A wire rod;

[0026] Figure 4 This is the standard diagram of segregation degree of Class B wire rod. DETAILED DESCRIPTION

[0027] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below through examples.

[0028] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0029] Secondly, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive of other embodiments.

[0030] Example:

[0031] See also Figure 1 This embodiment provides a method for evaluating the center segregation of medium and low carbon steel wire rods. The specific implementation steps are as follows:

[0032] (1) Cutting samples: Use a cutting machine to cut samples of about 15 mm to prepare metallographic samples to be observed and measured;

[0033] (2) Grinding the sample: Use a grinder to remove the burrs on the cross sections of both ends of the sample perpendicular to the rolling direction to facilitate mounting;

[0034] (3) Mounting the specimen: Place the specimen with the cross section perpendicular to the rolling direction facing downwards into the mounting machine for mounting;

[0035] (4) Grinding the sample: Grind the sample using a fully automatic plane pre-grinder;

[0036] (5) Grinding and polishing the sample: Use 400# or 800# water-abrasive sandpaper to coarsely grind the test surface of the sample on a grinding and polishing machine;

[0037] (6) Hand-grinded specimen: Use 400# or 600# metallographic sandpaper to manually finely grind the specimen test surface;

[0038] (7) Polishing the sample: polish the test surface of the sample with a polishing machine;

[0039] (8) Etching the sample: The sample was etched with 4% nitric acid alcohol solution for 20 seconds;

[0040] (9) Determine the detection position: Observe the entire cross section of the sample under a 25X optical microscope and find the location where the "black heart" is most serious as the detection position;

[0041] (10) Observe the sample: Place the determined test position under an optical microscope at 100X magnification for sample observation; use a microscope at a low magnification of 25X to observe the entire sample cross section, accurately determine the center segregation test position area, and then use an optical microscope at a magnification of 100X to observe the most serious "black heart" area of the determined test position, so as to accurately locate the center segregation area of the entire cross section;

[0042] (11) Method for evaluating the degree of central segregation

[0043] Determine the degree of center segregation: Assessment principle: assess the center segregation based on the color depth and relative diameter ratio of the "black heart" in the center area of the cross section, such as Figure 2 The regulations shown:

[0044] D=D2 / D1*100%

[0045] Where: D is the relative diameter ratio, D1 is the sample diameter, D2 is the average diameter of the pearlite aggregation area, D2 = (R1 + R2) / 2, R1 is the longest diameter of the black heart, R2 is the length of the black heart in the direction perpendicular to R1, and black hearts without obvious boundaries are not included.

[0046] The central segregation grade is divided into five grades: 0, 1.0, 2.0, 3.0 and 4.0. The assessment standard for grade 0 central segregation is no black heart, the assessment standard for grade 1.0 central segregation is slight black heart, the pearlite in the center is dispersed, 0.00%<D≤0.04%, the assessment standard for grade 2.0 central segregation is moderate black heart, the pearlite in the center is slightly aggregated, 0.04%<D≤0.07%, the assessment standard for grade 3.0 central segregation is obvious black heart, the pearlite in the center is moderately aggregated, 0.07%<D≤0.12%, and the assessment standard for grade 4.0 central segregation is severe black heart, the pearlite in the center is severely aggregated, D>0.12%.

[0047] like Figure 3 、 4 As shown, (12) prepare the test map: take metallographic pictures under an optical microscope at a magnification of 100X, and prepare standard pictures of the corresponding levels of central segregation of steel wire rods of type A: carbon content <0.30%, type B: 0.30% ≤ carbon content <0.4% as the test standard map, and analyze the segregation degree of the sample according to the standard test map.

[0048] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for evaluating center segregation of medium and low carbon steel wire rods, characterized in that: The method comprises the following steps in sequence: cutting a sample, grinding a sample, mounting a sample, grinding and polishing a sample, hand-grinding a sample, polishing a sample, etching a sample, determining a detection position, observing the sample, comparing with a standard detection spectrum, and determining a wire rod center segregation grade, wherein the center segregation grade is divided into five grades: 0, 1.0, 2.0, 3.0, and 4.

0. The evaluation standard for center segregation of grade 0 is no black center, the evaluation standard for center segregation of grade 1.0 is slight black center, pearlite is dispersed in the center, 0.00% < D ≤ 0.04%, 2.0 The assessment standard for grade 3.0 center segregation is moderate blackheart, with slight aggregation of pearlite in the center, 0.04%<D≤0.07%. The assessment standard for grade 3.0 center segregation is obvious blackheart, with moderate aggregation of pearlite in the center, 0.07%<D≤0.12%. The assessment standard for grade 4.0 center segregation is severe blackheart, with severe aggregation of pearlite in the center, D>0.12%. The "blackheart" refers to the area with high carbon content in the center caused by center segregation, abnormal increase in aggregation of pearlite or other non-ferrite structures, and darker color compared to the entire wire rod cross section. D=D2 / D1*100%, D2=(R1+R2) / 2, Where: D is the relative diameter ratio, D1 is the sample diameter, D2 is the average diameter of the pearlite aggregation area, R1 is the longest diameter of the black heart, R2 is the length of the black heart perpendicular to R1, and black hearts without obvious boundaries are not included.

2. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 1, characterized in that: The standard test patterns include two categories, A and B. Category A refers to carbon content less than 0.30%, and metallographic images of the corresponding levels of segregation at the five centers of the test position taken by an optical microscope at a magnification of 100X. Category B refers to carbon content 0.30% ≤ < 0.4%, and metallographic images of the corresponding levels of segregation at the five centers of the test position taken by an optical microscope at a magnification of 100X.

3. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 1, characterized in that: The method for cutting the sample is as follows: a sample of about 15 mm is cut with a cutting machine to prepare a metallographic sample to be observed and measured.

4. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 1, characterized in that: The method for grinding the sample is as follows: using a grinding machine to grind away burrs on the cross sections of both ends of the sample perpendicular to the rolling direction to facilitate inlaying.

5. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 1, characterized in that: The method for mounting the sample is as follows: the sample is placed in a mounting machine with the cross section perpendicular to the rolling direction facing downward for mounting.

6. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 1, characterized in that: The method for grinding the sample: grinding the sample with a full-automatic plane pre-grinder.

7. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 1, characterized in that: The method for grinding and polishing the sample is as follows: the sample detection surface is coarsely ground with 400# and 800# water-abrasive sandpaper on a grinding and polishing machine; the method for hand-grinding the sample is as follows: the sample detection surface is finely ground manually with 400# and 600# metallographic sandpaper.

8. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 1, characterized in that: The method for polishing the sample is as follows: polishing the test surface of the sample with a polishing machine.

9. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 1, characterized in that: The method for etching the sample is as follows: using 4% nitric acid alcohol solution, and the etching time is 20 seconds.

10. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 1, characterized in that: The method for determining the detection position is as follows: observing the entire sample cross section under a 25X optical microscope lens, and finding the position with the most serious black heart as the detection position; the method for observing the sample is as follows: placing the determined detection position under an optical microscope at 100X for sample observation.

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