High formability high carbon steel wire rod and process for producing the same

Abstract

This invention discloses a high-formability high-carbon steel wire rod and its production process. The chemical composition of the wire rod, by mass percentage, is as follows: [C] 0.70%–0.74%, [Si] 0.15%–0.25%, [Mn] 0.40%–0.55%, [Cr] 0.01%–0.05%, [P] ≤0.010%, [S] 0.0020%–0.010%, total oxygen 0.0010%–0.0020%, [Als] 0.0003%–0.0015%, [Co] 0.0001%–0.0009%, [Mg] 0.0002%–0.0010%. The production process includes steel smelting and continuous casting, continuous casting billet heating and rolling, wire rod rolling, wire rod wire drawing, and wire rod cooling. This invention adopts a chemical composition design scheme with low manganese, low silicon, and added chromium and cobalt elements. Through the design scheme of billet segregation and wire rod pearlite structure pellet orientation, the banded structure of large-specification wire rods with a diameter of 8-10mm is controlled below grade 0.5, meeting the user's requirements for the forming performance of high carbon steel wire rods.

Description

Technical Field

[0001] This invention relates to high-formability high-carbon steel wire rod and its production process, belonging to the field of wire rod production technology. Background Technology

[0002] High-carbon steel wire rod is widely used in the production of tire bead wire, steel cord, and steel rope. This wire rod undergoes drawing and other processing during user operations, therefore users have strict requirements regarding its processing performance.

[0003] Chinese invention patent CN102268596B discloses a "high-carbon steel wire rod for producing fine steel wire". The chemical composition of this wire rod by weight percentage is as follows: [C]: 0.6%~0.88%, [Si]: 0.1%~1.0%, [Mn]: 0.3%~1.0%, [P]≤0.015%, [S]≤0.010%, [N]≤0.004%, [O]≤0.002%, [Al]≤0.002%, [Ti]≤0.002%, [Mg]≤0.001%, other unavoidable impurities not exceeding 0.1%, and the remainder being iron. The high-carbon steel wire rod has a reasonable composition design, excellent processing performance, and the inclusions in the wire rod have low melting points. During hot rolling, the inclusions elongate along the rolling direction, with a length-to-width ratio greater than 3, which can reduce the wire breakage rate during steel wire processing by 16%-28%.

[0004] The wire rod production design described in the aforementioned patent does not consider the impact of banded structure on the wire rod's drawing performance. The severe banded structure in high-carbon steel wire rod causes stress concentration during processing. The difference in strain and stress between banded and non-banded structures can lead to cracking during drawing or torsion, reducing the formability of the wire rod or wire. To meet users' requirements for the formability of high-carbon steel wire rod, there is an urgent need to develop a technology to control the banded structure of high-carbon steel wire rod, thereby satisfying the quality requirements of metal product users. Summary of the Invention

[0005] The purpose of this invention is to provide a high-formability high-carbon steel wire rod and its production process, so that the strip structure of large-size wire rod is controlled below grade 0.5, thereby meeting the user's requirements for the processing performance of high-carbon steel wire rod.

[0006] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0007] A high-formability high-carbon steel wire rod has the following chemical composition by mass percentage: [C] 0.70%–0.74%, [Si] 0.15%–0.25%, [Mn] 0.40%–0.55%, [Cr] 0.01%–0.05%, [P] ≤0.010%, [S] 0.0020%–0.010%, total oxygen 0.0010%–0.0020%, [Als] 0.0003%–0.0015%, [Co] 0.0001%–0.0009%, [Mg] 0.0002%–0.0010%, with the balance being Fe and unavoidable impurities.

[0008] Furthermore, grains with an orientation difference of ≤10° of pearlite pellets account for 76% to 86%, grains with an orientation difference of 10° to 30° of pearlite pellets account for 7% to 12%, and grains with an orientation difference of ≥30° of pearlite pellets account for 4% to 15%.

[0009] Furthermore, the sorbitization rate of the wire rod is 75%–85%, and the thickness of the proeutectoid ferrite in the wire rod is ≤3μm.

[0010] Furthermore, the wire diameter is 8–10 mm, and the banded structure is controlled to be below grade 0.5.

[0011] A production process for high-formability high-carbon steel wire rod includes steel smelting and continuous casting, continuous casting billet heating and rolling, wire rod rolling, wire rod wire drawing, and wire rod cooling.

[0012] Heating and rolling of continuous casting billets: The total time for heating the continuous casting billets in the furnace is 3.5 to 4.5 hours; the temperature of the soaking zone is controlled at 1200 to 1260℃, and the holding time of the soaking zone is 30 to 50 minutes; after heating, the continuous casting billets are continuously rolled, and the final rolling temperature is controlled at 900 to 1050℃.

[0013] Wire rod rolling: The total furnace time for the continuously rolled billet is 140-160 min, the soaking zone temperature is 1100-1150℃, and the soaking zone holding time is 30-50 min; after heating, the wire rod is rolled through roughing, intermediate rolling, pre-finishing rolling, finishing rolling, and double-module rolling, and then the wire rod is produced; the wire rod exiting the pre-finishing rolling temperature is 950-1000℃, entering the finishing rolling temperature is 880-960℃, and entering the double-module rolling temperature is 890-940℃;

[0014] Wire rod spinning: The wire rod spinning temperature is controlled between 910℃ and 940℃;

[0015] Wire rod cooling: After spinning, the wire rod is cooled on the air-cooled roller conveyor. The cooling rate before the pearlite phase transformation of the wire rod is controlled at 12-18℃ / s, and the pearlite phase transformation temperature of the wire rod is controlled at 600℃-635℃.

[0016] Furthermore, the cross-sectional dimensions of the continuously rolled billet are (140~180)mm*(140~180)mm. The carbon segregation index at the center of the cross-section, at the 1 / 4 length position, and at the surface position are controlled at 0.96~1.09, 0.99~1.06, and 0.93~1.0, respectively, and the manganese segregation index at the corresponding positions is controlled at 0.98~1.02, 0.99~1.01, and 0.99~1.0.

[0017] Furthermore, the molten steel is smelted using a combination of molten iron and scrap steel. After smelting, the molten steel is refined using LF (sulfurized iron-smelting) refining. The refining time is controlled at 30–50 min, and the refining temperature is controlled at 1450–1550 °C. Argon gas is used for stirring during the molten steel refining process, and the argon gas flow rate is controlled at 200–500 NL / min.

[0018] Furthermore, after refining, the molten steel is continuously cast, and the cross-sectional dimensions of the continuously cast billet are (250~300)mm*(350~400)mm.

[0019] Furthermore, the proportion of scrap steel in the molten iron + scrap steel smelting process is 4% to 9%.

[0020] Compared with existing technologies, this invention adopts a chemical composition design scheme with low manganese and low silicon, as well as the addition of chromium and cobalt elements. Through the design scheme of billet segregation and pearlite structure pellet orientation of wire rod, the banded structure of large-specification wire rods with a diameter of 8-10mm is controlled below grade 0.5, which meets the user's requirements for the forming performance of high carbon steel wire rods. Detailed Implementation

[0021] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. The described embodiments are merely some, not all, of the embodiments of this invention. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0022] The chemical composition design principles of the wire rod described in this invention are as follows (by mass percentage):

[0023] The chemical composition of the wire rod applicable to this invention is as follows: [C] 0.70%~0.74%, [Si] 0.15%~0.25%, [Mn] 0.40%~0.55%, [Cr] 0.01%~0.05%, [P] ≤0.010%, [S] 0.0020%~0.010%, total oxygen 0.0010%~0.0020%, [Als] 0.0003%~0.0015%, [Co] 0.0001%~0.0009%, [Mg] 0.0002%~0.0010%, with the balance being Fe and unavoidable impurities.

[0024] The grain orientation characteristics of the wire rod are as follows: grains with a pearlite pellet orientation difference ≤10° account for 76%–86%; grains with a pearlite pellet orientation difference between 10° and 30° (excluding endpoint values) account for 7%–12%; and grains with a pearlite pellet orientation difference ≥30° account for 4%–15%. The sorbitization rate of the wire rod is 75%–85%, and the proeutectoid ferrite thickness is ≤3μm.

[0025] The rationale for setting the range of chemical composition and microstructure characteristics of the wire rod in this invention is as follows.

[0026] Carbon: If the carbon content in the wire is too low, it will not meet the strength requirements of the user when drawing the steel wire. If the carbon content is too high, it will increase the segregation of the billet and wire rod, increase the band structure level of the wire rod, and increase the breakage of high carbon steel wire during production. Therefore, the carbon content in this invention is controlled at 0.70% to 0.74%.

[0027] Silicon: Silicon is the main deoxidizing element in high-carbon steel. Low silicon content will result in insufficient deoxidation of the molten steel; excessive silicon content will lead to coarse silicate inclusions after deoxidation. Furthermore, silicon increases the eutectoid transformation temperature of steel, causing pearlite to transform at high temperatures, which is detrimental to the uniformity of the wire rod microstructure. Therefore, the silicon content in this invention is controlled at 0.15%–0.25%.

[0028] Manganese: Manganese is an element that improves the strength of wire rods. An appropriate amount of manganese in steel ensures that the wire rod produced from the wire rod meets the user's strength requirements. However, excessive manganese content leads to severe segregation in the wire rod, which is detrimental to the control of the banded structure. Manganese lowers the eutectoid transformation temperature of steel, resulting in a finer pearlite structure and a lower grade of the banded structure in the wire rod. Therefore, the manganese content in this invention is controlled at 0.40%–0.55%.

[0029] Chromium: Chromium in wire rod improves hardenability, refines the wire rod microstructure, and reduces the banded microstructure level. However, excessive chromium content leads to significant work hardening during processing, deteriorates the wire rod's machinability, and increases the likelihood of wire drawing or torsional fracture. Therefore, the chromium content in this invention is controlled at 0.01% to 0.05%.

[0030] Phosphorus: Phosphorus tends to form banded segregation in wire rods, increasing the grade of banded structure in the wire rods and causing cracking and breakage of the steel wires. Therefore, the phosphorus content in this invention is controlled at ≤0.010%.

[0031] Sulfur: High sulfur content in steel reduces the cold working performance of wire rod. Appropriate sulfur content in the steel controls the MnS inclusions in the wire rod within a suitable range. Taking advantage of MnS's high deformability and its ability to easily coordinate deformation with the steel matrix, the adverse effects of Al2O3 or spinel inclusions on the wire rod's deformability are reduced. Therefore, in this invention, the sulfur content is controlled between 0.0020% and 0.010%.

[0032] Oxygen: When the oxygen content in the wire rod is high, the inclusions in the steel are large in size and numerous, and the inclusions are prone to banded distribution after the steel billet is rolled into wire rod. When the oxygen content in the wire rod is low, the inclusions in the wire rod have poor deformation ability, which is not conducive to improving its deformation ability; therefore, in this invention, the total oxygen content of the wire rod is controlled at 0.0010% to 0.0020%.

[0033] Acid-soluble aluminum: When the acid-soluble aluminum content is high, large-sized Al2O3 inclusions will appear in the steel; when the acid-soluble aluminum content in the wire rod is too low, the melting point of the inclusions in the steel is high, and cracks are easily formed between the inclusions and the matrix during processing, reducing the machinability of the wire rod. Therefore, the acid-soluble aluminum content in this invention is controlled at 0.0003% to 0.0015%.

[0034] Cobalt: Cobalt can inhibit grain growth in steel billets during heating, thereby refining the pearlite structure of the wire rod and reducing its banded structure level. However, high cobalt content leads to severe work hardening during wire processing, making the wire prone to cracking and breakage. Therefore, the cobalt content in this invention is controlled at 0.0001% to 0.0009%.

[0035] Magnesium: Magnesium controls the oxygen content in steel and also the type of inclusions. Higher magnesium content leads to the formation of magnesium-aluminum spinel inclusions, reducing the machinability of the wire rod. Appropriate magnesium content improves the deformability of inclusions, expanding the range of inclusions with high deformability, which is beneficial for improving the coordinated deformation capability of the wire rod. Therefore, the magnesium content in this invention is controlled between 0.0002% and 0.0010%.

[0036] The orientation of pearlite grains in wire rods has a significant impact on the banded structure. A higher proportion of large-angle orientation relationships is beneficial for reducing the banded structure level of the wire rods. Therefore, the orientation characteristics of the wire rods in this invention are as follows: grains with pearlite cluster orientation differences ≤10° account for 76%–86%, grains with pearlite cluster orientation differences between 10° and 30° (excluding endpoint values) account for 7%–12%, and grains with pearlite cluster orientation differences ≥30° account for 4%–15%.

[0037] The higher sorbitization rate of the wire rod reduces the banded structure level, and the finer proeutectoid ferrite structure also makes the banded structure characteristics of the wire rod less prominent. Therefore, in this invention, the sorbitization rate of the wire rod is 75%–85%, and the proeutectoid ferrite thickness is ≤3μm.

[0038] The production process of the wire rod includes the following steps:

[0039] Steelmaking and Continuous Casting: The steel is smelted using a mixture of molten iron and scrap steel, with the scrap steel ratio ranging from 4% to 9%. After smelting, the steel is refined using an LF furnace. The LF furnace refining time is controlled at 30–50 minutes, and the refining temperature is controlled at 1450–1550℃. Argon gas is used for stirring during the refining process, with the argon gas flow rate controlled at 200–500 NL / min. After refining, the steel is continuously cast, with the continuous casting billet having a cross-sectional size of (250–300) mm * (350–400) mm.

[0040] Heating and rolling of continuously cast billets: The total furnace time for heating the continuously cast billets is 3.5–4.5 hours. The temperature of the soaking zone for the continuously cast billets is controlled at 1200–1260℃, and the holding time in the soaking zone is 30–50 minutes. After heating, the continuously cast billets are continuously rolled, and the final rolling temperature is controlled at 900–1050℃. The cross-sectional dimensions of the continuously rolled billets are (140–180) mm * (140–180) mm. The carbon segregation indices at the center of the cross-section, the 1 / 4 length position, and the surface position are controlled at 0.96–1.09, 0.99–1.06, and 0.93–1.0, respectively, and the manganese segregation indices at the corresponding positions are controlled at 0.98–1.02, 0.99–1.01, and 0.99–1.0. By controlling the segregation of carbon and manganese elements in the continuously rolled billets, the banded structure level of the final wire rod is reduced.

[0041] Wire rod rolling: The total furnace time for the continuously rolled billet is 140–160 min, with a soaking temperature of 1100–1150℃; high-temperature diffusion of the billet reduces the segregation of elements such as carbon and manganese; the soaking time is 30–50 min. After heating, the billet undergoes roughing, intermediate rolling, pre-finishing, finishing, and double-module rolling before wire rod production. The wire rod exits the pre-finishing rolling mill at 950–1000℃, enters the finishing rolling mill at 880–960℃, and enters the double-module rolling mill at 890–940℃. The wire rod is rolled to a diameter of 8-10 mm.

[0042] Wire rod spinning: The wire rod spinning temperature is controlled between 910℃ and 940℃. By spinning the wire rod at high temperature, the cooling rate of the wire rod on the air-cooled roller conveyor is increased, laying the foundation for controlling the strip structure level of the wire rod.

[0043] Wire rod cooling: After wire drawing, the wire rod is cooled on an air-cooled roller conveyor. The cooling rate before the pearlitic phase transformation is controlled at 12-18℃ / s, and the pearlitic phase transformation temperature is controlled at 600-635℃. After final cooling, the cord steel wire rod is mainly composed of sorbite, with a sorbitization rate of 75%-85%, which is beneficial for wire drawing by users.

[0044] Examples of the present invention are described below.

[0045] Example C，% Si,% Mn, % Cr，% P，% 1 0.71 0.24 0.49 0.05 0.009 2 0.73 0.21 0.45 0.03 0.010 3 0.74 0.22 0.55 0.04 0.009 4 0.72 0.15 0.52 0.02 0.007 5 0.72 0.19 0.48 0.04 0.008 6 0.71 0.18 0.53 0.05 0.008 Example S，% Total oxygen, % Acid-soluble aluminum, % Co,% Mg, % 1 0.009 0.0020 0.0008 0.0005 0.0009 2 0.007 0.0016 0.0007 0.0004 0.0007 3 0.008 0.0015 0.0012 0.0004 0.0008 4 0.005 0.0012 0.0011 0.0006 0.0005 5 0.003 0.0011 0.0011 0.0003 0.0005 6 0.008 0.0018 0.0012 0.0007 0.0009 Example Percentage of individuals with an orientation difference ≤10° Percentage of individuals with an orientation difference of 10°–30° Percentage of individuals with an orientation difference ≥30° Preeutectoid ferrite thickness, μm Band-like tissue, grade 1 78 8 14 ≤3 <0.5 2 84 11 4 ≤1 <0.5 3 82 10 8 ≤2 <0.5 4 80 9 11 ≤3 <0.5 5 79 11 10 ≤3 <0.5 6 83 11 6 ≤2 <0.5 Example Scrap steel ratio, % LF furnace refining time, min 1 5 47 2 7 50 3 6 38 4 8 43 5 7 44 6 8 45 Example LF refining temperature, °C Argon flow rate, NL / min <![CDATA[Continuous casting billet cross-sectional dimensions, mm 2 > Total time of continuously cast billet in furnace, h Holding time in the soaking zone of continuously cast billets, min 1 1522 445 280*380 3.7 44 2 1531 433 280*380 4.2 41 3 1495 392 280*380 4.2 39 4 1515 349 280*380 3.6 42 5 1512 375 280*380 4.2 38 6 1498 402 280*380 4.4 42 Example Temperature of the soaking zone for continuously cast billets, °C <![CDATA[Continuous rolling billet cross-sectional dimensions, mm 2 > Carbon segregation index of continuously rolled billet core Carbon segregation index at 1 / 4 position of continuous rolled billet length Carbon segregation index on the surface of continuously rolled billets 1 1245 160*160 1.04 1.05 0.99 2 1252 160*160 1.07 0.99 0.98 3 1243 160*160 0.97 1.01 0.96 4 1251 160*160 0.96 1.04 0.97 5 1251 160*160 1.03 1.05 0.98 6 1243 160*160 1.03 1.05 0.96 Example Carbon and manganese precipitation index of continuously rolled billet core Manganese segregation index at 1 / 4 position of continuous rolled billet length Manganese segregation index on the surface of continuously rolled billets Continuous rolling finishing temperature, °C Total furnace heating time for continuously rolled billets, min 1 1.01 1.00 0.99 965 154 2 1.01 0.99 0.99 951 153 3 0.99 1.00 1.0 962 147 4 0.99 0.99 0.99 963 152 5 1.01 0.99 0.99 957 152 6 1.01 1.01 0.99 948 147 Example Temperature of the heating and soaking section of the continuously rolled billet, ℃ The heating and homogenization time of the continuously rolled billet, in minutes Pre-finishing temperature of the rolled piece, ℃ Temperature of the rolled piece entering the finishing mill, ℃ Temperature of rolled piece entering dual module, ℃ 1 1136 44 983 943 917 2 1149 42 975 939 921 3 1143 47 990 955 916 4 1139 45 987 946 923 5 1142 41 973 942 922 6 1135 44 992 944 925 Example Wire rod specifications, mm Wire rod spinning temperature, °C Cooling rate before phase change, ℃ / s Phase transition temperature, °C Soxhletization rate, % 1 9 932 14 625 80 2 9 930 13 632 80 3 9 922 17 617 85 4 9 927 16 623 80 5 9 919 15 623 80 6 9 925 17 621 80

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high formability high carbon steel wire rod, characterized by, The chemical composition of the wire rod, by mass percentage, is as follows: [C] 0.70%–0.74%, [Si] 0.15%–0.25%, [Mn] 0.40%–0.55%, [Cr] 0.01%–0.05%, [P] ≤0.010%, [S] 0.0020%–0.010%, total oxygen 0.0010%–0.0020%, [Als] 0.0003%–0.0015%, [Co] 0.0001%–0.0009%, [Mg] 0.0002%–0.0010%, with the balance being Fe and unavoidable impurities. The production process includes steel smelting and continuous casting, continuous casting billet heating and rolling, wire rod rolling, wire rod wire splicing, and wire rod cooling. Heating and rolling of continuous casting billets: The total time for heating the continuous casting billets in the furnace is 3.5 to 4.5 hours; the temperature of the soaking zone is controlled at 1200 to 1260℃, and the holding time of the soaking zone is 30 to 50 minutes; after heating, the continuous casting billets are continuously rolled, and the final rolling temperature is controlled at 900 to 1050℃. Wire rod rolling: The total furnace time for the continuously rolled billet is 140-160 min, the soaking zone temperature is 1100-1150℃, and the soaking zone holding time is 30-50 min; after heating, the wire rod is rolled through roughing, intermediate rolling, pre-finishing rolling, finishing rolling, and double-module rolling, and then the wire rod is produced; the wire rod exiting the pre-finishing rolling temperature is 950-1000℃, entering the finishing rolling temperature is 880-960℃, and entering the double-module rolling temperature is 890-940℃; Wire rod spinning: The wire rod spinning temperature is controlled between 910℃ and 940℃; Wire rod cooling: After spinning, the wire rod is cooled on the air-cooled roller conveyor. The cooling rate before the pearlite phase transformation of the wire rod is controlled at 12-18℃ / s, and the pearlite phase transformation temperature of the wire rod is controlled at 600℃-635℃.

2. The high formability high carbon steel wire rod according to claim 1, characterized in that, Grains with a pearlite cluster orientation difference ≤10° account for 76% to 86%, grains with a pearlite cluster orientation difference between 10° and 30° but excluding 10° and 30° account for 7% to 12%, and grains with a pearlite cluster orientation difference ≥30° account for 4% to 15%.

3. The high-formability high-carbon steel wire rod according to claim 1, characterized in that, The sorbitization rate of the wire rod is 75%–85%, and the thickness of the proeutectoid ferrite is ≤3μm.

4. The high-formability high-carbon steel wire rod according to claim 1, characterized in that, The wire rod diameter is 8-10 mm, and the banded tissue is controlled to be below grade 0.

5.

5. The high-formability high-carbon steel wire rod according to claim 1, characterized in that, The cross-sectional dimensions of the continuously rolled billet are (140~180)mm*(140~180)mm. The carbon segregation index at the center of the cross-section, at the 1 / 4 position of the length, and at the surface position are controlled at 0.96~1.09, 0.99~1.06, and 0.93~1.0, respectively. The manganese segregation index at the corresponding positions is controlled at 0.98~1.02, 0.99~1.01, and 0.99~1.

0.

6. The high-formability high-carbon steel wire rod according to claim 1, characterized in that, The molten steel is smelted using molten iron and scrap steel. After smelting, the molten steel is refined using LF (sulfurized iron-smelting) refining. The refining time is controlled at 30-50 minutes, and the refining temperature is controlled at 1450-1550℃. Argon gas is used for stirring during the molten steel refining process, and the argon gas flow rate is controlled at 200-500 NL / min.

7. The high-formability high-carbon steel wire rod according to claim 1, characterized in that, After refining, the molten steel is continuously cast, and the cross-sectional dimensions of the continuously cast billet are (250~300)mm*(350~400)mm.

8. The high-formability high-carbon steel wire rod according to claim 1, characterized in that, The proportion of scrap steel in the smelting of molten iron and scrap steel is 4% to 9%.