Manufacturing method of steel bar or wire rod

Abstract

The present invention provides a method for producing a steel bar or wire rod that can produce a steel bar or wire rod with excellent surface properties even when a steel material containing Cu, Sn, and Ni is used. The method comprises heating a steel material containing Cu, Sn, and Ni in an atmosphere containing oxygen, water vapor, and carbon dioxide, with the balance being nitrogen and unavoidable impurities, and then rolling the heated steel material to produce a steel bar or wire rod. The heating temperature T (°C), oxygen concentration V O (vol%), water vapor concentration V W (vol%), and carbon dioxide concentration V C A method for manufacturing steel bars or wire rods in which the (vol%) satisfies certain conditions.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a steel bar or wire rod. [Background technology]

[0002] Steel bars and wire rods used in automobiles, industrial equipment, etc. are manufactured with the aim of achieving high performance, high strength, and high quality. However, there has been a recent review of manufacturing methods from the perspective of reducing CO2 emissions, and there is a demand for recycling scrap as a raw material.

[0003] Scrap used as raw material generally contains electrical wiring, plated products, etc., and therefore contains Cu, Ni, and Sn. These elements, known as tramp elements, are difficult to remove, and therefore inevitably end up in the steel material used to manufacture steel bars and wire rods.

[0004] However, when steel materials containing these elements are used to manufacture steel bars or wire rods, the surface characteristics of the final product can deteriorate. Therefore, various technologies have been proposed to prevent the deterioration of surface characteristics caused by these elements.

[0005] For example, Patent Document 1 proposes adding Si to a steel material containing Cu and Sn to prevent cracks during hot working.

[0006] Patent Document 2 proposes that when hot rolling steel containing Cu and Sn, the steel is cooled with cooling water immediately before the hot rolling to prevent the occurrence of surface defects.

[0007] Patent Document 3 proposes that the heating temperature when Cu-containing steel is heated prior to hot working be 1050 to 1200°C.

[0008] Patent Document 4 proposes that when Cu-containing steel is heated prior to hot rolling, the heating temperature or oxygen concentration is controlled to prevent deterioration of the surface properties.

[0009] Patent Document 5 proposes that when hot rolling steel containing Cu, the rolling temperature and rolling reduction in the first pass are controlled to prevent deterioration of the surface properties.

[0010] Patent Document 6 proposes that the heating temperature when heating steel containing Cu and Sn prior to hot rolling be set to 1150 to 1250°C. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 6-297026 [Patent Document 2] Japanese Patent Application Publication No. 7-292413 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-371337 [Patent Document 4] Japanese Patent Application Laid-Open No. 2004-223523 [Patent Document 5] Japanese Patent Application Laid-Open No. 2007-237194 [Patent Document 6] Japanese Patent Application Laid-Open No. 2008-229652 Summary of the Invention [Problem to be solved by the invention]

[0012] However, the conventional techniques proposed in Patent Documents 2 to 4 and 6 have not been able to sufficiently prevent the deterioration of surface properties when producing steel bars or wire rods using steel materials containing the above-mentioned tramp elements Cu, Sn, and Ni.

[0013] Furthermore, although Patent Document 1 prevents cracking by adding Si, the Si content of the product cannot be freely adjusted because it is regulated by JIS standards, etc. In addition, Si reduces workability, so the addition of Si alone cannot sufficiently prevent deterioration of surface properties.

[0014] Furthermore, in Patent Document 5, deterioration of surface quality is prevented by reducing the reduction ratio in the first pass of hot rolling. Therefore, if this method is used to suppress red brittleness, it is necessary to increase the reduction ratio in subsequent passes. However, because the temperature of the steel material is relatively low during rolling in the subsequent passes, deformation is unlikely to occur, and as a result, shape defects are more likely to occur.

[0015] The present invention was developed in view of the above circumstances, and aims to provide a method for producing a steel bar or wire rod that can produce a steel bar or wire rod with excellent surface properties even when a steel material containing Cu, Sn, and Ni is used, without resorting to methods such as adding Si or controlling the rolling reduction ratio. [Means for solving the problem]

[0016] As a result of extensive research conducted by the present inventors to solve the above problems, the following findings were obtained.

[0017] (1) Generally, steel bars and wire rods are manufactured by first heating the steel material and then rolling it. If the steel contains Cu and Sn, the Sn influences the Cu melting temperature, lowering it. As a result, under conventional heating conditions, the Cu melts and is left behind at the interface between the base steel and the scale.

[0018] (2) The Cu left at the interface is either incorporated into the scale as an alloy or concentrated at the grain boundaries in the surface layer of the base steel. Which reaction occurs depends on the temperature and atmosphere at the time. This is because the balance between the rate of scale formation and the rate of Cu diffusion changes depending on the temperature and atmosphere.

[0019] (3) Meanwhile, like Cu, Ni also melts and is left behind at the interface between the base steel and the scale. The left-behind Ni inhibits the penetration of molten Cu into the grain boundaries, delaying embrittlement. However, the presence of Ni at the interface increases the difference in oxidation rate between the grain boundaries and within the grains, and the higher the temperature, the more intense the oxidation at the grain boundaries. As a result, the product surface becomes more uneven.

[0020] (4) This can lead to grain boundary cracking during rolling, or scale adhesion due to unevenness at the interface between the base steel and the scale, which can result in scale being left behind during descaling, making it more likely that scale marks and overlaps will occur.

[0021] (5) Therefore, depending on the content of Cu, Sn, and Ni in the steel material, the heating temperature T and oxygen concentration V O , water vapor concentration V W , and carbon dioxide concentration V C By appropriately controlling the temperature, it is possible to produce steel bars or wire rods with excellent surface properties.

[0022] The present invention has been completed based on the above findings, and the gist and configuration of the present invention are as follows.

[0023] 1. A method for producing a steel bar or wire rod, comprising heating a steel material containing Cu, Sn, and Ni in an atmosphere containing oxygen, water vapor, and carbon dioxide, with the balance being nitrogen and unavoidable impurities, and then rolling the heated steel material to form a steel bar or wire rod, The heating temperature T (°C) in the heating satisfies the following formulas (1) and (2): The oxygen concentration V in the atmosphere O (vol%), water vapor concentration V W (vol%), and carbon dioxide concentration V C (vol%) satisfies the following formulas (3) to (10): (Y-30)≦T≦(Y+30) …(1) Y(℃)=1050+90([Cu]+10×[Sn]-[Ni] / 2) …(2) Vo≦V1…(3) V1(vol%)=8.6-10×[Cu]-5×[Sn]+[Ni] …(4) V2≦V W ≦V3…(5) V2(vol%)=2.5+15×[Cu]+10×[Sn]-1.0×[Ni] …(6) V3(vol%)=10+60×[Cu]+40×[Sn]-4.0×[Ni] …(7) V4≦V C ≦V5…(8) V4(vol%)=1.3+7.5×[Cu]+5×[Sn]-0.5×[Ni]…(9) V5(vol%)=5.0+30×[Cu]+20×[Sn]-2.0×[Ni] …(10) Here, the square brackets in the above formula represent the content (mass %) of the element written in the square brackets in the steel material.

[0024] 2. A method for producing a steel bar or wire rod as described in 1 above, wherein the high-temperature heating time t during the heating, during which the temperature of the steel material is 950°C or higher, is A (minutes) or less as defined by the following formula (11): A(min)=210-√[C](3[Si]+[Cr]+[Ni]-[Cu] / 2) …(11) Here, the square brackets in the above formula represent the content (mass%) of the element described in the square brackets in the steel material, and are set to 0 if the element is not contained.

[0025] 3. The steel material is, in mass%, C: 0.03 to 0.80%, Si: 0.03 to 2.00% Mn: 0.30 to 2.00% P: 0.100% or less, S: 0.400% or less, Cu: 0.50% or less, Ni: 2.0% or less, Sn: 0.050% or less, Cr: 0.03~2.50%, Al: 0.005 to 0.100%, and N: 0.002 to 0.030% The balance is Fe and unavoidable impurities, and 3. The method for producing a steel bar or wire rod according to 1 or 2 above, wherein the steel bar or wire rod has a chemical composition in which the X value defined by the following formula (12) is less than 0.60: X = [Cu] + 10 [Sn] - [Ni] / 2 … (12) Here, the square brackets in the above formula represent the content (mass %) of the element written in the square brackets in the steel material.

[0026] 4. The component range is in mass%. Mo: 0.80% or less V: 0.20% or less, Nb: 0.060% or less, Ti: 0.200% or less, B: 0.0080% or less, and Sb: 0.0050% or less 4. The method for producing a steel bar or wire rod according to 3 above, further comprising at least one selected from the group consisting of: [Effects of the Invention]

[0027] According to the present invention, deterioration of surface quality when producing steel bars or wire rods using steel materials containing Cu, Sn, and Ni can be effectively prevented without resorting to methods such as adding Si or controlling the rolling reduction. The steel bars or wire rods produced by the method of the present invention can be suitably used as materials for automobiles, industrial machinery, and the like. Furthermore, according to the present invention, scrap can be effectively utilized as a raw material, which also contributes to reducing CO2 emissions. DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention will be described in detail below. Note that the following description is of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below.

[0029] In a manufacturing method according to one embodiment of the present invention, a steel material containing Cu, Sn, and Ni is heated in an atmosphere containing oxygen, water vapor, and carbon dioxide, with the balance being nitrogen and unavoidable impurities, and then the heated steel material is rolled into a bar or wire. Each step will be described below.

[0030] [Steel material] The steel material is not particularly limited, and any steel material containing Cu, Sn, and Ni can be used. A billet can be typically used as the steel material.

[0031] The steel material can be produced, for example, by blooming a square slab (bloom) obtained by continuous casting. The bloom may typically be a square slab with a side length of 250 mm or more. The billet obtained by blooming may be a square billet with a side length of 130 mm or more.

[0032] The blooming may be carried out continuously after continuous casting, or may be carried out after reheating to 1000°C or higher. Immediately after blooming, it is preferable to subject the surface of the resulting steel material to spalling. This spalling can remove decarburized layers and surface defects present on the surface of the steel material. Furthermore, it is preferable to treat the surface of the steel material by grinding or other treatment to remove surface defects.

[0033] In the present invention, the desired effect is achieved by controlling the heating conditions according to the composition of the steel material used. Therefore, although the composition of the steel material is not particularly limited, it is preferable to use a steel material having the following composition. The composition of the steel bar and wire rod that are finally obtained is basically the same as the composition of the steel material used. In this specification, the unit of content, "%", represents "mass %" unless otherwise specified.

[0034] C: 0.03 to 0.80% C is an element that has the effect of improving strength. From the viewpoint of increasing internal hardness and improving machinability, the C content is preferably 0.03% or more. On the other hand, if the C content is excessively high, the machinability decreases and the machinability also deteriorates. Therefore, the C content is preferably 0.80% or less.

[0035] Si: 0.03 to 2.00% Si is an element that can be added for deoxidation. To enhance the deoxidation effect, the Si content is preferably 0.03% or more. On the other hand, because Si increases the deformation resistance during forging, it is desirable to keep the content low. Therefore, the Si content is preferably 2.00% or less.

[0036] Mn: 0.30 to 2.00% Mn is an element that increases the hardness of a material. To achieve this effect, the Mn content is preferably 0.30% or more. On the other hand, excessive addition of Mn increases the deformation resistance during forging. Therefore, the Mn content is preferably 2.00% or less.

[0037] P:0.100% or less P is an element contained in steel as an unavoidable impurity. If the P content exceeds 0.100%, P segregates at grain boundaries, embrittling the grain boundaries and resulting in a deterioration in durability. Therefore, the P content is preferably 0.100% or less. On the other hand, there is no lower limit for the P content, and it may be 0%. However, since an excessive reduction leads to an increase in manufacturing costs, the P content is preferably 0.005% or more.

[0038] S: 0.400% or less S is an element contained in steel as an unavoidable impurity. Furthermore, S combines with Mn in steel to form MnS, which improves machinability. However, if the S content exceeds 0.400%, fatigue fracture originating from MnS occurs at low strength, resulting in a deterioration in fatigue strength. Therefore, the S content is preferably 0.400% or less. Meanwhile, the lower limit of the S content is not limited and may be 0%. However, from the viewpoint of improving machinability, the S content is preferably 0.003% or more.

[0039] Cu: 0.50% or less Cu is an element that dissolves in steel and has the effect of increasing the strength of the steel material. Cu is also a major cause of surface defects during rolling due to hot brittleness. If the Cu content exceeds 0.50%, strength increases excessively and manufacturability decreases. Therefore, the Cu content is preferably 0.50% or less. On the other hand, although there is no particular lower limit for the Cu content, from the viewpoint of the strength-improving effect of Cu addition, the Cu content is preferably 0.02% or more.

[0040] Ni: 2.0% or less Ni is an element that dissolves in steel to increase strength, improves hardenability, and deepens the depth of the surface hardened layer. Ni also has the effect of suppressing hot embrittlement caused by Cu. Therefore, in the present invention, Ni is added along with Cu to suppress the occurrence of surface defects. However, Ni is expensive, and excessive addition leads to increased costs. Therefore, the Ni content is preferably 2.0% or less. On the other hand, although there is no particular lower limit for the Ni content, it is preferable that the Ni content be 0.02% or more in order to fully obtain the above-mentioned effects of adding Ni.

[0041] Sn: 0.050% or less Sn is an element that dissolves in steel to increase its strength. However, if the Sn content exceeds 0.050%, the strength increases excessively and workability decreases. Therefore, the Sn content is preferably 0.050% or less. On the other hand, although there is no particular lower limit for the Sn content, the Sn content is preferably 0.0001% or more in order to fully obtain the above-mentioned effects of adding Sn.

[0042] Cr: 0.03 to 2.50% Cr is an element that increases hardness and temper softening resistance. To achieve these effects, the Cr content is preferably 0.03% or more. On the other hand, if the Cr content exceeds 2.50%, the effect of increasing softening resistance saturates. In addition, the hardenability becomes too high, resulting in a deterioration of internal toughness. As a result, fatigue cracks propagate more quickly, reducing bending fatigue strength. Therefore, the Cr content is preferably 2.50% or less.

[0043] Al: 0.005 to 0.100% Al is an effective deoxidizing element, and this effect is exhibited when added at 0.005% or more. Furthermore, Al combines with N to form AlN, which acts to suppress coarsening of crystal grains. Therefore, it is preferable that the Al content be 0.005% or more. On the other hand, if the Al content exceeds 0.100%, manufacturability decreases. Therefore, it is preferable that the Al content be 0.100% or less.

[0044] N: 0.002 to 0.030% N combines with Al to form AlN, which has the effect of suppressing grain coarsening and ensuring fatigue strength. To achieve this effect, the N content is preferably 0.002% or more. On the other hand, if the N content exceeds 0.030%, not only will the effect saturate, but defects such as blowholes will occur inside the component, reducing pitting resistance. Therefore, the N content is preferably 0.030% or less.

[0045] The steel material according to one embodiment of the present invention has a composition containing the above elements, with the balance being Fe and unavoidable impurities.

[0046] In another embodiment of the present invention, in order to further improve the properties, the above-mentioned composition may optionally further contain at least one element selected from the group consisting of Mo, V, Nb, Ti, B, and Sb. Note that these elements are not essential components, and therefore the lower limit of the content of each element may be 0%.

[0047] Mo: 0.80% or less Mo is an element that has the effect of improving hardenability. However, if the Mo content exceeds 0.80%, the hardenability becomes too high, making quench cracking more likely to occur. Furthermore, Mo is expensive, which increases costs. Therefore, the Mo content is preferably 0.80% or less. On the other hand, although there is no particular lower limit for the Mo content, in order to fully obtain the hardenability-improving effect of Mo, the Mo content is preferably 0.01% or more.

[0048] V: 0.20% or less V improves the hardenability of steel and, like Si and Cr, is an element that has the effect of increasing temper softening resistance. V also forms carbonitrides to inhibit grain coarsening. However, if the V content exceeds 0.20%, the above effects saturate, and an increase in the V content does not provide sufficient benefits, resulting in increased manufacturing costs. Therefore, the V content is preferably 0.20% or less. While there is no particular lower limit for the V content, the V content is preferably 0.03% or more to fully obtain the above effects.

[0049] Nb: 0.060% or less Nb is an element that refines crystal grains by forming carbonitrides and has the effect of ensuring fatigue properties. However, if the Nb content exceeds 0.060%, this effect saturates, leading to unnecessary cost increases. Therefore, the Nb content is preferably 0.060% or less. On the other hand, although there is no particular lower limit for the Nb content, the Nb content is preferably 0.005% or more in order to fully obtain the above effect.

[0050] Ti: 0.200% or less Ti is an element that produces fine Ti compounds in steel, reducing the size of crystal grains after forging and increasing strength. However, if the Ti content exceeds 0.200%, Ti precipitates become coarse and act as starting points for fatigue fracture, shortening the lifespan. Therefore, the Ti content is preferably 0.200% or less. On the other hand, although there is no particular lower limit for the Ti content, in order to fully obtain the above-mentioned effect, it is preferable that the Ti content be 0.005% or more.

[0051] B: 0.0080% or less B is an element that dissolves in steel and has the effect of improving hardenability. However, if the B content exceeds 0.0080%, the effect of adding B becomes saturated. Therefore, the B content is preferably 0.0080% or less. On the other hand, although there is no particular lower limit for the B content, in order to fully obtain the above effect, the B content is preferably 0.0005% or more.

[0052] Sb: 0.0050% or less Sb is an element that has the effect of suppressing decarburization of the surface of steel material and ensuring surface hardness. However, if the Sb content exceeds 0.0050%, this effect saturates. Therefore, the Sb content is preferably 0.0050% or less. On the other hand, although there is no particular lower limit for the Sb content, in order to fully obtain the effect of adding Sb, the Sb content is preferably 0.0005% or more.

[0053] X-value Surface defects caused by Cu occur due to interaction with Sn. Furthermore, the addition of Ni suppresses the occurrence of such surface defects. Therefore, in order to further improve the surface quality, the X value defined by the following formula is preferably less than 0.60, more preferably 0.50 or less, and even more preferably 0.40 or less. On the other hand, the lower limit of the X value is not particularly limited, but may be, for example, -1.00 or more, -0.90 or more, or -0.80 or more. X = [Cu] + 10 [Sn] - [Ni] / 2 … (12) Here, the square brackets in the above formula represent the content (mass %) of the element written in the square brackets in the steel material.

[0054] [heating] Next, the steel material is heated in an atmosphere containing oxygen, water vapor, and carbon dioxide, with the balance being nitrogen and unavoidable impurities. In the present invention, the temperature T and oxygen concentration V during heating are determined according to the contents of Cu, Sn, and Ni in the steel material. O , water vapor concentration V W , and carbon dioxide concentration V C It is important to control the above. The reason for this is explained below.

[0055] ·Heating temperature T To prevent the occurrence of surface defects during rolling, it is necessary to take measures against melt embrittlement caused by Cu, and to do so, it is necessary to appropriately control the heating temperature before rolling. However, as a result of the inventors' investigations, it was found that the optimal heating temperature depends on the contents of Cu, Sn, and Ni in the steel material. To prevent the occurrence of surface defects, the heating temperature T (°C) during the heating must satisfy the following equations (1) and (2): (Y-30)≦T≦(Y+30) …(1) Y(℃)=1050+90([Cu]+10×[Sn]-[Ni] / 2) …(2) Here, the square brackets in the above formula represent the content (mass %) of the element written in the square brackets in the steel material.

[0056] If the heating temperature T is lower than (Y - 30) °C, melt embrittlement of Cu occurs, deteriorating the surface quality of the product. On the other hand, if the heating temperature T is higher than (Y + 30) °C, melt embrittlement of Cu can be suppressed, but the progress of grain boundary oxidation due to the influence of Ni, Si, etc. will accelerate, resulting in severe surface irregularities. As a result, it becomes difficult to remove the primary scale before rolling, and dents and wrinkles are more likely to occur.

[0057] Heated atmosphere The atmosphere in which the heating is performed (furnace atmosphere) affects the formation of scale. The oxygen concentration V O (vol%), water vapor concentration V W (vol%), and carbon dioxide concentration V C If the concentration (vol%) is too low, the rate of scale formation slows down and Cu is not properly incorporated into the scale. On the other hand, if the concentration is too high, the rate of scale formation becomes excessive, resulting in a significant decrease in yield and deterioration of surface quality. Therefore, in order to obtain good surface quality, it is necessary to appropriately control the heating atmosphere before rolling. However, as a result of the research by the present inventors, it was found that the optimal atmosphere depends on the contents of Cu, Sn, and Ni in the steel material. Specifically, the oxygen concentration V in the atmosphere O (vol%), water vapor concentration V W (vol%), and carbon dioxide concentration V C (vol %) must satisfy the following formulas (3) to (10). Vo≦V1…(3) V1=8.6-10×[Cu]-5×[Sn]+[Ni] …(4) V2≦V W ≦V3…(5) V2=2.5+15×[Cu]+10×[Sn]-1.0×[Ni] …(6) V3=10+60×[Cu]+40×[Sn]-4.0×[Ni]…(7) V4≦V C ≦V5…(8) V4=1.3+7.5×[Cu]+5×[Sn]-0.5×[Ni]…(9) V5=5.0+30×[Cu]+20×[Sn]-2.0×[Ni]…(10) Here, the square brackets in the above formula represent the content (mass %) of the element written in the square brackets in the steel material.

[0058] In addition, the oxygen concentration V O The lower limit of is not particularly limited and may be 0 vol%, but is preferably 0.1 vol% or more, and more preferably 0.2 vol% or more.

[0059] ·High temperature heating time t The heating time for the above heating is not particularly limited. From the viewpoint of suppressing melt embrittlement by incorporating molten Cu into the scale, a longer heating time is desirable. However, if the steel material is exposed to high temperatures for an excessively long time, surface decarburization becomes significant. Excessive decarburization can adversely affect the product characteristics and subsequent workability, so it is desirable to suppress it. Since decarburization mainly occurs at high temperatures of 950°C or higher, it is desirable to control the time t during which the steel material temperature is 950°C or higher (hereinafter referred to as high-temperature heating time) in order to suppress decarburization. However, the likelihood of decarburization also depends on the chemical composition of the steel material. Therefore, it is preferable to set the high-temperature heating time t to be equal to or less than A (minutes) defined by the following formula (11): A=210-√[C](3[Si]+[Cr]+[Ni]-[Cu] / 2) …(11) Here, the square brackets in the above formula represent the content (mass%) of the element described in the square brackets in the steel material, and are set to 0 if the element is not contained.

[0060] On the other hand, the lower limit of the high-temperature heating time t is not particularly limited, but from the viewpoint of suppressing melt embrittlement, it is preferably 60 minutes or more, more preferably 80 minutes or more, and even more preferably 100 minutes or more.

[0061] [rolling] The heated steel material is then rolled to form a bar or wire. The rolling conditions are not particularly limited, and the rolling can be carried out according to a conventional method. [Example]

[0062] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0063] First, steel materials were prepared having the chemical compositions shown in Tables 1 to 3. Specifically, steels having the chemical compositions were melted and cast into rectangular slabs with cross-sectional dimensions of 400 mm x 300 mm. The rectangular slabs were subjected to blooming (primary rolling) to produce steel materials (rectangular billets) having a size of 160 mm square. The steel materials were then surface ground.

[0064] For convenience, Tables 1 to 3 and Tables 4 to 6 shown below show values ​​obtained by rounding off the first decimal place for Y and A. For convenience, values ​​obtained by rounding off the second decimal place for X values ​​are shown. For convenience, Tables 1 to 6 also show values ​​obtained by rounding off the first decimal place for V1, V2, V3, V4, and V5.

[0065] Next, the steel material was heated under the conditions shown in Tables 4 to 6, and then hot-rolled to produce round steel bars with a diameter of 60 mm, round steel bars with a diameter of 25 mm, and wire rods with a diameter of 7 mm. The 25 mm round steel bars were finally wound into a coil to form what is known as a bar-in-coil. Similarly, the 7 mm diameter wire rod was also wound into a coil to form a wire rod coil.

[0066] Thereafter, the surface properties and decarburization depth of each of the obtained steel bars and wire rods were evaluated by the following procedure.

[0067] (Surface texture) The surface quality of the 60 mm diameter round steel bars was evaluated using Magnetic Leakage Flux Testing (MLFT). Specifically, MLFT was performed on each of the multiple round steel bars produced, and the number of round steel bars in which defects with a depth of 0.1 mm or more and a length of 0.1 mm or more were detected was determined. Next, the percentage of the number of round steel bars in which such defects were detected relative to the total number of round steel bars produced (hereinafter referred to as the MLFT defect rate) was calculated. If the MLFT defect rate was 1.0% or less, it was determined that good surface quality was obtained.

[0068] The surface quality of the 25 mm diameter steel bar and 7 mm diameter wire rod was evaluated by magnetic particle inspection. Specifically, one ring of samples was taken from the front end, rear end, and middle position of the coil and pickled. Then, magnetic particle inspection was performed to measure the number of defects per unit length (hereinafter referred to as the defect occurrence rate). Note that if the defect occurrence rate was 1.00 or less per meter, it was determined that good surface quality was obtained.

[0069] (Total decarburized layer depth) The total decarburized layer depth (DM-T) of each of the obtained steel bars and wire rods was measured based on the measurement method using a microscope specified in JIS G 0558. If the total decarburized layer depth was 0.4 mm or less, it was determined that decarburization was suppressed.

[0070] The measurement results are shown in Tables 7 to 9. As can be seen from these results, the steel bars and wire rods manufactured under the conditions of the present invention had excellent surface properties. In particular, when the component compositions were within the preferred range, the surface properties were even better. In contrast, the comparative examples that did not satisfy the conditions of the present invention had significantly poorer surface properties. Furthermore, when the high-temperature heating time t satisfied the preferred conditions, surface decarburization was also suppressed.

[0071] [Table 1]

[0072] [Table 2]

[0073] Table 3

[0074] Table 4

[0075] Table 5

[0076] Table 6

[0077] Table 7

[0078] Table 8

[0079] Table 9

Claims

1. A method for producing a steel bar or wire rod, comprising heating a steel material containing Cu, Sn, and Ni in an atmosphere containing oxygen, water vapor, and carbon dioxide, with the balance being nitrogen and unavoidable impurities, and then rolling the heated steel material to form a steel bar or wire rod, The heating temperature T (°C) in the heating satisfies the following formulas (1) and (2): The oxygen concentration V in the atmosphere Ｏ (vol%), water vapor concentration V Ｗ (vol%), and carbon dioxide concentration V Ｃ (vol%) satisfies the following formulas (3) to (10): (Y-30)≦T≦(Y+30)…(1) Y (℃) = 1050 + 90 ([Cu] + 10 x [Sn] - [Ni] / 2) ... (2) Vo≦V １ …(3) V １ (vu%%)=8.6-10×[C]-5×[S]+[N] …(4) V ２ ≦V Ｗ ≦V ３ …(5) V ２ (vja%)=2.5+15×[C]+10×[S]-100×[N] …(6) V ３ (vj%%)=10+60×[C]+40×[N]-4.0×[N] …(7) V ４ ≦V Ｃ ≦V ５ …(8) V ４ (Vol%) = 1.3 + 7.5 × [Cu] + 5 × [Sn] - 0.5 × [Ni] ... (9) V ５ (vj%%)=5.0+30×[C]+20×[S]-200×[N] …(10) Here, the square brackets in the above formula represent the content (mass %) of the element written in the square brackets in the steel material.

2. The method for producing a steel bar or wire rod according to claim 1, wherein a high-temperature heating time t during the heating, during which the temperature of the steel material is 950°C or higher, is A (minutes) or less defined by the following formula (11): A (minutes) = 210-√[C] (3[Si]+[Cr]+[Ni]-[Cu] / 2)...(11) Here, the square brackets in the above formula represent the content (mass%) of the element described in the square brackets in the steel material, and if the element is not contained, it is set to 0.

3. The steel material is, in mass%, C: 0.03 to 0.80%, Si: 0.03-2.00%, Mn: 0.30-2.00%, P: 0.100% or less, S: 0.400% or less, Cu: 0.50% or less, Ni: 2.0% or less, Sn: 0.050% or less, Cr: 0.03-2.50%, Al: 0.005 to 0.100%, and N: 0.002-0.030% The balance is Fe and unavoidable impurities, and The method for producing a steel bar or wire rod according to claim 1 or 2, wherein the steel bar or wire rod has a component composition in which the X value defined by the following formula (12) is less than 0.60: X=[Cu]+10[Sn]-[Ni] / 2...(12) Here, the square brackets in the above formula represent the content (mass %) of the element written in the square brackets in the steel material.

4. The component composition is, in mass%, Mo: 0.80% or less, V: 0.20% or less, Nb: 0.060% or less, Ti: 0.200% or less, B: 0.0080% or less, and Sb: 0.0050% or less The method for producing a steel bar or wire rod according to claim 3, further comprising at least one selected from the group consisting of:

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