Method for manufacturing steel bar or wire rod

Abstract

The present invention provides a method for manufacturing a steel bar or a wire rod, with which it is possible to obtain a steel bar and a wire rod that are excellent in terms of surface properties even in cases where a steel material that contains Cu, Sn, and Ni is used. Provided is a method for manufacturing a steel bar or a wire rod, in which a steel material that contains Cu, Sn, and Ni is heated in an atmosphere that contains oxygen, water vapor, and carbon dioxide, with the balance being made up of nitrogen and inevitable impurities, and the heated steel material is subsequently rolled so as to form a bar or a wire rod. In this method for manufacturing a steel bar or a wire rod, the heating temperature T (°C), the oxygen concentration VO (vol%), the water vapor concentration VW (vol%), and the carbon dioxide concentration VC (vol%) during the heating satisfy specific conditions.

Description

Manufacturing method of steel bar or wire rod

[0001] The present invention relates to a method for manufacturing steel bars or wire rods.

[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. ２ With a view to reducing emissions, a review of manufacturing methods is also being considered, and there is a demand to reuse 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 being included in the steel materials 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 may 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 be 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 1150 to 1250°C.

[0011] Japanese Patent Laid-Open No. 6-297026 Japanese Patent Laid-Open No. 7-292413 Japanese Patent Laid-Open No. 2002-371337 Japanese Patent Laid-Open No. 2004-223523 Japanese Patent Laid-Open No. 2007-237194 Japanese Patent Laid-Open No. 2008-229652

[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 a steel bar or wire rod using a steel material containing the above-mentioned tramp elements Cu, Sn, and Ni.

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

[0014] Furthermore, in Patent Document 5, the reduction ratio in the first pass of hot rolling is reduced to prevent deterioration of surface quality. Therefore, if this method is used to suppress red brittleness, it is necessary to increase the reduction ratio in subsequent passes. However, since 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 likely to occur.

[0015] The present invention was developed in view of the above-described circumstances, and aims to provide a method for producing a steel bar or wire rod that can obtain 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 rate.

[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 a steel material and then rolling the steel material. When Cu and Sn are contained in the steel, the melting temperature of Cu is lowered due to the influence of Sn. As a result, under conventional general heating conditions, 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) On the other hand, 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 and delays 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 surface irregularities of the product increase.

[0020] (4) This leads to grain boundary cracking during rolling, and 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, the heating temperature T and oxygen concentration V 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 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 produce a steel bar or wire rod, wherein the heating temperature T (°C) in the heating satisfies the following formulas (1) and (2), and the oxygen concentration V in the atmosphere is 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(°C)=1050+90([Cu]+10×[Sn]−[Ni] / 2) (2) Vo≦V １ …(3) V １ (vol%)=8.6-10×[Cu]-5×[Sn]+[Ni]…(4) V ２ ≦V Ｗ ≦V ３ …(5) V ２ (vol%)=2.5+15×[Cu]+10×[Sn]-1.0×[Ni]…(6) V ３ (vol%)=10+60×[Cu]+40×[Sn]-4.0×[Ni]…(7) V ４ ≦V Ｃ ≦V ５ …(8) V ４ (vol%)=1.3+7.5×[Cu]+5×[Sn]-0.5×[Ni]…(9) V ５ (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. The method for producing a steel bar or wire rod according to item 1 above, wherein the high-temperature heating time t during which the steel material is heated to a temperature of 950°C or higher is equal to or less than A (minutes) defined by the following formula (11): A (minutes) = 210 - √[C](3[Si] + [Cr] + [Ni] - [Cu] / 2) ... (11) Here, the square brackets in the formula represent the content (mass%) of the element shown within the square brackets in the steel material, and are set to 0 if the element is not contained.

[0025] 3. The method for producing a steel bar or wire rod according to item 1 or 2 above, wherein the steel material contains, 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 to 2.50%, Al: 0.005 to 0.100%, and N: 0.002 to 0.030%, with the balance being Fe and unavoidable impurities, and wherein the value of X 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 method for producing a steel bar or wire rod according to claim 3, wherein the component ranges further contain at least one selected from the group consisting of, 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.

[0027] According to the present invention, deterioration of surface properties 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, etc. Furthermore, according to the present invention, scrap can be effectively used as a raw material, and therefore CO ２ It also contributes to reducing emissions.

[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 remove surface defects by treating the surface of the steel material, such as by grinding.

[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 chemical composition of the steel material is not particularly limited, it is preferable to use a steel material having the following chemical composition. The chemical composition of the steel bar and wire rod that are finally obtained is basically the same as the chemical 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, it is preferable that the C content be 0.03% or more. On the other hand, if the C content is excessively high, the machinability decreases and the machinability also deteriorates. Therefore, it is preferable that the C content be 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, since 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 the 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 inevitable 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, the lower limit of the P content is not limited and 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 embrittlement. 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 the lower limit of the Cu content is not particularly limited, 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 and has the effect of increasing hardenability and deepening 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 together 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, in order to fully obtain the above-mentioned effects of adding Sn, the Sn content is preferably 0.0001% or more.

[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 element for deoxidation, and this effect is exhibited when added in an amount of 0.005% or more. Furthermore, Al bonds 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 does this effect saturate, but defects such as blowholes 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. In addition, Mo is expensive, which increases costs. Therefore, the Mo content is preferably 0.80% or less. On the other hand, there is no particular lower limit for the Mo content, but 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 is an element that improves the hardenability of steel and, like Si and Cr, has the effect of increasing temper softening resistance. V also forms carbonitrides and suppresses grain coarsening. However, if the V content exceeds 0.20%, the above effect saturates, 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, it is preferable to set the V content to 0.03% or more in order 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, in order to fully obtain the above effect, the Nb content is preferably 0.005% or more.

[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 life. 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 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, there is no particular lower limit for the B content, but 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 a 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, and 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 described within 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 In order 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 studies by the present inventors, it was found that the optimal heating temperature depends on the contents of Cu, Sn, and Ni in the steel material. In order to prevent the occurrence of surface defects, the heating temperature T (°C) in the heating needs to satisfy the following formulas (1) and (2): (Y - 30) ≦ T ≦ (Y + 30) ... (1) Y (°C) = 1050 + 90 ([Cu] + 10 × [Sn] - [Ni] / 2) ... (2) Here, the square brackets in the above formulas represent the content (mass %) of the element listed 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, and the surface quality of the product deteriorates. 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. is accelerated, 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] Heating atmosphere The atmosphere (furnace atmosphere) during heating affects the generation 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 conducted by the present inventors, it has been 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≦V １…(3) V １ =8.6-10×[Cu]-5×[Sn]+[Ni]…(4) V ２ ≦V Ｗ ≦V ３ …(5) V ２ =2.5+15×[Cu]+10×[Sn]-1.0×[Ni]…(6) V ３ =10+60×[Cu]+40×[Sn]-4.0×[Ni]…(7) V ４ ≦V Ｃ ≦V ５ …(8) V ４ =1.3+7.5×[Cu]+5×[Sn]−0.5×[Ni]…(9) V ５ = 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] 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 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 (hereinafter referred to as high-temperature heating time) during which the steel material temperature is 950°C or higher 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 A (minutes) or less, as 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 written in the square brackets in the steel material, and are set to 0 when 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, wire, or wire rod. The rolling conditions are not particularly limited, and the rolling can be carried out according to a conventional method.

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

[0064] In Tables 1 to 3 and Tables 4 to 6 described later, the values ​​of Y and A are rounded to the nearest tenth for convenience. The X value is rounded to the nearest tenth for convenience. In Tables 1 to 6, V １ , V ２ , V ３ , V ４ , V ５ The figures are rounded to the nearest tenth.

[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 Quality) The surface quality of the 60 mm diameter round steel bars was evaluated by magnetic leakage flux testing (MLFT). Specifically, MLFT was performed on each of the manufactured round steel bars, 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 the defects were detected relative to the number of manufactured round steel bars (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 amount). Note that if the defect occurrence amount was 1.00 or less per meter, it was determined that good surface quality was obtained.

[0069] (Total decarburized layer depth) For each of the obtained steel bars and wire rods, the total decarburized layer depth (DM-T) 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 produced 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 inferior surface properties. Furthermore, when the high-temperature heating time t satisfied the preferred conditions, surface decarburization was also suppressed.

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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 produce a steel bar or wire rod, wherein the heating temperature T (°C) in the heating satisfies the following formulas (1) and (2), and the oxygen concentration V in the atmosphere is 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(°C)=1050+90([Cu]+10×[Sn]−[Ni] / 2) (2) Vo≦V １ …(3) V １ (vol%)=8.6-10×[Cu]-5×[Sn]+[Ni]…(4) V ２ ≦V Ｗ ≦V ３ …(5) V ２ (vol%)=2.5+15×[Cu]+10×[Sn]-1.0×[Ni]…(6) V ３ (vol%)=10+60×[Cu]+40×[Sn]-4.0×[Ni]…(7) V ４ ≦V Ｃ ≦V ５ …(8) V ４ (vol%)=1.3+7.5×[Cu]+5×[Sn]-0.5×[Ni]…(9) V ５ (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.

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

3. A method for producing a steel bar or wire rod according to claim 1 or 2, wherein the steel material contains, 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 to 2.50%, Al: 0.005 to 0.100%, and N: 0.002 to 0.030%, with the balance being Fe and unavoidable impurities, and wherein the value of X, as 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. A method for producing a steel bar or wire rod according to claim 3, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of 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.

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