Manufacturing method of bar product

Abstract

To provide a manufacturing method capable of manufacturing a bar product in which crystal grains are sufficiently refined.SOLUTION: The manufacturing method is a method for manufacturing a bar product having a hollow or solid shape from a bar material having a hollow or solid shape corresponding to the shape of the bar product, and includes a preparation step (#5), a heating step (#10), a rolling step (#15), and a cooling step (#20). In the preparation step (#5), a bar material composed of Nb-containing low-alloy steel is prepared. In the heating step (#10), the bar material is heated to a temperature of 710°C or higher and an Ac1 point or lower. In the rolling step (#15), the bar product is manufactured by subjecting the bar material to inclined rolling by using an inclined rolling machine including a plurality of inclined rolls disposed at an inclination angle of 7° or more and 10° or less around a pass line. At this time, a temperature of the bar product is raised to a temperature equal to or higher than an Ac3 point and lower than 1,050°C due to processing heat generation during the inclined rolling. In the cooling step (#20), the bar product is water-cooled. At this time, the water cooling is started within 30 seconds from the end of the rolling step.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing bar products, and more particularly, to a method for manufacturing bar products having a hollow or solid shape.

Background Art

[0002] Examples of bar products having a hollow or solid shape include steel pipes and steel bars. This type of bar product is usually manufactured by hot working. Bar products are often required to have strength, toughness, corrosion resistance, etc. Grain refinement is effective for meeting these requirements.

[0003] For example, in oil wells and gas wells (hereinafter referred to as "oil wells"), oil well pipes are used as bar products, and seamless steel pipes are used as oil well pipes. Oil wells are often installed in a corrosive environment containing corrosive gases such as hydrogen sulfide. Therefore, seamless steel pipes are required to have excellent SSC (sulfide stress cracking) resistance. Naturally, seamless steel pipes are also required to have high strength. However, SSC resistance and strength are generally conflicting properties. Grain refinement contributes to the compatibility of these conflicting properties.

[0004] Also, for example, in machines, automobiles, buildings, etc., steel pipes and steel bars are used as bar products. Such bar products are required to have high strength and further excellent hydrogen embrittlement cracking resistance. However, hydrogen embrittlement cracking resistance and strength are generally conflicting properties. Grain refinement contributes to the compatibility of these conflicting properties.

[0005] Conventionally, various techniques have been proposed for manufacturing bar products having fine crystal grains. For example, Japanese Patent No. 2576254 (Patent Document 1), Japanese Patent No. 2591234 (Patent Document 2), and Japanese Patent No. 2913115 (Patent Document 3) disclose a manufacturing method including a step of inclined rolling a bar material into a bar product using an inclined rolling mill equipped with a plurality of inclined rolls. In this manufacturing method, during inclined rolling, the temperature of the bar material is raised to a temperature equal to or higher than the Ac3 point by utilizing the heat generated during processing, whereby the structure composed of ferrite is once reversely transformed into austenite. Patent Documents 1 to 3 describe that if the bar product obtained by such inclined rolling is cooled, it is possible to cause fine ferrite to appear.

[0006] However, in the manufacturing methods of Patent Documents 1 to 3, since the materials of the bar product and the bar material are not particularly limited, sufficient refinement of the crystal grains may be difficult. Further, in the inclined rolling mill used in the manufacturing methods of Patent Documents 1 to 3, the inclination angles of the respective inclined rolls are relatively large. In this case, since the bar material slips with respect to each inclined roll during inclined rolling, the manufacture of the bar product itself may become difficult.

[0007] Also, for example, International Publication No. 2019 / 107409 (Patent Document 4) discloses a manufacturing method including a heating step of heating a hollow pipe (bar material), a rolling step of stretch-rolling (inclined rolling) the hollow pipe using an elongator (inclined rolling mill) equipped with a plurality of inclined rolls, and a cooling step of water-cooling the stretch-rolled hollow pipe (bar product) immediately after completion of rolling. In this manufacturing method, the bar material is made of low alloy steel having a predetermined chemical composition. This low alloy steel contains Nb. In the heating step, the bar material is heated to 800 to 1030°C. In the rolling step, during inclined rolling using the inclined rolling mill, the bar material generates heat during processing and the temperature of the bar material rises. In this case, Nb carbides and Nb carbonitrides are formed in the bar material during heating and inclined rolling. Due to the pinning effect of these Nb carbides and the like, coarsening of the crystal grains is suppressed, and the crystal grains are refined in the bar product.

[0008] However, in the manufacturing method of Patent Document 4, the heating temperature of the bar stock is 800 - 1030°C, and it is assumed that due to the heat generated during inclined rolling, the temperature of the bar stock reaches 1050°C, that is, exceeds the melting point of Nb carbide, etc. In this case, more or less Nb carbide, etc. dissolves in the steel, and there is a risk that the pinning effect will decrease. Therefore, the refinement of crystal grains in the bar product is restricted.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0010] As described above, in any of the manufacturing methods of Patent Documents 1 to 4, it is difficult to manufacture a bar product with sufficiently refined crystal grains.

[0011] An object of the present disclosure is to provide a manufacturing method of a bar product capable of manufacturing a bar product with sufficiently refined crystal grains.

Means for Solving the Problems

[0012] The manufacturing method of the bar product according to the present disclosure is a method of manufacturing a bar product having a hollow or solid shape from a bar material having a hollow or solid shape corresponding to the shape of the bar product. The manufacturing method of the bar product includes a preparation step, a heating step, a rolling step, and a cooling step. The preparation step prepares a bar material made of low alloy steel containing Cr, Mo, and Nb. The heating step heats the bar material to a temperature of 710 °C or higher and below the Ac1 point. The rolling step uses an inclined rolling mill including a plurality of inclined rolls arranged at an inclination angle of 7 ° or more and 10 ° or less around the pass line to perform inclined rolling on the bar material to manufacture a bar product. In this rolling step, the temperature of the bar product is raised to 1050 °C or lower and above the Ac3 point due to the heat generated during the inclined rolling. The cooling step water-cools the bar product. In this cooling step, water cooling is started within 30 seconds from the end point of the rolling step.

Effects of the Invention

[0013] According to the manufacturing method of the bar product according to the present disclosure, a bar product with sufficiently refined crystal grains can be manufactured.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

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Figure 4

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Figure 8

Mode for Carrying Out the Invention

[0015] In order to solve the above problems, the present inventors have intensively studied, and as a result, obtained the following findings.

[0016] As a basic process, when hot-working a bar stock into a bar product, if the ferrite (α) structure is reversely transformed into an austenite (γ) structure and the bar product is rapidly cooled after hot-working to freeze the structure, a bar product with refined crystal grains can be manufactured. That is, by hot-working, a bar product having a temperature of 3 points or more above Ac3 is manufactured, and this bar product may be water-cooled. At that time, if coarsening of crystal grains is suppressed during hot-working and further coarsening of crystal grains is suppressed even after hot-working, sufficient refinement of crystal grains can be realized.

[0017] In order to suppress coarsening of crystal grains during hot-working, it is sufficient that the heating temperature of the bar stock, that is, the temperature of the bar stock before hot-working, is 1 point or less below Ac1. However, in this case, it is necessary to raise the temperature of the bar stock to 3 points or more above Ac3 during hot-working. For this reason, as hot-working, inclined rolling using an inclined rolling mill may be applied. In inclined rolling, since an extremely large shear strain is applied to the bar stock by a plurality of inclined rolls arranged around the pass line, the heat generation during processing is remarkable. Therefore, according to inclined rolling, during hot-working (inclined rolling), the temperature of the bar stock can be raised to 3 points or more above Ac3 by utilizing remarkable heat generation during processing. That is, by inclined rolling, a bar product having a temperature of 3 points or more above Ac3 can be manufactured.

[0018] Also, to suppress the coarsening of crystal grains during hot working (slant rolling), a low alloy steel containing Cr and Mo and further containing Nb may be applied as a bar product, that is, a bar material. In this specification, this low alloy steel may sometimes be referred to as "Nb-containing low alloy steel". In a bar material made of Nb-containing low alloy steel, Nb forms Nb carbides and Nb carbonitrides during heating and slant rolling. These Nb carbides and the like are fine grains, and the coarsening of crystal grains is suppressed by the pinning effect of the Nb carbides and the like. That is, the inclusion of Nb contributes to the refinement of crystal grains in the bar product.

[0019] Here, the melting point of Nb carbides and the like is 1050°C. When the temperature of the bar material rises above the Ac3 point and further rises above 1050°C due to the heat generated during slant rolling, the Nb carbides and the like dissolve in the steel, and the pinning effect decreases. In this case, the coarsening of crystal grains progresses. Therefore, it is necessary to keep the temperature of the bar material by slant rolling below 1050°C. For this reason, in slant rolling, the inclination angle of each slant roll may be 7° or more. If the inclination angle of each slant roll is too small, the shear strain applied to the bar material during slant rolling extremely increases, and the heat generation during processing becomes excessive. As a result, the temperature of the bar material may exceed 1050°C.

[0020] On the other hand, if the inclination angle of each slant roll is too large, the bar material slips with respect to each slant roll during slant rolling. In this case, the production of the bar product itself becomes impossible. For this reason, in slant rolling, the inclination angle of each slant roll may be 10° or less.

[0021] Also, although it can be said that the heating temperature of the bar material may be below the Ac1 point, if it is too low, the deformation resistance of the bar material increases, and the bar material sticks to the slant roll. In this case too, the production of the bar product itself becomes impossible. For this reason, the heating temperature of the bar material may be 710°C or more.

[0022] Furthermore, in order to suppress the coarsening of crystal grains after hot working (skew rolling), it is necessary to cool the bar product to a temperature below the Ar1 point at an early stage after skew rolling. This is because if the temperature of the bar product after skew rolling exceeds the Ar1 point for a long time, the crystal grains will gradually grow. Therefore, water cooling may be started within 30 seconds from the end point of skew rolling for the bar product.

[0023] In short, in order to manufacture a bar product with sufficiently refined crystal grains, the following configuration may be adopted. As the bar material, a Nb-containing low alloy steel capable of obtaining a pinning effect is applied, and as the hot working, skew rolling capable of obtaining significant processing heat generation is applied. On that premise, the heating temperature of the bar material subjected to skew rolling is limited to 710 °C or higher and below the Ac1 point, the inclination angle of each inclined roll used for skew rolling is limited to 7 ° or higher and 10 ° or lower, and the temperature of the bar product manufactured by skew rolling is limited to above the Ac3 point and less than 1050 °C. Furthermore, for the bar product manufactured by skew rolling, the start timing of water cooling is limited to within 30 seconds from the end point of skew rolling.

[0024] The piercing rolling method according to the embodiment of the present disclosure has been completed based on the above findings.

[0025] The manufacturing method of the bar product according to the present embodiment is a method for manufacturing a bar product having a hollow or solid shape from a bar material having a hollow or solid shape corresponding to the shape of the bar product. The manufacturing method of the bar product includes a preparation step, a heating step, a rolling step, and a cooling step. The preparation step prepares a bar material made of a low alloy steel containing Cr, Mo, and Nb. The heating step heats the bar material to a temperature of 710 °C or higher and below the Ac1 point. The rolling step uses a skew rolling mill including a plurality of inclined rolls arranged at an inclination angle of 7 ° or higher and 10 ° or lower around the pass line to skew roll the bar material to manufacture a bar product. In this rolling step, the temperature of the bar product is raised to above the Ac3 point and less than 1050 °C due to the heat generation during skew rolling. The cooling step water-cools the bar product. In this cooling step, water cooling is started within 30 seconds from the end point of the rolling step (first configuration).

[0026] In the manufacturing method according to the first configuration, a preparation step, a heating step, a rolling step, and a cooling step are carried out in this order. Specifically, in the preparation step, as the bar material, an Nb-containing low alloy steel capable of obtaining a pinning effect is applied. In the rolling step, as hot working, inclined rolling capable of obtaining significant processing heat generation is applied. On that premise, in the heating step, the heating temperature of the bar material to be subjected to inclined rolling is limited to 710 °C or higher and below the Ac1 point. In the rolling step, the inclination angle of each inclined roll used for inclined rolling is limited to 7 ° or more and 10 ° or less, and the temperature of the bar product manufactured by inclined rolling is limited to 3 points above Ac and less than 1050 °C. Further, in the cooling step, for the bar product manufactured by inclined rolling, the start timing of water cooling is limited to within 30 seconds from the end point of the rolling step. Therefore, according to the manufacturing method according to the first configuration, as described above, a bar product with sufficiently refined crystal grains can be manufactured.

[0027] In the above manufacturing method, for example, the bar material and the bar product have a hollow shape. In this case, the inclined rolling mill further includes a plug disposed on the pass line between the inclined rolls (second configuration).

[0028] In the second configuration, the bar product is a steel pipe, and a hollow plain pipe can be used as the bar material. In the rolling step, the inclined rolling mill is an elongator, and the bar material (hollow plain pipe) is stretch-rolled by each inclined roll and the plug, whereby the bar product (steel pipe) is manufactured.

[0029] In the manufacturing method according to the first configuration, the bar material and the bar product may have a solid shape. In this case, the number of inclined rolls is preferably three (third configuration).

[0030] In the third configuration, the bar product is a steel bar, and a round billet can be used as the bar material. In the rolling step, the bar material (round billet) is diameter-reduced and rolled by the inclined rolls, whereby the bar product (steel bar) is manufactured. At that time, since three inclined rolls are used, a solid round billet can be sufficiently diameter-reduced and rolled, and the cross-sectional shape of the manufactured bar product is likely to be close to a circle.

[0031] In the above manufacturing method, preferably, the chemical composition of the low alloy steel is by mass percentage, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 to 1.50%, Nb: 0.01 to 0.05%, B: 0.0003 to 0.0050%, Ti: 0 to 0.050%, V: 0 to 0.30%, Ca: 0 to 0.0050%, rare earth elements: 0 to 0.0050%, and the balance consists of Fe and impurities (the fourth configuration).

[0032] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. For the same or corresponding configurations in each figure, the same reference numerals are given, and duplicate explanations will not be repeated.

[0033] (First Embodiment) With reference to FIGS. 1 to 4, a method for manufacturing a bar product according to the first embodiment will be described. In the first embodiment, a bar product having a hollow shape is manufactured from a bar material having a hollow shape corresponding to the shape of the bar product. In this case, for example, the bar product is a steel pipe, and a hollow plain pipe can be used as the bar material.

[0034] FIG. 1 is a flowchart showing a manufacturing method according to the first embodiment. As shown in FIG. 1, the manufacturing method includes a preparation step (#5), a heating step (#10), a rolling step (#15), and a cooling step (#20). The preparation step (#5), the heating step (#10), the rolling step (#15), and the cooling step (#20) are carried out in this order. The preparation step (#5) prepares a bar material (hollow pipe). The heating step (#10) heats the bar material to a predetermined temperature. The rolling step (#15) is a hot working process, and uses a skew rolling mill equipped with a plurality of inclined rolls to skew roll the bar material to manufacture a bar product (steel pipe). The cooling step (#20) water cools the bar product. Hereinafter, the details of each step (#5, #10, #15, and #20) will be described in detail.

[0035] [Preparation Step (#5)] The preparation step (#5) prepares a bar material made of a low alloy steel containing Cr, Mo, and Nb. Specifically, the preparation step (#5) prepares a hollow pipe made of an Nb-containing low alloy steel. The Nb-containing low alloy steel is a Cr-Mo steel containing Cr and Mo, and further contains Nb as a specific element. The hollow pipe as the bar material can be manufactured, for example, by the Mannesmann piercing method.

[0036] [Heating Step (#10)] The heating step (#10) heats the bar material to a temperature of 710 °C or higher and below the Ac1 point. Specifically, the heating step (#10) heats the hollow pipe to a temperature of 710 °C or higher and below the Ac1 point. That is, the heating temperature of the hollow pipe as the bar material is limited to 710 °C or higher and below the Ac1 point. In the case of the Nb-containing low alloy steel, the Ac1 point is approximately 760 °C. The heating of the hollow pipe can be carried out, for example, using a heating furnace.

[0037] [Rolling Step (#15)] The rolling step (#15) uses a skew rolling mill to skew roll the bar material to manufacture a bar product. In this embodiment, since the bar product is a steel pipe and the bar material is a hollow pipe, the skew rolling mill is an elongator. Therefore, the rolling step (#15) uses an elongator to stretch and roll the hollow pipe to manufacture a steel pipe.

[0038] FIG. 2 and FIG. 3 are schematic views showing the tilting rolling mill 100 used in the rolling process in the manufacturing method according to the first embodiment. FIG. 2 is a top view of the tilting rolling mill 100, and FIG. 3 is a side view of the tilting rolling mill 100. In FIGS. 2 and 3, a part of the bar stock (hollow pipe) WP during tilting rolling is shown in a longitudinal section. In this specification, the longitudinal section means a section including the pass line PL. The cross section means a section perpendicular to the pass line PL.

[0039] Referring to FIGS. 2 and 3, the tilting rolling mill (elongator) 100 includes, as rolling tools, two tilting rolls 1 and a plug 2. Each tilting roll 1 has a cone shape or a barrel shape. The two tilting rolls 1 are arranged at equal intervals around the pass line PL. In the example shown in FIG. 2, the two tilting rolls 1 are arranged on the left and right of the pass line PL, respectively, and face each other with the pass line PL in between.

[0040] The central axis 1c of each tilting roll 1 is inclined with respect to the pass line PL. That is, as shown in FIG. 3, each tilting roll 1 is given an inclination angle FA. The inclination angle FA means the deflection angle of the central axis 1c of the tilting roll 1 in the circumferential direction centered on the pass line PL. Further, as shown in FIG. 2, each tilting roll 1 may be given an intersection angle CA. The intersection angle CA means the deflection angle of the central axis 1c of the tilting roll 1 in the radial direction centered on the pass line PL.

[0041] The plug 2 is arranged on the pass line PL between the tilting rolls 1. The plug 2 is held by a mandrel 3 extending along the pass line PL.

[0042] The tilting rolling mill 100 having two tilting rolls 1 as described above is called a two-roll type tilting rolling mill. The two-roll type tilting rolling mill 100 usually includes guide tools (e.g., disk rolls, guide shoes) not shown. The guide tools are provided between the tilting rolls 1 around the pass line PL. The guide tools play a role of restricting the overhang of the bar stock (hollow pipe) WP during tilting rolling.

[0043] The rolling process (#15) is carried out using such an inclined rolling mill (elongator) 100. Specifically, a bar material (hollow pipe) WP heated to a predetermined temperature in the heating process (#10) is placed on the pass line PL. The bar material WP is sent between the rotating inclined rolls 1 by a pusher or a conveying roller and engages with the inclined rolls 1. Then, the bar material WP rotates around its own axis on the pass line PL while advancing due to contact with the rotating inclined rolls 1 and is stretch-rolled by each inclined roll 1 and the plug 2. Thereby, a bar product (steel pipe) BP having a predetermined wall thickness and outer diameter is obtained.

[0044] FIG. 4 is a schematic diagram showing the state of the rolling process in the manufacturing method according to the first embodiment. In FIG. 4, a part of the bar material (hollow pipe) WP during inclined rolling is shown in cross section. As shown in FIG. 4, the bar material WP is strongly pressed and rolled by the rotating inclined roll 1 and the plug 2. At that time, the peripheral speed of the inclined roll 1 located on the outer surface side of the bar material WP is significantly different from the peripheral speed of the plug 2 located on the inner surface side of the bar material WP. Due to this peripheral speed difference, in the circumferential direction of the bar material WP, the material flow on the outer surface side of the bar material WP is in the opposite direction to the material flow on the inner surface side of the bar material WP. Therefore, in the circumferential direction of the bar material WP, the material flows on the outer surface side and the inner surface side of the bar material WP are extremely large with respect to the center of the wall thickness of the bar material WP (see the solid arrows in FIG. 4). Thereby, an extremely large shear strain is imparted to the bar material WP. Moreover, when the bar material WP is inclined and rolled, for example, compared with the case of simply rolling a plate, as the bar material WP rotates around its axis, the bar material WP comes into contact with the inclined roll 1 many times and is rolled in a short time. Therefore, during inclined rolling in the rolling process (#15), the processing heat generation is remarkable.

[0045] In the rolling process (#15), the inclined rolling mill 100 is supplied with a bar stock (hollow pipe) WP that has been heated to a temperature of 710 °C or higher and below the Ac1 point in the heating process (#10). Then, by inclined rolling using the inclined rolling mill 100, the temperature of the bar stock WP is raised to 1050 °C or higher and less than 1050 °C by utilizing significant processing heat generation. That is, in the rolling process (#15), the temperature of the bar product BP produced by inclined rolling is limited to 1050 °C or higher and less than 1050 °C. In the case of Nb-containing low alloy steel, the Ac3 point is approximately 835 °C.

[0046] Here, referring to FIG. 3, each inclined roll 1 is arranged at an inclination angle FA of 7 ° or more and 10 ° or less around the pass line PL. That is, in the rolling process (#15), the inclination angle FA of each inclined roll 1 is limited to 7 ° or more and 10 ° or less. This is to prevent the temperature of the bar product BP from unexpectedly exceeding 1050 °C during inclined rolling and to prevent the bar product BP from being impossible to manufacture by inclined rolling.

[0047] Specifically, during inclined rolling, the forward speed of the bar stock WP caused by contact with the rotating inclined roll 1 is generally lower than the theoretical speed derived from the dimensions, position, and rotational speed of the inclined roll 1. If the inclination angle FA of each inclined roll 1 is too small, the forward speed of the bar stock WP tends to decrease significantly. In this case, the number of times the bar stock WP contacts the inclined roll 1 during inclined rolling increases. As a result, the shear strain applied to the bar stock WP increases extremely, and the processing heat generation becomes excessive. Excessive processing heat generation leads to an excessive temperature rise of the bar stock WP. Therefore, in order to prevent the temperature of the bar product BP from unexpectedly exceeding 1050 °C, the inclination angle FA of each inclined roll 1 is set to 7 ° or more.

[0048] On the other hand, if the inclination angle FA of each inclined roll 1 is too large, the bar stock WP slips relative to each inclined roll 1 during inclined rolling. In this case, the bar stock WP adheres to the inclined roll 1, and the bar product BP cannot be manufactured at all. Therefore, in order to prevent the bar product BP from being impossible to manufacture by inclined rolling, the inclination angle FA of each inclined roll 1 is set to 10 ° or less.

[0049] Returning to FIG. 1, as described above, in the rolling process (#15), the inclined rolling mill 100 is supplied with the bar stock (hollow tube) WP heated to 710° C. or higher in the heating process (#10). Generally, if the heating temperature of the bar stock WP is too low, the deformation resistance of the bar stock WP increases. In this case, during inclined rolling, the bar stock WP sticks to the inclined roll 1, making it impossible to manufacture the bar product BP itself. In this regard, if the heating temperature of the bar stock WP is 710° C. or higher, it is possible to prevent the impossibility of manufacturing the bar product BP itself.

[0050] In the rolling process (#15), the elongation ratio is, for example, 1.20 or more and 2.80 or less. The elongation ratio means the value of the ratio of the length of the bar stock WP after inclined rolling to the length of the bar stock WP before inclined rolling.

[0051] [Cooling Process (#20)] Referring to FIG. 1, the cooling process (#20) water-cools the bar product (steel pipe) BP manufactured by the rolling process (#15). Specifically, the cooling process (#20) starts water-cooling within 30 seconds from the end point of the rolling process (#15). That is, for the bar product BP manufactured by inclined rolling, the start timing of water-cooling is limited to within 30 seconds from the end point of the rolling process (#15). In this specification, the end point of the rolling process (#15) means the time when the rear end of the bar stock WP passes the position of the inclined roll 1. Water-cooling of the bar product BP can be typically performed by immersing the bar product BP in a water tank. The bar product BP may be water-cooled by spraying water on the bar product BP.

[0052] In this specification, the temperature of the bar stock WP means the temperature of the outer surface of the bar stock WP. Similarly, the temperature of the bar product BP means the temperature of the outer surface of the bar product BP. These temperatures can be detected, for example, by infrared thermography or a radiation thermometer.

[0053] [Effect] In the manufacturing method according to this embodiment, a preparation step (#5), a heating step (#10), a rolling step (#15), and a cooling step (#20) are carried out in this order. Specifically, in the preparation step (#5), as the bar material WP, an Nb-containing low alloy steel capable of obtaining a pinning effect is applied. In the rolling step (#15), as hot working, inclined rolling capable of obtaining significant processing heat generation is applied. On that premise, in the heating step (#10), the heating temperature of the bar material WP to be subjected to inclined rolling is limited to 710 °C or higher and below the Ac1 point. In the rolling step (#15), the inclination angle of each inclined roll 1 used for inclined rolling is limited to 7 ° or higher and 10 ° or lower, and the temperature of the bar product BP manufactured by inclined rolling is limited to 3 points above Ac and less than 1050 °C. Further, in the cooling step (#20), for the bar product BP manufactured by inclined rolling, the start timing of water cooling is limited to within 30 seconds from the end point of the rolling step (#15). In this case, during the rolling step (#15) performed hot, the ferrite (α) structure can be reversely transformed into the austenite (γ) structure, and by the cooling step (#20) after the rolling step (#15), the bar product BP can be water-cooled to freeze the structure. At that time, the pinning effect due to the inclusion of Nb can be effectively exerted to refine the crystal grains, and furthermore, coarsening of the crystal grains can be suppressed during and after inclined rolling. Moreover, it is not impossible to manufacture the bar product BP itself during inclined rolling. Therefore, according to the manufacturing method according to this embodiment, a bar product BP with sufficiently refined crystal grains can be manufactured.

[0054] In this embodiment, the bar product BP is a steel pipe, and a hollow pipe is used as the bar material WP. In the rolling step (#15), the inclined rolling mill 100 is an elongator including two inclined rolls 1 and a plug 2, and the hollow pipe as the bar material WP is stretch-rolled by each inclined roll 1 and the plug 2. Thereby, a steel pipe with sufficiently refined crystal grains can be manufactured as the bar product BP.

[0055] [Regarding Nb-containing low alloy steel] The chemical composition of the Nb-containing low alloy steel applied in the manufacturing method according to this embodiment contains, for example, the following elements.

[0056] C: 0.21 to 0.35% Carbon (C) enhances the hardenability of steel and increases the strength of steel. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, the susceptibility to weld cracking of steel increases. If the C content is too high, furthermore, the toughness of steel may decrease. Therefore, the C content is 0.21 to 0.35%. The preferable lower limit of the C content is 0.23%, more preferably 0.25%. The preferable upper limit of the C content is 0.30%, more preferably 0.27%.

[0057] Si: 0.10 to 0.50% Silicon (Si) deoxidizes steel. Si further increases the strength of steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the SSC resistance and workability of steel decrease. Therefore, the Si content is 0.10 to 0.50%. The preferable lower limit of the Si content is 0.15%, more preferably 0.20%. The preferable upper limit of the Si content is 0.40%, more preferably 0.35%.

[0058] Mn: 0.05 to 1.00% Manganese (Mn) enhances the hardenability of steel and increases the strength of steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, Mn segregates at the grain boundaries and the SSC resistance of steel decreases. Therefore, the Mn content is 0.05 to 1.00%. The preferable lower limit of the Mn content is 0.30%, more preferably 0.40%. The preferable upper limit of the Mn content is 0.95%, more preferably 0.90%.

[0059] P: 0.025% or less Phosphorus (P) is an impurity and is inevitably contained in steel. Therefore, the P content is more than 0%. P segregates at grain boundaries, thereby reducing the SSC resistance of the steel. Therefore, the P content is 0.025% or less. The preferable upper limit of the P content is 0.020%, and more preferably 0.015%. The lower the P content, the better. However, an extreme reduction in the P content leads to an increase in manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the P content is 0.001%, and more preferably 0.002%.

[0060] S: 0.010% or less Sulfur (S) is an impurity and is inevitably contained in steel. Therefore, the S content is more than 0%. S combines with Mn to form sulfide-based inclusions, thereby reducing the SSC resistance of the steel. Therefore, the S content is 0.010% or less. The preferable upper limit of the S content is 0.006%, and more preferably 0.003%. The lower the S content, the better. However, an extreme reduction in the S content leads to an increase in manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the S content is 0.001%, and more preferably 0.002%.

[0061] Al: 0.005 - 0.100% Aluminum (Al) deoxidizes the steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, the effect saturates. If the Al content is too high, furthermore, a large number of coarse Al-based oxides are generated and the SSC resistance of the steel decreases. Therefore, the Al content is 0.005 - 0.100%. The preferable lower limit of the Al content is 0.010%, and more preferably 0.020%. The preferable upper limit of the Al content is 0.070%, and more preferably 0.050%. In this specification, the Al content means the content of so-called acid-soluble Al (sol.Al).

[0062] N: 0.010% or less Nitrogen (N) is inevitably contained in steel. Therefore, the N content is more than 0%. N forms nitrides. Fine nitrides prevent the coarsening of crystal grains. Therefore, N may be contained. On the other hand, coarse nitrides reduce the SSC resistance of steel. Therefore, the N content is 0.010% or less. The preferable upper limit of the N content is 0.004%, and more preferably 0.003%. The preferable lower limit of the N content for obtaining the pinning effect due to the precipitation of fine nitrides is 0.002%. Note that an extreme reduction in the N content leads to an increase in the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the N content is 0.001%, and more preferably 0.002%.

[0063] Cr: 0.05 - 1.50% Chromium (Cr) enhances the hardenability of steel and increases the strength of steel. If the Cr content is too low, this effect cannot be obtained. On the other hand, if the Cr content is too high, the SSC resistance of steel decreases. Therefore, the Cr content is 0.05 - 1.50%. The preferable lower limit of the Cr content is 0.20%, and more preferably 0.40%. The preferable upper limit of the Cr content is 1.20%, and more preferably 1.15%.

[0064] Mo: 0.10 - 1.50% Molybdenum (Mo) enhances the hardenability of steel and increases the strength of steel. Mo further enhances the temper softening resistance of steel and the SSC resistance by high-temperature tempering. If the Mo content is too low, this effect cannot be obtained. On the other hand, if the Mo content is too high, the effect saturates and the manufacturing cost increases. Therefore, the Mo content is 0.10 - 1.50%. The preferable lower limit of the Mo content is 0.15%, and more preferably 0.20%. The preferable upper limit of the Mo content is 0.80%, and more preferably 0.60%.

[0065] Nb: 0.01 - 0.05% Niobium (Nb) combines with C and N during heating or inclined rolling to form fine Nb carbides etc. (Nb carbides and Nb carbonitrides). These Nb carbides etc. refine the crystal grains due to the pinning effect, enhancing the SSC resistance of the steel or the hydrogen embrittlement cracking resistance of the steel. These carbonitrides etc. further suppress the variation in crystal grain size. If the Nb content is too low, this effect cannot be obtained. On the other hand, if the Nb content is too high, a large number of coarse Nb-based inclusions are generated, reducing the SSC resistance and the hydrogen embrittlement cracking resistance. Therefore, the Nb content is 0.01 - 0.05%. The preferable lower limit of the Nb content is 0.02%. The preferable upper limit of the Nb content is 0.04%.

[0066] B: 0.0003 - 0.0050% Boron (B) enhances the hardenability of the steel and increases the strength of the steel. If the B content is too low, this effect cannot be obtained. On the other hand, if the B content is too high, carbonitrides precipitate at the grain boundaries, reducing the SSC resistance of the steel. Therefore, the B content is 0.0003 - 0.0050%. The preferable lower limit of the B content is 0.0005%, more preferably 0.0008%. The preferable upper limit of the B content is 0.0030%, more preferably 0.0020%.

[0067] In the manufacturing method according to this embodiment, the remainder of the chemical composition of the Nb-containing low alloy steel consists of Fe and impurities. Here, the impurities are those mixed in from ores, scraps, or the manufacturing environment etc. as raw materials when industrially manufacturing the Nb-containing low alloy steel, and are those allowed within a range that does not adversely affect the Nb-containing low alloy steel. Among the impurities, the oxygen (O) content is 0.005% or less.

[0068] [Regarding optional elements] The chemical composition of the above Nb-containing low alloy steel may further contain one or more of Ti and V in place of a part of Fe.

[0069] Ti: 0 - 0.050% Titanium (Ti) is an optional element and may not be contained. Therefore, the Ti content may be 0%. When contained, it combines with C and N to form fine Ti carbonitrides and fixes N, which is an impurity. The formation of Ti nitrides refines the crystal grains and further increases the strength of the steel. When B is contained in the steel, Ti further suppresses the formation of B nitrides, thus promoting the improvement of hardenability by B. If the Ti content is too low, these effects cannot be obtained. On the other hand, if the Ti content is too high, Ti dissolves in the Nb-based inclusions, causing the Nb-based inclusions to coarsen. In this case, the SSC resistance and hydrogen embrittlement crack resistance of the steel decrease. Therefore, the Ti content is 0 to 0.050%. The preferable lower limit of the Ti content for more effectively obtaining the above effects is 0.002%, more preferably 0.004%. The preferable upper limit of the Ti content is 0.035%, more preferably 0.030%.

[0070] V: 0 to 0.30% Vanadium (V) is an optional element and may not be contained. Therefore, the V content may be 0%. When contained, V forms fine carbides to increase the tempering softening resistance and enable high-temperature tempering. Thereby, the SSC resistance of the steel is increased. However, if the V content is too high, excessive carbides are formed and the SSC resistance of the steel decreases instead. Therefore, the V content is 0 to 0.30%. The preferable lower limit of the V content for more effectively obtaining the above effects is 0.01%, more preferably 0.02%. The preferable upper limit of the V content is 0.25%, more preferably 0.20%.

[0071] The chemical composition of the above Nb-containing low alloy steel may further contain one or more selected from the group consisting of Ca and rare earth elements in place of a part of Fe.

[0072] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be contained. For this reason, Ca may be 0%. When contained, Ca spheroidizes sulfide-based inclusions in the steel. Thereby, the SSC resistance of the steel is enhanced. Even if a little Ca is contained, the above effect can be obtained. However, if the Ca content is too high, excessive inclusions are generated and the SSC resistance of the steel decreases. Therefore, the Ca content is 0 to 0.0050%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0015%. The preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%.

[0073] Rare earth elements (REM): 0 to 0.0050% Rare earth elements (REM) are optional elements and may not be contained. For this reason, REM may be 0%. When contained, REM spheroidizes sulfide-based inclusions in the steel. Thereby, the SSC resistance of the steel is enhanced. Even if a little REM is contained, the above effect can be obtained. However, if the REM content is too high, excessive inclusions are generated and the SSC resistance of the steel decreases. Therefore, the REM content is 0 to 0.0050%. The preferable lower limit of the REM content is 0.0001%, more preferably 0.0010%. The preferable upper limit of the REM content is 0.0040%, more preferably 0.0030%.

[0074] In this specification, REM means at least one or more elements of scandium (Sc), yttrium (Y), and lanthanoids (lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71). The REM content means the total content of these elements.

[0075] (Second Embodiment) Referring to FIGS. 5 to 8, a method for manufacturing a bar product according to the second embodiment will be described. In the second embodiment, a bar product having a solid shape is manufactured from a bar material having a solid shape corresponding to the shape of the bar product. In this case, for example, the bar product is a bar steel, and a solid round billet can be used as the bar material. Therefore, the manufacturing method of the second embodiment is different from the manufacturing method of the first embodiment in that the bar product and the bar material have a solid shape. Along with this difference, the manufacturing method of the second embodiment is different from the manufacturing method of the first embodiment in that an inclined rolling mill 100A (see FIGS. 6 and 7) is used in the rolling process (#15A) (see FIG. 5) as described later.

[0076] FIG. 5 is a flowchart showing the manufacturing method according to the second embodiment. As shown in FIG. 5, the manufacturing method includes a preparation process (#5A), a heating process (#10A), a rolling process (#15A), and a cooling process (#20A). The preparation process (#5A) prepares a bar material (round billet). The heating process (#10A) heats the bar material to a predetermined temperature. The rolling process (#15A) is a hot working process, and uses an inclined rolling mill having a plurality of inclined rolls to inclinedly roll the bar material to manufacture a bar product (bar steel). The cooling process (#20A) water-cools the bar product. Hereinafter, the details of each process (#5A, #10A, #15A, and #20A) will be described in detail.

[0077] [Preparation Process (#5A)] The preparation process (#5A) prepares a solid round billet as the bar material. The bar material (round billet) is made of an Nb-containing low alloy steel, similar to the first embodiment.

[0078] [Heating Process (#10A)] The heating process (#10A) heats the round billet as the bar material to a temperature of 710°C or higher and below the Ac1 point, similar to the first embodiment.

[0079] [Rolling Process (#15A)] The rolling process (#15A) uses an inclined rolling mill to inclinedly roll the round billet as the bar material to manufacture a bar steel as the bar product.

[0080] Figures 6 and 7 are schematic views showing the tilt rolling mill 100A used in the rolling process in the manufacturing method according to the second embodiment. Figure 6 is a perspective view of the tilt rolling mill 100A, and Figure 7 is a top view of the tilt rolling mill 100A. Only one tilt roll 1 arranged vertically above the pass line PL is shown in Figure 7, and the illustration of the two tilt rolls 1 arranged below is omitted.

[0081] Referring to Figure 6, the tilt rolling mill 100A includes three tilt rolls 1 as rolling tools. The three tilt rolls 1 are arranged at equal intervals around the pass line PL. That is, the three tilt rolls 1 are arranged at intervals of 120° from each other. One of the three tilt rolls 1 is arranged directly above the pass line PL (vertically upward). However, as long as the three tilt rolls 1 are arranged at equal intervals around the pass line PL, the positions of the three tilt rolls 1 are not limited. For example, one tilt roll 1 may be arranged directly below the pass line PL (vertically downward).

[0082] As shown in Figure 7, the central axis 1c of each tilt roll 1 is inclined with respect to the pass line PL, similar to the first embodiment. That is, an inclination angle FA is given to each tilt roll 1. Further, similar to the first embodiment, a crossing angle may be given to each tilt roll 1.

[0083] The tilt rolling mill 100A having three tilt rolls 1 in this way is called a three-roll type tilt rolling mill.

[0084] The rolling process (#15A) is carried out using such a tilt rolling mill 100A. Specifically, the bar stock (round billet) WPA heated to a predetermined temperature in the heating process (#10A) is placed on the pass line PL. The bar stock WPA is sent between the rotating tilt rolls 1 by a pusher or a conveying roller and engages with the tilt rolls 1. Then, the bar stock WPA rotates around its own axis while advancing on the pass line PL due to contact with the rotating tilt rolls 1, and is subjected to diameter-reducing rolling by each tilt roll 1. Thereby, a bar product (bar steel) BPA having a predetermined outer diameter is obtained.

[0085] FIG. 8 is a schematic view showing a state of a rolling process in the manufacturing method according to the second embodiment. In FIG. 8, a bar stock (round billet) WPA during inclined rolling is shown in a cross section. As shown in FIG. 8, the bar stock WPA is strongly pressed and rolled by the rotating inclined rolls 1. At that time, in the circumferential direction of the bar stock WPA, the material flow on the outer surface side of the bar stock WPA is extremely large with respect to the axis of the bar stock WP (see the solid line arrow in FIG. 8). As a result, an extremely large shear strain is imparted to the bar stock WPA. Therefore, as in the first embodiment, during inclined rolling in the rolling process (#15A), the heat generation during processing is remarkable.

[0086] In the rolling process (#15A), as in the first embodiment, by inclined rolling using the inclined rolling mill 100A, the temperature of the bar stock WPA is raised to 1050 ° C or less and more than the Ac3 point by utilizing remarkable heat generation during processing. At that time, referring to FIG. 7, as in the first embodiment, each inclined roll 1 is arranged at an inclination angle FA of 7 ° or more and 10 ° or less around the pass line PL.

[0087] [Cooling process (#20A)] Referring to FIG. 5, the cooling process (#20A) starts water cooling within 30 seconds from the end point of the rolling process (#15A), as in the first embodiment.

[0088] [Effect] In the manufacturing method according to the second embodiment, as in the first embodiment, the preparation process (#5A), the heating process (#10A), the rolling process (#15A), and the cooling process (#20A) are performed in this order. Therefore, according to the manufacturing method according to the second embodiment, the same effects as those of the first embodiment can be achieved.

[0089] In particular, in the second embodiment, the bar product BPA is a bar steel, and a round billet is used as the bar stock WPA. In the rolling process (#15A), the round billet as the bar stock WPA is reduced in diameter and rolled by each inclined roll 1, and the bar steel as the bar product BPA is manufactured. At that time, If a two-roll type inclined rolling mill is used, the cross-sectional shape of the produced bar product will be elliptical. In this regard, in the second embodiment, since a three-roll type inclined rolling mill 100A equipped with three inclined rolls 1 is used, a solid round billet can be sufficiently reduced in diameter by rolling, and the cross-sectional shape of the produced bar product (bar steel) WPA is likely to be close to circular.

Example

[0090] To verify the effect of the manufacturing method according to the first embodiment, a bar product was manufactured from a bar material along the flow shown in FIG. 1. The bar product was a steel pipe, and the bar material was a hollow plain pipe. For inclined rolling, a two-roll type inclined rolling mill 100 shown in FIGS. 2 and 3 was used. During the test, a plurality of hollow plain pipes were prepared. Table 1 shows the chemical compositions of three types of low alloy steels applied to the hollow plain pipes.

[0091]

Table 1

[0092] As shown in Table 1, Steel grades A and C were Cr-Mo steels containing Nb. On the other hand, Steel grade B was a Cr-Mo steel not containing Nb. For all of Steel grades A to C, the Ac1 point was 760 °C and the Ac3 point was 835 °C. Table 2 shows three types of setting conditions applied to inclined rolling.

[0093]

Table 2

[0094] As shown in Table 2, regarding the dimensions of the hollow plain pipes, for all of setting conditions a to c, the outer diameter was 65 mm and the wall thickness was 22 mm. The length of the hollow plain pipes was 300 mm for all of setting conditions a to c. Based on setting condition a, the target elongation ratio was decreased in the order of setting condition b and setting condition c. Specifically, in setting condition b, the plug diameter was smaller compared to setting condition a. In setting condition c, furthermore, the plug diameter was smaller compared to setting condition b.

[0095] For each hollow pipe with a test number, heating, inclined rolling (elongation rolling), and cooling were performed. At that time, the temperature of the hollow pipe before inclined rolling, the temperature of the hollow pipe (steel pipe) after inclined rolling, and the inclination angle of the inclined roll were variously changed. The temperature of the hollow pipe before inclined rolling was measured by infrared thermography on the inlet side of the inclined rolling mill. The temperature of the hollow pipe after inclined rolling was measured by infrared thermography on the outlet side of the inclined rolling mill. Here, the outer surface of the hollow pipe is often partially covered with oxide scale, and the temperature of the oxide scale is detected as low. Therefore, the hollow pipe was photographed from above with an infrared thermograph, and the highest temperature in the photographed field of view was adopted as the temperature of the hollow pipe. And within 30 seconds after the rear end of the hollow pipe (steel pipe) passed through the position of the inclined roll, water cooling was started. The water cooling was performed by immersing the steel pipe in a water tank.

[0096] For each steel pipe with a test number obtained in such a test, in order to determine whether the crystal grains were sufficiently refined, a test piece was taken from the central part of the wall thickness of the steel pipe, and the prior austenite grain size with a crystal grain size of less than 10 μm was investigated. This prior austenite grain size was calculated by reconstructing the austenite structure before transformation from the crystal orientation analysis results by EBSD (electron backscatter diffraction analysis method) as described in Patent Document 4 (austenite reconstruction method). Table 3 shows the test results.

[0097]

Table 3

[0098] As shown in Table 3, in Test No. 1, the temperature of the hollow steel pipe before rolling was too high, and the inclination angle of the inclined roll was also too large. Therefore, seizure occurred during inclined rolling, making rolling impossible. In Test Nos. 2 to 4, the temperature of the hollow steel pipe before rolling was too high, and the temperature of the hollow steel pipe after rolling was also too high. Therefore, the crystal grain size exceeded 5.0 μm. In Test Nos. 5 and 6, the temperature of the hollow steel pipe before rolling was too high. Therefore, the crystal grain size exceeded 5.0 μm. In Test No. 9, the hollow steel pipe did not contain Nb. Therefore, the crystal grain size exceeded 5.0 μm. In Test No. 13, the temperature of the hollow steel pipe after rolling was too low. Therefore, the crystal grain size exceeded 5.0 μm. In Test No. 15, the inclination angle of the inclined roll was too large. Therefore, seizure occurred during inclined rolling, making rolling impossible. In Test No. 17, the inclination angle of the inclined roll was too small, and the temperature of the hollow steel pipe after rolling was too high. Therefore, the crystal grain size exceeded 5.0 μm. In Test No. 18, the temperature of the hollow steel pipe before rolling was too low. Therefore, seizure occurred during inclined rolling, making rolling impossible.

[0099] In contrast, in Test Nos. 7, 8, 10, 11, 12, 14, and 16, all the requirements defined by the manufacturing method of the present embodiment were satisfied. Therefore, the crystal grain size was as fine as 5.0 μm or less.

[0100] Furthermore, for Test Nos. 7 and 10, separately from what was shown in Table 3, water cooling was started 35 seconds after the rear end of the hollow steel pipe (steel tube) passed through the position of the inclined roll. In this case, since the start timing of water cooling deviated from the requirement (within 30 seconds from the end point of inclined rolling) defined by the manufacturing method of the present embodiment, the crystal grain size exceeded 5.0 μm.

[0101] Therefore, it was demonstrated that the manufacturing method of the present embodiment can produce a steel pipe with sufficiently refined crystal grains.

Example

[0102] To verify the effects of the manufacturing method according to the second embodiment, a bar product was manufactured from a bar material along the flow shown in FIG. 5. The bar product was a steel bar, and the bar material was a solid round billet. For the inclined rolling, a three-roll type inclined rolling mill 100A shown in FIGS. 6 and 7 was used. In the test, a plurality of round billets were prepared. Three types of low alloy steels shown in Table 1 were applied to the round billets. Table 4 shows the three types of setting conditions applied to the inclined rolling.

[0103]

Table 4

[0104] As shown in Table 4, regarding the dimensions of the round billets, for all of the setting conditions a1 to c1, the outer diameter was 50 mm and the length was 300 mm. Based on the setting condition a1, the target elongation ratio was decreased in the order of the setting condition b1 and the setting condition c1. Specifically, in the setting condition b1, the interval between the inclined rolls was widened compared to the setting condition a1. In the setting condition c1, further, the interval between the inclined rolls was widened compared to the setting condition b1.

[0105] For each round billet of each test number, heating, inclined rolling (diameter reduction rolling), and cooling were performed. At that time, the temperature of the round billet before inclined rolling, the temperature of the round billet (steel bar) after inclined rolling, and the inclination angle of the inclined rolls were variously changed. Similar to Example 1, the temperature of the round billet before inclined rolling was measured by infrared thermography on the inlet side of the inclined rolling mill. The temperature of the round billet after inclined rolling was measured by infrared thermography on the outlet side of the inclined rolling mill. Then, similar to Example 1, water cooling was started within 30 seconds after the rear end of the round billet (steel bar) passed through the position of the inclined rolls.

[0106] For each steel bar of each test number obtained in such a test, in order to determine whether or not the crystal grains were sufficiently refined, a test piece was sampled from the 1 / 2 radius portion of the steel bar, and the prior austenite grain size with a crystal grain diameter of less than 10 μm was investigated. This prior austenite grain size was calculated by the same method as in Example 1 (austenite reconstruction method). Table 5 shows the test results.

[0107]

Table 5

[0108] As shown in Table 5, in Test No. 101, the temperature of the round billet before rolling was too high, and the tilt angle of the tilt roll was also too large. Therefore, seizure occurred during tilt rolling, making rolling impossible. In Test No. 102, the temperature of the round billet before rolling was too high, and the temperature of the round billet after rolling was also too high. Therefore, the crystal grain size exceeded 5.0 μm. In Test Nos. 103 and 104, the temperature of the round billet before rolling was too high. Therefore, the crystal grain size exceeded 5.0 μm. In Test No. 108, the temperature of the round billet after rolling was too low. Therefore, the crystal grain size exceeded 5.0 μm. In Test No. 111, the round billet did not contain Nb. Therefore, the crystal grain size exceeded 5.0 μm. In Test No. 113, the temperature of the round billet after rolling was too high. Therefore, the crystal grain size exceeded 5.0 μm. In Test No. 114, the temperature of the round billet before rolling was too low. Therefore, seizure occurred during tilt rolling, making rolling impossible.

[0109] In contrast, in Test Nos. 105, 106, 107, 109, 110, and 112, all the requirements defined by the manufacturing method of the present embodiment were satisfied. Therefore, the crystal grain size was as fine as 5.0 μm or less.

[0110] Therefore, it was demonstrated that the manufacturing method of the present embodiment can produce bar steel with sufficiently refined crystal grains.

[0111] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof. For example, in the tilt rolling mills 100 and 100A, the number of tilt rolls 1 may be plural. Specifically, in the tilt rolling mill (elongator) 100 of the first embodiment, the number of tilt rolls 1 may be three.

Explanation of Symbols

[0112] 100, 100A: Inclined Rolling Machine 1: Inclined Roll 2: Plug 3: Mandrel WP, WPA: Bar Stock BP, BPA: Bar Product FA: Inclination Angle

Claims

1. A method for manufacturing a bar product having a hollow or solid shape from a bar stock having a hollow or solid shape corresponding to the shape of the bar product, The manufacturing method of the bar product is as follows, A preparation step of preparing the bar stock made of low alloy steel containing Cr, Mo, and Nb, A heating step of heating the bar stock to a temperature of 710 °C or higher and below the Ac1 point, A rolling step of manufacturing the bar product by inclined rolling the bar stock using an inclined rolling mill equipped with a plurality of inclined rolls arranged at an inclination angle of 7 ° or more and 10 ° or less around the pass line, wherein the temperature of the bar product is raised to 1050 °C by the heat generated during the inclined rolling. above the Ac3 point and less than 1050 °C, said rolling step; A cooling step of water-cooling the bar product, wherein water-cooling is started within 30 seconds from the end of the rolling step, said cooling step, A method for manufacturing a bar product comprising:

2. The method for manufacturing a bar product according to claim 1, The bar stock and the bar product have a hollow shape, The inclined rolling mill further includes a plug disposed on the pass line between the inclined rolls. A method for manufacturing a bar product.

3. The method for manufacturing a bar product according to claim 1, The bar stock and the bar product have a solid shape, The number of the inclined rolls is three. A method for manufacturing a bar product.

4. The method for manufacturing a bar product according to any one of claims 1 to 3, The chemical composition of the low alloy steel is, By mass%, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 to 1.50%, Nb: 0.01 to 0.05%, B: 0.0003 to 0.0050%, Ti: 0 to 0.050%, V: 0 to 0.30%, Ca: 0 to 0.0050%, Rare earth elements: 0 to 0.0050%, and, The balance consists of Fe and impurities. A method for manufacturing a bar product.

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