JP3407704B2 - Manufacturing method of high carbon seamless steel pipe - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a seamless steel pipe (seamless steel pipe) made of high carbon steel containing C: 0.95 to 1.10% by mass. The present invention relates to a method for manufacturing a seamless steel pipe that suppresses the amount of heat generated by machining so that inner surface flaws do not occur when Mannesmann piercing and rolling high C bearing steel. [0002] Steel pipes for bearings that are required to have the required hardness, strength, toughness, wear resistance, dimensional stability, fatigue strength, etc. after quenching and tempering in the final use state include C A seamless steel pipe made of so-called high carbon steel containing about 1.00% by mass is widely used. Usually, the seamless steel pipe manufacturing method includes a hot rolling pipe manufacturing method represented by Mannesmann pipe and a hot extrusion pipe manufacturing method represented by Eugene Sejurne pipe. When both are compared, the hot-rolled pipe making method is excellent in productivity, and in particular, the Mannesmann pipe has good dimensional accuracy, so this seamless pipe making method is frequently used. [0003] Seamless steel pipes produced by Mannesmann pipes are manufactured from hollow billets by means of piercing and rolling mills by Mannesmann Piercer from solid billets, and these pipes are expanded by means of rolling mills such as mandrel mills and plug mills. After reducing the wall thickness, the outer diameter is reduced by a drawing mill such as a stretch reducer and finished into a steel pipe. [0004] When Mannesmann pipes are produced in the hot state of seamless steel pipes, since the melting temperature of high carbon steel is low, the central part and the vicinity of the material to be rolled are melted due to processing heat generation or overheating. Internal defects may occur in later steel pipes. If an inner surface defect (hereinafter referred to as “inner surface flaw”) occurs in a hot-rolled steel pipe, not only the yield of the product is reduced, but also a piercing and rolling machine, a drawing mill and a draw rolling. In some cases, it is necessary to stop the entire pipe mill composed of the machine. In such a case, the production efficiency is significantly hindered. Conventionally, as a measure for preventing the occurrence of internal flaws in Mannesmann pipes made of seamless steel pipes, the workability during pipe making is reduced, or the soaking time is increased by preheating the billet. Means such as performing a soaking process for reforming are employed. However, although these measures have the effect of suppressing certain internal flaws, all of them are based on the assumption that the productivity of hot pipe production is reduced or the manufacturing costs are soaring. It is hard to say. [0006] As described above, the conventional measures for preventing internal flaws used in the production of seamless steel pipes made of high carbon steel are reduction of workability and soaking treatment. In essence, it is an impediment to efficient production. [0007] The present invention has been made in view of the above-mentioned problems, and when producing a seamless steel pipe made of high carbon steel used as a bearing steel as a material to be rolled, without inhibiting the efficient production. An object of the present invention is to provide a method for producing a high-carbon seamless steel pipe capable of preventing the occurrence of internal flaws. In order to solve the above-mentioned problems, the present inventor conducted various studies on measures for preventing internal flaws in Mannesmann pipes made of high-carbon steel to be rolled. It has been found that the billet heating temperature should be as low as possible in order to prevent the grain boundaries from melting during hot pipe making due to the low temperature. In order to further improve the prevention of internal flaws due to melting of the material to be rolled by further study, the heating temperature of the billet is reduced at the same time as the billet heating temperature is lowered. It is necessary to suppress the increase in the material temperature of the material to be rolled, and to suppress the heat generated during piercing and rolling, the average strain rate in the axial direction must be controlled below a certain value. I found out. The present invention has been completed on the basis of the above findings, and the gist thereof is the following method for producing a high carbon seamless steel pipe. That is, when the seamless steel pipe made of high carbon steel containing C: 0.95 to 1.10% by mass% is pierced and rolled after billet heating, the billet heating temperature is set to 1200.
It is a method for producing a high carbon seamless steel pipe, characterized in that the average strain rate ε _AV in the axial direction obtained by the following formula (a) is 2.0 sec ⁻¹ or less. Ε _AV = ln (A ₀ / A) / (L / v) (a) where A: hollow steel tube cross-sectional area after rolling (mm ² ) A ₀ : billet cross-sectional area (mm ²⁾ L: Projected contact length between rolled material and piercer roll (mm) v: Drilling speed (mm / sec) In the present invention, bearing steel such as JIS G 48 is used as high carbon steel.
05 SUJ2 or DIN17230-3505, etc. is intended for pipe production. Among chemical compositions, C: 0.95
It is specified as ˜1.10% by mass. This C content prescribes that it is added in order to enhance hardenability and improve strength as bearing steel and to ensure wear resistance. However, if the content is less than 0.95%, the predetermined strength to be secured using high carbon steel cannot be secured. On the other hand, excessive addition of C worsens fertility,
The upper limit is 1.10%. The other components of the high carbon steel to be used in the present invention are usually in the component ranges allowed by the SUJ material. For example, the following chemical composition is exemplified. For example, Si: 0.15 to 0.70%, Mn: 0.50% or less, P: 0.025% or less, S: 0.025% or less, Cr: 0.90 to 1.
60%. Furthermore, Mo: 0.10 to 0.25% can be contained. In the production method of the present invention, the grain boundary melts at the center and the vicinity of the material to be rolled to prevent inner surface flaws generated in the hollow shell during Mannesmann piercing rolling. To do
It is assumed that the billet heating temperature is kept at a low temperature of 1200 ° C or lower. The reason why the billet heating temperature is thus limited will be described based on the test results. FIG. 1 is a graph showing the effect of billet heating temperature on defects caused by grain boundary melting in piercing and rolling. FIG. 1 (a) shows the case where the tube expansion ratio is 1.0, and FIG. 1 (b) shows the tube expansion ratio. 1.4
Shows the case. In either case, the outer diameter of the hollow shell on the exit side of the piercing and rolling mill is set to 80 mm, and the outer diameter / thickness ratio (t /
D) is varied. The C content of the billet used is 1.10% and the solidus temperature is 1270 ° C. Here, the defect generation limit is arranged based on the relationship between the billet heating temperature and the hollow shell size (t / D), with the object being a double crack (lamination) as a defect caused by grain boundary melting. . In the figure, under the perforation conditions indicated by ●, it is shown that a double crack has occurred. As is clear from FIG. 1, it is effective to lower the billet heating temperature to prevent the occurrence of double cracking. The smaller the t / D of the hollow shell after rolling, the lower the billet heating temperature. Need to be low. Furthermore, even if the hollow element tube after rolling has the same t / D, when the hollow element tube size on the piercing and rolling mill outlet side is constant, the billet diameter is reduced and the tube expansion ratio (rolling ratio) is reduced. By increasing the rear hollow shell outer diameter / billette outer diameter), it is possible to increase the temperature at which the two-piece cracking occurs and narrow the generation area. As will be described later, this is based on the fact that the axial strain rate is reduced as the tube expansion ratio is increased, and the processing heat generation amount is suppressed. In the production method of the present invention, it is necessary to suppress the billet heating temperature at the same time as well as to suppress the processing heat generation during Mannesmann piercing rolling and to suppress the rise in the material temperature of the material to be rolled. Here, the processing calorific value Q is determined by the following (b). Q = Kf ·] · V (b) where Kf: deformation resistance (kg / cm ² )]: equivalent strain rate (sec ⁻¹ ) V: total volume of rolled material (mm ³ ) As can be seen from the above equation (b), in order to reduce the processing calorific value Q, it is necessary to reduce the equivalent strain rate]. Here, in order to clarify the handling of strain and strain rate during piercing and rolling, the average value of the equivalent strain rate in the piercing and rolling step] is grasped as the average strain rate ε _AV . That is, since the average strain rate ε _AV in the axial direction can be expressed as an amount obtained by dividing the total strain amount in the piercing rolling by the time t required to generate the strain, it is expressed by the following equation (a). be able to. Ε _AV =] dt / t = ln (A ₀ / A) / t = ln (A ₀ / A) / (L / v) (a) where A is the hollow shell after rolling Cross-sectional area (mm ² ) A ₀ : Billet cross-sectional area (mm ² ) L: Projected contact length between rolled material and piercer roll (mm) v: Drilling speed (mm / sec) Therefore, expressed by the above equation (a) The average strain rate ε _{AV in the} axial direction is the cross sectional area A _{0 of the} billet subjected to Mannesmann piercing and rolling, the cross sectional area A of the hollow shell after rolling, the projected contact length L of the material to be rolled and the piercer roll, and the piercing speed. determined by v. The projected contact length L between the material to be rolled and the piercer roll is obtained as a contact length with the koji portion of the inclined roll interposed therebetween as shown in FIG. 3 described later. In order to clarify the relationship between the axial average strain rate ε _AV and the billet heating temperature, a very thin hollow element having an outer diameter of 196 mm and a wall thickness of 16 mm from a billet having an outer diameter of 191 mm containing 1.0% C is used. The state of occurrence of internal flaws when the pipe was rolled was investigated. FIG. 2 is a diagram showing the results of investigating the influence of the axial average strain rate ε _AV and the billet heating temperature on the occurrence of internal flaws when rolling a thin hollow shell.
In the figure, ○ indicates that the rate of internal flaws is 2.0% or less, × indicates that the rate of internal flaws is 3.0% or more, and Δ indicates that the rate of occurrence is 2.0% or more and less than 3.0%. Respectively. From the investigation results shown in FIG. 2, in the manufacturing method of the present invention for Mannesmann piercing and rolling such as high C bearing steel,
It can be seen that in order to prevent the occurrence of internal flaws, it is necessary to set the billet heating temperature to 1200 ° C. or less and at the same time to define the axial average strain rate ε _AV to 2.0 sec ⁻¹ or less. Further, as a means for suppressing the processing heat generation in Mannesmann piercing and rolling, it is effective to increase the tube expansion ratio as is clear from the comparison of FIGS. 1 (a) and 1 (b). This is because the equivalent strain rate in the axial direction is reduced by increasing the tube expansion ratio when the dimensions of the material to be rolled after piercing and rolling are the same, and the amount of heat generated by processing is suppressed from the above equation (b). This is because that. During Mannesmann piercing and rolling, seamless steel pipes made of high carbon steel rolled at the above-described billet heating temperature and the axial average strain rate ε _AV obtained by the above equation (a) are then processed in the usual manner. Even if the rolling is performed by drawing and drawing, it is possible to produce a product that suppresses internal melting and generates very little internal flaws. Hereinafter, the effects of the present invention will be described specifically by way of examples. In the examples, the steel grades (SU) having the chemical compositions shown in Table 1 were used.
J-2 equivalent) was melted, billets for Mannesmann pipes were manufactured from this, and the occurrence of flaws on the inner surface of the seamless steel pipes produced was investigated. [Table 1] FIG. 3 is a view for explaining the state of piercing and rolling by Mannesmann tube. The pair of inclined piercer rolls 1, 1 are disposed to face the target with respect to a pass line XX serving as a movement axis in the rolling direction of the billet 2 as a material to be rolled. In the piercing and rolling mill in which the piercer rolls 1 and 1 are arranged in this way, the billet 2 fed on the pass line XX is caught between the piercer rolls 1 and 1, and then swirled while turning. It moves on -X, and a hole is made in the shaft core portion by the plug 3 to form a hollow shell 2p.
During this time, the plug 3 is supported by the mandrel bar 4 so as to be positioned between the piercer rolls 1 and 1 along the pass line. In the piercing and rolling configured as described above, the projected contact length between the material to be rolled and the piercer roll represented by the formula (a) is indicated as L in FIG. The outer diameter of the prepared billet is 191Mmfai, a cross-sectional area A ₀ is set to one of 28,638mm ^2. The billet was charged into a heating furnace and heated in the range of 1180 ° C. to 1230 ° C., and after a soaking time of 1 Hr, piercing and rolling was performed with a piercer. The hollow shell after piercing and rolling has an outer diameter of 196mmφ x
The occurrence of inner surface flaws on the outlet side of piercing and rolling was investigated by varying the thickness from 16 mm to outer diameter 196 mmφ × thickness 25 mm. The survey results are shown in Table 2. [Table 2] As is apparent from the results in Table 2, when the billet heating temperature is 1200 ° C. or less, the axial average strain rate ε _AV is
Invention Examples 1 to 5 satisfying the condition of 2.0 sec ⁻¹ or less at the same time
The rate of occurrence of internal flaws was 1.5% or less, satisfying 2.0% or less, which is a guideline for the rate of internal flaws. On the other hand, in the comparative example 6, the billet heating temperature exceeds 1200 ° C., and in the comparative examples 7 and 8, the axial average strain rate ε _AV is 2.0 sec ⁻¹ or more. The amount of heat generated by processing increases, and internal flaws caused by melting frequently occur in the hollow shell after rolling. Therefore, in the production of a high carbon seamless steel pipe, the billet heating temperature is set to 1200 ° C. or less, and
The axial average strain rate ε _AV obtained by equation (a) is 2.0 sec.
It can be seen that the inner surface flaw generated in the seamless steel pipe can be effectively prevented by setting it to ^-1 or less. According to the method for producing a high carbon seamless steel pipe of the present invention, when piercing and rolling, the axial average strain rate ε _AV is controlled simultaneously with the billet heating temperature,
Since the amount of heat generated by processing can be suppressed and the temperature rise of the material to be rolled can be controlled, it is possible to prevent internal flaws caused by melting that occur in the hollow shell after rolling. As a result, even when a seamless steel pipe is manufactured using difficult-to-process bearing steel, it is possible to improve the product yield without impairing the productivity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the influence of billet heating temperature on defects caused by grain boundary melting in piercing and rolling. FIG. 2 is a diagram showing the results of investigating the influence of axial average strain rate ε _AV and billet heating temperature on the occurrence of internal flaws when rolling a thin hollow shell; FIG. 3 is a diagram for explaining a state of piercing and rolling by Mannesmann tube. [Explanation of symbols] 1: Piercer roll, 2: Billet 2p: Hollow tube, 3: Plug 4: Mandrel bar

──────────────────────────────────────────────────── ─── Continued from the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) B21B 19/04

Claims (1)

(57) [Claims] [Claim 1] When a seamless steel pipe made of high carbon steel containing C: 0.95-1.10% by mass% is pierced and rolled after billet heating, the billet heating temperature is Is 1200 ° C or less, and the average strain rate in the axial direction ε obtained by the following equation (a)
_A method for producing a high carbon seamless steel pipe, characterized in that _AV is 2.0 sec ^-1 or less. _{ε AV = ln (A 0 /} A) / (L / v) ··· (a) where, A: a hollow shell cross-sectional area after rolling (mm ²⁾ A _0: billet cross-sectional area (mm ²⁾ L: Projected contact length between rolled material and piercer roll (mm) v: Drilling speed (mm / sec)