JPH09300006A - Manufacture of seamless steel tube difficult to be worked - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a round cast billet having good endoplasm using existing continuous casting equipment and, next, to manufacture a seamless steel tube difficult to be worked without causing miss roll in piercing and, without generating the so-called Mannesmann crack at that. SOLUTION: By executing roll reduction, in the last stage of solidification of the round cast billet, especially at a solid phase rate in a shaft center part of >=0.3 and <1, and at the reduction of area in the direction C of 0.1-0.3% in one stage, the cast billet is manufactured. Desirably, molten steel is electromagnetically stirred in a mold. The cast billet is pierced at 1100-1300 deg.C. When P in the case based on that the inclined angle of a piercer roll 3 is 6-12 and roll gap at the tip of a piercing plug 4 is 93% of the diameter of the round billet 1 is expressed by P0 , piercing is executed under conditions by which the formula of P/P0 =0.973-1.027 is satisfied. P and P0 are the coefficients about the driving force of the billet which are imparted with the piercer roll and a function of the inclined angle of the piercer roll, shape of the plug, position of the plug, roll gap/billet diameter in the gorge part, face angle of the roll and diameter of the billet.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a seamless steel pipe from a steel slab having a poor hot workability such as high Cr steel, and particularly, a continuous round billet is simply cast by continuous casting,
Next, the present invention relates to a method for producing a seamless steel pipe for producing a pipe while suppressing the occurrence of flaws.

[0002]

2. Description of the Related Art Generally, a seamless steel pipe is produced by casting a bloom slab having a large cross-section square shape with a continuous casting machine, heating it, and then billeting it with a round cross-section with a slab mill, blooming mill, billeting mill, etc. After hot rolling into round billets),
Transport to a pipe manufacturing plant, or after casting a round billet with a continuous casting machine, transport to a pipe manufacturing plant, and Mannesmann perforation or press perforation, or to manufacture a hollow shell by hot extrusion, The product is drawn by a rolling machine such as an elongator, a plug mill, or a mandrel mill, and finally made into a product by sizing with a sizer or a stretch reducer.

A slab, which can be relatively easily cast by continuous casting and has a good hot workability like a general low carbon steel, is supplied as a raw material for a seamless steel pipe. In continuous casting, such as stainless steel, porosity and segregation are likely to occur in the shaft core of the slab, and the slab with poor hot workability is often supplied after large processing. The reason why such a step is performed in the case of a cast piece having poor hot workability is that the material billet is first subjected to severe processing such as Mannesmann drilling, and if such preprocessing is omitted, the porosity of the shaft core portion is omitted. And segregation causes flaws on the inner surface of the pipe. Therefore, it is said that processing before piercing is necessary not only for a slab called a difficult-to-work material, but also for a slab with a high carbon content and a slab to which Cr is added.

In particular, the internal quality of the slab has a great influence on the rolling performance in the rolling process. When the Cr content in the steel increases, segregation and porosity occur in the axial core of the continuous cast slab, and scaly surface flaws occur on the inner surface of the seamless steel pipe in the pipe manufacturing process. You have to remove the flaws,
This leads to an increase in manufacturing costs.

Generally, in the porosity generation mechanism of continuous cast slabs, molten steel is originally supplied with solidification shrinkage in the final solidification stage of the slab and voids do not occur, but the molten steel has a high viscosity and so on. It is said that porosity is generated when is not supplied. In particular, the viscosity of the molten steel increases as the Cr content in the steel increases, and the Cr content reaches a maximum around 13 wt.% As shown in FIG.

Therefore, conventionally, even if porosity is generated in a cast piece, porosity is pressure-bonded to a pipe-making material by slabbing, and then processed into a round shape by a billeting mill and supplied. In addition, in order to improve the quality of the slab and prevent the occurrence of defects during pipe manufacturing, an electromagnetic stirrer is installed in the mold of the continuous casting machine, and the molten steel in the mold is electromagnetically stirred to promote nucleation and final solidification. By forming equiaxed crystals in the part, center segregation of the slab and generation of porosity are suppressed.

As a means for improving the internal quality of the slab, there is a method of subjecting the slab to a light reduction in a continuous casting line. For example, Japanese Examined Patent Publication No. 59-16862 discloses that a slab is rolled at the end of solidification by the amount of solidification shrinkage, and the flow of concentrated molten steel in the solidification end region (crater end region) is suppressed to prevent center segregation. The technology (hereinafter referred to as "prior art 1") is disclosed.

[Materials and Processes] 1994 vol. 7
No. 1 p. 194 to 197 disclose a technique (hereinafter, referred to as "prior art 2") in which a method of lightly rolling a round bloom slab in two stages in a non-solidified state with a roll is applied to stainless steel SUS410. However, in the prior art 2, the density of the cast slab shaft core portion is 7.7, which does not reach the density 7.8 of the cast slab shaft core portion when porosity is not generated, and the porosity is not completely pressure-bonded. In addition, some porosity remains in the macro corrosion test results.

[Materials and Processes] 1994 vol. 7
No. 1 p. 179-182 has a C content of 0.4-
In order to improve the center segregation of the 0.6 wt.% Continuous casting bloom, a forging method (hereinafter referred to as "Prior art 3") in which a slab at the final stage of solidification is pushed into a die and continuously subjected to large reduction Disclosure.

On the other hand, many techniques for preventing the occurrence of internal flaws in the pipe manufacturing process have been proposed. For example, Japanese Patent Application Laid-Open No. 1-22
No. 8603 discloses heating a continuous cast round billet slab to 1200 to 1310 ° C. and then piercing-rolling it in order to manufacture a duplex stainless steel seamless steel pipe having good inner and outer surface properties from the round billet continuous cast slab. Machine, perforation ratio 1.40 or less, draft ratio ({(material diameter-gorge diameter) / material diameter} x 100
%) Is rolled at 5.0% or less (hereinafter referred to as "prior art 4").

Further, Japanese Unexamined Patent Publication No. 6-106209 discloses
A method of piercing and rolling stainless steel in a piercing and rolling process under conditions in which the geometrical relationship between various shape parameters of the piercing machine and the plug is optimized in order to reduce torsional deformation and occurrence of external surface flaws. (Hereinafter, referred to as "Prior Art 5") is disclosed, and the rolling material is not particularly limited. However, when subjected to processing under harsh conditions such as piercing and rolling, the internal quality of the billet shaft core that causes internal flaws greatly differs depending on the chemical composition of the material and the manufacturing method. A consistent and proper manufacturing method from the method to the piercing and rolling is desired. Therefore, it is not sufficient to pierce and roll a difficult-to-work material to produce a seamless steel pipe in which inner surface defects do not occur.

[0012]

The above-mentioned respective prior arts have the following problems. The prior art 1 is effective for improving the center segregation of the slab, but it has a problem that the equipment cost and the operating cost increase because the light reduction roll must be installed over a long section of the casting line. Prior art 2
Is effective for improving the solidification structure of the shaft core, but has a problem that the porosity disappearance is insufficient as described above. Further, the prior art 3 is an excellent process for pressure bonding of porosity, but has a drawback of high equipment cost and is not realistic.

In the prior art 4, since the round billet continuous cast slab can be directly rolled by a piercing and rolling mill, a seamless stainless steel pipe having excellent inner and outer surface properties can be produced with high efficiency. However, if the draft rate is 5% or less,
There is a problem with the biting property, and it is difficult to say that it is a condition that can always be operated stably. In Prior Art 5, as described above,
The consistent manufacturing conditions from the casting method of slabs to the piercing and rolling method, which are desired in the manufacture of difficult-to-work seamless steel pipes, are unknown.

Therefore, an object of the present invention is to solve the above-mentioned problems, and to easily produce a round billet slab of good quality with improved center segregation and porosity by using existing continuous casting equipment, and then, In particular, to provide a method for producing a difficult-to-work seamless steel pipe that does not cause misrolling in piercing rolling, and is hot-rolled under piercing conditions that does not cause so-called Mannesmann cracking, and thus does not cause internal flaws. It is in.

[0015]

In view of the above-mentioned circumstances, the inventors of the present invention have conducted extensive research into a technique for producing a difficult-to-work seamless steel pipe with inexpensive equipment. As a result, the following method was developed.

The method for producing a difficult-to-work seamless steel pipe according to claim 1 is to continuously cast alloy steel containing Cr in an amount of more than 0.5 wt.% In a round mold having a diameter of 340 mm or less, and solidify the slab. Characterized by a light reduction roll installed near the completion point, applying light reduction to the final solidification part of the obtained round billet slab, then heating the slab, then hot rolling, and pipe making Is to have.

According to a second aspect of the present invention, in the method for producing a seamless steel pipe that is difficult to process, in the method according to the first aspect, the molten steel is electromagnetically stirred in the mold during continuous casting, and the shaft core portion with respect to the cast piece is used. When the solid phase ratio is 0.3 or more and less than 1, the C-direction cross-sectional area reduction ratio is characterized by being subjected to a single stage of light reduction in the range of 0.1% or more and 3% or less.

The method for producing a difficult-to-work seamless steel pipe according to claim 3 is the method according to claim 2, wherein the slab is heated and rolled within a temperature range of 1100 ° C to 1300 ° C. Is piercing and rolling, and if the roll inclination angle of the pier is within the range of 6 ° to 12 ° and the roll spacing at the tip of the piercing plug is 93% of the round billet diameter, the following (1) It is characterized in that it is performed under the condition that satisfies the formula.

0.973 ≦ P / P ₀ ≦ 1.027 --------- (1) where P ₀ and P are coefficients relating to the billet driving force given by the roll of the punching machine, P = f (β, D _P , L _P , γ _P , γ _g , θ ₁ , θ ₂ , D _b ) P ₀ : The above P value when γ _P = 0.93 P / P ₀ : Perforator Number obtained by P and P ₀ when the roll tilt angle is constant at a certain value f: predetermined function β: punch roll roll angle D _P : plug diameter L _P : effective length of plug γ _P : plug position Γ _g : Roll spacing / billet diameter in the gorge section θ ₁ : Entrance side roll surface angle θ ₂ : Outgoing side roll surface angle D _b : Billet diameter

[0020]

Next, an embodiment of the present invention will be described. An embodiment of the invention described in claim 1 will be described.

A round billet slab having a Cr content of more than 0.5 wt.% And a diameter of 340 mm or less is cast by continuous casting, the slab is lightly pressed by a flat roll near the solidification completion point of the slab, and then The cast slab is rolled by a Mannesmann puncher to produce a hollow shell, stretched by an elongator, and sized by a sizer to produce a seamless steel tube.

When the diameter of the slab is 340 mm or less,
Since the solidification rate of the final solidified portion of the shaft core is high, porosity is easily generated, and further, the shaft core crack due to thermal stress is induced. Therefore, after piercing and rolling a slab with a diameter of 340 mm or less,
When manufacturing a difficult-to-work seamless steel pipe, the manufacturing process according to the method of the present invention is essential. Hereinafter, the reason will be described.

In the process of continuously casting a predetermined round billet,
It is necessary to lightly reduce the slab and press the porosity before piercing and rolling. The time of light reduction of the slab may be any time as long as the slab core has a liquid phase capable of pressing porosity and has not yet solidified. Regarding the chemical composition of the slab,
As the Cr content increases, the viscosity of molten steel increases, and according to the test results of the present inventors, in particular, the Cr content is 0.5 wt.%.
If it exceeds, internal flaws will occur in the seamless steel pipe. In addition, in the method of the present invention, the hollow shell is directly manufactured from the round billet slab without once rolling the large-section continuous cast slab into the round billet, which is also cost effective.

An embodiment of the invention described in claim 2 will be described. A round billet is cast while electromagnetically stirring molten steel in a continuous casting mold, and when the solid phase ratio f _S of the slab axial part is 0.3 or more and less than 1, the C-direction cross-section reduction rate is 0.1% or more and 3%.
The following reduction is performed by one-step reduction using a set of flat rolls installed in a continuous casting line to produce a cast slab for a seamless steel pipe. The round billet slab thus produced is rolled by a Mannesmann puncher to produce a hollow shell, stretched by an elongator, and then sized by a sizer to produce a seamless steel pipe.

It is essential to cast molten steel while electromagnetically stirring it in the mold, because it is effective in forming equiaxed crystals in the core of the cast slab. Regarding the timing of light reduction of the slab, since the porosity is not pressure-bonded after the shaft core is completely solidified, it is necessary to perform light reduction below the solidus temperature (solid fraction of less than 1).
However, when the solid fraction of the shaft core is 0.3 or less, even if the pressure is once lightly reduced, molten steel flow again occurs in the crater after the light pressure reduction, and as a result, center segregation and porosity are formed.

As for the method of lightly rolling down the cast slab, in addition to the above-mentioned solid fraction conditions, the reduction rate of the cross-sectional area in the C direction is limited to exert the effect of suppressing or crimping the formation of porosity in the shaft core. When the C-direction cross-sectional area reduction rate of the slab is 0.1% or less, segregation can be suppressed, but porosity cannot be suppressed or pressure bonding cannot be performed. On the other hand, if the C-direction cross-sectional area reduction rate exceeds 3%, the porosity can be pressure-bonded, but cracks occur inside the slab.

FIG. 1 is an example of an experimental result by the inventors of the present invention. In continuous casting of 330 mmφ billet slabs, a single step by a flat roll was performed at a position where the solid fraction f _s of the slab axial part was 0.52. It is a graph which shows the influence of the C direction cross-sectional area reduction rate (%) of a cast piece which acts on the internal quality of a cast piece when it is reduced. In the figure, the smaller the internal quality score, the better, and 1 and 2 are the acceptable ranges. From this experimental result, it can be seen that it is necessary to carry out the light reduction within the range in which the C-direction cross-sectional area reduction rate is 0.1% or more and 3% or less.

From the above, the light reduction of the cast slab should be such that the reduction rate of the cross-sectional area in the C direction is within 0.1% and 3% or less. Further, this light reduction method should be added under the condition that it is performed under a single stage pressure by a roll. The reason is that one-stage reduction is more effective in reducing porosity than multi-stage reduction.

An embodiment of the invention described in claim 3 will be described. In a continuous casting mold with a diameter of 340 mm or less, C
While electromagnetically stirring molten steel with an r content of more than 0.5 wt.%,
The round billet was cast, with a set of flat rolls installed in the vicinity of the cast piece solidification completion point of the continuous casting line, in the solid fraction f _S of the slab axial core portion is less than 1 0.3 or more, C-direction sectional An area reduction rate of 0.1% or more and 3% or less is performed under a single step reduction, and then the round billet slab thus produced is heated to a predetermined temperature between 1100 ° C and 1300 ° C, and a Mannesmann It is rolled with a piercing machine to manufacture a hollow shell,
A seamless steel pipe is manufactured by stretching with an elongator and calibrating with a sizer.

The heating temperature of 1100 ° C. or more and 1300 ° C. or less is a general heating temperature during piercing and rolling, and in order to normally pierce and roll a difficult-to-process slab while suppressing the occurrence of inner surface defects within this temperature range. Need to specify the drilling condition within an appropriate range. The concept of the drilling conditions specified to complete the present invention will be described.

In order to suppress the generation of inner surface defects, it is necessary to prevent so-called Mannesmann cracking. Mannesmann cracks occur due to the accumulation of strain until the billet reaches the tip of the plug during the process of rolling the billet with an inclined punching machine.Therefore, it is important for the hot workability of the billet material to suppress internal defects. It will be a condition. On the other hand, in order to prevent Mannesmann cracking, mechanically, if the distance from the position where the billet bites into the roll in the rolling direction to the tip of the plug is shortened, the strain accumulated in the billet becomes smaller and it is effective in suppressing cracking. is there. However,
If this distance is made too small, the driving force applied from the roll to the billet in the rolling direction becomes insufficient and the billet does not get caught in the plug, so-called misroll occurs. Therefore, the perforation condition that does not cause misroll and does not cause the inner surface flaw is limited to the case where the specific condition is satisfied. In addition, a billet material, particularly a slab with good internal quality in the vicinity of the shaft core, is required.

From the above viewpoints, the present inventors investigated and obtained the billet slab of the difficult-to-process material by the Mannesmann piercing machine. The piercing and rolling should be carried out under the condition that P satisfies the following formula (1). Is. Here, P is the roll load of the punching machine divided by the deformation resistance value of the punching material (dimensionless), and is a coefficient related to the billet driving force given by the roll.

0.973 ≦ P / P₀≦ 1.027 --------- (1) However, P₀And P are given by the roll of the punch
Is a coefficient related to the billet driving force, P = f (β, D_P, L_P, γ_P, Γ_g, Θ₁, θ_Two, D_b) P₀: Γ_PP value P / P when = 0.93₀: When the punch roll angle is constant at a certain value
Mushroom P and P₀The value obtained by P / P₀Γ as the base value of_P= 0.93
And the P value is P ₀As perforation and rolling condition P /
P₀Although the range of is limited as in the above equation (1), the base value
Is γ_PIt is not limited to when 0.93. That is, an example
For example, γ_P= 0.92 or γ_P= 0.94 mag
, P / P obtained using the P value at that time₀To
γ_PP / P when = 0.93₀The converted value of is (1) above
You just have to satisfy the formula.

F: predetermined function β: perforator roll inclination angle D _P : plug diameter L _P : effective length of the plug γ _P : value indicating the plug position γ _g : roll interval / billet diameter at the gorge portion θ ₁ : Entry-side roll surface angle θ ₂ : Exit-side roll surface angle D _b : Billet diameter FIG. 2 is a schematic diagram showing the state of perforation of a round billet in the longitudinal direction of the rolling direction, and FIG. FIG. 4 is a longitudinal cross-sectional view in the rolling direction for explaining the setting state of the machine roll inclination angle and the like, and FIG. 4 illustrates the rolling reduction of the tube thickness ΔH for each rolling cycle in the round billet piercing rolling. The right-angled sectional view is shown.

P can be obtained by calculating the roll load by a simple method under the following assumptions. The rolling pressure is determined by the balance of forces in the moving direction and the rotating direction of the billet.

As the yield condition, the Von-Mises condition is satisfied. The rolling reduction of each part of the material during rolling is mainly determined by the advancing angle and the advancing efficiency.

The contact length is approximated as a function of the amount of tube thickness reduced during half a turn of the roll, plug and billet.
Based on the above assumptions, the billet diameter D _b and the punch roll diameter D
_R , tube thickness, roll surface angles θ ₁ , θ ₂ , plug shape D _P , L
_P can be obtained from P 1 and the value γ _P indicating the plug position represented by the advanced plug amount or the plug tip reduction γ _P represented by the following equation (2).

Γ _P = (G _P / D _b ) --------- (2) However, G _P : Roll interval at the tip of the plug Hereinafter, a method of obtaining P will be described.

Regarding the contact length between the roll and the billet in the rolling direction, the contact length on the roll entry side is generally the roll face angle from the contact start point between the billet and the roll to the maximum diameter portion (gorge) of the punch roll. , The roll inclination angle, and the gorge reduction ratio obtained from the billet diameter and the gorge roll interval. The contact length on the roll outlet side is also found to be substantially the same, although there is a difference in the raw pipe diameter. Here, further simplifying this, if the roll surface angle is θ, the billet diameter is D _b , and the gorge reduction ratio is γ _g ,
The entrance side contact length L _xa , which is the projected length from the contact start point between the billet and the roll in the rolling direction to the gorge, is represented by the following formula (3).

L _xa = (D _b × (1−γ _g ) / 2) / tan θ --------- (3) Further, the tube thickness after rolling per half rotation of the billet is the roll interval and the plug. It is possible to simply obtain the difference from the diameter. That is, if the roll interval at a certain point x is G _x and the plug diameter is D _Px , the tube thickness H _x is expressed by the following equation (4).

H _x = (G _x −D _Px ) / 2 --------- (4) Originally, the stress in the rotational direction should also be taken into consideration, but for the sake of simple calculation, By substituting these into only the rolling direction stress solution derived from the above-described assumed stress balance equation, for example, the roll load P ′ in the vicinity of the plug tip, which is applied to the present invention, is obtained by the following equation (5). It becomes possible.

P ′ = {(m₁× Deformation resistance + m_Two× Deformation resistance + rolling direction stress)} / 2 --------- (5) where m₁, M_Two: Of Von-Mise's yield condition
Coefficient, m₁ ^Two+ M_Two ^Two-M₁m_Two= 1 value Note that the coefficient P related to the billet driving force is
Is the load P'divided by the deformation resistance value of the perforated material?
In addition, the variable element due to the billet material is also incorporated.
And In addition, the characteristics due to the tool surface angle of the punch are corrected.
Therefore, P / P ₀Use α for the range of values. In this case, P
/ P₀2.7%, which is the deviation from the value of 1, is multiplied by α
The range is.

[Example of P Calculation Method] Next, as an example of the P calculation method, the billet diameter = 5, which is the condition of the model experiment.
0 mmφ, roll diameter = 300 mmφ, entrance / exit angle =
The P value is calculated at 3 °, the inclination angle β = 9 °, the rolling reduction at the gorge = 10%, and the rolling reduction at the plug tip = 5.5%. The coefficient of friction with the roll μ ₁ = 0.5, the coefficient of friction with the plug μ ₂ = 0.3, m ₁ = 1.15, m ₂ = 0.
5 is assumed. The plug has a bottom radius of 20 mm and a length of 6
6 mm.

First, using the contact length L _{xa in the} longitudinal direction which has already been written in the present invention as the apparent surface angle due to the inclination of the roll from the entrance / exit side surface angle, the following equation (6) is used. Ask.

Tan θ ₁ ′ = (G _a −G ₀ ) / (L _xa cos β) -------------- (6) where θ ₁ ′: apparent face angle in calculation Further, the apparent face angle on the outlet side is obtained from the plug length L _P by using the length L _xb obtained by subtracting the plus advanced amount s and the outlet side outer diameter G _b in the same manner as in the equation (6). At this time, tan θ ₁ '= 0.0555, tan θ ₂ ' = 0.0658
9 However, θ ₂ ': an apparent face angle in calculation. Further, the advanced amount of the plug at this time (the length of the plug tip protruding from the gorge toward the entrance side: see FIG. 2) is calculated based on the numerical value of the rolling reduction at the plug tip.

S = (10−5.5) / 100 × (50/2) /tan3°=21.5 (mm) Further, the wall thickness during rolling is simply determined by the roll interval and the plug diameter. did. For example, the wall thickness at the gorge portion is: wall thickness at the gorge portion = (gorge interval / 2) − (plug radius at the gorge portion) = 16.4.

From this, the stress is calculated, but for simplicity, the load P is calculated with the stress distribution only in the longitudinal direction. First, as a boundary condition, the following formula group is used so that the stress becomes 0 (zero) at the point where the plug and the perforated billet are separated from the plug and the roll.

T _f = radius of bottom of plug / length of plug a ₁ = μ ₁ sin β- (t _f + μ ₂ sin β) / (1-
μ ₂ t _f ) g ₁ = t _f −tan θ ₂ ′ b ₁ = m ₂ a ₁ / (g ₁ + a ₁ ) b ₂ = outlet wall thickness / g ₁ b ₃ = − (g ₁ + a ₁ ) / g ₁ c ₁ = b ₁ b ₂ ^(-b3) After calculating these, the stress from the exit side to the gorge portion is obtained by the equation (7).

Stress = c ₁ {(L _xb −x) + b ₂ } ^b3 −b ₁ -------------- (7) where x: longitudinal position (L on the exit side _xb , 0 in Gorge) Next, since the geometric conditions are different between the input side and the output side, the above coefficient is obtained again. However, it is assumed that the stress of the gorge portion is known so that the stress value is continuous at the gorge portion, and it is adjusted to c ₁ .

G ₁ = t _f + tan θ ₁ 'b ₁ = m ₂ a ₁ / (g ₁ + a ₁ ) b ₂ = (wall thickness at gorge part) / g ₁ b ₃ =-(g ₁ + a ₁ ) / g ₁ c ₁ = (stress at the gorge part + b ₁ ) / (L _xb + b ₂ } ^b3 It is possible to obtain the entry-side stress value using this.

Therefore, in the vicinity of the tip of the plug, that is, x = (-
It is possible to obtain the stress value in the vicinity of s) by the equation (7). This value is put into the following equation (8) to calculate the load P. P = (m ₁ + m ₂ + stress value) / 2 -------------- (8) In this example, P = 1.241 is obtained. In the embodiment, similarly, after the P value is calculated, a reduction rate of 7% at the plug tip when a certain inclination angle β and a certain gorge reduction rate are set (in Tables 2 to 3, the plug position γ _p is 0.
93) is set as the base value P ₀ , and P / P ₀
The range of was limited.

By the above-mentioned method, the perforation condition that there is no misroll and no inner surface flaw is specified by considering the deformation resistance of the material according to the billet material at 1100 to 1300 ° C which is a general heating temperature during perforation. can do. In this way, it is possible to realize an improvement in rolling efficiency and a reduction in cost of a high alloy steel seamless steel pipe having a poor hot workability by using a pipe manufacturing facility which is completely the same as the conventional one.

In the present invention, the round billet slab lightly reduced in the continuous casting line is manufactured by hot rolling without a reduction process thereafter, so that the consistent manufacturing cost can be reduced.

[0054]

Next, the present invention will be described in more detail with reference to examples. (Test 1) In a continuous casting machine for curved billets with a radius of 11.5 m, a high Cr molten steel for a seamless steel pipe with a Cr content of 2.5 or 13.8 wt.% In a round mold having a slab cross section of 170 or 330 mmφ Was cast into a round billet slab. Then, the slab is heated to a predetermined temperature between 1100 ° C and 1300 ° C, rolled by a Mannesmann puncher to produce a hollow shell, stretched by an elongator, and then sized by a sizer to produce a seamless steel pipe. Was manufactured.

Table 1 shows Examples 1 to 8 which are manufacturing methods within the scope of the present invention, and Comparative Examples 1 to 3 which are manufacturing methods outside the scope of the present invention. Comparative Examples 1 and 3 are cases in which light reduction was not performed, and the solid fraction f _{s in} the table in this case indicates the axial core solid fraction at the light reduction roll position.

[0056]

[Table 1]

The chemical composition of each of the cast molten steels is a type of steel which is difficult to form and easily forms porosity and center segregation of the cast core. An electromagnetic stirrer is installed in the mold. The casting temperature was a temperature according to a conventional method according to the chemical composition of the molten steel, and the molten steel in the mold was stirred by an electromagnetic stirrer during casting. Also, the amount of slab cooling water was set to an appropriate amount according to the chemical composition of the molten steel.

To reduce the unsolidified slab in the final stage of solidification, 1
A light reduction device with a set of reduction rolls was installed in place on the continuous casting line. The rolling system is a hydraulic system, the roll diameter is 380 mm, and the type is a flat roll. The installation position of the light reduction roll is 1 from the molten steel meniscus in the mold.
Even if the molten steel composition and the slab diameter differ between casting Nos., It is possible to arbitrarily select the solid fraction f _s of the slab axial center by adjusting the casting speed etc. I chose

After the casting, predetermined test pieces were taken from the round billet slabs and tested for density and internal cracks at the center of the slab. Then, after the pipe was manufactured, the state of occurrence of internal flaws in the seamless steel pipe was observed, and the quality of the cast piece was evaluated.

[Axial core density measurement test] L including the axial center of the slab
A 10 mm square × 40 mm long block was cut out in the direction, subjected to replica treatment, then immersed in phthalic acid, and the density was measured by the Archimedes method. In addition, the density of the 1/4 part of the radius is measured from the surface of the cast piece by the same method as described above, and 1 / of the density difference between the _1/4 part and the shaft core part (ρ _1/4 −ρ _1/2 ). 4 parts ratio density _{(ρ 1/4) (Δρ} = ((ρ
_The following scores were given by 1 / ₄₋ ρ _1/2 ) / ρ _1/4 ) × 100).

Rating 1: Δρ <0.5% Rating 2: 0.5 ≦ Δρ <1% Rating 3: 1 ≦ Δρ <5% Rating 4: 5 ≦ Δρ <10% [Internal cracking test] C of cast slab Macro corrosion photo 3
The degree of internal cracking was determined by observing one sheet and one macro-corrosion photograph of a cross section in the L direction having a length of 500 mm, and a rating was given. A score of 1 indicates that no internal crack could be found, and a smaller score means better.

[Internal Quality Test] The inner surface of the seamless steel pipe is inspected,
The occurrence of internal flaws was investigated and used as an internal quality evaluation test for the corresponding cast pieces. The smaller the score, the better the internal quality, and the scores 1 and 2 indicate that no defects occur, and the scores 3 and 4 indicate that defects occur.

The above test results are also shown in Table 1. Table 1
The following matters can be understood from Comparative Examples 1 and 3 which are not subjected to light reduction are inferior in the axial core density index and the internal quality score.

Comparative Example 2, in which the shaft core was lightly pressed after being completely solidified, was not improved because there was no unsolidified phase, and the shaft core density index and the internal quality rating were inferior. As described above, the comparative example is inferior in at least one of the slab internal properties.

On the other hand, Example 1 according to the method of the present invention
˜8 is excellent in the internal quality characteristics such as the axial core density index, internal cracking and internal quality rating. In Examples 1 to 8, Example 8 in which the reduction rate in the C-direction cross-sectional area was set to a large value (3.30) within the scope of the present invention and the solid fraction f _s of the shaft core portion was set in the range of the present invention example was smaller in the inner 1 (0.21) than examples 2-7 in which the soft reduction within the scope of claim 2 wherein the C direction area reduction ratio and axial center solid phase ratio f _s are both present invention Is the axial core density index,
Both internal cracking and internal quality rating are equal or better.

(Test 2) Each round billet cast having the chemical composition shown in Examples 1, 3 and 7 and Comparative Examples 1 and 3 in Table 1 and produced under each casting condition. The piece was heated to a predetermined temperature between 1100 ° C. and 1300 ° C., and rolled by a Mannesmann perforator to produce a hollow shell, which was drawn and then sized to produce a seamless steel tube. The rolling conditions in the Mannesmann punching machine were such that the coefficient P / P ₀ satisfied or did not satisfy the condition of the formula (1), and the rolling conditions were appropriately combined with the manufacturing conditions of the round billet slab for piercing and rolling.

Perforation of the seamless steel pipes thus produced in Examples 11 to 34 which are methods within the scope of the present invention and Comparative Examples 11 to 13 which are methods outside the scope of the present invention (Especially, misroll) and the occurrence of internal flaws were investigated, and the rollability of the seamless steel pipe was evaluated based on the results.

Tables 2 and 3 show the manufacturing conditions (chemical composition and casting conditions) for round billet slabs and the rolling conditions for Mannesmann piers (each element and parameter P / P).
(Value of ₀ ) is shown, and the rating of the rollability of the seamless steel pipe is also shown.

[0069]

[Table 2]

[0070]

[Table 3]

The method for evaluating the rolling property is as follows. Perforability = Mainly the presence or absence of misrolling Good: ◎, Acceptable: ○, Not acceptable: × Pipe inner surface defect = Situation of inner surface defect Good: ◎, Acceptable: ○, Not acceptable: ×, Totally not acceptable: × × Overall rollability evaluation Best : Score 1 Good: Score 2 Acceptable: Score 3 Not acceptable:
Score 4 Further, in FIG. 5, the parameter P / P ₀ representing the punching condition is shown.
Shows the relationship between the total rolling property score of seamless steel pipe and.

The following matters can be seen from the results of Tables 1 to 3 and FIG. Comparative Examples 11 to 13, which are outside the scope of the present invention, are inferior in rollability. That is, when the manufacturing conditions of the round billet slab are out of the range of the present invention, the parameter P / P representing the drilling conditions is used.
_{Even if 0} is within the range of the present invention, (a) when drilling under the condition that no misroll occurs, pipe inner surface defects occur (Comparative Examples 11 and 12), while (b) pipe inner surface defects are as much as possible. Even if the punching condition is set so as not to cause the punching, the misrolling occurs (Comparative Example 13). Comparative Example 13
In the above, if P / P ₀ is further increased, that is, if a condition that does not cause further internal flaws is set, all misrolls occur, which is completely irrelevant to the workability of the raw material, which is meaningless.

On the other hand, in Examples 11 to 34 in which the round billet production conditions are within the scope of the present invention, all of them have excellent rolling property. Among these, the slab is lightly pressed at a C-direction cross-sectional area reduction rate within the range of 0.1% to 3% within the range of the axial core solid fraction f _s of 0.3 to less than 1. In addition, P / P ₀ of piercing and rolling was set within the range of 0.973 to 1.027 (Examples 12, 13, 16, 18, 1)
Nos. 9, 21, 22, 24, 25, 29 to 31, 33, and 34) are stable in both perforability and inner surface flaws of the pipe, and are more excellent and industrially suitable.

[0074] In the present invention, as the base value of P / P _0, treats P value when the gamma _P = 0.93 as P _0, but limit the scope of the piercing condition P / P _0, the aforementioned As described above, the base value is not limited to γ _P = 0.93, and even when γ _P is other than 0.93, P / P ₀ at that time is set to γ _P = 0.93. When it is converted into P / P ₀ at this time, it may be within the range of the present invention, and the effect of the present invention regarding the perforation property is sufficiently exerted.

Under the conventional piercing conditions, it is difficult to eliminate the generation of inner surface flaws by piercing and rolling because the internal quality of the difficult-to-work billet slab is not excellent. Since misrolls occur quite often, it has been difficult to obtain operating conditions that have no industrial problems. However, according to the method of the present invention, it becomes possible to perform piercing rolling with almost no inner surface flaw even under the rolling conditions that conventionally generate inner surface flaws, and further, misrolling occurs by changing the position of the plug to an appropriate position. It has become possible to manufacture seamless steel pipes of good quality without any problems.

[0076]

EFFECTS OF THE INVENTION Since the present invention is configured as described above, it is possible to manufacture a difficult-to-work seamless steel pipe having no internal flaws at a lower cost by a simpler and more integrated process from casting to pipe manufacturing. That is, according to the method of the present invention, continuous casting of a defect-free billet of a high-alloy steel with poor hot workability, particularly a high-Cr alloy steel, which has been conventionally considered difficult, is achieved without changing equipment. In addition, it is possible to manufacture a seamless steel pipe with no flaws on the inner surface of the pipe by a pipe-making process including piercing and rolling. Furthermore, by optimizing the piercing conditions, a seamless steel pipe with no flaws on the inner surface of the pipe and high added value can be further obtained. It is possible to provide a method for producing a difficult-to-work seamless steel pipe, which is advantageous in pipe production and can reduce the production cost, and brings an industrially useful effect.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of light reduction on the internal quality of a slab for the final stage of solidification of the slab.

FIG. 2 is a schematic diagram of perforation of a longitudinal section in the rolling direction, which explains each element of perforation conditions.

FIG. 3 is a longitudinal sectional view in a rolling direction of a main part for explaining a setting state of a punching machine roll.

FIG. 4 is a cross-sectional view perpendicular to the rolling direction, which illustrates the tube thickness reduction ΔH for each rolling cycle in the round billet piercing and rolling.

FIG. 5 is a graph showing a relationship between a parameter P / P ₀ indicating a piercing condition and a rollability comprehensive score of a pipe.

FIG. 6 is a graph showing the relationship between the Cr content in steel and the viscosity of molten steel.

[Explanation of symbols]

 1 Billet 2 Tube thickness 3 Punch roll 4 Plug 5 Punch roll axis

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location B22D 11/128 350 B22D 11/128 350A 11/20 11/20 C C21D 8/10 9270-4K C21D 8/10 A 9270-4K D C22C 38/00 301 C22C 38/00 301Z 302 302Z 38/18 38/18 (72) Inventor Takashi Arizumi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihonkokanko Ltd. (72) Inventor Rigo Nakagumi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd. (72) Inventor Takashi Itakura 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (3)

[Claims] 1. A high C containing more than 0.5 wt.% Cr.
r alloy steel was continuously cast in a round-shaped mold having a diameter of 340 mm or less, a light reduction was applied by a roll to the final solidification portion of the obtained round billet slab, and then the slab was heated and then hot-rolled. , And a method for producing a difficult-to-work seamless steel pipe. 2. In the continuous casting, electromagnetic stirring treatment of molten steel in a mold is added, and the light reduction of the slab is performed.
2. The reduction in the C-direction cross-sectional area reduction rate within the range of 0.1% or more and 3% or less is performed in a single-stage reduction when the axial core solid phase ratio is 0.3 or more and less than 1. A method for producing a seamless steel pipe having a difficult workability as described above. 3. The heating and hot rolling of the slab 1100
℃ or more and 1300 ℃ or less in the temperature range, and the hot rolling is piercing rolling, piercing roll angle of 6
Characterized in that the roll interval at the tip of the perforated plug is within a range of 90 ° or more and 12 ° or less, and the roll interval at the tip of the perforated plug is 93% of the diameter of the round billet as a reference under the condition that the following expression (1) is satisfied The method for producing a difficult-to-work seamless steel pipe according to claim 1. 0.973 ≦ P / P ₀ ≦ 1.027 --------- (1) where P ₀ and P are coefficients relating to the billet driving force given by the roll of the punch, and P = f (Β, D _P , L _P , γ _P , γ _g , θ ₁ , θ ₂ , D _b ) P ₀ : The above P value when γ _P = 0.93 P / P ₀ : Punch roll tilt angle Is a number obtained by P and P ₀ when is constant at a certain value f: predetermined function β: punch roll inclination angle D _P : plug diameter L _P : effective length of plug γ _P : value indicating plug position γ _g : Roll spacing / billet diameter in the gorge section θ ₁ : Entrance side roll surface angle θ ₂ : Outgoing side roll surface angle D _b : Billet diameter