JPH0457401B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The technical content described in this specification regarding the production of round billets for stainless steel pipes to be used in inclined rolling method pipe-making processing uses continuous casting stainless steel slabs as a starting material, and is compatible with the above-mentioned pipe-making processing. We propose a method to advantageously and appropriately obtain stainless steel pipes containing defects, that is, the results of development research based on elucidation of the effects of bloom hot processing conditions prior to hot rolling of the round billet. It's there to do. (Prior art) Obtaining stainless steel round billets by continuous bloom casting is described in Tetsu-to-Hagane 68 (1982).
It is reported in S910. However, bloom continuously cast materials generally have drawbacks in terms of segregation and center porosity, and in particular, because the total reduction into the round billet is relatively small, the hot workability of the resulting round billet is poor. However, it is difficult to say that this is necessarily sufficient, especially in the case of inclined rolling method pipe forming processing. (Problems to be Solved by the Invention) Seamless stainless steel pipes are generally manufactured by applying an inclined rolling method such as a plug mill method or a mandrel mill method, or a hot extrusion method such as a Eugene Sedziurne method or an Erhardt push bench method. Manufactured by. Stainless steel has poor hot workability, and when drilling with a piercing mill for application of the inclined rolling method,
Defects often occur on the inner surface. The hot extrusion method is suitable for making pipes of steel types that inherently have poor hot workability, but with direct drilling, which uses this method to drill billets, the length of the billet can be 5 to 7 times the diameter. In this case, the occurrence of uneven thickness increases, and therefore it is quite difficult to manufacture a long tube. On the other hand, there is also a known method for manufacturing long tubes using the so-called expansion method, in which a guide hole is formed in advance by machining in the center of a billet and then expanded.
Even in this case, the length of the billet is limited to about 15 times its diameter or less. In addition, the above-mentioned Eugene Sejourne method requires the use of a glass lubricant, and as a result, a step of peeling off the lubricant film after rolling is required, which adds to the effort and cost. It is obvious that productivity cannot be increased beyond a certain level under the above-mentioned constraints on billet length, and that the use of short billets lowers the yield and is disadvantageous in terms of cost. That said, the hot extrusion method can be used to make pipes without causing serious defects even with steel types with poor hot workability, so there is no major problem when applying continuous billet casting. is also true. On the other hand, it has become possible to manufacture stainless steel pipes using the inclined rolling method due to the progress and improvement of rolling technology, and in particular, the effective reduction of P, S, O, etc. through improvements in material refining. ,
This is largely due to the improvement of hot workability through the forging effect exerted on the blooming material under the advantageous addition of REM, Ca, B, etc. In other words, the heating temperature,
The influence of thermal history, strain rate, etc. on hot workability, along with the contribution of components in the steel, has been gradually elucidated mainly through small-scale tests, and the results have been applied to actual machines to determine the optimal rolling conditions. has now been established. As long as billets of bloomed material are used in this way, there are no particular problems in forming stainless steel pipes by the inclined rolling method. However, since billets by continuous casting have come to be applied to hot extrusion methods such as the Eugene Sejourne method, although it is insufficient, continuous casting materials cannot be applied to the inclined rolling method as well. The advantages unique to method pipe manufacturing will be halved. Accordingly, it is an object of the present invention to enable particularly advantageous application of continuous casting materials to inclined rolling type tube manufacturing. (Means for Solving the Problems) As a result of repeated experiments and studies regarding the application of billets using materials that have gone through the production history of continuous casting to inclined rolling method pipe manufacturing, the inventors have determined that the long and short side lengths The process basically consists of hot rolling a continuously cast slab with a ratio within a certain limit to form a bloom of a predetermined cross section, then rolling this bloom into a round billet, and then subjecting it to inclined rolling tube manufacturing. This led to the success of this invention. In other words, the above objectives will be advantageously fulfilled in accordance with the following matters: That is, the present invention provides a round billet for seamless stainless steel pipes to be subjected to inclined rolling method pipe manufacturing processing, C:
After passing through the intermediate stage of blooming by hot rolling, which begins with width reduction of a continuously cast slab made of stainless steel with a composition of stainless steel suppressed to 0.32wt% or less and N: 0.4wt% or less, hot rolling is applied to the bloom. In preparing the continuous casting slab, the width (W) is 1100 mm or less and the thickness (T) is at least 40 mm larger than the outer diameter (D) of the round billet to be made. The width/thickness ratio (W/T) is 2.5 or more and 3.67 or less, and the cross-sectional area (W/T) is at least 5 times larger than the cross-sectional area (πD ² /4) of the round billet. The above intermediate stage is 120~160mm/
By hot rolling including width reduction and subsequent thickness reduction of the continuous casting slab under pass conditions,
At least 30 mm compared to the outside diameter (D) of the round billet above
mm is large, the height (H) is at least 10 mm smaller than the thickness (T) of the continuous casting slab, and the width ratio (B/H) to this is 2.0 or less. This is a method for manufacturing round billets for seamless stainless steel pipes using a rolling method. Here, the stainless steel composition means that in addition to the above limitations regarding C and N, Cr: 11 to 30 wt%,
Needless to say, this means containing Ni: 35 wt% or less and Mo: 5 wt% or less. (Function) It is necessary to limit C to 0.32wt% or less in stainless steel composition to ensure strength, but
If it exceeds 0.32wt%, drilling workability deteriorates, and N also contributes to increased strength and corrosion resistance, but if it exceeds 0.4wt%, hot workability deteriorates and the slope This is because it becomes difficult to form pipes using the rolling method. Note that Cr makes a significant contribution to improving corrosion resistance at 11 wt% or more, but its effect has already reached saturation even when the content exceeds 30 wt%. Ni is also useful for corrosion resistance, but even if it is used in excess of 35 wt%, the effect will not be improved and it will be disadvantageous in terms of cost. Mo is useful for pitting corrosion resistance, but using more than 5wt% only increases cost. Next, the length of the long side of the continuous casting slab, that is, the width W
If it exceeds 1100mm, regardless of the slab thickness, the width reduction in the long side direction will cause a dent in the central area of the width, worsening the shape of the bloom. Also, the short side of the continuous casting slab, that is, the width W relative to the thickness T
The ratio W/T of 2.5 or more and 3.67 or less is 2.5
If it is less than 3.67, the degree of center segregation of the slab becomes significant and defects occur during drilling of round billets after the intermediate stage of blooming, and if it exceeds 3.67, buckling occurs during width reduction. This is because it hits the limit. In addition, the reason why this thickness T is made 40 mm or more larger than the outside diameter D of the round billet to be made is to apply sufficient thickness reduction in advance when rolling into a bloom to adjust the shape of the bloom, and thus to improve the shape of the round billet. This is because it is necessary to prevent shape deterioration. It is necessary to make the cross-sectional area W・T of ^the continuous casting slab at least 5 times the cross-sectional area π/4D ² of the round billet. This is because the hot workability at the center of the cross section of the billet cannot be sufficiently improved, and defects may occur on the inner surface of the tube during perforation by the inclined rolling method. Regarding this relationship, C: 0.19wt%, Si: 0.4wt
%, Mn: 0.6wt%, Cr: 13.0wt%, Ni: 0.2wt
%, P: 0.02wt%, S: 0.001wt%, N: 0.02wt
Depending on the molten steel with a composition of %, the length of the long side
Continuously cast slabs with various long side lengths and short side lengths within 1100 mm are made, and the round billet diameter is 230 mm or more.
Figure 1 summarizes the results of rolling experiments in which the rolling reduction ratio leading to these round billets ranges from 1.2 to 8.5 in each case of 230 mm or less.When the rolling reduction ratio is 5 or more, The occurrence of internal scratches has completely subsided. Next, the same procedure was followed except that the reduction applied in the width direction of the continuous casting slab was changed to a range of 90 to 160 mm/pass.
Two types of continuous casting slabs of 400 x 350 mm each
Figure 2 shows the results of investigating the effect on the indentation that occurs in the center of the long side surface when hot rolling a bloom of 400 mm x 200 mm. It was found that hot rolling without dents was possible. Moreover, if the amount of reduction exceeds 160 mm/pass, there will be a disadvantage due to collapse. The height H of the bloom should be increased by 30 mm or more compared to the outside diameter D of the round billet, and if it is less than this,
It becomes difficult to make round billets with good roundness. The height H of the bloom is made at least 10 mm smaller than the thickness T of the continuous casting slab in order to sufficiently reduce the thickness of the bloom, adjust the shape of the bloom, and prevent deterioration of the shape of the round billet. The ratio B/H of the width B to the height H of the bloom is
If it exceeds 2.0, the load in the width direction during billet rolling will increase and collapse may occur. (Example) Slabs were continuously cast into various sizes using test steels of test Nos. 1 to 22 whose chemical compositions are shown in Table 1, and were hot rolled to form blooms and then round billets. .

【table】

[Table] Here, the heating temperature for both bloom rolling and billet rolling is 1250°C, and the furnace time during which the temperature difference between the surface layer and the center part is 10°C or less is determined by heat transfer calculation, and the After heat retention, it is subjected to rolling, and the strain rate in each rolling is 1 to 5/s. In the conforming examples of Test Nos. 1 to 8 according to the present invention, no defects were observed when the inclined rolling method was applied after bloom rolling and round billet rolling. On the other hand, test No. 9 was C, and test No. 10 was N.
However, in both cases, defects occurred on the inner surface of the tube due to component incompatibility that exceeded the upper limit. Further, in Test Nos. 11 and 15, rolling into a round billet was abandoned due to poor shape with a significant dent in the center of the long side of the bloom due to the excessive width W of the continuously cast slab. Next, in Test Nos. 12, 17, and 20, the roundness was poor because the relationship condition of the bloom height to the outside diameter of the round billet was not satisfied. Next test No.13
Because the minimum width reduction per pass of the continuously cast slab was below the lower limit, the bloom was defective in shape with a dent in the center of the long side, so subsequent rolling was discontinued. Next, in Test Nos. 14 and 18, the width and thickness ratio W/T of the continuously cast slabs were too small, so defects occurred on the inner surface of the tube despite the large reduction ratio. In this case, in the continuous casting slab of the compatible example shown for test numbers 5 and 7, each side is 270 mm,
We have shown an example of non-conformity when a 100mm piece is cut and removed on both sides to make it narrower. Next, in Test Nos. 16 and 19, the height H of the bloom was equal to the thickness T of the continuous casting slab, so the reduction in the thickness direction was insufficient, and therefore the shape was defective due to the depression in the center of the long side of the bloom. This is an example of a nonconformity that resulted in the suspension of rolling. For test No. 12, the outer diameter of the round billet is
After machining to 300 mm to make the appearance similar to test No. 4, it was subjected to inclined rolling tube making, but defects on the inner surface of the tube were unavoidable because sufficient reduction to the round billet was not applied. On the contrary, regarding test No. 20,
When the outside diameter of the round billet was machined from 240 mm to 220 mm to correct the roundness defects and then pipe-formed, the reduction ratio to form a round billet was 6.33, which was sufficient, resulting in a defect-free pipe. was possible. Here, roundness is expressed as the difference between the maximum diameter and the minimum diameter, which are perpendicular to each other, and if it exceeds 1 mm, it is considered unsuitable. (Effects of the Invention) According to the present invention, it is possible to efficiently manufacture seamless stainless steel pipes using continuous cast stainless steel slabs by tilt rolling method pipe forming process without defects using long round billets. , the cost of this type of steel pipe can be reduced through improved yield and productivity.

[Brief explanation of the drawing]

Figure 1 is a graph showing the influence of the rolling reduction ratio from a continuously cast slab to a billet on the internal flaw occurrence rate.
The figure is a graph showing the influence of the amount of reduction in one pass on the occurrence of a dent in the center of the long side of the bloom.

Claims (1)

[Scope of Claims] 1. A round billet for seamless stainless steel pipes to be subjected to inclined rolling method pipe-making processing, C: 0.32 wt% or less,
Prepared by hot rolling the bloom after going through an intermediate stage of blooming by hot rolling starting with width reduction of a continuously cast slab made of stainless steel with N: 0.4wt% or less. In doing so, the width (W) of the above continuous casting slab is 1100 mm.
In the following, the thickness (T) is at least 40 mm larger than the outer diameter (D) of the round billet to be made.
The width/thickness ratio (W/T) of these is 2.5 or more,
3.67 or less, and which has a cross-sectional area (W・T) that is at least 5 times larger than the cross-sectional area (πD ² /4) of the above round billet. As a result of the above-mentioned width reduction and subsequent hot rolling including thickness reduction of the above-mentioned continuous casting slab under the following conditions, the continuous casting slab has a thickness that is at least 30 mm larger than the outer diameter (D) of the above-mentioned round billet and a thickness of the continuous casting slab ( T) at least 10
A method for producing a seamless round billet for stainless steel pipe using an inclined rolling method, characterized by processing the billet into a bloom having a small height (H) in mm and a width to width ratio (B/H) of 2.0 or less.