WO1998001589A1 - Process for producing weldless steel pipes - Google Patents

Abstract

The invention relates to a process for producing weldless steel pipes by hot-rolling of a round block in a plurality of stages. During said process, said round block consisting of interstitial-free (IF) steel is inserted and heated in a furnace; the heated round block is subjected to cross-rolling to produce a hollow block at a temperature (temperature at which said block is withdrawn from the furnace) in the region of 1060 to 1200 °C; the hollow block is subsequently subjected to breaking down at temperatures about Ar3; the drawn hollow block is subsequently used to homogenise the temperature thereof for further rolling and for grain refining by recrystallisation in a reheating furnace, the temperature being set in the region of 780 to 880° C in the hollow block; the homogenised hollow block is subjected to finishing rolling by stretch-reducing rolling or sizing rolling, the deformation level of finishing rolling being selected to be so high that a grain size of ASTM6 to ASTM9 is obtained; and the resultant weldless steel pipes are cooled in air.

Description

₅ Processes for the production of seamless steel tubes

The invention relates to a method for producing seamless steel tubes by hot rolling a round block in several steps according to the preamble of claim 1 ιo Such a method is from DE-FZ Steel and Iron 104, 1984, No. 25-26

Seιten1339-1343 known

The manufacture of seamless steel tubes has been part of the general state of the art for many years. The round block used as the input material is usually heated and heated in a furnace to a temperature in the range from 1220 to 1350 ° C.

15 then punched through diagonal rolling, so that a hollow block is obtained. This hollow block is then immediately stretched to a longer tubular intermediate product by stretching rollers. The stretching rolling can take place, for example, in a continuous tube rolling mill or in a plug rolling mill Temperature in the range of 30 to 50 K.

20 reheated above Ar3 Subsequently, a further hot forming takes place in a stretch-reducing mill or in a sizing mill, in which the pipe is formed into an already usable end product. It is also known to cold-form seamless steel pipes produced in this way in their dimensions (diameter and wall thickness) to further reduce seamless precision tubes

? 5

A method is known from US Pat. No. 5,200,005 with which sheets with increased strength and ductility can be produced from a so-called IF steel (Interstitial Free). Such steels have been known for the production of deep-drawn sheets for many years. The US sees a first variant 5 200 005 the production of steel strip 30 by pre-rolling at a temperature of 1260 ° C and finish-rolling at about 710 ° C. After a second process, the pre-rolling takes place at 850 ° C and the finish-rolling at about 700 ° C

The manufacture of steel tubes from IF steels has so far not become known 35 On the one hand, this may be due to the fact that IF steels are inherently 8/01589

- 2 -

have a comparatively low strength. On the other hand, such steels cannot be processed under the usual process conditions for producing seamless steel tubes by hot rolling, since their resistance to deformation is too low.

The object of the invention is to propose a method with which steel tubes with increased strength and comparatively high ductility can be produced. In particular, these tubes should be suitable for further processing by cold forming which can be carried out with as little effort as possible.

This object is achieved by a method having the features of patent claim 1. Advantageous developments of the invention are specified in subclaims 2 to 10.

The invention provides as a feedstock for the manufacture of seamless steel tubes

To use round blocks made of IF steel. Such IF steels are characterized by very low mass fractions of the elements carbon and nitrogen, which are (interstitially) stored in interstitial spaces in the iron crystal lattice. The round blocks are heated to rolling temperature in an oven. According to the heating temperature is considerably below the usual temperatures for

Round steel blocks. The furnace drawing temperature is in fact only in the range between 1060 and 1200 ° C, preferably 1080 to 1130 ° C. At this temperature, cross rolling begins, at which a hollow block is produced from the round steel block. After leaving the cross-rolling mill, this hollow block is stretched in a stretching mill. This stretch rolling is conveniently carried out on one

Pipe continuous rolling mill or on a plug rolling mill instead. The temperature is above Ar3. The stretched hollow block is then placed in a post-heating furnace to homogenize its temperature and to ensure refinement by recrystallization. The hollow block temperature is in the range of 780 to 880 ° C. A particularly preferred area for the

The homogenization temperature is 70 K below Ar1 to A. Immediately afterwards, the homogenized hollow block is finish-rolled, either by stretch-reducing rollers or by sizing rollers. The degree of deformation in this finish rolling is chosen sufficiently large to have a grain size of 98/01589

Achieve at least quality ASTM6 up to ASTM9. The seamless steel tubes produced in this way can then be cooled in air.

The composition of the IF steel used in the process according to the invention is expediently as follows:

expedient preferred

Element composition composition

C <0.005% <0.003%

Si <0.2% approx.0.02%

Mn 0.05 - 0.4% 0.05 - 0.15%

P <0.04% <0.01%

S <0.01% <0.005%

Alges 0.02 - 0.05% approx.0.02%

Cu <0.1% <0.1%

Cr <0.2% <0.1%

Ni <0.2% <0.1%

Mo <0.1% <0.01%

at least one of the (Ti 0.01-0.12% together both elements (Nb 0.01-0.24% approx.0.06%

B <0.0005% <0.0003%

N 0.0020 - 0.0120% approx.0.0050%

Rest iron and usual

Impurities

The seamless steel tubes produced according to the invention by hot rolling can be further processed in an excellent manner by cold forming into precision tubes. So far, such precision tubes have usually been made from seamless or welded

Blanks of conventional structural steels are produced by cold drawing. These structural steels contain carbon components that are significantly above 0.005% and are therefore referred to as carbon steels. As a result of their limited cold forming capacity, the cold drawing of such steel pipes has to be carried out in a very large number of sub-steps (individual pulls) if pipes with a particularly small diameter are to be produced, as is the case, for example, with precision pipes which are used as fuel injection pipes or as raw material for rivet production should be. In fuel injection pipes, normalization annealing is usually carried out after each cold draft, followed by pickling and bondering. The known procedure is extremely expensive because of the many steps. Added to this is the fact that in the material class of structural steels, any increase in strength is associated with a significant reduction in ductility. In many applications for precision pipes, however, you want to have good strength and ductility values at the same time. Here, precision tubes made of IF steels offer the advantage that the ductility is only comparatively little impaired due to cold forming.

In contrast, in the hot-worked seamless steel tubes produced according to the invention, very substantial cost savings result in subsequent further processing by cold forming. This cold forming can take place, for example, by cold drawing with or without an internal tool. With every single move there is an increase in strength, i.e. an increase in

Resistance to deformation. However, this increase is considerably less than that of carbon steel. For this reason, steel pipes made from IF steels can be reduced to a certain dimension with a comparatively significantly smaller number of individual passes than is the case with one. Carbon steel is the case. The degree of deformation for each individual train can therefore be greater. It is also important to note that, on average, significantly more individual moves can be carried out in succession without a reduction in the deformation resistance having to be brought about beforehand by annealing, so that the further cold forming can take place to even smaller dimensions. The ratio of the number of deformation steps to the number of anneals is expediently at least 3, preferably at least 3.5. Annealing preferably takes place below the normalization temperature, in particular at 680 - 720 ° C., so that the strength increases due to cold working are largely retained. In order to achieve a particularly high strength of the produced precision tube after the cold forming, a cold forming in at least 3 deformation steps can follow after the last annealing. As a result, strength levels can easily be achieved that are comparable to those of conventional structural steels (St37), whereby these favorable strength values are accompanied by excellent ductility values. In this respect, the seamless precision tubes produced by the method according to the invention are even superior to the known precision tubes made of carbon steel. Your Manufacturing is significantly cheaper overall, although more effort is required to manufacture the steel raw material, since the carbon and nitrogen contents have to be reduced to very low values.

The invention is explained in more detail below on the basis of two exemplary embodiments.

example 1

A round block with a diameter of 177 mm was produced from an IF steel by continuous casting on a round caster, which had the following composition:

C 0.003%

Si 0.01%

Mn 0.11%

P 0.007%

S 0.005%

AI 0.024%

Cu 0.05%

Cr 0.04%

Ni 0.03%

Mo 0.01%

V <0.002%

Ti 0.064%

Nb 0.0020%

B <0.0003%

N 0.0046%

Fe rest

This round block was placed in a rotary hearth furnace and heated to approximately 1130 ° C. The round block pulled out of the furnace at this temperature was then perforated on an inclined rolling mill and immediately stretched out on a tube continuous rolling mill to form a front pipe of 119 mm in diameter and 6.5 mm in wall thickness. This high deformation resulted in an average grain size in the front pipe, which corresponds approximately to the quality ASTM5 to ASTM6. To Passing through the tube continuous rolling mill, the preliminary tubes were for a period of

Homogenized for 10 min at about 820 ° C in a post-heating oven with respect to their temperature. The finish rolling was then carried out in a stretch-reducing mill to a diameter of 42.4 mm and a wall thickness of 5.6 mm. The seamless steel tube produced in this way had an average grain size of grade ASTM6. The steel tube could then be reduced in a total of six individual passes on a drawing bench to a final dimension of 6.0 mm in diameter and 2.0 mm in wall thickness. After the third cold draw, annealing was carried out at 680 to 720 ° C. If a comparable tube had been made from St37 steel, cold processing would also have been possible in six cold passes. However, at least three would have

Intermediate annealing must be carried out above Ac3 in order to reduce the material's resistance to deformation to a value permissible for the drawing bench used.

Example 2

A round tube of 119 mm in diameter and 6.5 mm in wall thickness was again produced from a round block of the same dimensions and the same composition as in Example 1. The temperature of this front pipe was also homogenized under the same conditions. A hot tube was then produced in a stretch-reducing mill with significantly greater deformation. It had a diameter of 33.7 mm and a wall thickness of 2.6 mm. The mean grain size of this seamless hot pipe was ASTM7. This seamless tube was also subjected to cold processing. There were a total

11 cold draws were carried out, the tubes being annealed at 680 to 720 ° C after the 3rd, 6th and 8th draw. The cold forming led to tubes with only 2.5 mm

Diameter and 0.25 mm wall thickness. If such precision tubes were produced in the conventional way from a St37, a total of 13 cold draws and 5 annealing operations would have been necessary.

Claims

8/01589 claims 1 Process for the production of seamless steel tubes by hot rolling a round block in several steps, wherein - a round block is inserted and heated in an oven, the heated round block is then subjected to cross-rolling to produce a hollow block, the hollow block is then subjected to a stretch rolling which is stretched Hollow block then for homogenizing its temperature for further rolling and for grain refinement Recrystallization is used in a reheating furnace, the homogenized hollow block is subjected to finish rolling by stretch-reducing rollers or sizing rollers, and the seamless steel tubes thus produced are cooled in air, characterized in that the round block is made of IF steel (interstitial free), that the round block is on a temperature (furnace drawing temperature) in the range from 1060 to 1200 C is heated, that the hollow block is stretch-rolled at temperatures above Ar3, that in the reheating furnace a temperature in the range from 780 to 880 C in Hollow block is set and that the degree of deformation of the finish rolling is selected so large that a grain size of ASTM6 to ASTM9 is reached in stretch-reducing rolling 2 The method according to claim 1, characterized in that the furnace drawing temperature is 1080 to 1130 ° C. 3 Method according to one of claims 1 to 2, characterized in that the homogenization temperature is set to a value which is at most Ar1 and at least 70 K below Ar1 - 8th - Method according to one of claims 1 to 3, characterized in that an IF steel with the following composition is used: C <0.005% Si <0.2% Mn 0.05 - 0.4% P <0.04% S <0.01% Alg _es 0.02 - 0.05% Cu <0.1% Cr <0.2% Ni <0.2% Mo <0.1% at least one of the (Ti 0.01 - 0.12% both elements (Nb 0.01 - 0.24% B <0.0005% N 0.0020 - 0.0120% Rest iron and usual Impurities 8/01589 9 - A method according to claim 4, characterized in that the IF steel has the following composition: C <0.003% Si approx.0.02% Mn 0.05 - 0.15% P <0.01% S <0.005% Algae approx. 0.02% Cu <0.1% Cr <0.1% Ni <0.1% Mo <0.01% at least one of the (Ti together two elements (Nb approx.0.06% B <0.0003% N approx. 0.0050% Rest iron and usual Impurities Method according to one of claims 1 to 5, characterized in that the steel pipes after the last hot forming step Be subjected to recrystallization annealing. Method according to one of claims 1 to 6, characterized in that the cooled steel tubes by cold forming with or without Inner tool in diameter and wall thickness can be further reduced. 8. The method according to claim 7, characterized in that the cold forming is carried out in a plurality of deformation steps. 9. The method according to claim 8, characterized in that annealing takes place at 680 to 720 ° C after several deformation steps, the ratio of the number of deformation steps to Number of anneals is at least 3. 10. The method according to claim 9, characterized in that at least 3 further deformation steps follow after the last annealing.