JP3451917B2 - Method for manufacturing seamless steel pipe having excellent surface properties without surface hardened layer - Google Patents

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 having no surface hardened layer and having excellent surface properties.

[0002]

2. Description of the Related Art First, a conventional method for manufacturing a seamless steel pipe will be described. Conventionally, C: 0.05 to 0.
5%, Si: 1.0% or less, Mn: 0.1 to 1.5%, Cr: 1.0%
When manufacturing a steel for general mechanical structures such as iron-based alloys containing the following into a seamless steel pipe by the Mannesmann pipe manufacturing method, a solid round bar-shaped metal piece (billet) with the above components heated to a predetermined temperature is used. First, it is fed to a piercing and rolling machine (piercer), and hot piercing is performed. After that, the hollow shell is transported to a succeeding rolling mill and hot-rolled to make a tube, and then finished to a final size.

In the manufacturing process carried out as described above, the surface temperature of the steel pipe is always in a high temperature range of 800 ° C. or higher, so that oxide scale is formed on the surface. Part of the oxide scale peels off during hot rolling, but since the final rolling temperature is as high as 700 to 1100 ° C, the oxide scale grows during the subsequent cooling and this oxidation occurs during bending correction by the rotary strainer performed after cooling. The scale is pushed in and the surface skin is deteriorated.

Therefore, in the past, as a measure against the deterioration of the surface skin due to oxide scale, a tunnel equipped with a large number of injection nozzles was installed in the middle of the conveying path to uniformly distribute high-pressure water on the surface of the steel pipe in the circumferential direction. By spraying on, the oxide scale is once removed, and the scale thickness is thinned to improve the surface texture.

Further, in the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-18332, a cooling furnace and a descaler whose atmosphere is adjusted after constant-diameter rolling are provided, and a steel pipe is inserted into the cooling furnace at 800 to 900 ° C. By cooling to ~ 600 ℃, then descaling and straightening in the temperature range of 450 ~ 550 ℃, we are trying to suppress the generation of surface defects caused by indentation of oxide scale.

[0006]

However, according to the descaling method in which high-pressure water is sprayed from the circumferential direction in the tunnel transport path as described above, the effect of removing the oxide scale can be surely seen, but on the other hand By spraying high-pressure water, the steel pipe surface is rapidly cooled. for that reason,
There is no temperature change at the center of the wall thickness of the steel pipe, and a hardened layer is formed only on the surface. At this time, when the heat removal of the surface portion of the steel pipe is large as in the above method, the surface portion and the central portion of the wall thickness have different microstructures. The presence of such a hardened layer causes deterioration of the tool life at the time of cutting the steel pipe and increase of deformation resistance at the time of working.

In order to manufacture such a steel pipe having no hardened layer, the descale temperature may be raised, but when the descale temperature is raised, the oxide scale grows after the descale and the surface quality deteriorates.

On the other hand, the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 7-18332 has the following problems when processing steel pipes having different sizes.
From the viewpoint of production efficiency, there is a drawback that a huge cooling furnace capable of controlling the atmosphere is required, and large-scale equipment addition and energy consumption are required.

The present invention has been made in view of the above circumstances, and it is possible to reliably remove oxide scale adhering to the outer surface of a steel pipe and to provide a seamless surface having a hardened layer on the surface of the steel pipe and having excellent surface properties. It is an object to provide a method for manufacturing a steel pipe.

[0010]

[Means for Solving the Problems] In order to achieve the above-mentioned objects, the inventors of the present invention have conducted extensive studies. As a result, they have found that such a problem can be solved by defining the steel composition and suppressing the outer surface temperature of the steel pipe and the steel pipe conveying speed during descaling, and completed the present invention.

That is, during descaling by blowing high-pressure water after pipe making, required descaling is carried out during the transportation period to the cooling bed, and the temperature of the outer surface of the steel pipe decreased by blowing high-pressure water is measured in the cooling bed. The present invention has been achieved by knowing that it is possible to prevent the formation of a hardened layer as described above by reheating the Ac ₃ point or more during cooling.

Further, when descaling the oxide scale produced after rolling on a Fe-based alloy steel pipe containing Cr: 1.0% or less, by controlling the steel pipe outer surface temperature and the steel pipe feed rate when spraying high-pressure water, It has been clarified that the micro metal structure of the surface portion of the steel pipe is similar to that of the central portion of the wall thickness of the steel pipe, the hardness is not increased, and excellent surface properties are obtained, and the present invention has been achieved.

In the present invention, the Cr content is 1.0 mass% or less.
Steel billet, after heating, hot piercing, after rolling to make a seamless steel pipe, on the outer surface of the resulting steel pipe, in the method for producing a seamless steel pipe to spray descaling high pressure water,
After hot rolling, during the transportation period to the cooling floor, the outer surface of the steel pipe can be sprayed with high-pressure water in the temperature range of 850 to 950 ℃.
After descaling more , cooling by air cooling in the cooling bed
Sometimes the front surface temperature of the manufacturing method excellent seamless steel pipe surface properties having no hardened surface layer, characterized in that Ru is recuperation _three or more points Ac.

The present invention further provides a method for producing a seamless steel pipe in which a billet made of steel containing Cr: 1.0 mass% or less is heated and then perforated by, for example, a piercer machine, and a constant-diameter finish is performed by hot rolling. After hot rolling, during air cooling, while the outer surface of the steel pipe is in the temperature range of 750 to 950 ° C, the steel pipe is conveyed at a steel pipe feed speed of not less than the R value shown below, and 150 to 250 kgf.
/ cm high-pressure water ² is sprayed on the steel pipe outer table surface descaling
Done , then transported to the cooling bed and by air cooling in the cooling bed
A method for producing a seamless steel pipe having an excellent surface quality, which does not have a surface-hardened layer and is characterized in that the surface temperature is reheated to an Ac ₃ point or more during cooling .

R = 25492.1 / (T + 273) −20.6 (1) where T: outer surface temperature of steel pipe (° C.), R: steel pipe feed rate
(m / s) As described above, according to the present invention, for example, a steel pipe finished at a final rolling temperature of 800 to 1100 ° C. has a steel pipe surface temperature of 750.
In the temperature range of up to 950 ° C., while conveying at a steel pipe feed rate equal to or higher than the R value that is a function of temperature, by spraying high-pressure water and descaling, the oxide scale is peeled off uniformly over the entire surface of the steel pipe, and The difference in hardness between the outer surface of the steel pipe and the center of the steel pipe wall thickness is the Vickers hardness (1 kg)
A steel pipe having a uniform hardness distribution of 30 or less can be obtained.

According to the present invention, the high-pressure water is sprayed during the transportation period to the cooling floor after the hot rolling, so that the high-pressure water is sprayed from the nozzle surrounding the running seamless steel pipe, for example. Not only is descaling possible over the entire circumference, but also after passing through the high-pressure water spray area, heat removal is eliminated, and an excessive decrease in surface temperature can be avoided.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory view showing the heat history in the method for producing a seamless steel pipe according to the present invention, in which the billet is first heated and then subjected to a pipe-making process of piercing and hot rolling. Manufactured into seamless steel pipe. These steps and conditions are not particularly limited in the present invention, and conventional ones may be used. After the tube is manufactured by finish rolling, it is cooled to room temperature, and the oxide scale is removed at that time. In the case of the present invention, the step of removing oxide scale described as descale in the figure is performed by spraying high-pressure water after pipe production.
Is done.

In the present invention, in claim 1, the temperature range of the steel pipe surface at the time of descaling by spraying high-pressure water is set to 850 to 950 ° C. at which the oxide scale can be uniformly peeled off. This is because if the temperature is lower than 850 ° C, the steel pipe surface temperature may not be able to reheat to the Ac ₃ point after spraying high-pressure water, and if it exceeds 950 ° C, the oxide scale becomes thick again after the oxide scale is removed. This is to generate. In claim 2, the temperature range of the steel pipe surface at the time of descaling by spraying high-pressure water is 750 to 950 ° C at which the oxide scale can be peeled most uniformly.
And This is because in the present invention, the pressure of the high-pressure water is usually about 150 to 250 kgf / cm ² at the original pressure, and when water of this pressure is used, oxide scale can be sufficiently removed at a temperature of less than 750 ° C. This is because the oxide scale cannot be formed, and above 950 ° C., the oxide scale is thickly formed again after the oxide scale is removed.

The high-pressure water is sprayed during the period of hot rolling and transportation to the cooling floor. At this time, the seamless steel pipe is
For example, as shown schematically in FIG. 2, a running seamless steel pipe
Circular nozzle, usually called Desuke water pipe, from all around 12
Descaling is effectively performed by spraying high-pressure water 14 with 10, and then recuperation is easily performed.
If desired, a plurality of such nozzles may be provided. It is not necessary to provide a cooling furnace and spray high-pressure water from the entire circumference of a steel pipe placed at a fixed position using a plurality of high-pressure water nozzles as in the conventional case. The position and number of nozzles at this time are not particularly limited as long as the oxide scale can be effectively removed.

[0020] In the preferred embodiment of the present invention, when sprayed with high pressure water at the above temperature range, as a generation method for preventing surface hardened layer, once reduced steel pipe outer surface temperature below Ac ₃ point from the Ac ₃ point or more However, so as to reheat to the Ac ₃ point or more again, for example, the steel pipe feed rate and the steel pipe outer surface temperature at the time of descaling are controlled, but in a further preferred aspect of the present invention, as a control method thereof, at the time of descaling, Established a formula to calculate the steel pipe feed rate as a function of the outer surface temperature of each steel pipe during descaling,
The critical steel pipe feed rate value R was determined. When the steel pipe feed rate is R value or more, descaling can be performed under the condition that the surface hardened layer is not formed.

When the microstructure of the steel pipe is descaled at a temperature of Ac ₃ point or higher, after the high pressure water is sprayed, the quenched steel pipe surface is reheated to a temperature of Ac ₃ point or higher again for a sufficient time. If the temperature is in the austenite region, the microstructures of the surface portion and the central portion of the wall thickness do not differ. However, after the high-pressure water is sprayed, if the temperature is not reheated to the Ac ₃ point or higher, the surface microstructure does not change when it is rapidly cooled, and has a microstructure different from the thickness center part.
In this case, the surface portion has a bainite structure, the central portion of the wall thickness has a mixed structure of ferrite and pearlite, and the hardness of the surface portion increases. Also, after spraying high-pressure water, when reheated to just above the Ac ₃ point, the surface portion once transformed into an austenite structure, the microstructure generated during quenching disappears, but for a sufficient time, austenite region Since it does not stay at the temperature, the growth of crystal grains does not proceed, and the surface portion has a microstructure with a grain diameter smaller than the central portion of the wall thickness, forming a hard layer on the surface portion.

In order to avoid such a situation, it is necessary to change the heat removal amount by spraying high-pressure water according to the outer surface temperature of the steel pipe during descaling. If the pressure or amount of water sprayed is reduced, the amount of oxide scale peeled off will decrease and the surface properties of the steel pipe will deteriorate.

Therefore, in the present invention, paying attention to the steel pipe feed rate which is easier to control in the actual operation, the steel pipe feed rate is controlled as a function of the steel pipe outer surface temperature by the above-described calculation formula. This makes it possible to manufacture a steel pipe having no hardened layer on its surface. Equation (1) above defines the relationship between them.

The cooling bed is cooled to room temperature by air cooling, but if necessary, rapid cooling such as water cooling may be performed, or the first half may be air cooled and the latter half may be rapidly cooled.
However, cooling in the cooling bed is to bring the reheated Ac ₃ point to room temperature over a sufficient time, and it is desirable to keep the holding time in the austenite region as long as possible.

Next, in the preferred embodiment of the present invention, the Cr content is 1
The reason why the content is less than mass% is that the properties of the oxide scale differ as the amount of Cr increases, and it becomes difficult to exfoliate the scale simply by spraying high-pressure water. Therefore, the steel of which the scale can be removed by the method of the present invention is 1 mass% or less.
Next, the working effects of the present invention will be described more specifically by way of examples.

[0026]

EXAMPLES Steels A to F having the compositional compositions shown in Table 1 were melted by a usual method, and billets having respective outer diameters obtained by continuous casting were used as raw materials.
The billet was heated at 00 to 1250 ° C, pierced by a piercer machine, and finish-rolled to various outer diameters and wall thicknesses by a rolling machine to obtain a seamless steel pipe.

After that, descaling was performed under various conditions shown in Table 2. The high-pressure water was sprayed at one place using the circular nozzle shown in FIG. The source pressure of the high-pressure water sprayed on the outer surface of the steel pipe during descaling was 150 to 250 kgf / cm ² .

The descaled steel pipe is air-cooled in a cooling bed, straightened and straightened by a rotary straightener, and then visually observed for surface properties of the steel pipe, and the occurrence frequency and depth of surface defects mainly due to oxide scale are observed. Was investigated. The surface defects at that time were evaluated according to the following criteria.

<Evaluation Criteria> Good (○): 7 pieces / m ² or less and all the flaw depths are 0.2.
mm or less unfavorable (x): In all cases other than the above, a part of the steel pipe was further cut to obtain a sample, and the cross section was etched for microstructure observation.
In addition, in addition, a Vickers hardness test was performed on the cross section of this sample. The load for the Vickers hardness test is 1 kg,
Three points were measured at a position of 0.5 mm from the outer surface, and the hardness of the outer surface portion was determined. In addition, three points were also measured at the center of the wall thickness, and this was taken as the hardness at the center of the wall thickness. The results are shown in Table 2 together with the descaling conditions.

FIG. 3 is a graph showing these results organized by the feed rate of the seamless steel pipe at the time of descaling and the outer surface temperature of the steel pipe. However, ◯: Good seamless steel pipe Δ: Seamless steel pipe having good surface properties but having a hardened layer ×: Seamless steel pipe having poor surface properties

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

As shown in Table 2, in the production method of the present invention,
Defects caused by indentation of oxide scale on the outer surface hardly occur and the surface quality is excellent. On the other hand, in the comparative materials shown in Table 3, the steel pipe surface temperature during descaling is 750 ° C.
When the temperature was less than 950 ° C. and when the temperature was higher than 950 ° C., numerous flaws presumed to be caused by indentation of oxide scale were observed on the surface, and the surface properties were poor.

In the method of the present invention shown in Table 2, even if the surface temperature of the steel pipe during descaling changes, the steel pipe surface does not have a microstructure different from the central portion of the wall thickness, and the Vickers hardness of the outer surface of the steel pipe is And the difference in Vickers hardness of the central portion of the wall thickness was 30 or less, no increase in hardness was observed. As described above, according to the present invention, it is understood that the outer surface temperature of the steel pipe, which has been lowered by spraying the high-pressure water, is reheated to the Ac ₃ point or higher.

On the other hand, in the comparative materials shown in Table 3, the temperature at the time of descaling is within the range of the present invention, and even if the steel pipe has a good outer surface property, the microscopic surface of the steel pipe is different from the central portion of the wall thickness. It has a texture and the difference in hardness is large. As described above, in the conventional descaling method in which only the temperature during descaling is controlled, a hardened layer is provided on the outer surface of the steel pipe. Further, it can be seen from FIG. 3 that the surface texture is improved by satisfying the above-mentioned formula (1).

[0037]

As described above, according to the present invention, by controlling the outer surface temperature of the steel pipe when the high-pressure water is sprayed during the steel pipe transportation period, and further by specifying the steel pipe feeding speed, the surface texture can be obtained. It is possible to produce a seamless steel pipe which is excellent and has no hardened layer.

[Brief description of drawings]

FIG. 1 is a schematic view of a manufacturing process of a seamless steel pipe in an example.

FIG. 2 is a schematic explanatory view of high-pressure water spraying means used in the present invention.

FIG. 3 is an evaluation diagram of a seamless steel pipe in comparison with a seamless steel pipe manufactured by the method of the present invention in Examples.

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C21D 9/00-9/44, 9/50 C21D 1/02-1/84 C21D 7/00-8 / 10 B21B 45/02 B21B 17/00-25/06

Claims (2)

(57) [Claims] 1. A steel billet containing Cr: 1.0 mass% or less,
After heating, perforating hot, after rolling to make a seamless steel pipe, on the outer surface of the resulting steel pipe, in the method for producing a seamless steel pipe to descale by blowing high pressure water, after hot rolling,
During transportation to the cooling floor, descaling is performed by spraying high-pressure water on the outer surface of the steel pipe in the temperature range of 850 to 950 ° C.
After, Ac surface temperature during cooling by air cooling in the cooling bed
Method for producing a good seamless steel pipe surface properties having no hardened surface layer, characterized in that to recuperation _three or more points. 2. A steel billet containing Cr: 1.0 mass% or less,
After heating, after hot piercing and rolling to make a seamless steel pipe, the steel pipe outer surface temperature is 750 to 950 ℃ on the outer surface of the obtained steel pipe.
In the temperature range, while conveying the value or more steel feed rate R shown below, by spraying high-pressure water of 150 ~250 kgf / cm ²
Descaling was performed , then the steel pipe was transported to the cooling bed, and the surface temperature during cooling by air cooling on the cooling bed was set to Ac ₃ points or less.
A method for producing a seamless steel pipe having an excellent surface property, which does not have a surface-hardened layer, which is characterized in that it is reheated . R = 25492.1 / (T + 273) -20.6 However, T: Steel pipe outer surface temperature (° C), R: Steel pipe feed rate
(m / s)