JP2001247931A - Non-heattreated high strength seamless steel pipe and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heattreated high strength seamless steel pipe having a high strength as hog-rolled without largely adding alloy elements or using supplementary such as heat treatment and cold working and to provide a production method therefor. SOLUTION: This non-heattreated high strength seamless steel pipe contains, by mass, 0.01 to 0.6% C, 0.01 to 1.0% Si and 0.1 to 2.0% Mn, and in which the average ferrite gain size in the cross-section vertical to the longitudinal direction of the steel tube is <=20 μm. Moreover, in the method for producing the seamless steel pipes so as to be fed to hot rolling of the Mannesmann system, a cast slab or steel slab is heated at Ac3 transformation point to 1,300 deg.C and is thereafter subjected to piercing and rolling, and, successively, the pipe stock is held at 650 to 750 deg.C and is thereafter subjected to finish rolling in which cumulative reduction of cross-section is >=20%, and the temperature of the tube stock at the time of the final rolling is 600 to 750 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat treated high strength seamless steel pipe and a method for producing the same.

[0002]

2. Description of the Related Art Cylinder tubes for hydraulic equipment used in civil engineering and construction machinery, steel tubes used for mechanical structures, automotive parts, etc., are subjected to heat treatment or cold working to improve strength. Used seamless steel pipes have been used frequently. However, these methods have a problem that the cost is high because an additional step such as a heat treatment step or a cold working step is required. For this reason, there is a demand for a high-strength seamless steel pipe having a strength equal to or higher than that of a heat-treated material or a cold-worked material as hot-rolled.

As a method of obtaining high strength as hot rolled, a method of adding a large amount of inexpensive alloy elements such as C, Si, and Mn can be considered. Since toughness is required, C content, Si
Amount, Mn amount and carbon equivalent Ceq (=% C +% Si / 24 +
% Mn / 6 +% Ni / 40 +% Cr / 5 +% Mo / 4 +
% V / 14) is often specified, and it is therefore difficult to solve the above problem by adding a large amount of inexpensive alloying elements.

Further, effective use of trace elements has been studied, and a non-heat treated high-strength seamless steel pipe is disclosed in Japanese Patent Laid-Open No. 5-20 / 1993.
No. 2447, etc., in addition to the precipitation strengthening action of V and the matrix strengthening of Mn and Cr,
There has been proposed a technique for obtaining a non-heat treated high strength seamless steel pipe by adding appropriate amounts of Ti and N. However, this steel pipe also has a problem that the weldability is insufficient due to the high carbon equivalent.

[0005] As a method for improving the strength of a hot-rolled material while maintaining the weldability and toughness, refinement of ferrite grains is a conventional means. In the field of thick plates, refinement of ferrite grains by controlled rolling is known. Widely used. However, in the hot rolling method of the Mannesmann system, which is a normal seamless steel pipe manufacturing method, it is difficult to apply controlled rolling as performed on a thick plate due to equipment problems. Therefore, the ferrite grain size of the seamless steel pipe as hot rolled is 20 to
It is only about 40 μm, and does not reach a sufficient fine grain level as compared with the ferrite grain size of the controlled rolled thick plate (10 μm can be obtained stably and the grain refinement limit is 5 μm).

[0006] Japanese Patent Application Laid-Open No. 10-306339 discloses that, mainly for electric resistance welded steel pipes (electrically welded pipes), the steel pipes after being formed are reheated off-line to a ferrite recrystallization temperature region and drawn and rolled. However, a technique has been proposed in which a ferritic steel pipe having a high toughness and a high ductility is obtained by reducing the ferrite particle diameter to 3 μm or less. However, when this technology is applied to a seamless steel pipe, a step of reheating the hot-finished steel pipe off-line and performing drawing and rolling is required, and thus there is a problem of high cost.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and does not require the addition of a large amount of alloying elements and uses ancillary steps such as heat treatment and cold working. It is another object of the present invention to provide a non-heat treated high-strength seamless steel pipe having high strength as it is hot-rolled, and a method for producing the same.

[0008]

Means for Solving the Problems The present inventors have repeated studies on the conditions for producing non-heat-treated high-strength seamless steel pipes by the Mannesmann hot rolling process for various materials having different components. As a result, the slab or slab was heated to a temperature of not less than the Ac3 transformation point and not more than 1300 ° C., followed by piercing and rolling.
C., and then perform finish rolling at a tube temperature of 600 to 750 ° C. during rolling at a cumulative cross-section reduction rate of 20% or more, so that the ferrite grain size in the hot-rolled state is 20 μm.
It has been found that a non-heat-treated high-strength seamless steel pipe having a diameter of not more than m can be manufactured.

The present invention has been made based on such knowledge, and the gist thereof is as follows. (1) In mass%, C: 0.01 to 0.6%, Si: 0.01 to 1.0%, Mn: 0.1 to 2.0%, perpendicular to the longitudinal direction of the steel pipe. A non-heat treated high-strength seamless steel pipe having an average ferrite grain size of 20 μm or less in cross section. (2) A cast or steel slab containing, by mass%, C: 0.01 to 0.6%, Si: 0.01 to 1.0%, and Mn: 0.1 to 2.0% A method for producing a seamless steel pipe by subjecting it to a hot rolling method of a system, wherein a slab or a slab is heated to a temperature not lower than the Ac3 transformation point but not higher than 1300 ° C., and then pierced and rolled.
Production of a non-heat treated high strength seamless steel pipe characterized in that it is maintained at a temperature of 50 to 750 ° C. and then finish rolled at a raw material temperature of 600 to 750 ° C. at the time of rolling at a cumulative reduction of area of 20% or more. Method. (3) A slab or a slab containing, by mass%, C: 0.01 to 0.6%, Si: 0.01 to 1.0%, and Mn: 0.1 to 2.0%,
This is a method for producing a seamless steel pipe by subjecting it to hot rolling of the Mannesmann method, in which a slab or a slab is heated to a temperature of not less than the Ac3 transformation point and not more than 1300 ° C., and thereafter, piercing and rolling are performed. The temperature is maintained at a temperature of 650 to 750 ° C. and a condition satisfying the following formula (1).
A method for producing a non-heat treated high-strength seamless steel pipe, characterized by performing finish rolling. 18700 ≦ T (logt + 20) ≦ 20,000 (1) where T: temperature (K) t: time (h) (4) After piercing and rolling, the raw tube is continuously cooled at a cooling rate of air cooling or higher. The method for producing a non-heat treated high-strength seamless steel pipe according to the above (2) or (3), wherein the pipe is cooled to a temperature of 650 to 750 ° C. (5) The temperature deviation in the longitudinal direction of the raw tube before holding at a temperature of 650 to 750 ° C. after piercing / rolling is 80 ° C. or less, the above (2) to (4). The method for producing a non-heat treated high-strength seamless steel pipe according to any one of the preceding claims. (6) The method for producing a non-heat treated high-strength seamless steel pipe according to any one of (2) to (5), wherein the finish rolling is performed under lubrication.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. First, the reasons for limiting the components of the non-heat treated high-strength seamless steel pipe in the present invention will be described. Component content is mass%
It is. C: C is added as an element that increases the strength of steel.
If it is less than 0.01%, it is difficult to secure the strength required for structural steel, and if it is added in excess of 0.6%, the toughness and weldability are significantly deteriorated. Therefore, the C content is 0.01 to 0.1.
6%.

Si: Si is added as a deoxidizing agent and as an element for increasing the strength of steel. If it is less than 0.01%, the effect of improving the strength cannot be effectively exhibited, and if it exceeds 1.0%, a coarse oxide is formed to deteriorate ductility and toughness. Therefore, the Si content is set to 0.01 to 1.0%.

Mn: Mn is added as an element that increases the strength of steel. If it is less than 0.1%, the effect of improving strength cannot be effectively exhibited, and if added in a large amount exceeding 2.0%, ductility and toughness are deteriorated. Therefore, the Mn content is set to 0.1 to 2.0%.

The above are the basic components of the steel of the present invention. Usually, the other components are Fe and unavoidable impurities. However, depending on the desired strength level and other necessary properties, Al: 0.001% by mass. 0.2%, N: 0.001 to 0.05%, Cr: 0.01 to 2.0%, Ni: 0.01 to 5.0%, Mo: 0.01 to 2.0%, Cu: 0.01 to 2.0%, B: 0.0003 to 0.02%, Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, REM: 0.001 to 0.05, Zr: 0.01-0.05%, Ti: 0.003-0.2%, Nb: 0.003-0.2%, V: 0.003-0.2%, W: 0.5 to 3%, O: 0.01% or less, P: 0.03% or less, S: 0.04% or less Good.

Next, the hot rolling method of the Mannesmann system will be described. The steel of the present invention is formed into a seamless pipe by a Mannesmann hot rolling method. FIG. 1 shows an example of a pipe forming process of a seamless pipe by a Mannesmann hot rolling method. The hot rolling method of the Mannesmann method here is a normal hot rolling method for producing a seamless steel pipe, using a material for pipe making having a rectangular or round cross section, and piercing with a press roll drilling machine or a Mannesmann drilling machine. After that, if necessary, inclined rolling mill (Elongator mill)
Elongation rolling is performed to reduce the wall thickness and increase the length, and if necessary, reduce the outer diameter with a rolling mill (hollow shell reducer), and further adjust the wall thickness with a mandrel mill or plug mill. This is a series of processes in which tubes are formed by a reeler mill, heated to a predetermined temperature by a reheating furnace, and formed into a predetermined outer diameter by a final finishing mill (stretch reducer or sizer mill) to form a tube.

Here, the reason why the raw tube is inserted into the reheating furnace before the finish rolling is that the hot rolling method of the Mannesmann method involves many rolling steps, so that the downstream rolling mill has a variation in the raw tube temperature in the longitudinal direction. . For this reason, it is necessary to equalize the temperature of the entire tube at the temperature of the austenite single phase in a reheating furnace before the final finish rolling, and a reheating process is required, and the finish of the tube during or after finish rolling is required. It has the function of preventing variations in transformation shrinkage and heat shrinkage and increasing the dimensional accuracy of the final product tube. This reheating process is the biggest difference between seamless steel pipe rolling and thick plate rolling, and since this reheating process exists, control rolling such as that performed on thick plates in seamless steel pipe rolling is applied. However, a sufficient fine grain effect cannot be obtained.

Next, the reasons for limiting the manufacturing conditions of the non-heat treated high strength seamless steel pipe in the present invention will be described. In the present invention, the heating temperature of the cast slab or the steel slab is set to the Ac3 transformation point or higher and 1300 ° C or lower. This is because the heating temperature is Ac3
If the temperature is less than the transformation point, the solution is not sufficiently formed. If the temperature exceeds 1300 C, the austenite grain size becomes extremely large, and it becomes difficult to reduce the size of ferrite grains.

Next, the pierced and rolled raw tube is 650 to 7
Hold at a two-phase temperature of 50 ° C. This is a retention temperature of 75
If the temperature exceeds 0 ° C., the crystal grains are coarsened. If the temperature is lower than 650 ° C., the generation of fine carbides serving as nuclei for recrystallization and transformation is suppressed, and the miniaturization of ferrite grains is suppressed. For this retention, a reheating furnace which has been conventionally used for equalizing the pipe temperature in the austenite region may be used at a low temperature, and there is no need to add a new process. Rather, the unit cost of energy can be reduced by lowering the temperature of the reheating furnace, which is advantageous in terms of cost.

Further, when keeping the raw tube at a temperature in the two-phase region of 650 to 750 ° C., it is desirable to keep it under the condition satisfying the following equation (1). Fine carbides that precipitate during austenite grain boundaries, austenite-ferrite grain boundaries and austenite grains during retention become nuclei for recrystallization and transformation, and promote recovery and recrystallization of ferrite after two-phase rolling. The effect of increasing the γ / α transformation ratio at the time of ferrite transformation of processed austenite is obtained, but T (1 oct +20) is 1
If it is less than 8700, precipitation of fine carbides is not always sufficient, and if it exceeds 20,000, the carbides are coarsened and the above effect tends to be suppressed. 18700 ≦ T (1ogt + 20) ≦ 20,000 (1) where T: temperature (K) t: time (h)

Further, by maintaining the temperature in the two-phase region, the temperature of the entire length of the raw tube can be made uniform, and although it is slightly inferior to the case where the temperature is maintained in the austenitic single-phase region, the dimensional variation and longitudinal strength variation of the final product tube are reduced. It can be reduced to a practically acceptable level.

In order to further reduce the dimensional variation and the strength variation in the longitudinal direction of the final product tube, it is desirable that the temperature deviation in the longitudinal direction of the raw tube before holding is 80 ° C. or less. After hot piercing and rolling, 650-75
The cooling rate until the temperature is maintained at the two-phase region temperature of 0 ° C. is desirably cooled at a cooling rate higher than air cooling in order to precipitate the ferrite finely.

In the finish rolling after the temperature is maintained at a two-phase temperature of 650 to 750 ° C., the cumulative cross-sectional reduction rate is 20% or more, and the tube temperature during rolling is 600 to 750 ° C. The reason why the cumulative area reduction rate is set to 20% or more is that the area reduction rate is 20%.
%, The recrystallization of ferrite is insufficient and a uniform fine grain structure cannot be obtained. In addition, the reason why the raw pipe temperature during rolling is set to 600 ° C. or more and 750 ° C. or less is that if the raw pipe temperature during rolling exceeds 750 ° C., the obtained fine-grained ferrite grows and becomes coarse, and If the tube temperature is lower than 600 ° C., recrystallization of ferrite does not sufficiently proceed, and a fine grain structure cannot be obtained.

The temperature of the raw tube during rolling is 600 ° C.
In finish rolling at 50 ° C or lower, a much better external surface can be obtained than in the case of finish rolling in the austenitic region, but in order to obtain a better external surface, finish rolling should be performed under lubrication. Is desirable. As the lubricant, a normal lubricant such as a mineral oil used as a rolling oil may be used, and there is no particular limitation.

According to the above-mentioned manufacturing method, a non-heat treated high-strength seamless steel pipe having an average ferrite grain size of 20 μm or less in a cross section orthogonal to the longitudinal direction of the steel pipe as hot rolled is obtained. When the ferrite particle size exceeds 20 μm, the effect of improving the strength by grain refinement is small, but when the ferrite particle size is 20 μm or less, the effect of improving the strength by grain refinement increases. In particular, when the ferrite particle size is 10 μm or less, the effect becomes remarkable. Therefore, the ferrite grain size can be stably reduced to 20 μm or less, desirably 10 μm or less as hot rolled.
By performing the following, a seamless steel pipe having non-heat treated and high strength can be obtained.

The limit of the ferrite grain size that can be achieved by the production method of the present invention is usually about 3 to 4 μm. In order to reduce the ferrite grain size to 3 μm or less using the production method of the present invention, it is necessary to further refine the austenite grain size before transformation and the ferrite grain size before finish rolling, and for that purpose, a more complicated It is necessary to take heat history,
There is a problem of high cost. Therefore, in consideration of the balance between productivity and product performance, the ferrite grain size is desirably set to more than 3 μm and 10 μm or less.

[0025]

EXAMPLES Continuous cast slabs having a composition shown in Table 1 and having a cross section of 217 mm × 217 mm were formed into seamless pipes by the hot rolling method of the Mannesmann system. After rolling, air-cool to room temperature,
Ferrite grain size, tensile properties and impact properties by Charpy test of the as-rolled steel pipe were investigated. As for the ferrite particle size, a cross section (C cross section) orthogonal to the longitudinal direction of the steel pipe was observed at 2,000 times in 10 visual fields, and the average particle size was measured. Impact characteristics by the Charpy test were evaluated at a 50% fracture surface transition temperature (vTrs). In addition, dimensional accuracy and outer surface skin were also investigated. Table 2 shows the results of these investigations together with the production conditions.

From Table 2, it can be seen that all of the examples of the present invention (Nos. 1 to 6) have an average ferrite grain size of 20 μm or less, and were manufactured by a conventional process of reheating to the austenite single phase region and finish rolling. Comparative Examples (Nos. 7 to 9) having the same components (Nos. 1, 2 and 7, No. 3).
4 and No. 8, No. Nos. 5, 6 and no. Compared with 9), each of them has significantly improved strength and the toughness is equal to or more than that, and the balance between strength and toughness is greatly improved. In Comparative Examples (Nos. 10 to 12) in which any of the conditions were outside the limited range of the present invention, the average ferrite particle diameter was 20 μm or more, and the present invention examples (No. In comparison with (2), the strength was significantly lower in each case, and a comparative example (N
o. It is about the same as 7). In Comparative Examples (Nos. 13 and 14) in which the components are beyond the limited range of the present invention, all have high strength, but have very low toughness.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

As described above, according to the present invention, a hot-rolled non-tempered state can be obtained without adding a large amount of alloying elements and without using ancillary steps such as heat treatment and cold working. A non-heat treated high strength seamless steel pipe having high strength is obtained.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a rolling process of a seamless steel pipe of the Mannesmann system.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Makoto Tamura 1-1 Futaba-cho, Tobata-ku, Kitakyushu Nippon Steel Corporation Yawata Works F-term (reference) 4K032 AA04 AA05 AA06 AA16 AA31 BA03 CA01 CA02 CA03 CC02 CD05

Claims (6)

[Claims] 1. A steel pipe containing, by mass%, C: 0.01 to 0.6%, Si: 0.01 to 1.0%, and Mn: 0.1 to 2.0%. A non-heat treated high-strength seamless steel pipe, wherein the average ferrite grain size in a cross section orthogonal to the cross section is 20 μm or less. 2. A slab or slab containing, by mass%, C: 0.01 to 0.6%, Si: 0.01 to 1.0%, and Mn: 0.1 to 2.0%. A method for producing a seamless steel pipe by subjecting it to hot rolling of the Mannesmann method, in which a slab or a slab is heated to a temperature of from the Ac3 transformation point to 1300 ° C., then pierced and rolled, and subsequently the raw pipe 6
Production of a non-heat treated high-strength seamless steel pipe characterized in that it is maintained at a temperature of 50 to 750 ° C., and then subjected to finish rolling at a raw material temperature of 600 to 750 ° C. at the time of rolling at a cumulative sectional reduction of 20% or more. Method. 3. A slab or slab containing, by mass%, C: 0.01 to 0.6%, Si: 0.01 to 1.0%, and Mn: 0.1 to 2.0%. A method for producing a seamless steel pipe by subjecting it to hot rolling of the Mannesmann method, in which a slab or a slab is heated to a temperature of from the Ac3 transformation point to 1300 ° C., then pierced and rolled, and subsequently the raw pipe 6
It is characterized by carrying out finish rolling at a temperature of 50 to 750 ° C and a condition satisfying the following formula (1), and thereafter, at a cumulative cross-section reduction rate of 20% or more, and at a raw tube temperature of 600 to 750 ° C during rolling. Of non-heat treated high strength seamless steel pipe. 18700 ≦ T (logt + 20) ≦ 20,000 (1) where T: temperature (K) t: time (h) 4. The non-heat treated high-strength seam according to claim 2, wherein after the piercing / rolling is performed, the raw tube is continuously cooled to a temperature of 650 to 750 ° C. at a cooling rate higher than air cooling. No steel pipe manufacturing method. 5. After piercing and rolling, 650 to 750
The temperature deviation in the longitudinal direction of the tube before holding at a temperature of
The method for producing a non-heat treated high-strength seamless steel pipe according to any one of claims 2 to 4, wherein the temperature is 0 ° C or lower. 6. The method for producing a non-heat treated high-strength seamless steel pipe according to claim 2, wherein the finish rolling is performed under lubrication.