JPS5948846B2 - Method for manufacturing large diameter steel pipes with excellent strength and toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing large diameter steel pipes with excellent strength and toughness.

In recent years, with the development of continental transportation of natural gas and crude oil, the diameter of transportation pipes has become larger and the walls thereof have become thicker.

On the one hand, this has exposed the difficulty of forming thick-walled, large-diameter pipes, and on the other hand, it has also brought about higher demands for strength and toughness.

In particular, the yield point is 37 kg/mA or more and the plate thickness is 15 m.
If the pipe has a large diameter of approximately 80 mm and requires high fracture toughness, it is not necessarily easy to use a cold pipe forming method such as ordinary UO forming.

The UO pipe making method involves first creating a bevel on both ends of a flat hot-rolled steel plate using an edge planer, then bending the ends using a crimping press. Furthermore, the steel plate is made into a U shape using U-ing press, and then O-ing press is applied.
s) to form an O shape, and then seam the inner and outer surfaces (sea
m) Perform suturing by welding, and then perform the pipe expansion process (mechan).
icalexpanding), water pressure tests, and various non-destructive tests before being shipped as complete pipes.

This UO pipe manufacturing method is considered to be the most efficient and accurate method for manufacturing thick plate large-diameter pipes, and is expected to continue to be the mainstream method for manufacturing large-diameter pipes in the future.

However, this UO pipe manufacturing method has also become increasingly thicker and larger in diameter in recent years.
It is known that the strength has reached its limit in terms of elastic forming.

The most important technical problem is that the required up-set rate in the O-press is extremely difficult to achieve in thick-walled, large-diameter pipes.

In other words, in order for the pipe to form a highly circular shape using an O-press, it is necessary to apply sufficient compressive strain in the circumferential direction, and this compressive strain is called the upset rate.

The definition is that the width of the material is 2b and the thickness of the material is t. , when the pipe diameter (outer diameter) is D and the circumference is π, then b 7゛7pupa7to ratio = (, (D to) - 1) x 100 (
%).

Normally, it is necessary to give an upset rate of about 0.2% or more, but it may not be possible to give a sufficient upset rate in thick-walled, large-diameter pipes due to the capacity of the O press.

Furthermore, as the strength increases, the required upset rate must be higher than that of conventional materials.

The current O-press load is around 3000 tons, which physically limits the upset rate of thick materials to a low level.

The present invention is an epoch-making method for manufacturing thick-walled, large-diameter pipes that solves these problems all at once.

That is, a steel containing one or more of Nb, ■, Ti, and Mo and containing hydrogen in the steel of 4.5 ppm or less was subjected to controlled rolling (
Controlled Rolling (hereinafter referred to as CR) is applied to produce a thick steel plate with a lower yield point and yield ratio, which is then reheated after being formed into a pipe using the UO forming method, which makes UO forming easier. It has become clear that both strength and toughness are dramatically improved by reheating.

For example, the yield point of products made under the manufacturing conditions of the present invention increases by nearly 10 kg/m4 due to reheating, so as long as the strength after reheating is guaranteed, the yield point will be about 10 kg/- lower than that when making UO pipes. It becomes possible to form the material, and the forming force for thick-walled, large-diameter pipes is much reduced.

What is important in the present invention is that it is essential that the metallurgical mechanism and machining are carefully combined.

In other words, in this component system, the yield ratio (yield point ÷ tensile strength) is as low as 0.85 or less if CR is used, but the dislocation defects of transformation products such as martensite and bainite in the structure regenerate after pipe forming. Pinning (p
(inning), the yield point increases rapidly.

In this case, the relationship with hydrogen is also important; if the hydrogen concentration in the steel exceeds 4.5 ppm, the diffusion activity of C and N slows down, and a sufficient rise in yield point is not observed even in the reheating process.

This is not so noticeable in conventional flat plates, but pipes have compressive strain on their inner surfaces, which affects the interstitial diffusion of C and H, and especially in pipes that require low hydrogen conditions. I think that the.

In addition to this, another reason is that if hydrogen exceeds 4.5 p\m, chain points are likely to appear on the steel plate after CR.

The lower the hydrogen content, the better the above characteristics.
When it exceeds m, it is preferably 3.5 p- or less.

Note that for the dehydrogenation treatment, a commonly used method such as a vacuum degassing treatment or a method of slowly cooling a slab under scorching heat for a long time may be appropriately employed.

The reason for limiting other manufacturing conditions will be described below.

C is important for strengthening steel, and a minimum content of 0.01% is required.

On the other hand, if the content exceeds 0.20%, weldability will be significantly deteriorated.

By the way, when fine carbides and carbonitrides of Nb, V, Ti, or MO are effectively used not only for precipitation hardening but also for suppressing the growth of austenite grains as in the present invention, it is necessary to sufficiently dissolve these elements in solid solution. Rather, it is preferable that C be about 0.10% or less.

The most preferable range is 0.03% to 0.08% in order to facilitate the diffusion of C into transformation dislocations and to ensure a sufficient rise in yield point.

Si is effective in assisting in deoxidizing steel, and a minimum content of 0.03% is required.

On the other hand, it works effectively to improve strength, but if it exceeds 1.0%, weldability deteriorates significantly.

Mn is effective in improving strength, and a minimum content of 0.5% is required.

On the other hand, if the content exceeds 5.0%, Mn segregation causes regions where the transformation point is significantly lowered, resulting in a significant deterioration in toughness.

The most preferable Mn is 1.0- from the balance of strength and toughness.
2.2% theal.

AI is essential for deoxidizing steel, and a minimum of 0.002% is required.

On the other hand, if it exceeds 0.3%, it is not preferable because it causes frequent sagging defects due to A t 203 clusters.

Nb, ■, Ti, and Mo are the main elements of the present invention, and they form fine carbides and carbonitrides after rolling and contribute to improving the tensile strength of steel. It is necessary to contain 0.005% or more.

On the other hand, if the content exceeds 1.0%, coarse precipitates will form, and the amount dissolved during heating before rolling will be small, rather the strength will be saturated and only the toughness will deteriorate.

Note that if the content is 0.8% or less, weldability will not deteriorate significantly, which is preferable.

The most preferable balance between strength and toughness is 0.01-0.5
It is 0%.

Although S is not specified in the present invention, the lower the S content, the better it is for toughness, and when it is about 0.010% or less, the toughness is particularly improved.

Although P is not specified intentionally, it is preferably 0.020% or less in order to suppress temper embrittlement due to reheating as small as possible.

Group A elements consisting of Ni, Cu, Cr, and Co are effective in improving strength, especially tensile strength, without causing a relatively large deterioration in toughness. It is preferable that the content is 0.3% or more.

On the other hand, if it exceeds 1%, it lowers the transformation point of the steel and promotes coarse precipitation of carbonitrides, which is unfavorable in terms of strength and toughness.

The most preferable balance between strength and toughness is 0.1 to 4.
5% is good.

Furthermore, elements of the B group consisting of W and B form fine nitrides, and although Nb, V, Ti, and Mo are not effective, they are effective in suppressing austenite grain growth during heating. A total of 0.0005% or more is required.

On the other hand, if the content exceeds 0.30%, coarse nitrides will be formed, resulting in Nb, V, Ti or M.

This is not preferable because it reduces the effect by half.

Group C elements consisting of rare earth elements, Ca, Mg, and Zr are effective in preventing the expansion of sulfides and improving the impact value, and for this purpose, one or more of them is required in a total amount of 0.001% or more. be.

On the other hand, if it is contained in an amount exceeding 0.1%, its oxide becomes too coarse, resulting in a decrease in toughness.

Next, we will discuss the reasons for the limitations on the manufacturing process.

First, regarding the rolling conditions, the cumulative reduction rate below 950℃ is 3
If it is too low, such as less than 0%, the transformation product becomes too coarse, resulting in a large deterioration in toughness.

If it exceeds 95%, Nb, V, Ti or Mo is added during rolling.
Precipitation of carbides and carbonitrides is promoted so much that not only can the yield point after rolling not be expected to decrease much, but also the effective amounts of C and N required during the reheating process are reduced.

The key is to use CR to create a fine ferrite structure and a fine transformation product that can be called a second phase, and also to create a suitable amount of Nb, ■, and T.
It is necessary to form carbides and carbonitrides of i or Mo, and the optimal cumulative reduction rate below 950°C is 50 to 8
It is 0%.

Note that rolling does not necessarily have to be carried out in the austenite region;
Rolling in a two-phase region with a ferrite content of about 50% or less may be used.

However, if the temperature is less than 590° C., the ferrite becomes a low-temperature two-phase region of more than 50%, resulting in a high yield ratio and the object of the present invention cannot be achieved.

The heating temperature in the reheating step after pipe making is sufficient to allow C and N to diffuse into dislocation defects in the transformed product after rolling, so it is sufficient that it is 100° C. or higher.

However, in the presence of Nb, ■, Ti, or Mo, diffusion is difficult due to their affinity; on the other hand, in order to diffuse between isolated transformation products, the temperature is 350°C.
℃ or higher is preferable.

On the other hand, if it exceeds 750°C, the transformation product will decompose, resulting in a decrease in strength.

Further, the temperature is preferably 650° C. or lower in order to prevent a decrease in strength due to coarsening of fine carbides and carbonitrides of Nb, V, Ti, and Mo.

Furthermore, what is noteworthy about the present invention is that reheating not only increases the yield point, but also significantly improves the toughness, especially the fracture propagation arresting properties, although the reason for this is not necessarily clear. One is Nb, V, and T, which are harmful to toughness.
By moderately coarsening the ultrafine precipitates of i or Mo, and by pinning the transformation products with C and N, they become even harder, increasing the hardness difference with the underlying ferrite structure. Two possible reasons are that it promotes separation and therefore improves fracture propagation arresting properties.

The most suitable temperature for improving this toughness is 450°C to 650°C.

From the above points, the most preferable temperature range from the balance of strength and toughness is 450°C to 650°C.

For the same reason, the holding time is preferably in the range of 1 minute to 10 hours, and most preferably 10 minutes to 2 hours from the viewpoint of the balance between strength and toughness.

Note that holding here does not necessarily mean holding the temperature constant, but cooling may start immediately after reaching the maximum temperature; in short, the maximum temperature is within the above temperature range,
Moreover, it is sufficient that the cumulative holding time within the specific temperature range falls within the above-mentioned specific time.

In this case, the cumulative retention time will be referred to as retention time.

Note that the plate thickness targeted by the present invention is 15 mm to 80 mm.

In other words, from the purpose of the present invention, a diameter of less than 15 mm cannot be covered by the invention because UO molding can be easily performed, whereas a diameter of more than 80 mm cannot be subjected to UO molding even if the method of the present invention is followed, so it is outside the scope of the invention.

In view of the balance between strength, toughness and UO formability, the preferred plate thickness range is about 25 to 48 mm.

On the other hand, the yield point after the reheating step is 37 kg/- or more.

Further, the yield point after the reheating process for a plate thickness of 25 to 48 mm is about 40 to 63 kg/mIt.

Next, examples of the present invention will be shown below.

[Example 1] Steel having the chemical composition shown in Table 1 was tapped in a converter (El and B2 are electric furnaces).

Among these, B1, B2, C1°C2, El, B2, Fl,
F2, ■, K, M, N, P, and Q were subjected to vacuum degassing treatment, and G1, G2, and G3 were subjected to vacuum degassing treatment after dephosphorization treatment in special refining treatment.

Note that the slabs other than Dl, B2, K, L, and 0 were made into slabs by ingot-forming and blooming rolling methods, while the slabs for D1, B2, K, L, and 0 were made into slabs by continuous casting.

Among them, A1, A2, El, B2, G1, G2, Hl,
For B2, J, and P, the slabs were soaked and slowly cooled for 24 hours after blooming, while for DI, B2, L, and 0, the slabs were also soaked and slowly cooled for 24 hours.

Note that vacuum degassing treatment and long-time soaking and slow cooling of slabs are both types of dehydrogenation treatment.

Hot rolling conditions, thickness of hot rolled sheet, limit upset rate during O pressing of UO forming, pipe forming diameter, yield point and base material toughness of hot rolled sheet, yield point of pipe after reheating treatment before UO forming Table 1 shows the yield point and base material toughness (yield point and base material toughness after UOO forming for conventional pipes that do not undergo reheating after UOO forming) and reheating treatment conditions after UOO forming.

In addition, in this experiment, the upset rate was 0.2.
Pipes with less than 0% had poor shape.

In addition, the tensile test is API tensile test 1 @ piece (direction perpendicular to rolling).
The yield point was measured at 0.5% under 1 oad.

The Charpy test is a 2 mm V1 impact test (in the direction perpendicular to rolling).

DWTT (Drop WeightTear Te5
t) Test (direction perpendicular to rolling) is 85% 5ATT (Shea
r Area Transition Tempera
It is evaluated at a test temperature showing a ductile fracture ratio of 85% by the API method, and the lower this temperature is, the better the fracture propagation arresting property is.

Now, as shown in Table 1, the chemical composition and manufacturing process of the present invention significantly improves the strength and toughness of the reheated pipe compared to that of the hot-rolled sheet before pipe forming. I understand.

On the other hand, in A3 and G3, which have a higher H content by 4.5 ppn, the other conditions are within the conditions of the present invention, but the strength and toughness characteristics of the pipe are not significantly improved compared to those of the hot rolled sheet.

On the other hand, even if the chemical components are included in the invention, the manufacturing process is outside the invention. P, or vice versa, Q and R, both have inferior pipe strength and toughness.

In addition, BP (British
Petroleum) tests showed that among the invention steels in Table 1, Dl, El, Fl, G1, ■, L, and M were all crack-free, while other comparative steels showed slight straight cracks or stepwise cracks. Ta.

[Example 2] The same tests as in Example 1 were conducted using steel having the chemical composition shown in Table 2.

The results are shown in Table 2. As shown in Table 2, the present invention steel The yield point and toughness of the pipe are greatly improved.

On the other hand, among the comparative steels, the pipe yield point of Y is similar to that of Xl, but since the method of the present invention is not applied, the yield point of the hot rolled sheet is too high and the upset rate during O-pressing is low. O
It is difficult to press and therefore the shape of the pipe is poor.

As shown above, the present invention is extremely effective in manufacturing thick-walled, large-diameter pipes that require high strength and high toughness.

Furthermore, considering the principle of the present invention, the present invention is not necessarily applied only to the production of pipes using the UO forming method, but it is also useful to use it for spiral pipes and other medium and small diameter pipes. It is.

It goes without saying that in view of the process, etc., it is most useful to apply the UO molding method to large diameter pipes.

It should be noted that any heat treatment method after UO molding may be used, and that a uniform temperature over the entire length of the pipe is not particularly required.

Therefore, it is believed that the industrial value of the present invention will be further expanded.

It should also be noted that the pipe of the present invention also has high resistance to hydrogen-induced cracking.

[Example 3] Steel having the chemical composition shown in Table 3 was tapped in a converter and subjected to vacuum degassing treatment.

Here, Z is a continuous casting method, and W is a slab made by a normal ingot-forming/blurring rolling method.

Hot rolling conditions, hot rolled sheet thickness, Monaca method elbow (Elb
ow) Table 4 shows the dimensions of the pipe after press forming and welding, the reheating treatment conditions after bone formation, the material of the hot rolled plate, and the material of the elbow pipe of the final product.

In addition, here, the shape of the elbow tube is determined by the roundness Dmax of the tube end.
-Dmin [Xl0Q (%), where O Dmax: maximum outer diameter, Dmin: minimum outer diameter, Do:
The smaller the outer diameter, the better the shape, and usually 1% or less is sufficient for the product.

Here, the heat input for welding is 45KJ/cm on the outside and 40K on the inside.
J/cm submerged arc welding.

Further, in the press forming, W is cold pressing at room temperature, and Z is cold pressing at room temperature and warm pressing after holding the stone at 00°C.

As shown in Tables 3 and 4, the present invention is extremely effective in manufacturing fittings such as Monaca type elbow pipes, which have excellent strength and crack propagation arresting properties, and can produce high-quality fittings with a small press force. It is possible to manufacture products with excellent strength, high toughness, and shape.

However, the toughness is slightly degraded by warm pressing at 500°C.

Needless to say, Fitzteindus is a type of thick-walled, large-diameter steel pipe that is used around pump stations for gas line pipes in cold regions.

Claims (1)

[Claims] 1C: 0.01-0.20%, Si: 0.03-1
．． 0%, Mn: 0.5-5.0%, Al: 0.00
2 to 0.30%, and further contains a total of 0.005 to 1.0% of one or more of Nb, V, Ti, and Mo, and the amount of hydrogen in the steel is 4.5 ppmJ! A steel plate with a thickness of 15 mm to 80 mm is obtained by hot rolling a steel consisting of the remainder Fe and unavoidable impurities at a cumulative reduction rate of 30 to 95% below 950 °C and a finishing temperature of 590 °C or above. None, after forming and welding the steel plate into a steel pipe, the temperature was 10
A method for producing a large-diameter steel pipe with excellent strength and toughness, which comprises performing heat treatment at 0°C to 750°C for a holding time of 1 minute to 10 hours. 2C: 0.01-0.20%, Si: 0.03-1
．． 0%, Mn: 0.5-5.0%, AI: 0.00
2 to 0.30%, further contains one or more of Nb, V, Ti, and Mo in a total of 0.005 to 1.0%, and further contains one of the elements of group A consisting of Ni, Cu, Cr, and Co. B consisting of B and W with a total of 0.03 to 10% of seeds or more
0.0005 to 0.30% in total of one or more elements of Group 0, and 0.001 to 0.1% of one or more of Group 0 elements consisting of rare earth elements, Ca, Mg, and Zr. any one of
group or contains two or more groups, and the amount of hydrogen in the steel is 4.5 pIl
A steel plate with a thickness of 15 mm to 80 mm is obtained by hot rolling a steel with a rr of 1 or less and the balance being Fe and unavoidable impurities at a cumulative reduction rate of 30 to 95% at 950 °C or less and a finishing temperature of 590 °C or higher. After the steel plate is formed and welded to form a steel pipe, it is heat treated at a temperature of 100°C to 750°C for a holding time of 1 minute to 10 hours.A large diameter pipe with excellent strength and toughness. Method of manufacturing steel pipes.