JPH0428779B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to seamless steel pipes mainly used for pipelines for transporting oil, gas, etc., and particularly to high-toughness seamless steel pipes manufactured by direct quenching and tempering. Conventional technology Recently, in the production of smooth steel pipes used as oil country tubular goods, line pipes, etc., mainly from the viewpoint of energy saving, the steel pipes after hot rolling are quenched from their original temperature and then heated. After that, a method of tempering the steel in the same way as ordinary tempered steel, that is, a so-called direct quenching/tempering method is increasingly being applied. However, when direct quenching and tempering are applied to manufacture seamless steel pipes, the austenite grains immediately before quenching are generally coarse;
The toughness after tempering is also inferior compared to conventional reheated and tempered materials. In other words, when applying direct quenching and tempering to the production of steel sheets, the austenite before direct quenching is It is possible to make the grains finer to some extent and obtain toughness equivalent to ordinary reheated and quenched/tempered steel sheets, but controlled rolling is difficult in seamless steel pipes and hot rolling processes (pipe manufacturing processes), so direct rolling is difficult. The reality is that it is unavoidable that the austenite before quenching becomes coarse and the toughness after tempering deteriorates. In particular, steel pipes for line pipes are required to have high toughness in the direction perpendicular to rolling, but the toughness in the direction perpendicular to rolling is compared to the toughness in the rolling direction, even in the case of ordinary reheat-quenched and tempered materials. Seamless steel pipes obtained by direct quenching and tempering have low toughness, especially in the direction perpendicular to the rolling direction, and therefore have problems when used as steel pipes for line pipes. . As a means to improve the toughness of such seamless steel pipes and directly quenched/tempered materials, for example, Japanese Patent Publication No. 11927/1983 describes a method to improve the toughness of seamless steel pipes and directly quenched/tempered materials by increasing the toughness by a considerable strain of 0.2 or more in secondary hot working immediately before direct quenching. A method of refining austenite grains by adding a process of
A method is described in which austenite grains are refined by cooling to an Ar point of ₁ or less and then heating to an Ac _{of 3} points or more. However, with the exception of stretch reducing mills used to manufacture small-diameter seamless steel pipes, in the manufacture of medium-diameter seamless steel pipes that are commonly used as line pipes, this is equivalent to secondary hot working. Normally, the degree of processing in the final sizing process is small, and large and special equipment is required to add significant strain of 0.2 or more, leading to the problem of increased equipment costs. In addition, the steel pipe after the first hot working is cooled to below ₁ point of Ar, and then
In the method of achieving grain refinement by heating to Ac ₃ points or higher, the process requires cooling once during hot rolling and then reheating. Additional time is spent in the cooling process and the subsequent reheating process, leading to a problem in which the rolling efficiency is reduced and the economic benefits of direct rolling are lost. Problems to be Solved by the Invention As already mentioned, when direct quenching and tempering treatments are applied to manufacture seamless steel pipes, the austenite grains before direct quenching are coarse;
It has been difficult to obtain high toughness in the past, and for this reason, direct quenching and tempering treatments are required for steel pipes that require high toughness, especially for line pipes that require high toughness in the direction perpendicular to rolling. The reality is that the seamless steel pipes manufactured in this way were considered unsuitable. Furthermore, the method described in the aforementioned Japanese Patent Publication No. 57-11927 for improving the toughness of directly quenched and tempered seamless steel pipes is not suitable for practical use due to problems such as increased equipment costs and reduced rolling efficiency. It was not possible to apply it to Therefore, an object of the present invention is to provide a seamless steel pipe that can obtain high toughness even through direct quenching and tempering treatment, and in particular, it is an object of the present invention to provide a seamless steel pipe that can obtain high toughness even when subjected to direct quenching and tempering treatments, and in particular, to avoid problems such as an increase in equipment costs and a decrease in rolling efficiency. The aim is to achieve high toughness by adjusting the chemical composition of steel pipes so that there is no corrosion. Means to Solve the Problems The main features of the seamless steel pipe of this invention are that the amount of C is kept low to achieve high toughness, and at the same time, the content of P and S as impurities is kept to a low level not found in conventional seamless steel pipes. The goal is to limit Also,
Add Ca as needed, and use elements that increase hardenability such as B and solid solution strengthening elements such as Cu and Ni to ensure strength, instead of relying on C or precipitation-strengthening elements. This is also one of the features of this invention. Some of these methods have been known to some extent individually as effective means for improving toughness, but an effective combination of these methods can improve the direct quenching and tempering of seamless steel pipes. It was not previously recognized that the toughness of the material could be improved to a level close to that of conventional reheated, quenched and tempered materials. In other words, as mentioned above, in the direct quenching of seamless steel pipes, poor toughness due to the coarse structure was noticeable, so direct quenching was previously thought to be inappropriate for line pipe steel pipes. Therefore, in the past, it has been difficult to combine the above-mentioned methods to apply direct quenching and tempering of seamless steel pipes mainly to steel pipes for pine pipes.
It had not been particularly attempted. Specifically, the seamless steel pipe of the first invention of the present application contains 0.01 to 0.08% C, 0.10 to 0.50% Si, and Mn.
0.50-2.00%, Al 0.001-0.100%, and P and S as impurities P 0.010% or less,
This is a directly quenched and tempered seamless steel pipe characterized by S content of 0.0020% or less, with the remainder consisting of Fe and other unavoidable impurities. Moreover, the seamless steel pipe of the second invention has C 0.01~
0.08%, Si 0.10~0.50%, Mn 0.50~200%, Al
Contains 0.001 to 0.100%, as well as 0.50% or less Cu, 1.0% or less Ni, 0.030% or less Ti, 0.0030
% or less of B and 0.0060% or less of Ca, and contains P and S as impurities of P 0.010% or less, S
This is a directly quenched and tempered seamless steel pipe characterized by a content of 0.0020% or less, with the remainder consisting of Fe and other unavoidable impurities. Detailed Description of the Invention First, the basic components of the seamless steel pipe of the present invention, that is, the reasons for limiting the components defined in the first invention will be explained. C: C is an element harmful to toughness, and in order to improve toughness, it is desirable that the amount of C be as low as possible. Therefore, the upper limit of C content is set at 0.08%, which is lower than that of ordinary seamless steel pipes. However 0.01
If it is less than 0.0%, it will be difficult to secure strength even if the alloying element is used extensively, so it was set within the range of 0.01 to 0.08%. Si: Si is a component added as a deoxidizing element in ordinary steel, and in the present invention, it is added within a range of 0.10 to 0.50%, which is generally considered to be an appropriate level from the viewpoint of toughness. Mn: Mn is an element that improves hardenability.
It is effective for ensuring strength while preventing a decrease in toughness, and for this purpose it is necessary to add at least 0.5% or more. However, Mn tends to segregate in a layered manner in rolled steel materials, and if excessively contained, the toughness in the direction perpendicular to the rolling direction deteriorates, so the upper limit was set at 2.00%, and it was decided to add within the range of 0.5 to 2.00%. . Al: Although Al is not directly involved in strength, it is an effective element for improving toughness. Especially in directly quenched materials, N, which is present in steel during quenching and is harmful to toughness, is an element that is effective in improving toughness.
It has the effect of improving toughness by precipitating as AlN during tempering. However, Al
If the concentration is less than 0.001%, the effect cannot be obtained.
The lower limit was set at 0.001%. On the other hand, considering that the nitrogen content as an impurity in normal steel is about 0.0100% at most, a maximum Al content of about 0.060% is sufficient, but up to about 0.100% does not have a negative effect on toughness, and B When containing aluminum, a large amount of Al may be added, so the upper limit of Al is 0.100.
%. P and S: Both of these elements are impurity components that are inevitably contained in steel, and both have a strong effect of deteriorating toughness. In the directly quenched and tempered material targeted by this invention, the toughness is inferior if the normal ingredients are used as they are, so in this invention it is particularly important to limit the contents of P and S to even lower levels than in the past. Therefore, it is preferable to keep the P content and S content as low as possible, but both levels are lower than normal components.
By limiting P to 0.010% or less and S to 0.0020% or less, the effect of improving toughness in directly quenched and tempered materials becomes significant, so the upper limits were set as such. In the case of the seamless steel pipe of the first invention of the present application,
The remainder of each of the above components may be Fe and unavoidable impurities other than the above-mentioned P and S, but in the case of the second invention, in addition to the above components, Cu, Ni,
It contains one or more selected from Ti, B, and Ca. The reasons for limiting these components will be explained below. Cu: Cu functions to improve strength while maintaining toughness. Therefore, by adding Cu, it becomes possible to reduce the amount of C, which is advantageous for improving toughness. however
If the amount of Cu exceeds 0.5%, hot workability deteriorates, so the upper limit of Cu was set at 0.5%. There is no particular lower limit when adding Cu, but it is usually
The content is preferably 0.10% or more. Ni: Like Cu, Ni has the effect of increasing yield stress without deteriorating toughness, and by correspondingly reducing other alloy components, it is advantageous for improving toughness. However, Ni is an expensive element, and if added in large amounts it reduces the economic benefits of direct quenching, so the upper limit was set at 1.0%. Furthermore, Ni
The lower limit for adding is not particularly limited, but it is usually desirable to set it to 0.10% or more. B: B is an element effective for improving strength, and C,
It is an indispensable element when the amount of Mn is small, when the pipe wall thickness is thick, or when the target strength is high, and it also has the effect of improving toughness by improving hardenability at the same time as improving strength. These effects can be expected even with a small amount of B, but if it is contained in excess, the toughness deteriorates, so the upper limit was set at 0.0030%. There is no particular lower limit for B, but if you want to actively obtain the effects of B, it is preferably 0.0005% or more. Ti: Ti has the effect of enhancing the effect of B by adding it simultaneously with B when it is added to the directly quenched material. The addition of Ti produces TiN, which is stable even at high temperatures.
This is because it traps N, which inhibits the effect of B. However, if Ti is added in excess, the effect of reducing toughness due to TiC precipitation etc. becomes stronger, so the upper limit of Ti was set at 0.030%. There is no particular lower limit when adding Ti, but in order to actively obtain the effect of adding Ti,
It is preferable to add 0.005% or more. Ca: Ca is an effective element for further improving toughness, but if its content is excessive, it will actually deteriorate toughness, so the upper limit was set at 0.0060%. Note that the lower limit of Ca addition is not particularly limited, but when the effect of Ca addition is actively expected, it is preferable to add 0.0010% or more. In addition, in the first and second inventions of the present application, it is possible to obtain a seamless steel pipe with high toughness by direct quenching and tempering by limiting the component elements as described above, but especially when high strength is required. In addition to each of the above component elements, 1 of Cr, Mo, Nb, and V is included in the range shown below.
To obtain a seamless steel pipe by direct quenching and tempering, which has high strength and high toughness without compromising toughness by containing these alloying elements. be able to. In other words, Cr improves hardenability and has a weak precipitation strengthening effect, but
Too much of it tends to deteriorate toughness, so
Acceptable within the range of 0.50% or less. Furthermore, although Mo has a large effect of improving hardenability and is effective, it also has a stronger effect of degrading toughness due to precipitation strengthening than Cr, so the allowable range is 0.20% or less. Nb is an element often used in steel materials for line pipes, and in the case of reheat-quenched and tempered materials, it can be expected to improve toughness by refining grains, but in the case of directly quenched and tempered materials, it can be expected to improve toughness. The precipitation strengthening effect becomes extremely pronounced, and therefore the toughness deterioration effect also appears strongly. Therefore, the tolerance range for Nb is lower than the normal tolerance range, 0.030
The upper limit is %. Furthermore, V also has a strong precipitation-strengthening effect, and too much V will result in greater toughness deterioration.
is within the range that does not cause significant toughness deterioration, that is,
Permissible within the range of 0.050% or less. As described above, each of the inventions of the present application suppresses C, which is harmful to toughness, as much as possible, and limits P and S as impurities to a lower range than before, and especially in the case of the second invention, In addition, by adding elements that improve hardenability and solid solution strengthening elements, direct quenching and tempering materials for seamless steel pipes have high toughness close to that of conventional reheating quenching and tempering materials. It became possible. In the production of the seamless steel pipe of this invention, the seamless steel pipe is produced (rolled) by hot working according to a conventional method, and then directly quenched using the retained temperature, and then tempered. good. In other words, in the seamless steel pipe of the present invention, after it is once cooled to below the Ar _point during rolling,
High toughness can be achieved without special operations such as reheating to Ac ₃ or higher points or extremely high processing rates in the final sizing process. Example Seamless steel pipes of steels A to F (conventional steels) and steels G to T having the compositions shown in Table 1 were rolled to the pipe sizes shown in Table 2 by the usual Mannesmann-plug mill rolling method. The tube was then directly quenched immediately after rolling. However, for conventional steel, a reheating and quenching process was also performed in which some of the rolled tubes were once cooled to room temperature and then heated to Ac ₃ or higher for quenching. Directly quenched steel pipes are not subjected to a grain refining process in which the pipe is cooled down to below ₁ point of Ar during rolling, but are simply rolled according to the normal process and directly quenched. . Steel pipe after being directly quenched as described above,
The reheated and quenched steel pipes were tempered at various temperatures from 620 to 720°C, and the strength and toughness of the steel pipes after tempering were examined. In addition, all the strengths were examined using API full-thickness test specimens parallel to the tube axis, and the toughness was measured using 10 mm It was investigated using a Charpy impact test piece. Figure 2 shows the strength and toughness of Steel A, a typical conventional steel, when tempered at various temperatures after direct quenching, and the strength and toughness when tempered at various temperatures after reheat quenching. This is a comparison of toughness. As is clear from Figure 2, in the case of conventional steel, the sharpy fracture surface transition temperature of the reheated quenched and tempered material is around -90℃, while the sharpy fracture surface transition temperature of the directly quenched and tempered material. is around -40°C, and even when compared at the same strength level, the directly quenched and tempered material has a shear py fracture transition temperature that is around 40°C higher than the reheated and tempered material, and its toughness has deteriorated. That is clear. On the other hand, Fig. 1 shows the present invention steel G, which is a representative steel type of this invention invented to improve the toughness deterioration of directly quenched and tempered materials in conventional steels as described above.
This is a comparison of the strength and toughness after direct quenching and tempering of this steel and typical conventional steel B. 1st
From the figure, it can be seen that the conventional steel B has a high sharpy fracture surface transition temperature of around -50°C after direct quenching and tempering, and its toughness is inferior, whereas in the case of the invention steel G, the same direct quenching and tempering It can be seen that the material's Charpy fracture surface transition temperature has been significantly improved to around -90℃. In other words, it is clear that in the steel of the present invention, even when direct quenching and tempering treatment was applied, high toughness comparable to the toughness of conventional steel after reheating and quenching and tempering was achieved. Furthermore, Table 2 shows the various steels A~ shown in Table 1.
F, G to T are rolled to each pipe size shown in the table,
Among the test results of tempering at various temperatures after direct quenching, the data after tempering at 650°C for 10 minutes are shown as a representative. From Table 2, it is clear that all of the steels of the present invention have superior toughness compared to conventional steels.

【table】

[Table] Depends.
Effects of the Invention As is clear from the examples described above, according to the present invention, it is possible to produce steel pipes with excellent toughness in directly quenched and tempered seamless steel pipes, for which it has been difficult to obtain high toughness in the past. Furthermore, since there is no need to make any changes to the regular seamless steel pipe rolling process (pipe manufacturing process), this direct quenching/tempering treatment provides high toughness without reducing equipment costs or rolling efficiency. Since a seamless steel pipe can be obtained, the economic advantages of direct quenching and tempering can be fully utilized.

[Brief explanation of drawings]

Figure 1 is a distribution diagram comparing the toughness and strength of the directly hardened and tempered materials of the invention steel G and conventional steel B when tempered at various temperatures. FIG. 2 is a distribution chart showing the toughness and strength of directly quenched/tempered materials and reheated quenched/tempered materials when tempered at various temperatures.

Claims (1)

[Claims] 1 C 0.01 to 0.08% (weight%, same hereinafter), Si
0.10~0.50%, Mn 0.50~2.00%, Al 0.001~
0.100%, and P and S as impurities are P 0.010% or less and S 0.0020% or less,
A high-toughness seamless steel pipe that is directly quenched and then tempered, the balance of which is Fe and other unavoidable impurities. 2C 0.01~0.08%, Si 0.10~0.50%, Mn
Contains 0.50-2.00%, Al 0.001-0.100%, as well as 0.50% or less Cu, 1.0% or less Ni, 0.030%
Contains one or more selected from the following Ti, 0.0030% or less B, and 0.0060% or less Ca, and contains P and S as impurities.
0.010% or less, S 0.0020% or less, and the balance is Fe.
A high-toughness seamless steel pipe that is directly quenched and then tempered, and is characterized by containing unavoidable impurities.