JPS6067623A - Preparation of high strength low carbon seamless steel pipe by direct hardening method - Google Patents

Abstract

PURPOSE:To prepare a low carbon seamless steel pipe having high strength and high toughness, by hot processing steel having a specific composition containing C, Si, Mn, Al, B, Ni, Cr, Mo, Cu, V, Nb, Ca, P, S, N and Ti before direct hardening. CONSTITUTION:Steel, which contains, on a wt. basis, 0.06-0.20% C, 0.10-0.50% Si, 0.5-2.0% Mn, 0.01-0.1% Al, 0.0005-0.0050% B, further, Tr-5% Ni, Tr- 1% Cr, Tr-1% Mo, Tr-0.5% Cu, Tr-0.1% V, Tr-0.1% Nb, Tr-0.0010% Ca, 0.03% or less P and 0.015% or less S and follows formula 3.42N-0.008<=Ti<= 3.42N+0.008, Ti>0 in 0.0010-0.0060% N and comprises the remainder of Fe and inevitable impruties, is heated to 1,200-1,300 deg.C and hot processed to mold a steel pipe having a predetermined shape. After this hot processing, the steel pipe is immediately hardened at 750 deg.C or more followed by tempering at a temp. of Ac1 or less to obtain a low carbon seamless steel pipe having excellent strength and toughness.

Description

[Detailed Description of the Invention] (Technical Field) The technical content described in this specification regarding the method of manufacturing low-carbon high-strength seamless steel pipes by so-called direct quenching is applicable to line pipes, marine or land-based borrowed products, etc. In order to achieve compatibility with the above direct quenching, we focused on the unique component behavior in the manufacturing stage of this type of seamless steel pipe that is used for It is located in the field of technology related to the production of seamless steel pipes.

(Problem) In the production of high-strength seamless steel pipes, quenching and tempering treatments are usually performed. At that time, it is common to add a small amount of boron (B) to improve the hardenability, and to add a suitable amount of Al to stabilize the hardenability.

``The quenching effect described in B can be achieved by the conventional method in which hot rolling is used to form a steel pipe of a predetermined size, then allow it to cool down, then reheat to above the AC point and undergo quenching treatment. In the case of so-called direct quenching, in which quenching is performed immediately after hot rolling, the hardenability effect of B cannot be stably obtained in practice.

In addition, Ti is added here to fix N, which inhibits the effect of B.
Although it is already known to add fM, it is still difficult to stably obtain this effect in the production of seamless fM tubes by direct quenching.

(Start of the idea) As a result of various studies, the inventors discovered that the cause of the above-mentioned problem was an inappropriate Ti-containing color N content value. By carrying out this process, we discovered that low-carbon, high-strength seamless steel pipes with excellent strength and notch toughness can be stably manufactured as follows.

Now, when manufacturing seamless steel pipes, billets are
Although it is heated to 1300° C., in this temperature range, Al has no effect of fixing N.

In other words, when manufacturing seamless steel pipes, the billet has a diameter of 1,200 to 1,300 to enable perforation using a perforation machine.
It is necessary to heat the steel at a temperature of When the temperature decreases during inter-rolling, B is combined with B to form BN, which segregates at austenite grain boundaries and reduces the solid solution B claws that should improve hardenability, thereby losing its effect.

After hot working, it is cooled to a low temperature of about 700℃ or less, and then A
It is well known that in the case of normal reheating and quenching, in which the material is heated to the c8 point or higher, Al fixes N as AlN during reheating, thereby increasing solid solution B and improving hardenability. However, in the case of direct quenching, the precipitation of AIN is inhibited as described above, so B
Therefore, the heating temperature before hot rolling is 1200 to 1300.
In the case of direct hardening, it is effective to add Ti, which stably combines with N in this temperature range, and an example of this effect is shown in FIG. 1.

In this way, although Ti has a fixed effect of N, excessive Ti
is precipitated as TiO in the tempering treatment after quenching and significantly deteriorates the notch toughness, so the amount of Ti must be strictly regulated.

In other words, since Ti fixes N in the form of TiN,
The amount of Ti required to fix all of the N contained in the steel is 3.42N, based on the theory of mathematics.

However, industrially, it is impossible to always maintain the amount of Ti, l(in steel) at this ratio.

(Experimental Facts) The inventors investigated the degree of fracture in the strength and impact tests using rusted scraps with various Ti and H ratios, and obtained the results shown in Figure 2. Ta.

ΔTi-=Ti-8.42 taken on the horizontal axis in Figure 2
N indicates excess or deficiency with respect to the amount of Ti theoretically required to fix N.

When ΔT1 is lower than -o, oos%, there is too much solid solution N due to the lack of Ti, which reduces the hardenability improvement effect of B and indicates poor strength and toughness.

On the other hand, when ΔTi exceeds +0.008%, although the effect of improving the hardenability of B is fully exerted by fixing N with Ti, surplus Ti precipitates as Tie during tempering after quenching. Although strength increases, toughness deteriorates significantly.

When ΔTi is between -o, oos and +o, oos, most of the N is fixed and the hardenability improvement effect of B provides cylindrical strength, while also preventing excessive TiC precipitation. (Objective of the invention) To manufacture high-strength seamless steel pipes with excellent strength and toughness by direct quenching by regulating the content of Ti and N within an appropriate range. is the purpose of this invention.

(Structure of the Invention) The above-mentioned object of the invention can be advantageously realized by a procedure based on the following matters.
Si: 0.10-0.50%, Mn: 0.5
~2.0%, Al: o, ox ~0.1%, B: 0.0
005 to 0.0050 and roughly Ni: Tr to 5%,
Cjr: Tr ~1'%, Mo:Tr ~1%,
Ou: Tr ~0.5%, V: Tr ~0.1%,
Hb: Tr ~0.1% and Oa: Tr
Contains ~0.010 chi, p: o, o 3% or less, S: 0.
Contains O15 or less and N: 0.0010 to 0.006
In the 0 part, steel containing T1 according to the following formula 0 according to the N content, and the remainder consisting of Fe and unavoidable impurities is heated to a temperature of 1200°C or more and 1300°C or less and hot worked to form a predetermined shape. A low-carbon, high-strength, seamless steel pipe created by the direct quenching method, which consists of forming a steel pipe into a steel pipe, immediately quenching it at a temperature of 750°C or higher after this hot working, and then tempering it at a temperature below AC processing. Method of manufacturing steel pipes.

In addition, in this case, the process of forming a seamless steel pipe into a predetermined shape by hot working is also carried out when the workpiece is reheated by charging it into a heating furnace maintained at a temperature of 850°C or higher during final processing. Included in the aspect 0 Next, the reason for specifying the component composition and process steps in this invention will be described below. 0 First, the reason for restricting the chemical components is as follows. It is essential for improving hardenability and obtaining high strength.
If it is less than 0.0%, there is no effect, and if it exceeds 0.20%, the carbon equivalent becomes excessive and increases the susceptibility to weld cracking, making it unsuitable for line pipes and structural steel pipes, which are the subject of this invention. 0 Si limited to 0.06~0.50%, 10~0.50% Si is necessary for deoxidizing steel, less than 0.10% has no effect, and over 0.50% it deoxidizes steel. O In: 0.10 to 0.50% since it may cause cracks in the pieces and impair weldability. Mn improves hardenability and increases strength. Although it is effective, if it is less than 0.5%, it is not effective, and if it exceeds 2.0%, weldability and workability are impaired, so it is limited to 0.5 to 2.0%.

Al: o, ol ~ 0.1% Al is a necessary element for deoxidation, but if it is less than 0.01% it is ineffective, and if it exceeds o, z% it remains in the steel as alumina inclusions. 0.01 because it deteriorates the toughness.
OB limited to ~0.1 inch: 0.0005~0.005
0% B is a very effective element in improving the hardenability of steel, but if it is less than 0.0005% it is ineffective, while if it exceeds o or ooso%, it may form precipitates. Since it deteriorates toughness, it is limited to a range of 0.00 (15 to 0.f) O50.

Ni: Tr S-5% N1 is particularly effective in improving the toughness of the base metal and weld zone, preferably at 0.2 mm or more, but it is also a very expensive element, so the lower limit has to be set. The upper limit was set at 5 inches.

Or: Tr~1% Or is effective for improving hardenability, preferably at 0.1%, but if it exceeds 1%, it impairs weldability, so the lower limit was set to Tr1, and the upper limit was set to 1%.

Mo: Tr-1 MO also has the same effect as Ni, and is particularly effective at improving hardenability and suppressing temper brittleness at 0.05% or more, but is very expensive and has a carbon equivalent of - Since it is an element that can be dissolved, the lower limit is T
r, the upper limit was set to 1%.

(!u: Tr-0.5% Ou is effective at 0.1% or more to increase corrosion resistance, but
If it exceeds 0.5%, the cracking susceptibility of the steel slab increases and weldability is impaired, so the lower limit is set to 0.5% from the upper limit of Tr1. It is effective to increase the strength after tempering at 0.01% or more;
If it exceeds this, it will cause cracking of the steel piece and impair the toughness, so the lower limit was set to Tr and the upper limit was set to 0.1.

Nb: Tr ~ 0.1% Nb contributes to the fine grain strength of austenite grains, and significantly contributes to increasing the strength due to precipitation strengthening at 0.01% or more, but when it exceeds 0.1%, it causes cracking in steel pieces. Since this causes a loss of weldability, the lower limit is set to Tr and the upper limit is set to 0.1 inch.

Ca: Tr ~ 0.010T.

Oa is effective at preferably 0.0010% or more to make the shape of the sulfide spherical, improve the toughness especially in the direction perpendicular to the pipe length, and furthermore prevent hydrogen Rfj cracking.
．． If the tensile strength exceeds 0.010, inclusions will form and the toughness will be impaired, so the lower limit and upper limit of Tr1 were set to 0.010.

P≦0.08 P is an impurity contained in steel, and the lower the P content, the better.

If it exceeds 0.03%, the toughness will be significantly deteriorated and tempering brittleness will occur, so the upper limit was set to 0.08% and O8≦0.
015 S is also an impurity contained in steel, and the lower the S content, the better.

If o, o exceeds 15%, toughness will be impaired, so set the upper limit to 0,01
It was set as 5.

N: 0.0010 to 0.0060% N is an impurity contained in steel and it is industrially difficult to reduce it to less than o, ooio%, so the lower limit is set to o, ooio%.
And so. Moreover, if it exceeds 0.0060 mm, the toughness of the weld will be impaired, and if the required amount of Ti is added, a huge Ti
Since N precipitates are formed and the toughness of the base metal is impaired, the upper limit is set to o, o.
As already described, OTi with oeo% is an essential element in this invention, and in order to ensure the hardenability effect of B and to prevent toughness deterioration due to excess T1, it was limited to the range defined by the following formula.

Next, the reason for limiting the manufacturing conditions will be explained.

In the production of seamless steel pipes, billets are
Heating in the range of 800°C is a necessary condition for processing with a drilling machine, and also serves as a prerequisite for keeping the T11N ratio within an appropriate range in this invention.
The heating temperature was limited to 1200-1300°C.

Next, direct quenching after hot working is widely practiced, for example in the field of thick plates, and is known to have economical effects due to thermal energy savings and higher strength due to increased burn resistance. On the other hand, although direct quenching is a well-known fact in the field of steel pipe manufacturing, it has not been put into practical use for steel pipes with low hardenability such as low carbon steel.

That is, in the case of seamless steel pipes, compared to thick plates,
The cause was that the billet was heated to a temperature as high as oo'c, making it impossible to effectively utilize the effect of B. This invention enables manufacturing of low-carbon seamless steel pipes by double quenching by using an appropriate ratio of components. This invention makes it possible to fully exhibit the economical and high-strength effects.Therefore, direct quenching is a prerequisite for this invention.

In direct quenching, the quenching temperature is important.

The temperature was limited to 750°C or higher because at temperatures lower than 750°C, a considerable amount of ferrite may be formed in the structure after quenching, and the strength is low under such conditions. be.

Hardening should originally be performed from the Ar8 transformation point or above, and the temperature of Ar8 should be determined depending on the components, but it is technically difficult to set the Ar8 point corresponding to the direct hardening process.

Even in the component system with the lowest hardenability included in this invention, it was recognized that if direct quenching is performed at a temperature of 750°C or higher, the structure after quenching will be composed of martensite, bentonite, etc. Therefore, the minimum quenching temperature is set to 750.
℃.

Seamless @ pipes are generally processed using a sizer or stretch reducer during hot processing (for each final addition, @H'F 11) is thin. Sometimes the temperature decreases and it may not be possible to maintain the temperature of 750° C. or higher required for direct quenching.

In such cases, it is necessary to raise the temperature of the entire tube by charging it into a reheating furnace before final processing, which is usually 850℃.
If the temperature is shifted to 750°C or higher before quenching, the lower limit of the reheating furnace temperature in this case is 850°C.
It is recommended as an implementation.

Various types of hardening equipment for steel pipes have been devised, including ring spray, immersion, and axial flow types, all of which can be used as a hardening method during direct hardening. A quenching device that provides a cooling water flow along the axial direction of the steel pipe at the inner and outer circumferences of the pipe (Japanese Patent Laid-Open No. 57-859)
No. 30, No. 114616) has a high hardenability and is well suited for direct hardening of low-carbon, high-strength seamless steel pipes.

(Example) Next, the effects of this invention will be explained by giving examples.

Table 1 shows the chemical components of the steel according to the invention and the comparative steel, and Table 2 is an example of the manufacturing conditions and mechanical properties obtained.

In Table 2, the fracture surface transition temperature v after direct quenching and tempering
Trs is very poor in the case of comparison @G, H, I, and J because the content range of TixN is not appropriate, whereas in the case of test steel A
, B, 0. It will be understood that D, E, and F have low vTrs and exhibit extremely excellent toughness.

Furthermore, it can be seen that even if each sample steel is used, when normal reheating and quenching is performed, the strength is slightly lower than when using the direct quenching method.

Note that /1615 is an example showing that sufficient strength and toughness cannot be obtained when the quenching temperature is lowered to 740°C even with steel having the composition according to the present invention.゛As shown in the examples, it is obvious that according to the present invention, a low carbon seamless steel pipe with excellent strength and toughness can be manufactured by the direct quenching method, and its economic and technical effects are extremely large. Some 0

[Brief explanation of the drawing]

Figure 1 shows the hardness part j after direct quenching, A11-B
In the Ti-B system, the hardenability is insufficient at 7F, whereas in the Ti-B system, sufficient hardening is achieved with a 7F roughness. When the transition temperature is in the jE range (-Q, QO8chi~+Q, QO8tsu), that is, the high toughness mEkata? This is a graph showing that the turtle is falling.

Claims (1)

[Claims] L a:o. 06-0.20% by weight, Si: 0.10-0.50% by weight, Mn: 0.5-2.0% by weight, Al: 0.01-0.1% by weight1 B: 0.0005- 0.0050 polydisperse, and further Ni: Tr ~ 5% by weight, Or + Tr ~ 1% by weight, MO: Tr ~ 1% by weight, Ou: Tr ~ 0.5% by weight, V: Tr ~ 0.1 wt%, Nb:Tr ~o,i wt% and (a:Tr ~0,010 wt%, P
: 0.03% by weight or less, S: 0.015% by weight or less, and N: 0.00
A steel containing 10 to 0.0060% by weight of Ti according to the following formula according to the N content, and the remainder consisting of ye and unavoidable impurities is heated to a temperature of 1200°C or more and 1800°C or less and hot worked. , formed into a steel pipe of a predetermined shape, immediately quenched at a temperature of 750°C or higher after this hot working, and then tempered at a humidity of AC□ or lower. A method for manufacturing carbon high-strength seamless steel pipes. λ The process of forming a seamless steel pipe of a predetermined shape by hot working includes the step of reheating the workpiece by charging it into a heating furnace maintained at a temperature of 850 ° C or higher during the final processing, The method described in 1.