JPH0250914A - Production of low ally high tension seamless steel pipe having fine grained structure - Google Patents

Abstract

PURPOSE:To produce a high tension seamless steel pipe having fine grained structure and excellent sulfide stress cracking resistance and low temp. toughness by controlling the steel components and hot rolling condition at the time of producing the seamless steel pipe with low alloy steel. CONSTITUTION:The low alloy steel billet of compositions containing 0.05-0.35wt.% C, 0.01-0.5% Si, 0.15-1. 5% Mn, <0.01% S, <0.02% P, 0.05-0.4% Mo and 0.005-0.1, Al or further, one or more kinds of the specific contents among Cr, Ni, V, Ti and B and singly or complexly containing one or more kinds among rare earth element, Ca, Co and Cu in >=1200 deg.C is hot-piercing- continuous-rolled and finishing-rolled at the temp. of >=Ar3 point and the obtd. hollow raw pipe is heated at 900-1100 deg.C. The hot-finishing-rolling is executed to this raw pipe at the temp. of >=Ar3 point + 50 deg.C to the last finishing temp. and after finishing-working to the seamless steel pipe, quenching is executed from the temp. of >=Ar3 point and after that, tempering treatment is executed at the temp. of <=Ac1 point, to produce the high tension seamless steel pipe having fine grained structure and excellent various characteristic.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing a low-alloy high-strength seamless steel pipe with a fine-grained structure.

[Prior Art] In order to obtain a low-alloy high-strength seamless steel pipe with a fine-grained structure using a hot-rolled seamless steel pipe, for example, as disclosed in Japanese Patent Application Laid-open No. 5277813, a hot rough-rolled hollow shell pipe is forcibly heated once. The temperature is lowered to below Ar1 point and heated again to the austenitizing temperature, followed by rapid cooling quenching and tempering immediately after the finish rolling, or reheating quenching and tempering after the completion of normal finish rolling. There was a way.

[Three Problems to be Solved by the Invention] However, all of the above-mentioned processes have the disadvantage of complicating the manufacturing process in addition to the problem of thermal efficiency. On the other hand, in the conventional direct quenching treatment after seamless hot rolling, the austenite grain size is ASTM No. 1 to 6.
Since the grains are coarse and the variation is large, there is a problem that it is not possible to obtain a low-alloy, high-strength, seamless steel with a fine-grained structure.

[Means for Solving the Problems] The present invention, Nagisa et al., has developed a low-alloy high-strength seamless steel pipe with a fine-grained structure, as well as an SMI (seamless) steel pipe with excellent sulfide stress cracking (hereinafter referred to as SSC) resistance. As a result of many experiments and studies aimed at manufacturing steel pipes, we found that by controlling the steel composition and hot rolling conditions, it is possible to produce low-alloy, high-strength seamless steel pipes with a fine-grained structure.

The present invention was constructed based on this knowledge, and the gist thereof is as follows: 0005-035% heated to a temperature of 1200°C or higher, sl: 0.01-0.5%, Mn: 0.15-1. 5%, S: 0.01% or less, P: 0.02% or less, Mo: 0.05-0.4%, ΔI: 0.005-01%, and further contains Cr: 0.01% if necessary. 1-1.5%, Ni: 0.1-2.
0%, ■001~0.1%, Ti:0.01~01%
, B・0.0003~0.003% of one or more types and rare earth elements: 0.001~0.05%, Ca: 0
．． 001-0.02%, Co: 0.05-0.5%
, Cu: A steel piece containing 0.1 to 0.5% of one kind or two or more kinds, the remainder being substantially Fe, is hot-pierced and continuously rolled at a finishing temperature of 3 or more points Ar. The finished steel pipe obtained by making a pipe and heating it to a temperature of 900 to 1000°C higher than the temperature of the raw pipe and hot finish rolling at the final finishing temperature or Ar3 points + 50°C or more is This is a method for producing a low-alloy, high-strength seamless steel pipe with a fine grained structure, which undergoes a quenching process in which it is rapidly cooled from a temperature, followed by a tempering process in which it is heated to a temperature below AC1 point and then cooled.

“Effect” The manufacturing method of the present invention will be explained in detail below.

First, the reason why the present invention is limited to the above-mentioned steel components will be explained.

C and Mn increase the hardening effect and increase the strength, yielding point 6
This is important in order to stably obtain high tensile strength steel of 0 to 80 kgf/mm2 and to achieve grain refinement. Too little will have no effect, and too much will cause quench cracking, so 0.05% to 0.35% and 0.15% to 1.5%, respectively.
And so.

Si is a residual deoxidizer and is an effective component for increasing strength. If it is too small, there will be no effect, and if it is too large, inclusions will increase and the properties of the steel will become brittle.
It was set at 5%.

P is a harmful component that causes grain boundary segregation and tends to cause cracks during processing, so its content was set to 0.02% or less.

S is a component harmful to low temperature toughness by forming MnS-based inclusions and stretching by hot rolling, and its content (2) was made to be 0.02% or less.

Mo is effective in increasing strength, improving toughness, etc. If it is too small, the effect will not be achieved, and if it is too large, the effect will be saturated, and it is very expensive.

The layer contains a residual deoxidizing agent similar to Si, which combines with N contained as an impurity component in the steel to suppress the growth of crystal grains, lower the transition temperature of the steel, and improve low-temperature toughness. If it is too small, there will be no effect, and if it is too large, inclusions will increase and the properties of the steel will become brittle, so the range was set at 0.005-01*.

When further increasing the strength of steel with the above composition C
Selective addition of other components as necessary. Cr, Ni
, V are added to increase the hardenability of the steel and increase its strength. If it is too small, it will not have the effect, and if it is too large, the effect will be saturated, and it is very expensive, so 0.01-1.5%, 0.1-2.0%, 0.0
The content was set at 1% to 0.1%.

T1 combines with N contained as an impurity component in the steel to suppress the growth of crystal grains and increase strength, and also exhibits the hardenability of B due to its deoxidizing and denitrifying effects. If it is too small, there is no effect, and if it is too large, Tic will precipitate and the steel will become brittle, and inclusions will increase and the properties of the steel will become brittle, so 0.0
The content was set at 1% to 0.1%.

B significantly improves hardenability and increases strength. If there is too little, there is no effect, and if there is too much, the effect remains the same.
0.0003 to 0.0.
003%.

Furthermore, in view of the usage environment of seamless steel pipes in recent years, the present invention selectively adds components such as rare earth elements as necessary to improve the SSC of steel having the above-mentioned composition.

The rare earth element, Ca, is an effective component that makes inclusions spherical and harmless. If it is too small, there will be no effect, and if it is too large, inclusions will increase and the SSC resistance will decrease.
It was set to 0.02%.

Co and Cu have the effect of suppressing hydrogen intrusion into steel and have S resistance.
Works effectively on SC characteristics. If the amount is too low, there will be no effect, and if it is too high, the effect will be saturated, so 0.05 to 0, respectively.
．． 5% and 0.1 to 0.5%.

Next, the reason why the rolling conditions in the final process of hot continuous continuous rolling are limited as described above will be explained.

Steel having the above-mentioned composition is melted in a melting furnace such as a converter or an electric furnace, or further subjected to vacuum descaling treatment, and steel slabs are manufactured by a continuous casting method or an ingot-blowing method. The steel billet is heated to a temperature of 1200° C. or higher, either immediately or after being cooled once. The heating temperature must be high enough to dissolve most of the C, Cr, ■, Ti, etc. into solid solution before hot piercing and continuous rolling. The heating temperature was set at 1200° C. or higher, as long as the temperature was within the range of the components specified in the present invention, all of the components would form a solid solution at a temperature of 1200° C. or higher, and there would be no problem in terms of hot forming efficiency.

The heated steel billet is transferred to a hot continuous rolling mill, where it is rolled to a target outer diameter and wall thickness, and roughly formed into a hollow tube. This rolling does not have a major effect on the material quality of the manufactured steel pipe, and the figure shows the effect of hot continuous rolling on austenite (hereinafter referred to as γ) grain size after direct quenching at a finishing temperature of 850°C. This figure shows the influence of the reheating furnace insertion temperature before the hot final finishing temperature. The γ grain size after direct quenching is determined by the finishing temperature of hot piercing continuous rolling or Ar3~Ar.
.

point, or even if the finishing temperature is Ar3 or higher, the reheating furnace insertion temperature before the hot final finishing temperature is Ar3 to Ar
, when it comes to points, it becomes coarse and becomes about ASTM No. 1. Therefore, in order to prevent coarsening of the γ grains, it is necessary to set the finishing temperature of hot piercing continuous rolling and the reheating furnace insertion temperature before the final hot finishing temperature to be at least Ar3 point.

Regarding the influence of such temperature, according to the inventors' speculation, the temperature of the mother pipe during the hot piercing continuous rolling process is Ar3.
~Ar, the temperature decreases to a point where it becomes a two-phase state of α + γ, and then A
When heated to a temperature above the C3 point, the final process of hot continuous continuous rolling changes from α to γ at a relatively low temperature and under a small rolling reduction.
It is considered that grain boundary movement occurs between the transformed reversely transformed γ grains and the heated untransformed γ grains, resulting in a coarsened structure. However, if the raw tube does not drop to the Ar point of 3 or less before reheating, the γ grains will only undergo normal secondary recrystallization during subsequent heating, and in the heating temperature range of the present invention, the γ grains will only undergo secondary recrystallization. It was found that the structure does not become coarse. Therefore, the finishing temperature of hot piercing continuous rolling and the subsequent temperature of insertion into the reheating furnace were set to Ar3 points or higher.

Hollow tubes made with such rolling temperature history are heated to 900~
Heat to 1000°C. If this heating temperature is below 900℃, the quenching temperature after final hot rolling can be secured.
At temperatures above 000°C, a large amount of oxide scale forms on the steel surface,
900 to 1 because it has a negative effect on ensuring the shape accuracy of steel pipes.
The temperature was limited to 000°C.

Also, if the hot final finishing temperature is too low,
Since the perfect γ state during quenching, which is required to obtain high strength, could not be ensured, the Ar 3 point was set at +50°C.

The starting temperature of the quenching process was set to 3 points or less in order to ensure a sufficient quenched structure and obtain the required strength. The cooling rate during quenching is not particularly limited, but it should be faster than air cooling. The tempering temperature is set to Ac due to the need to ensure stabilization of strength and toughness.
Scored 1 point or less. The heating method is not particularly limited.

The steel obtained under the above manufacturing conditions is effective in manufacturing high-strength seamless steel pipes with a fine-grained structure without containing coarse grains.

[Example code] Next, examples of the present invention will be described.

Table 1 shows the strength and toughness of steel pipes produced by melting them in a converter, continuously casting them, reheating them after hot drilling and continuous rolling, and then directly quenching and tempering them. , γ particle size and SSC resistance.

In addition, the SSC resistance was determined by a constant load method according to NACE TMOI-77.
ess) was determined and evaluated.

The steel pipe manufactured by the present invention has high strength, has a finer γ grain size than the conventional method, has high toughness, and has SSC resistance.
It can be seen that the performance improves by 0.2 ay or more in σth.

[Effect of the invention] The steel pipe manufactured by the method of the present invention as described above has high strength and coarsely fine grains, so it has good low temperature toughness and SSC resistance.
It has excellent properties and is used in the cold regions of the far north and in sulfide stress corrosion environments.

[Brief explanation of the drawing]

The figure shows the influence of the reheating furnace insertion temperature before the final finishing temperature at a finishing temperature of 850° C. during hot piercing continuous rolling on the γ grain size after direct quenching.

Claims (1)

[Claims] 1. C heated to a temperature of 1200°C or higher: 0.05 to 0.35% (weight %, same hereinafter), Si
: 0.01-0.5%, Mn: 0.15-1.5%, S: 0.01% or less, P: 0.02% or less, Mo: 0.05-0.4%, Al: A steel billet containing 0.005 to 0.1% Fe with the remainder being substantially Fe is used to form a hollow blank tube with a finishing temperature of Ar_3 points or higher during hot piercing and continuous rolling. Temperature 900-100 higher than temperature
The finished steel pipe obtained by heating to 0°C and hot finish rolling at a final finish temperature of Ar_3 point + 50°C or higher is subjected to a quenching treatment in which it is rapidly cooled from a temperature of Ar_3 point or higher, and then Ac_
A method for producing a low-alloy, high-tensile-strength seamless steel pipe with a fine-grained structure, characterized by subjecting it to a tempering treatment of heating to a temperature of 1 point or less and cooling. 2. C: 0.05-0.35%, Si: 0.01-0.5%, Mn: 0.15-1.5%, S: 0.01% or less, heated to a temperature of 1200°C or higher , P: 0.02% or less, Mo: 0.05-0.4%, Al: 0.005-0.1%, and further contains Cr: 0.1-1.5%, Ni: 0.1 ~2.0%, V
:0.01~0.1%, Ti:0.01~0.1%, B
: A steel billet containing 0.0003 to 0.003% of one or more kinds, the remainder being substantially Fe, is made into a hollow mother pipe with a finishing temperature of Ar_3 points or higher in hot drilling and continuous rolling. and a temperature of 900 to 1000°C higher than the temperature of the raw pipe.
The finished steel pipe obtained by heating it to a temperature of 50°C or higher to a final finishing temperature of Ar_3 point + 50°C or higher is subjected to a quenching treatment in which it is rapidly cooled from a temperature of Ar_3 point or higher, and then heated to a temperature of Ac_1 point or lower. A method for producing a low-alloy high-strength seamless steel pipe with a fine-grained structure, which is characterized by subjecting it to a tempering treatment in which it is cooled and then cooled. 3. C: 0.05-0.35%, Si: 0.01-0.5%, Mn: 0.15-1.5%, S: 0.01% or less, heated to a temperature of 1200°C or higher , P: 0.02% or less, Mo: 0.05-0.4%, Al: 0.005-0.1%, and further contains rare earth elements: 0.001-0.05%, Ca: 0 .00
1 to 0.02%, Co: 0.05 to 0.5%, Cu: 0
．． A steel billet containing 1 to 0.5% of one or more of the above elements, with the remainder substantially consisting of Fe, is made into a hollow blank tube with a finishing temperature of Ar_3 or higher in hot piercing and continuous rolling. Quenching in which the finished steel pipe obtained by heating to a temperature of 900 to 1000°C higher than the temperature of the raw pipe and hot finish rolling at a final finish temperature of Ar_3 point + 50°C or higher is rapidly cooled from a temperature of Ar_3 point or higher. A method for producing a low-alloy high-strength seamless steel pipe with a fine-grained structure, characterized by subjecting it to a tempering treatment, followed by a tempering treatment in which it is heated to a temperature of Ac_1 point or lower and then cooled. 4. C: 0.05-0.35%, Si: 0.01-0.5%, Mn: 0.15-1.5%, S: 0.01% or less, heated to a temperature of 1200°C or higher , P: 0.02% or less, Mo: 0.05-0.4%, Al: 0.005-0.1%, and further contains Cr: 0.1-1.5%, Ni: 0. 1-2.0%, V
:0.01~0.1%, Ti:0.01~0.1%, B
:0.0003~0.003% of one or more types, Rare earth element: 0.001~0.05%, Ca:0.00
1 to 0.02%, Co: 0.05 to 0.5%, Cu: 0
．． A steel billet containing 1 to 0.5% of one or more of the following, with the remainder substantially consisting of Fe, is made into a hollow blank tube with a finishing temperature of Ar_3 points or higher during hot piercing and continuous rolling. Temperature 900 to 1000 higher than the temperature of the raw pipe
The finished steel pipe selected by heating to ℃ and hot finish rolling at a final finish temperature of Ar_3 point + 50℃ or higher is subjected to a quenching treatment to rapidly cool it from a temperature of Ar_3 point or higher, followed by Ac_1.
A method for producing a low-alloy, high-tensile-strength seamless steel pipe with a fine-grained structure, which is characterized by subjecting it to a tempering treatment in which it is heated to a temperature below a point and then cooled.