JPS61127849A - Steel for pipe for working to bent pipe - Google Patents

Abstract

PURPOSE:To develop steel material without toughness deterioration due to various heat treatment in forming bent pipe, by using steel material contg. specified quantities of Mn, Ti, Al and Ni, Nb, etc. as the titled steel. CONSTITUTION:Steel sheet is obtd. from slab or billet by necessary rolling, hen are welded steel pipe, electrically seamed steel pipe and seamless steel pipe are manufactured by using said sheet, the necessary part is heated to 600-1,100 deg.C by high frequency, hot bending worked to manufacture bent pipe. Te material steel sheet is composed of, by weight 0.02-0.10% C, 0.20-0.50% Si, 0.50-2.00% Mn, 0.005-0.10% Ti, 0.005-0.19% Al, 0.0080% N and one or 2 kinds of 0.01-2.0% Ni and 0.01-0.10% Nb, under N <=Ti/3.4 in stoichiometric ratio. Bent pipe without deterioration in low temp. toughness, impacting characteristic after hot bending working is obtd.

Description

[Detailed Description of the Invention] (Industrial Application Field) The technical details described in this specification regarding the steel used for bent pipes used at the bends and branching parts of line pipes for oil and gas transportation are as follows: The aim is to obtain steel that can maintain high toughness by adjusting its composition.

BACKGROUND OF THE INVENTION Traditionally, bent pipes used for bends and branches of line pipes for oil and gas transportation have been manufactured by casting, forging, or welding assembly methods.

In recent years, the use of high frequencies has become popular industrially, and even in the production of single-curved pipes, an economical method of forming UOE steel pipes by applying high-frequency power, post-heat, and hot bending has been adopted.

Forming a curved tube using this high-frequency heating requires 815 minutes of bending, so the curved tube section is heated and quenched or heated and air-cooled, and the straight tube S remains as it was when the tube was formed.

The boundary between the curved pipe section and the straight pipe section is heated to various temperatures and subjected to various thermal histories over the entire length of the pipe, such as water cooling or air cooling. Furthermore, due to surface heating by high frequency waves, when the tube is heated, a temperature difference occurs between the outer surface and the inner surface, and it is estimated that the thicker the tube wall 1N, the larger this temperature difference becomes. Since cooling proceeds from the outside surface, different wall thicknesses result in different cooling rates, which causes changes in material properties.

In this way, when forming a curved tube using high-frequency heating, the heat treatment differs in the longitudinal direction, and the bending heating temperature and cooling rate also vary in the curved tube section, resulting in uneven material and curved tubes with poor toughness. becomes. Therefore, in order to prevent deterioration of toughness, it was necessary to strictly control the heating temperature, cooling rate, etc. when forming a single-curve tube.

(Prior art) As mentioned above, forming a curved pipe requires a special wiring line (NL), but if the copper for the curved pipe itself can be given properties that allow it to have excellent toughness even after hot bending. Although it does not require any special consideration, I have not found any steel that has been developed for the purpose of Jin.

(Problems to be Solved by the Invention) Therefore, the object of the present invention is to provide a steel that always has stable toughness even after various heating and cooling processes during the forming of a single curved tube.

(Means for Solving the Problems) In order to obtain a steel material that always has stable low-temperature toughness even after heat treatment, the inventors developed C from the viewpoint of quench hardenability.
and B, from the viewpoint of grain refinement in the heat affected zone, Nb, Ti,
Various experiments and studies were conducted focusing on the following alloying elements: REM and B, Cu and Ni from the perspective of improving the toughness of the matrix, and Nb and (2) from the perspective of improving the toughness of the weld metal.

In the experiment, steel materials that were only subjected to rolling (hereinafter referred to as "ma material") were heated and air cooled at a temperature of 550 to 1100 degrees Celsius after rolling.

Using various heat-treated materials that were heated to 600 to 1100° C. and then quenched, and then quenched and then tempered, each material was investigated and the variations in material quality were compared and studied. In addition, 8-electrode submerged arc bath welding (F, S, 81 K
J/cm, B, S, 8BKJ/FII) and F) A similar experiment was conducted for the weld heat affected zone.

An example of the experimental results is shown in FIG. In this experiment, steels A to H having the compositions shown in Table 1 were used, and every six steels were untreated after rolling, and heated and air-cooled at 650°C after rolling.

The fracture surface transition temperature was investigated when heat-quenching at 1050°C and tempering at 650°C after heat-quenching at 1050°C were performed.

From Figure 1 and Table 1, the low 0-Ti-Nb steel F
It is clear that Steel G, which is a low 0-Ti-Ni steel, has a low fracture surface transition temperature in both the as-rolled material and each heat-treated material, and that the dispersion in the fracture surface transition temperature after each treatment is small.

Regarding Ti added to Steels F and G, it is known that TiN improves the properties of high heat input welds through the effect of suppressing grain growth and the trapping effect of N in the steel. Steel F.

By combining low C and T1 addition as shown in G, the ferrite nose in the OCT curve is shifted to the short time side, and the toughness of the base metal and weld is improved by each heat treatment without greatly depending on the cooling rate. It turned out that it was possible to provide steel that remained stable even after the process.

On the other hand, rapid heating using high-frequency waves requires only a short time, so there is little grain growth during heating, and even higher toughness can be achieved.

When Nb + Nin is added to low a-Ti steel, N1 has the effect of improving the toughness of the matrix and can compensate for the deterioration of toughness caused by quenching.

In addition, Nb can improve toughness by suppressing the growth of Nb carbonitrides, and Nb carbonitrides themselves are stable, so they are effective when added to materials that are heated to relatively high temperatures and subjected to hot bending. It turned out that it was possible.

This invention is derived from the above knowledge.

That is, in this invention, c: 0.02 to 0.10w
t%.

Si: 0.20-0.50 wt%, Mn: 0
．． 50-2. IJOwt%.

xi: 0.005~O, 10wt%-AI! : 0.
005~0°100vt Chi and N: [1,008
Contains 0wt% or less, and Ni: 0.1-2. O
wL% (!: Nb: 0.01~0.10 wt
It is a steel for pipes for surface processing, which is characterized by containing two types of IPl in %, the remainder being Fe and unavoidable impurities, and partially hot oil having excellent toughness after processing.

Note that the ratio of T1 and N is determined from the stoichiometric equivalent to N≦Ti/B.
, 4 is desirable.

After obtaining the molten steel adjusted to the above composition range using the usual back copper method, it is subjected to the necessary rolling processing to form slabs and billets using the ingot-forming method or continuous casting method according to conventional methods to produce arc-welded steel pipes! If we manufacture sewn steel pipes and seamless steel pipes and use high-frequency heating to heat the parts of these steel pipes to 60υ to 1100°C and perform hot bending, we can create high-dust products with the same excellent impact properties at various parts of the steel pipes. You can get a sex tube.

(effect) Next, the reasons for limiting each component will be explained.

C is an essential component that most easily increases the strength of steel, and if it is less than 0.02 wt% (hereinafter simply referred to as C), no effect can be expected, while if it exceeds 0.10%, it will reduce hardenability. The content is set in the range of 0.02 to 0.10%, since this increases the toughness and decreases the coagulability.

Sl is 0.20% due to its deoxidizing effect and contribution to strength.
However, if it exceeds 0.50%, hardenability increases and low temperature toughness deteriorates, so 0.2 (1 to 1%) is required.
The range is 0.50%.

Mn needs to be 0.50% or more to ensure the specified strength, but if it exceeds 2.0096, the toughness during quenching will be large and the bath weldability and workability will be immediately impaired.
-2.00% range.

T1 is an important component that becomes a nitride and suppresses grain growth, which reduces nitrogen in the steel and improves toughness, as well as shifts the ferrite nose to the short-time side. Various heat treatments also have the effect of suppressing changes in material properties. This effect is small below 0.005%, and
If it exceeds 10%, it becomes saturated and the toughness deteriorates due to the precipitation of Ti carbides, which immediately impairs economic efficiency.
The range is 0.005 to 0.10%.

A/' has a deoxidizing effect and needs to be at least 0.005%, but if it exceeds 0.100%, weldability will deteriorate, so
．． The range is 0.005% to 0.100%.

If N exceeds o, oos%, the toughness deteriorates significantly, so it should be kept at 0.008% or less.

Ni and Nt) are effective in improving toughness.

Shows equivalent effects.

First, N1 is required to be 0.10% or more to improve toughness, but if it exceeds 2.0%, weldability and hot workability will decrease.
Since it is disadvantageous in terms of economy, it is set in the range of 0.10 to 2.0%.

Nb is required to be 0.01% or more because it becomes Nb carbonitride, suppresses grain growth, and improves toughness.

If it exceeds 0.10%, weldability decreases and toughness deteriorates due to improved hardenability.

The range is 0.01-2.0%.

(Example) Comparative steel 1 to 6° Invention steel 7 containing the chemical components shown in Table 2
~9 were melted into l00 kp steel ingots by vacuum melting, cut into l l Q mm thick slab steel pieces, and then 1
After heating to 150°C, a rolled steel plate with a thickness of 15 mm was obtained by controlled rolling at a finishing temperature of 780°C. Here, Charpy test pieces were taken from the as-rolled material for evaluation of impact properties. First, the as-rolled material was heated to a temperature of 600°C to 1150°C, and pressed with a punch whose tip has a radius of curvature of 180 mm at each temperature of 50°C to determine the relationship between the plate thickness t and the diameter of the bent pipe part. Hot working was performed such that t/D was 8:4%. Immediately thereafter, it was water-cooled at a cooling rate of about 80° C./s, and its impact properties were examined. The cooling rate at this time is considerably faster than the cooling rate of the actual bending machine (estimated at 5 to 15°C/S), and it is expected that the impact properties will be lower than that of the actual product. In addition, since toughness is improved by tempering after bending, the steel materials were evaluated here under the condition of rapid cooling immediately after bending, where the greatest degree of dust accumulation occurs. The results are shown in Table 8.

As is clear from Table 8, the invention steel showed less toughness due to quenching after hot working at each temperature, and showed a value on the same level as the fracture surface transition temperature of the rolled material.

It can be seen that it has high toughness. That is, it can be seen that even if the heating temperature varies over a wide range depending on the plate thickness, it is possible to manufacture a curved pipe having uniform stiffness in each part.

Furthermore, due to the relationship between the deformation resistance and bending of steel materials and the capacity of the equipment, heating temperatures of d900℃ or higher are often used in actual bending processes, and even when the heating temperature is kana p high, as with the invented steel, it has high toughness. However, the effect is large〇 (Effect of the invention) By using the steel of this invention, it is possible to create a high-tension steel that does not require special consideration during hot bending and has the same impact characteristics at each part after bending. It is possible to manufacture flexible curved pipes.

That is, there is no need for strict heating temperature control or consideration of temperature variations, and it is possible to provide a bent pipe with excellent stiffness without subjecting the entire pipe to tempering after bending.

In turn, it is possible to improve workability and productivity by relaxing processing conditions, and to reduce costs by reducing the heating unit required for tempering treatment.

Brief Explanation of Turning Surface FIG. 1 is a graph showing changes in fracture surface transition temperature due to various heating effects.

Claims (1)

[Claims] 1. C: 0.02 to 0.10 wt%, Si: 0.20 to 0.50 wt%, Mn: 0.50 to 2.00 wt%, Ti: 0.005 to 0.10 wt%. %, Al: 0.005 to 0.100 wt%, and N: 0.
Contains 0.080 wt% or less, including Ni: 0.1 to 2.0 wt% and Nb: 0
．． A pipe steel for forming curved pipes, characterized in that the steel contains one or two of the following: 01 to 0.10 wt%, the remainder being Fe and unavoidable impurities, and having excellent toughness after partial hot bending.