JPS5938357A - Electric welded pipe and its manufacture - Google Patents

Abstract

PURPOSE:To obtain an electric welded pipe having a small grain size number and high fatigue limit, by forming a pipe using a steel material having a composition consisting of prescribed percentages of C, Si, Mn, Cr, Nb, N and Al and the balance essentially Fe. CONSTITUTION:The desired electric welded pipe is obtd. by forming a pipe using a steel material contg. 0.35-0.55% C, <=0.50% Si, <=1.0% Mn, <=0.20% Cr, 0.005-0.030% Nb, 0.010-0.030% N and 0.010-0.050% Al. The steel material is heated to 1,030-1,100 deg.C and hot-rolled under conditions which bring the final rolling temp. to <=950 deg.C. The resulting material is formed into a pipe by electric welding. The electric welded pipe causes hardly quench hardening during cooling after the welding, so quenching crack can be avoided. The pipe is easily cut to a fixed size, and the edges are easily worked.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in medium carbon steel electrical resistance welded pipes, and also includes preferred methods of manufacturing the same.

In recent years, in response to the demand for lighter weight automobile and other mechanical structural parts, parts that were previously solid have been made hollow, for example, shafts for automobiles have been changed from round rods to tubular ones. However, many attempts have been made.

JISG3445iC is STKM 16, STK
M17 is a standard for medium carbon steel pipes containing 035-055% C, but these are usually provided as seamless steel pipes and L7, and the technology of electric resistance welded pipes with low manufacturing costs is used. is not used. There are two reasons why medium carbon steel electric resistance welded pipes are not manufactured. Firstly, the strength of the material is so high that it is difficult to roll form it, making it difficult to butt properly when welding, making the welding conditions unstable.Secondly, it is hardened by quenching during cooling after welding. However, there is a risk of quench cracking, and machining such as cutting to size and end face machining becomes difficult.

The first problem has been significantly improved by recent advances in technology, and molding has become possible, but the second problem cannot be said to have been solved yet. The present invention solves this problem and makes it possible to manufacture electric resistance welded pipes made of medium carbon steel.

The electric resistance welded pipe of the present invention is 0035-055%, 5i:050
% or less, Mn: 1.0% or less, Cr: 0.20
% or less, Nb: 0.005-0.030%, N:
0.010-0030% and /V: 0010-0
．． Contains 0.050%.

The remaining portion is made from a steel material whose composition is essentially Fe.

The reason why the alloy composition of the steel material was determined as described above is as follows.

C: 0.35-055% To obtain the strength required for mechanical structural parts.

0.35% or more is required. On the other hand, if the content is too large, the toughness decreases, so 055% is set as the -L limit.

Si: 0.50% or less Added as a deoxidizing agent during steel production, and increased as necessary to improve toughness and hardenability.

If it exceeds 0.50%, the moldability during electric resistance welding tube molding will deteriorate.

Mn: ], 0% or more F Added for the purpose of improving hardenability and also because it is useful for strength, toughness and ductility. shi power shi.

If the amount is too large, it will increase the susceptibility to quench cracking after welding.
The upper limit is 10%.

Cr: 0.20% or less Effective for improving hardenability, but to avoid quench cracking during pipe making, keep it within 0.2%. Depending on the size of the product, large items are added to the line.

Nb: 0. 005-0030%, N:0
．． 010-0.030% and AP: 0.01
0% to 0.050% are all important additive elements in the present invention and make austenite crystal grains fine. In addition to NbC, Nb combines with N to form NbN, and Nb also combines with N to form A-N, suppressing recrystallization during rolling of the material and forming a transformed structure (pearlite) during the cooling stage. or bainite). In this steel material, the amount of N was set higher than the content of conventional steel (usually 0005% or less) based on the results of detailed experiments, and the Nb
By making it exist in the range of 0005% to 0030%, they were able to successfully refine and maintain the grain size. A
In order to effectively precipitate gN.

AP is required to be at least 0.01%, but if it is too large, the amount of inclusions will increase, so 0.05% is set as the -F limit.

] - When manufacturing ERW pipes using the steel mentioned above, rolling the steel material under specially controlled conditions promotes grain refinement, resulting in the production of ERW pipes made of medium carbon steel. In other words, the preferable manufacturing method of the ERW pipe of the present invention is as follows: C: 0.35-0.55%,
Si: 0.50% or less, Mn: 10% or more F, Cr 20
．． 20% or less, Nb: 0.005-0.030%,
N:0.010~0030% and AP・0.010~
0050% with the remainder being substantially 1CFe at 11OO0CJ-L above 1030°C.

The material is heated to a temperature of 950°C and hot-rolled under conditions such that the final rolling temperature is 950°C.

The reason why the heating temperature is kept below 1100'C is to keep the initial grain size of austenite fine, and the reason why the final rolling temperature is limited to below 950°C is to delay recrystallization and prevent grain coarsening. This is to prevent this.

According to the present invention, by selection of alloy composition including micro-adjustment of Nb, N and Ae in medium carbon steel.

Since the hardenability of the material is reduced, the following advantages can be obtained.
That is, when the electric resistance welded pipe is cooled after welding, quench hardening occurs (it becomes tougher), so quench cracking can be avoided, and cutting to size and end face processing become easier.

Even when the product electrical resistance welded tube is machined into a mechanical structural component, such as a power transmission component, and subjected to heat treatment such as induction hardening, the crystal grains do not coarsen at all and high toughness can be maintained.

These effects can be obtained even more significantly by manufacturing the material for pipe making by controlled rolling according to the present invention.

EXAMPLES Hereinafter, the effects of the present invention will be specifically explained with reference to Examples.

Example [No.] Steel materials with the composition shown in the table are melted and hot rolled.

Cold rolling and annealing were performed to produce a raw material coil with a wall thickness of 4.5 mm and a width of 728 mm. The symbol E with a positive mark is the conventional steel 543C prepared for comparison.

An electric resistance welded pipe having the shape and dimensions shown in FIG. 1 was manufactured from this material under the following pipe-forming conditions.

Welding method: High frequency induction welding (450KHz) forming, vertical and horizontal rolls, 8 stages each Pipe forming speed: 40 m/
min.

In general, for electric resistance welded pipes, cooling of the forminog roll and squeeze roll and water cooling after welding are performed (therefore, rapid water cooling is performed, but here, cooling is not performed for 10 seconds after welding, and water cooling is performed after that). It is cooled through the pearlite and bainite transformation regions, reducing the quench hardness.

This was adopted to prevent quench cracking.

The grain size and hardness of the welded parts of the obtained electric resistance welded pipes were measured, and the presence or absence of quench cracking was observed. The results are shown in Table 2 along with the rolling conditions of the material. rC-
2J and rD-2J are examples demonstrating the conditions of the preferred manufacturing method of the present invention. Oh, the same material with a different diameter of 3
Rolled into a 2fi round bar and passed the Jominy test (JIS GO
'551) to determine the D+ value. The results are shown in Table 2, and as shown in the data in Table 2, all the electric resistance welded tubes of the present invention have fine crystal grains and low hardenability, so the hardness after tube production is also low. The present inventors have found empirically that in electric resistance welded pipes made of medium carbon steel, quench cracking occurs when the hardness after pipe production exceeds Hv600.
The present invention ((accordingly, all) (v 450 or less and no quench cracking occurred. This ((on the other hand, in the comparative example)
The crystal grains are large and the hardness after pipe making is Hv633,
No pearlite or bainite structure was observed, and quench cracking was observed.

The usefulness of the electric resistance welded tube according to the present invention as a material for mechanical structural parts was confirmed as follows.

That is, an electric resistance welded tube with an outer diameter of 25.4 MX and a wall thickness of 45 mm was cold-drawn to have an outer diameter of 16 mm and a wall thickness of 4.5 mm (C processing), and then subjected to induction hardening under the following conditions.

Induction hardening frequency 135 KHz Power +50Kw, feed speed 11 mm/sec.

It was tempered and processed into a rotary bending fatigue test piece having the shape and dimensions shown in FIG. In addition, the comparative example 343
The C steel test piece was manufactured from a round bar with a diameter of 32 mm by machining.

Table 3 shows the surface hardness, grain size, and rotary bending fatigue limit. It is clear that the crystal grains according to the invention are fine and the fatigue limit is high, and even better results are obtained, especially when the materials are manufactured by the preferred manufacturing method.

Table 3

[Brief explanation of drawings]

FIG. 1 shows an electric resistance welded tube manufactured in an example of the present invention.
FIG. 3 is a partial cross-sectional view showing the shape and dimensions. FIG. 2 is a partial sectional view similar to FIG. 1, showing the shape and dimensions of a rotary bending fatigue test piece made from the electric resistance welded tube of FIG. 1. Patent applicant: Daido Steel Co., Ltd. Agent: Patent attorney: Takeo Suga

Claims (1)

[Claims] If) C: 0.35 to 0.55%, S+:
0.50% or more F. Mn: 1.0% or less, Cr: 0.20% or more F, Nb: o
, oos ~oo3o=,;, N: 0010-00
30% and Ag: 0.010 to 0.050%, with the remainder being substantially Fe. (21C: 0.35-0.55%, Si' 0
．． 50%. 1vln: 1. 0% or more 'F',
Cr: 0.20%,
'! ”, Nb0.005~o, o3oz, N: 0
．． 010-0.030% and Ag: 0.010-00
A steel having a composition of 50% KFe and the remainder being substantially KFe is heated to
A method for producing an electric resistance welded pipe, which comprises forming an electric resistance welded pipe from a material obtained by hot rolling under conditions such that the final rolling temperature is 950°C or less.