JPS5819431A - Manufacture of steel for line pipe with superior characteristic of stopping propagation of brittle crack - Google Patents

Abstract

PURPOSE:To manufacture steel products with high strength and superior characteristics of stopping the propagation of brittle cracks by subjecting a steel contg. specified amounts of C, Si, Mn, Nb, S, V, Cu, Cr, Mo and Ti to rolling, hardening and tempering under specified conditions. CONSTITUTION:A steel contg. 0.04-0.18% C, 0.01-0.90% Si, 0.30-2.00% Mn, 0.008-0.060% Nb, 0.012-0.020% S, 1 or >=2 kinds of metals selected from 0.01- 0.10% V, 0.05-0.50% Cu, 0.05-1.0% Cr, 0.05-0.50% Mo and 0.005-0.050% Ti, and inevitable impurities or further contg. 0.20-2.00% Ni is heated to >=1,200 deg.C and hot-worked. The hot worked steel is cooled to a temp. below the Ar1 transformation point, austenitized again, cooled to a temp. close to ordinary temp. at a higher rate than air cooling rate, and tempered by heating to 400 deg.C- the Ac1 transformation point to convert most of the structure into a fine ferrite structure.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a steel for nine-twin arm isolators having excellent brittle crack propagation arresting properties.

Regarding welded end pipes and seamless steel pipe materials, the propagation arresting characteristics of brittle fractures and unstable ductile fractures have been attracting particular attention due to the recent increase in natural gas tube twins.Methods and techniques for evaluating this propagation arresting characteristic , B-DWT T (DropWei
ght T@ar Te5t) has been recognized as having good correspondence with the actual fracture characteristics of e-iso, and has been incorporated into the API standard. Conventionally, as a method for improving the brittle crack propagation arresting characteristics of steels containing Nb or riv, the slab is heated at a low temperature and rolled in the non-recrystallization temperature range, as typified by so-called troll rolling. By refining the crystal grains and further reducing KNh during cooling after rolling.
A rolling method in which carbonitrides such as , V, etc. are precipitated is widely used. However, in this method, the rolling conditions are strictly controlled, resulting in a large decrease in tooth quality, and it is difficult to apply to rounded seamless steel pipes, etc., which require low processing temperatures.

The present invention provides a method for producing line arm steel with excellent brittle crack propagation arresting properties by quenching and tempering heat treatment after rolling without using severe processing methods such as control)' sealing. The invention aims to make the structure after heat treatment to be a structure mainly composed of fine-grained ferrite. First, the austenite crystal grains are refined during reheating, and during rapid cooling, NbF is effectively used as precipitation nuclei of ferrite. and acts as a fine grain effect during α transformation (N
i1 This is a method of manufacturing steel materials with high strength and excellent brittle crack propagation arresting properties by combining with S, which further improves arrest performance. , it can be easily applied to any case.

Conventionally, the Charpy impact test has been used as a method for evaluating the toughness value of steel materials due to its simplicity.This test method does not always evaluate the hardened surface layer after heat treatment due to the test specimen collection regulations. The toughness value in this test method can be improved if the structure of the part excluding the rounded and quenched surface layer is made finer on average.

Therefore, elements (such as Nb) that work effectively to refine the structure.
Even when used alone, the effect of improving toughness can be seen in the Charpy impact test, but it cannot necessarily improve the structure of the quenched surface layer. That is, even in the range of constituent elements in which the fibers after quenching are mainly ferrite, the quenched surface layer tends to form an intermediate stage structure (hereinafter referred to as Bta weave) or an island-shaped martensitic weave because the cooling rate is fast. This Bu (upper bainite) and island-like rutinsite structure increase the strength, but are harmful to toughness.
In order to suppress as much as possible the appearance of Bu or island-like martensite structure in the phosphorized surface layer, which is harmful to the refinement of grain size and toughness, Nb and Ni were added at the same time, resulting in an additional effect of 8. The present invention relates to a method for manufacturing a steel for line pipes that has high strength and excellent brittle crack propagation arresting properties.

The components of the steel that is the object of the present invention and reasons for manufacturing limitations will be described below.

The lower limit was set at 0.04 N because it is inevitable to strengthen the Oti steel and refine the structure after quenching, but if it is too much, the toughness will deteriorate due to the appearance of Bu and island martensitic structure, and the i9 The upper limit is set to 0 because it does not adversely affect weldability.
．． 18N.

8I is necessary for steel manufacturing and for increasing strength, so the lower limit is set to 0.
．． The upper limit was set to 0.90 N since too much 9 would deteriorate the toughness.

Mn is necessary for increasing strength and toughness, so the lower limit is set to 0.
．． The upper limit was set to 30 N, but if it was too high, Bu or island-like martensite structure, which increases hardenability and causes deterioration of rice grain quality, would be likely to appear, so the upper limit was set to 2.0 QN.

Carbonitrides precipitated during Nbti hot forming and reheating prevent coarsening of austenite crystal grains during re-austenitization, and also serve as α-transformation nuclei during rapid cooling to form a fine-grained ferrite structure. The lower limit was set to 0.008 N because it was necessary for tightening, but the effect would be saturated if it was too large, so the upper limit was set to α060%.

S turns into Nakaaki A-based inclusions during rolling, and has the effect of improving brittle crack propagation arresting properties. The lower limit was set to 03012X because the effect would not appear if it was too small, but the effect would be saturated if it was too large, so the upper limit was set to 0.020X.

N1 lowers the α transformation temperature during quenching,
It is possible to refine the ferrite grains after quenching. If it is too small, the effect will not be achieved, and if it is too large, the following elements are 1
V, which is added as a seed or two or more types, is mainly precipitated as a carbide during tempering heat treatment and is intended to increase strength, but if it is too little, it will not be effective, and if it is too much, the effect will be saturated, so 0.
01-0, 10X.

Or 1 hb) is intended to increase strength, but if it is too little, it will not be effective, and if it is too much, the strength will be too high and the toughness will deteriorate. ON, 0.05~0.5
0 X Toshimaru.

Ti increases the strength by precipitation of I'i carbides, but if it is too little, it will not be effective and if it is too much, it will form giant nitrides and reduce toughness, so it is set as 0.005 to α050x.

(The purpose of lu is to increase strength as well as provide weather resistance, corrosion resistance, and hydrogen-induced cracking resistance, but if there is too little, the effect will be ineffective, and if there is too much, the effect will be saturated, so 005~α5ON
And so.

Among the hot working conditions for forming steel plates, seamless steel pipes, etc., the heating temperature must be high enough to dissolve most of the Nb V into solid solution before hot forming. This temperature is controlled by the solubility product of O, N and Nb, v, but is within the range of the present invention and the range of the example +7) O, N, Nb, V
The heating temperature was set at 1200° C. or higher, since all of the above-mentioned amounts will be solid-dissolved at a temperature of 1200° C. or higher, and there will be no problem in terms of hot forming efficiency. Furthermore, since the purpose of the present invention is to improve the brittle crack propagation arresting properties of steel plates, seamless steel pipes, etc. by quenching and tempering heat treatment rather than by rolling at low temperatures as in controlled trawling, the rolling conditions are particularly Not limited.

In this case, rolling, extrusion, etc. are used as a processing method, but the processing method is not limited in the present invention.

Since heat treatment is performed after the hot forming process is completed, it is necessary to cool the material to a temperature below the -mArx point, but the cooling rate is not limited to 41. In this case, the Ar1 point is the temperature at which all austenite finishes transforming into a low-temperature transformed structure (ferrite, ferrite, bainite, martensite). Next, when reheating from a temperature below Ar1 point to austenite, Nb (ON) precipitated during hot forming and reheating is used to prevent coarsening of austenite crystal grains and during subsequent rapid cooling. By acting as α-transformation nuclei, the fine-grain effect of KNI makes it possible to form a fine-grained ferrite structure from the quenched surface layer to the center of the wall thickness, thereby improving brittle crack propagation arresting properties. Austenitization + I: A, jl
It is desirable to keep the holding time to the minimum necessary to prevent coarsening of austenite crystal grains1, and the cooling rate during rapid cooling varies depending on the added element, but so that most of the structure after cooling becomes fine-grained ferrite. speed is required. Therefore, in the present invention, the cooling rate is not limited, but it is preferable that the cooling rate be faster than air cooling. Warm tempering heat treatment after quenching is necessary to stabilize the toughness of steel (4) at 400°C or higher, but if it exceeds Ac1 point, the original meaning of tempering is lost, so the upper limit was set to Ac1 point or less.

Further, the apparatus used for the quenching and tempering heat treatment includes furnace heating, conductive heating, flame radiation, salt bath, metal bath, etc., but is not particularly limited in the present invention. However, it is preferable in the present invention that the heating time required for austenitization, such as a spinning heating device, is shorter.
More than anything, the steel plates and seamless steel pipes with excellent brittle crack propagation arresting properties produced according to the present invention are particularly effective for use in thick-walled line nozzles.

Hereinafter, an example of a real gun of "1" will be explained.

Table 1 shows the chemical components of the example test materials, and Table 2 shows the hot forming and rolling conditions j... and heat treatment conditions.

The heat treatment equipment used was a furnace heating equipment and a salt gamma heating equipment.

As a means of evaluating brittle crack propagation arresting characteristics, Ll-D
W T T is carried out, and in addition, the toughness is 13-
D W T T was the entire wall thickness, and the Charpy test was the center of the wall thickness. Naori-1) Scarcity for W 1”r;
In order to more accurately evaluate the floating property, the notched portion of the test piece was previously subjected to TIG II bead treatment and then press notched. With this method, the reverse fracture surface (from the bottom of the notch to the
jL (ductile cracks that occur and then convert to brittle cracks) did not appear at all.

Table 3 shows the results of the Charpy test, B-DWTT, and tensile test. As is clear from the same table, in the Charpy test that does not evaluate part of the quenching table 1, the comparison steel also showed good values, and it was 8 degrees, which is the same as the steel obtained by the method of the present invention (hereinafter referred to as the steel of the present invention). , B-DWTT shows that the steel of the present invention is significantly improved compared to the comparative steel.

That is, according to the present invention, it is possible to modify the structure of the quenched surface layer, which could not be achieved in the past, in addition to the fine grain effect of Nb and teeth, and also to combine the improvement effect of S. In particular, it is possible to produce steel for lines with excellent brittle crack propagation stopping properties using the normal quenching and quenching heat treatment method, rather than using severe rolling methods at low temperatures such as controlled trawling. becomes.

Table 1 Chemical composition of sample material (wts
JTable 2 Experimental method

[Brief explanation of the drawing]

Figure 41 shows Ni content in steel and Charpy properties, DWTT
It is a figure which shows the 1st section of a characteristic. 0 θ, 5 1.0 /,
5Ni shoulder content (010) Figure A1 (Gold 1'l) 1978
8 years old? 5th Director General of the Japan Patent Office 1, Indication of the case Relationship to the case No. 115975 Applicant Address (residence) 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (fi65) Nippon Steel Corporation 4, Agent Residence 6, Taiyo Building, 2-38 Nihonka Kabuto-cho, Chuo-ku, Tokyo Number of inventions to be increased by the amendment None 8 Contents of the amendment As per the attached sheet; . Correction details: ``2 mn, V notch Charpy test'' 12 mmV
Notch Charpy Test".

Claims (2)

[Claims] (1) O:a04~(kill,81:aol~
01ON, Mm: α30~2.0ON, Nb: αeo
s~α06ON, 8:0011~aO! ON and V
:(Lot~αlQ*, Ou:αOS~α!!0*, O
r: αOs~1.0. X, Mo: ao 5-a8
G 1% "" αOOS is heated to 100℃ or more, and then cooled at a hot speed in a bottle near room temperature, and then heated to 400℃~
Production of steel for line piezos with excellent properties for stopping the propagation of sphincter, which is characterized by being heat-treated at 11, which is the temperature of the transformation point, and returned to the sieve to form a fine-grained 7-light structure. Method. (2) 0:0.04~α18.81: α01~α90
X, Mn: 0.30-2.0 ON, Nb: (1
008"'α06GN, 8: 0.012~0020X and V: 0.01~0.10X, Ou: 005~0.5
0X, Or: αOs~1. ON, Me: 0
．． 08~α50Xs? +: α005~o
, one or more types of oso depending on the purpose.
FCNi: Steel containing 0.201-400X and other unavoidable impurities is heated to a temperature of 1200°C or higher, followed by hot working, and then cooled to a temperature below the Ar1 transformation point. After being austenitized again, it is cooled to near room temperature at a faster rate than the air cooling rate, then 400℃~
A method for manufacturing line pipe steel with excellent properties for stopping the propagation of brittle china folds by performing return heat treatment to the temperature of the ACl transformation point to make the majority of the structure into a fine-grained ferrite structure.