JPS58189321A - Manufacture of high tension and high toughness steel material - Google Patents

Abstract

PURPOSE:To manufacture a high tension and high toughness steel material with high operational efficiency while eliminating a tempering stage, by hardening a steel material contg. specified amounts of C, Si, Mn, Nb, N and Al or further contg. Cu, etc. CONSTITUTION:A steel material consisting of, by weight, 0.002-0.06% C, 0.05- 0.8% Si, 0.8-2.2% Mn, 0.01-0.1% Nb, 0.002-0.008% N, 0.01-0.08% Al and the balance Fe with inevitable impurities or further contg. one or more among <=1.0% Cu, <=3.0% Ni, <=1.0% Cr, <=0.8% Mo, <=0.1% V, <=0.03% Ti, <=0.003% B and <=0.01% Ca is hardened by heating to a temp. above the Ac3 point to obtain a high tension and high toughness steel material such as a steel pipe, a steel bar or a steel rod without carrying out tempering. Since said steel is low C steel, it has superior weldability and resistance to unstable ductile rupture.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing high-tensile and high-toughness steel materials, particularly steel pipes, long steel bars, and steel bars.
By improving the toughness that was a problem with the quenching (hereinafter referred to as Q)-tempering (hereinafter referred to as T) type, and by omitting the T treatment process of the Q-T treatment mentioned above, we have improved the operational efficiency. The purpose is to improve efficiency.

Since Q/T type steel has a low carbon equivalent (Ceq) and can be made to have high tensile strength, it is known as a steel manufacturing method in areas where controlled rolling (hereinafter referred to as CR) cannot be applied, such as thick-walled high-tensile steel pipes. It is being However, in general, CR
Currently, it is inferior to other materials, such as gas A.
? Regarding the characteristics of DWTT (electrical drop test), which is adopted as an evaluation method for Eveline's brittle fracture propagation prevention criteria,
3211II + plate thickness: 85% S A T T (3
hear Area Transition Tempe
The temperature (rature) is usually 0° C. or higher, which is a problem when handling QT materials.

To solve these toughness problems, the CR+QT method (see JP-A-51-14814, etc.), the QQ'T method (see JP-A-55-9'7423, etc.), and the repeated QT method (Yagi et al. , Tetsu to Hagane VAT 66 (1980) N[L4
There are examples of attempts to improve the structure by making the previous structure finer or making the structure finer by heat treatment, such as S590), but these all involve a large number of heat treatment steps (at least Q and T), and are not necessarily appropriate in terms of operational efficiency. This was not a problem, but rather a problem from the standpoint of on-site operations.

Therefore, this invention was made to solve the above problems, and includes: C: 0.002 to 0.06%, Si: 0.05 to 0.8%, Mn: 0.8 to 2.2%. , Nb: 0.01-0.1%, N: 0.002-0.008%.

Contains At: 0.01 to 0.08%, and if necessary, Cu: 1.0% or less, Ni: 3. 0% or less, Cr:
1.0% or less, Mo2 0.8% or less, V: 0.1%
Below, Ti: 0.03% or less, B: 0.003%
A steel material containing one or more of the following and Ca: 0.01% or less, with the balance consisting of Fe and unavoidable impurities, is heated to AC 3 or higher and quenched, so that no tempering treatment is performed. This method is unique in that it is a manufacturing method for high-tensile, high-toughness steel materials.

The configuration of the present invention is roughly divided into the following two points.

1 First, reduce the amount of C to 0.061 or less to reduce the hardenability, and create a structure consisting mainly of fine-grained bainite (11 parts ferrite) up to Qt (as-quenched) (due to the decrease in the amount of C). The reduction in strength can be compensated for by adding alloys such as off-ground or Mor V).

2. Omit the tempering process.

The present invention will be explained below by taking a large diameter welded steel pipe as an example.

Conventionally, the general C content of heat-treated type steel has been 0.10% or more, and the premise has been to "inject as much hardening as possible."However, as the plate thickness increases, the toughness does not necessarily increase, depending on the composition system. is not good; in fact, the incompletely quenched structure is better.

By the way, 32 cabinet thick steel material (C-0-258i -1,35
Mn-0,02Nb-0,04V-0,03At), tensile strength (jS), toughness (v
Figure 1 shows the influence of the amount of C on Tr,s). The heat treatment conditions are Q: quenching in water after holding at 950℃ for 2 minutes, T
: Air cooling after holding at 640°C for 6 minutes. According to this, as the amount of C decreases, the toughness improves, and the difference between as-Q and after-QT becomes smaller. In other words, when the amount of C decreases,
It can be seen that the change in toughness due to tempering is small, and a high level of toughness can be obtained even at Q. On the other hand, regarding strength,
As the C content decreases, TS decreases, and the difference between QI and QT decreases, similar to the toughness behavior. In other words, although the hardenability is poor, the change in strength due to tempering is also small. As the C content decreases from the above, the toughness (vTrs
) is good, and of course it is seen that it is more advantageous than after Qt in terms of strength. In Figures 2(a) and (b), the values of co, o a % (Figure 2(a)) and C0111% (Figure 2(b)) in Figure 1 are selected and microscopy with the same Q is shown. Indicates organization. C is 0.11% below the conventional level.
It can be seen that the sample exhibits martensite + bainite up to Qt, whereas the sample containing 03% CO already exhibits a fine bainite + ferrite structure even at Qt. In other words, in a component system (Lowc system) in which the bainite nose is located on the short time side in the CCT curve,
As a result, a structure consisting mainly of bainite is obtained, and the toughness is improved even if Q remains unchanged.

This is one of the major features of the present invention.

On the other hand, as shown in FIG. 1, when the amount of C is reduced in order to improve toughness, the strength decreases. In that case, from the viewpoint of intensity compensation, the idea of the present invention is to add excitation 9M01v or the like according to the required intensity level. Figure 3 shows 32■
According to the formula Ceq [=C0Mn/6+ (Cu+Ni)/15+] for thick steel materials, even if C becomes low, if Ceq is made above a certain standard by adding other elements, the required amount can be achieved. It can be seen that the strength can be maintained. For example, from API5LX's tensile strength T S) i case,
60, Ceq≧0.25. Ceq'! on X70!
It becomes 0.275. On the other hand, with respect to the toughness level, vTrs of the conventional QT component is on the higher temperature side than -60℃, so to improve the toughness beyond this level, Ceq Riho 3
7 Preferably, it is necessary to keep Ceq <0.33.

Note that the relationship between vTs + Ts and Ceq holds true as shown in the figure, even for B-added steel that is not involved in the Ceq equation.

Considering the above, the reasons for limiting the components are described below.

(1) As shown in Figure 3, if C exceeds 0.06%, vTrs > -60°C with Q unchanged, and the toughness level obtained with conventional Q-T material cannot be improved, so the upper limit is 0.0
6%. As for the lower limit, the lower it is, the harder it is to develop strength and the supplementation of other elements is necessary, but in order to improve toughness, a lower limit is better. Therefore, from the viewpoint of actual operation, the lower limit is set to 00.
It was set at 2%.

(2) 5i Si is set to 0.051 or more from the viewpoint of deoxidizing effect, but 0.051 or more.
If the sail exceeds 8 inches, it will have a negative effect on the toughness, so 8 inches is the upper limit for the sail.

(3) Mn Mn is important as a strength compensating element when C is reduced, and at least 8 inches of sail is required.

Further, the upper limit was set at 2.2 inches in consideration of toughness (Ceq Riho 37) shown in Figure 3.

(4) Nb Since Nb is effective in refining the structure during controlled rolling (i.e., refining the structure before heat treatment) and in turn improving Q-toughness, it should be added in an amount of at least 0.01%. If it is added to the steel ingot, surface flaws will occur, so
The upper limit is %.

(5) At At is effective as a deoxidizing agent, and A! As N, Q
Since it has the effect of preventing coarsening of crystal grains during heating, it should be at least 01% or more. In addition, when B is contained, it combines with N in BN during Q heating to form solid solution B, which has the effect of increasing the hardenability of the steel, but if it exceeds 0.08%, surface defects will occur on the steel ingot. o, o s because there is a risk of
The upper limit is %.

(6) N N, as AtN, is required in an amount of at least 0.002 mm in order to keep the austenite grains fine at the quenching heating temperature, and if it is contained in an amount exceeding o, o o s s, the toughness will be deteriorated. Since it is harmful, it was set at 0.002 to 0.0s%.

The target steel in the present invention has the above composition as a basic component, but in addition to the above components, Cu, Ni, Cr, Mo,
V.

One or more of Ti, B, and Ca may be contained.

Cu may be added from the viewpoints of increasing strength, weather resistance, hydrogen-induced cracking resistance, etc., but since too much Cu impairs hot workability, the upper limit is set at 1.0%.

Ni is an effective element for obtaining strength and toughness in tempered steel, and also has the effect of preventing Cu flaws, but because it increases the possibility of hot cracking during welding and because it is a noble element,
The upper limit is 3.0%.

Cr is effective in improving strength, but too much Cr deteriorates toughness, so the upper limit is set at 1.0 cm.

Mo also contributes to improving the strength of steel and making the structure bainitic, but if it is too large it will actually impair toughness or weldability, so the upper limit is set at 0.8%.

(2) is also an important element for ensuring the strength of steel, but too much of it will adversely affect toughness, so the upper limit is set at 0.1%.

Here, Ti is finely precipitated in the matrix as TiN to prevent coarsening of austenite grains during Q heating and to ensure toughness, and to protect B from N when B is added. The upper limit is 0.03%.

B compensates for the decrease in hardenability (decreased strength) in the extremely low C region, but too much B impairs toughness, so it is added to 30 ppm.
is the upper limit.

Ca has the effect of spheroidizing sulfide-based inclusions and improving hydrogen-induced cracking resistance, but if it exceeds 100 ppm, calcium oxysulfide clusters are formed and the hydrogen cracking property deteriorates, so the upper limit is set at 1100 ppm.

Next, regarding the rolling conditions for steel having the above-mentioned components, controlled rolling is effective in improving toughness by refining the structure during heat treatment in the post-process, and preferably 50% below the non-recrystallization temperature.
It is preferable to use #1 gargle which ensures the above rolling reduction ratio, but it is not necessarily limited to this.

In addition, for industrial Q treatment of steel materials, short-time heating by induction heating (rapid heating) and short-time holding is desirable from the viewpoint of preventing coarsening of austenite grains, but this is not necessarily limited. .

The faster the heating rate up to the quenching heating temperature is 3°C/Bee or more, the better, and the retention time at the temperature is 10
It is preferable to set it within minutes. Further, the cooling rate during quenching is preferably 10° C./see or higher. In addition, for steel materials with welded parts such as seam welded parts such as large diameter steel pipes or welded parts in other steel materials, it is preferable to perform Q treatment after the welding process is completed because the welded part will also undergo Q treatment. I'm not good at explaining.

Next, an example in which the present invention is applied to a large diameter welded steel pipe will be described.

Steel plate having the composition shown in Table 1 (conventional example, example of the present invention)
After forming into a steel pipe shape using the UOE method and welding, 95
After being held for 2 to 2 minutes by induction heating to 0°C, it was quenched to room temperature at a cooling rate of 15 to 20°C for 7'sec. In addition, the conventional example is further 640 to 6
T) treatment of holding and tempering at 70° C. for approximately 6 minutes was performed. The results are also shown in Table 1.

From Table 1, in the conventional example, Charpy's sosw
It can be seen that the surface transition temperature is -60°C or higher, and DWTT85*5ATT is not very good as it is 0°C or higher. - Strength wood with a reduced amount of c, such as the example of the invention, has good toughness even when Q is maintained, s vTrs#i - 60°C or less, DW
It can be seen that the TT85%5ATT is also below 0°C, which is an improvement over the conventional method. Regarding strength, X65. Although a high grade X70 was used as an example, it is clear from FIG. 3 that the lower the required strength, the better the toughness (for example, vTrs). In addition, since the steel of the present invention has a lower C content than the conventional steel, it can be easily inferred that it is excellent not only in weldability but also in unstable ductile fracture characteristics. Further, since the steel of the present invention can obtain good performance even with Q as it is, it is possible to omit the tempering treatment step, which is preferable in terms of operational efficiency.

As explained above, in the present invention, high tensile and high toughness steel materials can be efficiently manufactured.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the amount of carbon in the steel plate after Qt and QT and Ta r vTrs, and Figures 2 (a) and (b) are micrographs of steel plates with different C contents. The structure photograph, Figure 3, shows the Ceq
It is a diagram showing the correlation between , Ts, and vTrs in C sentence. Applicant Nippon Steel Tube Co., Ltd. Agent Keita Tsutsumi and 1 other person Figure 1 et c@mu) Figure 3 Procedural amendment (voluntary) June 7, 1980 Commissioner of the Japan Patent Office Shima 1) Haruki Tono 1, Case Indication of Patent Application No. 57-71256 No. 2, Name of the invention Process for manufacturing high tensile and high toughness steel materials 3, Relationship to the case of the person making the amendment Patent ratio 1F 1 person Address 1-2 Marunouchi-chome, Chiyoda-ku, Tokyo mA< Am
> Representative Minoru Kaneo of Nippon Kokan Co., Ltd. 6. Subject of amendment (1) Specification, page 6, Detailed explanation of the invention, line 3, ``Show'' should be replaced with ``Show.'' Figure 2 Ca) (b) Both magnification is 400x)
. ” he corrected. (2) Submit one attached letter of commission. that's all

Claims (1)

[Claims] C: 0.002 to 0.06%, Si: 0.05 to 0.8% 1 Mn: 0.8 to 2-2'%, Nb: 0.01 to 0.1% , N: 0.002~o, o o schi, A/,: 0.
01-0.08%. Cu: 1.0% or less, Ni: 3.0% or less, Cr: 1-
O1 or less, Mo: 0.8% or less, V: 0.1
% or less, Ti: 0.03% or less, B: 0.00
3% or less, and ca: 0.01% or less of type 1 or 2
A steel material containing at least 100% Fe and the remainder consisting of Fe and unavoidable impurities is heated to a temperature above the Aca point and quenched.
A method for producing high-tensile and high-toughness steel material, characterized in that no tempering treatment is performed.