JPS58210125A - Production of strong and tough high tensile steel plate by direct hardening method - Google Patents

Abstract

PURPOSE:To produce a high tensile steel plate having high toughness by subjecting steel incorporated with specific ratios of C, Si, Mn, P, S, Nb, Ti, B, N, solAl in Fe to heating, rolling and cooling under specific conditions. CONSTITUTION:The steel contg., by weight %, 0.003-0.100% C, <=0.85% Si, 1.0- 3.0% Mn, <=0.03% P, <=0.009% S, 0.008-0.180% Nb, 0.004-0.040% Ti, 0.0004- 0.0025% B, 0.0005-0.0100% N, 0.005-0.090% solAl and consisting of the balance Fe and unavoidable impurities is prepd. After the steel is heated to 900-1,170 deg.C, the steel is rolled at >=50% cumulative draft at least <=900 deg.C and 800-650 deg.C finishing temp. After the finishing by rolling, the steel plate is cooled to a temp. range of the Ar1 transformation point or the neighborhood thereof-Ms point by air cooling or accelerated cooling of not exceeding 4 deg.C/sec cooling rate and is hardened directly from said temp. range.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high tensile strength steel plate with high toughness by a direct quenching method.

In recent years, as energy demand has increased, many gas and oil fields have been developed in harsh environments such as in extremely cold regions such as Alaska or on the ocean floor, and most of the natural gas and crude oil supplied from these fields is required through line pipes. Increasingly, it is being transported to different locations.

For this reason, the demand for line pipes is rapidly increasing, and there is a growing trend to improve transportation efficiency by adopting larger diameter pipes with excellent pressure resistance. Meat and strength.

Currently, there is an urgent need to develop line pipe materials with superior toughness and weldability.

In other words, submarine line pipes and cold region line pipes are required to have thick walls and high strength, and excellent weldability is also an essential requirement due to the harsh construction environment.

However, no matter how much we try to improve the composition of as-rolled steel sheets that have traditionally been used for line pipes, it has not been possible to obtain a product that satisfies the above requirements. Recognizing that the shortcut to solving the above problem is to combine the three factors of conditions, cooling conditions, and steel material composition, we have developed methods such as direct quenching and accelerated cooling that are effective immediately after rolling. A method of water-cooling steel plates to increase their strength has begun to be implemented.

The direct quenching method utilizes the high strength of the quenched structures of martensite and bainite, and generally finishes rolling in the austenite region of the steel (usually at 750°C or higher). This is then directly quenched at Ar 3 points or above, which is a method for manufacturing high-strength steel sheets.Although this method makes it possible to obtain high-strength steel materials, it still suffers from low-temperature toughness. be.

In addition, the accelerated cooling method is applied to the transformation region from austenite to ferrite of the steel plate at control IF /4 +i-, that is, Ar3
~Ar is cooled at a faster cooling rate than air cooling, and the high strength of fine paynite is sometimes utilized, but 600 ~
This is a method for manufacturing a high-strength steel sheet characterized by stopping the accelerated cooling at 550° C., and is intended to increase the strength by combining fine-grained ferrite or part of fine bainite. According to this method, tensile strength: 60
kg7) G1 class steel or x-70 class (API standard)
It is possible to obtain steel materials that are suitable for manufacturing up to line pipes, but strict restrictions on accelerated cooling conditions, such as the need to strictly adjust cooling start temperature, cooling rate, cooling end temperature, etc., make it difficult to manufacture steel materials. This had the problem of poor workability.

On the other hand, recently, a technology has been proposed to add Ti and B to controlled rolled steel to produce bainitic high-strength steel sheets with good toughness. It was found that even if an attempt was made to increase the toughness value by applying direct quenching or accelerated cooling as in conventional methods, the desired toughness could not be obtained, and on the contrary, the toughness deteriorated significantly.

Moreover, today's research has revealed that it is difficult to strengthen B-treated steel by controlled rolling by water cooling after rolling, even with conventionally known techniques.

From the above-mentioned viewpoints, the present inventors were able to ensure good manufacturing workability, a low carbon equivalent that is suitable for weldability, and a strength of class X-70 to X-100 (tensile strength: 6
0~80 Yu/-1 Yield strength 249~7 o 1g7't
d) As a result of various studies in order to produce a tough, high-strength steel plate that exhibits toughness of approximately -100°C or less at the Charpy fracture transition temperature (vTs), we have developed the following (a) to (d). This led to the discovery of new knowledge. That is, (a) Fine grain ferrite is generated by suppressing the C amount below a predetermined value in steel in which Nb, Ti, and B are added to promote the formation of fine martensite in order to obtain high strength. (b) When the steel shown in item (a) above is heated at a low temperature and then reduced to a large extent in a low temperature range, it becomes distorted. Fine austenite is formed. and,
- Fine ferrite will be generated from this, and the large reduction in this low temperature range will further promote the formation of fine ferrite. (C) When manufacturing steel sheets, as mentioned above, I
After rolling at a lower rate of F to sufficiently generate fine ferrite, the fine austenite existing untransformed due to the coexistence of solid solution Nb and solid solution B is reduced to 600 to 350.
When quenched from ℃ (above the Ms point), fine dispersed martinsite is produced from the retained austenite, and eventually
It is possible to obtain a steel with a two-phase mixed structure of ferrite and martensite, and to reliably obtain a strong, high-strength steel plate having not only the high toughness of fine-grained ferrite but also the high strength of fine martensite. In other words, extremely fine ferrite precipitates from the grain boundaries of fine-grained niostenite,
The residual austenite that remains in it becomes extremely fine martensite by quenching, and the structure becomes a mixed structure of extremely fine ferrite and martensite, resulting in particularly high strength of X-70-X-100 class. In addition, high toughness can be obtained even without a tempering process. in this case,
In the conventional direct quenching method, the steel plate is quenched immediately after rolling, so the cooling rate of the steel plate is too fast, preventing the formation of fine polygonal ferrite and turning it into bainite or martensite phase steel, resulting in high strength. However, the excellent toughness effect of fine-grained ferrite cannot be utilized. That is, in the present invention, after rolling is completed, fine ferrite is first precipitated into the steel sheet by cooling in the range of air cooling or accelerated cooling at about 4° C./sec to prepare a base for improving toughness. High strength can be achieved by converting untransformed austenite into martensite by direct quenching from a temperature range of 350°C.

If this is further tempered, the toughness will be further improved. Furthermore, in addition to the fact that ferrite is fine and has high strength, martensite provides even higher strength than bainite, resulting in lower C and lower Ceq (carbon equivalent).
(d) In addition to the above-mentioned steel, Cu, Cr, Mo,
Ni, Co, V.

The strength and toughness of a steel plate can be improved by adding a predetermined amount of one or more of Zr, La, Ce, and Ca.

Therefore, this invention was made based on the above findings, and includes: C: 0.003 to 0.100%, Si: 0.85% or less, Mn: 1.0 to 3. Os, P: 0.03
% or less, S: 0.009 or less, Nb: 0100
8~0180%, Ti:0.004~0040%, 1
3:O,0O04~0.0025s, N:O
,0O05~0.0100%, 5ot-Al: 0.0
05-0.090%.

and, if necessary, further contain Cu, C
r, and one or more kinds of MO: 0.05 to 0.50, one or more kinds of Ni, k, and CO: 0. l～l O%
.

■, and one or more types of Zr: 0.01 to '0.15
%, one or more of La, Ce, and Ca: O, 0O05
~00020% of steel, containing either alone or in combination, Fe and unavoidable impurities: (the above weight %, hereinafter, the content ratio of the composition component is referred to as weight%) is made of 900% ~1170
After heating to a temperature of at least 900°C, rolling is carried out at a cumulative reduction rate of at least 50 inches and a finishing temperature of 800 to 650°C, and after finishing the rolling, air cooling or accelerated cooling not exceeding 4°C/second is carried out to a temperature of 600°C. By cooling to a range of ~350°C and then directly quenching from this temperature range, a high tensile strength steel plate with excellent toughness and high strength is obtained.

That is, as mentioned above, the conventionally known direct quenching method
Rolling is finished at the r3 point or higher, and hardening is started at the Ar3 point or higher.
In contrast to the method in which a low C-Nb-Ti-B steel with a specific composition is rapidly cooled at 1 point to Ar and then allowed to cool, the present invention involves rolling a low C-Nb-Ti-B steel with a specific composition and quenching it from below the Ar point (600°C or less). By doing so, bainite transformation is prevented and a steel sheet with a two-phase mixed structure of fine ferrite and fine martensite is obtained.

Next, in the method for manufacturing a strong and high tensile strength steel plate of this invention,
The reason why the amount of chemical composition components, rolling conditions, cooling conditions, and quenching temperature are limited as described above will be explained.

■) Chemical composition component amount ■ C The C component has the effect of ensuring the strength of steel materials, but if its content is less than 0.003%, the desired effect cannot be obtained, and on the other hand, 0.100% If the content exceeds 0.03, the formation of fine ferrite will decrease and the amount of martensite will increase, making it impossible to obtain the desired toughness and also deteriorating weldability. ~
Q. It was limited to 100%.

■ 5I S1 component is an element that is inevitably contained in steel for deoxidation, but if its content exceeds 0.85%, it will have a negative effect on the toughness and weldability of steel. Its content was limited to 0.85% or less.

-■ Mn The Mn component has the effect of improving hardenability and improving the strength and toughness of steel, but if its content is less than 10 parts, it becomes difficult to generate martensite and the desired effect cannot be achieved. On the other hand, if the content exceeds 3.0%, the toughness will decrease, it will have a negative effect on weldability, and it will be difficult to generate fine-grained ferrite, so the content should be reduced to 1.
0-33. I limited it to ○chi.

■ PS component is normally contained as an unavoidable impurity and is not a desirable component, but especially if its content exceeds 0.03%, it may deteriorate weldability or cause segregation in the steel. The content was limited to 0.003% or less because the negative effects of internal quality deterioration would appear.

■ S The S component is also normally contained in steel as an unavoidable impurity, but if its content exceeds 0.009 inch, it will have a significant negative effect on Charpy's lateral energy absorption. However, the amount of honor is O, OO9
% or less.

■ Nb The Nb component promotes the formation of fine-grained ferrite and also promotes the formation of martensite in coexistence with [I], thereby improving strength and toughness.
If the O content is less than 8%, the desired effect cannot be obtained, and if the O content exceeds 180%, it will have a negative effect on weldability. Therefore, the content should be reduced to 0.0%.
It was limited to 08-0180%.

■ The Tl Ti component has the effect of refining the rolled structure and fixing N in the steel to ensure the hardenability improvement effect of the S component, but if its content is less than 0.004%, the above-mentioned The desired effect cannot be obtained, and if the content exceeds 0.040%, the toughness deteriorates, so the content is limited to O,OO-4 to 0°040%. did.

■ BS The S component has the effect of improving the hardenability of steel and ensuring strength and toughness, but when its content is 0, OO('1
If the content is less than 4%, the hardenability is insufficient and satisfactory toughness cannot be obtained, while if the content exceeds 0.0025%, it will adversely affect weldability. It was limited to 0.0004 to 0.0025.

■N The N component has the effect of forming TiN together with the Ti component and precipitating in the steel matrix to refine the rolled structure, but if its content is less than 5% of O,0O, the desired effect will not be achieved. On the other hand, if it is contained in excess of 0.0100%, it will combine with the S component and reduce the hardenability improvement effect of the S component. OOO5
It was limited to ~0.0100%.

[Phase] so’10M.

The M component has a deoxidizing effect and a grain refining effect, but if its content is less than 0.005%, the desired effects cannot be obtained, while if it is contained in an amount exceeding 0.090%, non-metal Since the amount of inclusions increases and the steel becomes brittle, the content should be reduced to 0. OO' was limited to 5-0, 090%.

OCu, Cr, and MO These components have the same effect of improving the hardenability of steel and helping to generate martensite, so they may be added as necessary when even greater toughness is required. However, if the content is less than 0.05% of each, the desired effect of improving toughness cannot be obtained, while if the content exceeds 0.50% of each, the formation of fine ferrite is suppressed and the toughness is deteriorated. The content was reduced to 0.05 because it caused a decrease in
~0.50 chi.

@N1. and C.

These components have the same effect of improving the toughness of steel, so they are included as necessary when even greater toughness is required, but if their content is less than 0.14, the desired toughness cannot be achieved. No toughness improvement effect was obtained, and on the other hand, 1. ,
If the content exceeds 0%, it will cause an economic disadvantage because the N1 and co element bodies are expensive, so the content was set at 0.1 to 10%.

Q■ V and Zr These components have a uniform effect of improving the strength of steel through precipitation, so they are included as necessary when even more strength is required, but their content is If each is less than 0.014, the desired strength improvement effect cannot be obtained.
On the other hand, if each content exceeds 0.15%, the toughness will deteriorate, so the content was limited to 0.01''-0,] and 5%, respectively.

(↓, La, Ce, and Ca) These components contain uniform additives that significantly improve Charpy absorption energy and further improve sulfide stress corrosion cracking resistance by controlling the morphology of sulfide-based nonmetallic inclusions. However, if the content is less than 5% each, the desired effect of spheroidizing the inclusions cannot be obtained;
If the content exceeds OO5%, the amount of nonmetallic inclusions will increase and the performance of these will tend to be impaired, so the content should be increased from 0005 to 0.0.
It was limited to 20%.

■) Rolling conditions ■ Heating temperature: 900 to 1170°C If the heating temperature is less than 900°C, it means that the temperature at which the steel becomes austenitic (ACs point) has not been reached, and the desired rolled structure cannot be obtained. On the other hand, if the heating temperature exceeds 11.70°C, the steel structure becomes coarse grained, resulting in poor toughness, so the heating temperature was set at 900-1]-70°C.

■ Reduction conditions: If the temperature at the time of reduction of 504 or more exceeds 900℃ at 900℃ or less, and the cumulative reduction rate is less than 50℃, it will create distorted, micro-M, i-stenite grains. Therefore, it becomes impossible to promote the formation of fine-grained ferrite. In other words, in order to obtain fine ferrite by air cooling or strong cooling to a certain extent after finish rolling, and to obtain finely dispersed martensite by subsequent quenching, cumulative rolling of 50 inches or more at a temperature of 90°C or less is required. Since rolling at a certain rate is essential, the rolling conditions were limited as described above.

■ Finishing temperature If the rolling finishing temperature exceeds 800°C, the rolling structure will not be sufficiently refined; If the temperature is lower than LO℃, the degree of rolling in the two-phase region becomes excessive, resulting in a decrease in toughness t'1 and ductility.
The temperature was set at 50°C.

■) Cooling conditions Air cooling or accelerated cooling not exceeding 4°C/sec was used for cooling after rolling finishing, as cooling rates faster than these values would inhibit ferrite transformation and cause a fine ferrite structure to form. Because you won't be able to get it. In particular, 4
The reason for setting the cooling rate as an upper limit of ℃/second is to promote cooling of thick plates until quenching and to improve efficiency. If it exceeds this, ferrite transformation will not occur smoothly as described above.

(2) If quenching is performed at a temperature exceeding 600° C., which is the quenching temperature Ar1 point, ferrite transformation will not proceed sufficiently and the structure will become a mere quenched structure, making it impossible to obtain good toughness. On the other hand, if the temperature is lower than 350°C, the hardening effect will not be sufficient because the temperature is below the so-called Ms point. Therefore, the quenching temperature was set at 600 to 350°C.

Next, the present invention will be explained by examples and in comparison with comparative examples.

Example 1 In this example, the influence of the quenching temperature after finish rolling on the mechanical and physical properties of steel materials was compared.

First, a slab having a chemical composition satisfying the method of the present invention as shown in Table 1 and having a thickness of 150+ m was manufactured by a conventional method.

Table 1 Next, this slab was heated and held at 1050° C. and then rough rolled, and then, when the temperature reached 900° C., it was rolled at a rolling rate f of 67 from a slab thickness of 75”. Then, rolling was completed at 740° C., followed by air cooling, and then quenching at various quenching temperatures as shown in Table 2 to obtain steel plates with a thickness of 225 mm.

The mechanical and physical properties of the various steel plates thus obtained were measured in order to compare their strength and toughness, and the results are also listed in Table 2.

Table 2 As is clear from the results shown in Table 2, quenched materials 2 and 3 with a quenching temperature of 700 to 630°C, which is higher than the range of the present invention, have the highest strength values. Although shown, Charpy fracture transition temperature (vTs).

It can be seen that the toughness as expressed by DWTT Drop Test 8 (FA'rT) is significantly inferior. This is because quenching was performed before the r→α transformation was completed, which prevented the formation of fine grain ferrite and significantly increased the amount of bainite and martensite.

On the other hand, quenched materials 4 to 7 whose quenching temperature was 600 to 350°C below the Ar1 transformation point, which is the range of the present invention, showed a strength increase of 10 kg/- or more compared to air-cooled material 1 after rolling. Despite this, almost no deterioration in toughness was observed.

° Example 2 In this example, the heating, rolling, and cooling conditions during steel sheet production are as follows:
The effects on the mechanical and physical properties of steel materials were compared.

First, similar to that shown in Table 1 in Example 1,
It has a chemical composition that satisfies the method of the present invention and has a thickness of 15
A slab of 0.0 was prepared.

After heating and holding one of these slabs at 1,1050°C,
When the temperature drops below 900°C, it is rolled at a reduction rate of 67%, then after finishing the rolling at 740°C, it is subsequently air cooled and quenched from a temperature of 550°C to a plate thickness of 25 mm. steel plate was obtained. The conditions at this time were taken as the standard conditions of the present invention, and sample number 5 obtained thereby
The mechanical and physical properties of the samples are shown in Table 3.

Next, for each of the above slabs, set the heating temperature. Pressure F, rate below 900°C.

The specimens were prepared by rolling under the same conditions as the standard conditions, except that the finishing temperature and cooling rate after finishing were changed as shown in Table 3, and quenching was performed from a temperature of 550"C. Steel plates numbered 9 to 14 were obtained.
Physical properties are also shown in Table 3.

From the results shown in Table 3, it is clear that when the heating, rolling, and cooling conditions are out of the range of the method of the present invention, the low-temperature toughness deteriorates significantly.

In particular, accelerated cooling between rolling finishing and quenching is a measure taken to prevent this, since it takes time to quench when the plate thickness is large, but if the cooling rate becomes too fast as a result of this, Comparing the toughness values of Sample No. 14 and Sample No. 13, it is clear that this is essentially the same as quenching immediately after rolling, leading to deterioration of toughness.

Example 3 In this example, the influence of chemical composition on mechanical and physical properties during steel sheet production is compared.

Steels having the chemical compositions shown in Table 4 are melted using normal melting methods, and cast into normal strips ('+).
After heating and holding the obtained slab at 1050°C, rough rolling was performed, and then rolling was restarted at 850°C with a rolling reduction of 7 cm at the slab thickness, and after finishing rolling at 700°C, air cooling was performed. , water quenched at a temperature of 550°C, resulting in a plate thickness of 19 m.

The mechanical and physical properties of each steel plate were measured. These results are also shown in Table 4.

From the results shown in Table 4, the present invention materials 21--50 are as follows:
All of them exhibit tensile strength of 70 kg/- class and have high low temperature toughness values, while comparative materials 51~
As seen in No. 63, it is clear that if any of the component compositions deviates from the scope of the present invention, at least one of the above characteristics will be inferior.

As described above, according to the method of the present invention, it is possible to ensure a low carbon content suitable for weldability, and the strength and toughness are improved by tensile strength: 60 to 80 kg/- and Charpy fracture transition, respectively. It is possible to manufacture steel that exhibits an excellent temperature knee of about 100°C or less, and therefore, when used as line pipe in the development of oil fields and gas fields under harsh environmental conditions, it has extremely excellent performance. It shows that.

Furthermore, tempering the high-strength steel sheet obtained by the method of the present invention to a temperature below the ACl transformation point does not impair the properties of the steel in any way, but rather improves its toughness and resistance to hydrogen sulfide cracking. is expected.

March 1711, 1981 Kazuo Wakasugi, Commissioner of the Japan Patent Office 1, Indication of the case, Patent Application No. 1987-93727 2, Name of the invention: Method for producing strong steel sheets by direct quenching method 3, Person making amendments Representative Noriyuki Kumagai 4. Contents of the agent's voluntary amendment (1) The scope of claims in the specification is amended in a separate notice.

(■ In the specification, page 6, line 19, it says [from 600 to 350°C (above the Ms point)], 1'-Ar, from the temperature Ir-range from near the transformation point to the Ms point.''
I am corrected.

(3) Specification, page 8, line 2 1600-350℃ That's what it says. l　Ar, near the transformation point ~ Ms point'' I am corrected.

(4) Specification, page 9, line 18: “600-35
"In the range of 0°C" should be corrected to "In the temperature range of Ar, near the transformation point to the Ms point."

(5) In the specification, page 1O, lines 7-8, the phrase "from Ar, below the point (600°C or less)" has been replaced with [from the Ar temperature near the end of the ferrite transformation (600°C or less)]. Correct it as 1.

(In the 0 specification, page 18, line 11, it says ``1 from the temperature exceeding 600°C, which is the point of Ar.''
, the point varies depending on the composition and cooling rate, but for the steel of the present invention, the temperature is considered to be 650 to 600 "C".

(η In the specification, page 18, line 14, it says [on the other hand, less than 350°C]. Corrected to ``On the other hand, below the MiE point temperature, that is, on average, less than 350°C.''

(In the 0 specification, page 18, line 17, it was set as 1600 to 350"C." is corrected to [The temperature range is set as Ar, near the transformation point to the Ms point.]

(9) Specification, page 20, 4th line from the bottom, “700
~630°C and the range of the present invention.'' 1700~630°C (Ar of the present invention steel, the point is approximately 600°C)
℃) and the scope of the present invention" is corrected as "2. Claim C: 0.003 to 0100%, Si: 0.85% or less,
Mn: 1.0-3.0%, p: 0.03% or less, S
: 0.009% or less.

Nb: 0.008~0.180%, Ti: 0.004~
0.040%.

B: 0.0004-0.0025%, N: 0.
0005~0.0100%, sol, AQ:
0.005-0.090%.

In addition to containing, if necessary, further Cu,
One or more of Cr and Mo: 0.05 to 0.50%
.

One or more of Ni and Co: 0.1 to 1.0%.

(1), and one or more types of Zr: 0.01 to 0.15%.

One or more of La, Ce, and Ca: 0.0005
~0.020%.

After heating a steel containing Fe and unavoidable impurities (at least % by weight) alone or in combination to 900 to 1170°C, the cumulative rolling reduction rate of at least 900°C is 50°C.
% or more and at a finishing temperature of 800 to 650° C., and after finishing the rolling, air cooling or direct quenching from the surroundings is performed. ”

Claims (1)

[Claims] C: 0003 to ○, 100%, Si: 0.85% or less,
Mn2 1.0-30%, P: 0.03 or less, S
: 0.009 inch or less, Nb: 0.008 to 0180 inch,
Ti: 0.004-0040%-, B: O, OOO now ~ O, OO25 Chi, N: 0.0005-0.01
00%, 5oLNl: 0.005-0.090
%. and, if necessary, further contain Cu, C
r, and one or more types of Mo: 0.05 to 0.50 h. One or more of Ni and co: O, 1 to 1.0%. One or more of v9 and Zr: 0.01 to 0.15%. one or more of La, Ce, and Ca 200005-0,
OO20%. After heating a steel containing Fe and unavoidable impurities (at least 2% by weight) alone or in combination to 900 to 1170°C, the cumulative rolling reduction rate below 900°C is at least 50%.
% or more and at a finishing temperature of 800 to 650°C, and after finishing the rolling, cool to a range of 600 to 350°C by air cooling or accelerated cooling not exceeding 4°C/sec, and then A method for producing strong, high-tensile steel sheets that is characterized by direct quenching.