JPS6092422A - Production of ni-containing low temperature steel having excellent crack opening displacement - Google Patents

Abstract

PURPOSE:To obtain a low temp. steel which exhibits an outstanding crack opening displacement and has high strength and toughness by subjecting a steel contg. a prescribed ratio of Ni and P as basic components to rolling and heat treatment under prescribed conditions. CONSTITUTION:A steel contg. 7.5-10wt% Ni and <=0.01wt% P as basic components is produced. The billet or casting billet of such steel is rolled after heating to <=1,200 deg.C. The rolled steel is then subjected to >=30% cumulative draft at <=950 deg.C and is finished at >=800 deg.C to form >=ASTM No.7 austenite grains. The steel is air cooled down to the temp. lower by >=50 deg.C than the above-described finishing temp. after the primary rolling and is rolled at 700-750 deg.C and 5-25% cumulative draft. The steel is cooled down to <=350 deg.C at >=2 deg.C/sec cooling rate right after the secondary rolling. The cooled steel is reheated to <=AC1 temp. and is tempered.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a Ni-containing low-temperature steel sheet with excellent crack opening displacement, which has an excellent crack opening displacement that has not been previously obtained as a Ni-containing low-temperature steel sheet with high strength and toughness. The aim is to stably produce a product having the following properties and to obtain a product suitable for use under temperature conditions such as liquefied natural gas temperature or lower.

With the diversification of energy sources triggered by the oil crisis, the demand for liquefied gas has been increasing year by year, and the growth in demand for liquefied natural gas (LNG) has been particularly remarkable.
Along with this, the demand for NI steel including 9%Nl @ as steel material for LNG storage and transportation containers is rapidly increasing. As the amount of steel used increases and the size of containers increases, the demands on the stability of Ni-containing low-temperature steel structures tend to become increasingly strict. However, the Ni-containing steel material conventionally used as a steel material for liquefied gas storage containers at temperatures below this LNG temperature (-162°C)
@ Warm temperature m (7.5~10%Nl steel) t, t A8T
It is standardized to M-A353 and A353, and its heat treatment method is twice normalized, once fired Mt, (NN
T) treatment, and the latter is quenching and tempering QT') treatment is the first to be adopted. By the way, low-temperature toughness, which is the most important performance for these low-temperature vehicles, is -195℃ or
The amount of lateral bulge or absorbed energy value of the 2+a+V notch mechanical impact test piece at -170°C (however, the absorbed energy value is Supplementary Q-mentary R,eg
ulrement). In general, there is a good correlation between the absorbed energy value of the 7 Jarby test and the amount of lateral bulge, and due to the relationship between the two, until now, the absorbed energy of the 7 Jarby test has not been used as a measure to improve low-temperature toughness. A variety of creative efforts have been made with a focus solely on improving the performance. However, in recent years, from the perspective of structural stability, the concept of fracture toughness has become widespread, and fracture stress or allowable defect size can be directly calculated, and KIO and Crack Opening Displacement (Crack Opening Displacement) have great significance in the design of structures.
Fracture toughness values such as increasing displacement (hereinafter referred to as COD) are attracting attention. Especially this
Among fracture toughness values, COD values have been increasingly included in steel material performance specifications in recent years due to the idea of minimizing the risk of brittle fracture occurrence in low-temperature structures.
In a recent example of 9% N1914, the limit CO
Many of them specify the D value as 0, 2 WIJ1 or more. By the way, no clear relationship has been found so far between this COD value and the amount of lateral bulge or absorbed energy value in the Charpy test. However, this does not necessarily lead to an improvement in the COD value. The reason for this is thought to be that while the COD value purely represents the occurrence characteristics of brittle fracture, the absorbed energy in the 7 Jarby test includes the propagation stopping characteristics as well as the occurrence of brittle fracture. By the way, from the viewpoint of obtaining high absorbed energy in the Charpy test, heat treatment has been carried out according to the method specified by ASTM described above, but it is not always easy to stably secure the above-mentioned COD value with such a method. On the other hand C
Although there are few examples of a method for producing Nl-containing low-temperature steel with an excellent OD value, the present inventors have already proposed JP-A-56-1.
No. 56715, etc., and this method is followed by accelerated cooling after completion of hot rolling, followed by two steps of AC, ~AC,
After being heated to a phase temperature range, it is quenched and finally tempered at a temperature below AC3, and it is necessary to heat it to a two-phase temperature range before tempering, which complicates the manufacturing process and makes it difficult to manufacture. There is a problem that the cost also increases. In addition, low-temperature steel containing Nl, such as 9% Ni steel, has a narrow two-phase region temperature range, so there is a disadvantage that the material quality deteriorates significantly due to slight fluctuations in temperature.

Furthermore, JP-A No. 55-76 (121) describes a method for producing steel sheets with stable strength and toughness by direct quenching and tempering. A technique of rapid cooling and then tempering has been disclosed, but as is clear from the name of the invention, the entire purpose of this is to reduce variations in strength and toughness depending on the position of the steel plate, and what is said about improving crack opening displacement? has not been fully shown.

The present invention was created after repeated studies in view of the above-mentioned circumstances, and it solves the above-mentioned problems and provides L
It is possible to obtain Ni-containing low-temperature steel that has excellent inter-crack displacement even at NG temperature (-162°C) or lower, and has uniform material properties with small anisotropy. Then, a Ni-containing low-temperature steel slab or cast slab containing Ni as a basic component in wt% (hereinafter simply referred to as 俤) N1 near, 5 to 10%, P: 0.010% or less is heated to a temperature of 1200°C or less. It is heated and then rolled, and in the rolling process 9
After performing primary rolling at a temperature of 50°C or lower with a cumulative reduction of at least 30% and a finishing temperature of 800°C or higher to make ausbunite grains as fine as ASTM Class 7 or higher, a temperature that is at least 50°C lower than the finishing temperature. air cooled to 700
Perform secondary rolling again at a temperature of ~750°C with a cumulative downstream rate of 5 to 25%, and immediately after the secondary rolling p, +TE, cool to a temperature of 350°C or more at a cooling rate of 2.0°C/dew or more, Then AC
, the idea is to reheat and temper the material to a temperature below.

That is, to further explain the contents of the present invention, first, as a steel for LNG, sufficient COD at -170°C is obtained.
At least 7.5% N is required to obtain the value in the base metal and welded joint.
On the other hand, even if it is contained in excess of N i fHll)%, the effect will be saturated and there will be no merit worth the cost increase, so the second limit is set to 10.

PI''i is an element that promotes embrittlement during high-temperature tempering, and in particular in steels with high hardenability such as 9% Nu steel, the absorbed energy in the Charpy test due to P segregated at the 111 austenite grain boundaries after tempering is First, the COD value also deteriorates.

Therefore, in order to suppress v8 return embrittlement caused by P and obtain an excellent COD value even in a low temperature range, the upper limit of P was set to 0.010%.

The essential requirements for the composition of the steel in the present invention are as described above, and the alloying elements of the steel are not particularly specified. However, the range in which the effects of the present invention can be preferably exhibited is as follows: C: 0.01 to 0.15%, SL: 0.01
~0.5%, Mn:(), 1~1.0%, and Cr', (above 511+ -de,
Mo: 0.5%1. F, B: 0.005
% or less of Group 1 elements, Ca
: 0.007% or more F, RgM: 0.1% or more, A4g: 0.007% or more, 7C: 01% or less of the second group element (1 type' or 2 or more types) It can contain one or both.

That is, here, C is an effective element for improving the overall strength and hardenability, and at least 0.01% is necessary to obtain the strength level specified by the ASTM standard, but 0.01% is necessary.
If it exceeds 15%, weldability deteriorates, so the upper limit is 0.
It shall be 15%.

SL is an essential element as a deoxidizing element in the steelmaking process, and is contained in steel in an amount of at least 0.01 eIb, but if it exceeds 0.5 eIb, toughness, especially COD properties, deteriorates. Since this is recognized, the upper limit is set at 0.5%.

un increases the strength and CO to improve the hardenability of steel.
Although it is an effective element for improving D characteristics, if it is less than 0.1%, these effects are hardly recognized, and if it exceeds 1.0%, it promotes high temperature tempering brittleness and conversely deteriorates COD characteristics. Therefore, the upper limit was set at 1.0%.

Cr, which is an element added as necessary, also improves hardenability.
However, if it exceeds 0.5%, carbon in the weld zone will increase.
01) In order to deteriorate the characteristics, the upper limit was set to (), 5chi.

Mo is also an element that similarly improves Soichiroshin properties and also completely reduces high temperature tempering brittleness, but even if it is contained in an amount exceeding 0.5%, the effect will be saturated, so 0.5% is the upper limit. did.

nb- is effective for improving strength and COD properties like the camphor element, but 0.00
Even if the content exceeds 5%, the effect of improving hardenability is saturated, and on the other hand, the formation of precipitates deteriorates the COD properties, so the upper limit is set at 0.005.

In addition to the above-mentioned elements, for the purpose of homogenizing the characteristics in the thickness direction, Ca with a VC of 0.007% or less, and 0.007% or less of VC are added.
Even if 1% or less of REM, 0.07% or less of gin, and 0.1% or less of n- are added, the effects of the present invention will not be impaired.

Next, to explain the reason for the limitation regarding the steel plate manufacturing conditions in the present invention, Fig. 1 shows the ASTM austenite grain size immediately before secondary rolling and the limit COD at 170C of a steel plate that has undergone subsequent processing within the scope of the present invention. value (hereinafter δc-1
However, in order to obtain excellent COD properties after tempering, it is necessary to make the austenite grains fine immediately before the secondary rolling, and after tempering, the stable L7 and δe-170℃≧0.2+s are required. Vi2 to secure
It is necessary to set the austenite grain size immediately before the next rolling to 7 or more in ASTM m.However, factors that affect the austenite grain size just before the second rolling include the slab heating temperature and the subsequent rolling conditions. In actual steel plate manufacturing, the heating temperature of the steel billet or cast slab is 1200℃.
Prevents coarsening of initial austenite grains as a temperature control,
In addition, it is necessary to add at least 30 degrees of downstream rolling at 950° C. or lower for the purpose of reducing the grain size to 1 by rolling. Note that the heating temperature F limit (・1, 950 in the above-mentioned primary rolling
There is no need to specifically limit the temperature as long as the temperature is such that the primary pressure treatment finishing temperature described below can be maintained while maintaining the cumulative pressure F of .degree. C. or less.

The reason for setting the temperature k of 800°C or higher for the 2nd rolling process is that
As is clear from the figure, when the temperature is lower than 800°C, the austenite that has been rolled does not recrystallize and forms crystal grains that extend in the rolling direction, resulting in significant anisotropy in strength. Furthermore, N, which was not yet in solid solution during heating of the slab, and MnS itself, which was extended during rolling, became the starting point for brittle fracture occurrence and promoted the embrittlement of the extended austenite grain boundaries, especially in the direction perpendicular to rolling (hereinafter referred to as This is because the C0DI characteristic (referred to as C direction) is significantly deteriorated. In addition, Figure 2 shows the temperature at 60 yen after the completion of primary rolling.
This is the result of air cooling to a temperature lower than ~80℃, immediate water cooling without secondary rolling, and then tempering. Even if the original goal is δC-170
≧0.2! It can be seen that it is difficult to stably secure the

, Figure 83 shows the tensile strength IW and COD test results of a steel plate that was air-cooled to 715°C after the primary rolling, then subjected to secondary rolling again, and water-cooled and tempered immediately after rolling. When the finishing temperature is 820℃ (0 in the figure)
It is clear that the strength and δe-170°C of the case (shown in the figure) are significantly improved as the rolling reduction in the secondary rolling increases.On the other hand, when the finishing temperature of the primary rolling is set to 770°C ( ) shows Je-1700 even if secondary rolling is performed.
There is a tendency for the properties to be not improved, but to be deteriorated, and the anisotropy to be increased.

In other words, the difference between the two is due to the difference in the austenite structure immediately before the secondary rolling, and as shown in the micrograph in Figure 5, the former shows that the austenite grains immediately before the secondary rolling are in a revo-zonal state where recrystallization has been completed. As a result, secondary rolling within the scope of the present invention results in less anisotropy, well-sized, and fine austenite.
Deterioration becomes noticeable at 0°C. Understanding the above phenomena (,
11 Important points in the structure of the present invention, namely 2
It is important to control the onutenite grains immediately before the next rolling and to select the optimal secondary rolling conditions.

The present invention also provides that the rolling finish temperature is at least 50
The reason for air cooling to a low temperature is to sufficiently recrystallize the austenite grains after the primary rolling, and also to minimize the temperature difference between the front and back surfaces of the steel sheet at the end of rolling and the temperature unevenness of the sheet wall, and to reduce the temperature of the steel sheet after water cooling. It is also effective in reducing distortion and material variations. Secondary rolling temperature range 700~7
The reason for setting the temperature to 50°C is that at temperatures higher than 750°C, austenite partially recrystallizes due to processing and becomes mixed grains, resulting in δe
At temperatures lower than 700°C, austenite is significantly elongated in the rolling direction, resulting in deterioration of -170°C.
This is because, together with the adverse effects of inclusions such as t- and MfIS, the δe-170° C. in the C direction is degraded. The reason why the cumulative reduction ratio during the secondary rolling was set to 5 to 25 inches is that if it is larger than 25%, it will deteriorate δe-170°C in the C direction for the same reason as the above-mentioned rolling temperature of less than 700°C. 5%, Il
, C is in a small range, h-crystalline austenite does not become fine grained, and an increase in strength due to the ausform effect is hardly expected in #1.

Next, immediately after the secondary rolling, 2. ．． 01:, The reason for water cooling at a cooling rate higher than /see is that the reason for water cooling immediately after secondary rolling is to quench the austenite obtained by the above-mentioned manufacturing method in its original state. This is to obtain a martensitic structure that is grain-sized, fine, and has a high dislocation density, which cannot be obtained with reheated and quenched materials.Furthermore, direct quenching after rolling prevents grain boundary segregation of impurity elements such as P. This is because it is also effective in improving low-temperature toughness after tempering. As can be seen from Figure 4, some payinite deformation occurs where the cooling rate is 2.0℃/especially slow, and as a result, the strength δl! -1
7 (1°C degrades, cooling rate 1f, Vi2.0°C/
It was assumed that the position was higher than that. The reason for cooling to 350° C. or lower at the above cooling rate is to completely transform into martensite. In addition, for the purpose of improving low-temperature toughness due to carbide imitation (111 dispersion, reduction in strength due to recovery of substracture, precipitation of fine stable austenite at prior austenite grain boundaries, lath boundaries, etc.), Perform tempering treatment.

Specific embodiments of the present invention will be described below.

That is, the chemical composition of the test steel used by the present inventors is as shown in Table 1 below. are also outside the scope of the present invention.

The following Table 2 summarizes the manufacturing conditions for the 6 steels as described above, and in this jfE2 table, the austenite grain size tiN
119 had an ASTM austenite grain size number of 5.5, and all the others had grain size numbers of 8 to 9.

Further, the cooling rate under the cooling conditions in Table 2 is shown as the average cooling rate from the water cooling start temperature to 300 U (water cooling was performed up to 4200°C or less). All sample materials were tempered at 575°C.

However, the following @3 table summarizes the results of testing and measuring the mechanical properties of each of the steel plates obtained as described above using test pieces taken along the C direction. +4[L3,5,8,12 produced by the method
Steel plates No. 18, 19, 23 and 26 all have high strength and an excellent limit COD value of δC≧()62 at -170°C. Nαi t for this tL
: The heating temperature, the primary rolling conditions for 9 and 10, and the secondary rolling conditions for 12, 11, 13 to 17 and 251, and the cooling rate at water cooling of 4° 20 t'' are all according to the present invention. It deviates from the range, and furthermore, @21 and 22 are conventional quenching and tempering treatments, and none of these /
The lowest value is 1c-170°C.

(As mentioned above, I, 7, and 24 are steels that deviate from the steel composition range in the present invention, and the
4-The entire scope of the present invention as one piece (tJI16゜24)
For δc-17On, only a low level value less than (1,2+mm VC) was obtained.

According to the present invention as explained above, it is possible to stably produce a high-strength, high-toughness N-containing low-temperature steel plate that exhibits excellent skewer and blade displacement.
This is a highly effective invention because it can appropriately provide a product suitable for use under the following temperature conditions, such as liquefied gas temperature or so.

[Brief explanation of drawings]

The drawings illustrate the technical content of the present invention, and the first
Figure t'' ASTM austenite grain N [just before the secondary pressure g
A diagram showing the relationship between L and ac-170℃, Figure 2 is 1
Next pressure sail finishing dedication and T8. A chart showing the relationship between dc and 170°C.
Figure 1.84 showing the relationship at 170°C + Z Cooling rate after 12th rolling and 1゛S. A diagram showing the relationship between δc and 170°C, and Figure 5 shows the secondary pressure [
i[Previous (715℃) showing austenite
It is a $00 microphotograph, and the primary rolling finishing temperature is (1
This is the case where k) is 820'0 and (b) is 770°C. Patent applicant: Nippon Kokan Co., Ltd. Inventor: Kazu Matsui, Yama 1) Hisashi Kawa, Yama 1) Mamukai, Noriyoshi Iwasaki, Hirodo, Tadashi Taira, MTr'l Ausdequi H Europe〉 弗 2 l l attack a member) 1 top j shi friend Hi prison J round 2nd rolling 5000 spicy (πr-710 rii small 41 22 arrows; me E dahii's) 勺 N and v Aisou OcunC. . Commissioner of the Patent Office: Ij Owa Failure 1. Indication of the incident 1975 qy vr - Application No. 1, 22, No. 3, 2, Name of the invention: Kamegon Shuguchi to the 46TC building N-, 2 countries, acupuncture, or ball 3, person making the amendment ( NJ patent with 1B, applicant name (name) Nippon Wi Kan Co., Ltd. 4, agent Showa year month 1” Correct J4 6, Subject of amendment” II, 1Jl19 7, Contents of amendment The amendment is as per the attached sheet. Content 1, from page 3, line 10 to line 111 of the answer to the specification of the present application, “Supplementary Requirement
t J Supplementary
Correct to Requirement J. 2. From the 10th line to the 111th line on page 13, it becomes noticeable that the grain size is regulated. '' is replaced by ``A well-sized and fine austenite can be obtained.
c -170°C is improved. On the other hand, in the latter case, the austenite immediately before the secondary rolling is in an unrecrystallized state that is elongated in the rolling direction, and is extended during the secondary rolling, so that the deterioration in the C direction at δc-170°C is particularly noticeable. Become. ” he corrected. 3. On page 18, from line 4 to line 6, the statement ``Austenite grain size was No. 9'' was replaced with ``The austenite grain size was 41 according to ASTM.
The austenite grain size number is 6, /I69 is the same grain size code and is 5.5, and the other grain size numbers are 7.5.
~It was number 9. ” he corrected. Procedural amendment written by Kazu Wakasugi, Commissioner of the Japan Patent Office Failure 1, case indication Showa 1Fr patent application No. 19 and 2. fj No. 2, invention name 1% "F'0'"' 1 noon JXr-8N + I
N "Js flll Kanamachi M made f;-i-;, f. 3. Person making the amendment Name of the applicant for the forgery in relation to the case (Mr. Button Nippon Koukan Co., Ltd. Association 4, Agent January 1939, f7B Contents of Shipping Amendment 12 From line 17 to line 19 on page 22 of the specification of the present application, the phrase [Figure 5 is .........] has been replaced with ``Figure 5 is...''. Immediately before secondary rolling in copper plate (715°
This is a micrograph at a magnification of 400 times showing the austenite structure of C).''

Claims (1)

[Claims] Ni near, 5-10 wt%, P: (1,01(l
A Nl-containing low-temperature steel billet or slab containing vrt% or less as a basic component is heated to a temperature of 1200°C or less and then rolled and polished, and in the rolling process, it is polished at least 30°C at a temperature of 950°C or less.
% cumulative reduction and a finishing temperature of 800°C or higher to make the austenite grains into fine grains of A37M ta 7 or higher, and then at least 50°C above the finishing temperature.
After air cooling to a low temperature, secondary rolling was performed again at a temperature of 7 (10 to 750 °C) with a cumulative reduction rate of 5 to 25%, and
Immediately after the next rolling, it is cooled to a temperature of 350°C or higher at a cooling rate of 2.0°C or higher, and then pre-heated and tempered to a temperature lower than AC.
i. Method for producing steel for low temperature use.