JPS6311652A - 80kgf/mm2 class low-carbon cr-mo steel plate - Google Patents

Abstract

PURPOSE:To obtain the titled steel plate having high strength, toughness and superior weld crack resistance and capable of being welded without preheating by specifying the amount of C, the ratio of Mn to C and the total amount of constituent elements represented by a specified formula. CONSTITUTION:This heat treated 80kgf/mm<2> class low-carbon Cr-Mo steel plate contains 0.05-0.10% C, 0.05-0.50% Si, 0.75-1.40% Mn, 0.50-1.40% Cr, 0.20%n0.80% Mo, 0.01-0.10% V and 0.0003-0.0018% B and has >=10 ratio of Mn to C and <=0.20% PcM defined by the formula. The Cr-Mo steel plate is suitably used in case of <=40mm thickness. The steel plate can be manufactured as usual by subjecting a steel ingot or slab having the above-mentioned composition to hot rolling, direct quenching and tempering.

Description

[Detailed Description of the Invention] The present invention is based on the low carbon tempered Cr-Mo system 80kgf/
Regarding mm2 class steel plate, for details, please refer to the 40m thick plate for city gas tanks, LPG tanks, etc., which has significantly improved weldability.
The following relates to a high tensile strength steel plate treated with a heat treatment of 80 kgf/mm.

(Conventional technology) In recent years, along with the trend toward larger city gas tanks and LPG tanks, there has been a strong demand for lighter weight.
The use of high-strength steel plates is increasing. These steel plates are
Since the susceptibility to welding cold cracking is high, preheating to about 100 to 150°C is usually performed during welding. However, especially in recent years, there has been an increasing demand for reductions in construction costs for such preheating, and 80 kgf, which does not cause cold cracking even without preheating, has been increasing.
/mm” sheet steel sheets are now in strong demand.

In order to reduce cold cracking susceptibility and improve weldability,
It is already well known that reducing CI in steel is effective in suppressing hardening of welds, and
Utilizing the hardenability improvement effect of 6, preheating is unnecessary.
0kgf/mm" steel plate has been put into practical use. However, reducing the 0th grade of steel leads to a decrease in strength, so for 80kgf/mm" steel plate with high guaranteed strength, the hardenability improving effect of B is used. However, simply by reducing Cl, it is difficult to simultaneously provide the required strength and excellent weldability that does not require preheating.

In order to solve this problem, for example, Japanese Patent Laid-Open No. 51-1
No. 03018, JP-A-53-28017, JP-A-53
As described in JP-A-29218 and JP-A-54-38216, the preheating temperature for preventing weld cracking can be suppressed to around 50°C by variously controlling the chemical composition and manufacturing conditions of the steel plate. 80 kgf/ms'' steel sheets have been proposed. In all of these, the decrease in strength due to the reduction in CI is compensated for by the addition of alloying elements such as Cu, Nis Cr, and Mn.

However, on the other hand, the above-mentioned steel plate is expensive when using Ni, for example, and is also highly susceptible to sulfide stress corrosion cracking, making it unsuitable as a tank material. Further, when the amount of Mn is relatively large, there are problems such as large temper embrittlement and SR (stress relief annealing) embrittlement.

Therefore, the present inventors investigated sulfide stress corrosion cracking susceptibility and S
As a result of intensive research to obtain an 80 kgf/mm" sheet steel plate with strong resistance to R embrittlement, Cr-M.

In the system 80 kgf/ram” steel plate, C3l is 0
．． 10% or less and the '-, M n Z C ratio within a predetermined range, it has been found that it is possible to obtain hardenability far greater than that expected from conventional knowledge, and thus carbon type, and
The present invention was achieved by discovering that it is possible to obtain a tempered Cr-MO type W4 board having a strength of 80 kgf/mmz class.

That is, the present invention has excellent weld cracking resistance and can be welded without preheating during welding work.
kgf/mm” class low carbon type T [quality CR-M.

The purpose is to provide a 80kgf/mm” steel plate.

Example for solving iJ 1 Low carbon type Cr-Mo system according to the present invention 80kgf/m
The first of the m'' steel plates contains, in weight percent, C 0.05-0.10%, Si 0.05-0.50%, Mn 0.75-1.40%, Cr 0150-1.40%, MOo, 20~0.80%, V'0.01~0.10%, B 0.0003~0.0018%, balance iron and unavoidable impurities, satisfies Mn/C≧10, and is defined by It is characterized by being a low carbon tempered Cr-MO 80kgf 7mm'' steel plate with a PCM content of 0.20% or less.

Low carbon type Cr-Mo system 80kgf/mm” according to the present invention
The second steel plate is a steel plate containing 0.0% Cu in addition to the above elements.
It is characterized by containing in the range of 0.05 to 0.50%.

The present invention will be explained in detail below.

In order to explore the possibility of reducing the amount of C in Cr-Mo 80 kgf/mm'' class steel sheets, as shown in Table 1,
5Cr-0,20Mo-0,040V-0,0O10B
The system steel is the basic steel, and C is 0.52% (constant).
A Jomini single-end quenching test was conducted using test pieces taken from as-rolled steels prepared by melting steels with various Mn contents and Mn contents.

As shown in FIG. 1, the Jomini curves of these circles show that the lower the C content, the lower the hardness of the hardened end, but on the other hand, there is a tendency for the Jomini curves to become more horizontal. In order to quantitatively evaluate this tendency, we calculated the ideal critical diameter DI+cat from the chemical components and the actual ideal critical diameter DI+ obtained from the Jomini curve in FIG. 1. ) was compared. As the results are shown in Table 2, cl is 0.
For steel A, which is 14%, DI (.III) is DI (
cmL), but for steel B-E with a C content of 0.12% or less, DI+. bil is Dl(c+at)
It is recognized that it is about 1.4 times that of Furthermore, D self c! L
, and DI (.bs) are shown in the footnotes of Table 2.

Next, the horizontal axis is the Mn/C ratio of each steel, and the vertical axis is DI+.

1. .

/D++ca. As shown in FIG. 2, when the M n /C ratio is 10 or more, D
It is understood that Dlo,L, is larger than Dlo,L.

In most cases, the thickness of steel plates for tanks is 4011 mm or less, but the position of the Jomini curve, which is cooled at the same cooling rate as the center of the 40 mm plate thickness, is about 13 m from the quenched end.
This is the position of The hardness at this position is Hv300 or higher for all the steels shown in Table 1, and the hardness is 80 kgf.
/mm This is almost sufficient as hardness as quenched for class steel sheets. Therefore, the amount of C and alloying elements can be further reduced than in the component system shown in Table 1.

Based on this knowledge, various steels with PCM of 0.20% or less were produced and subjected to the Jomini test. An example of the results is shown in Figure 3, 0.07C-0,96Mn-07
3Cr-0,35Mo-0,040V-0,0009B
−0,48χCeq−0,19! In the component system of PcM,
It is understood that at a position 13 mm from the quenched end, an as-quenched hardness of about HV315 can be obtained, and an 80 kgf/mm'' class steel plate with a plate thickness of 40 mm and no preheating required can be obtained.

Next, the reason for limiting the chemical components in the Cr-Mo1i plate according to the present invention will be explained.

In general, the smaller the amount of C added, the better the weldability. In order to make preheating unnecessary, it is necessary to keep the temperature below 0°10%. However, when there are too few,
Since it is not possible to obtain the required strength as an 80 kgf/mm'' class steel plate, the lower limit of the amount of C added is set to 0.05%.

Si is necessary for deoxidizing steel and ensuring strength, but when added in excess, it deteriorates weldability, so the amount added is in the range of 0.05 to 0.50%.

Mn is necessary to improve the hardenability of steel, and in the present invention it is necessary to add 0.75% or more, but if added in excess, it impairs weldability and Because it increases susceptibility to embrittlement and susceptibility to SR,
The upper limit of the amount added is 1.40%.

Furthermore, in the present invention, in order for the steel to have sufficient hardenability, Mn and C need to satisfy the relationship of M n /C≧10.

Cr needs to be added in a small amount (at least 0.50%) in order to improve the hardenability and corrosion resistance of steel. However, if it is added in excess, weldability deteriorates.
The upper limit of the amount added is 1.40%.

MO also increases the hardenability of steel, increases temper softening resistance,
It is also effective in reducing susceptibility to temper embrittlement. In order to effectively obtain these effects, it is necessary to add 0.20% or more, but when adding too much, it impairs weldability, and it is also an expensive element, so the amount of addition is limited. The upper limit of is 0.80%.

(2) is also effective in increasing the hardenability of the steel and increasing the resistance to temper softening. In order to effectively obtain these effects, it is necessary to add 0.01% or more, but
When added in excess, weldability and toughness deteriorate, so the upper limit of the amount added is 0°10%.

When added in a small amount, B is effective in improving hardenability and increasing strength. In order to effectively obtain this effect, 0.
It is necessary to add 0,003% or more. but,
When added in excess of more than 0.0018%, B compounds are produced, which lowers hardenability and at the same time causes deterioration of toughness.

In addition to the above-mentioned elements, the Cr-Mo steel sheet according to the present invention may contain Cu in a range of 0.05 to 0.50%.

Cu has a relatively small negative effect on cold weld cracking, but is effective in increasing strength through solid solution strengthening and precipitation strengthening.
It is necessary to add %.

However, when added in excess, hot workability deteriorates, so the amount added should be 0.50% or less.

Furthermore, in the present invention, it is necessary that the PCM defined by 30 21J blJ 11 11J be 0.20% or less.

As is well known, Pl:M is an index indicating the sensitivity to cold cracking during welding. In the board, Pc14 is set to 0.20% or less.

The Cr-MO steel plate according to the present invention is suitable for a plate thickness of 401 mm or less. The thickness of most steel plates for tanks is 40n.
In addition, in the case of steel plates with a thickness exceeding 40 mm, it is necessary to increase the amount of Cl and alloying elements in order to avoid incomplete quenching in the center due to the mass effect, and as a result, it is necessary to increase the amount of Cl and alloying elements. This is because preheating is required.

The Cr-Mo steel plate according to the present invention is obtained by hot rolling a steel ingot or slab having the predetermined chemical composition according to a conventional method,
Thereafter, it can be manufactured by directly quenching and tempering, or by quenching and tempering after reheating.

According to the present invention, Cr-Mo system 80 k
gf/mm" class steel plate, by setting the C amount to 0.10% or less and the M n / C ratio to 10 or more, the hardenability is significantly greater than that predicted from conventional knowledge. 80 kgf/mm, which is a low carbon type and has a PCM of 0.20% or less without using a large amount of alloying elements.
EXAMPLES The present invention will be described below with reference to one example in which a Z-class trained Cr-Mo steel sheet can be obtained, but the present invention is not limited in any way by these examples.

Example A Cr-Mo steel sheet having the chemical components shown in Table 3 was manufactured by a quenching and tempering method or a direct quenching and tempering method. Steel plate F- is a steel plate of the present invention, and steel plate L-0 is a comparison steel plate. Table 3 also shows PCM and Mn/S in these steel sheets.
Indicates the i ratio. Table 4 shows the base material properties of these steel plates and the results of the diagonal Y-shaped weld cracking test.

The steel plates of the present invention all have a yield strength of 70 kgf/mm.
"Tensile strength is 80 kgf/mm or more", vTrs
temperature is 170°C or less, and PCH is 0.20% or less. In addition, no cracking occurs at room temperature in the diagonal Y-shaped weld cracking test.

On the other hand, according to the comparative steel sheet, PcM is 0.20%
However, since the M n / Si ratio is smaller than 10 in all cases, the tensile strength is less than 80 kgf/mm2, or even if the tensile strength is 80 kgf/mm2 or more, vTrs is -10°C or higher, making it unsuitable as a tank material.

[Brief explanation of drawings]

Figure 1 shows 0.75Cr-0,20Mo-0,040V
-0,0010B steel is used as the basic steel, Ceq is 0.5
Figure 2 shows Jomini curves for test pieces taken from as-rolled steels of various compositions with a constant C content and Mn1t of 2% (constant), and which have the chemical compositions shown in Table 1. For a steel plate, the actual ideal critical diameter DI+ obtained from the Jomini curve in FIG. , ) and its chemical composition, the ideal critical diameter DIt. aL)
A graph showing the influence of the n/C ratio, Figure 3, is 0.0
7C-0,96Mn-0,73Cr-0,35Mo-0
,040V-0,00098-0゜48χCeq-0,
It is a graph showing the Jomini test results for steel with a composition of 19χPCM. Patent Applicant Kobe Steel Corporation Representative Patent Attorney Ittsu Makino Dept. Figure 2 M9L/c

Claims (2)

[Claims] (1) In weight%, C 0.05-0.10%, Si 0.05-0.50%, Mn 0.75-1.40%, Cr 0.50-1.40%, Mo 0.20 ~0.80%, V 0.01~0.10%, B 0.0003~0.0018%, the balance being iron and unavoidable impurities, satisfying Mn/C≧10, and P_C_M=C+(Si /30)+(Mn+Cu+Cr
)+(Ni/60)+(Mo/15)+(V/10)+
A low carbon tempered Cr-Mo 80kgf/mm^2 grade steel sheet with P_C_M defined as 5B (%) of 0.20% or less. (2) C 0.05-0.10%, Si 0.05-0.50%, Mn 0.75-1.40%, Cr 0.50-1.40%, Mo 0. 20-0.80%, V 0.01-0.10%, Cu 0.05-0.50%, B 0.0003-0.0018%, balance consisting of iron and inevitable impurities, Mn/C≧ 10 and P_C_M=C+(Si/30)+(Mn+Cu+Cr
/20)+(Ni/60M)+(Mo/15)+(V/
10) P_C_M defined as 5B (%) is 0.20%
Low carbon type tempered Cr-Mo system 80kgf/mm which is below
^2nd class steel plate.