JPS6140032B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a steel for deep-sea research vessels that is low in carbon, has a high yield point of 100 kg/mm ² or more, and has excellent weldability and toughness, and has high strength and toughness. It is applied to purposes that require performance. In recent years, with the development of marine applications, steels suitable for this purpose have been developed, but the deeper the sea goes, the more severe the properties required of steel materials become, requiring both high strength and excellent toughness, and of course, welded structures are not suitable for welded structures. Good weldability is a necessary condition. Yield point 100
Such steels with Kg/mm ² include 10Ni-80Co
There are steels, but the drawback is that they contain 8% Co, making them extremely expensive. On the other hand, Ni-Cr
- Mo steel and ordinary low-alloy steels that use precipitation hardening of V to increase their strength are cheap, but their strength is limited to 100 kg/mm ² at best. Reduce the amount of V to 0.05% of the normal amount
If it is added in a larger amount, the strength will increase, but the toughness will drop significantly. The inventors of the present invention have conducted various studies to overcome these difficulties, and have found that by making the combination of Mo, V, Cr, and Ni satisfy the special conditions shown below, sufficient strength can be achieved with low carbon. It has been found that a steel with very high toughness and strength can be obtained. Usually, high-strength materials with yield strength σy≧100Kg/mm ² are manufactured by quenching and tempering. The C content is determined from the viewpoint of weldability.
Conventionally, when trying to increase strength by lowering it to 0.15% or less,
A method is being used to increase the amount of Cr, Mo, V, etc. added. However, in order for σy to exceed 100 Kg/mm ² , it cannot be easily achieved unless the tempering temperature is kept as low as 550° to 600°C. Incidentally, although this temperature range is a region where precipitation hardening of Cr, Mo, V, etc. is remarkable, it is a well-known fact that the temperature at which precipitation hardening is greatest does not match depending on the element. Therefore, the desired strength cannot be obtained simply by combining these elements. Additionally, to take full advantage of Mo's enhancement, Cr
We found that the coexistence of these two types not only has no superimposed effect, but is actually harmful. An example is shown in FIG. As is clear from the figure, when 1% or more of Cr is added, the strength actually decreases. On the other hand, the plate thickness is 50
When quenching and tempering a steel plate with a diameter of mm or more, if the steel plate is not sufficiently quenched during quenching, the strength will not increase after tempering and the toughness will also decrease. Therefore, a certain amount of Cr is necessary to ensure hardenability. For this purpose, the amount of Cr that can be added without impairing the strengthening function of Mo is Cr≦
It has been empirically found that 0.8Mo, preferably Cr≦0.5Mo, is sufficient. On the other hand, since V undergoes precipitation strengthening in approximately the same temperature range as Mo, it can be added in the required amount independently of the amounts of Mo and Cr. By the way, the addition of alloying elements that are not effective for strengthening is not only ineffective for strengthening, but also creates large precipitates that impair toughness. Particularly in high-strength materials with σy = 100Kg/mm2 or ^more , the toughness is severely reduced due to harmful precipitates, so it is essential to keep the amounts of Cr, Mo, and V within a certain range, and the conventional component ranges That's not really achievable. As a result of various tests based on the above facts, σ
For y≧100Kg/ ^mm2, the fracture surface transition temperature of a 2mmV notched rupee is -80℃ or lower, and vE at -80℃ is 15Kg-m or higher, C+Mo/8+V≧0.26 and 0..2≦Cr ≦0.8Mo, 5≦Ni≦9.5S≦0.005, P≦
It was found that it is necessary to satisfy the conditions of 0.02% and O≦0.0030%. In order to obtain a high vE when σy≧100Kg/mm ^{2 ,} it is necessary that the oxygen content is particularly low. The required amounts of Mo and V vary depending on the C content, and lowering the C content causes a decrease in strength, so Mo and V must be increased accordingly. From the viewpoint of weldability, the maximum C content was set to 0.15%.
Furthermore, if Mo is less than 0.06%, the amount of C necessary for Mo ₂ C precipitation is insufficient in the case of Mo 1.0%, and even if more Mo is added, the necessary strength cannot be obtained. Therefore, C is required to be at least 0.06%. Furthermore, by tempering at 550 to 600℃, σy increases.
To achieve 100Kg/mm ² or more, a minimum Mo content of 0.7% is required; below this, the required strength cannot be obtained no matter how much V is increased. In order to stably obtain strength over a wide range of tempering temperatures, it is preferable to add Mo in an amount of 0.95% or more. The Ni content is determined by considering the precipitation of austenite during tempering. If Ni is 9.5% or more, unstable austenite precipitation will occur at temperatures below 600°C, which is not preferable. In addition, in order to have high strength and sufficient toughness of σy100Kg/mm ² or more, a minimum of 5%
The amount of Ni is required. When tempering at a low temperature of 600°C or lower, tempering brittleness becomes a problem, so the P content must be reduced as much as possible, but 0.02% or less is sufficient. However, since some parts are heated to 400°C or higher after production, the content is particularly preferably 0.01% or less. Similarly, Si is 0.35
% or less, preferably 0.20%. In order to obtain high strength and high toughness, it is essential to make the austenite crystal grains fine. Moreover, since too much content deteriorates toughness, the upper limit was set at 0.08%. Addition of Ti is preferable to prevent grain coarsening in the weld heat affected zone. However, since adding more than necessary will significantly reduce toughness, the upper limit was set at 0.02%. Addition of 0.03% or less of Nb and Ta is also effective for grain refinement. Since the object of the present invention is for deep sea use, corrosion in seawater naturally becomes a problem. To do this, you just need to paint it properly. It is more preferable to add Cu, W, and Co, but if Cu is 0.5% or more and W is 0.1% or more, the effect is small. Co is also effective, but due to its high cost, its addition is limited to 1% at most. Furthermore, the steel targeted by the present invention has a problem of hot cracking. In order to improve hot cracking susceptibility, in addition to lowering the S content, it is effective to add La, Ce, Ca, etc. Addition of these elements up to 0.003% has no effect on strength, but also improves toughness. Excessive addition will reduce toughness. Examples Table 1 shows the chemical composition of the steel used in the test.
A hot-rolled 12mm steel plate is quenched at 850℃,
Table 2 shows the mechanical properties when tempered at 560, 580, and 600°C. Components C and D of the steel of the present invention have high strength of σy≧110Kg/mm ² at any tempering temperature, and
vTrs<−150°C vE−20=25 to 29 Kg−m, showing very excellent characteristics. In addition, 0.7% Mo material A is also
0.5Cr shows a sufficient value. On the other hand, for 1.0Cr material E, when tempered at 600°C, σy is barely 100Kg/mm ² , which cannot be said to be a sufficient value considering the variation.
On the other hand, when Ni is 4.5o/oG, the strength is sufficient but the toughness is poor. Similarly, high-Si material H has somewhat high strength but poor toughness when tempered at low temperatures. I of the steel composition of the present invention,
As shown in materials J and K, the addition of 0.3% Cu, 0.1% W, and 1% Co all results in σy≧100Kg/mm ²
Shows excellent characteristics with vTrs<-120℃. In addition, as shown in the composition L, M, and N materials of the steel of the present invention, 0.03%
Ta, Nb, 0.03% Ce, La, and 0.0035% Ca additives also have sufficient strength and toughness.

【table】

【table】 [Brief explanation of the drawing]

Figure 1 shows the relationship between tempering temperature and hardness of Cr,
The effect of Mo and the combined addition of Cr and Mo is shown.

Claims (1)

[Claims] 1 C0.06~0.15%, Si0.35% or less, Mn0.15~1.5
%, Ni5~9.5%, V0.05~0.15%, Cr0.2~0.7%,
Contains Mo0.7-15%, A0.01-0.08%, C
(%) + 1/8 Mo (%) + V (%) [however, weight %] is 0.26 or more, and Cr≦0.8Mo is satisfied, with the balance consisting of iron and inevitable impurities. Yield strength 100
High-strength, high-toughness steel of Kg/mm ² or more. 2 C0.06~0.15%, Si0.35% or less, Mn0.15~1.5
%, Ni5~9.5%, V0.05~0.15%, Cr0.2~0.7%,
Contains Mo0.7~1.5%, A0.01~0.08%, C
(%) + 1/8 Mo (%) + V (%) [however, weight %] is 0.26 or more and satisfies Cr≦0.8Mo, Cu is 0.5% or less, 0.1
% or less W; 1% or less Co; 0.020% or less Ti;
High-strength, high-toughness with a yield strength of 100 Kg/mm ² or more, containing 0.030% or less of one or more of La, Ce, Ca, Nb, and Ta, with the remainder consisting of iron and unavoidable impurities. steel.