JPS59140355A - Extremely thick steel plate with high toughness and tension - Google Patents

Abstract

PURPOSE:To obtain the titled steel plate causing hardly any temper embrittlement and separation by refining an Ni steel having a specified composition contg. reduced amounts of impurity elements such as P, Sn, Sb and As, a reduced amount of Mn and a specified amount of B added to ensure high hardenability. CONSTITUTION:This extremely thick steel plate with high toughness and tension has a composition consisting of 0.05-0.30% C, 0.10-0.40% Si, <=0.20% Mn, 1.00-3.50% Cr, 2,50-4.00% Ni, 0.30-0.80% Mo, 0.005-0.05% V, 0.0001-0.0015% B, 0.005-0.10% sol.Al and the balance Fe with inevitable impurities including <=0.0080% N, <=0.013% P, <=0.008% S, <=0.007% Sb, <=0.007% As and <=0.007% Sn. To said composition may be added 0.005-0.05% Nb and/or 0.05-0.75% Cu. The steel plate maintains its superior toughness at low temp. and causes hardly any temper embrittlement.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an N1-added high-toughness, high-strength, extra-thick steel plate that suppresses temper brittleness and separation that appears on the fracture surface of an impact test specimen.

In recent years, there has been an increasing demand for high-toughness, high-strength steels that have both excellent toughness and high strength, and in order to meet these demands, various new steels have been developed and used in the past. High-toughness Nl steel, commonly known as "HY steel", is one of them.

However, although such high-toughness Ni steel is a material that exhibits excellent toughness even in extremely cold regions, it has the problem of extremely high temper brittleness. However, high-toughness 14IVi steel has rarely been used in parts that require stress-relief annealing, and the temperature at which it is used is often below room temperature, resulting in embrittlement during tempering. Temper brittleness itself was not often a problem because it was rarely held for a long time in the temperature range of 450 to 575°C, where the temperature of the steel progresses, or slowly cooled in the temperature range of 7c.

However, recently, as pressure vessels for oil refining and pressure vessels for synthetic fuel production are increasingly being constructed in cold regions, the demand for low tempering brittleness in Ni steel has suddenly increased. This is because the operating temperature of these pressure vessels is within the range of 450 to 575 degrees Celsius, where tempering brittleness tends to progress, and also because stress relief annealing and slow cooling are performed after melting during pressure vessel construction. This was because the problem of temper brittleness could no longer be avoided, and from this point of view, the future of Ni steel was by no means optimistic.

Regarding tempering embrittlement, once the basic composition of steel is determined, its tempering embrittlement is generally determined based on the amount of components in weight proportions, and J = (Sj, -1-Mn) (P + Sn) X 1
It can be evaluated using two parameters: 0' (however, chain production).
It has been reported that the smaller both values are, the lower the temper brittleness is, and similar results can be obtained with Ni-added steel. It is known that the susceptibility to tempering embrittlement can be extremely reduced by suppressing the susceptibility to temper embrittlement.

From the above-mentioned viewpoint, the present inventors have determined that while maintaining the extremely excellent low-temperature toughness of jJi steel, it is possible to prevent tempering embrittlement from occurring during stress relief annealing after welding or repeated heating to a specific temperature range. As a result of conducting research on high-toughness, high-tensile strength steel materials that do not have any That is, (a) Generally, when temper embrittlement occurs in steel, austenite grain boundaries present in positively segregated areas such as 14n become extremely brittle, and separation occurs on the fracture surface of a Charpy impact test piece. This separation is said to be an indicator of the toughness value in the plate thickness direction, and it is preferable that it does not occur, but lowering Mn is extremely effective in suppressing this separation. (b) For example, high Ni-Cr-Mo −
N1-added steel such as V steel gives the impression that it has sufficiently high hardenability, but in reality, the plate thickness is 100mx.
When a steel plate is made thick to about r, even if it is quenched, it will not necessarily have an ideal quenched structure.

When the Mn is further lowered, this tendency becomes more pronounced. By adding B to this steel, the hardenability is improved and the hardened structure approaches the ideal hardened structure, so its toughness is greatly improved. Lowering the Mn content in steel will also reduce the strength of the steel, which is disadvantageous for high-strength steel, but if B is added to it, the strength of the steel will be significantly restored.
The properties of high-strength steel can be sufficiently maintained.

The matters shown in the above three items (a) to (C) form the basis of the present invention.

Figure 1 shows (3.0-3.5%)Ni-2%Cr-0,
J factor (value of J in the above formula) for 5% MO steel (hereinafter referred to as weight), Charpy fracture surface transition salinity before step cooling: vTB, Charpy fracture surface transition salt after step cooling Degree: vTs' and its difference: △
This figure shows the relationship with VTS (i.e., vTs' - vTsO value), and it is clear from this figure that as the J factor increases, ΔvTs, that is, the tempering embrittlement susceptibility increases. .

Note that step cooling is a test method that is known as a test method that accelerates tempering brittleness that occurs due to changes over time when steel is repeatedly heated to a specific temperature range. The heat treatment is performed as shown by the heating/cooling curve. It is said that by such step cooling, the tempering brittleness index of the steel material due to changes over time can be determined.

Figure 3 is a diagram showing the effects of plate thickness, Mn content, and B addition treatment on the mechanical properties of 3%Nl steel quenched at 850°C.Figure 3 (a) Lfj, Mn content is 0.30
The high Mn steel in Figure 3 (b) has a Mn content of 0.10%.
Regarding low (cap) steel.

From this Figure 3, it can be seen that both the stress relief annealed steel as well as the stress relief annealed and step-cooled steel (in this case, the measurements were made only on the plate thickness of 100M) are superior to the low Mn steel. It can be seen that this shows an excellent toughness value.

In particular, it is clear that the low Mn-B addition treated steel has excellent performance after step cooling.

From this, it can be seen that the effect of addition of B is to improve hardenability.
It can be seen that this contributes to the improvement of toughness through 2. This is because when the plate thickness is 201LIn, there is no difference in toughness regardless of the presence or absence of B treatment, and regardless of the difference in Mn content and pel. This is clear from the fact that the toughness value of

Furthermore, Table 1 shows the occurrence of separation on the Charpy ductile fracture surface of similar Ni-added steel materials, and Table 1 also shows that if the Mn is reduced to about 0.10%, separation can be suppressed. It is clear that

This invention has been made based on the knowledge noted above, and for example, an extremely thick N1-added steel plate with a plate thickness of about 258 υ or more, C: 0.05-030%, Sj: 0.10-0. 040%5 Mn: 0.20% or less, Cr2 1.00-3.50%, Ni: 2.50-4. O0%, Mo: 0.3
0~0.80%, v: 0.005~
0.05%, B:○0001 NO,0015%
, sot, AR: 0.005-010%, and, if necessary, Nb: 0.005-0.05%, Cu: 0.05-0.05%. 'i', 5%, ] of 5 and further as an impurity, N: 0.0080
% or less, P: 0.013% or less, S, 0008% or less, sb: a, oo'y% or less, As: O, OO'7% or less, Sn: 0.00'7% including lead/do , Fe and other non-D5 avoidable impurities, and the remainder (in weight percent), and is characterized by extremely suppressing the occurrence of temper brittleness and separation.

Next, the reason why the compositional component amounts of the steel sheet of the present invention are limited as described above will be explained.

■ C The C component has the effect of ensuring the strength of the steel plate, but if its content is less than 0.05%, the desired effect cannot be obtained, while if it is contained in excess of 0.030%, the desired effect cannot be obtained. The content was determined to be 0.05 to 0.30% since it deteriorates the resistance to quench cracking or the resistance to welding cold cracking.

■S] As is clear from the formula for the J factor in 11f1, like bA n, tempering brittleness can be suppressed by reducing the amount of F3i component, but if it is reduced too much, the desired strength cannot be achieved. It becomes impossible. Therefore, from the point of view of ensuring strength, it is necessary to contain 0.10% or more, but if the content exceeds 0.40%, tempering brittleness will increase and the toughness of the matrix will also become tougher. , its content was determined to be 0.10-040%.

■ Mn The content of the Mn component is particularly important along with the B content in the steel sheet of this invention, and it is extremely difficult to manufacture steel with a content of less than 0.02% from the current steel manufacturing technology (,
However, it is preferable to lower this value as much as possible. When the Mn content exceeds 0.2%, the susceptibility to tempering embrittlement of the steel sheet becomes high, and separations are formed on the fracture surface of the Charpy impact test piece, resulting in slow cooling after stress relief annealing. The content has been reduced to 0.
．． It has been set at 20% lower.

■ Cr The Cr component has the effect of increasing hydrogen attack resistance when steel sheets are used for wide pressure vessels in hydrogen atmosphere, but if the content is less than 1.00%, the desired effect may not be achieved. On the other hand, if the content exceeds 3.50%, the weldability deteriorates, so the content was set as 00 to 3.50%.

■Ni The Ni component has the effect of increasing the toughness of the base of the steel plate, but if its content is less than 250%, the desired effect cannot be obtained; on the other hand, if it is contained in excess of 4.00%, Since this increases the susceptibility to temper embrittlement and makes weldability extremely difficult, the content is set at 25 to 40%.

■　M.

The 140 component has the effect of suppressing ferrite transformation and sufficiently hardening the thick plate to the center, but if its content is less than 030%, the desired effect cannot be obtained; If the content exceeds .80%, the toughness will deteriorate due to precipitation hardening of MO□C, so the content was set at 030 to 080%.

■ V ■ component has the effect of improving the resistance to softening of the steel plate during tempering treatment, but its content is 0%, OO5%
If the content is less than 0.05%, it will not be possible to obtain the desired effect, while if the content exceeds 0.05%, embrittlement will occur due to precipitation hardening.
It was set at 5%.

■ B The B component has the effect of extremely effectively increasing the hardenability of steel sheets when added in a small amount, so it is an important point of this invention along with low Mn and impurity reduction. If the amount is less than O, OOOl, the desired effect will not be obtained, while if it is contained in excess of 0.0015%, carbides will coarsen during tempering and the strength and toughness of the steel sheet will deteriorate. Therefore, the content was determined to be 0, OOO1 to 0.0015%.

■ 5ot-AI.

The sol, AP, and components have the effect of creating a base that allows the B component to effectively exhibit its effects. In other words, component B exhibits the desired effect only when it segregates at grain boundaries in a solid solution state, but it easily combines with N, and its effect disappears when it combines with N to form 13N. Resulting in. For the sake of Party B, N is fixed as AIIN by 5olA,
It is necessary to avoid combining N and B.

If the s'oA and AQ contents are less than 0.005%, the desired effects cannot be obtained;
If the content exceeds 10%, the toughness of the steel plate will deteriorate, so the content should be reduced to O, OO5-0, 100%.
It was determined that

■ N If the N content exceeds o, o08%, it will cause cracking of the steel ingot, so the content must be kept at 0.008% or less. Although it is extremely difficult to reduce the N content to less than 0.0015 h due to the limitations of current steel manufacturing technology, it is preferable for the content to be as low as possible for the above reasons.

■ P P is extremely important when discussing tempering embrittlement, and if its content exceeds 0.013%, it will cause tempering embrittlement in steel, and under the construction and usage conditions mentioned above, it will be extremely important. Because it becomes brittle and causes separation on the Charpy fracture surface, its content is reduced to 0.013.
% or less.

■ SS If the S content exceeds 0.008%, the ductility and toughness in the thickness direction will be significantly deteriorated, so the content was set at 0.008% or less.

■ 5b sb promotes tempering embrittlement and weld hot cracking along with P, so its content must be suppressed to 0007% or less. Especially, in the case of copper phase used at high temperatures, it is the main cause of creep embrittlement. Therefore, from this point of view, the sb content can be reduced to O, O
It is necessary to keep O7% or less.

■ As As is also an element that promotes tempering brittleness, its content should be reduced to 0.0000 to suppress the tempering brittleness of the steel sheet to a desired level or less.
It is necessary to suppress OO' to 7% or less.

(2) 5n Sn also promotes tempering brittleness when its content exceeds 7% of O,OO'. In particular, when steel sheets are used in pressure vessels, etc., stress relief annealing and slow cooling are always performed, so during slow cooling, the former austenite grain boundaries segregate and cause grain boundary embrittlement. The content is o, oo'
It is necessary to suppress it to y% or less.

■ Nb When added to tempered steel that undergoes quenching and tempering, the Nb component mainly has the effect of refining austenite grains and improving toughness, but if its content is less than 0.003%, the desired effect may not be achieved. On the other hand, if Nb is added in excess of 0.005%, Nb will migrate from the base metal into the weld metal during welding, significantly deteriorating the toughness of the weld metal. It was set at 5-005%.

■ The cu Cυ acid component precipitates fine ε-Cu particles in the base iron,
Since it blocks the movement of dislocations, it has the effect of increasing strength without deteriorating toughness, but its content is 0.05%.
If it is less than 075, the desired effect cannot be obtained in the action;
%, a CU concentrated layer will be formed just below the black scale on the surface of the steel sheet, and this will penetrate into the grain boundaries and cause tortoiseshell-like scratches on the surface, which requires an extremely large number of steps to remove. The content was determined to be 0.05 to 0.75% since it was necessary and would reduce the yield.

As mentioned above, this invention comprises P, Sn, Sb and A.
In addition to reducing the impurity element of S, the amount of Mn is lowered as much as possible, and the B addition treatment ensures good hardenability.
1 additive extra-thick steel plate (thickness: 25 mm or more), and is heat treated by heating to AC3 or higher and quenching.
This is based on the premise that a so-called heat refining treatment is performed, in which the steel is tempered to an Ac point of 1 or less. As is clear from FIG. 3, the properties of the steel plate become more pronounced when the thickness of the steel plate is 50 mm or more.

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

Example First, steel having the composition shown in Table 2 was melted by a conventional method.

Next, this was hot-rolled to obtain a steel plate with a thickness as shown in Table 3, which was then heat-treated as shown in Table 3, and the mechanical properties as heat-treated and further improved. The results of measuring the mechanical properties after step cooling are also shown in Table 3.

In addition, the heat treatment strip ('I) in Table 3 is ■...85
Water quenching after 0℃×1hr+air cooling after 650℃×1hr+6
Furnace cooling after 25°C x 12 hours, @...850°C x 1 hr followed by water quenching + 650°C x 1 br followed by air cooling + 625°C x 3 Ohr
Post-furnace cooling, ○...900℃X1hr post-water quenching +650
Air cooling after ℃ x 1 hr + Furnace cooling after 625 ℃ x 12 hr, ■...
・Water quenching after 900°C x 1 hr + air cooling after 650°C x ] hr + furnace cooling after 625°C x 3 hr.

Furthermore, each steel plate shown in Table 3 was examined for the presence or absence of separation, and the results are shown in Table 4. The presence or absence of separation can be investigated by observing the fracture surface of the Charpy impact test piece shown in Figure 4(a).
In the state shown in figure (b), there is no separation,
In the state shown in Figure 4 (C), Table 4 Separation! ,! It was determined to be 2.

From the results shown in Table 3, the Charpy fracture transition temperature (vTs
) is also good, but it is clear that the difference in fracture surface transition temperature (ΔvTs) before and after step I) is particularly excellent. In other words, the fracture surface transition temperature (v
Ts) and Charpy absorbed energy of 40 ft-A
Even if the temperature for b (vTao) is good, the risk of fracture increases if significant changes occur over time, and there are countless examples of fracture even if the initial toughness is extremely good. A small vTs means that there is no such danger, and the values shown in Table 3 can be said to clearly demonstrate the excellent properties of the steel sheet of the present invention.

From the results shown in Table 4, the excellent separation resistance of the steel sheet of the present invention can be clearly understood.

- As mentioned above, according to the present invention, it is possible to obtain an N1-added high-toughness, high-strength, extra-thick steel plate that maintains excellent low-temperature toughness and hardly causes temper brittleness.
It can be expected to have sufficient performance even when used as a material for pressure vessels and other materials in cold regions, resulting in industrially useful effects.

[Brief explanation of drawings]

Figure 1 shows the J factor and vTs, vTs', and ΔvT
s, Figure 2 is a heating/cooling curve during step cooling, and Figure 3 is a diagram showing the effects of plate thickness, Mn content, and B treatment on the mechanical properties of hardened materials. , Figure 3(a) is for high Mn steel,
FIG. 3(b) is a diagram related to low Mn steel, FIG. 4 is a schematic diagram explaining the procedure for determining the presence or absence of separation, and FIG. 4(a) is a diagram showing the shape of the Charpy impact specimen M before the test. Figure 4 (b) shows the shape of the fracture test piece without separation.
FIG. 4(c) shows the shape of a fracture test piece with separation. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo and 1 other person 3rd (
a) OIOo 200 3oo 4o. No IJ Tashakishi (mm) Ka 3 figure (e+)

Claims (1)

[Claims] (1) C: 0.05-0.30%, Si: 0.
10-0.40%, Mn: 0.20% or less, Cr: 1.00-3.30%, Ni: 2.50-400%, 1vIo: 0.30-0.80%, V: 0. 005
~005%, B:0000~0.0 O15%, 5ot-/V! : Contains 0.005-0.10% and as an impurity. N: 0.0080% or less, P: 0.013% or less, S: 0.008% or less. Contains St): 0.007% or less, As: 0.007% or less, Sn: 0.007% or less, Fe and other unavoidable impurities: Residue 5 Characterized by high toughness with extremely low temper brittleness and separation. High tensile strength extra thick steel plate. (21C: ○05~0.30%5 Si: 0.10~040%. Mn: 0.20% or less, Cr: 1.00~3.50%, Ni: 2.50~400%, lv' fo: o, 30-0180%, V: 0.00
5 to 0.05%, B: 0.0001 to O, OO 15%, 5al-AQ: O, OO 5 to O, ], O%, and Nb: 0.005 to 005%. It also contains one or more of Cu: 0.05 to 0.75%, and further contains as impurities: N: 0.0080% or less, P: 0.013% or less, S: 0008% or less, sb: o , oo''7% or less, As: 0.007% or less, Sn: 0.007% or less, and the remainder is Fe and other unavoidable impurities. A high-toughness, high-tensile, extra-thick steel plate with extremely low temper brittleness and separation.