JPS621456B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing high-strength steel that has excellent brittle fracture propagation arresting properties in the base material and excellent brittle fracture occurrence properties in the welded part. Steel materials for low-temperature containers that store energy sources such as LPG and LNG, and steel materials for line pipes used in cold regions, have high toughness to ensure the safety of their structures. value is required. As the environment in which steel materials are used becomes more severe, the tendency for higher toughness values to be required will become stronger in the future. To cope with this trend, alloying elements,
In particular, the current situation is that Ni is added, and the hot rolling method or heat treatment method is devised. By the way, in general steel structures, especially large steel structures, the main components are mainly manufactured by welding, and if there is a risk of brittle fracture occurring in these structures, it is almost always the welded parts. Therefore, it is important to provide steel with high weld toughness (resistance to brittle fracture occurrence).
If the aim is to ensure the safety of large steel structures, that is, to prevent collapse due to brittle fracture, the welds should mainly have resistance to brittle fractures, that is, excellent weld toughness, and the prevention of brittle cracks once generated in the base metal. It is important to use steel materials that have excellent properties to stop the propagation of brittle fractures, that is, to stop the propagation of brittle fractures. However, in the prior art, it is very difficult to manufacture a device that has both characteristics at a high level and at a manufacturing cost that allows mass use. For example, although the addition of Ni can improve brittle fracture propagation arresting properties, as the amount of Ni added increases, the maximum hardness of the weld increases, and the toughness of the weld, especially the limit COD, which is an indicator of brittle fracture occurrence, decreases. (Tetsu to Hagane, 68 (1982) S629, Figure 2). Based on these actual circumstances, the present inventors discovered a phenomenon that is completely different from the conventional wisdom regarding the brittle fracture propagation phenomenon of steel materials, and by adding detailed metallurgical studies, it was found that the COD value of the welded part This led to the invention of a method for manufacturing high-strength steel that is characterized by excellent brittle fracture occurrence characteristics and, in the base material, excellent brittle fracture propagation arresting characteristics. The details of the present invention will be described below, but first the reason for limiting the ingredients will be described. C is an essential element to ensure the required strength of steel materials, and if it is less than 0.05%, there is a risk that the strength will be insufficient, and the amount of finely dispersed martensite, which is the most characteristic of the present invention, will be too small. ,Also
If it exceeds 0.15%, the toughness of the weld zone will deteriorate, so the amount added is limited to 0.05% or more and 0.15% or less. Si is an element that promotes deoxidation in molten steel and can be expected to increase strength through solid solution strengthening, so it is added in an amount of 0.01% or more, but it often generates high-carbon martensite in welds and deteriorates weld toughness. The upper limit was set at 0.3%, as there is a large tendency to Mn is effective as an element for improving the low-temperature toughness of the base metal, so the lower limit was set at 0.5%; however, if added in excess, it increases the hardness of the weld, increases the cold cracking resistance, and also reduces the toughness of the weld. Because I'm inviting you,
In this invention, the upper limit is set at 2.0%. As is well known, Al is a strong deoxidizing agent and also functions effectively in refining crystal grains, so it is an essential element as a basic component. The upper limit was set at 0.1% because it forms harmful alumina clusters. In the present invention, addition of appropriate amounts of Ti and N is essential. This is because Ti and N form a TiN compound in the steel material, and appropriately dispersed TiN precipitates have the function of suppressing coarsening of austenite crystal grains. If the amount of added Ti is less than 0.005%, it is difficult to expect this effect, and if it exceeds 0.030%, it is not only ineffective in preventing austenite crystal grain coarsening, but also causes coarse grains that are harmful to the material, especially the weld toughness. Since the tendency of TiN compound formation becomes noticeable,
The upper limit was set at 0.030%. As is clear from the purpose of adding N, it is necessary to control the amount of N to a necessary and sufficient amount to form TiN, but taking into account the variations in steelmaking operations, the addition range was set as (0.2 to 0.5) x Ti%. . V is usually utilized as a precipitation strengthening element, but
Rather, in the spirit of the present invention, it is added if necessary from the viewpoint of improving hardenability. Excessive addition will deteriorate the brittle fracture propagation arresting properties of the base metal and the weld toughness.
It was limited to 0.2% or less. Although Nb is effective in refining austenite grains during hot rolling, it hardens the weld heat affected zone and ultimately causes deterioration of toughness, so the amount added was limited to 0.08% or less. Although it is effective to use appropriate amounts of both Cr and Mo from the viewpoint of hardenability and strengthening of the base metal, the hardenability improving function also significantly hardens the welded part, so
The upper limit was set at 1.0% for Cr and 0.5% for Mo. Ni is known as an element that significantly improves the brittle fracture propagation arresting properties of the base metal, but as mentioned above, it increases the hardness of the weld and reduces the toughness of the weld, and is added for the purposes of the present invention. Even if it does, it is set at 1.5% at most. Cu can be added as a solid solution strengthening element if necessary, but adding too much may cause deterioration of the toughness of the base metal and weld zone, as well as induction of hot embrittlement.
It was limited to 0.5% or less. In addition to the individual addition amounts of each element mentioned above, in the present invention, {C+Mn/6+Cr+Mo+V/5+Ni+C
u/15} (hereinafter referred to as carbon equivalent in the present invention) is an essential condition that the value is 0.40% or less. Carbon equivalent is
If it exceeds 0.40%, the hardness of the weld, especially when the welding heat input is relatively small, will increase significantly, and the weld toughness, especially the critical COD value, which is an indicator of brittle fracture occurrence characteristics, will decrease more significantly. Therefore, we set 0.40% as the upper limit. Next, the reasons for limiting the manufacturing conditions will be described. The purpose of heating a slab or steel slab to Ar ₃ or higher before hot rolling is to uniformly austenite the entire slab or steel slab, and to obtain the required mechanical property values after rolling. is essential. heating temperature
The reason is that by limiting the temperature to 1150℃ or less, it is possible to keep the austenite crystal grains fine and well-sized before rolling, together with the effectiveness of Ti and N added as essential elements. An upper limit was set on the heating temperature. In particular, in order to obtain excellent brittle fracture arresting properties, it is a necessary condition to obtain well-sized austenite crystal grains. The chemical composition of the steel material applicable to the present invention has a low carbon equivalent, which means that the Ar ₃ transformation point is relatively high. Therefore, in order to refine the austenite crystal grains after rolling, it is essential to ensure a rolling reduction of 30% or more at 900°C or lower. However, excessive rolling at temperatures below 900°C will cause the texture to develop and cracks parallel to the sheet surface, called separation, will likely occur, so the upper limit of the rolling reduction was limited to 80%. Although the apparent brittle fracture properties can be improved by introducing separation, in consideration of ensuring safety when stress is applied in the thickness direction, the occurrence of separation should be suppressed. In addition, the finishing temperature of rolling must be at least the Ar ₃ transformation point to suppress the precipitation of pro-eutectoid ferrite. By controlling the content of the steel components within the above range and limiting the carbon equivalent to 0.40% or less, the steel can be heated to ₃ or more Ac points and 1150℃ or less, and then hot rolled at ₃ or more Ar points. The main metallographic structure obtained by cooling in the air after rolling is mainly a mixed structure of ferrite and pearlite in the case of a steel material whose thickness is equivalent to that of a thick plate. However, in the present invention, after hot rolling at ₃ or more Ar points, the cooling rate from the temperature at ₃ or more Ar points is 35°C/sec to 90°C/sec (meaning the average cooling rate between 750°C and 200°C). ), the main structure is a mixed structure of ferrite and finely dispersed martensite, and the precipitation of fine precipitates in the ferrite is suppressed as much as possible. be. In the composition range of the present invention, martensite as a second phase cannot be obtained at a cooling rate of less than 35°C/sec, and precipitation of ferrite is suppressed at a cooling rate of more than 90°C/sec. Since it would be difficult to secure the characteristic mixed structure of ferrite and finely dispersed martensite, it was limited to the above range. In addition to grain refinement and the addition of Ni, which are conventionally known as metallurgical factors that control the brittle fracture propagation arresting characteristics of steel materials, the present inventors have investigated ferrite with few fine precipitates and finely dispersed martensite. It has been discovered that the surface energy for forming a brittle fracture surface can be increased by generating a mixed structure of , and this fact forms the basis of the present invention. In order to stop brittle fracture, it is necessary to absorb the kinetic energy of the propagating brittle crack, but based on detailed experiments and observations, the present inventor has devised two additional measures in addition to the existing technology. We found that it is possible to increase the surface energy for forming brittle fracture surfaces. That is, the first point is that when a brittle fracture surface propagates across a ferrite grain to an adjacent grain, a ductile fracture portion called a tear ridge is formed at the grain boundary, but the plastic deformation ability of the tear ridge is This is a method of increasing For this purpose, it is necessary to secure ferrite crystal grains with large plastic deformability. The most effective method is rapid cooling after rolling. When the cooling rate after rolling is about that of air cooling, numerous carbides and the like precipitate within the grains or form aggregates, resulting in a marked decrease in the plastic deformability of the ferrite crystal grains. The second point is that a high work hardening coefficient can be obtained by ferrite and finely dispersed martensite, which suppresses the localization of plastic deformation, which has the effect of increasing the plastic deformation ability of the tie ridge. As a result, this contributes to an increase in the energy for forming brittle fracture surfaces. Although the technical ideas of the first point and the second point are independent from each other, if the manufacturing method of the present invention is followed, both effects can be utilized at the same time, and the brittle fracture propagation stopping characteristics can be significantly improved. In the prior art, it has been thought that finely dispersed martensite has an adverse effect on toughness in a broad sense, but in the present invention, the reason why it does not have such an adverse effect will be explained. The adverse effect of finely dispersed martensite on toughness is a phenomenon known mainly in welds and has been analyzed. When rapidly cooling from the austenite temperature range, martensite is formed below the martensite transformation start temperature, which is determined mainly by the chemical components contained, but the formed martensite is accompanied by volume expansion, so the adjacent ferrite A dislocation is introduced. This fact has been confirmed by the present inventor through transmission electron microscopy. However, by continuing to rapidly cool the steel even after such dislocations have been introduced, carbon, nitrogen, etc. dissolved in the steel will diffuse into the introduced dislocations, reducing the probability that they will stick. Therefore, when rapid cooling is continued to below room temperature, finely dispersed martensite is formed, but it is characterized by the accompanying free dislocations within the adjacent ferrite grains, which are rich in plastic deformability. be. This means that even if finely dispersed martensite is brittle and fractured by an external force, it is unlikely to act as a Griffith crack, which is the source of brittle fracture, due to the movement of adjacent free dislocations. There is. In fact, there is almost no difference in the brittle fracture occurrence characteristics when steel materials with the same composition are manufactured according to the present invention and when they are air-cooled after rolling (ferrite/pearlite steel structure). In terms of COD value, the steel produced using the method of the present invention is even slightly better. The results are shown in FIG. On the other hand, the fine martensite generated in the weld zone is slowly cooled even after the martensite transformation, and the metal structure adjacent to the martensite is often upper bainite with low toughness, and the brittle martensite is There is a high possibility that this will act as a Griffiths crack, and as a result, it is thought that the toughness value evaluated by the Shapey test or COD test will be reduced. The brittle fracture propagation arresting characteristics of the base metal of steel manufactured based on the technical idea of the present invention described in detail above and the brittle fracture occurrence characteristics when subjected to a welding reproduction thermal cycle simulating a welded part were compared with conventional methods. A comparison is shown in Table 1. In the same table, the cooling rate was measured with a thermocouple attached to the center of the thickness of the steel plate.
Means the average cooling rate between ℃ and 200℃. The brittle fracture propagation arresting property is a simple type of DWTT (Drop
Weight Tear Test) was used, and the temperature at which the fracture surface of the test piece showed a brittle fracture ratio of 50% was used as an index. The specimen shape is 14 mm thick, 180 mm long, and 45 mm wide.
The 5mm deep notch facilitates brittle fracture,
In addition, in order to prevent reverse fracture surfaces, they are locally embrittled. The characteristics of brittle fracture occurrence in welds are
Evaluated by COD test in accordance with BS5762.
In addition, the Buitskars hardness of the steel material subjected to the welding simulated thermal cycle was measured under a load of 20 kg. As is clear from Table 1, the steel material according to the present invention has almost the same brittle fracture propagation arresting properties as steel containing 2% Ni, and the hardness of the welded part is low.
It is also clear that the critical COD, which is an indicator of weld toughness, is much better. It can be said that the low hardness of the welded part also has excellent properties against weld cold cracking and stress sulfide corrosion cracking of the welded part.

【table】

【table】

[Table] Although the present invention has mainly been described regarding brittle fracture propagation arresting characteristics, the large ductility value of the base material makes it effective in suppressing unstable ductile fractures, which are a problem mainly in gas line pipes. It is. It is known that there is a correlation between unstable ductile fracture and absorbed energy in DWTT, but the steel produced by the method of the present invention exhibits higher absorbed energy than that produced by the conventional method. In the present invention, the main target is the brittle fracture occurrence characteristics of welded parts, but the technical idea of the present invention can be utilized to improve the brittle fracture propagation stopping characteristics in welded parts by devising the welding method or subsequent heat treatment. is also possible. Although the facts described above mainly concern steel plates, they do not impose any restrictions on the shape and dimensions of the steel material as long as the requirements of the present invention are satisfied.

[Brief explanation of the drawing]

FIG. 1 is a diagram comparing the occurrence of brittle fracture in a steel material according to the present invention with that in a steel material produced by a conventional method. In the figure, the ○ mark is vTrs (50% fracture surface transition temperature in the Charpy test), and _the ● mark is Tδ _0.2
(The transition temperature at which the limit COD is 0.2 mm in the COD test. The COD test complies with BS5762).

Claims (1)

[Claims] 1 C: 0.05 to 0.15%, Si: 0.01 to 0.3 by weight
%, Mn: 0.5~2.0%, Al: 0.01~0.1%, Ti:
After heating a steel slab or billet containing 0.005 to 0.030%, N: (0.2 to 0.5) x Ti%, and the balance consisting of Fe and impurities to 1150℃ or less at ₃ Ac points, 900
The reduction rate is 30% in the temperature range of ₃ or more Ar points below ℃
After hot rolling to a temperature of 80% or more, the cooling rate is 35°C/sec or more and 90°C/sec or less from Ar ₃ or more points.
A method for producing high-strength steel with excellent weld toughness and brittle fracture propagation arresting properties, which is characterized by cooling to below 200℃. 2 C: 0.05-0.15%, Si: 0.01-0.3 in weight%
%, Mn: 0.5~2.0%, Al: 0.01~0.1%, Ti:
Contains 0.005 to 0.030%, N: (0.2 to 0.5) x Ti%, further V: 0.2% or less, Nb: 0.08% or less,
One or more of Cr: 1.0% or less, Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.5% or less {C+Mn/6+Cr+Mo+V/5+Ni+Cu/1
5}≦0.40%, with the remainder being Fe and impurities, after heating the steel slab or billet to Ac ₃ points or more and 1150°C or less, the reduction rate in the temperature range of Ar ₃ points or more of 900°C or less Weld toughness characterized by hot rolling where the temperature is 30% or more and 80% or less, and then cooling from ₃ Ar points or more to 200°C or less at a cooling rate of 35°C/sec or more and 90°C/sec or less. and a method for manufacturing high-strength steel with excellent brittle fracture propagation arresting properties.