JPH0527687B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing a steel plate with excellent low-temperature toughness. [Background Art] In recent years, energy development has been carried out in cold regions and polar regions, and the ocean is becoming deeper. As a result, marine structures are becoming significantly larger. Accordingly, the steel materials used are required to have unprecedented low-temperature toughness, especially in the center of the plate thickness, where the toughness decreases significantly, and toughness in the heat-affected zone of welded joints (HAZ toughness). Additionally, as structures become larger, the steel materials used are becoming thicker. As mentioned above, recent steel materials for offshore structures are
Regardless of the thickness of the material, toughness at the center of the thickness and HAZ
A material with good toughness is required. The central part of the steel plate is thicker (thick material here refers to a plate thickness of 50 mm).
Refers to steel plates of the above. ), the toughness decreases. Therefore, rolling methods and component systems that do not reduce toughness at the center of the plate thickness even when the plate is thick are being studied.
For example, as proposed in JP-A-59-211529
No. 3 is publicly known. [Problems to be Solved by the Invention] However, these measures are only for improving the toughness of the steel plate, that is, the base material. When looking at the welded structure as a whole, one of the areas where the toughness is most degraded is the HAZ. Therefore, in order to increase the safety of structures,
It is necessary to improve not only the toughness at the center of the thickness of the base material, but also the HAZ toughness. However, the reality is that it is difficult and there is no effective method to improve both characteristics at the same time. [Means for Solving the Problems] The present invention has been made to solve the above problems, and its gist is that C: 0.02 to 0.30% and Si: 0.05 to 0.05% by weight. 1.0%, Mn:
0.050~2.50%, Nb: 0.003~0.010%, Al: 0.005
Steel with ~0.10% balance Fe and unavoidable impurities, or one or two of the strength improving element group consisting of Cr: 1.0% or less, Mo: 1.0% or less, V: 0.20% or less, Cu: 2.0% or less Steel containing the above, and further containing one or more of the toughness improving element group consisting of Ni: 4.5% or less, Ti: 0.20% or less, and Ca: 0.0005 to 0.005%, with the balance being Fe and unavoidable impurities. heated to 1000-1050℃,
This is a method for producing a steel plate with excellent low-temperature toughness, which is characterized by rolling at a temperature of Ar ₃ to 850°C with a reduction rate of 50 to 70%, cooling with water after rolling, and then tempering. [Function] That is, the present inventors have determined the center-of-thickness toughness of thick materials based on the knowledge obtained as a result of numerous experiments.
We have discovered a method to improve both HAZ toughness. In order to improve the toughness at the center of the thickness of thick materials, as described in the well-known Japanese Patent Application Laid-open No. 59-211529, controlled rolling of Nb-added steel, which has the effect of suppressing recrystallization, and controlled rolling as necessary have been proposed. This was achieved by cooling. However, as will be described later, it is required to reduce the amount of added Nb as much as possible in order to improve HAZ toughness.
(below) also has a recrystallization suppressing effect, and furthermore, with such a small amount of Nb, even heating below 1050 ° C can sufficiently dissolve Nb as a solid solution, and there is a recrystallization suppressing effect in the subsequent rolling process. necessary
It has been found that a much smaller amount of Nb (0.003% or more by weight) is sufficient than previously thought. These points are decisively different from known documents. With regard to HAZ toughness, with the recent demands for strict low-temperature toughness, especially the CTOD test, which is a new evaluation method that is largely dominated by the localized embrittlement of the HAZ, micro-embrittlement areas are becoming more of a problem than before. I'm getting old. Regarding Nb, which was thought to be harmful to HAZ toughness, it was previously thought that Nb precipitates simply due to welding thermal cycles and embrittlement occurs. As a result of detailed investigation into the influence of Nb on HAZ toughness, we found that in multi-layer welding, which is commonly performed on thick materials, the first thermal cycle
When the part that has been heated above and has become coarse grained is heated to just above the A _C1 transformation point in the subsequent thermal cycle, a large number of high-carbon island-like martensite will be generated, which will significantly reduce the toughness. In actual multilayer welding, this high carbon martensite is also tempered in subsequent passes and rendered partially harmless, but the presence of Nb delays the decomposition of the island martensite due to this tempering. Therefore, the presence of Nb reduces HAZ toughness. As a result of investigating the limit value of this adverse effect of Nb, the weight ratio of Nb is 0.010%, preferably 0.007%. With the above-mentioned concept, both center-of-thickness toughness and HAZ toughness can be improved at the same time. The present invention is based on this knowledge. Next, the reason for limiting the components in the present invention will be described. C is an element that increases strength at a low cost, and 0.02% or more is required to ensure strength, but if added in large amounts, it will damage the toughness and weldability of the steel, so the upper limit should be set at 0.30%.
%. 0.05% or more of Si is required for deoxidizing steel, but
If it increases, weldability will be impaired, so the upper limit is set at 0.6%. Mn is required to be at least 0.50% to ensure strength, but as too much Mn causes deterioration in weldability and toughness, the upper limit is set at 2.5%. Al is required at least 0.005% for deoxidation, but
The upper limit is set at 0.1% because the toughness decreases significantly when the content increases. Nb is a main element in the present invention, but as shown in FIG. 1, if it is less than 0.003%, the effect of suppressing recrystallization by Nb disappears, and the toughness of the base material decreases. 0.010%
Above 0.003~ because the HAZ toughness decreases.
Limited to 0.010%. The chemical composition, rolling conditions, and welding conditions of the sample shown in FIG. 1 are as follows. Chemical composition (wt%) C Si Mn P S Al 0.07 0.21 1.1 0.007 0.003 0.04 Nb Fe 0 to 0.03 Residual rolling conditions 1050℃ heating, 850℃ or less rolling reduction 60%, plate thickness 50mm,
Start of water cooling at 720℃, stop of water cooling at 80℃, tempering at 550℃ for 30 minutes Welding conditions: Submerged arc welding Heat input: 40kJ/cm In addition to the above basic components, the following elements are selected according to the characteristics required for the steel. It can be added to V, Cu, Cr, Mo These elements have the uniform effect of improving the strength of steel, so one or more of them are selectively added; V is 0.2% or less, Cu is 2.0% or less, Cr is 1.0% or less, and Mo is 1.0% or less. Ti, Ni, Ca These elements have the uniform effect of improving the toughness of steel, so one or more of them are selectively added, but if they are in too large a quantity, weldability decreases.
Ti is 0.2% or less, Ca is 0.01% or less, and Ni is expensive, so it is 4.5% or less. Next, the reasons for limiting the heating, rolling, and cooling conditions will be described. The upper limit of the heating temperature is 1050°C in order to atomize the austenite grains. As the plate thickness increases, it becomes difficult for the grain refinement effect of rolling to reach the inside, so it becomes important to refine the austenite grains by heating. Therefore, the upper limit is set as low as 1050℃. If the lower limit is too low, Nb will not form a solid solution, so the lower limit should be 1000°C or higher. Hot rolling involves applying a reduction of 50% or more at Ar ₃ to 850°C. This is because rolling in a two-phase region with Ar less than ₃ lowers the toughness, and rolling at a high temperature of over 850° C. and a rolling reduction of less than 50% does not result in sufficient grain refinement. At the upper limit of the rolling reduction, the effect of controlled rolling begins to saturate.
70%. Next, for cooling after hot rolling, cooling water is injected from the high temperature state to rapidly cool it to a temperature of 250° C. or less, thereby increasing the strength. Although it is most efficient to perform this cooling on-line, it goes without saying that cooling may also be performed off-line. Next, tempering is performed by heating to a temperature in the range of 350 to 650° C., since an increase in strength is achieved by water cooling, but there is a decrease in toughness. This allows high strength and high toughness to be achieved at the same time. [Example] The chemical composition of the test material is shown in Table 1, the heating, rolling, cooling, and tempering conditions are shown in Table 2, and the mechanical properties of the obtained steel plate are also shown in Table 2. These steel plates are melted in a converter, then continuously cast into slabs with a thickness of 245 mm, and after ingot formation, they are bloomed and rolled to a thickness of 245 mm.
A thick plate was rolled using a slab with a diameter of 245 mm.

【table】

【table】

【table】

〔Effect of the invention〕

As described above, the present invention solves the problem of plate thickness 50, which was a problem in the past.
This product improves the toughness at the center of the plate thickness of mm or thicker steel plates, and at the same time improves the toughness at welded joints.It can provide steel plates for offshore structures that require low-temperature toughness, and is industrially effective. This is a great invention.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the relationship between Nb content and base metal and joint toughness.

Claims (1)

[Claims] 1 Steel with weight ratio of C: 0.02 to 0.30% Mn: 0.50 to 2.50% Al: 0.005 to 0.10% Si: 0.05 to 1.0% Nb: 0.003 to 0.010%, the balance being Fe and unavoidable impurities. 1000～
A method for producing a steel plate with excellent low-temperature toughness, which comprises heating to 1050°C, rolling at a reduction rate of 50 to 70% at a temperature of Ar ₃ to 850°C, cooling with water after rolling, and then tempering. 2 The basic components are C: 0.02-0.30% Mn: 0.50-2.50% Al: 0.005-0.10% Si: 0.05-1.0% Nb: 0.003-0.010%, furthermore Cr: 1.0% or less, Mo: 1.0 % or less, V: 0.20% or less, Cu: 2.0% or less, and further contains Ni: 4.5% or less, Ti: 0.20% or less, Ca: 0.0005
A steel containing one or more of the toughness-improving element group consisting of ~0.005%, with the remainder being Fe and unavoidable impurities, is heated to 1000-1050°C, and at a temperature of Ar ₃ ~850°C, a reduction rate of 50 ~ A method for manufacturing steel sheets with excellent low-temperature toughness, which is characterized by 70% rolling, water cooling after rolling, and then tempering.