JPS62196325A - Manufacture of high toughness acceleratedly cooled type 50kg class steel plate superior in weldability - Google Patents

Abstract

PURPOSE:To obtain the titled high toughness 50kg steel plate superior especially in low temp. characteristic, by heating steel slug having a specified component compsn., then hot rolling it while prescribing draft at a prescribed temp. or below and finishing temp. respectively, then immediately cooling the plate by a prescribed rate. CONSTITUTION:Steel slug having compsn. as basic components of, by weight 0.10-0.25% C, <=0.50% Si, <=1.00% Mn under <=0.30% carbon equivalent Ceq (shown by a formula) is prepd. Next, the slug is heated to 950-1,150 deg.C for obtaining fine ferrite and vainite, further hot rolled by >=60% draft at <=900 deg.C for refining austenite. Hot rolling is carried out while prescribing finishing temp. thereof to Ar3-20 deg.C-Ar3+50 deg.C, then immediately the plate is cooled to <=400 deg.C by >=2 deg.C/s rate to form bainite of 5-30mum fineness. Thereby, the aimed titled steel plate is obtd.

Description

[Detailed description of the invention]

(Field of Application of the Invention) The present invention relates to a lj method for producing a high toughness accelerated cooling type 50 Giga grade steel plate with excellent weldability. (Conventional technology and problems to be solved) Conventionally, when welding high-strength steel plates with low heat input in shipbuilding and construction of welded steel structures such as tanks, welding techniques were used to prevent welding cold cracking. Measures were taken such as preheating and limiting short beads. This cold cracking occurs in the weld heat affected zone (IIAZ).
This tends to occur more easily in those with higher hardenability of IIAZ, that is, those with higher carbon equivalent (Ceq). Therefore, in order to reduce the number of welding steps, there has been a demand for a low Ceq, high-strength steel plate with excellent weld cracking resistance, and as a result, it has become possible to manufacture a steel plate that satisfies this property by an accelerated cooling method. However, if Ceq is lowered, the Ar3 transformation point increases, so obtaining excellent low-temperature toughness is a national destiny. For example, Ceq for a 50 kg f/mm'' class steel plate is preferably 0.30% or less from the viewpoint of weld cracking resistance considering variations in welding process, but in that case, vTrs is about -30℃. In other words, it has been impossible to produce a low Ceq 50 kgf/mm'' class steel plate that has both excellent weld cracking resistance and low temperature toughness. The purpose of the present invention is to produce a 50 kg class steel plate with excellent weldability and high toughness, and to propose a filtration method (11) for the +i construction of a 50 kg class steel plate that is acceleratedly cooled after hot working. (Means for solving the problem) The most effective means for improving the base metal toughness of J9 in 50kg steel plates is to refine the grains of Ferrite 1. However, in steel plates with low Ceq, Ar: As I becomes higher, the following problems occur, so lateralization of ferrite cannot be fully utilized. ■Un(4) Since the crystal region becomes narrower, the ferrite nucleation size 1~ decreases. ■ Ferrite transformation starts at a high temperature, resulting in significant ferrite growth.Therefore, in order to achieve the second objective, the present inventors have made extensive efforts to find the optimal rolling and cooling method to obtain high toughness with low Ceq material. As a result of our research, we have found that by controlling the heating source degree, rolling reduction rate, finishing temperature, cooling rate, and cooling stop temperature during hot rolling, we can prevent coarsening of ferrite and eliminate pearlite. It has been found that it is effective to finely disperse bainite, which was thought to have a negative effect.The appropriate size of bainite is 5 to 30 μl, and
It has also been found that this fine bainite is easier to produce in a high C-low Mn system. Based on the above, the present invention has discovered a method that can easily produce 50 kg-class steel plates with low Ceq and high toughness, which have been thought to be difficult until now. It is the law. The gist of it is: C:O, LO~0.25%,
Contains Si≦0.50% and Mn≦0.00%, and
Carbon equivalent Ccq (Ceq==C+Mn/6+(C
r+Mo+V)15+(Cu+Ni)/15) is Cc
The basic composition is q≦0.30%, and if necessary, 1゛j≦0.020%, Ca≦0.0050% and C
A slab of steel containing one or more of c≦0.0050%, with the balance consisting of iron and unavoidable impurities.
Heating to 50~1150℃, reduction rate at 900℃ or below 60% or more, finishing temperature Ar, -20℃~Ar3+50
After hot rolling at ℃, it is immediately heated to 400℃ or less by 2℃/
A method for producing a high-toughness accelerated cooling type 50 kg-class steel plate with excellent weldability, characterized by cooling at a cooling rate of 5 to 30 μm or higher to produce fine bainite of 5 to 30 μm. The present invention will be described in detail below with reference to Examples. First, the composition of the steel to which the present invention is applied and
The reason for limitation 1 is shown below. C: C is the most effective element for ensuring strength. Especially for low Ceq materials, high Chk is advantageous. However, if it exceeds 0.25%, a large amount of coarse bainite is generated, which not only impairs toughness but also reduces weld cracking resistance and HAZ
In order to prevent deterioration of toughness, the lower limit of C is set to 0.25%. On the other hand, the lower limit of C is set to 0.10%, which can ensure strength with low Ct: +1 material. SL: S↓ is effective for deoxidizing and ensuring strength, but 0,
If it exceeds 50%, the ferrite will harden and the toughness of the base material will deteriorate, so the content should be 0.50% or less. Mn: Like C, Mn is an effective element for ensuring strength. However, the contribution of Mn addition to strength is smaller than that of C, and in order to ensure strength in a low Ceq material, it is desirable that the contribution is smaller. Therefore, it should be 1.00% or less. Ceq: Q for carbon Ceq is for estimating the hardenability of IIAZ and is one index of weld cracking resistance.
eq=C+Mn/6+(Cr+Mo+V)150(Cu
+Ni)/15. CeqK, which is desirable in terms of weld cracking resistance, varies depending on the structure, the type of welding rod used, etc., but it is set to 0.30% or less, taking into account the control variations during JM work. The above C, Si, Mn and Ceq are the basic components,
One or more of the following "l'i", Ca, and Ca can be added as necessary mainly to improve toughness. Ti: Ti has the effect of suppressing coarsening of austenite grains and It is effective for refining ferrite grains due to intragranular nucleation during
Base material toughness and IIAZ toughness can be improved. However, when it exceeds 0.020%, IIAZ toughness deteriorates. Therefore, when added, it should be limited to 0.020% or less. Ca, Ca: Both of these elements are effective in spheroidizing inclusions and can improve ductility. However, if it is added to 1), coarse inclusions will be formed, which will trap internal defects. Therefore, when added, each should be 0.0050% or less. Next, the reasons for limiting the heating and rolling conditions and cooling conditions in hot rolling of the steel slab having the above-mentioned composition will be described. Heating 7!11 degrees: The heating temperature of the steel slab needs to be regulated to 1150 degrees Celsius in order to obtain fine ferrite and bainite, but 950 degrees
The heating temperature should be set at 95°C because rolling will be hindered if the temperature is lower than 95°C.
The temperature shall be 0 to 1150°C. Pressure at 900°C or below]: Rate: In order to obtain fine ferrite 1- and bainite, since steel with the above composition has a narrow non-recrystallized region, it is necessary to fully utilize recrystallization to make austenite fine grains. important and 90
A rolling reduction of 60% or more is required at a temperature of 0° C. or less. Therefore, the rolling reduction rate at 900° C. or lower is set to 60%. Finishing temperature: When the finishing temperature is set to Ar and lower than -20°C, rolling occurs in a two-phase region, and the ferrite undergoes work hardening, resulting in a significant deterioration of toughness. Moreover, if Ar exceeds +50°C, coarse bainite is generated and toughness deteriorates, so the finishing temperature is set in the range of Ar, -20°C to Ar, +50°C. Cooling rate: The present invention is directed to accelerated cooling materials, and in order to increase the strength, it is desirable that the cooling rate be as high as possible. Plate thickness 11
Considering 00IB, a cooling rate of at least 2"C/s or more is required. Cooling stop temperature: The lower the cooling rate is, the more remarkable the strength increase effect is, and the cooling rate is 400℃ or less. is necessary. In this way, the high C-low Mn system with the above component composition has low Ceq.
By hot rolling and cooling a steel slab under the above conditions, there is no barley 1-, and fine ferrite has 5
A structure in which fine bainite of ~730 μm is dispersed can be easily obtained. W4 temporary, which has such a structure, has sufficient strength of 50 kg class, and although it is a low Ceq material, it has excellent low-temperature toughness at -30℃ or below, and of course has good weld cracking resistance. Are better. (Example 1) A steel plate having a thickness of 25+ am was manufactured using a steel slab having the chemical composition shown in Table 1 under the rolling conditions and cooling conditions shown in the same table. Cut a test piece from each steel plate. A tensile test, an impact test and a y-slit test were conducted. The results are shown in the same table. As is clear from Table 1, all of the steels of the present invention having chemical compositions within the range of the present invention have a strength of 50 kg class, are particularly excellent in low-temperature toughness, and have good weld cracking resistance. . On the other hand, comparative steels in which any of C1SL, Mn, or Ceq is outside the range of the present invention have poor low-temperature toughness even if they have strength (F, H), or have poor low-temperature toughness even if they have good low-temperature toughness. - minutes (G), or intensity (- minutes 1:
), the low Lj toughness and weld cracking resistance are poor (I
).

[Left below]

(Example 2) Next, a steel slab having a chemical composition within the range of the present invention was hot rolled by varying rolling conditions and cooling conditions.
Cooling was performed to investigate the effects of rolling and cooling conditions. In addition,
The base material is steel with the symbol. 1:) 25non
A test piece was cut from a thick steel plate and subjected to a tensile test and an impact test, and the average bainite diameter was determined to be 1 tl by microstructural examination.
It was determined. These results are shown in Table 2 and FIGS. 1 and 2. Note that each item 1 is a compilation of the results shown in Table 2. As is clear from FIG. 1, if the manufacturing is performed under heating conditions within the range of the present invention, a reduction rate of 900°C or less, and a finishing temperature,
It has excellent low-temperature toughness (v T rs ≦ -60°C) and can sufficiently secure 55 kg class strength (TS ≧ 50), and in particular, as shown in Figure 2, the bainite size of the obtained steel sheet is also small and fine. Excellent low temperature toughness is guaranteed. Furthermore, if the cooling conditions are within the range of the present invention, as is clear from Table 2, similarly excellent low-temperature toughness and strength can be obtained. (Effects of the Invention) As detailed above, according to the present invention, despite being a low Ceq material, it has high toughness, particularly excellent low temperature toughness, and has excellent weldability such as weld cracking resistance. It is possible to produce 50 kg class steel plates.

[Brief explanation of drawings]

Figure 1 shows the relationship between hot rolling conditions (heating temperature, rolling reduction below 900℃, finishing temperature), strength, low-temperature toughness, etc. of steel slabs. Figure 2 shows the bainite dimensions in steel plates and low-temperature toughness. FIG. Patent applicant Takashi Nakamura, Patent attorney representing Kobe Steel, Ltd. (See Figure 2 (Night QiAC voice #I)

Claims (2)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.10 to 0.2
5%, Si≦0.50% and Mn≦1.00%,
And carbon equivalent Ceq (however, Ceq=C+Mn/6+
(Cr+Mo+V)/5+(Cu+Ni)/15) is C
A steel slab with a basic composition of eq≦0.30% and the balance consisting of iron and unavoidable impurities is 950 to 1150.
After heating to ℃ and hot rolling at a rolling reduction rate of 60% or more at 900℃ or less and a finishing temperature of Ar_3-20℃ to Ar_3+50℃, immediately cool to 400℃ or less at a cooling rate of 2℃/s or more. A method for producing a high toughness accelerated cooling type 50 kg class steel plate with excellent weldability, which is characterized by producing fine bainite of 5 to 30 μm. (2) Contains C: 0.10 to 0.25%, Si≦0.50% and Mn≦1.00%, and carbon equivalent Ceq (Ceq=C+Mn/6+(Cr+Mo+V)/5+
(Cu+Ni)/15) has a composition of Ceq≦0.30% as a basic component, and furthermore, Ti≦0.020% and Ca≦0.0.
050% and one or two of Ce≦0.0050%
A slab of steel containing more than 100% of carbon dioxide and the remainder consisting of iron and unavoidable impurities is heated to 950 to 1150°C, the rolling reduction is 60% or more at 900°C or less, and the finishing temperature is Ar_3-20°C.
〜Ar_3+50℃ immediately after hot rolling at 400℃
℃ or less at a cooling rate of 2℃/s or more, 5 to 30℃
A method for producing a high-toughness accelerated cooling type 50 kg-class steel plate with excellent weldability, which is characterized by producing μm-sized fine bainite.