JPS625215B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing high-strength steel with excellent weldability and low-temperature toughness, and in particular, the present invention relates to a method for manufacturing high-strength steel with excellent weldability and low-temperature toughness. The present invention relates to a method for manufacturing steel plates, steel plates for pressure vessels such as tanks for butane and propane, steel plates for line pipes for cold regions, etc., without thermal refining. Conventionally, high-strength steel sheets with excellent low-temperature toughness that involve welding have been manufactured by Norma treatment and QT treatment, but they have the disadvantage of increasing manufacturing costs due to the rise in heat treatment costs. Controlled rolling (hereinafter referred to as CR) is a manufacturing method that uses so-called non-thermal treatment that does not involve heat treatment to achieve high tensile strength and high toughness. In order to increase the tension, it is necessary to lower the finish rolling temperature of CR, which not only significantly reduces rolling efficiency but also causes separation on the shear py impact fracture surface of the resulting steel plate, which is disliked by users. The drawback is that it is difficult to expand the range of applicable steel types. CR that improves the above drawbacks of CR up to low temperature range
There is a method of accelerated cooling after rolling that achieves high tensile strength without requiring C0.06%, as shown in Figure 1.
As can be seen from the relationship between cooling rate and strength (hereinafter referred to as TS) and yield strength (hereinafter referred to as YS) for a steel plate containing 1.4% Mn and 0.29% Geq, increasing the cooling rate Therefore, TS can be easily increased, but YS decreases when the cooling rate is relatively slow, and conversely increases when the cooling rate increases, but the disadvantage is that the amount of increase is very small. Due to the lack of YS in steel produced by cooling, there are only a limited number of steel types that can be used as substitutes for Norma and QT materials, and they are still not fully satisfied. The purpose of the present invention is to provide a method for manufacturing high-strength steel with excellent weldability and low-temperature toughness, which eliminates the drawbacks seen in the conventional manufacturing methods, without heat treatment, and which improves productivity and is inexpensive. The above object can be achieved by providing the method as described in the claims. Next, the present invention will be explained in detail. The present inventors investigated a method of increasing YS without heat treatment, and found that accelerated cooling is performed immediately after rolling, and accelerated cooling is stopped when the rolled steel sheet reaches a temperature of less than 500°C. We have newly discovered that YS increases significantly by applying light reduction within the range of 0.5% to less than 20% in the above temperature range. In other words, a rapid increase in YS can be obtained by applying light pressure reduction again after accelerated cooling. By the way, by applying this light reduction, the TS becomes
Although it does not reach the rate of increase of YS, it has the advantage of increasing considerably, and also has the advantage that separation does not occur on the sharpey impact fracture surface, but on the other hand, this light reduction has the disadvantage of worsening toughness. A problem arose in that it was difficult to apply to steel types that required low-temperature toughness. The present inventors have studied day and night on a method for improving low-temperature toughness, and as a result, by incorporating Ti into the component composition described in the claims of the present invention,
Since the fine γ grains can be maintained during slab heating, the structure of the steel sheet that has been subjected to CR and then accelerated cooling is refined, so even if light reduction is applied after the accelerated cooling has stopped, there is less deterioration in toughness. , Y.S.
The present invention was conceived after further discovering that TS increases. Next, the present invention will be explained using experimental data. Figure 2 contains 0.06-0.07% C and 1.4% Mn.
Same as Ti-containing steel (i.e. steel of the present invention, marked with ○)
YS with Ti-free steel (i.e. comparative steel, marked △),
The relationship between TS and transition temperature (hereinafter referred to as VTrs) is
A comparison was made by changing the rolling reduction rate at 400°C. According to the figure, it can be seen that Ti-containing steel can significantly improve vTrs without adversely affecting TS and YS compared to Ti-free steel. In other words, by applying CR to Ti-containing steel and immediately performing accelerated cooling, TS increases, and by subsequently stopping accelerated cooling and applying light reduction.
YS increases, so it is high without heat treatment.
TS and YS can be obtained, and the low-temperature toughness is also very high, so high tensile strength can be obtained with a carbon equivalent (hereinafter referred to as Ceq) lower than or at most the same amount as Norma and QT materials. Next, the reason for limiting the component composition of the present invention will be explained. If C is less than 0.005%, the strength of the steel plate will decrease and the weld heat affected zone (hereinafter referred to as HAZ) will become significantly softened, while if it exceeds 0.15%, the toughness of the base metal will deteriorate, and the weld will harden and become resistant. Since deterioration of crackability becomes significant, C needs to be within the range of 0.005 to 0.15%. Si is an element that is inevitably included for deoxidation during steel refining, but if it is less than 0.1%, the toughness of the base material will deteriorate, while if it exceeds 0.5%, the cleanliness of the steel will deteriorate and the toughness will decrease. must be within the range of 0.1-0.5%. If Mn is less than 0.8%, the strength and toughness of the steel plate will decrease, and the softening of the HAZ will increase;
%, the toughness of HAZ deteriorates, so Mn
It must be within the range of 0.8 to 2.0%. Al needs to be added in such a way that at least 0.005% of Al becomes a solid solution for deoxidizing the steel.On the other hand, if it exceeds 0.08%, not only the toughness of the HAZ but also the toughness of the weld metal will deteriorate significantly. Must be within the range of 0.005-0.08%. If S exceeds 0.008%, the absorbed energy in the C direction will drop significantly, so S must be kept at 0.008% or less. Ti forms TiN precipitates and has the effect of refining γ grains and refining ferrite and bainite grains.
If it is less than 0.003%, the amount of TiN precipitates will be insufficient and there will be no fine grain effect, while if it exceeds 0.04%, the amount of TiN precipitates will be excessive and the toughness will deteriorate, so Ti is 0.003 to 0.04%.
Must be within the range. N needs to be limited in order to prevent deterioration of the weld toughness. In other words, for HAZ toughness, it is better to have as little solid solution N as possible, and N flows into the weld metal during welding, which also deteriorates the toughness of the weld metal.
If it is less than 0.0010%, the amount of TiN precipitates required for fine grains will be insufficient, while if it is more than 0.010%, the amount of TiN precipitates will be excessive or solid solution N will remain, and in either case, the toughness of the weld will deteriorate, so N is 0.0010. over %
It must be within the range of less than 0.010%, and Ti
In relation to the content, the N amount is limited to the following formula: (Ti%/3.4-0.0020%) <N< (Ti%/3.4+0.0020%) This is to reduce solid solution N. It is. In other words, in order to form TiN precipitates with just the right amount of both elements in the relationship between N and Ti, the N content should theoretically be Ti%/3.4, but the N content should be Ti%/3.4. Since it is virtually impossible to adjust, N is set to (Ti%/3.4
- 0.0020%) and less than (Ti%/3.4 + 0.0020%). The above are the basic components of the steel slab used in the present invention, and if necessary, Ni, Mo, Cu, etc.
At least one selected from V, Cr, Ca, and REN can be added, and proper inclusion of each element adds a unique effect as described later. Ni improves the strength and toughness of the base metal without adversely affecting the hardenability and toughness of HAZ.
If Ni is added in an amount exceeding 0.5%, it will increase the manufacturing cost and is not necessary to achieve the objects and effects of the present invention, so the content of Ni should be 0.5% or less. Cu not only has almost the same effect as Ni, but also
It also improves corrosion resistance, but if it exceeds 0.5%, cracks are likely to occur during hot rolling and the surface quality of the steel sheet deteriorates, so it is necessary to keep Cu at 0.5% or less. Mo makes the γ grains regular during rolling and also produces fine bainite, which improves strength and toughness, but in order to achieve the purpose of this invention, 0.5% Mo
It is not necessary to add more than 0.5%, and any more will increase the manufacturing cost, so the content of Mo should be 0.5% or less. Cr is added to ensure the strength of the base metal of the steel plate and the strength of the joint, but if it exceeds 0.5%, not only the toughness of the base metal but also the toughness of the weld will deteriorate, so Cr should be added at 0.5%.
It is necessary to do the following. V is added to improve the strength and toughness of the base material of steel sheets and ensure joint strength, but if it is less than 0.01%, it has no effect, while if it exceeds 0.10%, it will damage the base material and the strength of the joint.
Since it significantly deteriorates the toughness of HAZ, V is 0.01~
Must be within 0.10%. If Ca is less than 0.002%, it is insufficient to control the morphology of MnS and has no effect on improving the toughness in the C direction;
If it exceeds Ca, the cleanliness of the steel will deteriorate and cause internal defects, so Ca must be within the range of 0.002 to 0.010%. If REM is less than 0.005%, it is insufficient to control the morphology of MnS and is not effective in improving the toughness of the steel plate in the C direction.
If it exceeds 0.010%, the cleanliness of the steel will deteriorate and it will also be disadvantageous for arc welding, so REM is 0.005%.
Must be within the range of ~0.010%. Next, the reason for limiting the manufacturing conditions of the present invention will be explained. After heating the steel slab, it is rolled down by 50 to 90% in the non-recrystallized γ range from Ar ₃ +70°C to Ar ₃ . The reason why the finishing rolling temperature is limited from Ar ₃ +70℃ to Ar ₃ is that the grain refining mechanism caused by rolling in this temperature range generates many deformed bands that become ferrite nuclei within the austenite grains. Ar ₃ Rolling only in the temperature range exceeding +70°C does not produce deformation bands within the austenite grains. Since the ferrite grains cannot be made sufficiently fine, it is not possible to obtain high toughness due to the fine grains.On the other hand, if rolling is performed in a temperature range below the Ar ₃ point, separation will occur on the shear pie impact fracture surface, so the rolling temperature range is ₃ Must be within the range of +70℃~ _Ar3 . Furthermore, the reason why the rolling reduction rate is limited to 50 to 90% during rolling in the above temperature range is that if the rolling reduction rate is less than 50%, the generation of deformation bands in the austenite grains is insufficient, so accelerated cooling after rolling, which will be described later, is performed. As a result, the ferrite grains do not become finer, but massive bainite is produced, resulting in a significant deterioration in toughness. On the other hand, when rolling is carried out at a reduction rate exceeding 90%, the introduced deformation band becomes saturated, and even if subsequent accelerated cooling is performed, the effect of improving toughness becomes small, so the reduction rate in the non-recrystallized γ region is It should be within the range of 50-90%. Immediately after rolling, heat to 500℃ at a cooling rate of 2 to 40℃/sec.
The reason for performing accelerated cooling to the temperature range below is (1) γ→α
(2) Mainly to increase TS by suppressing the growth of ferrite grains after transformation and improving toughness, and (2) transforming the transformation region that becomes pearlite structure into bainite structure or island-like martensite structure. If the speed is less than 2℃/sec, there is no effect of forming bainite structure, etc., while if the rate is less than 2℃/sec,
If the cooling rate exceeds sec, a lumpy martensitic structure will be generated and the toughness will deteriorate significantly, so the cooling rate should be set to 2.
It is necessary to keep it within the range of ~40℃/sec. Furthermore, if the cooling stop temperature is 500°C or higher, the amount of bainite and martensitic structures generated is insufficient, and the TS rises to less than 5 kgf/mm ² compared to air-cooled materials. The stop temperature must be less than 500℃. The reason for performing light reduction at a reduction rate of 0.5% to less than 20% in the temperature range from less than 500℃ to 200℃ or more after cooling is stopped is mainly to increase YS, YS under light pressure
On the other hand, if light pressure is applied in a temperature range lower than 200℃, hydrogen defects will occur because hydrogen cannot be removed sufficiently, so the temperature range for applying light pressure is 500℃.
It must be within the range of below 200℃.
The rolling reduction rate of light rolling is as shown in Figure 2.
If it is less than 0.5%, there is no noticeable effect on increasing YS, while if it is more than 20%, separation will occur on the shear peace impact fracture surface, so the reduction rate in the temperature range from less than 500℃ to more than 200℃ is 0.5% to 20%. Must be within the range below. The reason why air cooling or slow cooling is performed in a temperature range below 200°C is to facilitate the removal of hydrogen and prevent hydrogen defects. Next, the present invention will be explained with reference to examples. Examples The mechanical properties of steel plates obtained by processing the test steel types whose compositions are shown in Table 1 under the rolling-cooling conditions shown in Table 2 are shown in the same table.

【table】

【table】

[Table] Experimental Examples 1 to 10 shown in Table 2 were produced using A1 steel slabs having the composition of the present invention under various rolling-cooling conditions. According to Table 2,
In Experimental Example 1, accelerated cooling was not performed after rolling, and in Experimental Example 5, accelerated cooling was performed, but cooling was stopped at 500°C or higher, so it was found that the TS was less than 50 Kgf/mm ² in both cases, and the experiment In Example 2, LYS was found to be 36Kgf/mm ² because there was no light reduction after cooling, and in Experimental Example 3, vTrs was -40℃ because the reduction rate from Ar ₃ +70℃ to Ar ₃ was less than 50%. It was found that in Experimental Example 9, the cooling stop temperature was below 200°C, so cracking occurred due to the contained _H2 , and in Experimental Example 10, rolling was performed in the (α + γ) two-phase region. It was found that separation occurred due to the presence of steel, and Experimental Examples 4, 6, 7, and 8 were manufactured within the scope of all the constituent requirements of the present invention, so this is one of the goals for expanding the applicable steel types. LYS36Kg shown in the standard of YP36kg steel for shipbuilding
It can be seen that the conditions of f/ ^mm2 or more, TS50Kgf/ ^mm2 or more, and vTrs-40°C or less are all fully satisfied. Experimental Example 11 was manufactured within the range of rolling-cooling conditions of the present invention, but since it used B1 steel whose composition is shown in Table 1, it did not contain Ti, so vTrs was -40°C or higher. I understand that. Experimental examples 12 and 13 are Norma material, which is the conventional manufacturing method.
It shows the mechanical properties of QT's 50 kg class comparative steel, and the Ceq of the invention steel is Normal compared to the Ceq of the comparative steel.
In the case of QT material, it is less than 0.08%.
It can be seen that it is less than 0.03%. Experimental Examples 14 to 22 were manufactured within the scope of the constituent requirements of the present invention, and in particular, the component composition appropriately contained Ni, Cu, Cr, Ca, etc. According to Experimental Example 14, TS50 It can be seen that it is possible to obtain high tensile strength steel of kg class, TS60 kg class according to Experimental Examples 15 to 20, and TS70 kg class according to Experimental Examples 21 to 22. As can be seen from the above examples, the manufacturing method of the present invention has a low carbon equivalent and high yield strength, and there is no separation phenomenon on the shear peace impact fracture surface.
High-strength steel plates with low vTrs weldability and excellent low-temperature toughness can be produced inexpensively and stably.

[Brief explanation of the drawing]

Figure 1 shows the influence of accelerated cooling conditions (cooling rate, cooling stop temperature) on tensile properties and sharpie impact properties after controlled rolling, and Figure 2 shows the effects of light reduction at 400°C on tensile properties after accelerated cooling. Effect on Shapey impact properties (10℃/after controlled rolling)
FIG.

Claims (1)

[Claims] 1 C0.005-0.15%, Si0.1-0.5%, Mn0.8-2.0
%, Ti0.003~0.04%, Al0.005~0.08%, S0.008
% or less, more than 0.0010% and less than 0.010%, and in relation to the Ti content, the N content is within the range of the following formula, Ti%/3.4-0.0020%<N<Ti% /3.4+0.00
A steel billet consisting of 20% balance Fe and unavoidable impurities is heated to Ar ₃
Rolling is performed at a reduction rate of 50 to 90% in the temperature range from +70°C to Ar ₃ , followed by immediate cooling at an accelerated cooling rate of 2 to 40°C/sec until the temperature is below 500°C, and then accelerated cooling. After stopping, the reduction rate is 0.5 or more to 20% in the temperature range from less than 500℃ to 200℃ or more.
1. A method for producing high-strength steel with excellent weldability and low-temperature toughness, characterized by applying a reduction within a range of less than or equal to 100 ml, followed by air cooling or gradual cooling. 2 C0.005~0.15%, Si0.1~0.5%, Mn0.8~2.0
%, Ti0.003~0.04%, Al0.005~0.08%, S0.008
% or less, more than 0.0010% and less than 0.010%, and in relation to the Ti content, the N content must be within the range of the following formula: Ti%/3.4-0.0020%<N<Ti%/3.4+0 .00
20% A steel billet containing one or two groups selected from the following groups (a) and (b), with the balance consisting of Fe and unavoidable impurities, is heated at a temperature from Ar ₃ +70℃ to Ar ₃ Rolling is performed at a rolling reduction rate of 50 to 90% in the area, and then immediately cooled at an accelerated cooling rate of 2 to 40°C/sec until the temperature is below 500°C, and then after stopping the accelerated cooling, the rolling process is performed from below 500°C. A method for manufacturing high-strength steel with excellent weldability and low-temperature toughness, characterized by applying rolling reduction in the range of 0.5% or more to less than 20% in a temperature range of 200°C or higher, followed by air cooling or slow cooling. . Group (a): One or more selected from V0.01-0.10%, Cu0.5% or less, Ni0.5% or less, Cr0.5% or less, Mo0.5% or less. Group (b): Any one or two selected from Ca0.002-0.010% and REM0.005-0.010%.