JPS622613B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing low-carbon equivalent high-strength steel with excellent weldability and low-temperature toughness, and in particular, the present invention relates to a method for manufacturing a low-carbon equivalent high-strength steel with excellent weldability and low-temperature toughness. The present invention relates to a method for manufacturing high-strength steel plates for shipbuilding, steel plates for pressure vessels such as tanks for butane and propane, steel plates for line pipes for cold regions, etc., without thermal treatment. Conventionally, high-strength steel plates 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. In addition, controlled rolling (hereinafter referred to as CR) is a manufacturing method that does not apply heat treatment, so-called non-thermal treatment, to achieve high tensile strength and high toughness.
However, in order to achieve high tensile strength with CR material that can replace Norma and QT materials, it is necessary to lower the finish rolling temperature of CR, which not only significantly reduces rolling efficiency but also reduces the Separation occurs on the shear peace impact fracture surface, which is disliked by users and makes it difficult to expand the range of applicable steel types. CR that improves the above drawbacks of CR up to low temperature range
As a manufacturing method that achieves high tensile strength without requiring rolling, there is a method of accelerated cooling after rolling.According to this accelerated cooling method, the C0.06
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 steel plates containing %, Mn1.4%, and Ceq0.29%, increasing the cooling rate TS can be easily increased by , but on the other hand, 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, steel produced by accelerated cooling is limited in steel types that can be substituted for Norma and QT materials, and is still not fully satisfied. The present invention provides a low-carbon equivalent high-strength steel with excellent weldability and low-temperature toughness, which eliminates the drawbacks seen in the conventional manufacturing methods described above, and provides improved productivity and an inexpensive manufacturing method without heat treatment. The object can be achieved by providing the method described in the claims. Next, the present invention will be explained in detail. As a result of studying a method to increase YS without heat treatment, the inventors of the present invention found that accelerated cooling is performed immediately after rolling, and the rolling reduction rate is from 0.5% to less than 20% in the temperature range from less than 500°C to more than 200°C. We have newly found that YS increases significantly by applying light pressure reduction. 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
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. However, a problem arose in that it was difficult to apply to steel types that required low-temperature toughness. As a result of day and night research into a method for improving low-temperature toughness, the present inventors have incorporated Nb as shown in the component composition described in the claims of the present invention, and further applied CR with Nb in a solid solution state, Immediately after that, accelerated cooling further refines the structure of ferrite, bainite, etc., so even if light reduction is applied after stopping accelerated cooling, there is less deterioration in toughness, and YS,
The present invention was conceived after further discovering that TS increases. In other words, by applying CR to Nb-containing steel and then immediately applying 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 at less than the carbon equivalent (hereinafter referred to as Ceq) of Norma and QT materials, making it easy to expand the applicable steel types listed above. can be done. 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 crack resistance will increase. C should be within the range of 0.005 to 0.15% since the deterioration of properties becomes significant. 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. Si must be within the range of 0.1 to 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. Nb is effective in refining ferrite, but 0.005
If it is less than 0.1%, it has no effect, while if it exceeds 0.1%, it will diffuse into the weld metal during welding and reduce the toughness of the weld metal, so Nb needs to be in the range of 0.005 to 0.1%. The above are the basic components of the steel slab used in the present invention, and if necessary, Ni, Mo, Cu,
At least one selected from V, Cr, Ca, and _REM can be added, and proper inclusion of each element adds a unique effect as described below. 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 Mo in excess of 0.5%, and any more will increase manufacturing costs, so Mo should be added at 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 must be kept at 0.5% or less. V is added to improve the strength and toughness of the base metal of steel plates and ensure joint strength, but if it is less than 0.01%, it has no effect, while if it exceeds 0.10%, it will significantly deteriorate the toughness of the base metal and HAZ. V is 0.01~0.10%
Must be within the range. 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 R _EM 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 R _EM 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 to a temperature at which at least 0.005% Nb is dissolved in solid solution, heating in the temperature range from Ar ₃ + 150℃ to Ar ₃
The reason why rolling is performed at a rolling reduction ratio of 90% is that the onset temperature of the non-recrystallized γ region of solid-dissolved Nb is higher at Ar ₃ +150°C, compared to the above-mentioned starting temperature of Ar ₃ +70°C for undissolved Nb. The unrecrystallized γ region can be expanded, and deformation bands can be effectively generated by rolling down the unrecrystallized γ region, both of which have the effect of promoting grain refinement of ferrite and increasing toughness. However, if the solid solution Nb is less than 0.005%, the effect cannot be obtained sufficiently, so it is necessary to heat to a temperature at which at least 0.005% Nb is dissolved. The reason why the rolling finishing temperature range is limited from Ar ₃ +150°C to Ar ₃ is that by rolling from Ar ₃ +150°C, which is the starting temperature of the non-recrystallized γ region of solid solute Nb, a deformation zone that becomes a ferrite nucleus is formed. generated, while
If rolling is performed at a temperature lower than Ar ₃ , separation will occur on the Shapey impact fracture surface, so the rolling temperature range needs to be within the range of Ar ₃ +150°C to _{Ar 3} .
Furthermore, during rolling in the above temperature range, the rolling reduction rate was increased to 50
The reason for limiting the rolling reduction to ~90% is that if the rolling reduction is less than 50%, the formation of deformation bands within the austenite grains is insufficient, so by applying accelerated cooling after rolling, which will be described later, the ferrite grains will not become finer. Since massive bainite is generated, toughness is significantly deteriorated. On the other hand, 90
If rolling is performed at a reduction rate exceeding 50%, the introduced deformation band will be saturated, and even if subsequent accelerated cooling is performed, the effect of improving toughness will be small. Must be within the range of %. Immediately after rolling, heat to 500℃ at a cooling rate of 2 to 40℃/sec.
The purpose of accelerated cooling to a temperature below The main purpose is to increase TS by transforming into TS, but if the cooling rate is less than 2°C/sec, there is no effect of generating bainite structure, etc., while if it exceeds 40°C/sec, a lumpy martensite structure is generated. The cooling rate should be 2 to 40℃/sec as it will significantly deteriorate the toughness.
Must be within the range. In addition, when the accelerated cooling stop temperature is 500℃ or higher, the amount of bainite and martensitic structure generated is insufficient, and the TS rise is less than 5Kgf/mm ² compared to air-cooled materials, and the strength is insufficient and it cannot be used as a substitute for QT steel. The stop temperature must be less than 500℃. The reason for performing light reduction at a reduction rate of 0.5% or more to less than 20% in the temperature range from less than 500°C to more than 200°C after stopping accelerated cooling is mainly to increase YS, and to under pressure
If the increase in YS is small and 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 in which light pressure is applied is 500℃.
It must be within the range of below ℃ to 200℃ or higher. If the reduction rate of light reduction is less than 0.5%, it will not have a significant 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 should be within the range of 0.5% or more and less than 20%. It is necessary to 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 sheets prepared 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 Nos. 1 to 11 shown in Table 2 are those produced using A1 steel slabs having the composition of the present invention under various heating-rolling-cooling conditions. Examples Nos. 1 to 4 are manufactured within the scope of all the constituent requirements of the present invention, including Nb solid solution conditions, rolling conditions, cooling conditions, light reduction conditions after cooling, and slow cooling start conditions, so they are applicable. Goal 1 of expanding steel types
As shown in the standards for high-strength steel plates for structural use for welding.
WES-135 is the standard for mechanical properties of HW36
It can be seen that values of LYS36Kgf/mm ² or higher and TS53Kgf/mm2 ^or higher can be obtained with a very low carbon equivalent, and the low-temperature toughness is also excellent. It can be seen that Experimental Examples Nos. 5 to 14 cannot satisfy the standards for high-strength structural steel for welding (for example, HW36) because any of the constituent requirements of the present invention are not satisfied. In other words, Experimental Example No. 5 was air-cooled (hereinafter referred to as AC) without accelerated cooling after rolling, and Experimental Example No. 11 had a solid solution Nb amount of 0.005.
Since it is less than %, both targets are TS53Kgf/
mm ² was not satisfied, and in Experimental Example No. 6, the LYS was 36 kg because light compression was not applied after cooling.
It was found that f/mm ² was not satisfied, and Experimental Example No. 7
Since the rolling reduction rate from Ar ₃ +150℃ to Ar ₃ is less than 50%, it can be seen that vTrs is higher than the target -40℃, and in Experimental Example No. 9, the slow cooling start temperature after light pressure reduction is 200℃. It can be seen that hydrogen cracking occurs because the temperature is less than 0.degree. C., and in Experimental Example No. 10, separation occurs because rolling was performed at a temperature of Ar ₃ or less with a reduction ratio of 30%. Experimental examples Nos. 15 to 23 were manufactured within the scope of the constituent requirements of the present invention, especially in terms of component composition.
It contains Ni, Cu, Cr, Mo, V, Ca, REM, etc. appropriately, so according to Experimental Examples 15 to 19, it is 60Kgf/mm ²
According to Experimental Examples 20 and 21, the steel plate is 70Kgf/mm ^2nd grade, and according to Experimental Examples 22 and 23, 80Kgf/mm ^2nd grade steel plate has a very low carbon equivalent, a good balance of LYS and TS, and a low temperature. It can be seen that excellent toughness can be obtained with no separation occurring on the Charpey impact fracture surface. As can be seen from the above examples, the manufacturing method of the present invention has a low carbon equivalent, excellent weldability, and no separation on the shear py impact fracture surface.
Low carbon equivalent high tensile strength steel with low vTrs and excellent low temperature toughness can be produced inexpensively and stably.

[Brief explanation of the drawing]

The figure shows the influence of accelerated cooling conditions (cooling rate, cooling stop temperature) after controlled rolling on tensile properties and Charpy impact properties.

Claims (1)

[Claims] 1 C0.005-0.15%, Si0.1-0.5%, Mn0.8-2.0
%, Nb0.005~0.1%, Al 0.005~0.08%, S0.008
% or less, with the remainder consisting of Fe and unavoidable impurities _.
Rolling is performed at a reduction rate of 50 to 90% in the temperature range from +150℃ to Ar ₃ , and then immediately cooled to a temperature of less than 500℃ at a cooling rate of 2 to 40℃/sec, and after cooling is stopped, it is less than 500℃. 0.5% in the temperature range from to 200℃ or higher
From the above, a method for producing low carbon equivalent high tensile strength steel with excellent weldability and heat retention toughness, which is characterized by rolling at a reduction rate of less than 20% and subsequent air cooling or slow cooling. 2 C0.005~0.15%, Si0.1~0.5%, Mn0.8~2.0
%, Nb0.005~0.1%, Al 0.005~0.08%, S0.008
% or less, and further contains one or two groups selected from the following groups (a) and (b), and the remainder
After heating a steel slab consisting of Fe and unavoidable impurities to a temperature at which at least 0.005% of Nb contained in the steel slab becomes a solid solution, it is reduced by 50 to 90% in a temperature range from Ar ₃ +150℃ to Ar ₃ . Rolling is carried out at a cooling rate of 2 to 40℃/sec.
A method for producing a low carbon equivalent high tensile strength steel with excellent weldability and low temperature toughness, which is characterized by rolling at a reduction rate of 0.5% or more to less than 20% in a temperature range of ℃ or higher, followed by air cooling or slow cooling. Group (a): V0.01~0.10%, Cu0.5% or less, Ni0.5%
One or more types selected from the following: Cr 0.5% or less, Mo 0.5% or less. Group (b): Ca0.002-0.010%, REM0.005-0.010%
One or two selected from
seed.