JPH0517287B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing non-temperature high-strength steel with excellent weldability and low-temperature toughness, and more particularly to a method for manufacturing high-strength steel with low carbon equivalent and less distortion within a steel plate. Conventionally, high tensile strength steel plates that require low-temperature toughness due to welding, such as steel plates for line pipes in cold regions, steel plates for pressure vessels such as tanks for butane and propane, and steel plates for structures in icy areas, have been processed by quenching and tempering. However, the drawback is that the manufacturing cost is high due to the rise in heat treatment costs. In addition, there is a method using controlled rolling (hereinafter referred to as CR) as a manufacturing method that aims to achieve high tensile strength and high toughness without heat treatment, so-called non-temperature treatment, but when the finishing temperature of CR is lowered, the rolling efficiency is significantly reduced. Not only will it decline,
Separation occurs on the shear py impact fracture surface of the obtained steel plate, which is disliked by customers and makes it difficult to expand the range of applicable steel types. CR that improves the above problems caused by CR up to low temperature range
As a manufacturing method for achieving high tensile strength and high toughness without requiring the above, there is a method of performing accelerated cooling after rolling, as disclosed in JP-A-57-134514, for example. C: 0.10%,
After rolling steel containing 1.4% Mn and 0.03% Nb, accelerated cooling was performed by changing the cooling stop temperature and cooling rate.
Tensile strength TS, yield strength YS, fracture surface transition temperature (hereinafter
vTrs) and the results are shown in Figure 1. As is clear from Figure 1, as the cooling rate increases, the tensile strength easily increases, but on the other hand, the yield strength decreases as the cooling rate increases. There are very few types of steel that can be used as a substitute for return material. A possible method for improving the drawback of reduced yield strength due to accelerated cooling is to apply light reduction after accelerated cooling. However, with this method, the cooling stop temperature is
Because it is less than 500℃, accelerated cooling time is longer,
Even cooling within the steel plate tends to occur, and furthermore, the steel plate tends to become distorted due to heat generation and differences in thermal expansion due to bainite and martensitic transformation, and productivity also decreases due to the long accelerated cooling time. be. The purpose of the present invention is to solve the above-mentioned problems of the prior art, and to produce a high-strength steel with less distortion in the steel plate and excellent weldability and low-temperature toughness without heat treatment, with improved productivity and low cost. is to provide. This object of the present invention is achieved by the following three inventions. The gist of the first invention is as follows. That is, it contains C: 0.005 to 0.15% Si: 0.1 to 0.5% Mn: 0.8 to 2.0% Nb: 0.005 to 0.1% Al: 0.005 to 0.1% S: 0.008% or less, and the balance is Fe and unavoidable impurities. heating a steel slab consisting of (Ar ₃ transformation point + 150
°C) to _Ar3 transformation point with a rolling reduction of at least 50%, and the hot-rolled sheet is immediately rolled at a temperature range of 2 to
A stage of accelerated cooling to 500°C or more at a cooling rate of 40°C/sec, and a step of cooling the cooling plate in a temperature range of 600 to 200°C.
A step of lightly rolling down at a rolling reduction rate of 0.5% or more and less than 20%;
and a step of air cooling or slow cooling the lightly rolled plate from a temperature of 200°C or higher. This is the manufacturing method. The second invention has the same basic components as the first invention,
Furthermore, it contains at least one selected from Cr, Ni, Mo, V, and Cu in a range of 0.5% or less for each of Cr, Ni, Mo, and Cu, and 0.01 to 0.10% for V, with the balance being Fe and Cu. A non-thermal high tensile steel with low distortion in the steel plate and excellent weldability and low-temperature toughness, characterized in that the steel plate containing unavoidable impurities is subjected to the same controlled rolling process as in the first invention. This is a manufacturing method. Furthermore, the third invention contains, in addition to the steel components having the same composition as the second invention, Ca or rare earth metals in the range of Ca: 0.002 to 0.010% and rare earth metals: 0.005 to 0.010%, with the balance being Fe and unavoidable impurities. A method for manufacturing a non-tempered high tensile strength steel with little distortion in the steel plate and excellent weldability and low-temperature toughness, characterized by comprising the same controlled rolling process as in the first invention for a steel slab made of It is. The inventors of the present invention have conducted various studies with the aim of reducing strain within the steel sheet. As a result, they have found that if the stop temperature after accelerated cooling is set to 500°C or higher, the cooling time is short, so there is less uneven cooling within the steel sheet, and furthermore, bainite is reduced. In particular, it has become clear that since no martensite is generated, the difference in heat generation and thermal expansion is reduced, which reduces the occurrence of strain within the steel sheet. However, as shown in FIG. 1, when the cooling stop temperature is set to 500° C. or higher, the fracture surface transition temperature improves, but there is a drawback that the amount of increase in tensile strength is small. Therefore, we investigated a method that would increase tensile strength and obtain high tensile strength even if accelerated cooling was applied immediately after CR and stopped at temperatures above 500°C. As a result, we found a method that would increase tensile strength and obtain high tensile strength even if accelerated cooling was stopped above 500°C. and then 600~
We have newly discovered that tensile strength increases significantly by applying light reduction in the range of 0.5% to less than 20% at a temperature of 200°C. Applying this light reduction has the advantage of increasing not only the tensile strength but also the yield strength, and also has the characteristic that separation does not occur on the Charpey impact fracture surface, but on the other hand, it is said that this light reduction deteriorates the toughness. It has become clear that this method has drawbacks and is difficult to apply to steel types that require low-temperature toughness. As a result of various investigations into methods for improving low-temperature toughness, the present inventors found that a limited amount of Nb was contained, CR was applied to form a solid solution state of Nb, and then immediately accelerated cooling was performed at a cooling stop temperature of 500°C or higher. The present invention was achieved based on the new finding that even if light reduction is applied thereafter, there is little deterioration in toughness and the tensile strength and yield strength increase. Next, the experiments that formed the basis of the present invention will be explained. The composition is shown in Table 1 in the Examples below.
The present invention steel containing Nb (○ mark, A1 steel) and the comparison steel not containing Nb (△ mark, B1 steel) were acceleratedly cooled to 600 °C and rolled at 500 °C with varying reduction ratios.
The relationship between the tensile strength, yield strength, and fracture surface transition temperature (vTrs) was investigated, and the results are shown in Figure 2. From Figure 2, it can be seen that the vTrs of Nb-containing steel can be significantly improved compared to Nb-free steel without adversely affecting tensile strength and yield strength. Furthermore, it is also effective to eliminate distortion of the steel sheet due to uneven cooling, which cannot be avoided by applying accelerated cooling after rolling, by light rolling after stopping accelerated cooling. In other words, by applying CR to Nb-containing steel and immediately accelerated cooling, yield strength and vTrs are improved,
Furthermore, the tensile strength can be increased by applying light reduction after cooling has stopped.
By appropriately combining accelerated cooling and light reduction, it is possible to produce low-carbon equivalent high-strength steel with low distortion in the steel plate and excellent weldability and low-temperature toughness without heat treatment, with high productivity and at low cost. can. Next, the reason for limiting the component composition of the present invention will be explained. C: 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 softened, while if it exceeds 0.15%, the toughness of the base metal will deteriorate and the weld will harden. Since the deterioration of cracking resistance becomes significant, C needs to be within the range of 0.005 to 0.15%. Si: 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 should be within the range of 0.1-0.5%. Mn: 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;
Since the toughness of HAZ deteriorates when the Mn exceeds
It must be within the range of 0.8 to 2.0%. Nb: Nb is effective in making ferrite grains finer, but
If it is less than 0.005%, 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 was limited to a range of 0.005 to 0.1%. Al: It is necessary to add at least 0.005% of Al as a solid solution for deoxidizing the steel.On the other hand, if the amount 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 0.005-0.8%. S: When S exceeds 0.008%, the absorbed energy in the direction perpendicular to rolling decreases significantly, so S is 0.008%.
% or less. The above are the basic components of the steel slab used in the present invention, but if necessary, limited amounts of Cr, Ni,
At least one selected from Mo, V, Cu, Ca, and rare earth metals can be added and contained,
By appropriately containing each of them, specific effects can be added as will be described later. The reasons for limiting these additive elements are as follows. Cr: Cr is added to ensure the strength of the base metal and joints of steel plates, but if it exceeds 0.5%, not only the toughness of the base metal but also the toughness of the weld will deteriorate, so it must be kept below 0.5%. . Ni: Ni improves the strength and toughness of the base material without adversely affecting the hardenability and toughness of HAZ.
If added in an amount exceeding 0.5%, manufacturing costs will increase, and it is not necessary to achieve the objects and effects of the present invention, so the content should be 0.5% or less. Mo: Mn makes the γ grains regular during rolling and also produces fine bainite, improving strength and toughness, but in order to achieve the purpose of this invention, 0.5% Mn
It is not necessary to add more than 0.5%, and any more will increase the manufacturing cost, so Mo is limited to 0.5% or less. V: 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
V is 0.01 as it will significantly deteriorate the toughness of HAZ.
Limit within the range of ~0.10%. Cu: 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. Ca: If Ca is less than 0.002%, it is insufficient to control the morphology of MnS and is not effective in improving the toughness of the steel plate in the direction perpendicular to rolling.On the other hand, if it exceeds 0.010%, the cleanliness of the steel deteriorates and causes internal defects. , Ca is 0.002~0.010
It was set within the range of %. Rare earth metals (hereinafter referred to as REM): If REM is less than 0.005%, it is insufficient for controlling the morphology of MnS and is not effective in improving the toughness of the steel plate in the direction perpendicular to the rolling direction.On the other hand, if it exceeds 0.010%, the cleanliness of the steel deteriorates. , since it is also disadvantageous in terms of arc welding,
REM should be within the range of 0.005-0.010%. Next, the reason for limiting the manufacturing conditions of the present invention will be explained. These manufacturing conditions of the present invention can be commonly applied to all of the first, second, and third inventions having different steel components. The reason why Nb is heated to a temperature at which at least 0.005% of Nb is dissolved in solid solution and then rolled at a reduction rate of at least 50% in the temperature range of (Ar ₃ + 150℃) to Ar ₃ is that the solid solution Nb
The starting temperature of the unrecrystallized γ region of unrecrystallized Nb is higher at Ar ₃ +150°C than that of unresolved Nb, which is Ar ₃ +70°C. Deformation bands can be effectively generated by rolling down in the crystal γ region, and both of these have the effect of promoting grain refinement of ferrite and increasing toughness, but if the solid solution Nb is less than 0.005%, the effect is not sufficient. Since it is not possible to obtain Nb at least 0.005%
It is necessary to heat it to a temperature that will form a solid solution. The reason why rolling is performed in the temperature range 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, the deformation zone that becomes the 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 must be within the range of (Ar ₃ +150°C) to _{Ar 3} .
If the rolling reduction in the above temperature range is less than 50%, the fracture surface transition temperature cannot be lowered to -40°C or less, so the rolling reduction in the above temperature range must be at least 50%. The reason why this rolled plate is immediately accelerated cooled to a temperature range of 500℃ or higher at a cooling rate of 2 to 40℃/sec is as follows.
The purpose is to suppress the growth of ferrite grains after the γ→α transformation, improve toughness, and transform the transformation region that becomes a pearlite structure into a bainite structure, thereby increasing the yield strength. If the cooling rate is less than sec, there is no effect of forming a bainite structure, while if it exceeds 40℃/sec, a lumpy bainite or martensite structure will be generated, significantly deteriorating the toughness, so the cooling rate should be within the range of 2 to 40℃/sec. There is a need. Also, the cooling stop temperature is
At temperatures below 500°C, a large amount of bainite and martensite structures are generated, resulting in a significant decrease in yield strength, and the longer cooling time causes uneven cooling, which tends to cause distortion within the steel sheet, which is the object of the present invention. Since it is not possible to obtain a steel plate with low distortion,
The cooling stop temperature was limited to 500°C or higher. The reason for performing light reduction with a reduction rate of 0.5% or more and less than 20% in the temperature range from below 600℃ to above 200℃ after cooling is stopped is mainly to increase the tensile strength, and when the temperature exceeds 600℃. Under light pressure in the temperature range, the increase in tensile strength is small, while at 200℃
The temperature range under light pressure was limited to 600 to 200°C because if light pressure was applied at a temperature lower than that, hydrogen defects would occur because hydrogen could not be removed sufficiently. As shown in Figure 2, if the rolling reduction rate of light rolling is less than 0.5%, there will be no effect of increasing the tensile strength, while if it is more than 20%, separation will occur on the shear peace impact fracture surface, so light rolling in the temperature range of 600 to 200°C will not be effective. The rolling reduction rate must be within the range of 0.5% or more and less than 20%. Furthermore, the reason why the lightly rolled plate is air-cooled or gradually cooled from a temperature of 200° C. or higher is to facilitate the removal of hydrogen and prevent hydrogen defects. Examples Test steel types whose compositions are shown in Table 1 were processed under the heating-rolling-cooling conditions shown in Table 2, and the mechanical properties of the steel sheets were investigated, and the results are also shown in Table 2. In Table 2, test materials No. 1 to 10 are manufactured by various heating-rolling-cooling methods of A1 steel slabs having the composition of the present invention, and No. 1 is manufactured by accelerated cooling after rolling. No. 2 was not subjected to light reduction rolling after accelerated cooling, and No. 10 had a tensile strength of 60 Kgf/mm because the heating temperature of the slab was low and Nb was not solidly dissolved. ² was not satisfied, and No. 3 had a reduction rate of less than 50% in the temperature range of (Ar ₃ +150℃) to Ar ₃ , so the fracture surface transition temperature was -40℃ or higher. No.7 has a cooling stop temperature of 500℃
Even if light reduction is applied, the strain in the steel plate is not completely removed. No. 8 has an annealing start temperature of 200
Since the temperature is less than ℃, cracks occur due to the contained _H2 , and No. 9 has Ar below ₃ points (γ + α)

【table】

【table】

【table】

[Table] Although separation occurred due to rolling in the two-phase region, Nos. 4, 5, and 6 were manufactured within the range of all the constituent requirements of the present invention, so they did not achieve the tensile strength of 60 Kgf/, which is the target of the present invention. mm ² or more, fracture surface transition temperature −40
All conditions below ℃ are satisfied. Although sample material No. 11 satisfies the limiting requirements of the present invention under manufacturing conditions, it does not contain Nb in its chemical composition, which is one of the other constituent requirements.
The fracture surface transition temperature is -40℃ or higher. Test materials No. 12 and 13 satisfy all the constituent requirements of the present invention, especially in terms of component composition, V, Cu,
It appropriately contains Ni, Mo, Ca, etc., and all of them satisfy the target characteristics of the present invention. Test material No. 14 shows the mechanical properties of comparison steel made by quenching and tempering, which is a conventional manufacturing method.
It can be seen that the carbon equivalent of the steel of the present invention is 0.06 to 0.07% lower than that of the comparative steel subjected to this quenching and tempering treatment. As is clear from the above examples, the present invention limits the ingredients, especially contains an appropriate amount of Nb, and
Heating to a temperature at which 0.005% or more solid solution occurs, (Ar ₃ +150
Perform controlled rolling of 50% or more in the temperature range of ℃) ~ A ₃ ,
Accelerated cooling to a temperature of 500℃ or higher, followed by
By applying a light reduction of 0.5% to less than 20% in a temperature range of 600 to 200°C, and then air cooling or slow cooling, a high-strength steel with little distortion in the steel plate and excellent weldability and low-temperature toughness is produced without heat refining. could be manufactured cheaply and stably.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the influence of accelerated cooling conditions after controlled rolling on tensile properties and Charpy impact properties. Figure 2 is a diagram showing the influence of accelerated cooling conditions after controlled rolling on tensile properties and Charpy impact properties.
The reduction rate of rolling performed at 500℃ is the tensile property,
FIG. 3 is a diagram showing the influence on the Charpey impact characteristics.

Claims (1)

[Claims] 1 Contains C: 0.005 to 0.15% Si: 0.1 to 0.5% Mn: 0.8 to 2.0% Nb: 0.005 to 0.1% Al: 0.005 to 0.08% S: 0.008% or less, and the balance heating a steel slab consisting of Fe and unavoidable impurities to a temperature at which 0.005% or more of Nb becomes _a solid solution;
150℃) to at least 50% in the temperature range of Ar ₃ transformation point
rolling at a rolling reduction ratio of 0.5% or more, immediately accelerating cooling the hot-rolled sheet to 500°C or higher at a cooling rate of 2 to 40°C/sec, and rolling the hot-rolled plate at a temperature range of 600 to 200°C to a temperature of 0.5% or more. Weldability with less distortion in the steel plate, characterized by comprising the steps of lightly rolling at a rolling reduction rate of less than 20%, and cooling or slowly cooling the lightly rolled plate from a temperature of 200°C or higher. and a method for producing non-thermal high tensile strength steel with excellent low-temperature toughness. 2 Contains C: 0.005-0.15% Si: 0.1-0.5% Mn: 0.8-2.0% Nb: 0.005-0.1% Al: 0.005-0.08% S: 0.008% or less, and further contains Cr, Ni, Mo, and V , Cu: Cr, Ni, Mo, Cu: 0.5% or less each, V: 0.01~0.10%, and the balance is Fe and unavoidable impurities. heating the heated steel piece to a temperature at which _0.005 % or _more of
% rolling, immediately accelerated cooling of the hot-rolled sheet to 500°C or higher at a cooling rate of 2 to 40°C/sec, and rolling the cooling plate at a reduction rate of 0.5% in the temperature range of 600 to 200°C. Welding with less distortion in the steel plate, characterized by comprising the steps of lightly rolling the plate at a rolling reduction rate of less than 20%, and air cooling or slowly cooling the lightly rolled plate from a temperature of 200°C or higher. A method for producing non-thermal high tensile strength steel with excellent toughness and low-temperature toughness. 3 Contains C: 0.005-0.15% Si: 0.1-0.5% Mn: 0.8-2.0% Nb: 0.005-0.1% Al: 0.005-0.08% S: 0.008% or less, and further contains Cr, Ni, Mo, and V Contains at least one selected from Cr, Ni, Mo, Cu: 0.5% or less, V: 0.01 to 0.10%, and further contains Ca or a rare earth metal from Ca: 0.002 to 0.10%. A step of heating a steel billet containing 0.010% rare earth metal in the range of 0.005 to 0.010% with the balance consisting of Fe and unavoidable impurities to a temperature at which 0.005% or more of Nb is dissolved in solid solution _; At least in the temperature range of transformation point + 150℃) to Ar ₃ transformation point
rolling at a reduction rate of 50%; immediately accelerated cooling of the hot-rolled sheet to 500°C or higher at a cooling rate of 2 to 40°C/sec; % or more and less than 20%, and a step of air-cooling or slowly cooling the light-reduced plate from a temperature of 200°C or higher, with less strain in the steel plate. A method for manufacturing non-thermal high tensile strength steel with excellent weldability and low-temperature toughness.