JPH07207335A - Manufacturing method of high strength steel with excellent weldability and large uniform elongation - Google Patents

Abstract

(57) [Summary] (Modified) [Purpose] High-strength steel sheet with excellent weldability and large uniform elongation, which is mainly used for steel structures in the fields of construction and civil engineering. [Structure] C: 0.04 to 0.10, Si:
0.6 or less, Mn: 0.8 to 1.5, P: 0.02 or less, S: 0.008 or less, Cu: 0.85 to 1.8, N
i: 0.3 to 2.0, Mo: 0.3 to 0.7, Nb:
0.005-0.05, V: 0.02-0.08, T
i: 0.005 to 0.03, Al: 0.10 or less, N:
0.01 or less, Pcm = C + Si / 30 + Mn / 20 + C
u / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V /
10 + 5 B is 0.28 or less, the balance is iron and inevitable impurities.
After heating to 000 ℃, rolling with a reduction ratio of 2 or more,
After finishing rolling at 650-800 ° C, 700-
A method for producing a high-strength steel excellent in weldability and having a large uniform elongation, which is reheated to 850 ° C., then air-cooled, then heated to 400 to 650 ° C. if necessary, and then air-cooled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing high-strength steel having a tensile strength of 70 kgf / mm ² or more, excellent weldability, and excellent plastic deformability (large uniform elongation). Is. The steel manufactured by this method can be used for steel structures such as construction and civil engineering.

[0002]

2. Description of the Related Art In recent years, in structural steel materials such as steel materials (steel plates, steel pipes, shaped steels, etc.) used for building structures, high tensile strength steels have been used because of the increase in the size of structures and the accompanying energy saving. The weight reduction of the structure by applying is progressing. Most of the conventional high-strength steels exceeding 60 kgf / mm ² class were manufactured by quenching and tempering B-added steel. However, the weldability of B-added high-strength steel is 60 kgf / mm.
It was significantly inferior to the grade ² steel. Therefore, during welding, preheating at about 200 ° C. (keeping the temperature of the steel sheet at a constant temperature during welding) is required to prevent weld cracking, resulting in a significant decrease in the working efficiency. To solve this problem, JP-A-2-129317 and JP-A-4-314825
No. B, No. 4-333516, B-free 80 kgf /
mm ² grade steel and the like are disclosed.

On the other hand, along with the application of the seismic design method to building structures, high tensile steel with a low yield ratio is required from the viewpoint of seismic resistance. More recently, particularly in high-strength steel, steel materials having a large uniform elongation have become extremely important from the viewpoint of plastic deformability. As a method for producing a high-strength steel having a low yield ratio, for example, JP-A-55-41927 or JP-A-55-97425 has been proposed. The former JP-A-55-41927 is a method utilizing a combination of controlled rolling and controlled cooling, and the latter JP-A-55-97.
No. 425 is of the so-called tempering (QT) type. In any case, a 60 kgf / mm ² class steel has a good low yield ratio, but a high yield steel of 70 kgf / mm ² or more cannot obtain a sufficiently low yield ratio (yield ratio less than 70%).
Also, the uniform elongation is extremely small at about 8 to 12%. With respect to this problem, some of the inventors of the present invention disclosed in Japanese Unexamined Patent Publication No. 4-31482.
Low yield ratio 70 kgf / mm ² with excellent weldability as shown in No. 4
The production method of high grade high strength steel is shown. However, in this case, although the yield ratio is low, the actual uniform elongation is small at about 8 to 12%. In any case, good weldability, low yield ratio of less than 80% and uniform elongation of more than 15% 70 kgf / mm ²
Until now, there is no technology for producing the above high-strength steel.

[0004]

SUMMARY OF THE INVENTION The object of the present invention is to solve the problems of the conventional method and to clarify the optimum composition and manufacturing conditions, so that the weldability is good and the uniform elongation is large. The present invention provides a method for producing high-strength steel (70 kgf / mm ² or more).

[0005]

SUMMARY OF THE INVENTION The gist of the present invention is the weight%
And C: 0.04 to 0.10%, Si: 0.6% or less,
Mn: 0.8-1.5%, P: 0.02% or less, S:
0.008% or less, Cu: 0.85-1.8%, Ni:
0.3-2.0%, Mo: 0.3-0.7%, Nb:
0.005-0.05%, V: 0.02-0.08%,
Ti: 0.005 to 0.03%, Al: 0.10% or less, N: 0.01% or less, and further C as necessary.
r: 0.05 to 0.4%, Ca: 0.0005 to 0.0
Contains 050% of one or two, Pcm = C + Si
/ 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr /
20 + Mo / 15 + V / 10 + 5B (%) is 0.28%
Hereinafter, after heating a steel slab substantially containing B, the balance of which is iron and unavoidable impurities, to a temperature range of 800 ° C. or higher and 1000 ° C. or lower, rolling is performed at a reduction ratio of 2 or more,
After finishing rolling in a temperature range of 650 ° C. or higher and 800 ° C. or lower, it is then reheated to 700 to 850 ° C., then air-cooled, and then heated to a temperature of 400 ° C. or higher and 650 ° C. or lower, and thereafter. It is air cooling.

[0006]

It is generally known that as strength increases, weldability deteriorates and the uniform elongation of steel material decreases. That is, the higher the strength, the worse the weldability and plastic deformability, so it was difficult to apply to steel structures such as construction and civil engineering. Therefore, the inventors have earnestly studied the optimum components and rolling conditions for producing a steel having excellent weldability and plastic deformability and having a strength of 70 kgf / mm ² or more, and arrived at the present invention. In the present invention, as the high-strength steel of 70 kgf / mm ² or more, the strength and toughness of the base material are secured, and HAZ
With a maximum hardness of 330 Hv or less, a low yield ratio of less than 80%,
The composition and rolling conditions as necessary conditions for achieving uniform elongation of 15% or more were clarified.

In the present invention, 1) the amount of C is reduced to ensure good weldability, and B is not added, and the Pcm value of the steel component is suppressed to 0.28% or less. 2) In order to secure the base material strength and uniform elongation, a steel containing a certain amount of Cu, Nb, V, Mo is heated to a low temperature range, and then appropriately rolled to obtain a fine ferrite-marten structure. Make it a site. 3) To obtain a steel sheet having high strength and large uniform elongation by reheating to an austenite-ferrite two-phase coexisting region to ensure uniform elongation and then air cooling.

The present invention will be described below. In order to secure good weldability, it is extremely important to add no B and reduce the amount of C. In addition to this, the total of the constituent elements is 0.28 in Pcm.
It must be less than or equal to%. Within this limited range, the hardening of the welding heat affected zone (HAZ) is small under normal welding conditions, and preheating during welding can be reduced. Further, in order to secure the strength of the base material in such a low C and B-free state, first, the structure is made of ferrite-martensite.
It is necessary to form a phased organization.

By containing Mo and Nb, the austenite non-recrystallization effect is remarkably promoted, and even if the slab reheating temperature is lowered to refine the initial austenite grains, the austenite unrecrystallization and It is extremely effective in miniaturizing ferrite. Further, Mo is effective in forming fine martensite subsequent to the formation of fine ferrite during the transformation from the stretched unrecrystallized austenite, and making the structure into a two-phase structure of ferrite-martensite. The dislocation density inside this fine martensite is very high, and high strength can be easily achieved. A large uniform elongation can be secured by the generation of fine ferrite.

In order to produce these effects, M
The amount of o should be 0.3 to 0.7%. If the content is less than 0.3%, the effect is small, and the addition of 0.7% or more is not preferable for weldability, so the upper limit was made 0.7%. Further, the amount of Nb needs to be 0.005 to 0.05%. 0.005
%, The effect is small, and addition of 0.05% or more is not preferable for weldability, so the upper limit was made 0.05%.

Cu and V are known as precipitation hardening elements, but they are easy to form a solid solution even when the slab reheating temperature is low, and are effective for increasing strength. In order to produce this effect, the amount of Cu needs to be 0.85 to 1.8%. If it is less than 0.85%, the effect of increasing strength is small, and addition of 1.8% or more impairs HAZ toughness, so the upper limit was made 1.8%. Further, the V amount needs to be 0.02 to 0.08%. If it is less than 0.02%, the effect is small, and
Since the addition of 08% or more impairs the HAZ toughness, the upper limit is set to 0.
It was set to 08%.

Not only the amounts of Mo, Nb, Cu and V, but also the heating and rolling conditions are important. The reheating temperature of the slab needs to be 800 to 1000 ° C. This is because the initial austenite grains during heating are kept small and the rolling structure is refined. Furthermore, the smaller the initial austenite grains, the easier the two-phase organization of fine ferrite-martensite occurs. 1000 ° C. is an upper limit temperature at which austenite grains do not become coarse during heating. On the other hand, if the heating temperature is too low, the additional alloying elements are not sufficiently solutionized,
The desired material cannot be obtained. Further, since it is necessary to heat the slab for a long time in order to uniformly heat the slab, and further, the deformation resistance at the time of rolling increases, the energy cost increases, which is not preferable. Therefore, it is necessary to set the lower limit to 800 ° C.

However, even if the heating temperature is limited to a low value as described above, if the rolling conditions are not appropriate, a good material cannot be obtained. Therefore, it is necessary to carry out rolling at a reduction ratio of 2 or more. This is because the austenite grains are thoroughly refined and stretched by adding sufficient rolling in the non-recrystallization temperature range to low-temperature heating, and further, a two-phase structure of ferrite-martensite is achieved. If the reduction ratio is less than 2, sufficient ferrite-martensite two-phase organization is not achieved. Furthermore, the rolling end temperature needs to be 650 ° C. or higher and 800 ° C. or lower. This is because, when the rolling is completed at a temperature higher than 800 ° C., the structure is not refined and the two-phase structure is not sufficiently formed, and good strength, toughness and large uniform elongation cannot be obtained.

Cooling after rolling includes a method of air cooling after rolling and a method of accelerating cooling to a desired temperature of 550 ° C. or less at a cooling rate of 5 to 40 ° C./second and then air cooling after rolling. It is possible to select either from the viewpoints of plate thickness, strength level and cost. Especially when thick and high strength and high toughness are required, after rolling, cooling rate is 5 to 40 ° C /
It is desirable to perform accelerated cooling to an arbitrary temperature of 550 ° C. or less per second and then air cooling. The reason for setting the cooling rate to 5 to 40 ° C./sec is that a fine martensite structure is less likely to be generated at less than 5 ° C./sec and strength improvement cannot be expected, and if it exceeds 40 ° C./sec, it is coarse and large. This is because martensite of is generated and deteriorates ductility.

The cooling stop temperature is designated as an arbitrary temperature of 550 ° C. or lower because the dehydrogenation effect and sufficient precipitation hardening cannot be obtained if the cooling temperature is too low. In this case, it is desirable to stop cooling around 350 to 550 ° C. and air cool. However, if the cooling stop temperature exceeds 550 ° C, a sufficient increase in strength cannot be expected.

In order to obtain a larger uniform elongation, it is necessary to air-cool after heating to the two-phase coexisting region of austenite-ferrite. This is because the concentration of C from ferrite to austenite is promoted and ferrite having an extremely small amount of C is generated. However, the purpose cannot be achieved unless the reheating temperature is appropriate. If the heating temperature exceeds 850 ° C., the proportion of ferrite phase decreases, and the effect of improving uniform elongation cannot be expected. Again 70
When the heating temperature is lower than 0 ° C., the concentration of C from ferrite to austenite is not promoted, and the effect of improving uniform elongation cannot be expected.

Thereafter, if necessary, a tempering treatment is continued at a temperature of 400 ° C. or higher and 650 ° C. or lower. This is for decomposing martensite and improving the toughness as stable carbide. If the temperature is less than 400 ° C, tempering is insufficient and the temperature is 650 ° C.
If it exceeds the limit, the strength will be reduced. Therefore, the tempering temperature should be 40
The temperature was set to 0 ° C or higher and 650 ° C or lower. In the present invention, the thick steel plate means a steel plate having a plate thickness of 6 mm or more.

Next, the reasons for limiting the other components will be described. It is necessary to add 0.04% or more of C in order to obtain the required tensile strength. However, excessive addition of C causes deterioration of weldability, so the upper limit is 0.10%.
And Si is an element contained in deoxidized upper steel, but excessive addition thereof impairs weldability and weld heat affected zone (HAZ) toughness. Therefore, it is necessary to set the upper limit to 0.6% or less. Mn is an element useful for ensuring strength, toughness, and hardenability, and it is necessary to add 0.8% or more. However, if the amount of Mn is too large, weldability and HAZ toughness are deteriorated, so the upper limit is made 1.5%.

The reason why the impurities P and S in the steel of the present invention are 0.02% and 0.008% or less, respectively, is to improve the low temperature toughness of the base material and the welded portion. Reduction of P prevents grain boundary fracture, and reduction of S content prevents deterioration of toughness due to MnS. Ni has little adverse effect on weldability, improves strength and toughness, and is also effective for preventing Cu cracks. However, if it exceeds 2%, the weldability is not preferable, so the upper limit is made 2.0%. If less than 0.3%,
Since the effect is small, the lower limit is made 0.3%. Ti suppresses coarsening of austenite grains during welding and is useful for securing HAZ toughness. However, addition of less than 0.005% has no effect, and addition of 0.03% or more causes deterioration of HAZ toughness due to precipitation hardening of TiC, so the addition amount is 0.005 to 0.03%. And

Al is generally an element contained in deoxidized upper steel, but since deoxidation is also performed by Si and Mn or Ti, the lower limit of Al is not limited in the present invention. However, if the amount of Al increases, the cleanliness of the steel deteriorates and the HAZ toughness deteriorates, so the upper limit is made 0.1%.
Although N is generally contained in steel as an unavoidable impurity,
The excessive addition of causes deterioration of HAZ toughness, so the upper limit is made 0.01%. In the steel of the present invention, if necessary, Cr: 0.05-0.4%, Ca: 0.0005-
Any one or two of 0.0050% is contained. The main purpose of containing these elements is to enable improvement of strength, toughness and expansion of production plate thickness without impairing the characteristics of the steel of the present invention, and the addition amount thereof is in terms of weldability and HAZ toughness. It is of a nature that should be naturally limited. Cr is an element that enhances the strength of the base material and needs to be added in an amount of 0.05% or more. However, Cr
If the amount exceeds 0.4%, the weldability and HAZ toughness deteriorate, so the upper limit is made 0.4%. Ca controls the morphology of sulfides, increases Charpy absorbed energy to improve low temperature toughness, and is also effective in improving hydrogen-induced cracking resistance. However, if the amount of Ca is 0.0005% or less, there is no practical effect, and if it exceeds 0.005%,
O and CaS are generated in large amounts to form large inclusions, which not only impairs the toughness of steel but also the cleanliness and also adversely affects the weldability, so the range of Ca addition amount is 0.0005-0.0.
050%.

[0021]

[Examples] Table 1 shows the chemical composition, manufacturing conditions and mechanical properties of the test steel. Steel sheets with various thicknesses were manufactured, and their mechanical properties and weldability (HAZ maximum hardness test) were investigated.

[0022]

[Table 1]

[0023]

[Table 2]

In Table 1, Steels 1 to 5 are steels of the present invention, Steel 6
-20 shows comparative steel. The invention steels 1 to 5 are 70 kgf / mm ²
It has the above tensile strength, the HAZ maximum hardness of 330 Hv or less, a low yield ratio of less than 80%, and a uniform elongation of 15% or more. On the other hand, Comparative Steel 6 does not have a fine ferrite-martensite two-phase structure because the amount of Mo is small, and the strength and uniform elongation are reduced. Comparative Steel 7 is a ferrite with a fine structure because the amount of Nb is small.
The two-phase structure of martensite is not formed, and the strength and uniform elongation are reduced. Comparative Steel 8 has a small amount of Cu and has a reduced strength.

The comparative steel 9 has a small amount of V and has a reduced strength. Since the comparative steel 10 has B added thereto, the maximum hardness is high and the weldability is deteriorated. Since the comparative steel 11 has a high Pcm, the maximum hardness is high and the weldability is deteriorated. Since the comparative steel 12 has a low heating temperature, the alloying elements that contribute to precipitation strengthening are not sufficiently solution-treated and the strength is lowered.
Comparative Steel 13 had a large initial austenite grain due to the high heating temperature, and had a fine ferrite-martensite content of 2
Phase organization does not occur, and strength and uniform elongation are reduced. Since the comparative steel 14 has a small reduction ratio, a sufficient ferrite-martensite two-phase texture cannot be obtained, and the strength and the uniform elongation are lowered.

Comparative steel 15 has a low rolling end temperature, so that the amount of ferrite processed is large and the low temperature toughness is deteriorated. Since Comparative Steel 16 has a high rolling finish temperature, the structure is not refined and the two-phase structure is not sufficiently formed, and good strength, low temperature toughness and large uniform elongation cannot be obtained. Since the comparative steel 17 has a low heating temperature after rolling, the proportion of the ferrite phase is small and the uniform elongation is low. In Comparative Steel 18, since the heating temperature after rolling is high, the concentration of C from ferrite to austenite is not promoted, and the uniform elongation is reduced. Comparative Steel 19 has a low tempering temperature, so that the tempering is insufficient and the low temperature toughness is reduced. Since the comparative steel 20 has a high tempering temperature, its strength is lowered.

[0027]

The present invention provides means for producing an epoch-making high-strength steel having good weldability, a low yield ratio, a large uniform elongation, and a high strength of 70 kgf / mm ² or more. Therefore, it is possible to reduce the weight of buildings and civil engineering structures and to ensure the safety against earthquakes.

Claims (3)

[Claims] 1. By weight%, C: 0.04 to 0.10%, Si: 0.6% or less, Mn: 0.8 to 1.5%, P: 0.02% or less, S: 0 0.008% or less, Cu: 0.85-1.8%, Ni: 0.3-2.0%, Mo: 0.3-0.7%, Nb: 0.005-0.05%, V : 0.02-0.08%, Ti: 0.005-0.03%, Al: 0.10% or less, N: 0.01% or less, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
(%) Is 0.28% or less, and the balance is a steel slab containing substantially iron and unavoidable impurities, which is substantially free of B, and is heated to a temperature range of 800 ° C. to 1000 ° C., and then rolled at a reduction ratio of 2 or more. 650
After finishing rolling in the temperature range of ℃ to 800 ℃, reheat to 700 to 850 ℃, then air-cool, then heat to 400 ℃ to 650 ℃ or less, if necessary, and then air-cool. A method for producing high-strength steel with excellent weldability and high uniform elongation. 2. The method for producing a high-strength steel excellent in weldability and having a large uniform elongation, which contains Cr: 0.05 to 0.4% by weight. 3. Ca: 0.0005 to 0.0 by weight.
The method for producing a high-strength steel having a high uniform elongation excellent in weldability according to claim 1 or 2, which contains 050%.