JPS6366368B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for efficiently manufacturing high-strength steel plates having high strength and excellent toughness at low cost. In recent years, the world's economic situation has shown signs of becoming more and more complex, and as a result, welded structures such as various buildings and machinery have become larger and can be used in a wider range of environments. Ta. In order to meet these demands, the demand for high-strength steel sheets is also increasing. By the way, these high-strength steel plates are naturally required to have excellent toughness, weldability, economic efficiency, etc., and the manufacturing method is as follows.
Controlled rolling methods and quenching and tempering methods are widely used to obtain products with the above-mentioned properties as fully as possible. However, the above-mentioned controlled rolling method has problems such as there is a limit to the improvement in steel sheet strength that can be obtained by it, and rolling takes too much time, leading to a decrease in productivity. However, since it requires reheating and quenching, it has had the disadvantage of lowering economic efficiency. For this reason, in recent years, direct quenching and tempering methods, in which the material is quenched and tempered immediately without cooling, have been implemented, which increases strength without reducing economic efficiency. It has started to attract a lot of attention because of its ability to For example, it is known that the effects of direct quenching and tempering are noticeable in structural alloy steel containing 0.005 to 0.016% Nb, and it is possible to achieve a significant improvement in strength compared to normal quenching and tempering. The heating temperature of the direct quenching method is higher than that of the normal quenching and tempering method, and therefore it is easier to dissolve carbonitrides such as Nb (C, N) into a solid solution, and the amount of Nb in the solid bath in the austenitic state can be reduced. can be increased,
Therefore, during tempering treatment after quenching, NbC or
This is because the amount of Nb (C, N) precipitated increases and the strength is improved. However, as a result of long-term experiments and studies conducted by the present inventors based on actual operations, it has been found that the above-mentioned direct quenching and tempering method not only cannot avoid the occurrence of strength unevenness in the resulting steel sheet, but also does not ensure the expected toughness. We have come to realize that there is a problem in that it cannot be done. In other words, when direct quenching is performed, since ordinary steel contains a relatively large amount of N, the steel is heated to a high temperature in order to fully dissolve the existing NbC and Nb (C, N). Then, AlN in the steel is also decomposed at the same time, and the amount of solid solution N increases. Therefore,
During rolling in a low temperature range or during direct quenching after rolling, precipitation of Nb (C, N) etc. occurs, reducing the amount of solid solute Nb.
The amount of NbC precipitated during tempering decreases, making it impossible to obtain a sufficient increase in strength.
When precipitation of (C, N), Al, etc. begins, this promotes the nucleation of ferrite and reduces hardenability, which reduces strength and toughness. It is necessary to supplement this by adding alloying elements such as Even by rolling, it is difficult to make the grains fine enough, so the crystal grains become coarser than when normal quenching and tempering is performed, which also causes disadvantages such as deterioration of toughness. It was released. The present inventors aimed to solve the above-mentioned problems found in the conventional direct quenching and tempering method, and to find a method for manufacturing high-strength steel sheets with excellent strength and toughness at low cost and with high efficiency. As a result of detailed research focusing on N in steel, which is thought to be the cause of the need for high-temperature heating, we found that (a) N in steel combines with Al to form AlN, but there is too much AlN; This precipitates during rolling or before quenching after rolling and promotes nucleation of ferrite as described above, leading to deterioration of toughness. Therefore, if the amount of N is lowered, during rolling,
(b) However, because AlN does not precipitate between the end of rolling and quenching, and AlN precipitates only during tempering, strength can be improved without significantly deteriorating toughness. Lowering the austenite grains causes the austenite grains to become coarser, which hinders the improvement of toughness, but when a small amount of Ti is added to the steel, this creates fine TiN, which prevents the austenite grains from becoming coarser during high-temperature heating. and greatly improves the toughness of steel. However, in this case, the amount of Ti added is 0.01%, which is the same as in conventional general Ti-added steel.
There are times when it exceeds. Because it is a low-N steel, excess Ti precipitates as TiC during tempering and significantly deteriorates the toughness of the steel material, so it is important to limit the amount of Ti added to a particularly small amount; In addition, when a steel with low N content and a small amount of Ti added with an appropriate amount of Nb is hot rolled, directly quenched and tempered, Nb (C, N) etc. are added in addition to AlN in the steel structure. In this case, NbC and Nb(C,N) are precipitated by Ti(C,N).
Since it forms a solid solution in steel at a lower temperature than N), if the heating temperature is appropriately selected, only a small amount of TiN will precipitate and prevent the coarsening of austenite grains.
Only during tempering does NbC and Nb (C, N) precipitate, which further improves the strength of the steel material. (d) Generally speaking, if a low N steel containing a small amount of Ti and an appropriate amount of Nb is hot rolled and then directly quenched and tempered, the austenite grains during heating will be reduced due to the addition of Ti. In addition to preventing coarsening of the austenite, hot rolling also refines the austenite grains, and furthermore, when rolling is finished above a certain temperature, austenite recrystallization is promoted and deterioration of hardenability due to processing strain is also prevented. In addition, NbC and Nb (C, N) during tempering
We have come to the knowledge shown in (a) to (d) above that the strength is further improved by the precipitation of , and a high tensile strength steel plate with an extremely excellent balance of strength and toughness can be obtained. Next, in the method of the present invention, the reason why the composition of the target steel and other steel sheet manufacturing conditions are numerically limited as described above will be explained. (A) Amount of chemical components C The C component has the effect of ensuring the hardenability and strength of the steel plate, but if its content is less than 0.03%, the desired effect cannot be obtained; If the content exceeds 0.03 to 0.03, weldability will be deteriorated.
It was set at 0.20%. Si The Si component has a deoxidizing effect and a strength-enhancing effect on steel, but if its content is less than 0.03%, the desired effect cannot be obtained, and on the other hand, if the content is less than 0.50%, the desired effect cannot be obtained.
Since the toughness deteriorates if the content exceeds 0.03% to 0.50%. Mn The Mn component has the effect of improving the hardenability of copper, and in order to ensure the desired hardenability, it is necessary to add 0.60% or more. on the other hand,
If the content exceeds 1.80%, the weldability and toughness of the steel will deteriorate, so the content was set at 0.60 to 1.80%. The purpose of adding a small amount of Ti component is to form TiN, but TiN does not dissolve easily even when heated to about 1250℃, and it is necessary to nail the austenite grain boundaries and prevent coarsening of the austenite grains. It has a preventive effect. However, if the content is less than 0.003%, the desired effect cannot be obtained, while if the content exceeds 0.010%, solid solution Ti increases and precipitates as TiC during tempering, significantly reducing toughness. In order to degrade its content,
It was set at 0.003-0.010%. Nb The Nb component has the effect of increasing the hardenability during direct hardening and improving the strength of the steel plate through secondary precipitation during tempering, but if its content is less than 0.01%, the desired effect is not obtained. On the other hand, if the content exceeds 0.10%, the toughness will be significantly deteriorated, so the content was set at 0.01 to 0.10%. sol.Al The sol.Al component is an important component in direct quenching and tempering, rather than refining austenite grains, it fixes solid solution N present in the steel as AlN and improves toughness. However, if the content is less than 0.002%, satisfactory effects cannot be obtained;
If the content exceeds 0.100%, it will deteriorate the toughness of the steel, so the content should be controlled.
It was set at 0.002-0.100%. N The N component has the important effect of preventing coarsening of austenite crystal grains by combining with Ti and producing TiN, and also
It also has the effect of combining with Al and Nb to improve the strength of the steel plate after tempering, but the content is
If the content is less than 0.0020%, the desired effect cannot be obtained; on the other hand, if the content exceeds 0.0040%, Nb (C, N) and AlN are etc. will precipitate, reducing the hardenability of the steel and deteriorating its strength and toughness.
When Nb (C, N) precipitates, the amount of Nb in the solid bath decreases, reducing the amount of NbC precipitated during tempering and further lowering the strength, so the content was determined to be 0.0020 to 0.0040%. Cr The Cr component has the effect of ensuring the hardenability and strength of steel, so it is included as necessary. However, if its content is less than 0.05%, the desired effect cannot be obtained, and on the other hand,
If the content exceeds 0.75%, the toughness will deteriorate, so the content should be reduced to 0.05% or more.
It was set at 0.75%. Ni The Ni component has the effect of ensuring the hardenability of the steel and particularly improving the low-temperature toughness, so it is added as necessary, but its content is
If the content is less than 0.05%, the desired effect cannot be obtained, while if the content exceeds 1.50%, it will be economically disadvantageous, so the content should be reduced to 0.05%.
~1.50%. Mo Mo component increases the hardenability of steel,
It is added as necessary because it has the effect of increasing resistance to temper softening, but if the content is less than 0.05%, the desired effect cannot be obtained; on the other hand, if the content is less than 0.50%, it is added. This will significantly deteriorate weldability.
Its content was set at 0.05-0.50%. V Since the V component has the effect of increasing the strength of steel, it is added when a higher strength steel plate is required, but if its content is less than 0.01%, the desired effect of the above effect is not achieved. However, if the content exceeds 0.10%, the toughness deteriorates, so the content was set at 0.01 to 0.10%. B Component B has the effect of greatly improving the hardenability of steel when added in small amounts, so it is added when it is necessary to further improve the strength and toughness of steel sheets, but its content is If it exceeds 0.0020%, coarse M ₂₃ (C, B) ₆ is produced and the toughness of the steel deteriorates, so the content was limited to 0.0020% or less. In addition, in this invention, the amount of N is
Since B is limited to 0.0020 to 0.0040%, it is desirable that the amount of B added be 0.0010% or less. Cu The Cu component has the effect of improving the hardenability and strength of steel, but if the content is less than 0.05%, the desired effect cannot be obtained, whereas if the content exceeds 0.50%, the desired effect cannot be obtained. Since it begins to show hot brittleness, its content must be reduced.
It was set at 0.05-0.50%. (B) Heating temperature During hot rolling, if the heating temperature is less than 1050°C, poorly soluble carbides such as NbC cannot be sufficiently dissolved in solid solution, while if the heating temperature exceeds 1250°C, TiN will become coarse. Since the growth of austenite grains could no longer be suppressed, the heating temperature was set at 1050 to 1250°C. (C) Finishing rolling temperature When the finishing rolling temperature is less than 900°C, recrystallization of worked austenite is suppressed and working strain remains. If processing strain remains, the formation of ferrite or bainite is promoted and the hardenability is reduced, resulting in deterioration of the strength and toughness of the steel sheet. Therefore,
Promote recrystallization of processed austenite,
To prevent residual machining distortion, the rolling finishing temperature was set at 900°C.
℃ or higher. (D) Quenching temperature after rolling After rolling, quenching is performed at a temperature above the Ar ₃ transformation point.

【table】

[Table] (Note) *mark indicates that it is outside the scope of the present invention.

【table】

【table】

[Table] *mark indicates that the conditions of the present invention are not met.
The reason for this is to quench immediately after rolling to amplify the quenching effect and obtain sufficient strength and toughness, and this is because the quenching effect cannot be obtained below the above temperature. (E) Tempering Treatment The reason for performing tempering treatment below the Ac ₁ transformation point is to precipitate NbC etc. into the steel sheet structure and further improve its strength and toughness. Next, the present invention will be explained by examples and in comparison with comparative examples. Example First, steel slabs A to U having the composition shown in Table 1 and having a thickness of 100 mm were manufactured by a conventional method.
Next, this was heated to 1150℃ and hot rolled to form a steel plate with a thickness of 30mm at a finishing temperature of 930℃.
After directly water quenching from that temperature, further
Tempering was performed at 630°C. On the other hand, apart from this, steel types A~ shown in Table 1
After heating a 100mm thick steel slab to 1150℃, hot rolling it into a 30mm thick steel plate at a finishing temperature of 830℃, directly water quenching it at that temperature, and then further heating it at 630℃. We also prepared steel plates that had been subjected to conventional low-temperature finish rolling called tempering. Then, from the center of the thickness of each of these steel plates,
JIS No. 4 2mm V notched pea test piece and 8.5
Round bar tensile test pieces with mmφ and parallel length: 50 mm were taken in the rolling direction, and their mechanical properties were investigated. The results are shown in Table 2 along with the processing conditions. From the results shown in Table 2, it is clear that if steel having a composition within the range of the present invention is directly quenched and tempered, a steel plate with high strength and good toughness can be produced. It can be seen that the steel plate at a lower temperature of 830°C has inferior strength and toughness compared to the steel plate obtained by the method of the present invention, and cannot achieve a good balance of strength and toughness. Furthermore, even if comparative steel (conventional steel) is directly quenched and tempered under the treatment conditions of the present invention, the resulting steel plate will not have good toughness, and it may still be impossible to obtain a steel plate with a satisfactory balance of strength and toughness. It's obvious. As mentioned above, this invention involves manufacturing high-strength steel sheets by controlling the amount of N contained in Nb-containing high-strength steel and adding a small amount of Ti, followed by rolling under specific conditions and direct quenching and tempering treatment. According to the above, products with excellent strength and toughness and low susceptibility to weld cracking can be obtained through a simple process without using large amounts of expensive additive alloying elements, and can be used for bridges, pressure vessels, pen stocks, various tanks, etc. This brings about industrially useful effects such as increasing the safety of welded structures and making it possible to achieve lower prices.

Claims (1)

[Claims] 1 C: 0.03-0.20%, Si: 0.03-0.50%, Mn: 0.60-1.80%, Ti: 0.003-0.010%, Nb: 0.01-0.10%, sol.Al: 0.002-0.100% , N: 0.0020~0.0040%, Fe and unavoidable impurities: the remainder, steel with a composition (more than 1050% by weight) consisting of 1050~0.0040%.
After heating to a temperature range of 1250°C and then hot rolling at a temperature range of 900°C or higher to finish the steel plate to the specified dimensions, it is directly quenched at a temperature higher than the Ar ₃ transformation point, and then A method for producing a high-strength steel sheet with excellent strength and toughness, characterized by tempering at a temperature below the Ac ₁ transformation point. 2 C: 0.03-0.20%, Si: 0.03-0.50%, Mn: 0.60-1.80%, Ti: 0.003-0.010%, Nb: 0.01-0.10%, sol.Al: 0.002-0.100%, N: 0.0020-0.0040 %, and further contains Cr: 0.05-0.75%, Ni: 0.05-1.50%, Mo: 0.05-0.50%, V: 0.01-0.10%, B: 0.0020% or less, Cu: 0.05-0.50%, Containing one or more of the following, with the remainder being Fe and unavoidable impurities, steel with a composition (more than 1% by weight) consisting of 1050~
After heating to a temperature range of 1250°C and then hot rolling at a temperature range of 900°C or higher to finish the steel plate to the specified dimensions, it is directly quenched at a temperature higher than the Ar ₃ transformation point, and then A method for producing a high-strength steel sheet with excellent strength and toughness, characterized by tempering at a temperature below the Ac ₁ transformation point.