JP3975674B2 - Low alloy steel manufacturing method - Google Patents

Description

【００４１】
【表２】

【００４２】
【表３】

【００４３】
【表４】

【００４４】
【発明の効果】
本発明によれば、高強度とともに低温靱性が確保でき、しかもこれまでの圧延設備を使用することでその製造が可能となるなど、その実際上の効果は大きい。
【００４５】
このように本発明の鋼材は、溶接性と安定した低温靭性を有し、且つ飛来塩分量が多い環境下においても充分な耐大気腐食性を発揮することから、溶接構造物におけるメンテナンスコストを大幅に減少することのできる優れた経済性を発揮でき工業上大変有用なものである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a low alloy steel. Specifically, the present invention relates to a method for producing a high-strength low-alloy steel material suitable as a steel material for welded structures such as offshore structures, bridges, shipbuilding, large industrial machines and the like.
[0002]
[Prior art]
In recent years, from the viewpoints of economy and safety, the strength of welded structures such as offshore structures, bridges, shipbuilding, large industrial machinery, etc. has been increasing, and the required characteristics for steel materials used in these fields have increased. On the other hand. Steel materials used for these applications are potentially required to have weldability and high toughness in terms of characteristics, and economically in terms of applications.
[0003]
Generally, in order to improve the toughness of a steel material, lowering C is effective, and in order to compensate for strength reduction due to lowering C, various alloy element additions and higher strength from the process side are attempted. For example, ASTM A710 and U.S. Pat. No. 3692514 disclose steel materials using Cu precipitation strengthening. These steel materials are characterized by excellent weldability, but it is difficult to say that they have sufficient low temperature toughness.
[0004]
As a technique for improving the low temperature toughness, pressurized El way pressure between cold or warm after normal hot-rolling in Japanese Patent No. 2611565 Patent or the second 2,690,578 is disclosed. However, these techniques apply cold or warm reduction in a thick plate mill that normally performs hot rolling, and have a large equipment load, and are not widely applicable techniques. In addition, from the economical aspect, it requires painting treatment every several years in order to minimize the inevitable corrosion problem in steel materials, which is a main cause of an increase in maintenance costs.
[0005]
[Problems to be solved by the invention]
A general problem of the present invention is to develop a high-strength steel material excellent in weldability, toughness, and corrosion resistance, particularly a steel material for welded structures.
[0006]
[Means for Solving the Problems]
As a result of various studies to solve the above-mentioned problems, the present inventors have achieved a weldability and stable toughness by requiring a steel composition in a specific range in which additive elements are regulated, and require a coating treatment or the like. The present invention was completed by knowing that it was possible to provide a high-strength, low-alloy steel material that was excellent in economy and suppressed corrosion to a practically sufficient level.
[0007]
The gist of the present invention is as follows.
(1) In mass%, C: 0.02 to 0.1%, Mn: 0.50 to 2.5%, P: 0.025% or less, S: 0.01% or less, Cu: 0.50% or more, Less than 2.5 %, Ni: 0.25 to 2%, Mo: 0.05 to 0.30%, Nb: 0.03% or less, Ti: 0.05% or less, Se: 0.005% or less, and Al : A steel slab containing 0.1% or less and having a steel composition composed of the remaining Fe and impurities is heated to a temperature range of 950 to 1250 ° C., hot-rolled, and then cooled to a temperature range of 580 ° C. or less. And then reheating to 450 to 680 ° C. and cooling, a method for producing a low alloy steel material for welded structures ( excluding steel plates for line pipes ) .
[0008]
(2) By mass%, C: 0.02 to 0.1%, Mn: 0.50 to 2.5%, P: 0.025% or less, S: 0.01% or less, Cu: 0.50 % : Less than 2.5 %, Ni: 0.25 to 2%, Mo: 0.05 to 0.30%, Nb: 0.03% or less, Ti: 0.05% or less, Se: 0.005% or less , And Al: A steel slab containing 0.1% or less and having a steel composition consisting of the remainder Fe and impurities is heated to a temperature range of 950 to 1250 ° C. and hot-rolled, then 1 to 50 ° C. / A method for producing a low alloy steel material for welded structures ( excluding steel plates for line pipes ) , characterized by cooling to a temperature range of 580 ° C. or lower at a cooling rate of sec, and then reheating to 450 to 680 ° C. .
[0009]
(3) In the above (1) or (2), the steel composition further comprises Cr: 0.05 to 0.5%, Ca: 0.0005 to 0.005%, REM: 0.02% in mass%. The manufacturing method of the low alloy steel material for welded structures characterized by containing the following 1 type, or 2 or more types.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Next, the reason why the steel composition and production conditions are defined as described above in the present invention will be described in detail. In this specification, “%” means “mass%” unless otherwise specified.
[0011]
C: 0.02-0.1%
C is an element that contributes to an increase in strength, but if it is less than 0.02%, it is difficult to ensure the strength. On the other hand, if the addition amount is large, weldability and toughness are deteriorated. From the aspect of corrosion resistance, when Cr carbide is formed, it acts as a cathode and deteriorates weather resistance. Therefore, the C content is limited to 0.02% or more and 0.1% or less. Desirable C content is 0.03% or more and 0.08% or less from the viewpoint of economical efficiency and higher performance.
[0012]
Mn: 0.50 to 2.5%
Mn is an element necessary for ensuring the strength and toughness of steel, but if it is less than 0.50%, such an effect is small, while if it is added in excess of 2.5%, the weldability is deteriorated. Let Moreover, when added in a large amount, MnS is formed and becomes a starting point of corrosion, and weather resistance is deteriorated. Therefore, the Mn content is limited to 0.50% to 2.5%. The lower limit of the Mn content is preferably 0, 60%, more preferably 0.9%, while the upper limit is 1.8%.
[0013]
P: 0.025% or less, S: 0.01% or less Both P and S are elements that easily cause segregation during solidification of the steel, and the segregation causes the weld to become brittle and reduce toughness. For this reason, it is desirable to reduce the content of both P and S, but a significant reduction requires a corresponding processing cost. Moreover, although P is an element which improves a weather resistance, as above-mentioned, the negative factor in terms of weldability and toughness is large. Therefore, in the present invention, the P content is limited to 0.025% or less. Desirable P addition amount is 0.005% or more and 0.02% or less from the viewpoint of obtaining the effect in terms of weather resistance and not impairing the weldability. On the other hand, S becomes MnS, which is an A-based inclusion, and precipitates in the steel, and is stretched during rolling to lower toughness, and further becomes a starting point of corrosion and deteriorates weather resistance. Therefore, in the present invention, the S content is 0.01% or less.
[0014]
Cu: 0.50% or more and less than 2.5% Cu has an effect of improving the weather resistance by addition of 0.50% or more. Further, by utilizing the strengthening effect by precipitation strengthening after rolling and cooling, low C is realized, and weldability and low-temperature toughness, as well as fracture toughness are improved. In order to effectively obtain precipitation strengthening, the lower limit is limited to 0.50%. On the contrary, excessive addition causes a decrease in toughness, and the adverse effect of promoting the generation of Cu cracks during hot rolling of steel and the intergranular cracking of HAZ becomes remarkable. For this reason, the upper limit is limited to less than 2.5%. The desirable Cu content is 0.7% or more and 1.8% or less in view of economical efficiency and higher performance.
[0015]
Ni: 0.25% to 2%
Ni, without adversely affecting the substantially similarly to weldability and HAZ toughness and Cu, the strength of the base material, improves the toughness, very expensive made for economy as two percent of the added pressure in the structural steel Therefore, the addition amount is limited to 2% or less. In addition, when Ni is contained from the aspect of corrosion resistance, the X-ray amorphous rust or α-FeOOH becomes fine, and has the effect of suppressing the dissolution reaction of steel and also suppressing the permeation of chloride ions to some extent. To do. Therefore, a weathering effect can be obtained even in a high-flying salinity environment where corrosion resistance cannot be ensured with the weathering steel generally known as JlSG3114. However, when added in a large amount, this effect is saturated at about 6% and the economy is lost. In either case, the effect is small when the addition amount is less than 0.2%. Furthermore, Ni has a Cu crack suppressing effect when Cu is added. The amount of Ni added from the surface exhibiting this effect needs to be 1/2 or more of the amount of Cu. Desirable Ni content is 0.25% to 2% from the viewpoint of obtaining the above-described effects and obtaining excellent economic efficiency. More desirably, the upper limit of the Ni content is 1.5%, and the lower limit is 0.40%.
[0016]
Mo: 0.05% to 0.30 %
Mo is an element that increases the strength. Furthermore, it adsorbs to rust in the form of oxyacid ions, and is effective in improving weather resistance by suppressing the permeation of chloride ions, but the effect is small at an addition amount of less than 0.05%, exceeding 0.30 % The effect is saturated with the added amount. The Mo content is limited to 0.05% to 0.30 % from the viewpoint of obtaining both economic efficiency and effect .
[0017]
Nb: 0.03% or less Nb is an element effective for improving toughness by precipitation hardening and fine graining, but if it exceeds 0.03 %, the toughness of the welded portion deteriorates. Therefore, the amount of Nb addition is limited to 0.03 % or less from the viewpoint of economical efficiency and higher performance .
[0018]
Ti: 0.05% or less Ti is an element effective for preventing austenite grain coarsening, but if it exceeds 0.05%, toughness deteriorates. Therefore, the Ti addition amount is limited to 0.05% or less. Desirable Ti content is 0.03% or less from the viewpoint of economical efficiency and higher performance. In particular, Ti is preferably contained in an amount of 0.007% or more for improving the low-temperature toughness because it exhibits its effect.
[0019]
Se: 0.005% or less Se is an element effective for improving the cutting property of steel by addition, but it is intended by the present invention to form hard and brittle inclusions and reduce fracture toughness. it is not necessary to seek actively machinability improvement of steel, but does not perform an intentional added pressure of Se, the trace contaminants from ores and scrap to be used for steel making, about 0.007% or even at normal conditions May be included.
[0020]
In the case of the present invention, the presence of an excessive amount of Se causes serious deterioration of fracture toughness, so the content is limited to 0.005% or less. The smaller the Se content, the more desirable it is to obtain higher performance for low temperature toughness.
[0021]
Al: 0.1% or less Al is an element effective for deoxidation and prevention of austenite grain coarsening, but with respect to the deoxidation effect, Si and Mn, and with respect to the austenite grain coarsening prevention effect, Nb and Ti. It can be replaced by the addition of such trace elements, and there is a concern that the cleanliness in the steel is deteriorated when a large amount is added. Therefore, the Al content is limited to 0.1% or less. Desirable Al content is 0.06% or less from the viewpoint of economical efficiency and higher performance.
[0022]
Other Elements In the present invention, a limited amount of one or two of Cr, Ca and REM may be contained. Further, other than the elements further is limited, elements added generally in the production of steel, the V, B, etc. For example, the is not limited to be added in a range not impairing the effect of the present invention. In other words, if the addition amount is economically acceptable, the effect of the present invention is promoted and is not hindered, so that the addition amount is not limited.
[0023]
The addition amount of Cr, Ca, and REM and the reasons for limitation are as follows.
Cr: 0.05% to 0.5%
Cr is an element that increases the strength. Furthermore, although it is effective in improving the weather resistance, the effect is small when the addition amount is less than 0.05%, and when the addition amount exceeds 0.5%, sufficient weather resistance in a high flying salinity environment cannot be obtained. For this reason, the Cr content is limited to 0.05% to 0.5%. In consideration of both the material cost and the obtained performance improvement, the desirable Cr content is 0.1% to 0.3%.
[0024]
Ca: 0.0005% to 0.005%
Ca exists as an oxide or sulfide in steel, promotes fine precipitation and aggregation of rust particles, and contributes to improved weather resistance. Further, when forming sulfides, the formation of MnS is suppressed as a result of the decrease in S, so that adverse effects due to MnS are eliminated. On the other hand, excessive hydrogenation pressure is through the formation of excessive oxide or sulfide, causing cleanliness deterioration of toughness decreases and steel. For this reason, Ca content is limited as 0.0005%-0.005%.
[0025]
REM: 0.02% or less REM has the effect of improving weldability and can be added as necessary. However, the addition of a large amount degrades the cleanliness of the steel material, so 0.02% is made the upper limit.
[0026]
In addition, the present invention may contain inevitable impurities and elements that are generally added to steel to adjust strength, Ceq, Pcm, and the like.
(Hot Rolling) In the present invention, the steel slab thus manufactured is heated to a temperature range of 950 to 1250 ° C. to perform hot rolling. When the heating temperature is less than 950 ° C., Nb is not sufficiently dissolved in the matrix, so that recrystallization of austenite cannot be suppressed in the subsequent hot rolling, resulting in insufficient refinement of the structure. Toughness cannot be obtained. On the other hand, the heating temperature is coarsened austenite grains during pressure heat continuous casting slab exceeds 1250 ° C., the toughness of the entire base material not only the center of plate thickness is reduced. Therefore, in the present invention, the heating temperature of the continuously cast slab is 950 ° C to 1250 ° C. Preferably, the upper limit temperature is 1180 ° C and the lower limit temperature is 1000 ° C. Other hot rolling conditions may be those in normal steel production, but for example, the rolling end temperature is preferably in the range of 700 to 850 ° C.
[0027]
(cooling)
After finishing the hot rolling, it is allowed to cool or cooled to a temperature range of 580 ° C. or lower at a cooling rate of 1 to 50 ° C./sec as necessary. The cooling is preferably performed by air cooling. Also at this time, the cooling stop temperature may be 580 ° C. or lower, and is not particularly limited as long as it is limited.
[0028]
The term “as needed” as used herein is determined in terms of both the target steel thickness and low temperature toughness securing, for example, steel or thick steel that requires high toughness at lower temperatures. If the cooling rate during cooling is slow, it means that it is effective to perform accelerated cooling.
[0029]
In the case of forced cooling, a cooling start temperature corresponding to the rolling end temperature is selected, but usually cooling starts immediately after the end of rolling. However, in some cases, it may be desirable to start cooling at a lower temperature, and the cooling condition to the cooling start temperature at that time is not particularly limited.
[0030]
If the average cooling rate in this cooling is less than 1 ° C./sec, a bainite structure or the like with coarse carbides is likely to be generated, so that it is not possible to ensure a sufficient yield strength particularly at the center of the steel sheet. On the other hand, when the cooling rate exceeds 50 ° C./sec, the toughness of the surface layer may be lowered because baking is likely to occur near the surface layer portion of the steel sheet. Therefore, in the present invention, the average cooling rate up to a temperature range of 580 ° C. or less is limited to 1 ° C./sec or more and 50 ° C./sec or less. A preferred lower limit is 5 ° C./sec.
[0031]
Here, the “cooling rate” is the average cooling rate of the steel material.
If the cooling stop temperature in this cooling exceeds 580 ° C., the strength cannot be ensured because the formation of martensite or lower bainite becomes insufficient not only in the central part of the steel sheet but also in the surface layer part. Therefore, in the present invention, the cooling stop temperature is set to 580 ° C. or lower.
[0032]
(Heat treatment)
The steel that has been allowed to cool or accelerate is reheated to 450 to 680 ° C. and then cooled. The conditions for the cooling treatment at this time are not particularly limited. This heat treatment step is for efficiently and stably exerting precipitation strengthening of Cu, and is performed in a temperature range of 450 to 680 ° C. according to the intended strength and low temperature toughness. Here, the lower limit is limited to 450 ° C. The reason for this is that at a temperature lower than this, Cu precipitation is not sufficiently completed during the heat treatment, and then the base metal near the joint is welded by welding heat when processed into a structure or the like. This is because there is a concern that the characteristics may change significantly. Moreover, the upper limit is limited to 680 ° C., if it exceeds the upper limit, sufficient strength cannot be secured due to overaging.
[0033]
In addition, the reheating in this heat treatment process means reheating to a higher temperature than the cooling stop temperature in the cooling process preceding that. Further, when the cooling stop temperature and the reheating temperature are the same, it is only necessary to hold at that temperature.
[0034]
【Example】
Next, the effects of the present invention will be described more specifically with reference to examples.
The target performance in this example is YS ≧ 550 MPa, vTrs ≦ −60 ° C., but this does not suggest that the present invention is limited to this strength / toughness level, and strength / toughness as required. The level can be appropriately achieved by adjusting the steel composition and production conditions.
[0035]
73 types of steel plates having the steel compositions and production conditions shown in Tables 1 and 3 were produced. In the embodiment, the case where the “steel material” is a “steel plate” is taken as an example. However, the effect is not changed even in other forms such as a bar steel material, a steel bar material, and a steel pipe material.
[0036]
Example Nos. 1 to 51 in Table 1 are examples of the present invention, and Example Nos. 52 to 73 in Table 3 are comparative examples.
The results are shown in Table 2 and Table 4. In the table, “corrosion resistance” is the amount of reduction in plate thickness compared to JlSG3114 SMA570W steel, which is a comparative material, after horizontal exposure for 1.5 years in an atmospheric corrosion environment equivalent to an incoming salt content of 0.5 mg / dm ² / day. Is indicated by ○ mark as good.
[0037]
In addition, “surface properties” are indicated by ◯ marks when there is no harmful surface flaw as a product by visual inspection and penetrant flaw detection.
Evaluation of the "inner quality" is according to the JlSG0801, sensitivity was evaluated on the 24d B Gaité. “Weldability” was measured by the Vickers hardness test when welding was performed at a heat input of 0.7 kJ / mm by the Vickers hardness test.
[0038]
In either case, the defective ones are indicated by x.
As is apparent from the results in Table 2, the present invention examples (Example Nos. 1 to 51) show YS exceeding the target value and good low temperature toughness, and in any evaluation of corrosion resistance, surface properties, internal properties and weldability. It was good.
[0039]
On the other hand, the comparative examples (Example Nos. 52 to 73) shown in Table 4 cannot secure the target value of YS or vTrs because either the steel composition or the manufacturing conditions are out of the scope of the present invention, or the corrosion resistance, the surface The level intended by the present invention could not be exceeded in any of properties, internal properties and weldability.
[0040]
[Table 1]

[0041]
[Table 2]

[0042]
[Table 3]

[0043]
[Table 4]

[0044]
【The invention's effect】
According to the present invention, the practical effects are great, such as high strength and low temperature toughness can be secured, and the production can be achieved by using the conventional rolling equipment.
[0045]
As described above, the steel material of the present invention has weldability and stable low temperature toughness and exhibits sufficient atmospheric corrosion resistance even in an environment with a large amount of incoming salt, greatly increasing the maintenance cost of the welded structure. It is very useful industrially because it can exhibit excellent economic efficiency that can be reduced significantly.

Claims (3)

In mass%, C: 0.02 to 0.1%, Mn: 0.50 to 2.5%, P: 0.025% or less, S: 0.01% or less, Cu: 0.50% or more, Less than 2.5%, Ni: 0.25 to 2%, Mo: 0.05 to 0.30%, Nb: 0.03% or less, Ti: 0.05% or less, Se: 0.005% or less, and Al : A steel slab containing 0.1% or less and having a steel composition composed of the remaining Fe and impurities is heated to a temperature range of 950 to 1250 ° C., hot-rolled, and then cooled to a temperature range of 580 ° C. or less. And then reheating to 450 to 680 ° C. and cooling, a method for producing a low alloy steel material for welded structures ( excluding steel plates for line pipes ) . In mass%, C: 0.02 to 0.1%, Mn: 0.50 to 2.5%, P: 0.025% or less, S: 0.01% or less, Cu: 0.50% or more, Less than 2.5%, Ni: 0.25 to 2%, Mo: 0.05 to 0.30%, Nb: 0.03% or less, Ti: 0.05% or less, Se: 0.005% or less, and Al : A steel slab containing 0.1% or less and having a steel composition composed of the balance Fe and impurities is heated to a temperature range of 950 to 1250 ° C., hot-rolled, and then cooled at 1 to 50 ° C./sec. A method for producing a low alloy steel material for a welded structure ( excluding a steel plate for a line pipe ) , characterized by cooling at a speed to a temperature range of 580 ° C. or lower, then reheating to 450 to 680 ° C. and cooling. 3. The steel composition according to claim 1, wherein the steel composition further includes, in mass%, Cr: 0.05 to 0.5%, Ca: 0.0005 to 0.005%, REM: 0.02% or less. A method for producing a low-alloy steel material for welded structures, comprising at least a seed.