JPS62256947A - Tempered high-phosphorus-type weather-resisting steel plate excellent in weldability and toughness at low temperature - Google Patents

Abstract

PURPOSE:To develop a tempered high-phosphorus-type weather-resisting steel plate capable of hardening and tempering treatments and excellent in weldability and toughness at low temp., by adding trace amounts of B to a P-Cu-Cr steel plate as a welding construction material and by controlling a hardened structure. CONSTITUTION:As the welding construction material for use in atmospheric corrosion environment, such as bridges, etc., a steel having a composition which contains, by weight, 0.02-0.10% C, <0.75% Si, 0.5-1.5% Mn, 0.04-0.15% P, 0.15-0.60% Cu, 0.1-1.0% Cr, and 0.0003-0.0020% B or further contains specific trace amounts of Ce and/or Ca or 1 or >=2 elements among specific trace amounts of Ti, Zr, Ni, Mo, Nb, and V independently or in combination and in which carbon equivalent represented by (C+1/24Si+1/6Mn+1/40Ni+1/5Cr+1/4Mo+1/14V)% is regulated to <=0.40% is subjected to hardening so that it has a structure in which the amount of martensite + bainite is regulated to >=90%, followed by temperating. In this way, a tempered 50-60kgf/mm<2> class weather-resisting steel plate improved in weldability and toughness at low temp. can be obtained.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention is a steel plate with excellent weldability and low-temperature toughness that is suitable for use as a welding structural material in general atmospheric corrosive environments such as bridges, etc. The present invention relates to a high phosphorus type weathering steel sheet manufactured by off-line quenching and tempering treatment. (Prior Art) Conventionally, weathering steel sheets have a P content of 0.
It is already well known that P-Cu-Cr steel sheets containing 0.07 to 0.15% have excellent weather resistance against corrosion in the atmosphere. However, since this steel sheet contains a large amount of P as mentioned above, when subjected to tempering heat treatment such as normal quenching and tempering, tempering embrittlement appears markedly and low-temperature toughness is impaired. There's a problem. Therefore, a method has been adopted for this type of steel plate to ensure a predetermined strength as rolled without heat treatment. (Problem to be solved by the invention) However, in order to increase the strength of steel as rolled,
It is necessary to increase the amount of alloying elements in the steel, but since increasing the amount of alloying elements deteriorates the weldability of the steel or the low-temperature toughness of the welded joint, there is a limit to the amount that can be added. Therefore, in consideration of weldability and low-temperature toughness, high-phosphorus weathering steel sheets have conventionally been produced with a tensile strength of up to 50 kg. On the other hand, the tensile strength required for this type of weathering steel sheet has recently reached 60 kg, and customers are demanding high phosphorus type 60 kg weathering steel sheet with excellent weldability and low-temperature toughness. There is something strong about it. The present invention has been made in response to such demands, and solves the problem of temper embrittlement of high phosphorus weathering steel sheets by utilizing a trace amount of B and quenching without adding expensive alloying elements. To provide tempered 50 kg class and 60 kg class high phosphorous type weathering steel sheets with excellent weldability and low-temperature toughness that can be solved by microstructural control and can be manufactured by performing quenching and tempering treatments that have not been performed in the past. The purpose is to (Means for solving the problem) As mentioned above, when conventional high-phosphorus weathering steel is quenched and tempered, it becomes brittle compared to as-rolled material, but this embrittlement is due to so-called tempering embrittlement. It is something. One of the causes of temper embrittlement is said to be that impurity elements such as P segregate at prior γ grain boundaries when heated to a temperature below the AC working point, making the grain boundaries brittle and inducing intergranular fracture. It is being said. Therefore, the present inventors focused on the segregation of B to the γ grain boundaries,
Conventional quenching and tempering treatments were performed on high-phosphorus weathering steel with and without B added, and the low-temperature toughness was investigated. In addition, the so-called 0.07%C-0.08%P-0
．． Hardening (930°C) and tempering (640°C) were performed using 3% Cu-0.4% Cr as the basic component and varying the amount of B added thereto. As a result, as shown in Figure 1, the B-added product was
Fracture surface transition temperature (vTrs) compared to that without B addition
It has become clear that the temperature has shifted to the lower temperature side of 40° C., indicating excellent low-temperature toughness. The reason for this is that when heated to a temperature of 900℃ or higher, B
This is thought to be due to the fact that P segregates at the γ grain boundaries before P, lowering the grain boundary energy, and suppressing the segregation of P at the γ grain boundaries, so that the grain boundaries do not become brittle. On the other hand, the research conducted by the present inventors has revealed that the low-temperature toughness of a material obtained by quenching and tempering B-containing high-phosphorus weathering steel corresponds well to the quenched structure. That is,
As shown in Figure 2, when the amount of martensite + bainite in the structure of the as-quenched material is 90% or more, the transition temperature (vTrs) of the steel plate after tempering is -60°C or less, and it has good low-temperature toughness. have. The causes of this have not yet been fully elucidated, but are thought to be as follows. ■ As is clear from the Fe-P phase diagram shown in Figure 3,
When the heating temperature is 930°C, P content: to. The structure of 10% steel is a two-phase structure consisting of γ+α, as shown in the figure. In this case, the degree of pa in α is significantly higher than that in γ. ■ The toughness of α, which has a high P concentration, is expected to be lower than that of other structures with a low P concentration, and this becomes the starting point for fracture, and if the amount exceeds 10%, the low temperature toughness decreases rapidly. . In this way, by adding an appropriate amount of B to high-phosphorous weathering steel that is prone to temper embrittlement and further controlling the quenched structure, it is possible to achieve a good transition temperature (vTrs) of 60°C or less. It was discovered that a tempered high phosphorus type weathering steel sheet having low temperature toughness and excellent weldability can be obtained, and based on this knowledge, further experimental research was carried out, and the present invention was hereby made. . That is, the tempered high phosphorus type weathering steel sheet having excellent weldability and low-temperature toughness according to the present invention has C: 0.02 to 0.10%, S
L: 0.75% or less, Mn: 0.5-1.5%, P: 0
．． 04-0.15%, Cu:Q, 15-0.6%, C
r: 0.1-1.0% and B: 0.0OO3-0.00
20%, and further contains Ce:0. OOl
~0. One or two of L% and Ca: 0.004% or less, and L or Ti: 0.002 to 0.05
%, Zr: 0. OO2~0.05%, Ni:0.05~
1.0%, Mo: 0.05-0.5%, V: 0.01-
0.15% and Nb: 1 of 0.01-0.15%
Contains one species or two or more species. And carbon equivalent Ceq (=C+1/24Si+1/6
Mn+1/40Ni+115Cr+1/4Mo+1/1
4V) Hardened and tempered steel with (%) of 0.40% or less, the balance being iron and unavoidable impurities, and made by tempering a steel in which the amount of martensite + bainite in the quenched structure is 90% or more. It is characterized by: The present invention will be explained in detail below based on examples. First, the reason for limiting the components of the high phosphorus weathering steel sheet according to the present invention will be explained. It is desirable that C be as small as possible from the standpoint of both weldability and low-temperature toughness.In order to achieve the object of the present invention, the upper limit should be 0.10%. However, C
If the content is less than 0.02%, the strength of the base material cannot be ensured, so the lower limit is set to 0.02%. Si is an effective element for deoxidizing molten steel and improving the strength of steel sheets, but adding more than 0.75% will have little effect and will actually deteriorate weldability and low-temperature toughness, so the upper limit should not be set. It shall be 0.75%. When the content of Mn is less than 0.5%, what is the content of Mn in high phosphorus weathering steel sheet? It is not possible to improve the hot cracking resistance during 8 welding and ensure the strength of the base metal, and even if it is added in excess of 1.5%, the effect tends to be saturated, and on the contrary, the toughness of the welded joint deteriorates. Therefore, the range is set to 0.5 to 1.5%. P is a preferable element from the viewpoint of improving weather resistance, and if its content is reduced, weather resistance in the atmosphere will decrease, so at least 0.04% is required. but,
Even if it is added in an amount exceeding 0.15%, the effect of improving weather resistance is saturated, and on the contrary, the deterioration of weldability and low-temperature toughness becomes significant. Therefore, the range is set to 0.04 to 0.15%. Cu, together with P, is an element effective in improving weather resistance, and a minimum content of 0.15% is required. However, even if it is added in an amount exceeding 0.60%, no effect can be expected, and on the contrary, aging properties and surface cracks during hot rolling will occur, so the range is set to 0.15 to 0.60%. Cr improves weather resistance as its content increases, but if it is less than 0.1%, the effect on weather resistance is small;
Adding more than 0% is not preferable as it will significantly deteriorate weldability. Therefore, the range should be reduced to 0.1 to 1.0%.
shall be. As mentioned above, B is effective in significantly improving the low-temperature toughness of quenched and tempered high-phosphorus weathering steel sheets, and a minimum amount of 0.0003% is required. However, 0.0
If added in excess of 0.020%, the B compound will increase and will significantly deteriorate not only the toughness of the weld joint but also the toughness of the base metal, so the upper limit is set at 0.0020%. Carbon equivalent Ceq (=C+1/24Si+1/6Mn+1
/40Ni+115Cr+1/4Mo+1/14V)(
%) is set to 0.40% or less because reducing Ceq is extremely effective in improving the weldability of high-phosphorus weathering steel sheets and the toughness of welded joints, and when Ceq exceeds 0.40%, these This is because the characteristics of the material deteriorate significantly, making it impossible to achieve the object of the present invention. In addition, the reason why the amount of martensite + bainite is restricted to 90% or more (the amount of ferrite is 10% or less) in the quenched structure is because, as mentioned above, when the amount of martensite + bainite is less than 90%, good low-temperature toughness is achieved. This is because you will not be able to obtain it. Such a structure is heated at a heating temperature of, for example, 95
A method of quenching at a temperature of 0℃ or higher, or a method of quenching at 950℃ after rolling.
It can be obtained by a method such as direct quenching at a temperature of about ℃. Further, it is preferable that the above-mentioned hardened steel be tempered at a temperature below the AC1 point. In addition to the above-mentioned essential elements, the tempered high-phosphorus weathering steel sheet having excellent weldability and low-temperature toughness according to the present invention further contains at least one element selected from the group consisting of Ca and Ce, if necessary. Seed elements can be contained. These elements are oxysulfide-forming elements, so their addition improves the properties and shape of inclusions, improves the hot cracking susceptibility of the weld, the Z-direction properties of the base metal, and the relationship between the base metal and HA.
The toughness of Z (heat affected zone) can be further improved. Ca is, for example, Ca-3i, Ca(CN), CaC
. 0.5-20k in molten steel in the form of alloy compounds such as
1 usually by adding about g/ton of molten steel. Ca remains in the resulting steel at a content of 0.004% or less. As mentioned above, in order to control the properties and shapes of inclusions, it is not necessary to add more than the above amount, so the upper limit of the amount of Ca added is 0°004%. On the other hand, when the amount of Ce added is less than 0.001%, the effect of controlling inclusions as described above is poor. If added in a large amount exceeding 0.1%, Ce will be added to the bottom of the m block.
Since large inclusions such as S accumulate and cause ultrasonic flaw detection defects in steel plates, the amount added is in the range of 00O1 to 0.1%. Furthermore, the steel of the present invention may contain Tj, in addition to or together with the above-mentioned optionally added elements, if necessary. At least one element selected from the group consisting of Zr, Ni, Mo, V, and Nb can be contained to an extent that does not impair HAZ toughness. Both T1 and Zr form nitrides and are effective elements for improving HAZ toughness during high heat input welding. In order to effectively express such effects, each of at least 0.00
It is necessary to add 2%, but on the other hand, each 0
．． If added in excess of 0.5%, large nitrides will precipitate as well as non-metallic inclusions of Ti or Zr, resulting in HAZ
Not only the toughness deteriorates, but also the toughness of the base material decreases.6 Therefore, in the present invention, the amount of Ti or Zr added is in the range of 0.002 to 0.05%, respectively. Ni improves weather resistance, base metal strength, base metal notch toughness, and HAZ
Since it is effective in improving toughness, it can be added in a wide range of amounts depending on these requirements. In order to effectively obtain the above effect, at least 0.05
% or more, but if too much is added, the effect will be saturated and it is also unfavorable from an economic point of view, so the upper limit of the amount added is set at 1.0%. Mo significantly improves the strength of the base material when added in a small amount, so it is an effective element for ensuring the strength of thick products. In order to effectively obtain this effect, at least 0.05%
It is necessary to add the above, but if too large a amount is added, the HAZ toughness will deteriorate, so the upper limit of the amount added is 0.5%.
shall be. (2) and Nb together form nitrides, which can prevent softening of the weld zone, improve base metal strength, and further improve weld cracking resistance by lowering Ceq. In order to effectively exhibit such effects, it is necessary to add at least 0.01% of each. However, each 0.15%
Even if it is added in excess of more than 100%, the effect of improving base metal strength will not increase significantly, but will instead cause deterioration of weldability and decrease in HAZ toughness. Therefore, in the present invention, Nb and V are each added in a range of 0.01% to 0.15%. The steel sheet of the present invention that satisfies the above conditions is a tempered high-phosphorous weathering steel sheet that has excellent weldability and low-temperature toughness. It can be manufactured by ordinary steel manufacturing, blooming rolling, quenching and tempering treatment, or directly quenching and tempering treatment after rolling, and can be given a tensile strength of 50 to 60 kgf/mm'' class. (Example) ) Next, one example of the present invention will be shown, but it goes without saying that the present invention is not limited in any way by these examples. The chemical composition, quenched structure, base material properties, and weldability of conventional fllNα9 and comparative steels Na1o to 12 are shown by direct quenching after rolling (quenching temperature 950"C) or off-line quenching (quenching temperature 930"C).
950'C) and then tempered at 640°C. The weldability was evaluated by a diagonal Y-type restrained butt weld cracking test (low-temperature cracking test) established in JIS standards. According to the test results shown in the same table, the base material of the steel of the present invention was 40%
Absorbed energy (vE-4°) at °C is 12 kgf
−m or more, conventional steel Nα9 and comparative steel Nα10.11
It is clear that the value is much better than that of . In addition, while the preheat temperature for preventing cold cracking of conventional steel is 100°C, the steel of the present invention has a preheat temperature of 50°C or less,
It is clear that it has excellent cold cracking resistance and susceptibility. Regarding the strength level, the steel of the present invention has a strength level of 50 kgf/mm" class and 6.
A tensile strength of 0 kgf/mm'' class was obtained.

[Left below]

Next, regarding the steel plate shown in Table 1, the heat input is 45KJ/
Table 2 shows the absorbed energy (vE-2a) of the bond part of the cm submerged arc welded joint at 120°C and the degree of corrosion determined by the dew corrosion test. From the same table, it is clear that the steel of the present invention has much better impact properties than the conventional steel and comparative steel with respect to the impact properties of the bonded portion of submerged arc welded joints. On the other hand, comparative gllNα10 has a lower amount of martensite + bainite in the quenched structure than the inventive steel, at 80%, so even though the weldability and weather resistance are comparable to the inventive steel, the base metal toughness and The impact properties of the joint bond area are extremely poor. Furthermore, according to the results of an accelerated test conducted over a 6-week period using a dew-type corrosion tester under conditions simulating a marine industrial zone, the steel of the present invention has weather resistance equal to or higher than that of the conventional steel Nα9. Moreover, the steel of the present invention has a comparative steel (JIS 5
It is clear that it has 1.5 times better weather resistance than MA30). Table 2 (Effects of the Invention) As detailed above, according to the present invention, P-Cu-Cr
In high phosphorus type weathering steel sheets, especially low C and low C
By adding a small amount of B and controlling the quenched structure, it is possible to obtain a heat-treated 50-60 kgf/m2 grade steel plate with significantly improved weldability and low-temperature toughness. In particular, the weathering steel plate is suitable for use in structures such as bridges, buildings, and steel towers that require weather resistance, high strength, weldability, and low-temperature toughness.

[Brief explanation of drawings]

Figure 1 shows 0.07%C-0.08%P-0.30%Cu
A diagram showing the effect of B content on the base metal impact properties (vTrs) when normal quenching and tempering treatments were carried out on -0.4% Cr steel with and without B added. Figure 2 shows the relationship between the amount of martensite + bainite in the quenched structure and the base metal impact characteristic value (vTrs) obtained by varying the quenching temperature for the 0,0010% B-containing steel shown in Figure 1. The above figure and FIG. 3 are the original drawings of the Fe-P shape. Patent applicant Takashi Nakamura, Patent attorney representing Kobe Steel, Ltd. Figure 1 B Combined amount (PPm) Figure 2

Claims (4)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.02 to 0.1
0%, Si: 0.75% or less, Mn: 0.5-1.5%
, P: 0.04-0.15%, Cu: 0.15-0.6
0%, Cr: 0.1-1.0% and B: 0.0003-
0.0020%, and carbon equivalent Ceq (=C
+1/24Si+1/6Mn+1/40Ni+1/5C
Hardened and tempered steel in which r+1/4Mo+1/14V) (%) is 0.40% or less, the balance being iron and unavoidable impurities, and the amount of martensite + bainite in the quenched structure is 90% or more. A heat-treated high phosphorus weathering steel sheet with excellent weldability and low-temperature toughness, characterized by being tempered. (2) C: 0.02-0.10%, Si: 0.75% or less, Mn: 0.5-1.5%, P: 0.04-0.15
%, Cu: 0.15-0.60%, Cr: 0.1-1.
0% and B: 0.0003 to 0.0020%,
Furthermore, Ce: 0.001 to 0.1% and Ca: 0.004
% or less, and carbon equivalent Ceq (=C + 1/24Si + 1/6Mn + 1/40N
i+1/5Cr+1/4Mo+1/14V) (%) is 0
．． 40% or less, the balance being iron and unavoidable impurities, the weldability and tempering steel is made by tempering a steel whose quenched structure has a martensite + bainite content of 90% or more. Tempered high phosphorus weathering steel plate with excellent low temperature toughness. (3) C: 0.02-0.10%, Si: 0.75% or less, Mn: 0.5-1.5%, P: 0.04-0.15
%, Cu: 0.15-0.60%, Cr: 0.1-1.
0% and B: 0.0003 to 0.0020%,
Furthermore, Ti: 0.002 to 0.05%, Zr: 0.002
~0.05%, Ni: 0.05~1.0%, Mo: 0.
05-0.50%, V: 0.01-0.15% and Nb
: Contains one or more of 0.01 to 0.15%, and carbon equivalent Ceq (=C+1/24Si+1
/6Mn+1/40Ni+1/5Cr+1/4Mo+1
/14V) (%) is 0.40% or less, the balance is iron and unavoidable impurities, and the tempered steel has a martensite + bainite content of 90% or more in the quenched structure. A tempered high phosphorus type weathering steel sheet with excellent weldability and low temperature toughness. (4) C: 0.02-0.10%, Si: 0.75% or less, Mn: 0.5-1.5%, P: 0.04-0.15
%, Cu: 0.15-0.60%, Cr: 0.1-1.
0% and B: 0.0003 to 0.0020%,
Furthermore, Ce: 0.001 to 0.1% and Ca: 0.004
% or less and Ti: 0.002 to 0
．． 05%, Zr: 0.002-0.05%, Ni: 0.05%, Zr: 0.002-0.05%, Ni: 0.
05-1.0%, Mo: 0.05-0.50%, V: 0
．． 01 to 0.15% and Nb: 0.01 to 0.15%, and carbon equivalent Ce
q(=C+1/24Si+1/6Mn+1/40Ni+
1/5Cr+1/4Mo+1/14V) (%) is 0.4
0% or less, the balance being iron and unavoidable impurities, the hardened and tempered steel is made by tempering a steel whose quenched structure has a martensite + bainite content of 90% or more. Tempered high phosphorus weathering steel plate with excellent low temperature toughness.