JP2013019015A - Steel plate having weld heat-affected zone excellent in toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel plate suitable for weld structure, which has a weld heat-affected zone excellent in toughness in welding with large-heat input exceeding 500 kJ/cm, and has a tensile strength of ≥590 MPa.SOLUTION: The steel plate contains by mass%: 0.025-0.050% C; ≤0.6% Si; 0.9-2.3% Mn; 1.0-4.9% Cr; ≤0.05% Al; 0.005-0.050% Ti; 0.01-0.07% Nb; ≤2.0% (including 0%) Ni; ≥0.03% and <0.10% Mo; if necessary, one or two selected from ≤0.09% V, ≤0.005% Ca, and ≤0.5% Cu, while satisfying the following formulas (1)-(3); and the balance iron with inevitable impurities. The formulas are (1): 5.5≤Mn+0.7Ni+14Nb+Cr+4Si+2Mo≤7.0; (2): Mn+0.7Ni+14Nb≥1.0; and (3): Cr+4Si+2Mo≥2.9, in each formula, Mn, Cr, Ni, Nb, Si, and Mo are the contents (mass%).

Description

  The present invention is a steel plate excellent in the toughness of the heat affected zone, particularly when subjected to high heat input welding exceeding 500 kJ / cm, excellent in the toughness of the weld heat affected zone, with a tensile strength of 590 MPa or more and for welded structures It is related with a suitable steel plate.

  In submerged arc welding, electroslag welding, and the like, which are applied to assembly welding of box columns of building structures, large heat input welding exceeding 500 kJ / cm may be performed to improve construction efficiency. In general, when the heat input of welding increases, the structure of the weld heat affected zone (hereinafter sometimes referred to as HAZ) becomes coarse and the toughness decreases, and various methods for improving the HAZ toughness have been proposed.

  For example, Patent Document 1 relates to a technique for suppressing the HAZ toughness deterioration (heat input amount of about 500 kJ / cm) of a steel having a tensile strength of 590 MPa class, and the formation of island martensite (MA) is suppressed by extremely reducing the steel composition. In addition, it is disclosed that Mn, Ni and Cr, which are hardenability improving elements, are appropriately contained to suppress the formation of ferrite at the γ grain boundary and to refine the block size of the transformed structure in the grains.

  Patent Document 2 relates to a HAZ toughness improvement technique (heat input amount of about 500 kJ / cm) for steel having a tensile strength of 780 MPa class, and positively contains Mn, Ni, and Cu as steel compositions, and contains Mo, Nb, and V contents. It is disclosed that by limiting, a structure mainly composed of bainitec ferrite is used, the amount of Ti and N is optimized, and TiN is stabilized at a high temperature to prevent coarsening of austenite crystal grains in HAZ.

JP 2007-126725 A JP 2006-118007 A

  However, the tensile strength 590 MPa class steel described in Patent Document 1 needs to contain about 1% Ni by mass% in order to improve HAZ toughness (hereinafter,% means mass%). Since Ni is an expensive element, the alloy cost is considerably high, and cost competitiveness is low as 590 MPa class steel. Further, since Mn is contained in a large amount of about 2%, solidification segregation of Mn is strong at the slab 1/2 · t position (t: slab thickness), and the toughness at the 1/2 position of the plate thickness is easily impaired. .

  In order to improve the HAZ toughness, the tensile strength 780 MPa class steel described in Patent Document 2 needs to contain about 1-2% Ni, and further needs to contain 0.2-0.5% Mo. Since Ni and Mo are expensive elements, the alloy cost is high and the cost competitiveness is low.

  Further, since Mn is contained in a large amount of about 2%, the solidification segregation of Mn at the 1/2 · t position of the slab is strong as in Patent Document 1, and the toughness at the 1/2 position of the plate thickness is easily impaired. .

  Therefore, the present invention has an object to provide a steel having a tensile strength of 590 MPa or more, which is excellent in high heat input HAZ toughness, and has a high cost competitiveness and a component composition in which toughness deterioration does not occur at a half position of the plate thickness. To do.

The present inventor has intensively studied in order to solve the above problems, and has obtained the following knowledge.
1. In order to stably secure high toughness of the heat-affected zone with high heat input welding in steels with a tensile strength of 590 MPa or more, the formation of island martensite (hereinafter referred to as MA) is suppressed by extremely low C, and Mn, Cr The basis of the component design guideline is to contain elements that enhance the hardenability such as Ni, Si, Nb, and Mo to lower the transformation temperature and make the entire structure a uniform bainite structure.
2. HAZ toughness is improved by using an austenite-forming element (Mn, Nb, Ni) and a ferrite-forming element (Cr, Si, Mo), and a component composition in which the ferrite-forming element amount is appropriate relative to the austenite-generating element amount. To do. Although Nb is not generally called an austenite-forming element, Nb is regarded as an austenite-stabilizing element as in the case of Mn and Ni in order to significantly delay the transformation from austenite to ferrite.

That is, when a ferrite stabilizing element is contained in addition to an austenite-forming element, excessive austenite stabilization is relaxed, MA between bainite laths is reduced, and a region having excellent HAZ toughness appears in a combination of component compositions.
3. When Mn and Nb are mainly used as an austenite generating element and Ni is used supplementarily, the alloy cost is reduced and it is effective for improving the HAZ toughness.

  That is, Mn is a strong austenite stabilizing element and lowers the transformation temperature and promotes bainite transformation. However, inclusion of Mn alone causes MA to precipitate between bainite laths and lowers HAZ toughness.

  Nb significantly delays the ferrite transformation and promotes the bainite transformation. However, when Nb is added, MA tends to precipitate between bainite laths, and the effect of improving the HAZ toughness is limited.

  Ni is a strong austenite stabilizing element next to Mn. It lowers the transformation temperature and promotes bainite transformation, but its effect is smaller than that of Mn, so if it contains only Ni, its effect is not enough.

When Ni is contained in a large amount, HAZ has a uniform bainite structure, and unlike Mn content, there is almost no formation of MA harmful to toughness between bainite laths, so that excellent HAZ toughness can be obtained. However, since it is expensive, the alloying cost of the steel material becomes extremely high due to the inclusion of a large amount thereof.
5. It is practical to mainly use Cr as a ferrite-generating element and use Si and Mo as auxiliary elements. Cr is effective in suppressing excessive austenite stabilization by Mn. Mo is a strong ferrite-forming element and has an effect similar to that of Cr, but is expensive. Moreover, since it is also a strong hardenability element, the addition of a large amount promotes the formation of MA. Si is also a strong ferrite-forming element. However, in order to strongly suppress the formation of cementite, when it is excessively contained, the formation of MA is promoted.

The present invention has been made based on further studies based on the above findings.
1. Steel composition is mass%, C: 0.025 to 0.050%, Si: 0.6% or less, Mn: 0.9 to 2.3%, Cr: 1.0 to 4.9%, Al : 0.05% or less, Ti: 0.005 to 0.050%, Nb: 0.01 to 0.07%, Ni: 2.0% or less (including 0%), Mo: 0.03 to 0 Less than 10%, a steel plate satisfying the following formulas (1) to (3) and having excellent toughness in the heat affected zone consisting of the remaining iron and inevitable impurities.
5.5 ≦ Mn + 0.7Ni + 14Nb + Cr + 4Si + 2Mo ≦ 7.0 (1)
Mn + 0.7Ni + 14Nb ≧ 1.0 (2)
Cr + 4Si + 2Mo ≧ 2.9 (3)
In each formula, Mn, Cr, Ni, Nb, Si, and Mo indicate the content (% by mass) of each element.

  2. Furthermore, the toughness of the weld heat-affected zone according to 1, which contains one or two or more of V: 0.09% or less, Ca: 0.005% or less, and Cu: 0.5% or less as a steel composition. Excellent steel plate.

  According to the present invention, the content of expensive alloy elements such as Ni and Mo, which have soared in recent years, is small, and excellent HAZ toughness can be secured even when high heat input welding exceeding 500 kJ / cm is performed. Since a steel plate can be obtained, a highly safe building structure and the like can be produced efficiently and economically, which is extremely useful industrially.

  The steel sheet according to the present invention contains austenite-forming elements (Mn, Nb, Ni) and ferrite-forming elements (Cr, Si, Mo) in the component composition from the viewpoint of improving the HAZ toughness and ensuring the strength and toughness of the base material. It is characterized by optimizing the amount and the balance between the two. Hereinafter, the reason for component limitation will be described. In the description,% is mass%.

C: 0.025 to 0.050%
C is an element necessary for ensuring the strength of the base material and suppressing the coarsening of the γ grains to ensure the HAZ toughness, and is contained in an amount of 0.025% or more in order to exert this effect. On the other hand, since MA will increase and HAZ toughness will deteriorate when content increases, it is set as 0.025 to 0.050%.

Si: 0.6% or less Si is an element necessary for deoxidation at the time of steelmaking, and it is preferable to add 0.02% or more for this purpose. However, if the purpose of deoxidation is achieved, the content is at least good. Moreover, although it is a strong ferrite formation element, it suppresses the excessive stabilization of the austenite by Mn and suppresses the production | generation of MA, On the other hand, the production | generation of cementite is suppressed strongly. Therefore, if the content increases, MA increases and the HAZ toughness deteriorates, so the content is made 0.6% or less. Preferably it is 0.3% or less.

Mn: 0.9 to 2.3%
Mn is a strong austenite stabilizing element and is an element useful for reducing the transformation point and ensuring the strength of the base material. It is an element that promotes bainite transformation. In order to ensure the strength of the base material, it is contained in an amount of 0.9% or more. However, if Mn is excessive, HAZ becomes too hard and HAZ toughness deteriorates, so the content is made 2.3% or less.

Cr: 1.0 to 4.9%
Cr is an element useful for improving the hardenability and ensuring the strength and toughness of the base material and is a ferrite stabilizing element, and prevents excessive austenite stabilization due to Mn content and suppresses the generation of MA. It is a useful element. In order to exert these effects, 1.0% or more is contained. However, if the Cr content increases, the hardness of the HAZ increases and the HAZ toughness deteriorates, so the content is made 4.9% or less.

Al: 0.05% or less Al is an element necessary for deoxidation at the time of steelmaking. For this purpose, addition of 0.01% or more is preferable. However, if the purpose of deoxidation is achieved, the content is at least good. . Moreover, when content increases, coarse inclusions, such as an alumina, will increase and base material toughness will deteriorate. In addition, MA increases and HAZ toughness also deteriorates, so 0.05% or less.

Ti: 0.005 to 0.050%
Ti is an element that combines with N to form TiN, and TiN suppresses the growth of γ grains of HAZ and contributes to the improvement of HAZ toughness. In order to exhibit this effect, 0.005% or more, preferably 0.010% or more is contained. On the other hand, when the content increases, TiN becomes coarse and both the base material toughness and the HAZ toughness deteriorate, so the content is made 0.050% or less, preferably 0.025% or less.

Nb: 0.01 to 0.07%
Nb is an element that significantly delays the transformation from austenite to ferrite and promotes bainite transformation. To ensure the strength of the base material, 0.01% or more is contained. However, if Nb is excessive, the hardness of the HAZ becomes too hard and the HAZ toughness deteriorates. Therefore, the Nb amount is suppressed to 0.07% or less.

Ni: 2.0% or less (including 0%)
Ni is also a strong austenite stabilizing element, and is an element useful for reducing the transformation point and securing the strength of the base material. It is an element that promotes bainite transformation. However, since it is an expensive element, it is not necessary to contain it as a selective element used as an auxiliary to Mn in the present invention, when the desired strength and HAZ toughness can be obtained with Mn alone. Even if contained, the content is made 2.0% or less from the viewpoint of alloy cost.

Mo: 0.03 to less than 0.10% Mo is an element useful for improving the hardenability and securing the strength and toughness of the base material. Further, it is a ferrite stabilizing element, and is an element useful for preventing the excessive stabilization of austenite by addition of Mn and suppressing the generation of MA. In order to exert these effects, 0.03% or more is contained. However, since it is an expensive element, it is used as an auxiliary of Cr, and if it is excessively contained, MA increases and HAZ toughness deteriorates, so the content is made less than 0.10%.

5.5 ≦ Mn + 0.7Ni + 14Nb + Cr + 4Si + 2Mo ≦ 7.0 (1)
Mn + 0.7Ni + 14Nb ≧ 1.0 (2)
Cr + 4Si + 2Mo ≧ 2.9 (3)
In each formula, Mn, Cr, Ni, Nb, Si, and Mo indicate the content (% by mass) of each element.

Each formula of (1) to (3) defines the balance of the contents of Mn, Nb, Ni that are austenite forming elements and Cr, Si, Mo that are ferrite forming elements in the component composition from the viewpoint of HAZ toughness. It is a parameter expression. The contents of Mn, Nb and Ni as austenite forming elements satisfy Mn + 0.7Ni + 14Nb ≧ 1.0, the contents of Cr and Si as ferrite forming elements satisfy Cr + 4Si + 2Mo ≧ 2.9, and 5.5 ≦ In the case of Mn + 0.7Ni + 14Nb + Cr + 4Si + 2Mo ≦ 7.0, good HAZ toughness is obtained.
The above is the basic component of the present invention, and the balance is Fe and inevitable impurities. P and S are treated as inevitable impurities, and the content is not controlled. Furthermore, in the present invention, in addition to the above basic components, the following elements can be selectively added as necessary.

V: 0.09% or less V is an element effective for improving the strength of the base material, and is an element that suppresses ferrite transformation and promotes bainite transformation. However, if over 0.09% is added, precipitates are precipitated with HAZ and the HAZ toughness is lowered. Therefore, when added, the content is preferably made 0.09% or less.

Ca: 0.005% or less Ca is an element that improves the HAZ toughness by controlling the form of inclusions. However, even if it is added in excess of 0.005%, inclusions are coarsened and the HAZ toughness is lowered. Therefore, when added, the content is preferably 0.005% or less.

Cu: 0.5% or less Cu is an element effective in improving the strength of the base material, and is an element that suppresses ferrite transformation and promotes bainite transformation. However, if over 0.5% is added, surface flaws (Cu wrinkles) are generated on the surface of the steel sheet by hot rolling, so when added, the content is preferably made 0.5% or less. Moreover, when adding Cu, it is preferable to add Ni at least 1/2 of Cu addition amount in order to prevent surface flaws.

  The steel sheet according to the present invention can be manufactured by melting and rolling by a conventional method. Due to the component composition having high hardenability, the tensile strength can be 590 MPa or more as it is rolled. Hereinafter, the present invention will be described more specifically with reference to examples.

  Steel materials with various component compositions were melted into ingots, heated to 1200 ° C, hot-rolled to a rolling start temperature of 1150 ° C, a rolling end temperature of 910 ° C, a rolling reduction of 80%, and a sheet thickness of 15 mm, and then air-cooled. . About the obtained steel plate, base material characteristics (strength, toughness) and HAZ toughness were evaluated.

[Base material properties]
For each steel plate, a round bar test piece (parallel portion: diameter 6.0 mm × distance between gauge points 25.0 mm) is taken from the rolling direction, and subjected to a tensile test according to JISZ2241, yield strength (YS), tensile strength. (TS), yield ratio (YR), and elongation (EL) were measured. For reference, a Charpy impact test was conducted using a V-notch test piece (standard test piece test piece width 10 mm) in accordance with JIS Z2242, and the absorbed energy (vE ₀ ) (average value of 3 pieces) and brittleness at a test temperature of 0 ° C. The fracture surface ratio (%) was measured.

[HAZ toughness]
HAZ toughness was evaluated by a reproducible thermal cycle test of an electroslag weld. The reproducible heat cycle is a combination of skin plate material (50mm thickness) and diaphragm material (50mm thickness), and the heat history corresponding to the heat affected zone near the bond when electroslag welding with welding heat input of 550kJ / cm is performed. Simulated.

A 12 mm thick × 12 mm wide square bar specimen taken from the rolling direction of each steel plate was held at 1400 ° C. for 1 second by a high-frequency induction heating device and subjected to a heat cycle of 800 to 500 ° C. for a cooling time of 510 seconds. Then, three V-notch test pieces (standard test piece test piece width 10 mm) are sampled from each test piece, subjected to Charpy impact test, and the absorbed energy (vE ₀ ) at a test temperature of 0 ° C. is measured in accordance with JISZ2242. Then, the brittle fracture surface ratio was obtained for reference.

  The range of the present invention is that the tensile strength is 590 MPa or more, the average of the three absorbed energy in the reproducible thermal cycle Charpy impact test is 70 J or more, and the individual minimum value is 50 J or more.

Table 1 shows the component composition of the test steel. Steels A1 to A7 are examples of the present invention. A8 to A39 are comparative examples whose components are outside the scope of the present invention. Steels A8 to A20 are comparative examples in which at least one of C (carbon), F, and A + F is outside the scope of the present invention. Steels A21 to A36 have F and / or A + F values outside the scope of the present invention. Steels A37 and 38 have Cr, F and A + F values outside the scope of the present invention. Steel A39 has F and A + F values out of range.
Note that F = Cr + 4Si + 2Mo, A = Mn + 0.7Ni + 14Nb, and A + F = Mn + 0.7Ni + 14Nb + Cr + 4Si + 2Mo, which are parameters corresponding to equations (3) to (1), respectively.

  Table 2 shows the base material properties (strength and toughness). Steels A1 to A7 are examples of the present invention. A8 to A39 are comparative examples whose components are outside the scope of the present invention. Except for the steels A12 and A13, all have sufficient strength, but the comparative example does not have sufficient HAZ toughness by a reproducible thermal cycle Charpy impact test as described later.

  Table 3 shows the reproduced thermal cycle Charpy impact test results. As is clear from the table, all of the inventive examples (steel A1 to A7) have excellent HAZ toughness with an average of three absorbed energy of 70 J or more and a minimum value of three of 50 J or more. However, the comparative examples (steel A8 to 39) are inferior in HAZ toughness except for steel A13 having a tensile strength (TS) of 453 MPa.

Claims (2)

Steel composition is mass%, C: 0.025 to 0.050%, Si: 0.6% or less, Mn: 0.9 to 2.3%, Cr: 1.0 to 4.9%, Al : 0.05% or less, Ti: 0.005 to 0.050%, Nb: 0.01 to 0.07%, Ni: 2.0% or less (including 0%), Mo: 0.03 to 0 Less than 10%, a steel plate satisfying the following formulas (1) to (3) and having excellent toughness in the heat affected zone consisting of the remaining iron and inevitable impurities.
5.5 ≦ Mn + 0.7Ni + 14Nb + Cr + 4Si + 2Mo ≦ 7.0 (1)
Mn + 0.7Ni + 14Nb ≧ 1.0 (2)
Cr + 4Si + 2Mo ≧ 2.9 (3)
In each formula, Mn, Cr, Ni, Nb, Si, and Mo indicate the content (% by mass) of each element.   The weld heat-affected zone according to claim 1, further comprising one or more of V: 0.09% or less, Ca: 0.005% or less, and Cu: 0.5% or less as a steel composition in mass%. Steel sheet with excellent toughness.