JP2005256135A - High strength steel for welding having excellent corrosion resistance and machinability and method for producing high strength steel sheet for welding - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high strength steel for welding having excellent corrosion resistance, and to provide a method for producing a high strength steel sheet for welding. <P>SOLUTION: The high strength steel for welding has a composition containing, by mass, >0.03 to 0.3% C, 0.01 to 1.0% Si, 0.1 to <3% Mn, ≤0.03% P, ≤0.01% S, 3 to 7% Cr, 0.1 to 2% Al and ≤0.02% N, and also selectively containing one or more kinds of metals selected from 0.01 to 5% Cu, 0.01 to 5% Ni, 0.005 to 1% Mo, 0.005 to 0.1% V, 0.005 to 0.050% Nb, 0.005 to 0..03% Ti, 0.0005 to 0.05% Ca, 0.0005 to 0.05% Mg and 0.001 to 0.1% rare earth metals, and containing components satisfying Tp value, expressed by expression, of ≥1,150 and also satisfying Tc value, expressed by the expression, of ≥600. Further, the steel ingot thereof is subjected to hot rolling, and is thereafter air-cooled, so that the high strength steel sheet for welding is obtained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing high-strength steel for welding and high-strength steel plate for welding excellent in corrosion resistance and machinability.

In recent years, the use of stainless steel has been expanded in various building structures and storage containers. However, although stainless steel, particularly austenitic stainless steel, is excellent in corrosion resistance, it is expensive and has disadvantages such as low yield strength and difficulty in machining such as cutting and drilling. In recent years, steel in which Al is added to Cr for the purpose of improving corrosion resistance or improving corrosion resistance and workability has been proposed (for example, Patent Documents 1 to 8). These steels are technologies made to improve corrosion resistance or corrosion resistance and workability, and some effectiveness is recognized, but on the other hand, cracks occur in the heat affected zone due to the formation of coarse ferrite, In addition, the machinability such as machinability has not been improved so much.
JP 5-279791 A JP-A-6-179949 Japanese Patent Laid-Open No. 6-17950 JP-A-6-179951 JP-A-6-212256 JP-A-6-212257 Japanese Unexamined Patent Publication No. 7-3388 Japanese Patent Laid-Open No. 11-350082

  In view of such a current situation, the present invention aims to provide a steel sheet that has a high corrosion resistance and improves machinability in an indoor environment or an atmospheric environment, and at the same time has a good weldability and can be manufactured at low cost. Yes.

  In order to achieve the above object, the present inventors have developed various steels having excellent corrosion resistance in a corrosive environment such as a condensation corrosive environment, an atmospheric corrosive environment, a tap water corrosive environment, a concrete corrosive environment, and a seawater corrosive environment. We examined from the viewpoint of. First, as a result of various investigations on means for improving the corrosion resistance in the corrosive environment as well as excellent toughness of the welded portion, a steel in which 0.1 to 2% of Al is added to steel containing 2 to 7% of Cr It has been found that it exhibits excellent corrosion resistance in many of the above-mentioned corrosive environments. However, such a steel has a wide ferrite phase transformation region. For example, when it is heated to 1200 ° C. or higher during welding, coarse ferrite is generated, and there is a concern that the toughness is greatly reduced. Therefore, as a result of repeating many experiments, the inventors succeeded in quantifying the relationship with the amount of additive alloy element as a means for suppressing the generation of ferrite phase transformation that occurs during welding, as shown below. It has been found that the formation of ferrite in the high temperature range is suppressed when the alloy addition amount satisfies the range of the expression Tp.

  Furthermore, focusing on the base material structure for improving machinability, based on the knowledge that it is more important to mix the soft phase and the hard phase appropriately than the single phase structure, after hot rolling, during the air cooling of the steel sheet An alloy addition amount for generating an appropriate matrix structure, a Tc formula, was introduced. That is, in this invention, the component range which can improve machinability by this Tc type | formula was obtained.

（２）、前記した（１）記載の耐食性および切削性に優れた溶接用高強度鋼の鋼塊を加熱後、熱間圧延し700 ℃以上の温度から空冷することを特徴とする耐食性および切削性に優れた溶接用高強度鋼板の製造法、
（３）、熱間圧延後、Ａｃ₁ 変態点以下の温度で焼戻しをすることを特徴とする（２）に記載の耐食性および切削性に優れた溶接用高強度鋼板の製造法、 である。 The gist of the present invention is
(1) By mass%
C: 0.03% to 0.3% or less Si: 0.01 to 1.0%
Mn: Less than 0.1 to 3% P: 0.03% or less S: 0.01% or less Cr: 2-7%
Al: 0.1 to 2%
N: 0.02% or less, and optionally,
Cu: 0.01 to 5%
Ni: 0.01 to 5%
Mo: 0.005 to 1%
V: 0.005-0.1%
Nb: 0.005 to 0.050%
Ti: 0.005 to 0.03%
Ca: 0.0005 to 0.05%
Mg: 0.0005 to 0.05%
REM: 0.001 to 0.1%
A steel containing one or two of the above, containing a component having a Tp value of 1150 or more and a Tc value of 600 or more represented by the following formula, and having excellent corrosion resistance and machinability High strength steel,

(2) Corrosion resistance and cutting characterized by heating the steel ingot of high strength steel for welding excellent in corrosion resistance and machinability described in (1) above, followed by hot rolling and air cooling from a temperature of 700 ° C. or higher. Manufacturing method of high-strength steel sheets for welding with excellent properties,
(3) A method for producing a high-strength steel sheet for welding excellent in corrosion resistance and machinability according to (2), characterized in that after hot rolling, tempering is performed at a temperature equal to or lower than the Ac ₁ transformation point.

  The present invention is excellent in corrosion resistance in various corrosive environments such as a dew corrosive environment, an atmospheric corrosive environment, a seawater corrosive environment, etc., as well as in the indoor environment, and at the same time, toughness of an important base material part and a weld heat affected zone in a welded structure. This makes it possible to provide excellent high-strength steel and high-strength steel sheet manufacturing methods at an extremely low cost, and is extremely important in contributing to industrial development.

(Essential ingredients)
C: C is an element that improves the strength and at the same time increases the hardness of the hard phase of the base material and can improve the machinability. Therefore, it is necessary to add more than 0.03%. However, if added over 0.3%, the HAZ part is effective and the weldability is impaired, so the upper limit was made 0.3%.

  Si: It is effective to add Si as a deoxidizer and strengthening element to steel containing 2% or more of Cr. However, if the content is less than 0.01%, the deoxidation effect is not sufficient. If the content exceeds 0%, the effect is saturated and the weld heat-affected zone toughness is lowered. Therefore, the content range is limited to 0.01% to 1.0%.

  Cr: Cr needs to be contained in an amount of 2% or more in order to ensure corrosion resistance, but if it exceeds 7%, the weld heat-affected zone toughness will be hindered, so the upper limit is made 7%.

  Al: Al is an important element along with Cr in order to ensure corrosion resistance in the present invention, and the content of Al is required to be 0.1% or more from the viewpoint of ensuring corrosion resistance, If added over 2%, it dissolves and inhibits the weld heat affected zone toughness, so its content is limited to 0.1% or more and 2% or less.

  Mn: In the present invention, Mn is added mainly to improve the strength and to act as an austenite forming element, and to suppress the formation of coarse ferrite promoted by Cr and Al added from the viewpoint of corrosion resistance. That is, as is well known, Cr and Al are ferrite-forming elements, and when they are added in large amounts, they undergo a transformation from solidification to room temperature and become a ferrite single-phase structure. In the heat affected zone, weldability may be significantly impaired. Therefore, the inventors have conducted a systematic experiment for the purpose of improving the base material and toughness without impairing the corrosion resistance, and as a result, found that it can be avoided by adding Mn. The specific constraint conditions will be described later. According to this, it is necessary to add 0.1% or more of Mn, but the addition of 3% or more increases the hardenability of the base material structure. Less than 3%.

  N: If N is added in a large amount in the steel sheet, the toughness of the base metal and the weld heat-affected zone is lowered, so a smaller amount is desirable, and the upper limit content is 0.02%.

  P: If P is present in a large amount, the toughness is lowered, so a smaller amount is desirable, and the upper limit content is 0.03%. It is preferable to reduce the content inevitably mixed in as much as possible.

  S: When S is also present in a large amount, the pitting corrosion resistance is deteriorated, so the smaller one is desirable. The upper limit content is 0.01%. S, like P, should have the unavoidable amount of contamination as small as possible.

Furthermore, in the present invention, one or more of the following elements can be selected and added.
Cu, Ni: Both Cu and Ni have the effect of improving the strength and suppressing the formation of ferrite. In particular, Ni has the effect of improving the toughness of the base material and the heat-affected zone. Both of these effects require the addition of 0.01% or more. However, since the embrittlement occurs when both are added in excess of 5%, in both cases, the limited range is 0.01 to 5%. And

  Mo: When steel is added with 0.01% or more in steel to which Cr and Al are added, the effect of suppressing the occurrence and growth of pitting corrosion is recognized without impairing the toughness of the base material. Even if added over 0%, the effect is not only saturated but also the toughness is lowered, so the range is made 0.005% to 1.0%.

  Nb: Nb is an element that improves strength and toughness without impairing corrosion resistance, and its effect is recognized from 0.005%. The range is 0.005% to 0.05%.

  V: V is an element that improves the strength without impairing the corrosion resistance, and an effect is recognized at 0.005% or more. However, as is well known, addition of a large amount inhibits toughness. 1%.

  Ti: Ti is an element that contributes to the refinement of the crystal grain size at a high temperature through the formation of nitrides. Therefore, addition of 0.005% or more is necessary, but if it exceeds 0.03%, it is coarse. Since precipitates are generated, the toughness is particularly significantly reduced. Therefore, the range is set to 0.005% to 0.03% for both elements.

  Ca, Mg: Ca and Mg are elements that can improve the corrosion resistance of steel containing Cr and Al. At present, there are many unclear points in the mechanism, but the corrosion resistance is further improved when both of them are 5 ppm or more, and as the amount increases, the corrosion resistance is improved. In addition to this, the tendency to lower the toughness has been clarified, and the addition amount is limited to 5 ppm or more and 500 ppm or less.

  REM: Furthermore, in the present invention, it is possible to improve the properties of the base material and the welded part without damaging the corrosion resistance even if a rare earth element (REM) is added as appropriate. The addition amount needs to be 0.001% or more, but addition of a large amount inhibits toughness and the like, so the upper limit is made 0.1%.

図１は、0.035 ％Ｃ−0.15％Ｓｉ−0.0050Ｎ％の鋼を基本として、Ｍｎ、Ｃｒ、Ａｌまた、場合によりＣｕ、Ｎｉ添加した素材に溶接サイクルを与え、その時の変態点と粗大なフェライトの生成挙動を観察した結果である。すなわち、横軸で示すＴp 式が、１１５０以上になると、粗大なフェライト相の生成が抑制されることが分かる。 Further, in the present invention, both Tp and Tc are introduced in order to improve the weldability and the machinability that are the core of the present invention.

Figure 1 shows 0.035% C-0.15% Si-0.0050N% steel as a base, Mn, Cr, Al, and optionally Cu and Ni-added materials are subjected to a welding cycle, the transformation point and coarse ferrite at that time. It is the result of having observed the production | generation behavior of. That is, it can be seen that when the Tp expression shown on the horizontal axis is 1150 or more, the formation of a coarse ferrite phase is suppressed.

  The Tc equation is related to the transformation point during cooling of the steel sheet. When this equation is less than 600, a ferrite structure is not generated during air cooling of the steel sheet after rolling, and an appropriate mixed structure is obtained. I can't.

In addition, when producing the steel of the present invention, it is produced as a steel ingot by the ingot-splitting method or the continuous casting method and other methods, and after that, it is made into a steel plate by hot rolling or hot forging and air-cooled as it is. In some cases, it is desirable that tempering is performed at a temperature equal to or lower than the transformation point after hot rolling. Air cooling after hot rolling is a process necessary for obtaining a mixed structure containing ferrite necessary for imparting machinability of the base material. When the Tc formula has 600 or more components, the machinability is improved. Tempering also reduces the strength of the steel sheet and is effective in improving machinability.
The steel sheet produced in this way can be applied as a high-strength steel sheet for welding having excellent corrosion resistance and machinability.

  After melting the component steels shown in Table 1, the steel ingot is hot-rolled to a steel plate with a thickness of 20 mm to 50 mm, and tempering after rolling as appropriate under the production conditions shown in Table 2. The following tests were carried out on steel sheets that had been subjected to water-cooling and tempering immediately after rolling, or subjected to quenching and tempering by reheating after rolling.

(1) Evaluation of base metal tensile properties JIS4 test specimens were collected from as-rolled steel sheets from 1/2 part thickness -C direction and tested.
(2) Weldability evaluation
A 12mmφ test piece is cut out from the center of the plate thickness, and after the welding heat cycle [maximum heating temperature: 140 ° C cooling rate: 15 ° C / s], a circumferential notch (10mmφ 0.25mm V notch depth 1mm) test piece is taken. After that, the load was increased at 0 ° C., and the stress was obtained from the load at the time of fracture. The fracture stress was evaluated by comparing the yield strength of the material.
(3) Corrosion resistance evaluation A 5 mm thick corrosion test was sampled by cutting from a test steel plate and tested under the following conditions.
Indoor environment: 100-day exposure test without painting in an air-conditioned room Wet environment: Hold for 2 hours at 20 ° C and then hold for 4 hours in an environment of 95%-25 ° C for 1300 times.
Salt damage environment: Expose the specimen to the coastal splash zone for 12 months.
In both cases, the size of rust spots is expressed as a score.
(4) Evaluation of machinability A steel plate with a thickness of 18 mm (1 mm from the surface) is taken from 1 mm below the surface layer of the steel plate, and a commercially available high-speed drill (6 mmφ) is used. Drill through holes while supplying the appropriate cutting oil, and record the number of drilled holes without changing the tool. ◎: More than 200 pieces, Δ: 100 to 200 pieces, X: less than 100 pieces are evaluated.

Table 2 also shows the results of the test.
Steels A to K are all within the scope of the present invention. That is, all the tensile strength of the base metal is 600 MPa or more, and the result of the notched round bar tensile test simulating the effect of welding heat evaluated as weldability is 80% of the yield strength (0 ° C) of the base material. In addition to the above high breaking stress, only a small amount of rusting of 2 mm or less was observed in part of the corrosion resistance, and all showed good characteristics. In addition, all of the machinability can drill more than 200 holes.

  On the other hand, L steel to V steel are all comparative steels. That is, L, M, N, O, P, Q, R, U, and T steels are those in which the additive elements depart from the scope of the present invention within the scope of the present invention. That is, L steel and U steel each have C amount deviating from the lower limit and the upper limit, and both have good corrosion resistance and weldability, but L steel has a strength as low as 500 MPa or less, and U steel has weldability. It is falling. V steel is a case where Si addition amount exceeds the upper limit of the invention range, and weldability is lowered.

  Further, the M steel deviates from the upper limit of Mn, and at the same time, because the Tc value is 526, which is below the range of the present invention, the weldability and the machinability are deteriorated. N and O steels have P and S added in excess of the upper limit, and both have low weldability. P and Q are cases where the amount of Al added from Cr is lower than that of the present invention. In this case, the corrosion resistance is significantly reduced. In the case of R steel, the amount of Al exceeds the upper limit, and the weldability is reduced.

  In S and T steels, the amount of each alloy is within the range of the present invention, but the Tp value indicating weldability and the Tc value indicating machinability deviate from the range of the invention. As a result, it is recognized that S steel has poor weldability and T steel has low machinability.

It is a figure which shows the relationship between the value defined by Tp, and an austenite-> delta ferrite transformation point.

Claims (3)

% By mass
C: 0.03% over 0.3%
Si: 0.01 to 1.0%
Mn: Less than 0.1 to 3% P: 0.03% or less S: 0.01% or less Cr: 2-7%
Al: 0.1 to 2%
N: 0.02% or less, and optionally,
Cu: 0.01 to 5%
Ni: 0.01 to 5%
Mo: 0.005 to 1%
V: 0.005-0.1%
Nb: 0.005 to 0.050%
Ti: 0.005 to 0.03%
Ca: 0.0005 to 0.05%
Mg: 0.0005 to 0.05%
REM: 0.001 to 0.1%
In a steel containing one or two of the above, a weld having excellent corrosion resistance and machinability characterized by containing a component having a Tp value of 1150 or more represented by the following formula and a Tc value of 600 or more For high strength steel.

  The high strength steel ingot for welding excellent in corrosion resistance and machinability according to claim 1 is heated, then hot-rolled, and air-cooled from a temperature of 700 ° C. or higher. Manufacturing method of high strength steel sheet. After hot rolling, the preparation of corrosion resistance and machinability in excellent high strength steel plate for welded according to claim 2, characterized in that the tempering at a temperature of Ac ₁ transformation point.

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