Martensitic heat-resisting steel excellent in HAZ-softening resistance and process for producing the same

• US 5,650,024 A
• Filed: 08/25/1995
• Issued: 07/22/1997
• Est. Priority Date: 12/28/1993
• Status: Expired due to Fees

• Current Assignee: Nippon Steel Corporation
• Original Assignee: Nippon Steel Corporation, Toshio Fujita
• Inventors: Fujita, Toshio, Mizuhashi, Nobuo, Hasegawa, Yasushi, Ohgami, Masahiro, Naoi, Hisashi
• Primary Examiner(s): Yee, Deborah
• Application Number:
  US08/513,999
• Time in Patent Office: 697 Days
• Field of Search: 148/326, 148/328, 148/542, 148/548
• US Class Current: 148/326
• CPC Class Codes: C21D 2211/008 Martensite
• CPC Class Codes: C21D 6/002 containing Cr C21D6/004 takes precedence
• CPC Class Codes: C21D 8/0205 of ferrous alloys
• CPC Class Codes: C21D 8/10 during manufacturing of tubular bodies
• CPC Class Codes: C22C 38/22 with molybdenum or tungsten
• CPC Class Codes: C22C 38/24 with vanadium
• CPC Class Codes: C22C 38/26 with niobium or tantalum
• CPC Class Codes: C22C 38/28 with titanium or zirconium
• CPC Class Codes: C22C 38/40 with nickel
• CPC Class Codes: C22C 38/42 with copper
• CPC Class Codes: C22C 38/44 with molybdenum or tungsten
• CPC Class Codes: C22C 38/46 with vanadium
• CPC Class Codes: C22C 38/52 with cobalt

Abstract

A martensitic heat-resisting steel comprises, in terms of % by mass, 0.01 to 0.30% of C, 0.02 of 0.80% of Si, 0.20 to 1.00% of Mn, 5.00 to 18.00% of Cr, 0.005 to 1.00% of Mo, 0.20 to 3.50% of W, 0.02 to 1.00% of V, 0.01 to 0.50% of Nb, 0.01 to 0.25% of N, and at least one element selected from the group consisting of Ti, Zr, Ta and Hf in an amount of 0.005 to 2.0% for each of the elements, the volume of (Ti %+Zr %+Ta %+Hf %) in the metal component M of M₂₃ C₆ type carbides therein being from 5 to 65%. The heat-resisting steel is produced by a process comprising the steps of

adding Ti, Zr, Ta and Hf to a molten steel having chemical components as mentioned above, during the period from 10 minutes before completion of refining to completion of refining, casting said molten steel, working the resulting casting, solution treating said worked product, subjecting said worked product to temporary cooling stop at a temperature from 950° to 1,000° C. in the course of cooling said hot worked product, and holding said worked product at the temperature for 5 to 60 minutes. The heat-resisting steel is excellent in HAZ-softening resistance, and exhibits a high creep strength at high temperature of at least 550° C.

Claims

1. A martensitic heat-resisting steel having a tempered martensite structure and excellent in HAZ-softening resistance consisting essentially of, in terms of % by mass, 0.01 to 0.30% of C, 0.02 to 0.80% of Si, 0.20 to 1.00% of Mn, 5.00 to 18.00% of Cr, 0.005 to 1.00% of Mo, 0.20 to 3.50% of W, 0.02 to 1.00% of V, 0.01 to 0.50% of Nb, 0.01 to 0.25% of N, up to 0.030% of P, up to 0.010% of S, up to 0.020% of O, at least one element selected from the group consisting of Ti, Zr, Ta and Hf in an amount of 0.005 to 2.0% for each of the elements, and the balance Fe and unavoidable impurities, the volume of (Ti %+Zr %+Ta %+Hf %) in the metal component M of M₂₃ C₆ type carbides precipitated in the tempered martensite structure of the steel being from 5 to 65%.

2. The martensitic heat-resisting steel according to claim 1, wherein said steel further consists essentially of, in terms of % by mass, at least one element selected from the group consisting of Co, Ni and Cu in an amount of 0.1 to 5.0% for Co or Ni, and 0.1 to 2.0% for Cu.

3. A process for producing a martensitic heat-resisting steel excellent in HAZ-softening resistance, comprising the steps of

adding at least one element selected from the group consisting of Ti, Zr, Ta and Hf in an amount of 0.005 to 2.0% for each of the elements, in terms of % by mass, to a molten steel consisting essentially of 0.01 to 0.30% of C, 0.02 to 0.80% of Si, 0.20 to 1.00% of Mn, 5.00 to 18.00% of Cr, 0.005 to 1.00% of Mo, 0.20 to 3.50% of W, 0.02 to 1.00% of V, 0.01 to 0.50% of Nb, 0.01 to 0.25% of N, up to 0.030% of P, up to 0.010% of S, up to 0.020% of O, and the balance Fe and unavoidable impurities, during the period from 10 minutes before completion of refining to completion of refining,

casting said molten steel,

hot working the resulting casting,

solution treating the hot worked product thus obtained,

subjecting said hot worked product having been solution treated to cooling stop at a temperature from 950°

to 1,000°

C. in the course of cooling said hot worked product from the solution treating temperature to room temperature,

holding said hot worked product at the temperature for 5 to 60 minutes, and

tempering said worked product.

4. The process for producing a martensitic heat-resisting steel according to claim 3, wherein said molten steel further consisting essentially of, in terms of % by mass, at least one element selected from the group consisting of Co, Ni and Cu in an amount of 0.1 to 5.0% for Co or Ni, and 0.1 to 2.0% for Cu.

5. The process for producing a martensitic heat-resisting steel according to claim 3, wherein said hot working is rolling for producing a plate product and a tube product.

6. The process for producing a martensitic heat-resisting steel according to claim 3, wherein said hot working is forging.