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WO2018061101A1 - Acier - Google Patents

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Publication number
WO2018061101A1
WO2018061101A1 PCT/JP2016/078558 JP2016078558W WO2018061101A1 WO 2018061101 A1 WO2018061101 A1 WO 2018061101A1 JP 2016078558 W JP2016078558 W JP 2016078558W WO 2018061101 A1 WO2018061101 A1 WO 2018061101A1
Authority
WO
WIPO (PCT)
Prior art keywords
steel
content
less
present
hardness
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/JP2016/078558
Other languages
English (en)
Japanese (ja)
Inventor
久保田　学
聡 志賀
一 長谷川
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel and Sumitomo Metal Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nippon Steel and Sumitomo Metal Corp filed Critical Nippon Steel and Sumitomo Metal Corp
Priority to US16/329,463 priority Critical patent/US20190256957A1/en
Priority to EP16917654.2A priority patent/EP3521469A4/fr
Priority to JP2018541768A priority patent/JP6798557B2/ja
Priority to KR1020197007851A priority patent/KR20190041502A/ko
Priority to CN201680089493.8A priority patent/CN109790602B/zh
Priority to PCT/JP2016/078558 priority patent/WO2018061101A1/fr
Publication of WO2018061101A1 publication Critical patent/WO2018061101A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium

Definitions

  • the present invention relates to steel.
  • Cold forging can improve the surface texture and dimensional accuracy of the product compared to hot forging, and also has a good yield, so it is a relatively small machine such as a bolt. It is widely applied as a manufacturing method for parts.
  • machine parts carbon steel and alloys for medium-carbon mechanical structures specified in JIS G 4051, JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106, etc. as materials.
  • Steel is often used as a final product through manufacturing processes such as hot wire rolling, annealing (or spheroidizing annealing), wire drawing, cold forging, quenching, and tempering.
  • the general manufacturing process is characterized by adding an annealing or spheroidizing annealing process before cold forging.
  • the reason why annealing or spheroidizing annealing is added before cold forging is the reason why medium carbon carbon steel and alloy steel remain hot rolled (that is, when air cooled without heat treatment after hot rolling) ),
  • the hardness of the rolled material is high, and the die is worn at the time of cold forging, resulting in an increase in manufacturing cost, and cracking occurs at the time of cold forging because the ductility of the material is insufficient with hot rolling. This is because there is a manufacturing problem such as a reduction in yield due to facilitation.
  • Patent Document 4 describes that the precipitation of BN is suppressed by setting Ti / N (mass% ratio) to 4 or more. In principle, if Ti / N is set to 3.42 or more, precipitation of BN can be suppressed.
  • the general boron steel as described above tends to generate so-called coarse grains in which some austenite crystal grains are coarsened by abnormal grain growth during quenching heating as compared with conventional steels.
  • coarse grains In a part in which coarse grains are generated, deterioration in dimensional accuracy due to an increase in heat treatment strain generated during quenching, and deterioration in part characteristics such as impact value, fatigue strength, and delayed fracture characteristics occur. Therefore, especially in the case of high strength bolts having a tensile strength of 800 MPa or more, preventing the generation of coarse grains is a major practical issue.
  • N in the steel is fixed as TiN by the addition of Ti, so that AlN that effectively acts as pinning particles in carbon steel and alloy steel as conventional steel is not generated.
  • TiN is coarser than AlN, it cannot be finely dispersed, and it is difficult to secure the number of pinning particles necessary for preventing coarse particles.
  • the above factor (1) is unavoidable in order to omit the annealing process. Therefore, how to secure the number of pinning particles in boron steel to improve the factor (2) It has been a point of prevention.
  • Patent Document 5 and Patent Document 6 describe using TiC and Ti (CN), which are finer precipitates than TiN, instead of AlN and TiN as pinning particles.
  • TiC and Ti (CN) having a diameter of 0.2 ⁇ m or less in steel before quenching and after hot rolling are used. Is dispersed in a total number of 20/100 ⁇ m 2 or more. By dispersing a large amount of such fine precipitates before quenching and heating, these precipitates function as pinning particles that pin the austenite grain boundaries during quenching and heating. Since this technology makes it possible to stably prevent the generation of coarse grains in boron steel, steel to which this technology is applied is currently widely used as an inexpensive bolt steel material that can omit the annealing process.
  • Patent Document 7 also describes a technical idea similar to the technique for preventing the generation of coarse grains of the boron steel. That is, it is a technique for preventing the coarsening of crystal grains by dispersing the carbonitrides of these elements in steel by making the relationship among the contents of Ti, Nb, Al, and N within a certain range. Patent Document 7 further describes the effect of improving the machinability by adding 0.01% or more of Bi. However, in Patent Document 7, only the effect of improving the machinability is disclosed as the effect of Bi. There is no description of the relationship between Bi and the coarsening characteristics of crystal grains. Since Bi is added for the purpose of improving the machinability, Patent Document 7 only considers adding a relatively large amount of Bi. In this case, as described in Patent Document 7, there is a concern about a decrease in hot workability due to Bi addition.
  • Patent Document 8 discloses a case-hardening steel that exhibits excellent grain coarsening characteristics even when carburized at a higher temperature than conventional examples, and exhibits excellent cold workability without soft annealing. Steel for case hardening intended to provide is disclosed. However, Patent Document 8 proposes only the use of fine Ti carbides, Ti-containing composite carbides, and the like as means for ensuring the crystal grain coarsening resistance. In Patent Document 8, the hot rolling temperature is extremely low in order to ensure cold workability, and thus the productivity of case hardening steel is impaired.
  • the present invention has been made in view of the above problems. That is, the present invention suppresses the generation of coarse grains during quenching without using Ti carbides and Ti carbonitrides such as TiC and Ti (CN), thereby improving productivity, cold forgeability, and quenching. It is an object to provide a steel excellent in all mechanical properties.
  • the gist of the present invention is as follows.
  • the chemical composition is unit mass%, C: 0.15% to 0.40%, Mn: 0.10% to 1.50%, S: 0.00. 002 to 0.020%, Ti: 0.005% to 0.050%, B: 0.0005 to 0.0050%, Bi: 0.0010% to 0.0100%, P: 0.020% or less, N: 0.0100% or less, Si: 0% or more and less than 0.30%, Cr: 0 to 0.050%, Mo: 0 to 0.20%, Cu: 0 to It contains 0.20%, Ni: 0 to 0.20%, and Nb: 0 to 0.030%, with the balance being Fe and impurities.
  • the chemical component is unit mass%, Si: 0.01% or more and less than 0.30%, Cr: 0.01 to 1.50%, and Al: One or more selected from the group consisting of 0.001 to 0.050% may be contained.
  • the chemical component is unit mass%, Mo: 0.02 to 0.20%, Cu: 0.02 to 0.20%, Ni One or two or more selected from the group consisting of: 0.02 to 0.20% and Nb: 0.002 to 0.030% may be contained.
  • the steel according to any one of (1) to (3) may have an N fixed index I FN defined by the following formula 1 of 0 or more.
  • I FN [Ti] ⁇ 3.5 ⁇ [N] (Formula 1)
  • [Ti] is the Ti content in unit mass%
  • [N] is the N content in unit mass%.
  • I P 0.3 ⁇ [Ti] + 0.15 ⁇ [Nb] ⁇ [N] (Formula 2)
  • [Ti] is the Ti content in unit mass%
  • [Nb] is the Nb content in unit mass%
  • [N] is the N content in unit mass%.
  • the steel according to the present invention it is possible to provide steel that can achieve both softening before cold forging and suppression of generation of coarse grains during quenching after cold forging. Moreover, the steel according to the present invention is excellent in manufacturability because it is not cracked during casting or rolling, and can be manufactured under conditions that do not place a load on the manufacturing equipment.
  • wear of the mold during cold forging can be suppressed, and the life of the mold can be improved.
  • the steel according to the present invention to cold forged parts, the cost of expensive dies can be reduced, which can contribute to the reduction of the manufacturing cost of high-strength bolts having a tensile strength of 800 MPa or more. it can.
  • the steel according to the present invention is excellent in machinability. Therefore, the present invention has a great industrial contribution.
  • the steel according to one embodiment of the present invention will be described.
  • the steel according to this embodiment has the following characteristics.
  • the chemical composition is unit mass%, C: 0.15% to 0.40%, Mn: 0.10% to 1.50%, S: 0.002 to 0.020%, Ti: 0.005% to 0.050%, B: 0.0005 to 0.0050%, Bi: 0.0010% to 0.0100%, P: 0.020% or less, N: 0.0100% or less, Si: 0% or more and less than 0.30%, Cr: 0 to 0.050%, Mo: 0 to 0.20%, Cu: 0 to 0.0. 20%, Ni: 0 to 0.20%, and Nb: 0 to 0.030%, with the balance being Fe and impurities.
  • the chemical component is unit mass%, Si: 0.01% or more and less than 0.30%, Cr: 0.01 to 1.50%, and Al: One or more selected from the group consisting of 0.001 to 0.050% may be contained.
  • the chemical component is unit mass%, Mo: 0.02 to 0.20%, Cu: 0.02 to 0.20%, Ni One or two or more selected from the group consisting of: 0.02 to 0.20% and Nb: 0.002 to 0.030% may be contained.
  • the steel according to any one of the above (a) to (c) may have an N fixed index I FN defined by the following formula 1 of 0 or more.
  • I FN [Ti] ⁇ 3.5 ⁇ [N] (Formula 1)
  • [Ti] is the Ti content in unit mass%
  • [N] is the N content in unit mass%.
  • I P 0.3 ⁇ [Ti] + 0.15 ⁇ [Nb] ⁇ [N]
  • [Ti] is the Ti content in unit mass%
  • [Nb] is the Nb content in unit mass%
  • [N] is the N content in unit mass%.
  • produce a coarse grain with excellent productivity is obtained by performing bolt processing, quenching, and tempering with respect to the steel which concerns on this embodiment by a well-known method.
  • the inventors have noticed a significant increase in ferrite hardness due to precipitation strengthening, and hence TiC and Ti (CN), etc., which are particles that cause an increase in steel hardness and impair the cold workability of steel.
  • TiC and Ti (CN), etc. are particles that cause an increase in steel hardness and impair the cold workability of steel.
  • the above features are based on the following findings obtained by the present inventors by earnestly studying the technology for suppressing abnormal grain growth of austenite grains during quenching and heating of steel.
  • C is an element necessary for increasing the strength of steel having a tempered martensite structure.
  • the C content needs to be 0.15% or more.
  • the lower limit of the preferred C content is 0.17%, 0.19%, or 0.23%.
  • the upper limit of C content is 0.40%.
  • the upper limit of the preferable C content is 0.35%, 0.34%, 0.33%, or 0.30%.
  • Mn is an element effective for improving the hardenability of steel.
  • the Mn content needs to be 0.10% or more.
  • the lower limit of the preferable Mn content is 0.20%, 0.35%, or 0.40%.
  • the upper limit of the Mn content is 1.50%.
  • the upper limit of the preferable Mn content is 1.30%, 1.00%, or 0.80%.
  • S exists in steel as MnS, TiS, and Ti 2 C 2 S, and has the effect of suppressing abnormal grain growth of austenite crystal grains by acting as pinning particles during quenching heating. For this reason, it is necessary to make S content 0.002% or more.
  • the lower limit of the preferable S content is 0.003%.
  • the steel according to the present embodiment uses Bi to suppress abnormal grain growth, the S content may be smaller than that of the prior art.
  • S content exceeds 0.020%, S causes embrittlement of the prior austenite grain boundaries of the steel after quenching, and deteriorates delayed fracture resistance (hydrogen embrittlement resistance).
  • Ti 2 C 2 S described above is a particle that impairs the machinability of the steel, if the S content exceeds 0.020%, the machinability of the steel may be deteriorated. Therefore, it is necessary to limit the S content to 0.020% or less.
  • the upper limit of the S content is 0.015%, 0.010%, or 0.005%.
  • Ti forms a compound with C, N, and S in steel and exists in steel as Ti-based inclusions such as TiN, Ti (CN), TiC, TiS, and Ti 2 C 2 S. By acting as a stop particle, it has the effect of suppressing abnormal grain growth of austenite grains. Further, Ti has a strong affinity for solute N in steel, and is therefore an extremely effective element for preliminarily fixing solute N in steel as TiN and suppressing the formation of BN. In boron steel, it is necessary to suppress the formation of BN in order to ensure the content of solute B that is effective in improving hardenability. Therefore, the Ti content needs to be 0.005% or more.
  • the lower limit of the preferable Ti content is 0.010%, 0.015%, or 0.020%.
  • the Ti content may be smaller than that of the prior art.
  • Ti-based inclusion particles cause precipitation strengthening, and the hardness of the rolled material after hot rolling becomes too high. The service life is significantly reduced.
  • the upper limit of the Ti content is 0.050%.
  • a preferable Ti content is 0.040% or less, 0.030% or less, less than 0.030%, or 0.025% or less.
  • B is an element that contributes to improving the hardenability of steel when contained in a trace amount, and improves hardenability without increasing the hardness of the rolled material after hot rolling and before cold forging. An effect can be acquired and the hardness after cold forging and hardening can be increased.
  • B is an essential element particularly for boron steel for bolts. Further, B has an effect of suppressing grain boundary fracture by segregating at the prior austenite grain boundaries and strengthening the prior austenite grain boundaries. In order to obtain the above effect, the B content needs to be 0.0005% or more. Preferably, the lower limit of the B content is 0.0010%, 0.0012%, or 0.0015%.
  • the B content exceeds 0.0050%, the effect is saturated. Therefore, the B content is 0.0050% or less.
  • the upper limit of the B content is 0.0030%, 0.0025%, 0.0020%, or 0.0018%.
  • Bi 0.0010% to 0.0100%
  • the lower limit of Bi content is preferably 0.0020%, 0.0025%, or 0.0030%.
  • the Bi content exceeds 0.0100%, not only the effect is saturated, but also the hot ductility of the steel decreases, so cracks and flaws occur in the steel manufacturing process (casting, rolling process, etc.). It becomes easier and the yield decreases.
  • the Bi content exceeds 0.0100%, grain boundary embrittlement occurs in the steel after quenching, and the mechanical properties of the steel are impaired. Therefore, the Bi content is 0.0100% or less.
  • the Bi content is preferably less than 0.0100%, 0.0080% or less, or 0.0060% or less.
  • P is an impurity and is an element that embrittles the old ⁇ grain boundary and lowers the delayed fracture resistance (hydrogen embrittlement resistance) of the steel. Therefore, it is necessary to limit the P content to 0.020% or less.
  • the upper limit of the P content is 0.015%, 0.013%, or 0.010%. Since P is not required to solve the problem of the steel according to the present embodiment, the lower limit value of the P content is 0%. However, in order to suppress the cost of the refining process for reducing the P content, the lower limit value of the P content may be 0.001%.
  • the lower limit of the N content is 0%.
  • the lower limit value of the N content may be 0.0001%, 0.0005%, or 0.0010%.
  • the upper limit of the N content is 0.0070%, 0.0050%, or 0.0040%.
  • the spring steel according to the present embodiment may further contain one or more selected from the group consisting of Si, Cr, and Al as necessary, within a range described below. However, since Si, Cr, and Al are not essential, the lower limit of the content of each of Si, Cr, and Al is 0%.
  • the lower limit value of the Si content is 0%.
  • Si is an element effective in improving the hardenability of steel and improving the temper softening resistance of martensite.
  • the Si content is preferably more than 0% or 0.01% or more.
  • the lower limit value of the Si content may be 0.05% or 0.15%.
  • the Si content is less than 0.30%.
  • the upper limit of the preferable Si content is 0.27%, 0.25%, or 0.20%.
  • the lower limit value of the Cr content is 0%.
  • Cr is an effective element for improving the hardenability of steel and improving the temper softening resistance of martensite.
  • the Cr content is preferably more than 0% or 0.01% or more.
  • the lower limit of the Cr content may be 0.10%, 0.20%, or 0.30%.
  • the upper limit of the Cr content is 1.50%.
  • the upper limit of the preferable Cr content is 1.20%, 1.00%, or 0.80%.
  • Al is an element effective for deoxidation of steel.
  • the lower limit of the Al content is 0%.
  • the Al content exceeds 0.050%, problems such as generation of coarse inclusions and deterioration of the toughness of steel become significant. Therefore, even when Al is contained, the upper limit of the Al content is 0.050%.
  • the upper limit of the Al content is preferably 0.040%, 0.030%, or 0.025%.
  • the spring steel according to the present embodiment may further contain one or more selected from the group consisting of Mo, Cu, Ni, and Nb as necessary, within a range described below. However, since Mo, Cu, Ni, and Nb are not essential, the lower limit of the contents of Mo, Cu, Ni, and Nb is 0%.
  • the lower limit value of the Mo content is 0%.
  • Mo is an element that contributes to improving the hardenability of steel even if its content is small.
  • the Mo content is preferably set to 0.02% or more. More preferably, the lower limit of the Mo content is 0.03%, 0.04%, or 0.05%.
  • the Mo content is set to 0.20% or less.
  • the upper limit of the Mo content is 0.16%, 0.13%, or 0.10%.
  • the lower limit value of the Cu content is 0%.
  • Cu is an element that improves the corrosion resistance of steel.
  • the Cu content is preferably set to 0.02% or more. More preferably, the lower limit of the Cu content is 0.05%.
  • the upper limit of Cu content is 0.15%, 0.10%, or 0.08%.
  • the lower limit value of the Ni content is 0%.
  • Ni is an element that improves the corrosion resistance of steel and is also an effective element for improving the toughness of steel.
  • the Ni content is preferably 0.02% or more. More preferably, the lower limit of the Ni content is 0.03%, 0.04%, or 0.05%.
  • the upper limit of the Ni content is 0.15%, 0.12%, 0.10%, or 0.08%.
  • the lower limit value of the Nb content is 0%.
  • Nb forms a compound with C in steel and exists in steel as Nb-based inclusions such as NbC or TiNb (CN), and suppresses abnormal growth of austenite grains as pinning particles during quenching heating.
  • the Nb content is preferably set to 0.002% or more. More preferably, the lower limit of Nb content is 0.003%, 0.005%, or 0.006%.
  • the Nb content exceeds 0.030%, not only the effect is saturated, but also the Nb-based inclusions cause precipitation strengthening, so that the manufacturability during continuous casting is impaired.
  • Nb-based inclusions cause precipitation strengthening, so that the hardness of the rolled material after hot rolling becomes too high. Therefore, when the Nb content exceeds 0.030%, problems such as a decrease in manufacturability and a significant decrease in the life of a cold forging die become prominent. Therefore, even when Nb is contained, the Nb content is set to 0.030% or less.
  • the upper limit of Nb content is 0.015%, 0.013%, or 0.010%.
  • the steel according to this embodiment contains the above alloy components, and the balance of the chemical components contains Fe and impurities.
  • impurities are components mixed due to raw materials such as ores and scraps and other factors when industrially producing steel materials, and do not impair the effects of the steel according to the present embodiment. Means what is the amount.
  • N fixed index I FN preferably 0 or more
  • N fixed index IFN defined by the following formula 1 to 0 or more.
  • the lower limit of the N fixed index I FN 0.0005,0.0010,0.0014, or may be 0.0050.
  • the steel according to the present embodiment is softened before cold forging as long as the Ti content and the N content are controlled within the above-described range even if the N fixed index IFN is not particularly limited.
  • the generation of coarse grains during quenching can be suppressed.
  • I FN [Ti] ⁇ 3.5 ⁇ [N] (Formula 1)
  • [Ti] and [N] in the above formula 1 indicate the Ti content and N content in the steel in unit mass%, and 0% when these elements are not contained.
  • Ti—Nb-based precipitate formation index I P preferably 0.0100 or less
  • I P the amount of solid solution N
  • Ti combines with C and S to form fine precipitates, and these fine precipitates may adversely affect the properties of the steel according to the present embodiment.
  • the present inventors have also found that Nb has the same function as Ti.
  • Ti—Nb-based precipitates such as Ti 2 C 2 S, which are precipitates existing in steel, are pinned during quenching heating.
  • Ti—Nb-based precipitates such as Ti 2 C 2 S, which are precipitates existing in steel.
  • these Ti—Nb-based precipitate particles are dispersed in a large amount in the structure after hot rolling, there is a side effect that the hardness of the ferrite increases due to precipitation strengthening by fine precipitate particles. . Therefore, when these Ti—Nb-based precipitate particles are dispersed in an excessively large amount in the steel, the hardness of the rolled material after hot rolling becomes too high.
  • Ti 2 C 2 S causes deterioration of machinability. Therefore, in the steel according to the present embodiment, it is preferable to limit the amount of these Ti—Nb-based precipitate particles.
  • the Ti-Nb-based precipitates generated index I P which is calculated by the following equation 2 and 0.0100 or less.
  • the Ti-Nb-based precipitates generated index I P 0.0075 or less, less than 0.0050, 0.0045 or less, 0.0040 or less, or 0.0035 may be less.
  • Steel is softened before cold forging and can suppress the generation of coarse grains during quenching.
  • the suitable manufacturing method of the steel of this embodiment is demonstrated.
  • the above-described chemical component steel is melted in a converter, and a slab is obtained by continuous casting through a secondary refining process as necessary.
  • the slab is reheated and subjected to ingot rolling to obtain a wire rolling material (steel slab) having a cross section of, for example, 162 mm square (vertical 162 mm ⁇ lateral 162 mm).
  • the steel slab is heated at a temperature of about 1000 to 1280 ° C. and subsequently subjected to wire rod rolling to form a wire rod having a diameter of 6 to 20 mm.
  • it after being hotly wound into a coil shape by a winding device, it is cooled to room temperature. In this way, the steel of this embodiment is obtained.
  • the amount of Ti-based precipitated particles that cause precipitation strengthening is suppressed. Therefore, in the steel manufacturing method according to the present embodiment, hot rolling is performed in order to suppress the hardness of the steel. It is not necessary to apply a load to the hot rolling equipment by lowering the temperature, and defects such as cracks and wrinkles due to an increase in hardness are less likely to occur in the steel. Furthermore, the hardness of the steel according to the present embodiment is suppressed without performing annealing after hot rolling. Therefore, the steel according to this embodiment is excellent in terms of high productivity.
  • both softening before cold forging and suppression of the generation of coarse grains during quenching can be achieved.
  • the steel of this embodiment is excellent in manufacturability without cracking during casting or rolling.
  • the hardness of the steel according to the present embodiment is not particularly limited because it can be appropriately adjusted according to the application. However, when it is necessary to ensure cold forgeability, the hardness of the steel according to this embodiment is preferably Hv 180 or less, and more preferably Hv 170 or less, or Hv 160 or less. .
  • the lower limit value of the hardness of the steel according to the present embodiment is not particularly limited, but is considered to be substantially about Hv130 or about Hv140 in view of its chemical composition. Even if it does not anneal after hot rolling, the steel which concerns on this embodiment can make the hardness into the above-mentioned suitable range. Moreover, the steel according to the present embodiment is excellent in machinability.
  • the steel according to the present embodiment is heated to a temperature of, for example, 840 ° C. to 1100 ° C., held for 30 minutes, and then quenched under water cooling or oil cooling, and further in a temperature range of 150 ° C. to 450 ° C.
  • the tensile strength can be 800 MPa or more. Therefore, the steel according to the present embodiment is suitable as a material for parts that require high strength.
  • heat processing conditions are not specifically limited, It can select suitably according to a use.
  • the use of the steel according to the present embodiment is not particularly limited, it is preferable that the steel is applied to high-strength mechanical parts manufactured by cold forging and quenching, particularly high-strength bolts.
  • the steel according to the present embodiment having high cold forgeability is used as a material for high-strength mechanical parts, wear of the mold during cold forging can be suppressed and the life of the mold can be improved.
  • die can be reduced, it can contribute to the reduction of the manufacturing cost of the high intensity
  • the cast slab was subjected to soaking diffusion treatment and partial rolling as necessary to obtain a wire rolling material (steel piece) having a cross section of 162 mm square (vertical 162 mm ⁇ lateral 162 mm).
  • a wire rolling material steel piece
  • the steel slab was heated at a temperature of about 1000 to 1280 ° C. and subsequently subjected to wire rod rolling to obtain a wire rod (spring steel) having a diameter of 10 mm.
  • a test piece for Vickers hardness measurement was cut out from the rolled wire. Specifically, a test piece having a cross section including the central axis of the wire in a direction parallel to the rolling direction was cut out. After the cut cross section was polished, the Vickers hardness of a portion (1/4 part) having a depth of 1/4 of the diameter of the wire from the surface of the wire was measured.
  • the test load is 10 kgf, and the average value obtained by measuring four points is described as “hardness after rolling” in Tables 2-1 and 2-2. This is an index for predicting the life of a die for cold forging. did.
  • the prior austenite grain boundaries appeared by corrosion and observed with an optical microscope to measure the prior austenite grain size after quenching and tempering.
  • the prior austenite grain size was measured according to JISG0551.
  • the measurement field of view was 400 times magnification and 10 fields or more, and a test piece in which even one large crystal grain having a prior austenite grain size of No. 5 or less was present was determined to have coarse grains.
  • the results of measuring the crystal grain coarsening temperature are shown in Tables 2-1 and 2-2.
  • A1 to A32 which are examples of the present invention, have low hardness of the wire after rolling and can be expected to improve the life of the cold forging die.
  • Slab debris rate because no coarse grains are generated even when heated above 900 ° C during quenching and heating after cold working, and surface cracks of the slab do not occur during continuous casting It is clear that it is low and therefore is excellent in manufacturability.
  • the inventive examples A1 to A32 after the heat treatment for measuring the prior austenite grain size described above all had a tensile strength of 800 MPa or more.
  • any of the cold forgeability, coarse grain prevention characteristics, and manufacturability is inferior. That is, since B1 to B4 have too much Bi added, the hot ductility is lowered and the manufacturability is poor. In B5 to B7, Bi was not added, or the addition amount was too small, so the coarse grain preventing properties were inferior. In B8 and B9, the added amount of Ti is too large, or the N content is small relative to the added Ti amount and the Ti—Nb-based precipitate formation index IP is exceeded. It was inferior to the forgeability.
  • the steel according to the present invention it is possible to provide steel that can achieve both softening during cold forging and suppression of generation of coarse grains during quenching after cold forging.
  • the steel according to the present invention is excellent in manufacturability because it is not cracked during casting or rolling, and can be manufactured under conditions that do not impose a load on manufacturing equipment.
  • wear of the mold during cold forging can be suppressed, and the life of the mold can be improved.
  • the steel according to the present invention to cold forged parts, the cost of expensive dies can be reduced, which can contribute to the reduction of the manufacturing cost of high-strength bolts having a tensile strength of 800 MPa or more. it can.
  • the steel according to the present invention is excellent in machinability. Therefore, the present invention has a great industrial contribution.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

Dans un mode de réalisation, l'invention concerne de l'acier comprenant en unités de pourcentage en masse, comme composants chimiques : de 0,15 à 0,40% de C, de 0,10 à 1,50% de Mn, de 0,002 à 0.020% de S, de 0,005 à 0,050% de Ti, de 0,0005 à 0,0050% de B, de 0,0010 à 0,0100% de Bi, 0,020% ou moins de P, 0,0100 ou moins de N, de 0 à moins de 0,30% de Si, de 0 à 1,50% de Cr, de 0 à 0,050% d'Al, de 0 à 0,20% de Mo, de 0 à 0,20% de Cu, de 0 à 0,20% de Ni et de 0 à 0,030% de Nb, le reste étant constitué de Fe et d'impuretés.
PCT/JP2016/078558 2016-09-28 2016-09-28 Acier Ceased WO2018061101A1 (fr)

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US16/329,463 US20190256957A1 (en) 2016-09-28 2016-09-28 Steel
EP16917654.2A EP3521469A4 (fr) 2016-09-28 2016-09-28 Acier
JP2018541768A JP6798557B2 (ja) 2016-09-28 2016-09-28 鋼
KR1020197007851A KR20190041502A (ko) 2016-09-28 2016-09-28 강
CN201680089493.8A CN109790602B (zh) 2016-09-28 2016-09-28 钢
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JP2019218584A (ja) * 2018-06-18 2019-12-26 日本製鉄株式会社 ボルト
CN111100976A (zh) * 2019-09-20 2020-05-05 河南中原特钢装备制造有限公司 玻璃模具用钢锻后防止开裂的热处理工艺

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KR20230082090A (ko) * 2021-12-01 2023-06-08 주식회사 포스코 내지연파괴 저항성이 향상된 냉간단조용 선재, 강부품 및 이들의 제조방법
CN114855093B (zh) * 2022-03-28 2023-10-03 本钢板材股份有限公司 一种高冷镦成型性低碳低硅含铝冷镦钢热轧盘条及其制备方法

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JP7155644B2 (ja) 2018-06-18 2022-10-19 日本製鉄株式会社 ボルト
CN111100976A (zh) * 2019-09-20 2020-05-05 河南中原特钢装备制造有限公司 玻璃模具用钢锻后防止开裂的热处理工艺

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EP3521469A4 (fr) 2020-03-11
CN109790602A (zh) 2019-05-21
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KR20190041502A (ko) 2019-04-22
JP6798557B2 (ja) 2020-12-09
CN109790602B (zh) 2021-03-02
EP3521469A1 (fr) 2019-08-07

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