JP2009221539A - High-strength steel having excellent delayed cracking resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high-strength steel having a delayed cracking resistance used for the part at which delayed cracking becomes a problem in the fields of building, civil engineering, construction industry machines, line pipes, marine structures, energy plants or the like, and suitable as wear resistant steel with a TS of ≥1,200 MPa particularly used in a construction industrial machine field. <P>SOLUTION: The high-strength steel having the delayed cracking resistance has a composition comprising, by mass, 0.20 to 0.40% C, 0.05 to 0.50% Si, ≤0.05% (including 0%) Mn, ≤0.030% P, ≤0.005% S, 0.005 to 0.05% Nb, 0.005 to 0.05% Ti, 0.0003 to 0.0030% B, ≤0.1% Al and ≤0.0060% N, and, if required, containing one or more selected from among Cu, Ni, Cr, Mo, W, V, Ca and REM, and the balance Fe with inevitable impurities, and has a martensitic structure or a tempered martensitic structure in which the average grain size of prior austenite grains is ≤30 μm as the main structure. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to high-strength steel used in construction, civil engineering, construction industrial machinery, line pipes, offshore structures, energy plant fields where delayed fracture is a problem, and particularly TS1200 MPa or more used in the construction industrial machinery field. It is related with what is suitable as wear-resistant steel.

  Delayed fracture is a phenomenon in which a steel plate placed under static load suddenly breaks after a lapse of time, and is more likely to occur with higher strength steels and often becomes a problem with high strength steels of 1200 MPa or more. It has been clarified that delayed fracture involves hydrogen present in steel and residual stress.

  Various methods for suppressing delayed fracture of high-strength steel have been proposed, and Patent Documents 1 to 4 propose techniques for improving delayed fracture resistance by reducing Mn. Patent Document 4 describes a steel for bolts having excellent delayed fracture characteristics, in which Mn is dissolved in cementite and promotes the growth of precipitated cementite, and therefore the delayed fracture characteristics are reduced. .

  Patent Document 5 relates to high-strength steel excellent in delayed fracture resistance, by refining the crystal grain size, and further adding Zr to the component composition to finely disperse carbides that become hydrogen accumulation sites in the steel. It describes the improvement of delayed fracture characteristics. The austenite grain size is determined according to ASTM No. Is regulated to 8.5 or more and about 20 μm or less.

Moreover, in patent document 6, the structure | tissue is refined | miniaturized and the delayed fracture-proof characteristic is improved by the ausfoam effect by taking sufficient reduction in the non-recrystallization temperature range.
JP-A-60-59019 JP-A-5-51691 Japanese Unexamined Patent Publication No. Sho 63-317623 Japanese Patent Laid-Open No. 5-148580 JP-A 61-223168 Japanese Patent Application Laid-Open No. 2002-115024

  However, in Patent Document 3, low temperature tempering heat treatment at 200 to 500 ° C. is required, and it is a problem to prevent deterioration of ductility and toughness due to low temperature tempering brittleness. Therefore, the same problem arises.

  Moreover, in patent document 5, since high temperature tempering of 600 degreeC or more is implemented, compared with the state as-quenched, intensity | strength and hardness fall and it is going to obtain the same strength level as the state as-quenched. However, it is necessary to increase the amount of alloying elements, and an increase in alloy costs is a problem.

  Further, in Patent Documents 1, 2, and 5, P is set to 0.018% or less, 0.010% or less, and 0.020% or less, respectively, and the load in the dephosphorization process increases. In Patent Document 6, austen foam is utilized and the structure is refined by direct quenching. However, strict temperature control during rolling is required for refinement of the rolled structure to suppress delayed fracture.

  The present invention has been made in view of the problems of the prior art, and an object of the present invention is to provide a high-strength steel excellent in delayed fracture resistance.

  In order to achieve the above-mentioned problems, the inventors conducted an SSRT test on steels having various compositional compositions, and conducted extensive research on delayed fracture characteristics of martensitic steels. As a result, even in a steel having a chemical component with a P content exceeding 0.020%, the Mn content is 0.05% or less, and an appropriate amount of Nb, Ti, B, etc. is added, and the prior austenite grain size is 30 μm or less. It was found that the delayed fracture characteristics were improved when the martensitic structure was used.

  Table 1 shows the test steel No. used in the SSRT test. 1 to 3 chemical components were shown, and a 12 mm steel plate was reheated at 900 ° C. and then subjected to water quenching as a test material.

In the SSRT test, a round bar tensile test piece having a parallel part of 3.4 mmΦ × 25 mmL collected from the above test material was used in a 3% sodium chloride-0.3 g / L ammonium thiocyanate aqueous solution with a cathode current density of 0.04 mA / After performing cathode hydrogen charging at cm ^{2 for} 24 hours and galvanizing, holding for 24 hours and applying a tensile load at a strain rate of 3.3 × 10 ⁻⁶ / s at room temperature .

  Table 2 shows the room temperature strength and SSRT test results of the test materials (test steels Nos. 1 to 3). From the test material (sample Nos. 2 and 3) in which the Mn content was 0.45% or more and the P content was reduced to 0.020% or less, the Mn content was reduced to 0 even if the P content exceeded 0.020%. It can be confirmed that the test material (test steel No. 1) reduced to 0.05% or less has excellent time to break, break strength and drawing value, and improved delayed fracture characteristics. The room temperature strength of the test materials is almost the same.

The present invention has been completed by further investigation based on the obtained knowledge. That is, the gist of the invention is as follows.
1. In mass%, C: 0.20 to 0.40%, Si: 0.05 to 0.50%, Mn: 0.05% or less (including 0%), P: 0.030% or less, S: 0 0.005% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0030%, Al: 0.1% or less, N: 0.00. Delayed fracture resistance with excellent delayed fracture resistance, mainly composed of martensite structure or tempered martensite structure, including 0060% or less, balance Fe and inevitable impurities, and average austenite grain size of 30 μm or less High strength steel with excellent properties.
2. Furthermore, by mass%, Cu: 0.05-1.0%, Ni: 0.05-2.0%, Cr: 0.05-1.0%, Mo: 0.05-1.0%, W: 0.05 to 2.0%, V: 0.005 to 0.1% One or more compositions are contained, and excellent in delayed fracture resistance according to 1. High strength steel.
3. The delayed fracture resistance according to 1 or 2, further comprising a composition containing one or two of Ca: 0.0005 to 0.0050% and REM: 0.0005 to 0.0050%. Excellent high strength steel.
4). 4. The high strength steel excellent in delayed fracture resistance according to any one of 1 to 3, wherein the martensite structure or tempered martensite structure is 90% or more in volume fraction.

  According to the present invention, a steel having high strength and excellent delayed fracture resistance can be obtained without using a large amount of expensive alloy elements, and a remarkable industrial effect can be obtained.

Hereinafter, the present invention will be described in detail.
[Chemical component] In the following%, all indicate mass%.
C: 0.20 to 0.40%
C is an element effective for ensuring the strength of high-strength steel or the surface hardness of wear-resistant steel, and 0.20% or more is necessary to exert its effect. However, if it exceeds 0.40%, the weldability is remarkably deteriorated. Therefore, it is set to 0.20 to 0.40%.

Si: 0.05 to 0.50%
Si is a deoxidizing element and is an element effective for securing strength as a solid solution strengthening. In order to exhibit the effect, 0.05% or more is necessary. However, if it exceeds 0.50%, the weldability is remarkably deteriorated. Therefore, it is 0.05 to 0.50%.

Mn: 0.05% or less Mn significantly deteriorates delayed fracture characteristics. Therefore, it is 0.05% or less (including no addition).

P: 0.030% or less P segregates at grain boundaries, weakens grain boundary strength, and deteriorates delayed fracture characteristics. For this reason, a lower value is desirable, but a load is imposed on the dephosphorization work in the steel making process.

  In the present invention, in order to reduce this load, the amount of Mn is reduced. However, if the amount of P exceeds 0.030%, even if the amount of Mn is reduced, the grain boundary strength is lowered and the delayed fracture characteristics are deteriorated. Therefore, it is 0.030% or less.

S: 0.005% or less S exists as MnS, becomes a starting point of destruction, and deteriorates delayed fracture characteristics. Reduction of S is important because MnS is generated even with a slight amount of Mn of 0.05% or less (including no addition) specified in the present invention. Therefore, it is made 0.005% or less.

Nb: 0.005 to 0.05%
Nb is an element effective for refining the crystal grain size. In order to exert the effect, 0.005% or more is necessary. However, if added over 0.05%, coarse Nb (CN) or the like remains and deteriorates the base material toughness. Therefore, it is made 0.005 to 0.05%.

Ti: 0.005 to 0.05%
Ti is an element that fixes N as TiN and effectively exhibits the effect of improving the hardenability of boron, and 0.005% or more is necessary to exert the effect. However, if added over 0.05%, coarse TiN is generated and the base metal toughness deteriorates. Therefore, it is made 0.005 to 0.05%.

B: 0.0003 to 0.0030%
B segregates at the grain boundaries, increases the grain boundary strength, and improves the base metal toughness and delayed fracture characteristics. Furthermore, hardenability is remarkably improved by adding a small amount. In order to exhibit the effect, 0.0003% or more is necessary. However, by adding over 0.0030%, it precipitates as a carbon boride and deteriorates the base metal toughness. Therefore, it is set as 0.0003 to 0.0030%.

Al: 0.1% or less Al is a deoxidizing material, and by fixing N as AlN, the crystal grain size is refined and the base material toughness is improved. Containing a large amount exceeding 0.1% reduces the cleanliness of the steel. For this reason, Al is limited to 0.1% or less. When Al is used for the deoxidizing material, it is desirable that the content be 0.0020% or more.

N: 0.0060% or less N binds to boron and exists as BN, thereby inhibiting the effect of improving the hardenability of boron. Therefore, it is made 0.0060% or less.

  The above is the basic component system of the present invention, but when further improving the characteristics, one or more of Cu, Ni, Cr, Mo, W, V, Ca, and REM are contained.

Cu: 0.05 to 1.0%
Cu is an element effective for solid solution strengthening. In order to exhibit the effect, 0.05% or more is necessary. However, addition exceeding 1.0% causes an increase in alloy cost. Therefore, when adding, it is 0.05 to 1.0%.

Ni: 0.05-2.0%
Ni is an element effective for solid solution strengthening and has the effect of improving the base material toughness. In order to exhibit the effect, 0.05% or more is necessary. However, addition over 2.0% causes an increase in alloy costs. Therefore, when adding, it is 0.05 to 2.0%.

Cr: 0.05-1.0%
Cr is an element effective for solid solution strengthening. In order to exhibit the effect, 0.05% or more is necessary. However, addition exceeding 1.0% causes an increase in alloy cost. Therefore, when adding, it is 0.05 to 1.0%.

Mo: 0.05-1.0%
Mo is an element effective for solid solution strengthening. In order to exhibit the effect, 0.05% or more is necessary. However, addition exceeding 1.0% causes an increase in alloy cost. Therefore, when adding, it is 0.05 to 1.0%.

W: 0.05-2.0%
W is an element effective for solid solution strengthening. In order to exhibit the effect, 0.05% or more is necessary. However, addition over 2.0% causes an increase in alloy costs. Therefore, when adding, it is 0.05 to 2.0%.

V: 0.005 to 0.1%
V is an element effective for solid solution strengthening. In order to exert the effect, 0.005% or more is necessary. However, if added over 0.1%, the base material toughness deteriorates. Therefore, when adding, it is made 0.005 to 0.1%.

Ca: 0.0005 to 0.0050%
Ca has an effect of reducing MnS, which can be a fracture starting point, by fixing S. In order to exhibit the effect, 0.0005% or more is necessary. On the other hand, adding more than 0.0050% reduces the cleanliness of the steel. Therefore, when adding, it is 0.0005 to 0.0050%.

REM: 0.0005 to 0.0050%
REM has the effect of reducing MnS, which can be a fracture starting point, by fixing S. In order to exhibit the effect, 0.0005% or more is necessary. On the other hand, adding more than 0.0050% reduces the cleanliness of the steel. Therefore, when adding, it is 0.0005 to 0.0050%.

The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include O: 0.040% or less, Pb: 0.01% or less, Sn: 0.01% or less, and Sb: 0.01% or less. It should be noted that the smaller the inevitable impurities, the better.
[Microstructure]
The microstructure is mainly a martensite structure or a tempered martensite structure in which the average particle size of the prior austenite grains is 30 μm or less. Here, the main structure is a structure that accounts for 80% or more of the volume fraction, and the finer the microstructure, the delayed fracture characteristics are improved. If the average particle size exceeds 30 μm, the delayed fracture characteristics deteriorate rapidly, so the thickness is made 30 μm or less. In order to obtain excellent base material toughness, it is desirable that the main structure of the matrix has a high martensite fraction, and the volume fraction of martensite or tempered martensite is desirably 90% or more.

  The remaining structure is not particularly defined. However, since the base material toughness deteriorates in a structure in which upper bainite is mixed, it is desirable that the structure does not contain as much as possible. Next, preferable production conditions of the present invention will be described.

  As for the production conditions, it is desirable to carry out a quenching heat treatment after rolling the steel sheet, air-cooling to room temperature, reheating to the austenite single phase region. Moreover, you may use the direct hardening which cools immediately after rolling. In the case of direct quenching, in order to obtain a prior austenite grain size having an average grain size of 30 μm or less, it is desirable that the rolling finishing temperature is 950 ° C. or less.

  Furthermore, you may implement tempering below 500-Ac1 point. It is also acceptable to perform tempering at 300 ° C. or lower for dehydrogenation treatment and distortion removal.

Molten steel having the composition shown in Table 3 was melted in a vacuum melting furnace to obtain a small steel ingot (150 kg) (steel material). After rolling these steel materials to 12 mm, they were reheated to 900 ° C. and subjected to quenching heat treatment. The obtained steel sheet was subjected to a structure observation, a tensile test, and an SSRT test.
[Structure Observation] A specimen for structure observation was collected from the obtained steel sheet, polished, corroded with nital, and then observed with an electron microscope to obtain a martensite fraction (volume fraction).

  Furthermore, it corroded with the picric acid aqueous solution, and the prior austenite particle size was measured using the optical microscope about the position of the board thickness 1 / 4t part. As for the prior austenite grain size, about 200 prior austenite grains were observed, the circle equivalent grain size of each grain was determined, and the average value thereof was taken as the mean grain size of the steel sheet.

  [Tensile test] About the obtained steel plate, a round bar tensile test piece having a parallel part of 6 mmΦ x 30 mmL was taken from 1/2 t part, and the strength was measured.

[SSRT test] Delayed fracture characteristics were evaluated by the SSRT test. From the 1 / 2t position of the obtained steel plate, a round bar tensile test piece having a parallel portion of 3.4 mmΦ × 25 mmL was prepared in a 3% sodium chloride-0.3 g / L ammonium thiocyanate aqueous solution with a cathode current density of 0.04 mA. The cathode hydrogen was charged for 24 hours at / cm <2> and galvanized, then held for 24 hours, and a tensile load was applied at room temperature at a strain rate of 3.3 * 10 < ^-6 > / s.

  Table 4 shows the results of the structure observation, tensile test, and SSRT test. As a result of the structure observation, the main structure of all the test steels (symbols A to G) was martensite having a volume fraction of 90% or more, and the average grain size of the prior austenite grains was 30 μm or less excluding the symbol G.

  The steel of the present invention (symbols A, B, C, D) has a delayed fracture property with a ratio of fracture strength at SSRT to TS (strength during normal tension) of 70% or more and an RA value ratio of 25% or more. In contrast, the comparative steels (symbols E, F, and G) have a delayed fracture with a ratio of fracture strength in SSRT to TS (normal tensile strength) of less than 70% and RA value ratio of less than 25%. The characteristics are inferior.

  The comparative steels (symbols E and F) satisfy the requirements of the present invention in terms of microstructure, but the component composition is outside the scope of the present invention, and the comparative steel (symbol G) has a microstructure and component composition outside the scope of the present invention. It was. In particular, the comparative steel (symbol E) had a high Mn content of 0.42% and was inferior in delayed fracture characteristics even when the P content was 0.008%.

Claims (4)

  In mass%, C: 0.20 to 0.40%, Si: 0.05 to 0.50%, Mn: 0.05% or less (including 0%), P: 0.030% or less, S: 0 0.005% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0030%, Al: 0.1% or less, N: 0.00. High-strength steel excellent in delayed fracture resistance, mainly composed of martensite structure or tempered martensite structure, comprising 0060% or less, the balance being Fe and inevitable impurities, and the average grain size of prior austenite grains being 30 μm or less .   In addition to the above composition, Cu: 0.05 to 1.0%, Ni: 0.05 to 2.0%, Cr: 0.05 to 1.0%, Mo: 0.05 to A composition containing one or more selected from 1.0%, W: 0.05 to 2.0%, and V: 0.005 to 0.1%. 1. High strength steel excellent in delayed fracture resistance as described in 1.   The delayed fracture resistance according to 1 or 2, further comprising a composition containing one or two of Ca: 0.0005 to 0.0050% and REM: 0.0005 to 0.0050%. Excellent high strength steel.   The high-strength steel excellent in delayed fracture resistance according to any one of claims 1 to 3, wherein the martensite structure or tempered martensite structure has a volume fraction of 90% or more.