JP4657128B2 - High strength structural steel with excellent hydrogen embrittlement resistance and toughness and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength structural steel having sufficient hydrogen embrittlement resistance and toughness that can be used universally as a high-strength structural steel requiring a tensile strength of 1250 MPa or more, and the high-strength structural steel. The present invention relates to a manufacturing method that can be manufactured relatively easily.

  In recent years, lightening, increasing the size and extending the life of transportation equipment and building structures have been studied mainly for the purpose of reducing the environmental impact. There is a demand for improvement in material properties such as lifetime.

  However, generally, as the strength increases, the toughness and fatigue characteristics deteriorate. Further, in high-strength steel having a tensile strength of 1200 MPa or more used for high-strength members such as bolts, PC steel bars, and suspension springs, hydrogen-induced embrittlement represented by delayed fracture becomes significant. For this reason, it has been a challenge to maintain high toughness and hydrogen embrittlement resistance and to achieve ultra-high strength.

  Various attempts have been made for this purpose.

  For example, Patent Document 1 discloses a spring steel having a tensile strength of 1700 MPa or more with good hydrogen fatigue resistance by adding V, Mo, Ti, Nb, and Zr and introducing hydrogen trap sites due to these precipitates. It is disclosed.

  Patent Document 2 describes that by drawing pearlite steel, high strength with a tensile strength of 1200 MPa or more and excellent delayed fracture resistance can be obtained.

Patent Document 3 describes the austenite grain size according to ASTM No. It is described that a high strength steel having excellent delayed fracture resistance can be obtained by tempering at 580 ° C. or higher. There are many other examples in which the austenite grain size is controlled in order to achieve high strength. In Patent Document 4, the old austenite average crystal grain size is controlled to 1.0 while controlling the carbide shape of the steel wire for spring. It is described that high fatigue resistance and corrosion fatigue resistance can be obtained by setting to ˜18.0 μm. In Patent Document 5, old austenite is obtained by performing a special heat treatment on a steel material containing 7 to 12 wt% of Ni. It is described that a high-strength steel excellent in delayed fracture characteristics with an average crystal grain size of 5 μm or less and a tensile strength of 1400 MPa or more can be obtained.
JP 2001-288539 A JP-A-11-315347 JP-A 64-4566 Japanese Patent Laid-Open No. 2002-19496 Japanese Patent Laid-Open No. 11-80903

  As described above, many technologies have been developed for the problem of increasing the strength. However, for example, an increase in strength of 11 T or more for bolts has not yet been achieved. The reasons for this are the high costs associated with the positive addition of microalloys and Ni, and the fact that it is difficult to actually manufacture because it makes use of special and less versatile heat treatments such as rapid heating. Can be mentioned. Until now, only limited material properties have been noticed and their improvement has been studied, and considering practical application, various material properties such as hydrogen embrittlement resistance and toughness can be improved in a balanced manner. is necessary.

  The present invention has been made in view of such circumstances, and has high hydrogen embrittlement resistance and tough ductility that can be used universally as a high-strength structural steel that requires a tensile strength of 1250 MPa or more. It is an object of the present invention to provide a high-strength structural steel and a manufacturing method capable of manufacturing the high-strength structural steel relatively easily.

  In order to solve the above-mentioned problems, the present invention is as follows. First, C is 0.35 to 0.65 wt%, Si is 3.0 wt% or less, Mn is 0.10 to 0.70 wt%, Ni is 0 ~ 3.0wt%, Mo 0.1-1.9wt%, Cr 0-2.0wt%, Nb 0-0.1wt%, S 0.025wt% or less, P 0.025wt% or less In addition, Al is contained in 0.040 wt% or less, N is contained in 0.0035 wt% or less, O is contained in 0.0040 wt% or less, the balance is Fe and inevitable impurities, and the average grain size of prior austenite is 5.5 μm. Hereinafter, the maximum diameter of the nonmetallic inclusion is 5 μm or less, the tensile strength is 1250 MPa or more, and the yield ratio is 0.95 or more.

Secondly, the present invention is a method for producing a high-strength structural steel excellent in hydrogen embrittlement resistance and toughness having the first characteristic, and a step of performing a homogenization treatment by heating to 1200 ° C. or higher A pre-processing process including a process including heating at 600 to 1100 ° C. and a process of performing warming / hot working at a temperature of 700 ° C. or less and a reduction in area of 80% or more, and a heating temperature T ₁ is T _1. It is characterized by including a post-processing treatment including a quenching step of <850 ° C. and a tempering step in which the heating temperature T ₂ is 200 ° C. <T ₂ .

  According to the present invention, it is possible to manufacture a high-quality, high-strength structural steel having high strength and excellent ductility, toughness, and delayed fracture resistance by an inexpensive chemical component and a relatively easy manufacturing process.

The chemical components contained in the high-strength structural steel excellent in hydrogen embrittlement resistance and toughness of the present invention and the composition range thereof are based on the following reasons.
(1) C
It is an element deeply involved in the strength of steel materials. In order to stably obtain a tensile strength of 1250 MPa or more, it is necessary to contain 0.35 wt% or more. Preferably it is 0.37 wt% or more, more preferably 0.38 wt% or more. On the other hand, when the amount is increased, the amount of retained austenite increases, which may adversely affect the characteristics. Moreover, workability deteriorates. Therefore, the upper limit is 0.65 wt%. Preferably it is 0.62 wt%, More preferably, it is 0.60 wt%.
(2) Si
It is added for deoxidation during steelmaking. When added positively, remarkable softening resistance is obtained in low temperature tempering, and it is an element effective for securing strength. It also has the effect of improving corrosion resistance. On the other hand, when tempering at a relatively high temperature, grain boundary embrittlement is promoted. Further, a large amount of addition promotes decarburization and is not preferable in terms of manufacturability. Therefore, the upper limit is 3.0 wt%. Preferably it is 2.5 wt%, More preferably, it is 2.0 wt%.
(3) Mn
It forms S and MnS, which are harmful elements, and suppresses embrittlement due to S. Further, softening resistance is obtained by tempering at a relatively high temperature, which is useful. On the other hand, solute Mn promotes grain boundary segregation of P, causes grain boundary embrittlement, and deteriorates toughness. Therefore, the upper limit is set to 0.70 wt%. Preferably it is 0.50 wt%, More preferably, it is 0.30 wt%. The lower limit was 0.10 wt% in order to fix S. Preferably it is 0.12 wt%, More preferably, it is 0.15 wt%.
(4) Ni
Significantly improves corrosion resistance. In addition, since the transformation point is lowered, it is effective for refining crystal grains. On the other hand, excessive addition increases residual austenite and may adversely affect toughness. In addition, since the cost is high, it is appropriate to refrain from adding as much as possible. Since the improvement of toughness is saturated at 3.0 wt%, the upper limit is 3.0 wt%. From the cost aspect, it is preferably 2.0 wt%, more preferably 1.0 wt%. The lower limit is 0 wt% because necessary characteristics can be obtained without positive addition, and is 0.2 wt%, preferably 0.3 wt%, more preferably 0.5 wt% from the viewpoint of corrosion resistance.
(5) Mo
Suppresses embrittlement due to P grain boundary segregation. Moreover, precipitation of the carbide | carbonized_material to a prior-austenite grain boundary can be suppressed and there exists an effect which suppresses a grain-boundary fracture. Furthermore, since the hardenability is remarkably improved, there is an advantage that a hardened structure can be easily obtained even with a fine structure having a low hardenability. Furthermore, carbonitride has a hydrogen trap effect. The lower limit is 0.1 wt%, preferably 0.4 wt%, more preferably 0.6 wt%. In particular, if it is 0.9 wt% or more, a certain effect can be obtained. On the other hand, since a large amount of addition deteriorates workability, the upper limit is set to 1.9 wt%. Preferably it is 1.7 wt%, More preferably, it is 1.5 wt%.
(6) Cr
Improve corrosion resistance. Further, since temper softening resistance is obtained, it is effective when the target strength is high. On the other hand, undissolved carbide tends to remain, causing problems such as insufficient strength. Therefore, the upper limit is set to 2.0 wt%. Considering the saturation of the effect and cost, it is preferably 1.8 wt%, more preferably 1.5 wt%. The lower limit is 0 wt% because aggressive addition is not necessary, and is added as appropriate when corrosion resistance and high strength are particularly desired. In this case, it is preferably 0.5 wt%, more preferably 1.0 wt%.
(7) Nb
It forms carbonitride and is effective for refining crystal grains. However, if added in excess, coarse nitrides are formed, which adversely affects fatigue properties and toughness. Therefore, the upper limit is 0.1 wt%. From the viewpoint of manufacturability and cost, it is preferably 0.07 wt%, more preferably 0.05 wt%. Since it is not an essential element, the lower limit is 0 wt%, preferably 0.02 wt%, more preferably 0.03 wt%.
(8) S
It is a harmful element that promotes grain boundary embrittlement and degrades toughness, and is fixed as a sulfide by adding Mn or the like. However, when the amount and size of the sulfide increase, it becomes a starting point of fracture, which impairs toughness. Therefore, the upper limit is 0.025 wt%. Preferably, it is limited to 0.010 wt%, more preferably 0.005 wt%.
(9) P
It is a harmful element that promotes grain boundary embrittlement and degrades toughness. Grain boundary embrittlement is promoted by the addition of Mn, Cr, Si, Ni and the like. Therefore, the upper limit is 0.025 wt%, preferably 0.010 wt%, more preferably 0.005 wt%.
(10) Al
Although used as a deoxidizing element, the generated oxide is hard and tends to be a starting point of destruction. For high-strength steel, it is appropriate to avoid deoxidation with Al and substitute with Si or the like. On the other hand, a nitride is formed, which effectively works to refine the structure and fixes harmful N. Therefore, the upper limit is 0.040 wt%, preferably 0.030 wt%, more preferably 0.005 wt%.
(11) N
The formation of nitride contributes to the refinement of the structure. However, excess N degrades mechanical properties as solute N and also forms coarse nitrides. Therefore, the upper limit is set to 0.0035 wt%, preferably 0.0030 wt%, and more preferably 0.0025 wt%.
(12) O
Oxides are the starting point for fatigue failure. Therefore, the amount of oxygen is reduced as much as possible to suppress the formation of coarse oxides. The upper limit is 0.0040 wt%, preferably 0.0030 wt%, and more preferably 0.0015 wt%.

The high strength structural steel excellent in hydrogen embrittlement resistance and toughness of the present invention containing the chemical component in the above composition range has the following characteristics in terms of structure.
(A) The average value of the crystal grain size of prior austenite is 5.5 μm or less.

If the average crystal grain size of prior austenite is 5.5 μm or less, the tensile ductility and the notch toughness are remarkably improved. Preferably it is 4.5 micrometers or less, More preferably, it is 3.0 micrometers or less.
(B) The maximum diameter of the nonmetallic inclusion is 5 μm or less.

  If coarse inclusions are present, the toughness deteriorates. When the maximum diameter of nonmetallic inclusions in the steel is controlled to 5 μm or less, good toughness is ensured. In particular, in a hydrogen environment, coarse inclusions become hydrogen accumulation sites and are likely to be embrittled.

Further, the characteristics have the following characteristics.
(A) Tensile strength of 1250 MPa or more The high strength structural steel excellent in hydrogen embrittlement resistance and toughness of the present invention is required to have a tensile strength of 1250 MPa or more. As structural steel, the higher the strength, the more lightweight the structure can contribute. Preferably it is 1300 MPa or more, More preferably, it is 1350 MPa or more.
(B) Yield ratio 0.95 or more It is known that the hydrogen embrittlement resistance deteriorates when the strength is increased. In order to improve the hydrogen embrittlement resistance, the inventors have found that it is effective to set the yield ratio (tensile strength / 0.2% proof stress) to 0.95 or more. Preferably it is 0.97 or more, More preferably, it is 0.98 or more.

The manufacturing method of the high strength structural steel excellent in hydrogen embrittlement resistance and toughness of the present invention includes a step of performing a homogenization treatment of heating to 1200 ° C. or higher, heating of 600 to 1100 ° C., and warm / hot. A pre-processing process including a process including a process and a process of performing processing with a surface reduction rate of 80% or more at 700 ° C. or less, a quenching process where the heating temperature T ₁ is T ₁ <850 ° C., and the heating temperature T ₂ is 200 ° C. <including processing after including the tempering process of T _2.

  In order to obtain stable and excellent characteristics, it is necessary to make the metal structure and inclusions fine and uniform. In order to obtain a uniform structure, it is important to reduce casting segregation as much as possible. If casting segregation remains, the structure becomes non-uniform by subsequent heat treatment, and inclusions and precipitates are also concentrated locally, so the characteristics are likely to vary. Therefore, in order to reduce casting segregation, a homogenization process is performed in which preheating is performed at a high temperature of 1200 ° C. or higher. The temperature of the homogenization treatment is preferably 1250 ° C. or higher, more preferably 1300 ° C. or higher. However, if homogenization is performed at an excessively high temperature, non-metallic inclusions such as sulfides may be dissolved, resulting in coarse inclusions in the subsequent process, which may deteriorate toughness. is there. Such deterioration of toughness can be avoided by finely reprecipitating non-metallic inclusions that have been solid-solutioned by homogenization treatment by heating to 600 to 1100 ° C. and then processing warm or hot. . By this process, casting segregation can be reduced while avoiding coarsening of inclusions, and steel can be made uniform.

  In order to uniformly refine the martensite structure, it is necessary to obtain uniform and fine austenite grains during quenching heating. By subjecting the steel to a processing with a reduction in area of 80% or more at a relatively low temperature of 700 ° C. or less in the pre-processing treatment, uniform austenite grains having an average crystal grain size of 5.5 μm or less can be obtained during quenching heating. The processing temperature is preferably 650 ° C. or lower, more preferably 600 ° C. or lower. The area reduction rate is preferably 85% or more, more preferably 90% or more.

Then, in the post-processing is a heating temperature T ₁ of the quenching process is less than 850 ° C.. This is because the growth rate of the prior austenite crystal grains increases as the heating temperature increases in the quenching process, and particularly when the temperature is 850 ° C. or higher, the decomposition and solid solution of the pinning particles become remarkable, and the prior austenite crystal grain size becomes coarse. .

Further, in order to uniformly disperse the carbides precipitated at tempering treatment, a tempering temperature T ₂ in the post-processing to 200 ℃ <T _2. Tempering temperature T ₂ is sufficiently carbide does not precipitate at less than 200 ° C.. The upper limit of the tempering temperature T _2, since the combination and strength characteristics by the method of tempering of the alloy elements is changed variously, not specifically defined. However, tempering near 600 ° C. is preferred for the purpose of effectively using a hydrogen trap by carbonitride, and for the purpose of simultaneously increasing the strength, secondary hardening phenomenon occurs due to precipitation of alloy carbides related to Mo and the like. Tempering at less than 580 ° C. is more preferred.

  Through the above series of steps, the structure and characteristics are stabilized to high quality, and the high strength structural steel excellent in hydrogen embrittlement resistance and toughness of the present invention is manufactured.

  Steel materials having the chemical composition shown in Table 1 were melted in a vacuum melting furnace.

そして、各鋼材に図１に示す前加工処理、後加工処理を順次行い、以下の各試験に供した。処理条件を表２に示す。表２に記載された焼入れ処理条件および焼戻し条件は後加工処理における焼入れ工程と焼戻し工程の条件である。

Each steel material was sequentially subjected to the pre-processing and post-processing shown in FIG. Table 2 shows the processing conditions. The quenching treatment conditions and tempering conditions described in Table 2 are the conditions for the quenching step and the tempering step in the post-processing treatment.

１．旧オーステナイトの結晶粒径の平均値の測定
結晶組織から旧オーステナイトの結晶粒径の平均値を測定した。

1. Measurement of average value of crystal grain size of prior austenite The average value of crystal grain size of prior austenite was measured from the crystal structure.

A sample that had been quenched or tempered after quenching was embedded in a resin and wet-polished to give a mirror finish. Next, the prior austenite grain boundaries were revealed using a corrosive solution mainly composed of picric acid, and observed with an optical microscope or a scanning electron microscope. At least 4 visual fields of tissue photographs were taken, and the average value of the crystal grain size was calculated from the obtained photographs by the Heyn method. At this time, at least two pairs or more of perpendicular line segments were drawn in one field of view, and the average value of the crystal grain size was calculated from at least 400 sections. For inclusions, sample preparation (cutting, embedding, polishing) was performed with the plane perpendicular to the longitudinal direction of the steel material as the observation plane, and non-metallic inclusions in the steel were observed with an optical microscope. The presence or absence was investigated. The results are shown in Table 2.
2. Evaluation of toughness (1) Smooth tensile test After tempering, a smooth round bar specimen was prepared and subjected to a tensile test. 0.2% proof stress (σ0.2), tensile strength (σUTS), total elongation (et), uniform elongation (eu), and drawing (RA) were measured to evaluate the strength-ductility balance. In the tensile test, the crosshead speed was all 0.5 mm / min. The elongation was measured by attaching an extensometer (GL = 17.5 mm) to the test piece.

  The yield ratio σ0.2 / σUTS was calculated from the obtained 0.2% yield strength and tensile strength.

The results are shown in Table 2. FIG. 2 shows the relationship between the tensile strength σUTS and the aperture value RA. Those satisfying the requirements of the present invention have a strength level of TS1250 MPa or higher, an aperture value RA of 50% or higher, and an excellent strength-ductility balance.
(2) Notch tensile test After tempering, a notch specimen was prepared and subjected to a tensile test, and the notch tensile strength (σnUTS) was measured. In addition, the fracture surface was observed after the notch tensile test, and the notch sensitivity was evaluated along with the occurrence of brittle fracture surface. In the notch tensile test, the crosshead speed was 0.5 to 0.0005 mm / min.

The results are shown in Table 2. Those satisfying the requirements of the present invention are all dimples in the fracture surface form. On the other hand, those that did not satisfy the requirements were brittle fractures such as cleavage fractures and old austenite grain boundary fractures.
3. Evaluation of delayed fracture resistance After tempering, notched tensile specimens were prepared, and a hydrogen embrittlement susceptibility test was performed according to the procedure shown in FIG. From this test, the relationship between the amount of hydrogen in steel and the fracture characteristics is obtained. Since delayed fracture is caused by hydrogen entering from the environment, the hydrogen storage capacity of steel is important for evaluating delayed fracture. In this example, the fracture characteristics were compared under the hydrogen content condition in steel, which is considered to be appropriate from the results of the hydrogen storage capacity evaluation test that was conducted separately. That is, in the case of tempering temperature 350-500 ° C. with low hydrogen storage capacity, about 0.2 ppm of hydrogen in steel, and in the case of tempering temperatures 250 ° C. and 550 ° C. with high hydrogen storage capacity, about 0.8 ppm of hydrogen in steel. The breaking strength was evaluated.

  The results are shown in Table 2 and FIG. In FIG. 4, the horizontal axis represents the yield ratio obtained in the smooth tensile test, and the vertical axis represents the fracture strength in the hydrogen embrittlement susceptibility test. As shown in FIG. 4, the higher the yield ratio, the higher the breaking strength. When the yield ratio is 0.95 or more, the breaking strength increases remarkably. Those satisfying the requirements of the present invention have a breaking strength exceeding 1000 MPa and are excellent in resistance to hydrogen embrittlement. On the other hand, those that do not meet the requirements of the present invention have a low yield ratio and thus a low breaking strength, or even when the yield ratio is 0.95 or higher, a high breaking strength exceeding 1000 MPa cannot be obtained. For example, R-11 satisfies the requirements of the present invention except for the presence of coarse inclusions, and has undergone ductile fracture in the notch test. However, in the delayed fracture test, the inclusion is due to the presence of coarse inclusions. Stress concentration occurred at the object, and hydrogen accumulated, causing low stress fracture starting from inclusions. R-11 is an example in which variations occur in characteristics.

It is a figure which shows the process given to each steel materials in the Example. It is a figure which shows the relationship between tensile strength (sigma) UTS and aperture value RA. It is a figure which shows the procedure of a hydrogen embrittlement sensitivity test. It is a figure which shows the relationship between a yield ratio and breaking strength.

Claims (2)

  C is 0.35 to 0.65 wt%, Si is 3.0 wt% or less, Mn is 0.10 to 0.70 wt%, Ni is 0 to 3.0 wt%, Mo is 0.1 to 1.9 wt%, Cr is 0 to 2.0 wt%, Nb is 0 to 0.1 wt%, S is 0.025 wt% or less, P is 0.025 wt% or less, Al is 0.040 wt% or less, N is 0.0035 wt% or less, O is contained in an amount of 0.0040 wt% or less, the balance is Fe and inevitable impurities, the average grain size of prior austenite is 5.5 μm or less, the maximum diameter of nonmetallic inclusions is 5 μm or less, and the tensile strength is A high-strength structural steel excellent in hydrogen embrittlement resistance and toughness, characterized by being 1250 MPa or more and a yield ratio of 0.95 or more. A method for producing a high-strength structural steel excellent in hydrogen embrittlement resistance and toughness according to claim 1, comprising a step of performing a homogenization treatment of heating to 1200 ° C or higher, heating at 600 to 1100 ° C, and A pre-processing process including a process including warm / hot processing and a process of performing processing with a surface reduction rate of 80% or more at 700 ° C. or less, a quenching process in which the heating temperature T ₁ is T ₁ <850 ° C., and heating A method for producing a high-strength structural steel excellent in hydrogen embrittlement resistance and toughness, characterized in that it includes a post-processing treatment including a tempering step in which the temperature T ₂ is 200 ° C. <T ₂ .

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