JP2019173160A - Low alloy steel excellent in hydrogen embrittlement resistance - Google Patents

Abstract

A low alloy steel excellent in hydrogen embrittlement resistance is provided. SOLUTION: In mass%, C: 0.010% or less, Si: 0.05 to 1.00% and / or Al: 0.005 to 0.100%, B: 0.010% 1.000%, P: 0.010 to 1.000%, S: 0.0100% or less, O: 0.005% or less, N: 0.010% or less, if necessary, a predetermined amount of Mn, etc. And the balance is Fe and impurities, the total content of P, Ge, As, Se, Sb and Te is 0.050 to 1.000%, and the solid solution in the low alloy steel The total content of P, Ge, As, Se, Sb, and Te present is 0.050 to 1.000 in mass%, and P, Ge, As segregated at the grain boundary of the low alloy steel. Embrittlement resistance in which the total amount of, Se, Sb and Te is 2.0% or less in atomic% Excellent low alloy steel. [Selection diagram] None

Description

  The present invention relates to a low alloy steel.

  In various fields such as thin steel plates, thick steel plates, steel pipes and rods, hydrogen embrittlement has become a problem as the strength increases and the usage environment becomes severe. Applications in which hydrogen embrittlement is a problem include, for example, high-strength steel sheets for automobiles, thick steel sheets for building materials, steel pipes used in harsh environments such as oil well pipes and line pipes, and high-strength bars, bars and bolts. Etc.

  As structural measures to improve the hydrogen embrittlement resistance of carbon steel and low alloy steel having a bcc structure, for example, in martensitic materials, spheroidization of grain boundary carbides by increasing the tempering temperature, hydrogen trap Utilization of fine alloy carbide having an effect is known (Non-Patent Document 1 and Non-Patent Document 2).

  The above measures pay attention to the action of carbides generated during tempering and control this. On the other hand, when an alloy element other than Fe is added, a part of it is dissolved in the matrix phase of the bcc structure, and if this solid solution element is used, universal hydrogen resistance can be achieved without depending on heat treatment. It is expected that embrittlement characteristics can be obtained.

  However, solid solution alloy elements other than Fe are generally considered to have a negative effect on hydrogen embrittlement by lowering the hydrogen diffusion coefficient and increasing the concentration of occluded hydrogen in the use environment (Non-patent Documents 3 and 3). Non-patent document 4).

  Patent Document 1 discloses an invention related to a steel material for cold working in which the amount of dissolved nitrogen is limited to 0.004 to 0.03% by mass. According to this steel material for cold work, since high strength can be obtained only by cold work, heat treatment can be omitted.

  Patent Document 2 discloses an electromagnetic field in which a steel slab having a predetermined chemical composition is hot-rolled and cold-rolled, and thereafter an intermetallic compound is used as an inhibitor to control the progress of secondary recrystallization and perform finish annealing. A method for manufacturing a steel sheet is disclosed.

  Patent Document 3 discloses an invention related to a seamless steel pipe for an oil well having a predetermined chemical composition, containing 0.40% or more of solid solution Mo, and having a tempered martensite phase as a main phase.

JP, 2006-056418, A JP 2007-031793 A Japanese Patent Laying-Open No. 2015-038247

Takahiro Kushida, Hitoshi Matsumoto, Naoyuki Kuratomi, Terutaka Tsumura, Fukukazu Nakazato, Ikuo Kudo, Iron and Steel Vol.82, No. 4 (1996) pp. 297-302 Yamazaki Masaki, Takahashi Yasuhiko, Iron and Steel Vol.83, No. 7 (1997) 454-459 Hideki Hagi, Journal of the Japan Institute of Metals, Vol. 55, No. 12 (1991), pages 1283-1290 Takahiro Kushida, Ikuo Kudo, Materia Vol. 33, No. 7 (1994), pages 932-939

  As a result of intensive studies using the pure iron-based material and an alloy in which various alloy elements are completely dissolved, the present inventors have mainly studied P, Ge, As, Se, Sb, and Te. (Hereinafter also referred to as “improvement element such as P”) has been found to have an effect of improving hydrogen embrittlement resistance without lowering the hydrogen diffusion coefficient even in a solid solution state. The details of the action mechanism of the above improvement effect are unknown, but when an improvement element such as P is dissolved in iron, it repels the hydrogen atom that has diffused in the iron and approached it. By promoting, it is estimated that hydrogen atoms are prevented from accumulating at the embrittlement starting points (interfaces such as stress concentration portions and inclusions), and hydrogen embrittlement is suppressed.

  In the invention of Patent Document 1, no consideration is given to the hydrogen embrittlement resistance by ensuring the content of an improving element such as P (hereinafter also referred to as “solid solution amount”) present in a solid solution state. In addition, it is assumed that heat treatment for tempering, so-called quenching / tempering heat treatment, is omitted, but in the case of such non-tempered steel in which heat treatment is omitted, P and the like are improved in the slow cooling process during the manufacturing process. Since the element is precipitated, this technique cannot secure the solid solution amount of the improving element such as P.

  The invention of Patent Document 2 is said to hold for 20 seconds or more in a temperature range of 800 ° C. or higher in order to cause secondary crystals. However, when heat treatment is carried out in such a temperature range, the content of an improving element such as P that segregates at grain boundaries (hereinafter also referred to as “grain boundary segregation amount”) increases, resulting in resistance to hydrogen embrittlement. May deteriorate.

  Since the invention of Patent Document 3 contains 0.15 to 0.50% of C, undissolved coarse carbide (mainly iron carbide) remains during annealing, and the hydrogen embrittlement resistance deteriorates.

  It is an object of the present invention to provide a low alloy steel having improved hydrogen embrittlement resistance by allowing an improvement element such as P to be present in a solid solution state in a steel in which C is limited to 0.010% or less. To do.

  In order to achieve the above-mentioned object, the present inventors conducted intensive research on a method for increasing the solid solution amount of an improving element such as P effective for suppressing hydrogen embrittlement, and as a result, obtained the following knowledge. .

  (A) In order to increase the solid solution amount of an improving element such as P, it is necessary to reduce the amount of grain boundary segregation of the improving element such as P. Further, when the content of the improving element such as P (hereinafter also referred to as “precipitation amount”) present in the precipitate, inclusions and intermetallic compound (hereinafter also referred to as “precipitation”) is large. , The amount of solid solution of the improving element such as P decreases. Therefore, in order to increase the solid solution amount of the improving element such as P, it is necessary to reduce the precipitation amount of the improving element such as P.

  (B) When a predetermined amount of B is contained in the steel, B is present at the grain boundary before the improving element such as P, and the grain boundary segregation amount of the improving element such as P can be reduced.

  (C) Performing soaking after hot forging is effective in reducing the amount of grain boundary segregation of improving elements such as P, and is also effective in reducing the amount of precipitation of improving elements such as P.

  (D) If the final heat treatment temperature is too high, the grain boundary segregation amount of an improving element such as P becomes excessive. Therefore, it is important to reduce the final heat treatment temperature as much as possible within the temperature range where a single phase structure of ferrite can be obtained.

  (E) It is important that the cooling process after the final heat treatment is performed at a cooling rate as fast as possible in order to prevent the formation of precipitates and to reduce the amount of improving elements such as P deposited.

  As described above, the present inventors have succeeded in suppressing hydrogen embrittlement by increasing the solid solution amount of an improving element such as P, thereby completing the present invention.

The gist of the present invention is the following low alloy steel.
[1] By mass%
C: 0.010% or less,
Si: 0.05 to 1.00% and Al: 0.005 to 0.100% and / or either
B: 0.010 to 1.000%,
P: 0.0510 to 1.000% or less,
S: 0.0100% or less,
O: 0.005% or less,
N: 0.010% or less,
Mn: 0 to 1.00%,
Cr: 0 to 5.00%,
Mo: 0 to 5.00%,
V: 0 to 5.00%
W: 0 to 5.00%,
Nb: 0 to 0.100%,
Ti: 0 to 0.10%,
Zr: 0 to 0.20%,
Hf: 0 to 0.20%,
Ta: 0 to 0.200%,
Cu: 0 to 3.00%,
Ni: 0 to 5.00%,
Co: 0 to 3.00%
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.50%,
Ge: 0 to 1.000%,
As: 0 to 1.000%,
Se: 0 to 0.100%,
Sb: 0 to 1.000%,
Te: 0 to 0.100%,
Balance: Fe and impurities,
Having a chemical composition in which the total content of P, Ge, As, Se, Sb and Te is 0.050 to 1.000%;
The total content of P, Ge, As, Se, Sb and Te present in a solid solution state in the low alloy steel is 0.050 to 1.000 in mass%, and the low alloy steel The total amount of P, Ge, As, Se, Sb, and Te segregated at the grain boundary The amount of P segregated at the grain boundary is 2.0% or less in atomic%.
Low alloy steel with excellent hydrogen embrittlement resistance.

[2] In mass%,
Mn: 0.10 to 1.00%,
Cr: 0.01 to 5.00%,
Mo: 0.01 to 5.00%,
V: 0.01 to 5.00%
W: 0.01 to 5.00%
Nb: 0.001 to 0.100%,
Ti: 0.001 to 0.10%,
Zr: 0.001 to 0.20%,
Hf: 0.001 to 0.20%,
Ta: 0.001 to 0.200%,
Cu: 0.10 to 3.00%,
Ni: 0.10 to 5.00%,
Co: 0.10 to 3.00%
Ca: 0.0001 to 0.0100%,
Containing one or more selected from Mg: 0.0001 to 0.0100% and REM: 0.0001 to 0.50%,
The low alloy steel of [1] above.

[3] The total amount of precipitates, inclusions and intermetallic compounds is 0.500% by mass or less.
The low alloy steel according to the above [1] or [2].

  According to the present invention, a low alloy steel excellent in hydrogen embrittlement resistance can be obtained.

1. Chemical Composition The low alloy steel according to the present invention has the following chemical composition. The range of the content of each element and the reason for limitation will be described. % Of the content of each element means “mass%”.

C: 0.010% or less C (carbon) is effective in increasing the strength of the steel by solid solution strengthening, but if it exceeds 0.010%, it is completely dissolved in the ferrite phase of the parent phase. Disappears and forms coarse carbides during annealing, reducing the hydrogen embrittlement resistance. From this viewpoint, the C content is set to 0.010% or less. The lower the C content, the better. However, the extreme reduction of the C content greatly increases the manufacturing cost. Therefore, when industrial production is considered, the minimum with preferable C content is 0.0003%, More preferably, it is 0.0005%.

Si: 0.05 to 1.00% and Al: 0.005 to 0.100% or both Si (silicon) and Al (aluminum) are effective elements for deoxidation of steel, both Or any one is contained. In order to obtain a deoxidizing effect, Si is contained in an amount of 0.05% or more and Al is contained in an amount of 0.005% or more. On the other hand, since the effect is saturated even if any element is contained excessively, the upper limit of the Si content is 1.00% and the upper limit of the Al content is 0.100%. In order to obtain the above effect, the lower limit of the Si content is preferably 0.10%, the lower limit of the Al content is preferably 0.010%, more preferably 0.015%. preferable.

B: 0.010 to 1.000%
B (boron), like C, is effective in increasing the strength of steel. Furthermore, there is an effect of suppressing grain boundary segregation of an improving element such as P and suppressing embrittlement by the improving element such as P segregated at the grain boundary. In order to obtain these effects, B is contained by 0.010% or more. However, even if the content exceeds 1.000%, these effects are not only saturated, but also increase the recrystallization temperature and require high-temperature annealing, leading to an increase in manufacturing cost and further deterioration in workability. Let For this reason, the upper limit of the B content is set to 1.000%. A desirable upper limit of B is 0.600%, and a more desirable upper limit is 0.300%.

P: 0.050 to 1.000% or less P (phosphorus) is an element usually present as an impurity in steel. Further, P is an element that segregates at the grain boundary and reduces the resistance to hydrogen embrittlement. Therefore, it is desirable that the P content is as low as possible, but it is an important element in the present invention. The amount of P grain boundary segregation and precipitation is reduced, and the content of P existing in a solid solution state in the matrix phase ferrite is increased, thereby improving the hydrogen embrittlement resistance. For that purpose, the P content is set to 0.050% or more. On the other hand, when the P content is excessive, grain boundary embrittlement is caused, so the P content is made 1.000% or less.

Ge: 0 to 1.000%
As: 0 to 1.000%
Se: 0 to 0.100%
Sb: 0 to 1.000%
Te: 0 to 0.100%
Like P, Ge, As, Se, Sb, and Te are elements that segregate at grain boundaries and reduce the resistance to hydrogen embrittlement. Therefore, it is desirable that the content of these elements is as low as possible. However, hydrogen embrittlement resistance is improved by reducing the amount of grain boundary segregation and precipitation of these elements and increasing the content of these elements in solid solution in the ferrite phase of the parent phase. It becomes possible. Therefore, you may contain 1 or more types of these elements. However, when these elements are contained excessively, the amount of segregation at the grain boundaries is increased, and there is a risk of causing grain boundary embrittlement. Therefore, when these elements are contained, Ge is 1.000% or less, As is 1.000% or less, Se is 0.100% or less, Sb is 1.000% or less, and Te is 0.100% or less. To do. In order to obtain the above effect, the content of any element is preferably 0.05% or more. In addition, when 2 or more types selected from P, Ge, As, Se, Sb, and Te are contained, the total content is set to 0.050 to 1.000%.

  Regarding the improving element such as P, it is necessary to manage not only the total amount contained in the steel but also the solid solution amount of the improving element such as P. That is, when the solid solution amount of an improving element such as P is less than 0.050%, the hydrogen embrittlement resistance is hardly improved. On the other hand, as the solid solution amount of the improving element such as P increases, the hydrogen embrittlement resistance improves, but if it is excessive, it causes grain boundary embrittlement, so it is made 1.000% or less. The upper limit of the solid solution amount of the improving element such as P is preferably 0.500%, and more preferably 0.200%.

  As for the improvement element such as P, there is a problem that when the grain boundary segregation amount of the improvement element such as P increases, the solid solution amount of the improvement element such as P is relatively reduced. When the amount of segregation at the boundary is excessive, there is a problem that grain boundary embrittlement is caused. Therefore, the grain boundary segregation amount of the improving element such as P is set to 2.0% or less in atomic%. A preferable upper limit is 1.0%, and a more preferable upper limit is 0.5%. The grain boundary segregation amount of an improving element such as P is preferably as small as possible, and may be 0%, but in practice, 0.001 atomic% is a practical lower limit.

  The grain boundary segregation amount of an improving element such as P can be measured by a three-dimensional atom probe (3DAP). 3DAP is a field ion microscope (FIM) equipped with a position sensitive mass analyzer. With such a configuration, an atom can be identified and its position can be specified.

  In the measurement by 3DAP, first, a portion including a grain boundary of a steel plate is taken as a sample and processed into a needle sample (10 μm × 10 μm × 100 μm) by a focused ion beam method (FIB). Using 3DAP, a voltage is applied to the needle sample, and ions such as P released at that time are analyzed with a coordinate detector, and the concentration (atomic%) of the improving element such as P at the grain boundary is set at a grain boundary width of 1 nm. calculate. The concentration of the improving element such as P is calculated at five or more grain boundaries, and the average value thereof is defined as the grain boundary segregation amount of the improving element such as P. The measurement conditions by 3DAP are as follows: voltage is DC pulse (pulse ratio is 20% or more), and sample temperature is 70K or less.

S: 0.0100% or less S (sulfur) is an element present as an impurity in steel. Since S is an element that segregates at the grain boundary and reduces the resistance to hydrogen embrittlement, its content needs to be 0.0100% or less. It is desirable that the S content is as low as possible. However, the extreme reduction of the S content greatly increases the manufacturing cost. Therefore, when industrial production is considered, the minimum with preferable S content is 0.0001%, More preferably, it is 0.0003%.

O: 0.005% or less O (oxygen) is an element present as an impurity in steel. When the content exceeds 0.005%, a coarse oxide is formed, and mechanical properties such as toughness are lowered. Therefore, O is 0.005% or less. It is desirable that the O content be as low as possible. The upper limit is desirably 0.004%, and more desirably 0.003%. However, the extreme reduction of the O content greatly increases the manufacturing cost. Therefore, when industrial production is considered, the minimum with preferable O content is 0.0001%, More preferably, it is 0.0003%.

N: 0.010% or less N (nitrogen) is an element present as an impurity in steel. When the content exceeds 0.010%, coarse nitrides are formed, and mechanical properties such as toughness are lowered. Therefore, N is set to 0.010% or less. The N content is desirably as low as possible. The upper limit is desirably 0.008%, and more desirably 0.006%. However, the extreme reduction of the N content greatly increases the manufacturing cost. Therefore, when industrial production is considered, the minimum with preferable N content is 0.0001%, More preferably, it is 0.0003%.

Mn: 0 to 1.00%
Since Mn (manganese) has the effect of solid solution strengthening, it may be contained. However, since an effect is saturated even if it contains excessively, the upper limit in the case of making it contain shall be 1.00%. In order to acquire said effect, it is preferable to make it contain 0.10% or more.

Cr: 0 to 5.00%
Mo: 0 to 5.00%
V: 0 to 5.00%
W: 0 to 5.00%
Nb: 0 to 0.10%
Ti: 0 to 0.10%
Zr: 0 to 0.20%
Hf: 0 to 0.20%
Ta: 0 to 0.20%
Cr (chromium), Mo (molybdenum), V (vanadium), W (tungsten), Nb (niobium), Ti (titanium), Zr (zirconium), Hf (hafnium) and Ta (tantalum) (hereinafter these 9) The two elements are collectively referred to as “first group elements”.) Is a ferrite-forming element and has a solid solution strengthening ability. Further, Nb, Ti, Zr, Hf and Ta have a strong ability to form carbonitrides and have the effect of forming fine carbonitrides during annealing and reducing solid solution C and solid solution N. For this reason, you may contain 1 or more types of these elements. However, since the effect is saturated when the content of each element is excessive, when these elements are contained, Cr is 5.00% or less, Mo is 5.00% or less, and V is 5. 00% or less, W is 5.00% or less, Nb is 0.10% or less, Ti is 0.10% or less, Zr is 0.20% or less, Hf is 0.20% or less, Ta is 0.20% The following. In order to obtain the above effect, Cr is 0.01% or more, Mo is 0.01% or more, V is 0.01% or more, W is 0.01% or more, and Nb is 0.001% or more. , Ti is preferably 0.001% or more, Zr is 0.001% or more, Hf is 0.001% or more, and Ta is preferably 0.001% or more.

Cu: 0 to 3.00%
Ni: 0 to 5.00%
Co: 0 to 3.00%
Cu (copper), Ni (nickel) and Co (cobalt) are all effective for solid solution strengthening of steel. For this reason, you may contain 1 or more types selected from these elements. However, since the effect is saturated even if contained excessively, the upper limit of Cu and Co is set to 3.00%, and the upper limit of Ni is set to 5.00%. Moreover, in order to acquire said effect, it is preferable to contain any element 0.10% or more.

Ca: 0 to 0.0100%
Mg: 0 to 0.0100%
REM: 0 to 0.50%
Ca (calcium), Mg (magnesium) and REM (rare earth elements) combine with S in steel to form sulfides, improve the shape of inclusions and improve mechanical properties such as toughness, You may make it contain. Since this effect is saturated even if contained excessively, the upper limit of Ca and Mg is 0.0100%, and the upper limit of REM is 0.50%. In order to acquire said effect, it is preferable to contain 0.0001% or more of any element.

  Note that REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and the REM content refers to the content when REM is 1 type, and the total content thereof when 2 or more types. REM is also supplied as misch metal, which is generally an alloy of a plurality of types of REM. For this reason, one or more individual elements may be added so that the REM content falls within the above range. For example, the REM content may be added in the form of misch metal. You may make it contain so that it may become this range.

  The chemical composition of the present invention contains each of the above elements in a prescribed range, with the balance being Fe and impurities. An impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing steel materials industrially.

2. Metal structure The steel of the present invention is intended for a ferrite single-phase structure in which 99% or more of the volume is ferrite. There is no particular restriction on the structure other than ferrite, but it works as a starting point and a development path for hydrogen embrittlement, so the amount is preferably as small as possible. Therefore, the total amount of precipitates, inclusions and intermetallic compounds is preferably 0.500% by mass or less.

3. Production Method The steel of the present invention can be produced by melting (melting and casting) by a normal method, hot forging, and further hot rolling as necessary. However, the heat history after hot forging is important in order to increase the solid solution amount of an improving element such as P and reduce the amount of segregation and precipitation at the grain boundaries. That is, the steel of the present invention needs to be subjected to a soaking heat treatment and a final heat treatment that satisfy the following conditions after hot forging.

Soaking treatment: 1200 to 1300 ° C. x 1 to 168 hours In order to reduce the amount of grain boundary segregation of improving elements such as P and to suppress the formation of intermetallic compounds, after hot forging, hold at 1200 ° C. or more for 1 hour or more. Soak heat treatment. If the soaking temperature is too high, the processing cost increases, so the upper limit of the soaking temperature is 1300 ° C. In addition, if the soaking time is too long, the processing cost is increased, so the upper limit of the soaking time is 168 hours. Here, the soaking temperature corresponds to the temperature of the furnace used when heating and holding the steel material after hot forging.

Final heat treatment: 650-750 ° C x 5 minutes or more soaking + soaking temperature-400 ° C cooled at a cooling rate of 10 ° C / s or more The soaking temperature of the final heat treatment is to obtain a single phase structure of ferrite. 650 ° C. or higher. However, if the soaking temperature is too high, the grain boundary segregation amount of an improving element such as P becomes excessive, so the upper limit is made 750 ° C. or less. If the holding time is too short, the above effect cannot be obtained. If the holding time is too long, the processing cost is increased.

  If the cooling rate after soaking is too slow, the formation of precipitates and the like cannot be suppressed, so the cooling rate at the soaking temperature to 400 ° C. is 10 ° C./s or more. Preferably, it is 15 ° C. or higher, more preferably 20 ° C. or higher.

  As a cooling method, for example, the steel material is continuously forcedly cooled from the soaking temperature, and the surface temperature of the steel material is continuously reduced. As such a continuous cooling treatment, for example, there are a method of immersing a steel material in a water tank and cooling, a method of accelerated cooling of the steel material by shower water cooling, mist cooling, forced air cooling, and the like. In addition, the cooling rate after the soaking is measured at a portion that is cooled most slowly in the cross section of the steel material. For example, when the steel material is a steel plate and forced cooling is performed from both surfaces, a cooling rate after holding the soaking can be measured by inserting a sheath-type thermocouple in the center of the plate thickness and measuring the temperature. .

  Examples in which the effects of the present invention are verified will be described below.

  Each of the low alloy steels having the chemical compositions shown in Table 1 was vacuum-melted by 50 kg. From the obtained slab, a plate material having a thickness of 30 mm was obtained through hot forging, soaking, and hot rolling, and then a final heat treatment was performed to obtain a test material (annealed material).

<Measurement of total content of precipitates, etc.>
The solid solution amount of an improving element such as P and the total content of precipitates were determined by the following procedure.
A round bar specimen for extraction analysis having a diameter of 1 to 10 mm and a length of 50 mm was collected from the center of the test material (annealed material). This test piece is subjected to anodic electrolysis to dissolve the matrix, precipitates and the like are extracted, ICP (High Frequency Inductively Coupled Plasma) emission analysis is performed using the extracted residue, and the content of P and the like in the residue is measured. . The anodic electrolysis was performed by constant current electrolysis using an electrolytic solution containing 1% by mass of tartaric acid. By dividing this content by the difference in mass of the test piece before and after dissolution of the matrix by anodic electrolysis (that is, the dissolution amount of the test piece), the content of P and the like deposited in the matrix (amount of precipitated P etc.) ( Mass%) was calculated. The amount of precipitation P and the like was subtracted from the content of P and the like in the steel material, and the solid solution amount of the improving element such as P was calculated. Moreover, the total amount of the residue obtained by dissolving the matrix is divided by the difference in mass of the test piece before and after dissolution of the matrix by anodic electrolysis (dissolution amount of the test piece), and the content (mass%) of precipitates, etc. Asked.

<Measurement of grain boundary segregation amount of improving elements such as P>
A part including a grain boundary is sampled from the center of the thickness of the test material (annealed material) and processed into a needle sample (10 μm × 10 μm × 100 μm). The concentration of the improving element was determined, and the average value of these was taken as the grain boundary segregation amount of the improving element such as P. At this time, the voltage was DC pulse (pulse ratio 20%), and the sample temperature was 70K.

<Measurement of hydrogen embrittlement resistance>
A plate-shaped tensile test piece having a width of 2 mm × thickness of 2 mm, or a width of 2 mm × thickness of 1 mm is taken from the central portion of the thickness of the test material (annealed material), and low under a cathode charge in an aqueous solution. The hydrogen embrittlement resistance was evaluated by a strain rate tensile test. A 3% NaCl + 3 g / L ammonium thiocyanate aqueous solution at room temperature was used as the solution, and a tensile test was performed while performing cathode hydrogen charging at −1.2 (V) against a saturated calomel electrode. The strain rate was 3 × 10 ⁻⁴ (s ⁻¹ ). Relative elongation at break (%) was calculated by measuring the elongation at break under cathodic charge and dividing this by the elongation measured in air. The higher the relative elongation at break, the better the hydrogen embrittlement resistance. In the present embodiment, it was determined that a test material having a relative elongation at break of 55% or more was excellent in hydrogen embrittlement resistance.

<Measuring mechanical properties>
A plate-shaped tensile test piece having a thickness of 1 to 2 mm and a width of 6 mm is taken from a test material (annealed material), and a tensile test is performed according to JIS Z 2241: 2011 to obtain TS (tensile strength). Asked.

  Table 2 shows the conditions for soaking and final heat treatment, and the test results described above.

  As shown in Table 2, Examples 1 to 25 of the present invention had a relative elongation at break of 55% or more, and had excellent hydrogen embrittlement resistance. On the other hand, Comparative Example 26 was inferior in hydrogen embrittlement resistance because the total amount and solid solution amount of P in the steel material were low. In Comparative Examples 27 to 29, the total amount of P in the steel material satisfied the range specified in the present invention, but because a large amount of precipitates and the like were produced, the solid solution amount of P became low, and hydrogen embrittlement resistance It was inferior to. In Comparative Example 30, the C content exceeded 0.01%, undissolved coarse carbide (mainly iron carbide) remained during annealing, and the hydrogen embrittlement resistance was poor. In Comparative Example 31, the total amount and solid solution amount of P in the steel material exceeds 1.000%, and the grain boundary segregation amount of P is 2.0 atomic% or more. It was inferior. In Comparative Example 32, the B content was lower than the range specified in the present invention, the P grain boundary segregation amount was more than 2.0 atomic%, and the hydrogen embrittlement resistance was inferior. In Comparative Examples 33 and 34, since the soaking temperature of the final heat treatment was high, the grain boundary segregation amount of P was more than 2.0 atomic%, and the hydrogen embrittlement resistance was inferior.

  Each of the low alloy steels having chemical compositions shown in Tables 3 to 5 was vacuum-melted by 50 kg. The obtained slab was subjected to hot forging, soaking, and hot rolling to obtain a plate material having a thickness of 30 mm, and then subjected to final treatment to obtain a test material (annealed material). In Tables 3 to 5, this means that the underlined portion is outside the range defined by the present invention or the range recommended in the present specification.

  Tables 6 and 7 show conditions for soaking and final heat treatment. Tables 6 and 7 also show the results of the same tests as in Example 1. In Tables 6 and 7, it is meant that the underlined portion is outside the range defined by the present invention or the range recommended in this specification.

  As shown in Tables 6 and 7, all of the inventive examples had a relative elongation at break of 55% or more and had excellent hydrogen embrittlement resistance. On the other hand, Comparative Examples 51, 53, 56, 61, 71, 84, 88, 89, and 93 were inferior in hydrogen embrittlement resistance because the total content and solid solution content of improving elements such as P were high. Comparative Example 78 was inferior in hydrogen embrittlement resistance because the amount of solid solution of an improving element such as P was low. In Comparative Examples 54, 67, 68, 72, 91, 94, 95 and 96, the total content and solid solution amount of the improving element such as P satisfied the range defined in the present invention, but the improvement of P and the like Since the grain boundary segregation amount of the element exceeded 2.0 atomic%, the hydrogen embrittlement resistance was inferior. In Comparative Examples 97 and 98, the total content of the improving elements such as P in the steel material satisfied the range defined by the present invention, but a large amount of precipitates and the like were formed, so The amount was low and the hydrogen embrittlement resistance was poor.

  According to the present invention, a low alloy steel excellent in hydrogen embrittlement resistance can be obtained.

Claims (3)

% By mass
C: 0.010% or less,
Si: 0.05 to 1.00% and Al: 0.005 to 0.100% and / or either
B: 0.010 to 1.000%,
P: 0.010 to 1.000%
S: 0.0100% or less,
O: 0.005% or less,
N: 0.010% or less,
Mn: 0 to 1.00%,
Cr: 0 to 5.00%,
Mo: 0 to 5.00%,
V: 0 to 5.00%
W: 0 to 5.00%,
Nb: 0 to 0.100%,
Ti: 0 to 0.10%,
Zr: 0 to 0.20%,
Hf: 0 to 0.20%,
Ta: 0 to 0.200%,
Cu: 0 to 3.00%,
Ni: 0 to 5.00%,
Co: 0 to 3.00%
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.50%,
Ge: 0 to 1.000%,
As: 0 to 1.000%,
Se: 0 to 0.100%,
Sb: 0 to 1.000%,
Te: 0 to 0.100%,
Balance: Fe and impurities,
Having a chemical composition in which the total content of P, Ge, As, Se, Sb and Te is 0.050 to 1.000%;
The total content of P, Ge, As, Se, Sb and Te present in a solid solution state in the low alloy steel is 0.050 to 1.000 in mass%, and the low alloy steel The total amount of P, Ge, As, Se, Sb and Te segregated at the grain boundaries is 2.0% or less in atomic%.
Low alloy steel with excellent hydrogen embrittlement resistance. % By mass
Mn: 0.10 to 1.00%,
Cr: 0.01 to 5.00%,
Mo: 0.01 to 5.00%,
V: 0.01 to 5.00%
W: 0.01 to 5.00%
Nb: 0.001 to 0.100%,
Ti: 0.001 to 0.10%,
Zr: 0.001 to 0.20%,
Hf: 0.001 to 0.20%,
Ta: 0.001 to 0.200%,
Cu: 0.10 to 3.00%,
Ni: 0.10 to 5.00%,
Co: 0.10 to 3.00%
Ca: 0.0001 to 0.0100%,
Containing one or more selected from Mg: 0.0001 to 0.0100% and REM: 0.0001 to 0.50%,
The low alloy steel according to claim 1. The total amount of precipitates, inclusions and intermetallic compounds is 0.500% by mass or less,
The low alloy steel according to claim 1 or 2.