JP2018059193A - Manufacturing method of high strength low alloy steel - Google Patents

Abstract

[PROBLEMS] To provide high strength, low content of expensive alloy elements, tensile strength of 1500MPa or more, excellent hydrogen embrittlement resistance and productivity, and suitable for use in automobiles, industrial machinery, building structures, etc. A method for producing a low alloy steel material is provided. The chemical composition is, by mass%, C: more than 0.60% to 1.0%, Si: 1.2 to 2.0%, Mn: less than 0.30 to 1.0%, Cr: 0.5 to 1.5%, Al: 0.005 to 0.10%, Mo: 0 to less than 0.30%, Ti: 0 to 0.10%, Nb: 0 to 0.10%, V: 0 to 0.10%, Zr: 0 to 0.20%, balance: Fe and impurities, P, S as impurities For steel materials in which N and O are P ≦ 0.030%, S ≦ 0.030%, N ≦ 0.030%, and O ≦ 0.010%, [austeniteizing step of heating at 850 to 1050 ° C. for 20 to 60 minutes], [30 ° C. / Cool to a temperature range of 400 to 300 ° C. at a cooling rate of at least 2 seconds and hold at that temperature range for 10 to 100 minutes, a cooling step to cool to room temperature, and a cooling at a total processing rate of 10.0% or more. Cold working process] is performed in order. [Selection figure] None

Description

  The present invention relates to a method for producing a high-strength low-alloy steel, particularly a high-strength low-alloy having a tensile strength of 1500 MPa or more and excellent hydrogen embrittlement resistance, and suitable for use in automobiles, industrial machines, building structures, etc. The present invention relates to a method for manufacturing a steel material.

  In recent years, high strength steel materials having a tensile strength exceeding 1000 MPa tend to be used from the viewpoint of weight reduction, function, and the like. However, when the tensile strength of steel materials exceeds 1000 MPa, hydrogen embrittlement becomes a serious problem. Hydrogen embrittlement is a phenomenon in which mechanical properties deteriorate from the original value due to hydrogen intrusion into a steel material. In addition, since the atomic radius of hydrogen is the smallest among all the elements, penetration into the steel material is inevitable.

Regarding a high-strength steel material excellent in hydrogen embrittlement resistance and a method for producing the same, for example, Patent Document 1 discloses a technique that suppresses delayed fracture characteristics, which is one form of hydrogen embrittlement characteristics. Specifically, it is made of a steel material containing a specific amount of C, the area ratio of the bainite structure is 80% or more, and then subjected to strong wire drawing to have a strength of 1200 MPa (1200 N / mm ² ) or more and excellent resistance. A technique related to a high strength steel wire having delayed fracture resistance and excellent forgeability is disclosed. In Patent Document 1, steel having the above-described chemical composition is hot-rolled or forged, then rapidly cooled to a temperature of 300 to 500 ° C., and at an average cooling rate of 1 ° C./second or less from that temperature for 200 seconds or more. A manufacturing method in which the wire is cooled and then subjected to strong wire drawing is shown.

JP 2002-241899 A

  The high-strength steel wire disclosed in Patent Document 1 has room for improvement in hydrogen embrittlement resistance from the viewpoint of the fracture limit hydrogen concentration.

  The present invention is used for automobiles, industrial machines, building structures, etc., which have a low content of expensive alloy elements, a tensile strength of 1500 MPa or more, excellent hydrogen embrittlement resistance and excellent productivity. An object of the present invention is to provide a suitable method for producing a high-strength low-alloy steel material.

  The gist of the present invention resides in a method for producing a high-strength low-alloy steel material having a tensile strength of 1500 MPa or more as shown below.

(1) A method for producing a high strength low alloy steel material,
Chemical composition is mass%,
C: more than 0.60% and 1.0% or less,
Si: 1.2-2.0%,
Mn: 0.30% or more and less than 1.0%,
Cr: 0.5 to 1.5%
Al: 0.005 to 0.10%,
Mo: 0 to less than 0.30%,
Ti: 0 to 0.10%,
Nb: 0 to 0.10%,
V: 0 to 0.10%,
Zr: 0 to 0.20%,
The balance is Fe and impurities,
In steel materials in which P, S, N and O as impurities are P: 0.030% or less, S: 0.030% or less, N: 0.030% or less and O: 0.010% or less,
The following steps (i) to (iv) are sequentially performed.
A method for producing a high-strength low-alloy steel material having a tensile strength of 1500 MPa or more.
(I): Austenitizing step of heating at 850 to 1050 ° C. for 20 to 60 minutes (ii): Cooling to a temperature range of 400 to 300 ° C. at a cooling rate of 30 ° C./second or more, and 10 to 100 in the temperature range Isothermal transformation step (iii): cooling to room temperature, cooling step (iv): cold working step for cold working at a total working rate of 10.0% or more

  (2) The tensile strength according to (1) above, wherein the cold working process of (iv) is a cold working process in which a drawing process is performed with a total area reduction rate of 10.0 to 20.0%. A method for producing a high-strength low-alloy steel material of 1500 MPa or more.

  (3) The tensile strength according to (1) above, wherein the cold working process of (iv) is a cold working process in which sheet rolling is performed at a total rolling reduction of 10.0 to 40.0%. A method for producing a high-strength low-alloy steel material of 1500 MPa or more.

(4) The chemical composition of the steel material is mass%,
Mo: A method for producing a high-strength low-alloy steel material having a tensile strength of 1500 MPa or more, according to any one of (1) to (3), which is 0.05% or more and less than 0.30%.

(5) The chemical composition of the steel material is mass%,
Ti: 0.005 to 0.10%,
Nb: 0.005 to 0.10%,
V: 0.005 to 0.10%, and
Zr: 0.010 to 0.20%,
A method for producing a high-strength low-alloy steel material having a tensile strength of 1500 MPa or more, according to any one of the above (1) to (4), which contains one or more selected from:

  According to the present invention, the content of expensive alloy elements is low, the hydrogen embrittlement resistance is excellent, and the tensile strength is 1500 MPa or more, which can be suitably used for automobiles, industrial machines, building structures and the like. High strength low alloy steel can be manufactured with high productivity.

It is a figure which shows the shape of the notched tensile test piece used for evaluation of the hydrogen embrittlement resistance property in (Example 1). The numerical value in a figure shows a dimension (unit: mm). It is a figure which shows the shape of the board tensile test piece used for evaluation of tensile strength in (Example 2). The numerical value in a figure shows a dimension (unit: mm). It is a figure which shows the shape of the notched tensile test piece used for evaluation of the hydrogen embrittlement resistance property in (Example 2). The numerical value in a figure shows a dimension (unit: mm).

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) About chemical composition of steel materials:
C: more than 0.60% and 1.0% or less C is the most important element in the present invention. Since C has the effect of reducing the stored hydrogen concentration even at the same strength, the hydrogen embrittlement resistance can be improved even when the content of expensive elements such as Mo and Ni is lowered. C is an element that is important for ensuring high strength, and can ensure the strength without excessive cold working. Therefore, less dislocations are introduced by cold working, reducing hydrogen embrittlement resistance. Hateful. For this reason, C must be contained exceeding 0.60%. Further, C lowers the Ac ₃ point, so that the steel is easily converted to austenite by heating at a relatively low temperature, so that the prior austenite crystal grain size is reduced, and in this respect also the hydrogen embrittlement resistance is improved. To do. On the other hand, when the C content increases and exceeds 1.0%, the deterioration of toughness becomes significant. Therefore, the C content is more than 0.60% and 1.0% or less. A desirable lower limit of the C content is 0.65%, and a desirable upper limit is 0.80%.

Si: 1.2-2.0%
Si has a deoxidizing action and also has an effect of improving strength and hardenability. The improvement in strength is effective for securing a tensile strength of 1500 MPa or more. Si also has an effect of improving hydrogen embrittlement resistance by isothermal transformation at a low temperature. In order to obtain these effects, the Si content needs to be 1.2% or more. On the other hand, even if Si is contained exceeding 2.0%, the effect is saturated and toughness is deteriorated. Therefore, the Si content is set to 1.2 to 2.0%. A desirable lower limit of the Si content is 1.3%, and a desirable upper limit is 1.5%.

Mn: 0.30% or more and less than 1.0% Mn has an effect of improving hardenability and strength. The improvement in strength is effective for securing a tensile strength of 1500 MPa or more, and the improvement in hardenability is advantageous from the viewpoint of manufacturing because a desired strength can be easily obtained. Further, Mn also has an effect of improving hydrogen embrittlement resistance by combining with S to form sulfides and suppressing S grain boundary segregation. In order to obtain these effects, the Mn content needs to be 0.30% or more. On the other hand, when Mn is contained excessively, it segregates at the grain boundary and promotes the intergranular cracking type hydrogen embrittlement fracture. Therefore, the Mn content is set to 0.30% or more and less than 1.0%. A desirable lower limit of the Mn content is 0.40%, and a desirable upper limit is 0.60%.

Cr: 0.5 to 1.5%
Cr is an element effective for improving the strength. Cr also has an effect of improving the hardenability, and the improvement of the hardenability is advantageous from the viewpoint of manufacturing because a desired strength can be easily obtained. In order to obtain these effects, it is necessary to contain 0.5% or more of Cr. On the other hand, when Cr is excessively contained, toughness is deteriorated. Therefore, the Cr content is set to 0.5 to 1.5%. A desirable lower limit of the Cr content is 0.8%, and a desirable upper limit is 1.2%.

Al: 0.005-0.10%
Al is an element having a deoxidizing action. In order to sufficiently secure this effect, it is necessary to contain Al by 0.005% or more. On the other hand, the effect is saturated even if Al is contained exceeding 0.10%. Therefore, the content of Al is set to 0.005 to 0.10%. The Al content of the present invention refers to the content of acid-soluble Al (so-called “sol. Al”).

Mo: 0 to less than 0.30% Mo is an element that improves the stability of the Fe carbide and improves the hydrogen embrittlement resistance. For this reason, you may contain Mo as needed. However, in the present invention, good hydrogen embrittlement resistance can be ensured by optimizing the content of other elements such as C, and since Mo is a very expensive element, a large amount of Mo The inclusion of slashes the economy. Therefore, the Mo content in the case of inclusion is less than 0.30%. The upper limit of the Mo content is desirably 0.20%. In order to obtain the above effect stably, the lower limit of the Mo content is desirably 0.05%, and more desirably 0.10%.

Ti: 0 to 0.10%
Ti is an element that combines with C or / and N, forms fine precipitates, refines prior austenite crystal grains, and improves hydrogen embrittlement resistance. For this reason, you may contain Ti as needed. However, if Ti is contained in an amount exceeding 0.10%, the amount of precipitates increases and the toughness is deteriorated. Therefore, the upper limit of the Ti content when contained is 0.10%. The upper limit of the Ti content is preferably 0.06%. In order to obtain the above effect stably, the lower limit of the Ti content is preferably 0.005%, and more preferably 0.03%.

Nb: 0 to 0.10%
Nb is an element that combines with C or / and N to form fine precipitates and refines prior austenite crystal grains to improve hydrogen embrittlement resistance. For this reason, you may contain Nb as needed. However, when an amount of Nb exceeding 0.10% is contained, the amount of precipitates increases and the toughness is deteriorated. Therefore, the upper limit of the Nb content when contained is 0.10%. The upper limit of the Nb content is preferably 0.06%. In order to obtain the above effect stably, the lower limit of the Nb content is preferably 0.005%, and more preferably 0.03%.

V: 0 to 0.10%
V is an element that combines with C or / and N, forms fine precipitates, refines prior austenite crystal grains, and improves hydrogen embrittlement resistance. For this reason, you may contain V as needed. However, even if V is contained in an amount exceeding 0.10%, the effect of making the prior austenite crystal grains fine is saturated and only the cost is increased. Therefore, the upper limit of the V content when contained is 0.10%. The upper limit of the V content is preferably 0.06%. In order to obtain the above effect stably, the lower limit of the V content is preferably 0.005%, and more preferably 0.03%.

Zr: 0 to 0.20%
Zr is an element that combines with C or / and N to form fine precipitates and refines prior austenite crystal grains to improve hydrogen embrittlement resistance. For this reason, you may contain Zr as needed. However, when Zr is contained in an amount exceeding 0.20%, the amount of precipitates increases and the toughness is deteriorated. Therefore, the upper limit of the Zr content when contained is 0.20%. The upper limit of the Zr content is preferably 0.12%. In order to obtain the above effect stably, the lower limit of the Zr content is preferably 0.010%, and more preferably 0.06%.

  The total amount when Ti, Nb, V and Zr are contained in combination is preferably 0.08% or less.

  The steel material according to the present invention is composed of the above-described elements, the balance being Fe and impurities, and P, S, N and O as impurities are P: 0.030% or less, S: 0.030% or less, The chemical composition is N: 0.030% or less and O: 0.010% or less.

  Here, “impurities” are components mixed in due to various factors of raw materials such as ores and scraps and manufacturing processes when industrially producing steel materials, and are permitted within a range that does not adversely affect the present invention. Means what will be done.

P: 0.030% or less P is contained as an impurity and segregates at the grain boundary to lower toughness and / or hydrogen embrittlement resistance. When the P content exceeds 0.030%, the above-described adverse effects become remarkable. For this reason, content of P shall be 0.030% or less. The P content is desirably as low as possible.

S: 0.030% or less S is contained as an impurity and, like P, segregates at the grain boundary and deteriorates the hydrogen embrittlement resistance. When the S content exceeds 0.030%, the above-described adverse effects become remarkable. For this reason, content of S shall be 0.030% or less. It is desirable that the S content be as low as possible.

N: 0.030% or less N is contained as an impurity, and when its content is excessive and exceeds 0.030%, the deterioration of toughness becomes remarkable. Therefore, the N content is 0.030% or less. The N content is desirably as low as possible.

O: 0.010% or less O (oxygen) is contained as an impurity and is combined with Al to form an oxide. When the content increases and exceeds 0.010%, problems such as excessive formation of oxides and reduction in toughness occur. Therefore, the content of O is set to 0.010% or less. The O content is desirably as low as possible.

(B) Strength of high strength low alloy steel:
The high strength low alloy steel according to the present invention has a tensile strength of 1500 MPa or more. If the tensile strength is 1500 MPa or more, in recent years, it is possible to sufficiently meet the increase in strength required for automobiles, industrial machines, building structures and the like from the viewpoints of weight reduction, function, and the like. The upper limit of the tensile strength is desirably 2500 MPa, and more desirably 2000 MPa.

(C) About manufacturing method:
The high strength low alloy steel material having a tensile strength of 1500 MPa or more according to the present invention is produced by the following method.

  After melting the low alloy steel having the chemical composition described in the above section (A), an ingot or slab is formed by casting. The cast ingot or slab is subjected to hot working such as hot rolling, hot extrusion, hot forging, etc., and if necessary, cold working is performed to obtain the required round bar, steel wire, steel plate, etc. Finish the steel with a shape. Thereafter, the steps (i) to (iv) described below are sequentially performed on the steel material.

(I): Austenitizing step of heating at 850 to 1050 ° C. for 20 to 60 minutes The above-described steel material is heated at 850 to 1050 ° C. for 20 to 60 minutes to completely austenite. If the heating temperature of the steel material is lower than 850 ° C., it may not be possible to completely austenite. On the other hand, when the heating temperature exceeds 1050 ° C., the prior austenite grains become coarse, so that the ductility is lowered and it becomes difficult to perform cold working with a total working rate of 10.0% or more as described later (iv). Or important properties such as tensile strength of 1500 MPa or more and good hydrogen embrittlement resistance cannot be secured at the same time. Even if the heating temperature of the steel material is in the above range, if the heating time is less than 20 minutes, the steel material may not be completely austenitic, and if it exceeds 60 minutes, in addition to increasing the energy cost, It may be difficult to obtain new prior austenite grains. Therefore, an austenitization process shall heat a steel material at 850-1050 degreeC for 20 to 60 minutes. In addition, the heating temperature in this process (i) points out the temperature in the surface of steel materials. A desirable lower limit of the heating temperature of the steel material is 870 ° C. The desirable upper limit of the heating temperature is 1000 ° C., more desirably 950 ° C. A desirable lower limit of the heating time is 30 minutes, and a desirable upper limit is 45 minutes.

(Ii): An isothermal transformation step in which the steel material is cooled to a temperature range of 400 to 300 ° C. at a cooling rate of 30 ° C./second or more and held in the temperature range for 10 to 100 minutes. Then, the cooling rate is set to 30 ° C./second or more, the temperature is cooled to a temperature range of 400 to 300 ° C., and the temperature is maintained for 10 to 100 minutes to perform isothermal transformation. When the cooling rate after austenitization is less than 30 ° C./second, the tensile strength of 1500 MPa or more may not be reached even if cold working (iv) described later is performed. The upper limit of the cooling rate after austenitization is about 80 ° C./second industrially. Even at the cooling rate of 30 ° C./second or more, it is difficult to obtain a strength of 1500 MPa or more when the cooling temperature exceeds 400 ° C. Moreover, when the said temperature will be less than 300 degreeC, it will become weak in the holding time of 10 to 100 minutes, and defects, such as a crack, may be produced at the time of the cold processing of (iv). Further, in the case of the chemical composition described in the above section (A), the following steps (iii) and (iv) are usually performed by maintaining the temperature within the above-described temperature range for 10 to 100 minutes after complete austenite formation. If it passes, the high intensity | strength of 1500 Mpa or more can be stably comprised with the tensile strength as described in (B) term. In addition, because of the influence of the size or / and contained elements of the steel material, defects such as cracks may occur during the cold working of (iv), or desired hydrogen embrittlement resistance may not be obtained. The lower limit of the retention time in is desirably 30 minutes, and more desirably 60 minutes. The upper limit is preferably about 80 minutes. In addition, the cooling rate and temperature in the step (ii) indicate the cooling rate and temperature based on the surface of the steel material.

(Iii): Cooling step for cooling to room temperature The steel material that has been isothermally transformed in the step (ii) is cooled to room temperature. There is no particular limitation on the cooling rate at this time. The cooling temperature in the step (iii) also refers to the temperature on the surface of the steel material.

(Iv): Cold working step for performing cold working at a total working rate of 10.0% or more Cold working at a total working rate of 10.0% or more is applied to the steel material cooled to room temperature in the step (iii) above. Apply. When the total processing rate in cold working is less than 10.0%, desired tensile strength and hydrogen embrittlement resistance (good hydrogen embrittlement resistance at a tensile strength of 1500 MPa or more) cannot be obtained. Note that the cold working in the step (iv) needs to be performed without softening the steel material cooled to room temperature in the step (iii). Specific processing methods in the cold processing step (iv) include “drawing”, “plate rolling”, and the like, and the upper limit of the total processing rate varies depending on the processing method. Hereinafter, the case of “drawing” and “plate rolling” will be described as an example.

  In the present invention, “drawing” refers to a plastic working method in which a steel material is drawn through a die and stretched in one direction, and includes wire drawing of a wire coil. The “total processing rate” in “drawing” is expressed by “total area reduction rate”. As described above, when the “total area reduction rate”, which is the total processing rate, is less than 10.0%, it is desired. The tensile strength and hydrogen embrittlement resistance cannot be obtained. On the other hand, the upper limit of the “total area reduction rate” is preferably 20.0% in order to suppress the occurrence of processing defects such as cracks and breaks. The lower limit of the “total area reduction ratio” is preferably 12.0%, and the upper limit is more preferably 18.0%.

  If the total area reduction ratio is 10.0 to 20.0%, the number of drawing processes is not particularly limited, and may be one or more times.

The “area reduction ratio” in the n-th “drawing” is the cross-sectional area of the steel material before and after the _n-th drawing (where n is a positive integer), respectively, “S _n-1 ”. And “S _n ”, {(S _n−1 −S _n ) / S _n−1 } × 100
The value represented by The “total area reduction ratio” means that the cross-sectional area of the steel material before the first drawing process is “S ₀ ”, and the cross-sectional area of the steel material after the final drawing process is “S _f ”. {(S ₀ −S _f ) / S ₀ } × 100
The value represented by

  If necessary, the steel material cooled to room temperature may be subjected to mechanical processing such as cutting or peeling before performing “pulling”. In the “drawing process”, it is preferable to perform a lubrication treatment by an appropriate method.

  In the present invention, “plate rolling” refers to a plastic working method in which a steel material is stretched in one direction using a rolling roll. In addition, the “total reduction ratio” in the “sheet rolling” is represented by the “total reduction ratio”. As described above, when the “total reduction ratio” that is the total reduction ratio is less than 10.0%, the desired reduction ratio Tensile strength and hydrogen embrittlement resistance cannot be obtained. On the other hand, when the total rolling reduction is increased, the tensile strength is improved, but the hydrogen embrittlement resistance is deteriorated. Therefore, the upper limit of the “total rolling reduction” is desirably 40.0%. The lower limit of the “total rolling reduction” is preferably 15.0%, and the upper limit is more preferably 35.0%.

If the total rolling reduction is 10.0 to 40.0%, the number of plate rolling processes is not particularly limited, and may be one or more times. The “total rolling reduction” is defined as “t ₀ ” for the thickness of the steel before the first plate rolling and “t _f ” for the steel after the final plate rolling. {(T ₀ −t _f ) / t ₀ } × 100
The value represented by

  If necessary, the steel material cooled to room temperature may be subjected to descaling treatment such as cutting, blasting, and pickling before performing “sheet rolling”. In the “plate rolling process”, it is preferable to perform a lubrication treatment by an appropriate method.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steels A to O having the chemical composition shown in Table 1 were melted, and the ingot obtained by casting into a mold was heated to 1250 ° C., and then a round bar having a diameter of 25 mm was formed by hot forging.

  Steels A to I in Table 1 are steels whose chemical compositions are within the range defined by the present invention, while Steels J to O are steels whose chemical compositions deviate from the conditions defined by the present invention.

(Example 1) When the cold working process is "drawing":
The round bar having a diameter of 25 mm obtained as described above was heated to the austenite at the temperature shown in Table 2 for 45 minutes. Each round bar of test numbers 1-1 to 1-12 and test numbers 1-14 to 1-20 was austenitized, and then immersed in a lead bath or a salt bath within 5 seconds from the temperature and subjected to isothermal transformation treatment. . Moreover, the round bar of test number 1-13 was austenitized, cooled in the atmosphere from the temperature, and immersed in a lead bath when 30 seconds had elapsed, and subjected to an isothermal transformation treatment. Details of the cooling rate from the austenitizing temperature and the temperature and holding time of the isothermal transformation treatment (the temperature of the lead bath or salt bath (that is, the temperature at which cooling by the cooling rate is stopped) and the holding time at the temperature) are as follows: As shown in Table 2. In addition, except test number 1-12, after isothermal transformation process, it stood to cool to room temperature in air | atmosphere. On the other hand, Test Nos. 1-12 were cooled to room temperature at a cooling rate of 30 ° C./second after the isothermal transformation treatment.

  For each test number, a part of a round bar with a diameter of 25 mm cooled to the above room temperature was used to investigate the mechanical characteristics shown in <1> described later. Further, for each test number, the remaining area of the round bar having a diameter of 25 mm cooled to room temperature was peeled to a diameter of 23 mm, and then the first area reduction was 8.5% or 16.6% without applying a softening treatment. As shown in Table 2, it was cold-drawn under the conditions shown in Table 2. Lubrication during the drawing process was performed with a wet lubricant.

  In Test Nos. 1-12 and 1-14 shown in Table 2, cracks occurred in the first drawing process in which the above-mentioned area reduction rate was 16.6%. Moreover, although the test number 1-10 did not produce a crack in the first drawing process, it produced a crack in the next second drawing process in which the total area reduction rate was 28.1%.

  Except for Test No. 1-12 and Test No. 1-14 that caused cracks in the above-mentioned first drawing, and Test No. 1-10 that caused cracks in the second drawing, For the round bar, the mechanical properties shown in the following <1> and the hydrogen embrittlement resistance shown in <2> were investigated.

<1> Mechanical properties:
For each steel, about a part of the round bar with a diameter of 25 mm cooled to the room temperature and a round bar subjected to drawing processing, the diameter of the parallel part in the longitudinal direction is 6 mm and the gauge distance is 40 mm from the center part. A round bar tensile test piece was cut out and subjected to a tensile test at room temperature to determine the tensile strength.

<2> Hydrogen embrittlement resistance:
A notched tensile test piece having a shape shown in FIG. 1 is cut out in the longitudinal direction from the center of each of the round bars subjected to the drawing process in which a tensile strength of 1500 MPa or more was obtained in the investigation of <1> above. The hydrogen embrittlement characteristics were investigated. Specifically, first, the presence or absence of breakage was investigated when a constant load test with a stress of 900 MPa was performed for 200 hours under the condition of cathodic charging a 3% NaCl solution at a current density of 1 mA / cm ² .

  Next, for the test piece that did not break, the 10 mm parallel portion shown in FIG. The hydrogen concentration was measured and regarded as the “breaking limit hydrogen concentration”.

  In addition, it determined with having a favorable hydrogen embrittlement-proof characteristic, when it does not fracture | rupture by said constant load test and a fracture | rupture limit hydrogen concentration is 0.50 ppm or more.

  Table 2 summarizes the results of each of the above investigations.

  From Table 2, the round bars with test numbers 1-1 to 1-9 of the examples of the present invention produced by subjecting the round bars whose chemical composition is within the range defined by the present invention to the process defined by the present invention are drawn out. Cracking did not occur when processing was performed, and despite having a high tensile strength of 1500 MPa or more, no breakage occurred in a constant load test under a cathode charge, and the breaking hydrogen concentration at that time was also 0 It is clear that it has a good hydrogen embrittlement resistance, and exceeds the standard of 0.50 ppm at 0.56 ppm or more.

  On the other hand, the round bars with test numbers 1 to 10 in the reference example have a chemical composition and a heat treatment process from (i) to (iii) within the range defined by the present invention, but with a total area reduction rate. Cracks occurred when the second drawing process of 28.1% was performed.

  Further, in the case of the round bars of test numbers 1-11 to 1-20 of the comparative example, cracks are generated when the drawing process is performed, and the tensile strength of 1500 MPa or more and good hydrogen embrittlement resistance are important. Simultaneous characteristics cannot be secured.

  Although the round bar of the test number 1-11 has a chemical composition and the heat treatment process from (i) to (iii) is within the range specified in the present invention, the total area reduction rate is 8.5%, Since it deviates from the conditions stipulated by the present invention, the fracture limit hydrogen concentration is as low as 0.48 ppm, and the hydrogen embrittlement resistance is inferior.

  The round bar of test number 1-12, although the chemical composition of the steel A used is within the range specified by the present invention, the isothermal transformation treatment temperature is 250 ° C., and thus deviates from the conditions specified by the present invention. Cracks occurred when the drawing process was performed.

  In the round bar of test number 1-13, the chemical composition of the steel C used and the drawing process are within the range specified by the present invention, but the cooling rate after austenitization deviates from the conditions specified by the present invention. The tensile strength was as low as 1383 MPa and was less than 1500 MPa.

  Although the round bar of test number 1-14 has the chemical composition of steel F used within the range specified in the present invention, the austenitizing temperature is 1100 ° C., which is out of the conditions specified by the present invention. The austenite grain size became large, and cracking occurred when drawing.

  The round bar of test number 1-15 was manufactured by performing the process specified in the present invention, but the C content of the steel J used was as low as 0.47%, and deviated from the conditions specified in the present invention. Therefore, the tensile strength was as low as 1435 MPa and was less than 1500 MPa.

  The round bars of test numbers 1-16 were manufactured by performing the process specified in the present invention, but the Si content of the steel K used was as low as 0.89%, and deviated from the conditions specified in the present invention. Therefore, the fracture limit hydrogen concentration is as low as 0.29 ppm, and the hydrogen embrittlement resistance is inferior.

  The round bar of test number 1-17 was manufactured by applying the process specified in the present invention, but the Mn content of the steel L used was as large as 1.22%, which deviated from the conditions specified in the present invention. For this reason, it breaks in a constant load test under cathodic charge and is inferior in hydrogen embrittlement resistance.

  The round bar of test number 1-18 was manufactured by applying the process specified in the present invention, but the Cr content of the steel M used was as low as 0.36%, which was not within the conditions specified in the present invention. Since the hardenability was poor, the tensile strength was as low as 1427 MPa and less than 1500 MPa.

  The round bars of Test No. 1-19 were manufactured by applying the process specified in the present invention, and the contents of P and S in the impurities of the steel N used were 0.042% and 0. 0%, respectively. Since it is as high as 040% and deviates from the conditions stipulated in the present invention, it is broken in a constant load test under a cathode charge and inferior in hydrogen embrittlement resistance.

  The round bar of test number 1-20 has a low Si content of 0.19% in steel O used, and does not contain Cr, which is out of the chemical composition conditions defined in the present invention. The temperature and holding time of the isothermal transformation treatment as conditions are 450 ° C. and 4 minutes, respectively, which are also outside the conditions of the isothermal transformation step (ii) defined in the present invention. For this reason, although a total area reduction rate of 45.4% can be ensured without cracking, the fracture limit hydrogen concentration is as low as 0.32 ppm and the hydrogen embrittlement resistance is poor.

(Example 2) When the cold working process is “sheet rolling”:
A plate material having a thickness of 3 mm, a width of 20 mm, and a length of 300 mm was cut out from the remainder of the round bar having a diameter of 25 mm cooled to room temperature as described in Example 1 above. Next, without subjecting the above plate material to softening heat treatment, a plurality of passes of plate rolling until the total rolling reduction shown in Table 3 is 6.7 to 50.0% (thickness 2.8 to 1.5 mm). Was given. Lubrication during plate rolling was performed with a wet lubricant. In addition, “tensile strength before cold drawing” in Table 2 obtained in (Example 1) was used as it is for “tensile strength before cold plate rolling” in Table 3.

  In Test No. 2-13 shown in Table 3, ear cracks occurred in the process of setting the total rolling reduction to 16.7% (thickness 2.5 mm).

  Except for test number 2-13 which caused the above-mentioned ear cracking, the mechanical properties shown in the following <2-1> and the hydrogen embrittlement resistance shown in <2-2> for each plate after plate rolling. The characteristics were investigated.

<2-1> Mechanical properties:
About each steel, about the plate which gave the plate rolling process, the width of the parallel part shown in FIG. 2 is 5 mm, the gauge distance is 20 mm, and the thickness is the plate after the plate rolling. A tensile test piece was cut out and subjected to a tensile test at room temperature to determine the tensile strength.

<2-2> Hydrogen embrittlement resistance:
Notches having the shape shown in FIG. 3 in the longitudinal direction from the center of the plate width and thickness of each plate subjected to the plate rolling process in which the tensile strength of 1500 MPa or more was obtained in the investigation of <2-1> above. The attached tensile test piece was cut out and examined for hydrogen embrittlement resistance by the same method as in (Example 1). Specifically, first, the presence or absence of breakage was investigated when a constant load test with a stress of 900 MPa was performed for 200 hours under the condition of cathodic charging a 3% NaCl solution at a current density of 1 mA / cm ² .

  Next, for the test piece that did not break, the parallel portion 8 mm shown in FIG. The hydrogen concentration was measured and regarded as the “breaking limit hydrogen concentration”.

  In addition, it determined with having a favorable hydrogen embrittlement-proof characteristic, when it does not fracture | rupture by said constant load test and a fracture | rupture limit hydrogen concentration is 0.50 ppm or more.

  Table 3 summarizes the results of the above investigations.

  From Table 3, a book produced by subjecting a plate material cut out from a round bar with a diameter of 25 mm cooled to room temperature (Example 1) whose chemical composition is within the range specified by the present invention, by subjecting it to the steps specified by the present invention. The plates of Test Nos. 2-1 to 2-10 of the invention example were not cracked when subjected to plate rolling, and had a high tensile strength of 1500 MPa or more, but under cathodic charge. In the constant load test, no breakage occurred, and the hydrogen concentration at break at that time was 0.61 ppm or more, exceeding the standard of 0.50 ppm, and it is clear that it has good hydrogen embrittlement resistance. .

  On the other hand, in the case of the plates having test numbers 2-11 to 2-22 of the comparative example, the ear cracks occurred when the plate rolling was performed, the tensile strength of 1500 MPa or more and the good hydrogen embrittlement resistance. At the same time, it is not possible to secure the important characteristic of the conversion characteristic.

  The plate of test number 2-11 had a chemical composition and a heat treatment step from (i) to (iii) within the range specified by the present invention, but the total rolling reduction was 6.7%, and the present invention Therefore, in addition to the low tensile strength of 1485 MPa, the fracture limit hydrogen concentration is as low as 0.42 ppm, and the hydrogen embrittlement resistance is inferior.

  Although the plate of test number 2-12 has the chemical composition and the heat treatment steps from (i) to (iii) within the range specified in the present invention, the total rolling reduction is 50.0%, and the present invention Therefore, it breaks in a constant load test under cathodic charge and is inferior in hydrogen embrittlement resistance.

  The plate of test number 2-13 has the chemical composition of steel A used within the range specified in the present invention, but the isothermal transformation treatment temperature is 250 ° C., which is outside the conditions specified by the present invention. Ear cracks occurred when a plate rolling process with a rolling reduction of 16.7% was performed.

  The plate of Test No. 2-14 has the chemical composition of Steel A used within the range specified by the present invention, but the isothermal transformation temperature is 250 ° C. Tensile strength exceeding 1500 MPa was obtained without causing ear cracks at 6.7% below, but the hydrogen concentration at break was as low as 0.38 ppm, and the hydrogen embrittlement resistance was inferior.

  Although the plate of Test No. 2-15 has the chemical composition of Steel C used and the plate rolling process within the range specified by the present invention, the cooling rate after austenitization deviates from the conditions specified by the present invention. The tensile strength was as low as 1457 MPa and was less than 1500 MPa.

  The plate of Test No. 2-16 has a chemical composition of Steel F used within the range specified by the present invention, but the austenitizing temperature is 1100 ° C. and deviates from the conditions specified by the present invention. The particle size becomes large, it is broken in a constant load test under a cathode charge, and the hydrogen embrittlement resistance is inferior.

  The plate of Test No. 2-17 was manufactured by performing the process specified in the present invention, but the C content of the steel J used was as low as 0.47%, which is outside the conditions specified in the present invention. In addition, the fracture limit hydrogen concentration is as low as 0.41 ppm, and the hydrogen embrittlement resistance is inferior.

  The plate of test number 2-18 was manufactured by performing the process specified in the present invention, but the Si content of the steel K used was as low as 0.89%, and thus deviated from the conditions specified in the present invention. In addition, the fracture limit hydrogen concentration is as low as 0.36 ppm, and the hydrogen embrittlement resistance is inferior.

  The plate of test number 2-19 was manufactured by performing the process specified in the present invention, but the Mn content of the steel L used was as large as 1.22%, and thus deviated from the conditions specified in the present invention. It breaks in a constant load test under a cathode charge, and is inferior in hydrogen embrittlement resistance.

  The plate of test number 2-20 was manufactured by performing the process specified in the present invention, but the Cr content of the steel M used was as low as 0.36%, which deviated from the conditions specified in the present invention. Since it was inferior in hardenability, the tensile strength was as low as 1418 MPa and was less than 1500 MPa.

  The plate of test number 2-21 was manufactured by performing the process specified in the present invention, and the contents of P and S in the impurities of steel N used were 0.042% and 0.040, respectively. %, Which is outside the conditions specified in the present invention, it was broken in a constant load test under a cathode charge, and was inferior in hydrogen embrittlement resistance.

  The plate of test No. 2-22 has a low Si content of 0.19% in steel O used, and does not contain Cr, deviating from the chemical composition conditions defined in the present invention, and further manufacturing conditions The temperature and holding time of the isothermal transformation treatment are 450 ° C. and 4 minutes, respectively, which are also outside the conditions of the isothermal transformation step (ii) defined in the present invention. For this reason, the tensile strength was as low as 1479 MPa and less than 1500 MPa even when sheet rolling with a total rolling reduction of 33.3% satisfying the provisions of the present invention was performed.

According to the present invention, the content of expensive alloy elements is low, the hydrogen embrittlement resistance is excellent, and the tensile strength is 1500 MPa or more, which can be suitably used for automobiles, industrial machines, building structures and the like. High strength low alloy steel can be manufactured with high productivity.

Claims (5)

A method for producing a high strength low alloy steel material,
Chemical composition is mass%,
C: more than 0.60% and 1.0% or less,
Si: 1.2-2.0%,
Mn: 0.30% or more and less than 1.0%,
Cr: 0.5 to 1.5%
Al: 0.005 to 0.10%,
Mo: 0 to less than 0.30%,
Ti: 0 to 0.10%,
Nb: 0 to 0.10%,
V: 0 to 0.10%,
Zr: 0 to 0.20%,
The balance is Fe and impurities,
In steel materials in which P, S, N and O as impurities are P: 0.030% or less, S: 0.030% or less, N: 0.030% or less and O: 0.010% or less,
The following steps (i) to (iv) are sequentially performed.
A method for producing a high-strength low-alloy steel material having a tensile strength of 1500 MPa or more.
(I): Austenitizing step of heating at 850 to 1050 ° C. for 20 to 60 minutes (ii): Cooling to a temperature range of 400 to 300 ° C. at a cooling rate of 30 ° C./second or more, and 10 to 100 in the temperature range Isothermal transformation step (iii): cooling to room temperature, cooling step (iv): cold working step for cold working at a total working rate of 10.0% or more   The tensile strength according to claim 1, wherein the cold working step (iv) is a cold working step in which drawing is performed at a total area reduction of 10.0 to 20.0%. A manufacturing method of high strength low alloy steel.   The tensile strength according to claim 1, wherein the cold working step (iv) is a cold working step in which sheet rolling is performed at a total rolling reduction of 10.0 to 40.0%. A manufacturing method of high strength low alloy steel. The chemical composition of steel is mass%,
Mo: 0.05% or more and less than 0.30%, The manufacturing method of the high strength low alloy steel materials in any one of Claim 1 to 3 whose tensile strength is 1500 Mpa or more. The chemical composition of steel is mass%,
Ti: 0.005 to 0.10%,
Nb: 0.005 to 0.10%,
V: 0.005 to 0.10%, and
Zr: 0.010 to 0.20%,
The manufacturing method of the high intensity | strength low alloy steel material in which tensile strength is 1500 Mpa or more in any one of Claim 1 to 4 containing 1 or more types selected from these.

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