JP6879145B2 - Manufacturing method of high-strength low-alloy steel - Google Patents

Description

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

In recent years, there has been a tendency to use high-strength steel materials having a tensile strength of more than 1000 MPa from the viewpoint of weight reduction and function. However, when the tensile strength of the steel material exceeds 1000 MPa, hydrogen embrittlement becomes a serious problem. Hydrogen embrittlement is a phenomenon in which the mechanical properties deteriorate from the original values due to the intrusion of hydrogen into the steel material. Since hydrogen has the smallest atomic radius among all elements, it is inevitable that hydrogen will enter the steel material.

Regarding a high-strength steel material having excellent hydrogen embrittlement resistance and a method for producing the same, for example, Patent Document 1 discloses a technique for suppressing delayed fracture property, which is one form of hydrogen embrittlement property. Specifically, it is made of a steel material containing a specific amount of C, and the area ratio of the bainite structure is set to 80% or more, and then strong wire drawing is performed to achieve ^{a strength of 1200 MPa (1200 N / mm 2} ) or more and excellent resistance. A technique relating to a high-strength steel wire having excellent delayed fracture resistance and forging property, which has delayed fracture resistance, is disclosed. In Patent Document 1, after hot rolling or forging a steel having the above-mentioned chemical composition, it is rapidly cooled to a temperature of 300 to 500 ° C., and the average cooling rate of 1 ° C./sec or less from that temperature is 200 seconds or more. A manufacturing method is shown in which the material is cooled by applying and then subjected to strong wire drawing.

JP-A-2002-241899

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

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

The gist of the present invention is 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 high-strength low-alloy steel materials.
The chemical composition is by 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: 0.5-1.5%,
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: 0-0.20%,
The rest is Fe and impurities,
For 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 performed in order.
A method for producing a high-strength low-alloy steel material having a tensile strength of 1500 MPa or more.
(I): Austenizing step of heating at 850 to 150 ° C. for 20 to 60 minutes (ii): Cooling to a temperature range of 400 to 300 ° C. at a cooling rate of 30 ° C./sec or higher, and 10 to 100 in that temperature range. Isothermal transformation step (iii): Cooling to room temperature, Cooling step (iv): Cold working step with a total working rate of 10.0% or more

(2) The tensile strength according to (1) above, wherein the cold working step of (iv) is a cold working step of drawing with a total surface 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 step of (iv) is a cold working step of rolling a plate at a total reduction ratio 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: The 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) above, which contains 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%,
The 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 (4) above, which contains one or more selected from the above.

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

It is a figure which shows the shape of the tensile test piece with a notch used for the evaluation of the hydrogen embrittlement resistance property in (Example 1). The numerical values in the figure indicate the dimensions (unit: mm). It is a figure which shows the shape of the plate tensile test piece used for the evaluation of the tensile strength in (Example 2). The numerical values in the figure indicate the dimensions (unit: mm). It is a figure which shows the shape of the tensile test piece with a notch used for the evaluation of the hydrogen embrittlement resistance property in (Example 2). The numerical values in the figure indicate the dimensions (unit: mm).

Hereinafter, each requirement of the present invention will be described in detail. The "%" of the content of each element means "mass%".

(A) 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 with the same strength, the hydrogen embrittlement resistance can be improved even if the content of expensive elements such as Mo and Ni is lowered. C is an important element for ensuring high strength, and since strength can be ensured without excessive cold working, less dislocations are introduced by cold working and hydrogen embrittlement resistance is deteriorated. Hateful. Therefore, C must be contained in an amount of more than 0.60%. Furthermore, since C _{lowers the Ac 3} point, the steel tends to be completely austenite when heated at a relatively low temperature, so that the old austenite crystal grain size becomes smaller, and the hydrogen embrittlement resistance is also improved in this respect. To do. On the other hand, when the C content increases and exceeds 1.0%, the toughness is significantly deteriorated. Therefore, the content of C is set to more than 0.60% and 1.0% or less. The desirable lower limit of the C content is 0.65%, and the desirable upper limit is 0.80%.

Si: 1.2-2.0%
Si has a deoxidizing effect and also has an effect of improving strength and hardenability. Improving the strength is effective in ensuring a tensile strength of 1500 MPa or more. In addition, Si also has the effect of improving hydrogen embrittlement resistance by performing 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 in excess of 2.0%, the effect is saturated and the toughness is deteriorated. Therefore, the Si content is set to 1.2 to 2.0%. The desirable lower limit of the Si content is 1.3%, and the 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 of the strength is effective for securing the tensile strength of 1500 MPa or more, and the improvement of the hardenability is advantageous from the viewpoint of production because the desired strength can be easily obtained. In addition, Mn also has the effect of forming sulfide by combining with S, suppressing grain boundary segregation of S, and improving hydrogen embrittlement resistance. In order to obtain these effects, the Mn content needs to be 0.30% or more. On the other hand, if Mn is excessively contained, segregation occurs at the grain boundaries and promotes grain boundary cracking type hydrogen embrittlement fracture. Therefore, the Mn content is set to 0.30% or more and less than 1.0%. The desirable lower limit of the Mn content is 0.40%, and the desirable upper limit is 0.60%.

Cr: 0.5-1.5%
Cr is an element effective for improving the strength. Further, Cr also has an action of improving hardenability, and improvement of hardenability is advantageous from the viewpoint of production because it is easy to obtain desired strength. In order to obtain these effects, it is necessary to contain Cr in an amount of 0.5% or more. On the other hand, excessive inclusion of Cr causes deterioration of toughness. Therefore, the Cr content is set to 0.5 to 1.5%. The desirable lower limit of the Cr content is 0.8%, and the 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 0.005% or more of Al. On the other hand, even if Al is contained in an amount of more than 0.10%, the effect is saturated. Therefore, the Al content 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 enhances the stability of Fe carbides and improves hydrogen embrittlement resistance. Therefore, Mo may be contained if necessary. 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 is used. The content of is greatly impairing economic efficiency. Therefore, the Mo content when contained is set to less than 0.30%. The upper limit of the Mo content is preferably 0.20%. In order to obtain the above effect stably, the lower limit of the Mo content is preferably 0.05%, more preferably 0.10%.

Ti: 0 to 0.10%
Ti is an element that binds to C and / and N to form fine precipitates and refine the former austenite crystal grains to improve hydrogen embrittlement resistance. Therefore, Ti may be contained if necessary. However, if an amount of Ti exceeding 0.10% is contained, the amount of precipitates increases and the toughness deteriorates. Therefore, the upper limit of the Ti content when it is contained is set to 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%, more preferably 0.03%.

Nb: 0 to 0.10%
Nb is an element that binds to C and / and N to form fine precipitates and refine the former austenite crystal grains to improve hydrogen embrittlement resistance. Therefore, Nb may be contained if necessary. However, if an amount of Nb exceeding 0.10% is contained, the amount of precipitates increases and the toughness deteriorates. Therefore, the upper limit of the Nb content when it is contained is set to 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%, more preferably 0.03%.

V: 0 to 0.10%
V is an element that binds to C or / and N to form fine precipitates, refine the former austenite crystal grains, and improve hydrogen embrittlement resistance. Therefore, V may be contained if necessary. However, even if an amount of V exceeding 0.10% is contained, the effect of making the old austenite crystal grains fine is saturated, and the cost is only increased. Therefore, the upper limit of the V content when it is contained is set to 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%, more preferably 0.03%.

Zr: 0-0.20%
Zr is an element that binds to C and / and N to form fine precipitates and refine the former austenite crystal grains to improve hydrogen embrittlement resistance. Therefore, Zr may be contained if necessary. However, if an amount of Zr exceeding 0.20% is contained, the amount of precipitates increases and the toughness deteriorates. Therefore, the upper limit of the Zr content when it is contained is set to 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%, more preferably 0.06%.

When the above Ti, Nb, V and Zr are compounded and contained, the total amount is preferably 0.08% or less.

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

Here, the "impurity" is a component mixed by various factors in the raw material such as ore and scrap, and various factors in the manufacturing process when the steel material is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means what is done.

P: 0.030% or less P is contained as an impurity and segregates at grain boundaries to reduce toughness and / or hydrogen embrittlement resistance. When the P content exceeds 0.030%, the above adverse effect becomes remarkable. Therefore, the P content is set to 0.030% or less. It is desirable that the content of P is as low as possible.

S: 0.030% or less S is contained as an impurity and segregates at the grain boundaries like P to lower the hydrogen embrittlement resistance. When the S content exceeds 0.030%, the above adverse effect becomes remarkable. Therefore, the S content is set to 0.030% or less. It is desirable that the S content is as low as possible.

N: 0.030% or less N is contained as an impurity, and when the content becomes excessive and exceeds 0.030%, the deterioration of toughness becomes remarkable. Therefore, the N content is set to 0.030% or less. It is desirable that the N content is as low as possible.

O: 0.010% or less O (oxygen) is contained as an impurity and combines with Al to form an oxide. If the content increases and exceeds 0.010%, problems such as excessive formation of oxides and a decrease in toughness occur. Therefore, the content of O is set to 0.010% or less. It is desirable that the O content is as low as possible.

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

(C) 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 item (A), it is cast into an ingot or a slab. 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 round bars, steel wires, steel plates, etc. Finish the steel material with a shape. Then, the steps (i) to (iv) described below are sequentially applied to the steel material.

(I): Austenitizing step of heating at 850 to 1050 ° C. for 20 to 60 minutes The above-mentioned steel material is heated at 850 to 1050 ° C. for 20 to 60 minutes to completely austenitize. If the heating temperature of the steel material is lower than 850 ° C., it may not be completely austenitized. On the other hand, when the heating temperature exceeds 1050 ° C., the old austenite grains become coarse, so that the ductility is lowered and it becomes difficult to perform cold working with a total processing rate of 10.0% or more, which will be described later (iv). Or, it becomes impossible to simultaneously secure important properties such as a tensile strength of 1500 MPa or more and good hydrogen embrittlement resistance. Even if the heating temperature of the steel material is within the above range, if the heating time is less than 20 minutes, the steel material may not be completely austenitized, and if it exceeds 60 minutes, the energy cost increases and the fineness is increased. It may be difficult to obtain old austenite grains. Therefore, in the austenitizing step, the steel material is heated at 850 to 1050 ° C. for 20 to 60 minutes. The heating temperature in the step (i) refers to the temperature on the surface of the steel material. The desirable lower limit of the heating temperature of the steel material is 870 ° C. Further, the desirable upper limit of the heating temperature is 1000 ° C., and 950 ° C. is more desirable. The desired lower limit of the heating time is 30 minutes, and the desired upper limit is 45 minutes.

(Ii): Isothermal transformation step of cooling to a temperature range of 400 to 300 ° C. at a cooling rate of 30 ° C./sec or higher and holding in the temperature range for 10 to 100 minutes. With a cooling rate of 30 ° C./sec or higher, the product is cooled to a temperature range of 400 to 300 ° C. and held in that temperature range for 10 to 100 minutes for isothermal transformation. When the cooling rate after austenitization is less than 30 ° C./sec, the tensile strength of 1500 MPa or more may not be reached even if the cold working of (iv) described later is performed. The upper limit of the cooling rate after austenitization is industrially about 80 ° C./sec. Even if the cooling rate is 30 ° C./sec or higher, if the cooling temperature exceeds 400 ° C., it becomes difficult to obtain a strength of 1500 MPa or higher. Further, when the temperature is less than 300 ° C., the temperature becomes brittle with a holding time of 10 to 100 minutes, and defects such as cracks may occur during the cold working of (iv). Further, in the case of the chemical composition described in the above item (A), the subsequent steps (iii) and (iv) are usually carried out by completely austenitizing and then holding the mixture in the above temperature range for 10 to 100 minutes. After that, it is possible to stably provide a high strength of 1500 MPa or more with the tensile strength described in item (B). In addition, due to the influence of the size and / and the contained elements of the steel material, defects such as cracks may occur during the cold working of (iv), or the desired hydrogen embrittlement resistance may not be obtained. The lower limit of the holding time in is preferably 30 minutes, more preferably 60 minutes. Moreover, it is desirable that the upper limit is about 80 minutes. The cooling rate and temperature in the step (ii) refer to the cooling rate and temperature based on the surface of the steel material.

(Iii): Cooling step of cooling to room temperature The steel material that has been isothermally transformed in the step of (iii) above 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 in which cold working is performed at a total working rate of 10.0% or more Cold working is performed on a steel material cooled to room temperature in the above step (iii) at a total working rate of 10.0% or more. Give. When the total processing ratio in cold working is less than 10.0%, the desired tensile strength and hydrogen embrittlement resistance (good hydrogen embrittlement resistance at a tensile strength of 1500 MPa or more) cannot be obtained. The cold working in the step (iv) needs to be applied to the steel material cooled to room temperature in the step (iii) without being softened. Specific processing methods of the cold processing process of (iv) include "drawing processing", "plate rolling processing", and the like, and the upper limit of the total processing rate differs depending on the processing method. Hereinafter, the cases of "drawing" and "plate rolling" will be described as an example.

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

When the total surface reduction rate is 10.0 to 20.0%, the number of drawing processes is not particularly limited, and may be once or a plurality of times.

The "surface reduction rate" in the nth "drawing process" is the cross-sectional area of the steel material before and after the _{nth drawing process (where n is a positive integer) is "S n-1} ". And when "S _n " is set, {(S _n-1 −S _n ) / S _n-1 } × 100
Refers to the value represented by. The "total surface reduction rate" is the case where 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
Refers to 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 "drawing". In the "drawing process", it is preferable to perform the lubrication process 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. The "total processing rate" in the "plate rolling process" is represented by the "total reduction rate", and as described above, when the "total reduction rate", which is the total processing rate, is less than 10.0%, it is desired. Tensile strength and hydrogen embrittlement resistance cannot be obtained. On the other hand, as the total reduction rate increases, the tensile strength improves, but the hydrogen embrittlement resistance deteriorates. Therefore, it is desirable that the upper limit of the "total reduction rate" is 40.0%. The lower limit of the "total reduction rate" is preferably 15.0%, and the upper limit is more preferably 35.0%.

When the total reduction ratio is 10.0 to 40.0%, the number of plate rolling processes is not particularly limited, and may be one or a plurality of times. _{The "total reduction ratio" is defined as "t 0} " for the thickness of the steel material before the first plate rolling process _{and "t f} " for the thickness of the steel material after the final plate rolling process. In the case of {(t ₀ −t _f ) / t ₀ } × 100
Refers to the value represented by.

If necessary, the steel material cooled to room temperature may be descaled by cutting, blasting, pickling or the like before the "plate rolling". In the case of "plate rolling", it is preferable to perform lubrication treatment by an appropriate method.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Steels A to O having the chemical compositions shown in Table 1 were melted and cast into a mold, and the obtained ingot was heated to 1250 ° C. and then hot forged to obtain a round bar having a diameter of 25 mm.

Steels A to I in Table 1 are steels whose chemical composition is within the range specified in the present invention, while steels J to O are steels whose chemical composition does not meet the conditions specified in 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 at the temperature shown in Table 2 for 45 minutes to form austenite. The round bars of Test Nos. 1-1 to 1-12 and Test Nos. 1 to 14 to 1-20 were austenitized and then immersed in a lead bath or a salt bath within 5 seconds from the temperature to undergo an isothermal transformation treatment. .. The round bar of test number 1-13 was austenitized, cooled in the air from the temperature, and after 30 seconds, was immersed in a lead bath for isothermal transformation treatment. Details of the cooling rate from the austenizing 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 at the above cooling rate was stopped) and the holding time at that temperature) can be found. It is as shown in Table 2. Except for test numbers 1-12, after the isothermal transformation treatment, the mixture was allowed to cool to room temperature in the air. On the other hand, Test No. 1-12 was cooled to room temperature at a cooling rate of 30 ° C./sec after the isothermal transformation treatment.

For each test number, the mechanical properties shown in <1> described later were investigated using a part of the above-mentioned round bar having a diameter of 25 mm cooled to room temperature. Further, for each test number, after peeling the rest of the round bar having a diameter of 25 mm cooled to room temperature to a diameter of 23 mm, the first surface reduction rate is 8.5% or 16.6% without any softening treatment. As a result, the drawing process was performed cold under the conditions shown in Table 2. Lubrication during the drawing process was performed with a wet lubricating oil.

Test numbers 1-12 and test numbers 1-14 shown in Table 2 were cracked in the first drawing process in which the surface reduction rate was 16.6%. Further, Test No. 1-10 did not crack in the first drawing process, but cracked in the next second drawing process in which the total surface reduction rate was 28.1%.

Except for test numbers 1-12 and 1-14, which were cracked in the first drawing process, and test numbers 1-10, which were cracked in the second drawing process, each after drawing. The mechanical properties shown in <1> and the hydrogen embrittlement resistance shown in <2> below were investigated for the round bar.

<1> Mechanical characteristics:
For each steel, a part of the round bar having a diameter of 25 mm cooled to room temperature and the round bar subjected to the drawing process have a diameter of 6 mm and a gauge point distance of 40 mm in the longitudinal direction from the center of the round bar. 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 tensile test piece with a notch having the shape shown in FIG. 1 is cut out in the longitudinal direction from the center of each of the drawn round bars whose tensile strength of 1500 MPa or more was obtained in the investigation of <1> above, and has resistance. The hydrogen embrittlement characteristics were investigated. Specifically, first, the presence or absence of breakage was investigated when a constant load test in which a stress of 900 MPa was applied was carried out for 200 hours under the condition that the 3% NaCl solution was ^{charged with a cathode at a current density of 1 mA / cm 2.}

Next, with respect to the test piece that did not break, the parallel portion 10 mm shown in FIG. 1 was cut out with a low temperature cutting machine, and the hydrogen concentration released to 500 ° C. when the temperature was raised at 10 ° C./min by a temperature raising desorption device was determined. The hydrogen concentration was measured and regarded as the "breaking limit hydrogen concentration".

In the above constant load test, it was determined that the product did not break and had good hydrogen embrittlement resistance when the breaking limit hydrogen concentration was 0.50 ppm or more.

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

From Table 2, the round bars of Test Nos. 1-1 to 1-9 of the example of the present invention produced by subjecting the round bars having the chemical composition within the range specified in the present invention to the steps specified in the present invention are drawn out. No cracking occurred during processing, and despite having a high tensile strength of 1500 MPa or more, no fracture occurred in the constant load test under cathode charge, and the fracture limit hydrogen concentration at that time was also 0. At .56 ppm or higher, it exceeds the standard of 0.50 ppm, and it is clear that it has good hydrogen embrittlement resistance.

On the other hand, in the round bar of test number 1-10 of the reference example, although the chemical composition and the heat treatment steps from (i) to (iii) are within the range specified in the present invention, the total surface reduction rate is increased. Cracks occurred when the second drawing process, which was 28.1%, was performed.

Further, in the case of the round bar of test numbers 1-11 to 1-20 of the comparative example, cracks occur when the drawing process is performed, and the tensile strength of 1500 MPa or more and the good hydrogen embrittlement resistance are important. It is not possible to secure the same characteristics at the same time.

The round bar of test number 1-11 has a total surface reduction rate of 8.5%, although the chemical composition and the heat treatment steps from (i) to (iii) are within the range specified in the present invention. Since the conditions specified by the present invention are not met, the breaking limit hydrogen concentration is as low as 0.48 ppm, and the hydrogen embrittlement resistance is inferior.

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

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

The round bar of test number 1-14 is old because the chemical composition of the steel F used is within the range specified in the present invention, but the austenitizing temperature is 1100 ° C., which deviates from the conditions specified in the present invention. The austenite particle size became large, and cracks occurred when the drawing process was performed.

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%, which deviates from the conditions specified in the present invention. Therefore, the tensile strength was as low as 1435 MPa, which was less than 1500 MPa.

The round bar of test number 1-16 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%, which deviates from the conditions specified in the present invention. Therefore, the breaking 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 performing the process specified in the present invention, but the Mn content of the steel L used was as high as 1.22%, which deviates from the conditions specified in the present invention. Therefore, it breaks in a constant load test under cathode charge and is inferior in hydrogen embrittlement resistance.

The round bar of test number 1-18 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 is inferior in hardenability, the tensile strength is as low as 1427 MPa, which is less than 1500 MPa.

The round bar of test number 1-19 was manufactured by performing the steps 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, respectively. Since it is as high as 040% and deviates 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 round bar of Test No. 1-20 has a low Si content of 0.19% and does not contain Cr in the steel O used, which deviates from the chemical composition conditions specified in the present invention, and is further manufactured. The temperature and holding time of the isothermal transformation treatment as conditions are 450 ° C. and 4 minutes, respectively, which are different from the conditions of the isothermal transformation step of (ii) specified in the present invention. Therefore, although the total surface reduction rate of 45.4% can be secured without breaking, the breaking limit hydrogen concentration is as low as 0.32 ppm, and the hydrogen embrittlement resistance is inferior.

(Example 2) When the cold working process is "plate 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 rest of the round bar having a diameter of 25 mm cooled to room temperature described above (Example 1). Next, a plurality of passes of plate rolling until the total reduction ratio shown in Table 3 is 6.7 to 50.0% (thickness 2.8 to 1.5 mm) without subjecting the above plate material to softening heat treatment. Was given. Lubrication during plate rolling was performed with a wet lubricating oil. As the "tensile strength before cold plate rolling" in Table 3, the "tensile strength before cold drawing" in Table 2 obtained in (Example 1) was used as it was.

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

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

<2-1> Mechanical characteristics:
For each steel, for a plate that has been subjected to plate rolling, the width of the parallel portion shown in FIG. 2 is 5 mm, the gauge distance is 20 mm, and the thickness is the same as after plate rolling in the longitudinal direction from the center of the plate width. 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:
A notch having a shape shown in FIG. 3 in the longitudinal direction from the center of the plate width and thickness of each plate that has been subjected to plate rolling processing for which a tensile strength of 1500 MPa or more was obtained in the above investigation of <2-1>. A tensile test piece was cut out, and the hydrogen embrittlement resistance was investigated by the same method as in (Example 1). Specifically, first, the presence or absence of breakage was investigated when a constant load test in which a stress of 900 MPa was applied was carried out for 200 hours under the condition that the 3% NaCl solution was ^{charged with a cathode at a current density of 1 mA / cm 2.}

Next, with respect to the test piece that did not break, the parallel portion 8 mm shown in FIG. 3 was cut out with a low temperature cutting machine, and the hydrogen concentration released to 500 ° C. when the temperature was raised at 10 ° C./min by a temperature raising desorption device was determined. The hydrogen concentration was measured and regarded as the "breaking limit hydrogen concentration".

In the above constant load test, it was determined that the product did not break and had good hydrogen embrittlement resistance when the breaking limit hydrogen concentration was 0.50 ppm or more.

Table 3 summarizes the results of each of the above surveys.

From Table 3, a book produced by subjecting a plate material cut out from a round bar having a diameter of 25 mm cooled to room temperature in which the chemical composition is within the range specified in the present invention (Example 1) to the steps specified in the present invention. The plates of Test Nos. 2-1 to 2-10 of the invention example did not crack when the plate was rolled and had a high tensile strength of 1500 MPa or more, but under a cathode charge. No fracture occurred in the constant load test, and the fracture limit hydrogen concentration at that time was 0.61 ppm or more, which exceeded 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 of test numbers 2-11 to 2-22 of the comparative example, ear cracks occur when the plate is rolled, the tensile strength is 1500 MPa or more, and good hydrogen embrittlement resistance. It is not possible to secure important characteristics such as chemical characteristics at the same time.

The plate of test number 2-11 has a chemical composition and the heat treatment steps from (i) to (iii) are within the range specified in the present invention, but the total reduction ratio is 6.7%, and the present invention. In addition to the low tensile strength of 1485 MPa, the breaking limit hydrogen concentration is as low as 0.42 ppm, and the hydrogen embrittlement resistance is also inferior.

The plate of test number 2-12 has a chemical composition and the heat treatment steps from (i) to (iii) are within the range specified in the present invention, but the total reduction ratio is 50.0%, and the present invention. Since it deviates from the conditions specified in the above, it breaks in a constant load test under a cathode charge and is inferior in hydrogen embrittlement resistance.

Although the chemical composition of the steel A used in the plate of test number 2-13 is within the range specified in the present invention, the isothermal transformation treatment temperature is 250 ° C., which deviates from the conditions specified in the present invention. Ear cracks occurred when the plate was rolled with a rolling reduction of 16.7%.

Although the chemical composition of the steel A used in the plate of test number 2-14 is within the range specified in the present invention, the isothermal transformation treatment temperature is 250 ° C., so that the total reduction rate is the condition specified in the present invention. At 6.7%, which is lower than that, a tensile strength exceeding 1500 MPa was obtained without causing ear cracking, but the breaking limit hydrogen concentration was as low as 0.38 ppm, and the hydrogen embrittlement resistance was inferior.

For the plate of test number 2-15, although the chemical composition of the steel C used and the plate rolling process are within the range specified in the present invention, the cooling rate after austenitization deviates from the conditions specified in the present invention. The tensile strength was as low as 1457 MPa, which was less than 1500 MPa.

In the plate of Test No. 2-16, although the chemical composition of the steel F used was within the range specified in the present invention, the austeniticization temperature was 1100 ° C., which deviated from the conditions specified in the present invention. It has a large particle size, breaks in a constant load test under a cathode charge, and is inferior in hydrogen embrittlement resistance.

The plate of test number 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 deviates from the conditions specified in the present invention. The breaking 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%, which deviates from the conditions specified in the present invention. , The breaking 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 high as 1.22%, which deviates from the conditions specified in the present invention. , It broke in a constant load test under cathode charge and was 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 is inferior in hardenability, the tensile strength is as low as 1418 MPa, which is less than 1500 MPa.

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

The plate of Test No. 2-22 has a low Si content of 0.19% and does not contain Cr in the steel O used, which deviates from the chemical composition conditions specified in the present invention, and further, the production conditions. The temperature and holding time of the isothermal transformation treatment are 450 ° C. and 4 minutes, respectively, which are different from the conditions of the isothermal transformation step of (ii) specified in the present invention. Therefore, even if the plate was rolled with a total reduction ratio of 33.3% satisfying the provisions of the present invention, the tensile strength was as low as 1479 MPa, which was less than 1500 MPa.

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

Claims (3)

A method for producing high-strength, low-alloy steel materials.
The chemical composition is by 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: 0.5-1.5%,
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: 0-0.20%,
The rest is Fe and impurities,
P, S, N and O as impurities are P: 0.030% or less, S: 0.030% or less,
For steel materials with N: 0.030% or less and O: 0.010% or less,
The following steps (i) to (iv) are performed in order.
A method for producing a high-strength low-alloy steel material having a tensile strength of 1500 MPa or more.
(I): Austenizing step of heating at 850 to 150 ° C. for 20 to 60 minutes (ii): Cooling to a temperature range of 400 to 300 ° C. at a cooling rate of 30 ° C./sec or higher, and 10 to 100 in that temperature range. Isothermal transformation step (iii): cooling to room temperature, cooling step (iv): drawing with a total surface reduction rate of 10.0 to 20.0%, or a total rolling reduction rate of 10.0 to 40 Cold working process that performs plate rolling at 0.0% The chemical composition of the steel material is mass%,
Mo: containing less than 0.30% 0.05% or more, of claim 1, the tensile strength of the manufacturing method of the above high-strength low-alloy steel 1500 MPa. 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%,
The method for producing a high-strength low-alloy steel material having a tensile strength of 1500 MPa or more , according to claim 1 or 2 , which contains one or more selected from the above.

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