JP2009287072A - Steel sheet for low temperature tempering - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for hardening/tempering having excellent workability. <P>SOLUTION: A steel sheet for low temperature tempering has a chemical composition including, by mass, 0.30 to 0.47% C, ≤0.30% Si, 0.30 to 1.00% Mn, 0.0010 to 0.02% S, 0.002 to 0.030% Ti, ≤0.050% Al and ≤0.0070% N, wherein the contents of P and B respectively satisfy equations (1) to (4), and the balance Fe with inevitable impurities, and the steel sheet is subjected to tempering at ≤400°C after hardening: (1) P≤0.2×(31/11)×(B*)<SP>0.5</SP>; (2)0.0005≤B*≤0.0050; (3) B*=max[B-(11/14)×N*, 0]; and (4) N*-max[N-(14/48)×Ti, 0]; wherein, the P, N and B in the respective equations denote the contents (units: mass%) of the respective elements, and the max[ ] denote the functions returning the maximum value of the arguments in the [ ]. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steel sheet that is subjected to quenching and tempering. More specifically, the present invention relates to a steel plate for quenching and tempering, which has excellent workability before quenching and can obtain a steel plate member having high strength and excellent toughness after quenching and tempering.

  Chains used in automobiles, motorcycles, bicycles, and other industrial machines and various industrial machines are punched from steel plates that are used as materials, heat-treated by quenching and tempering and austempering, and conditioned to a hardness of 43 to 47 with HRC. It is manufactured. And as such a steel plate, the pickled steel plate and cold-rolled steel plate which pickled the hot-rolled steel plate are used.

  Here, the tempering is performed in a temperature range slightly exceeding 400 ° C. in order to avoid a temperature range in which temper embrittlement occurs. In order to ensure the hardness after tempering in such a high temperature range, it is necessary to use a steel type containing 0.50% by mass or more of C. For this reason, steel types, such as S50C-S65C steel, are applied to the steel plate for chains.

  However, since S50C to S65C steel has a high C content, the steel plate before quenching has high strength and poor workability, and the steel plate member after quenching and tempering has poor toughness. Moreover, since tempering must be performed in a high temperature range as mentioned above, manufacturing cost increases.

  On the other hand, Patent Document 1 and Patent Document 2 ensure the toughness equivalent to that of the conventional material by reducing the C content as compared with the conventional material, and the hardenability by reducing the C content. A steel plate for a chain that compensates for the decrease in the amount by adding B is disclosed.

On the other hand, Patent Document 3, Patent Document 4, Patent Document 5 and Patent Document 6 also disclose a steel sheet for quenching in which B is added.
JP 05-98356 A JP 05-98357 A JP 11-256268 A Japanese Patent Laid-Open No. 11-256272 JP 2000-248330 A JP 2000-265240 A

  The steel sheets described in the above publications are all used as-quenched and are not subjected to tempering heat treatment, so that it is difficult to obtain stable characteristics after heat treatment.

  The present invention has been made in view of the above situation, and it is possible to obtain a steel plate member having excellent workability before quenching and stably having high strength and excellent toughness after quenching and tempering. It is an object of the present invention to provide a steel sheet for quenching and tempering that has excellent workability.

  In order to solve the above-mentioned problems, the inventor conducted a detailed investigation on the influence of the chemical composition and tempering condition of the steel sheet on the workability of the steel sheet before quenching and the strength and toughness of the steel sheet member after quenching and tempering.

  As a result, the present inventors have newly found a chemical composition that can ensure good toughness even in a low temperature range of 400 ° C. or lower, which has been avoided from the viewpoint of ensuring brittleness in the prior art, and that can be tempered in that temperature range. As a result, it has been newly found that excellent workability can be ensured before quenching and high strength and excellent toughness can be ensured after quenching and tempering.

That is, the present invention is based on the following new findings.
(1) With a material having a high C content such as S50C to S65C steel that has been used conventionally, excellent workability is ensured before quenching, and excellent toughness is secured in a steel plate member after quenching and tempering. It is not possible.

  (2) Although it is necessary to reduce the C content in order to ensure these characteristics, it is possible to reduce the C content on the extension line of the conventional technical idea, or even if it is possible, it is excellent before quenching Workability could not be ensured.

(3) The reason is as follows.
1 shows that Si: 0.21%, Mn: 0.61%, P: 0.012%, S: 0.007%, Cr: 0.15%, Ti: 0.001%, sol. A hot-rolled steel sheet having a chemical composition in which only C content is changed using Al: 0.034% and N: 0.0040% as the basic composition is subjected to oil quenching after holding at 870 ° C. for 20 minutes, and thereafter This shows the influence of the tempering temperature on the surface hardness of the steel plate after quenching and tempering when tempering is performed for 30 minutes at a temperature. As shown in the figure, the as-quenched hardness is determined by the C content, and the surface hardness decreases as the tempering temperature increases. From the figure, in order to ensure a hardness of 43 to 47 by HRC after tempering, at least the C content needs to be 0.30% or more. However, low temperature brittleness occurs in a temperature range of 400 ° C. or lower, and excellent toughness cannot be ensured after quenching and tempering. Therefore, it is necessary to set the tempering temperature to 400 ° C. or higher. Assuming temperature, the C content is 0.50% or more.

On the other hand, as a method for compensating for a decrease in hardenability due to a decrease in C content and a decrease in strength (hardness) of a steel sheet member after quenching and tempering, a method of containing an element such as Mn is conceivable. Therefore, it is necessary to contain a large amount of elements, which increases the strength of the steel sheet before quenching and deteriorates workability.
Therefore, the C content cannot be reduced on the extension line of the conventional technical idea, or even if it can, the excellent workability before quenching cannot be ensured.

  (4) By including B, the present inventor (a) can compensate for a decrease in hardenability associated with a reduction in C content, and (b) suppresses grain boundary segregation of P causing low temperature brittleness. Since tempering at low temperature is possible, the strength (hardness) reduction of the steel sheet member after quenching and tempering due to the reduction of the C content can be compensated for, and (c) the above effect can be obtained with a small amount of content. It was newly found out that the strength of the steel sheet before quenching can be reduced and the workability can be remarkably improved.

  (5) Since the hardenability improvement by B and the grain boundary segregation suppression of P are brought about by B in a solid solution state, to ensure a predetermined hardenability and enable tempering in a low temperature region of 400 ° C. or lower. While determining the lower limit of the B content according to the N content, it is necessary to determine the upper limit of the P content according to the B content in the solid solution state.

  FIG. 1-10 and no. 18-20 hot-rolled steel sheet is subjected to a quenching process in which oil quenching is performed after being held at 870 ° C. for 20 minutes, and further tempering process is performed in a temperature range of 250-400 ° C. for 30 minutes so as to be about 45 at HRC. It shows the influence of the chemical composition on the Charpy impact value after applying.

As shown in the figure, it is necessary to obtain a chemical composition that satisfies the following formulas (1) to (4) in order to reliably obtain the effect of improving the hardenability by B and suppressing the grain boundary segregation of P. .
P ≦ 0.2 × (31/11) × (B ^* ) ^0.5 (1)
0.0005 ≦ B ^* ≦ 0.0050 (2)
B ^* = max [B− (11/14) × N ^* , 0] (3)
N ^* = max [N− (14/48) × Ti, 0] (4)
Here, P, N, and B in each formula represent the content (unit: mass%) of each element, and max [] is a function that returns the maximum value of arguments in [].

The present invention completed based on the above knowledge is as follows.
(1) By mass%, C: 0.30 to 0.47%, Si: 0.30% or less, Mn: 0.30 to 1.00%, S: 0.0010 to 0.02%, Ti: 0.002 to 0.030%, Al: 0.050% or less and N: 0.0070% or less, and the contents of P and B satisfy the following formulas (1) to (4), and the balance A steel sheet for low-temperature tempering, which has a chemical composition comprising Fe and impurities, and is tempered at 400 ° C. or lower after quenching.

P ≦ 0.2 × (31/11) × (B ^* ) ^0.5 (1)
0.0005 ≦ B ^* ≦ 0.0050 (2)
B ^* = max [B− (11/14) × N ^* , 0] (3)
N ^* = max [N− (14/48) × Ti, 0] (4)
Here, P, N, and B in each formula represent the content (unit: mass%) of each element, and max [] is a function that returns the maximum value of arguments in [].

  (2) The low-temperature tempering steel sheet according to (1), wherein the chemical composition contains Cr: 0.50% or less in mass% instead of part of Fe.

  (3) The steel composition for low-temperature tempering as described in (1) or (2) above, wherein the chemical composition contains Cu: 0.15% or less in mass% instead of part of Fe. .

  (4) The chemical composition is selected from the group consisting of Ni: 0.15% or less, Mo: 0.30% or less, and Nb: 0.030% or less in mass% instead of part of Fe. The steel sheet for low-temperature tempering according to any one of the above (1) to (3), comprising seeds or two or more kinds.

  According to the present invention, a steel plate for quenching and tempering having excellent workability, which has excellent workability before quenching and can obtain a steel plate member having high strength and excellent toughness after quenching and tempering. And a manufacturing method thereof.

  Below, the best form of the steel plate which concerns on this invention, the range of manufacturing conditions, and these setting reasons are demonstrated. In the present specification, “%” representing the chemical composition of steel means mass% unless otherwise specified.

1. Chemical composition (1) C: 0.30 to 0.47%
C is an important element that determines the strength (hardness) of the steel sheet member after quenching and tempering and has a great influence on the workability (strength) of the steel sheet before quenching. When the C content is less than 0.30%, it is difficult to sufficiently increase the strength of the steel plate member after quenching and tempering. For example, when the steel plate member is a chain, it is difficult to make the hardness of the chain core part 43 to 47 by HRC. On the other hand, if the C content exceeds 0.47%, the steel plate before quenching has high strength and poor workability, and the steel plate member after quenching and tempering has poor toughness. Therefore, the C content is set to 0.30% or more and 0.47% or less.

(2) Si: 0.30% or less Si is an element contained in steel as an impurity. However, since Si is an effective element as a deoxidizer, it may be added as necessary. On the other hand, Si is a solid solution strengthening element, and if it is contained in excess, the steel sheet before quenching has high strength and is inferior in workability. Therefore, the Si content is set to 0.30% or less.

(3) Mn: 0.30 to 1.00%
Mn is an element that is effective as a deoxidizer, and is also an element that is effective for increasing the quenching factor during quenching and increasing the depth of curing. For this reason, Mn content shall be 0.30% or more. On the other hand, when it contains excessively, the strength of the steel plate before quenching becomes high and the workability deteriorates. For this reason, Mn content shall be 1.00% or less.

(4) P: P ≦ 0.2 × (31/11) × (B ^* ) ^0.5
P is an important element in the present invention.
P is an element contained in the steel as an impurity, and easily segregates at the austenite grain boundaries, thereby lowering the grain boundary strength and remarkably degrading toughness. Therefore, the smaller the P content, the better.

  However, in the present invention, B is contained as will be described later. Among these B, B in a solid solution state that does not form BN by combining with N is austenite grains in preference to P during quenching. It was found that segregated at the boundary and suppressed grain boundary segregation of P, and the effect was confirmed to be about 20 times that of P per atom. Therefore, the upper limit of the P content is relaxed according to the B content, and the P content satisfies the following formulas (1) to (4).

P ≦ 0.2 × (31/11) × (B ^* ) ^0.5 (1)
0.0005 ≦ B ^* ≦ 0.0050 (2)
B ^* = max [B− (11/14) × N ^* , 0] (3)
N ^* = max [N− (14/48) × Ti, 0] (4)
Here, P, N, and B in each formula represent the content (unit: mass%) of each element, and max [] is a function that returns the maximum value of arguments in [].

(5) S: 0.0010 to 0.02%
S is an element contained in steel as an impurity, and if the S content exceeds 0.02%, the toughness of the steel sheet member after quenching and tempering is deteriorated. On the other hand, S has the effect | action which improves the punching workability of the steel plate before hardening, and if S content is less than 0.0010%, the punching workability of the steel plate before hardening will deteriorate. Therefore, the S content is set to be 0.0010% or more and 0.02% or less. Preferably, it is 0.004% or more and 0.01% or less.

(6) B: 0.0005 ≦ B ^* ≦ 0.0050
B is the most important element in the present invention.
B is an element effective in increasing the quenching factor during quenching and quenching to the center of the plate thickness of the steel plate member. Further, it is an element that segregates at the austenite grain boundaries in preference to P during quenching and suppresses the grain boundary segregation of P causing low temperature brittleness. By utilizing such an action of B, not only compensates for a decrease in hardenability accompanying a reduction in C content, but also by low temperature tempering at 400 ° C. or lower, which has been conventionally avoided to avoid low temperature brittleness. In addition, it is possible to ensure good toughness and to secure a high strength after tempering by applying such low temperature tempering while having a low C content. That is, by enabling the reduction of the C content and the application of low temperature tempering, a steel plate member having excellent workability before quenching and having high strength and excellent toughness after quenching and tempering is obtained. Is possible.

These actions by B are brought about by effective B in a solid solution state (hereinafter also referred to as “B ^* ”), and B which forms BN by combining with N improves the hardenability and improves the P content. Does not contribute to suppression of grain boundary segregation. Therefore, in order to surely obtain the effect of improving the hardenability by B and the effect of suppressing the grain boundary segregation of P, B ^* defined by the following formulas (3) and (4) is set to 0.0005 or more. Containing. On the other hand, when the B content is excessive, cracks at the slab stage and narrowing during hot rolling are likely to occur, which causes problems such as difficulty in production. For this reason, B is contained so that the B ^* is 0.0050 or less.

B ^* = max [B− (11/14) × N ^* , 0] (3)
N ^* = max [N− (14/48) × Ti, 0] (4)
Here, N and B in each formula represent the content (unit: mass%) of each element, and max [] is a function that returns the maximum value of the arguments in [].

(7) Al: 0.050% or less Since Al is an element effective as a deoxidizer, it may be added as necessary. On the other hand, when it contains excessively, it will become easy to produce a surface defect, the intensity | strength of the steel plate before quenching will become high, and workability will deteriorate. Therefore, the Al content is set to 0.050% or less. For the purpose of deoxidation with Al, the Al content is preferably 0.005% or more. When deoxidation is performed only with Si, Al need not be added.

(8) N: 0.0070% or less N is combined with B to form BN as described above, and reduces the effective B in a solid solution state. Therefore, the smaller the N content, the better. Preferably it is 0.0050% or less.

(9) Ti: 0.002 to 0.030%
Ti binds to N at a higher temperature than B and has the effect of fixing N as TiN. Therefore, the amount of B consumed by combining with N is reduced and effective for ensuring effective B. Element. Therefore, the Ti content is set to 0.002% or more. Preferably it is 0.005% or more. However, when it contains excessively, excessive Ti will couple | bond C with N, TiC will be formed, the intensity | strength of the steel plate before quenching will become high, and workability will deteriorate. Further, the formation of carbonitride causes deterioration of toughness and hardenability. Therefore, the Ti content is 0.030% or less. Preferably it is 0.025% or less.

(10) Cr: 0.50% or less Cr, as well as Mn, is an effective element that increases the quenching factor at the time of quenching and increases the depth of curing, and may be added as necessary. On the other hand, when it contains excessively, the strength of the steel plate before quenching becomes high and the workability deteriorates. In addition, the cost increases. For this reason, Cr content shall be 0.50% or less. In order to obtain the above effect more reliably, the Cr content is preferably 0.05% or more.

(11) Cu: 0.15% or less Since Cu has an action of suppressing peroxidation during pickling and stabilizing the surface state after pickling, it may be added as necessary. On the other hand, when it contains excessively, the strength of the steel plate before quenching becomes high and the workability deteriorates. In addition, the cost increases. For this reason, Cu content is made into 0.15% or less. Preferably it is 0.12% or less. In order to acquire the said effect more reliably, it is preferable to make Cu content 0.05% or more, and it is further more preferable to set it as 0.08% or more.

(12) Ni: 0.15% or less Since Ni is an element effective for improving toughness, it may be added as necessary. On the other hand, Ni is an expensive element, and excessive addition causes a significant increase in cost. Therefore, the Ni content is 0.15% or less. Preferably it is 0.12% or less. In order to acquire the said effect more reliably, it is preferable to make Ni content into 0.04% or more, and it is further more preferable to set it as 0.06% or more.

(13) Mo: 0.30% or less Since Mo is also an element effective for improving toughness, it may be added as necessary. On the other hand, when it contains excessively, the strength of the steel plate before quenching becomes high and the workability deteriorates. In addition, the cost increases. Therefore, the Mo content is set to 0.30% or less. Preferably it is 0.28% or less. In order to acquire the said effect more reliably, it is preferable to make Mo content into 0.02% or more, and it is more preferable to set it as 0.05% or more.

(14) Nb: 0.030% or less Nb is an effective element that has the effect of refining austenite crystal grains and improving toughness during quenching, and may be added as necessary. On the other hand, when it contains excessively, a carbide | carbonized_material will be formed and the intensity | strength of the steel plate before quenching will become high, workability will deteriorate, or the hardenability at the time of quenching may be reduced. Therefore, the Nb content is 0.030% or less. Preferably it is 0.025% or less. In order to acquire the said effect more reliably, it is preferable to make Nb content into 0.005% or more, and it is more preferable to set it as 0.010% or more.

2. Applications The steel sheet according to the present invention not only compensates for the decrease in hardenability accompanying the reduction of the C content by containing B, but also suppresses the grain boundary segregation of P causing low temperature brittleness at low temperatures. It enables tempering, thereby compensating for a decrease in the strength (hardness) of the steel sheet member after quenching and tempering due to a reduction in the C content, thereby having excellent workability before quenching and after quenching and tempering. Makes it possible to obtain a steel plate member having high strength and excellent toughness.

  Therefore, the steel sheet according to the present invention is used for an application in which tempering is performed after quenching, and the tempering is low-temperature tempering at 400 ° C. or lower. This is because if the tempering temperature is higher than 400 ° C. as in the prior art, it is difficult to ensure the desired strength after tempering. In order to obtain the original tempering effect, the lower limit of the tempering temperature is preferably 250 ° C. or higher. The tempering time may be appropriately determined according to the tempering temperature, and may be, for example, 15 to 60 minutes.

3. Manufacturing method Since the steel sheet according to the present invention has the above-described chemical composition, it has excellent workability before quenching and high strength and excellent after quenching and tempering by performing a hot rolling process according to a conventional method. It is possible to obtain a steel plate member having high toughness. Therefore, the production method is not particularly limited, but a preferred production method of the steel plate for quenching and tempering according to the present invention is exemplified below.

(1) Hot rolling process i) Hot rolling completion temperature: 850-910 ° C
When the hot rolling completion temperature is higher than 910 ° C., the scale thickness becomes too thick, so that the pickling efficiency and the yield may decrease, or the surface quality may deteriorate. On the other hand, when the hot rolling completion temperature is less than 850 ° C., the deformation resistance of the steel ingot or steel slab becomes large, and it may be difficult to perform the hot rolling itself. Therefore, the hot rolling completion temperature is preferably 850 to 910 ° C.

ii) Winding temperature: 550-660 ° C
If the coiling temperature is too low, the steel sheet becomes high in strength, and when the hot-rolled steel sheet is subjected to quenching, the workability of the steel sheet before quenching deteriorates, so the coiling temperature is preferably 550 ° C or higher. . On the other hand, if the coiling temperature is too high, the scale thickness becomes too thick, and the pickling efficiency and yield may decrease, or the surface quality may deteriorate. It is preferable.

(2) Other steps The steel sheet according to the present invention may be a hot-rolled steel sheet or a cold-rolled steel sheet.
In the case of a hot-rolled steel sheet, it may be a hot-rolled steel sheet, or a steel sheet that has been softened by hot-rolled sheet annealing. When hot-rolled sheet annealing is performed, it is preferable that the annealing temperature is 650 ° C. or more and 760 ° C. or less, and the annealing time is 0.1 hours or more and 30 hours or less. Usually, after pickling treatment and processing into a steel plate member, quenching and tempering are performed.

  In the case of a cold-rolled steel sheet, it may be a cold-rolled steel sheet or a steel sheet that has been softened by annealing. Here, the hot-rolled steel sheet used for cold rolling may be a steel sheet as hot-rolled as described above, or may be a steel sheet softened by hot-rolled sheet annealing. Examples of the cold pressure ratio of cold rolling include 20 to 70%. When annealing is performed, it is preferable that the annealing temperature is 650 ° C. or more and 760 ° C. or less, and the annealing time is 0.1 hours or more and 30 hours or less. Cold rolling and annealing may be repeated a plurality of times.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.
A slab having the chemical composition shown in Table 1 was heated to 1250 ° C. and hot-rolled under conditions of a finishing temperature of 870 ° C. and a winding temperature of 620 ° C. to obtain a 2.6 mm thick hot rolled steel sheet. The obtained hot-rolled steel sheet was pickled and subjected to each test.

  As an evaluation method, first, for hot-rolled steel sheets, YS, TS, and El are measured by a tensile test to evaluate mechanical properties, and a punching test is performed by punching to 30 mmφ under a clearance condition of 15% of the plate thickness. The punchability was evaluated by measuring the thickness. The tensile test was performed based on JIS Z 2241.

  Next, after quenching the hot-rolled steel sheet at 870 ° C. for 20 minutes, a quenching treatment is performed in which oil quenching is performed, and further tempering is performed in a temperature range of 250 to 400 ° C. for 30 minutes so that the surface hardness is about 45 in HRC. Treated. However, no. For No. 11, the hardness after quenching was 30.5 in HRC, so the treatment and test after tempering were omitted.

  The steel sheet thus quenched and tempered was measured for surface hardness and evaluated for toughness by a Charpy test. However, the TS of the steel plate before quenching is over 630 MPa and the workability is inferior. Nos. 15, 17 and 23, in which the burr height in the punching test of the steel sheet before quenching is less than 5 μm and the workability is poor. No. 21, which was insufficient in hardness after quenching due to insufficient hardenability. For 16, 24 and 25, the Charpy test was omitted.

  The Charpy test was performed based on JIS Z 2241 by collecting test pieces in the rolling direction and confirming the hardness, and then adjusting the plate thickness to 2.5 mm by electrolytic polishing so as not to give strain to the surface.

  The results are shown in Table 2.

As shown in Table 2, the steel sheet according to the present invention is excellent in workability because TS is 630 MPa or less and the burrs height is 5 μm or less before quenching. Further, after quenching and tempering, when the HRC is about 45, it is 34 J / cm ² or more, and is excellent in toughness. When the hardness after quenching and tempering is the same, the steel sheet according to the present invention is remarkably superior in toughness as compared with the conventional materials S50C and S60C, which means that some toughness deterioration with increasing strength is expected. The steel sheet according to the present invention has high strength and excellent strength after quenching and tempering because it is possible to obtain higher strength than the conventional material while maintaining the toughness superior to that of the conventional material. It can be said that a steel plate member having toughness can be obtained.

  According to the present invention, a steel plate for quenching and tempering having excellent workability, which has excellent workability before quenching and can obtain a steel plate member having high strength and excellent toughness after quenching and tempering. Is obtained. The steel plate is suitable as a material steel plate for chains used in vehicles such as automobiles, motorcycles and bicycles, and various industrial machines.

It is a graph which shows the influence of C content and tempering temperature which it has on the surface hardness of the steel plate after quenching and tempering. It is a graph which shows the influence of the chemical composition which gives to the Charpy impact value.

Claims (4)

In mass%, C: 0.30 to 0.47%, Si: 0.30% or less, Mn: 0.30 to 1.00%, S: 0.0010 to 0.02%, Ti: 0.002 ~ 0.030%, Al: 0.050% or less and N: 0.0070% or less, and the contents of P and B satisfy the following formulas (1) to (4), with the balance being Fe and A steel plate for low-temperature tempering, which has a chemical composition comprising impurities and is tempered at 400 ° C. or lower after quenching.
P ≦ 0.2 × (31/11) × (B ^* ) ^0.5 (1)
0.0005 ≦ B ^* ≦ 0.0050 (2)
B ^* = max [B− (11/14) × N ^* , 0] (3)
N ^* = max [N− (14/48) × Ti, 0] (4)
Here, P, N, and B in each formula represent the content (unit: mass%) of each element, and max [] is a function that returns the maximum value of arguments in [].   The steel composition for low-temperature tempering according to claim 1, wherein the chemical composition contains Cr: 0.50% or less in mass% instead of part of Fe.   The steel composition for low-temperature tempering according to claim 1 or 2, wherein the chemical composition contains Cu: 0.15% or less in mass% instead of part of Fe.   The chemical composition is one or two selected from the group consisting of Ni: 0.15% or less, Mo: 0.30% or less, and Nb: 0.030% or less in mass% instead of part of Fe. The steel plate for low-temperature tempering according to any one of claims 1 to 3, comprising a seed or more.

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