EP3674426A1

EP3674426A1 - Method for production of ni-containing steel sheet

Info

Publication number: EP3674426A1
Application number: EP18848632.8A
Authority: EP
Inventors: Akito TABATA
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2017-08-25
Filing date: 2018-08-13
Publication date: 2020-07-01
Anticipated expiration: 2038-08-13
Also published as: EP3674426B1; KR20200033291A; WO2019039339A1; EP3674426A4; KR102351770B1

Abstract

The method for production according to the embodiment of the present invention includes the steps of, in sequence: quenching a steel including a predetermined chemical composition, with the balance being Fe and inevitable impurities, from a quenching temperature of 800°C or higher and 820°C or lower after hot-rolling the steel; applying an intermediate heat treatment to the quenched steel by holding the steel at a heating temperature of 690°C or higher and 710°C or lower and then cooling the steel at an average cooling rate of 5°C/sec or more until a cooling end temperature of 200°C or lower; and tempering the steel at a tempering temperature of 570°C or higher and 600°C or lower, wherein a parameter H defined by a heating and holding time during the intermediate heat treatment, and a heating and holding time during the tempering is set at 1.73 × 10<sup>-6</sup>or more and 1.96 × 10<sup>-6</sup>or less.

Description

Technical Field

The present disclosure relates to a method for production of a Ni-containing steel plate.

Background Art

Global environmental deterioration, together with an increase in the global energy demand, has recently become an issue. Thus, the demand for natural gas (LNG) as a clean energy source is rapidly increasing. Following an increase in the demand for the natural gas (LNG), constructing storage tanks for LNG has been actively promoted domestically and overseas these days. Under such circumstances, there is an increasing demand for high-Ni steel plates (hereinafter sometimes simply referred to as "steel plates") with excellent low-temperature toughness, which are usable in the main bodies of the storage tanks for LNG.
The high-Ni steel plates are relatively inexpensive and are known to exhibit excellent low-temperature toughness by having the functions and effects, such as improvement of the toughness of a matrix of the steel by addition of Ni, the refinement of the microstructure of the steel by a heat treatment, and improvement of the toughness of the steel due to the presence of stable residual austenite (hereinafter sometimes referred to as "residual γ") even under an ultralow temperature. Among the high-Ni steel plates, a steel plate (9% Ni steel) having a Ni content of about 9% by mass has been put into practical use in many cases as a material for tanks since it was used in a storage tank for LNG in 1963. In addition, the amount of usage of the high-Ni steel plate is also expected to increase in the future.
As mentioned above, in the high-Ni steel plate, its low-temperature toughness is significantly improved by the presence of the residual γ. However, when the steel plate is processed to be subjected to a large plastic strain, the residual γ may experience a process-induced transformation to become martensite. When the process-induced transformation occurs, the amount of residual γ may decrease, deteriorating the low-temperature toughness of the steel.
In such a situation, various technologies have been investigated which do not deteriorate the low-temperature toughness even when a large plastic strain is applied to a high-Ni steel plate.
For example, Patent Document 1 discloses a method for production of a steel plate of a Ni-containing steel that has a sufficient low-temperature toughness even though the steel plate is an extremely thick member with a thickness exceeding 40 mm. In Patent Document 1, the Ni-containing steel as a raw material contains C, Ni and Mn in a predetermined content range, and each of the contents of P and S in the impurities is restricted to an extremely low level of 0.001 % by weight (mass) or less. Then, this steel is subjected to hot-rolling, followed by quenching twice and tempering under specific conditions. Thus, the steel has improved low-temperature toughness.
In Patent Document 1, FIG. 2 shows the results of examining the Charpy impact energy in a rolling direction (L direction) at -196°C and in a direction (C direction) orthogonal to the rolling direction for steel plates which have already been subjected to the above-mentioned heat treatment and then tensile pre-strain of 5%, followed by an aging treatment at 250°C for one hour. Patent Document 1 describes that as shown in FIG. 2, the low-temperature toughnesses of the steel plate itself and a welding joint are drastically improved by setting the P content to 0.001% by weight (mass) or less.

Prior Art Document

Patent Document

Patent Document 1: JP 6-179909 A

Disclosure of the Invention

Problems to be Solved by the Invention

The upper limit of P content needs to be restricted to 0.001% by mass as mentioned in Patent Document 1. However, when the cleanliness of a steel plate is enhanced in order to restrict the upper limit of P content to 0.001% by mass, there occurs a problem of deterioration in the productivity of steel plates.
An embodiment of the present invention has been made in light of such circumstances and has an object to provide a method for production of a Ni-containing steel plate that has excellent low-temperature toughness after a plastic strain is applied thereto even when a P content exceeds 0.001% by mass.

Means for Solving the Problems

A first aspect of an embodiment of the present invention is directed to a method for production of a Ni-containing steel plate, wherein a steel includes:

C: 0.040% by mass or more and 0.060% by mass or less;
Si: 0.10% by mass or more and 0.30% by mass or less;
Mn: 0.50% by mass or more and 0.70% by mass or less;
P: 0.0010% by mass or more and 0.0025% by mass or less;
S: 0.0010% by mass or less;
Ni: 9.10% by mass or more and 9.40% by mass or less;
Al: 0.020% by mass or more and 0.050% by mass or less; and
N: 0.0050% by mass or less, with the balance being Fe and inevitable impurities, the method including the steps of, in sequence:
- quenching the steel from a quenching temperature of 800°C or higher and 820°C or lower after hot-rolling the steel;
- applying an intermediate heat treatment to the quenched steel by holding the steel at a heating temperature of 690°C or higher and 710°C or lower and then cooling the steel at an average cooling rate of 5°C/sec or more until a cooling end temperature of 200°C or lower; and
- tempering the steel at a tempering temperature of 570°C or higher and 600°C or lower,
- wherein in the intermediate heat treatment step and the tempering step, a parameter H represented by formula (1) below is set at 1.73 × 10^-6 or more and 1.96 × 10^-6 or less: $H = \{{(D_{Ni, L} \times t_{L})}^{0.5} + {(D_{Ni, T} \times t_{T})}^{0.5}\} \times [Ni] + \{{(D_{C, L} \times t_{L})}^{0.5} + {(D_{C, T} \times t_{T})}^{0.5}\} \times [C]$
  where
  - t_L (seconds) is a heating and holding time in the intermediate heat treatment step,
  - t_T (seconds) is a heating and holding time in the tempering step,
  - [Ni] (% by mass) is a Ni content,
  - [C] (% by mass) is a C content, $D_{Ni, L} = 1.4 \times 10^{- 4} \times \exp (- 29.58 \times 1,000 / T_{L})$
    $D_{Ni, T} = 1.4 \times 10^{- 4} \times \exp (- 29.58 \times 1,000 / T_{T})$
    $D_{C, L} = 0.45 \times 10^{- 4} \times \exp (- 18.54 \times 1,000 / T_{L})$
    $D_{C, T} = 0.45 \times 10^{- 4} \times \exp (- 18.54 \times 1,000 / T_{T})$
    where
    - T_L (K) is a heating temperature in the intermediate heat treatment step, and
    - T_T (K) is a tempering temperature.

A second aspect of the embodiment of the present invention is directed to the method for production according to the first aspect, wherein the steel satisfies at least one of the following (a) to (d):

(a) the C content is 0.045% by mass or more and 0.060% by mass or less;
(b) the Si content is 0.15% by mass or more and 0.30% by mass or less;
(c) the Mn content is 0.60% by mass or more and 0.70% by mass or less; and
(d) the Al content is 0.020% by mass or more and 0.045% by mass or less.

A third aspect of the embodiment of the present invention is directed to the method for production according to the first or second aspect, where the steel further includes one or more elements of Cu: 0.01% by mass or more and 0.20% by mass or less, Cr: 0.01% by mass or more and 0.20% by mass or less, Mo: 0.01% by mass or more and 0.20% by mass or less, V: 0.1% by mass or less, Nb: 0.1% by mass or less, Ti: 0.1% by mass or less, and B: 0.005% by mass or less.

Effects of the Invention

According to the embodiment of the present invention, it is possible to produce a Ni-containing steel plate that has excellent low-temperature toughness after the plastic strain is applied thereto even when the P content exceeds 0.001% by mass.

Brief Description of the Drawings

FIG. 1 is a diagram showing a relationship between the parameter H and the brittle fracture rate after the application of a plastic strain in an embodiment of the present invention.

Mode for Carrying Out the Invention

As a result of intensive studies conducted by the present inventors, it has been found that a steel plate excellent in the low-temperature toughness after the application of the plastic strain (hereinafter sometimes referred to as a "strain aging property") can be produced even when the P content exceeds 0.001% by mass, by controlling a parameter H specified by a heating temperature and a holding time during an intermediate heat treatment as well as a heating temperature and a holding time during a tempering treatment.
FIG. 1 is a diagram showing a relationship between the parameter H and the brittle fracture rate of a steel plate after the application of the plastic strain, as an index of the strain aging property. As shown in FIG. 1, the present inventors have found that by setting the parameter H to 1.73 × 10^-6 or more and 1.96 × 10^-6 or less, the brittle fracture rate of the steel plate after the application of the plastic strain can be set to 5% or less, thereby making it possible to produce the steel plate with excellent strain aging property.
The details of the mechanism for improving the strain aging property by controlling the parameter H even when the P content exceeds 0.001% by mass is not certain. However, the present inventors understand the mechanism at the moment as follows.
When the P content increases, P segregates more in austenite grain boundaries, which generally embrittles the grain boundaries. Thus, when the P content increases, the strain aging property of the steel plate could be deteriorated.
The diffusion of C and Ni into the metallographic structure of the steel plate is controlled in the embodiment of the present invention by controlling the parameter H (that is, by controlling the heating temperature and the holding time during the intermediate heat treatment and the heating temperature and the holding time during the tempering treatment). Specifically, when the parameter H increases, the diffusion of C and Ni is promoted during the intermediate heat treatment and the tempering treatment, whereas when the parameter H decreases, the diffusion of C and Ni is suppressed during the intermediate heat treatment and the tempering treatment. In the embodiment of the present invention, by controlling the parameter H within a predetermined range, the diffusion of C and Ni is controlled during the intermediate heat treatment and the tempering treatment, thereby consequently controlling the enrichment of C and Ni into the residual γ.
Here, the enrichment of C and Ni into the residual γ significantly contributes to the stability of the residual γ, in which the residual γ remains in the steel plate without experiencing the process-induced transformation even when the plastic strain is applied to the steel plate. In the embodiment of the present invention that controls and optimizes the enrichment of C and Ni into the residual γ, it is thought that the steel plate with excellent strain aging property can be produced by compensating for the deterioration of the strain aging property due to an increase in the P content.

1. Chemical Composition

The chemical composition of the steel plate produced by the embodiment of the present invention will be described below.
Basic elements, i.e., C, Si, Mn, P, S, Ni, Al, and N will be first described, and further elements which may be selectively added will be then described.

[C: 0.040% by mass or more and 0.060% by mass or less]

Carbon (C) is an element that increases the strength of a steel plate. To ensure a desired high strength of the steel plate, the C content needs to be 0.040% by mass or more. The C content exceeding 0.060% by mass leads to reduction in the low-temperature toughness. Thus, the C content is set at 0.040% by mass or more and 0.060% by mass or less. The lower limit of C content is preferably 0.045% by mass in order to more contribute to an increase in the strength of the steel plate.

[Si: 0.10% by mass or more and 0.30% by mass or less]

Silicon (Si) is an element that acts as a deoxidizer and improves the strength of steel. To obtain these effects, the Si content needs to be 0.10% by mass or more. If the Si content is extremely large, exceeds 0.30% by mass, the temper embrittlement susceptibility of the steel will be enhanced. Thus, the Si content is set at 0.10% by mass or more and 0.30% by mass or less. The lower limit of Si content is preferably 0.15% by mass in order to more contribute to an increase in the strength of steel.

[Mn: 0.50% by mass or more and 0.70% by mass or less]

Manganese (Mn) needs to be added in an amount of 0.50% by mass or more in order to contribute to an increase in the strength of steel. The addition of Mn in an amount exceeding 0.70% by mass leads to enhanced temper embrittlement susceptibility, thus reducing the toughness of steel. Thus, the Mn content is set at 0.50% by mass or more and 0.70% by mass or less. The lower limit of Mn content is preferably 0.60% by mass in order to more contribute to an increase in the strength of steel.

[P: 0.0010% by mass or more and 0.0025% by mass or less; S: 0.0010% by mass or less]

Phosphorus (P) and sulfur (S) both are elements that reduce the toughness of steel, and thus the contents of P and S are desirably lowered as much as possible. The P content is allowed to be 0.0025% by mass or less (not including 0% by mass), and the S content is allowed to be 0.0010% by mass or less (not including 0% by mass).
From an economic viewpoint, P is added in an amount of 0.0010% by mass or more and 0.0025% by mass or less. In consideration of the economical efficiency, the P content is more preferably 0.0015% by mass or more and 0.0025% by mass or less.

[Ni: 9.10% by mass or more and 9.40% by mass or less]

Nickel (Ni) is an essential element in the embodiment of the present invention and has the effect of imparting high toughness to a steel plate at low temperature. If the Ni content is less than 9.10% by mass, its effect becomes lower. If Ni is added in a large amount exceeding 9.40% by mass, its effect is saturated and also uneconomical. Thus, the Ni content is set at 9.10% by mass or more and 9.40% by mass or less.

[Al: 0.020% by mass or more and 0.050% by mass or less]

Aluminum (Al) needs to be added in an amount of 0.0020% by mass or more as a deoxidizer. However, if Al is added in an amount exceeding 0.050% by mass, the cleanliness of the steel is degraded. Thus, the Al content is set at 0.020% by mass or more and 0.050% by mass or less. To further enhance the cleanliness, the upper limit of Al content is preferably 0.045% by mass.

[N: 0.0050% by mass or less]

Nitrogen (N) reduces the toughness of steel in a solid-solution state and also has the effect of refining crystal grains in the form of AlN. Therefore, the N content is reduced as much as possible to the extent that it does not coarsen crystal grains. Thus, the N content is set at 0.0050% by mass or less (not including 0% by mass).

[Balance]

In a preferred embodiment, the balance includes iron and inevitable impurities. Inevitable impurities are trace elements (for example, As, Sb, Sn, and the like) brought into situations, including raw materials, source materials, manufacturing facilities, and the like, which are allowed to be mixed in the steel plate. For example, there are elements, such as P and S, whose content ranges are separately specified as mentioned above, despite being inevitable impurities which are preferably contained in a smaller amount in general. Because of this, as used herein, the term "inevitable impurities" constituting the balance is based on the concept that excludes the elements whose content ranges are specified separately.
However, the present invention is not limited to this embodiment. The steel plate produced by the production method according to the embodiment of the present invention may further contain any other elements as long as the certain properties of the steel plate can be maintained. The other elements selectively contained in such a manner can be exemplified as follows.

[One or more elements of Cu: 0.01% by mass or more and 0.20% by mass or less, Cr: 0.01% by mass or more and 0.20% by mass or less, Mo: 0.01% by mass or more and 0.20% by mass or less, V: 0.1% by mass or less, Nb: 0.1% by mass or less, Ti: 0.1% by mass or less, and B: 0.005% by mass or less]

Cu, Cr, Mo, V, Nb, Ti, and B are elements contributing to the improvement of the strength of steel. One or more of these elements may be selected as necessary to be contained in the steel. In order to contribute to the improvement of the strength of the steel, it is preferable to add Cu in an amount of 0.01% by mass or more, Cr in an amount of 0.01% by mass or more, and Mo in an amount of 0.01% by mass or more. On the other hand, in consideration of the fact that these elements could become a cause that reduces the toughness of a base material, it is preferable to restrictedly add Cu in an amount of 0.20% by mass or less, Cr in an amount of 0.20% by mass or less, Mo in an amount of 0.20% by mass or less, V in an amount of 0.1% by mass or less, Nb in an amount of 0.1% by mass or less, Ti in an amount of 0.1% by mass or less, and B in an amount of 0.005% by mass or less.

2. Manufacturing Method

The production method according to the embodiment of the present invention will be described below.
The following description of the production method will be given on mechanisms in which a desired metallographic structure can be obtained with various improved properties by such a production method. It should be noted that these mechanisms are considered by the inventors based on the findings presently available, but are not intended to limit the technical scope of the present invention.
The inventors of the present application have found that a steel plate with excellent strain aging property can be produced even when the P content exceeds 0.001% by mass, by quenching a rolled material having a predetermined chemical composition from a predetermined quenching temperature, and performing an intermediate heat treatment and tempering on the quenched material by strictly controlling the heating time and the holding time such that the parameter H to be mentioned later in detail falls within a predetermined range.
The details thereof will be described below.
First, preferably, a raw material for production of steel that satisfies the requirements for the above-mentioned chemical composition is molten by a conventional method in a usual blasting furnace such as a converter and is cast into a slab (raw material steel) by a continuous casting method. The obtained raw material steel is heated to a temperature that enables hot-rolling by a conventional method, and then subjected to the hot-rolling (AR: As-Roll) to obtain a steel plate having a desired thickness (for example, 32 mm).

[Quenching step (quenching temperature: 800°C or higher and 820°C or lower]

Subsequently, to obtain the uniform martensite microstructure, the steel plate is subjected to a quenching treatment by reheating to a quenching temperature of 800°C or higher and 820°C or lower and then quenching. The quenching is preferably performed at an average cooling rate of 5°C/sec or more to a cooling end temperature of 200°C or lower. The quenching is performed by, for example, water cooling or the like. For example, in the case of water cooling, the average cooling rate sufficiently becomes 5°C/sec or more until the cooling end temperature of 200°C or lower. When the quenching temperature exceeds 820°C, austenite grains are coarsened by recrystallization, and as a result, the low-temperature toughness of the steel plate may be deteriorated. In contrast, when the quenching temperature is lower than 800°C, the quenching becomes insufficient, which could deteriorate the strain aging property and make the strength of the steel plate insufficient.

[Intermediate heat treatment step (holding the steel plate at a heating temperature of 690°C or higher and 710°C or lower and then cooling it at an average cooling rate of 5°C/sec or more until the cooling end temperature of 200°C or lower]

Subsequently, the steel plate is reheated to a heating temperature (intermediate heat treatment temperature) to 690°C or higher and 710°C or lower which corresponds to a two-phase region where ferrite and austenite coexist. After the heating temperature is reached, the steel plate is held for a predetermined period of time and thereafter cooled. The cooling is performed at an average cooling rate of 5°C/sec or more to a cooling end temperature of 200°C or lower. The cooling is performed by, for example, water cooling or the like. For example, in the case of water cooling, the average cooling rate sufficiently becomes 5°C/sec or more until the cooling end temperature of 200°C or lower.
The uniform martensite microstructure obtained in the above-mentioned quenching step is transformed into a ferrite microstructure and an austenite microstructure when being heated to the heating temperature of the two-phase region. Through the heating and holding step, C and Ni are diffused into the austenite microstructure, resulting in the enrichment of C and Ni into the austenite microstructure. Thereafter, by quenching, the austenite microstructure is transformed into the martensite microstructure to form a mixed microstructure of a clean ferrite microstructure and a martensite microstructure containing the enrichment of C and Ni.
If the intermediate heat treatment temperature is lower than 690°C, the amount of austenite formed in the subsequent tempering step becomes insufficient, which leads to the deterioration of the strain aging property. On the other hand, the intermediate heat treatment temperature exceeding 710°C falls within a temperature range of a single-phase region, whereby no ferrite microstructure is formed there. Thus, an austenite microstructure containing the enrichment of C and Ni cannot be obtained. As a result, austenite is not formed in the sequent tempering step, leading to the deterioration of the strain aging property.
When the cooling end temperature exceeds 200°C or the average cooling rate is lower than 5°C/sec, no martensite microstructure is not obtained.

[Tempering step (tempering temperature: 570°C or higher and 600°C or lower)]

Subsequently, the steel plate is subjected to the tempering treatment by being reheated to a tempering temperature of 570°C or higher and 600°C or lower and held for a predetermined period of time after the tempering temperature is reached. A cooling method is not particularly limited and is preferably, for example, water cooling or the like.
When tempering the ferrite microstructure and the martensite microstructure containing the enrichment of C and Ni, which has been obtained by the above-mentioned intermediate heat treatment, part of the martensite microstructure is reversely transformed into the austenite microstructure. The reversely transformed austenite microstructure becomes the residual austenite. In more detail, the martensite microstructure obtained in the intermediate heat treatment step also includes a portion where C and Ni are densely enriched and a portion where C and Ni are not densely enriched. When the martensite microstructure is tempered, the portion thereof where C and Ni are densely enriched is reversely transformed into the austenite microstructure even at a temperature around the tempering temperature because A_s point (reverse transformation start temperature) is lowered. In the reverse austenite microstructure, C and Ni are densely enriched. The portion where C and Ni are not densely enriched does not experience the reverse transformation because the A_s point thereof is not lowered so much, and thus is subjected to a usual tempering treatment for adjusting the hardness of steel or the like.
As mentioned above, the final metallographic structure obtained after the tempering step includes a ferrite microstructure, a martensite microstructure, and a residual γ microstructure. It is considered that in the reverse transformed austenite microstructure obtained by heating to the tempering temperature, C and Ni are further enriched through the heating and holding processes. In the residual γ obtained in this way according to the embodiment of the present invention, C and Ni are densely enriched. Thus, the steel plate obtained by the production method according to the embodiment of the present invention has improved strain aging property.
When the tempering temperature is lower than 570°C, the amount of residual γ in the obtained steel plate is small, leading to the deterioration of the strain aging property. On the other hand, when the tempering temperature exceeds 600°C, both the size and quantity of the residue γ increases, leading to the deterioration of the strain aging property. The tempering temperature exceeding 600°C is not preferable also from the viewpoint of ensuring the strength of the steel plate.

[Parameter H: 1.73 × 10^-6 or more and 1.96 × 10^-6 or less]

In the embodiment of the present invention, in order to improve the strain aging property, the parameter H represented by the following formula (1) is set at 1.73 × 10^-6 or more and 1.96 × 10^-6 or less in the above-mentioned intermediate heat treatment step and tempering step. $H = \{{(D_{Ni, L} \times t_{L})}^{0.5} + {(D_{Ni, T} \times t_{T})}^{0.5}\} \times [Ni] + \{{(D_{C, L} \times t_{L})}^{0.5} + {(D_{C, T} \times t_{T})}^{0.5}\} \times [C]$
where

t_L (seconds) is a heating and holding time in the intermediate heat treatment step,
t_T (seconds) is a heating and holding time in the tempering step,
[Ni] (% by mass) is a Ni content,
[C] (% by mass) is a C content, $D_{Ni, L} = 1.4 \times 10^{- 4} \times \exp (- 29.58 \times 1,000 / T_{L})$
$D_{Ni, T} = 1.4 \times 10^{- 4} \times \exp (- 29.58 \times 1,000 / T_{T})$
$D_{C, L} = 0.45 \times 10^{- 4} \times \exp (- 18.54 \times 1,000 / T_{L})$
$D_{C, T} = 0.45 \times 10^{- 4} \times \exp (- 18.54 \times 1,000 / T_{T})$
where
- T_L (K) is a heating temperature in the intermediate heat treatment step, and
- T_T (K) is a tempering temperature.

To improve the strain aging property, it is important to form the residual γ in the steel plate and to improve the stability of the residual γ so as not to cause the process-induced transformation in the residual γ. To form the residual γ in the steel plate, it is important to enrich C and Ni in the austenite microstructure during the intermediate heat treatment. Further, to improve the stability of the residual γ, it is important to appropriately control the enrichment of C and Ni in the residual γ. As mentioned later, the excessive enrichment of C and Ni in the residual γ may deteriorate the strain aging property. In this way, the enrichment of C and Ni in the austenite microstructure significantly contributes to both the formation of the residual γ and the stability of the residual γ. In addition, the enrichment of C and Ni in austenite is related to the diffusion of C and Ni. Thus, the embodiment of the present invention is configured by focusing on the diffusion of C and Ni.
The diffusion of an element is basically substantially proportional to the square root of the product of the diffusion coefficient and the time. Thus, the square root of this product is determined for each element of C and Ni, and then the formula for adding these square roots together is defined as the parameter H. When the square root of the product of the diffusion coefficient and the time is determined for each element of C and Ni, then the parameter H is defined by considering each heat treatment of the intermediate heat treatment and the tempering treatment. The parameter H defined in this way becomes an index indicative of the extent of diffusion of C and Ni during the intermediate heat treatment and the tempering treatment. If the parameter H is less than 1.73 × 10^-6, the diffusion of C and Ni into the austenite microstructure becomes insufficient during the intermediate heat treatment, and the amount of residual γ also becomes insufficient in the steel plate, thus deteriorating the strain aging property. If the parameter H exceeds 1.96 × 10^-6, C and Ni are excessively diffused into the austenite microstructure, and the amount of residual γ is decreased, thus deteriorating the strain aging property. Examples

1. Sample Preparation

A test steel plate was produced by smelting a steel containing a chemical composition shown in Table 1, hot-rolling a cast steel, and then applying a heat treatment shown in Table 2 to the rolled steel piece obtained. All the produced steel plates had a thickness of 32 mm. Samples were taken out of these steel plates. During both the quenching treatment and the intermediate heat treatment, the cooling was performed by water cooling.

Numeral values underlined in Table 2 means that they deviated from the range specified by the embodiment of the present invention.

[Table 1]

Raw material steel	Chemical composition (% by mass, Balance: Fe and inevitable impurities)
Raw material steel	C	Si	Mn	P	S	Ni	Al	N
A	0.051	0.21	0.65	0.0021	0.0006	9.17	0.025	0.0036
B	0.052	0.22	0.65	0.0018	0.0009	9.20	0.030	0.0031
C	0.049	0.22	0.64	0.0023	0.0005	9.23	0.027	0.0027

[Table 2]

Sample No.	Raw material steel	Quenching	Intermediate heat treatment		Tempering		Parameter H	Category
Sample No.	Raw material steel	Temperatun (°C)	Temperature (°C)	Holding time (min)	Temperature (°C)	Holding time (min)	Parameter H	Category
1	A	810	700	15	580	15	1.85×10^-6	Examples o f the invent ion
2	A	810	700	15	600	15	1.96×10^-6
3	B	810	700	15	590	5	1.78×10^-6
4	B	810	700	15	590	15	1.92×10^-6
5	B	810	700	15	600	5	1.81×10^-6
6	A	810	700	15	540	15	1.72×10 ^-6	Comparative Example
7	B	810	700	15	590	60	2.27×10 ^-6
8	B	810	700	15	600	15	1.98×10 ^-6
9	B	810	700	15	600	60	2.38×10 ^-6
10	C	810	600	15	600	15	0.77×10 ^-6	Comparative Example
11	C	810	710	15	600	15	2.13×10 ^-6
12	C	810	800	15	600	15	5.51×10 ^-6
13	C	810	710	15	650	15	1.54×10 ^-6
14	C	750	700	15	600	15	1.92×10 ^-6
15	C	810	690	15	600	15	1.74×10 ^-6	Examples of the invention

2. Evaluation of Properties

Next, various properties of the steel plates were evaluated on the conditions below.

[Tensile test]

Tensile test pieces in conformity with JIS No. 4 were taken out of each steel plate from a t/4 position (t: plate thickness) of the steel plate such that the direction perpendicular to the rolling direction of the steel plate was the longitudinal direction, and then the yield strength and tensile strength of the test piece were measured in accordance with a method specified by JIS Z2241:2011. The results are shown in Table 3.

[Charpy impact test after application of plastic strain]

After applying a plastic strain of 5% to each steel plate, an aging treatment was performed thereon at 250°C for one hour. Then, three Charpy impact test pieces (V-notched test pieces in accordance with JIS Z2242:2005) were taken out from a t/4 position (t: plate thickness) of each steel plate such that the direction perpendicular to the rolling direction of the steel plate was the longitudinal direction. Then, the brittle fracture rates (%) of the test pieces at -196°C were measured by the method mentioned in JIS Z2242:2005. The sample of three test pieces that had the brittle fracture rate of 5% or less was evaluated to be excellent in the strain aging property. It is noted that Table 3 shows three measured values which were measured by using three test pieces of each sample.

[Table 3]

Sample No.	Yield strength (MPa)	Tensile strength (MPa)	Brittle fracture rate in Charpy impact test at - 196°C after application of plastic strain of 5% (%)	Category
Sample No.	Yield strength (MPa)	Tensile strength (MPa)	Measured value	Category
1	673	720	0, 0, 0	Examples of the invention
2	668	723	0, 0, 0
3	658	730	0, 0, 0
4	649	729	0, 0, 0
5	672	741	0, 0, 0
6	740	749	30, 30, 30	Comparative Example
7	619	727	10, 15, 15
8	619	745	10, 15, 15
9	627	732	20, 20, 15
10	620	684	25, 20, 25
11	651	715	10, 0, 10
12	651	703	5, 15, 0
13	507	941	40, 50, 45
14	634	719	10, 10, 15
15	594	711	0, 0, 0	Examples of the invention

From the results shown in Table 3, the following consideration can be made.
Samples Nos. 1 to 5 and 15 were samples produced by the production method that satisfied the requirements of the embodiment of the present invention. All three test pieces of each of these samples had a brittle fracture rate of 5% or less and exhibited excellent strain aging property.
It is noted that all samples Nos. 1 to 5 and 15 had excellent yield strength and tensile strength as well as high strength.
Samples Nos. 6 to 14 were samples produced by a production method that did not satisfy the requirements of the embodiment of the present invention. At least one of three test pieces of each of these samples had a brittle fracture rate exceeding 5% and was inferior in strain aging property.
Sample No. 6 was inferior in the strain aging property because of a low tempering temperature and a low parameter H.
Samples Nos. 7 to 9 was inferior in the strain aging property because of a high parameter H.
Sample No. 10 was inferior in the strain aging property because of a low intermediate heat treatment temperature and a low parameter H.
Sample No. 11 was inferior in the strain aging property because of a high parameter H.
Sample No. 12 was inferior in the strain aging property because of a high intermediate heat treatment temperature and a high parameter H.
Sample No. 13 was inferior in the strain aging property because of a high tempering temperature and a high parameter H.
Sample No. 14 was inferior in the strain aging property because of a low quenching temperature.
The present application claims priority to Japanese Patent Application No. 2017-162740 filed on August 25, 2017 , and Japanese Patent Application No. 2018-131749 filed on July 11, 2018 , the disclosures of which are incorporated herein by reference in its entirety.

Claims

A method for production of a Ni-containing steel plate, wherein a steel comprises:
C: 0.040% by mass or more and 0.060% by mass or less;

Si: 0.10% by mass or more and 0.30% by mass or less;

Mn: 0.50% by mass or more and 0.70% by mass or less;

P: 0.0010% by mass or more and 0.0025% by mass or less;

S: 0.0010% by mass or less;

Ni: 9.10% by mass or more and 9.40% by mass or less;

Al: 0.020% by mass or more and 0.050% by mass or less; and

N: 0.0050% by mass or less, with the balance being Fe and inevitable impurities, the method comprising the steps of, in sequence:
quenching the steel from a quenching temperature of 800°C or higher and 820°C or lower after hot-rolling the steel;

applying an intermediate heat treatment to the quenched steel by holding the steel at a heating temperature of 690°C or higher and 710°C or lower and then cooling the steel at an average cooling rate of 5°C/sec or more until a cooling end temperature of 200°C or lower; and

tempering the steel at a tempering temperature of 570°C or higher and 600°C or lower,

wherein in the intermediate heat treatment step and the tempering step, a parameter H represented by formula (1) below is set at 1.73 × 10^-6 or more and 1.96 × 10^-6 or less: $H = \{{(D_{Ni, L} \times t_{L})}^{0.5} + {(D_{Ni, T} \times t_{T})}^{0.5}\} \times [Ni] + \{{(D_{C, L} \times t_{L})}^{0.5} + {(D_{C, T} \times t_{T})}^{0.5}\} \times [C]$
where
t_L (seconds) is a heating and holding time in the intermediate heat treatment step,

t_T (seconds) is a heating and holding time in the tempering step,

[Ni] (% by mass) is a Ni content,

[C] (% by mass) is a C content, $D_{Ni, L} = 1.4 \times 10^{- 4} \times \exp (- 29.58 \times 1,000 / T_{L})$
$D_{Ni, T} = 1.4 \times 10^{- 4} \times \exp (- 29.58 \times 1,000 / T_{T})$
$D_{C, L} = 0.45 \times 10^{- 4} \times \exp (- 18.54 \times 1,000 / T_{L})$
$D_{C, T} = 0.45 \times 10^{- 4} \times \exp (- 18.54 \times 1,000 / T_{T})$
where
T_L (K) is a heating temperature in the intermediate heat treatment step, and

T_T (K) is a tempering temperature.
The method for production according to claim 1, wherein the steel satisfies at least one of the following (a) to (d):
(a) the C content is 0.045% by mass or more and 0.060% by mass or less;

(b) the Si content is 0.15% by mass or more and 0.30% by mass or less;

(c) the Mn content is 0.60% by mass or more and 0.70% by mass or less; and

(d) the Al content is 0.020% by mass or more and 0.045% by mass or less.
The method for production according to claim 1 or 2, wherein the steel further comprises one or more elements of Cu: 0.01% by mass or more and 0.20% by mass or less, Cr: 0.01% by mass or more and 0.20% by mass or less, Mo: 0.01% by mass or more and 0.20% by mass or less, V: 0.1% by mass or less, Nb: 0.1% by mass or less, Ti: 0.1% by mass or less, and B: 0.005% by mass or less.