JPH09194989A - Thick 610 N / mm 2 class high-strength steel sheet with excellent NRL drop weight characteristics and method for producing the same - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] A lower bainite structure at the center of plate thickness of a thick steel plate,
Quenching and tempering with NRL drop weight characteristics of T _NDT less than -40 ℃ 61
A method for producing 0 N / mm ² class high-strength steel is provided. SOLUTION: C: 0.08-0.16, Si: 0.15-0.55, Mn: 0.
9-1.5, P ≤ 0.01, S ≤ 0.005, Ni: 0.15-0.6, Mo: 0.01-0.5, N
b: 0.005-0.03, V: 0.01-0.1, B: 0.0005-0.002
Steel with Hp ≥ 66.04 (mm) defined by the following formula (1) and balance of iron and unavoidable impurities is heated in the temperature range of 1000-1200 ℃ and rolled in the range of 800-1050 ℃. Is completed to form a steel plate with a thickness of 40-80 mm, and this steel plate is heated at a heating temperature of 900-950
° C., and quenched with soaking time 30 minutes or more, then, thick 610N / mm ² class high manufacturing method of tensile steel sheet excellent in NRL drop weight characteristic tempered at a temperature of 550-680 ℃. However, Hp = 0.32 × (% solid solution C) ^0.5
× (1-8.90% unsolNb) × (1 + 0.64% Si) × (1 + 4.10% Mn) × (1
+ 0.27% Cu) x (1 + 0.52% Ni) x (1 + 2.33% Cr) x (1 + 3.14% M
o) × (1 + 1.75% V) × 33.02 (mm)… (1)

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quenching / tempering 61 having good NRL drop weight characteristics even in the center of plate thickness.
The present invention relates to 0 N / mm ² class high-strength steel and its manufacturing method.

[0002]

^2. Description of the Related Art Conventional quenching and tempering 610 N / mm ² class high-strength steel sheets show a lower bainite structure with sufficient quenching up to the center of the sheet thickness when the sheet thickness is less than 40 mm. Also, in the toughness evaluation by the NRL drop weight test, which emphasizes the crack propagation stopping property, good values are obtained on the other side.

[0003]

However, in the case where the carbon equivalent CeqWES is limited to 0.45% or less in order to improve the weldability, the plate thickness is 40 as long as it is manufactured by the conventional method.
It was confirmed that the NRL weight drop characteristics deteriorated extremely as the Charpy characteristics were improved as the value increased from mm to beyond. Such bainite thick product 610N /
In order to solve the deterioration of NRL weight drop characteristics at the center of the plate thickness of mm ² steel, it is essential to secure hardenability and refine the grain size of bainite structure by optimizing the addition amount of synthetic components.

[0004] In contrast, in the Japanese Patent Laid-Open 4-198418 discloses has secured limited to hardenability the component composition by quenching hardening index _(N D _I), Ya _N D _I-out strength ensure an effective V The effect of Nb, which is effective for the refinement of the structure, is not taken into consideration, and it is insufficient as a hardenability evaluation index in V or Nb-containing steel, and bainite thick material 60 that sufficiently satisfies T _NDT ≦ −40 ° C. At present, kilo steel has not been obtained.

According to the present invention, in a thick steel plate having a plate thickness of 40 mm or more, a lower bainite structure is formed in the central portion of the plate thickness where it is hard to quench and the NRL drop weight characteristic such that T _{NDT of} this part is -40 ° C or less. Quenching / tempering 610N
/ Mm ² class high-strength steel and its manufacturing method.

[0006]

In order to solve the above problems, the quenching / tempering 610 N / mm ² class high-strength steel of the present invention is
% By weight, C: 0.08 to 0.16%, Si: 0.15
~ 0.55%, Mn: 0.9-1.5%, P: 0.01
% Or less, S: 0.005% or less, Ni: 0.15 to 0.
6%, Mo: 0.01-0.5%, Nb: 0.005-
0.03%, V: 0.01 to 0.1%, B: 0.000
5 to 0.002%, Hp = 0.32 × (% solid solution C) ^0.5 × (1-8.90% undissolved Nb) × (1 + 0.
64% Si) x (1 + 4.10% Mn) x (1 + 0.2
7% Cu) x (1 + 0.52% Ni) x (1 + 2.33)
% Cr) x (1 + 3.14% Mo) x (1 + 1.75%
V) × 33.02 (mm), Hp ≧ 66.04 (mm), and the balance is iron and inevitable impurities.
It is a 610 N / mm ² class high-strength steel sheet characterized by being excellent in NRL drop weight characteristics of 80 mm or less, and after heating the steel of the above composition to a temperature range of 1000 to 1200 ° C., 80
Plate thickness 40mm after rolling is completed within the range of 0-1050 ℃
A steel plate of 80 mm or more and a heating temperature of 900 to 950 ° C.,
Quench for more than 30 minutes soaking time, then 550-680
A method for producing 610 N / mm ² class high-strength steel, which is characterized by being excellent in NRL drop weight characteristics after tempering at a temperature of ° C.

The steel further has Cu: 0.05 to 0.3.
%, Cr: 0.01: 0.3%, and it is desirable to contain one or two of them. In the present invention, “excellent in NRL drop weight characteristics” means that the material satisfies T _NDT ≦ −40 ° C.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for adding and limiting the components of the present invention are as follows. 1) Component composition 1. C: 0.08 to 0.16% C requires 0.08% or more to secure strength,
If it exceeds 0.16%, the NRL weight drop characteristics are significantly deteriorated and the weldability is impaired, so the content is made 0.08 to 0.16%.

[0009] 2. Si: 0.15 to 0.55% Si is made 0.55% or less for the same reason as C, but 0.
If it is less than 15%, the strength is insufficient, so 0.15 to 0.55.
%.

3. Mn: 0.9 to 1.5% Mn requires 0.9% or more from the viewpoint of ensuring hardenability and NRL weight drop characteristics, but if it exceeds 1.5%, weldability deteriorates, so 0 9 to 1.5%.

4. P: 0.01% or less Since P, which is an impurity element, exceeds 0.01%, weldability and toughness are significantly deteriorated, so P is set to 0.01% or less.

5. S: 0.005% or less When S, which is an impurity element, exceeds 0.005%, the weldability is
The toughness is significantly deteriorated, so the content is made 0.005% or less. .

6. Ni: 0.15 to 0.6% Ni requires 0.15% or more to secure strength and toughness and improve NRL weight drop characteristics, but the upper limit is 0.6% or less in consideration of economy. And

7. Mo: 0.01 to 0.5% Mo is required to be 0.01% or more in order to secure strength and hardenability, but addition of a large amount causes deterioration of weldability, so the upper limit is made 0.5%. To do.

[8] Nb: 0.005 to 0.03% Nb is required to be 0.005% or more in order to improve NRL weight drop characteristics due to the refinement of the structure and to secure strength by precipitation strengthening, but a large amount of addition is hardenability and welding. The upper limit is made 0.03% because it deteriorates the toughness of the part.

9. V: 0.01 to 0.1% V is required to be 0.01% or more to secure the strength, but addition of a large amount causes deterioration of weld toughness, so the upper limit is 0.1%.
And

10. B: 0.0005 to 0.002% B needs to be 0.0005% or more to secure hardenability, but addition of a large amount causes deterioration of weldability, so the upper limit is set to 0.
002%.

11. N: 0.001 to 0.006% N is preferably added in a predetermined amount in order to combine with Al to form a (carbon) nitride and improve grain refinement of austenite grains. The preferred addition amount is 0.001% or more, but
If the addition exceeds 006%, the toughness deteriorates.

In the present invention, preferable results can be obtained by containing, in addition to the above alloying elements, one or two selected from the following Cu and Cr. 12. Cu: 0.05 to 0.3% Cu requires 0.05% or more to secure the strength, but if added in a large amount, toughness is deteriorated, so the upper limit is made 0.3%.

13. Cr: 0.01 to 0.3% Cr is required to be 0.01% or more in order to secure strength and hardenability, but addition of a large amount causes deterioration of toughness, so the upper limit is made 0.3%. .

14. CeqWES (carbon equivalent value): 0.
42 to 0.45% Ceq is preferably 0.42 or more in order to secure strength, toughness and hardenability. However, it is necessary to limit the content to 0.45% or less in order to improve the weldability.
However, CeqEWS = (C + Mn / 6 + Si / 24 + Ni / 4
0 + Cr / 5 + Mo / 4 + V / 14).

2) Hp (hardenability index): 66.04 mm or more In order to improve the hardenability of the central portion of the plate thickness, the hardenability index has a lower limit of (Hp) of 66.04 mm.

Hp is calculated by the following equation. Hp = 0.32 × (% solid solution C) ^0.5 × (1-8.90%
Undissolved Nb) × (1 + 0.64% Si) × (1 + 4.1
0% Mn) x (1 + 0.27% Cu) x (1 + 0.52
% Ni) x (1 + 2.33% Cr) x (1 + 3.14%
Mo) × (1 + 1.75% V) × 33.02 (mm) However,% solid solution C,% undissolved Nb amount is the following formula and heating temperature T
More estimate.

Log (% solid solution Nb) (% solid solution C + 6/7)
% Solid solution N) =-6770 / T + 2.26 (% solid solution C + 6/7% solid solution N)-(% C + 6/7% N)
= 24/93 (% solid solution Nb-% Nb) 3) Plate thickness The present invention is intended for a thick steel plate in which quenching is hard to occur in the central part of the plate thickness, and the plate thickness lower limit value is set to 40 mm. on the other hand,
When the plate thickness exceeds 80 mm, even if all the other conditions are set within the range of the present invention, a fully baked lower bainite structure at the center of the plate thickness cannot be obtained, so the upper limit is 80 mm. And

4) Manufacturing conditions 1. Heating temperature of steel: 1000 to 1200 ° C. When the heating temperature of steel exceeds the upper limit, N is caused by crystal grain coarsening.
The RL drop weight characteristics deteriorate, and if it is less than the lower limit, the hardenability deteriorates and the strength decreases, so the above range was made.

2. Steel rolling end temperature: 800 to 1050
C. If the rolling end temperature of steel exceeds the upper limit, the NRL weight drop characteristics deteriorate, and if it is less than the lower limit, the strength decreases, so the above range was made.

3. Quenching temperature of steel sheet, soaking time: 9
00 to 950 ° C., 30 minutes In order to improve the hardenability of the steel sheet having the above thickness, it is necessary to set the quenching heating temperature to 900 to 950 ° C. and the quenching soaking time to 30 minutes or more.

4. Tempering treatment: 550 to 680 ° C. The tempering temperature is 5 because it is necessary to balance strength and toughness.
It is necessary to set within the range of 50 to 680 ° C.

5) Austenite (γ) grain size number:
6-8.5 In order to secure toughness and NRL drop weight characteristics, it is preferable that the γ crystal grain size number is 6 or more (average crystal grain size = 0.050 mm). However, since it causes deterioration of hardenability, 8.
It is necessary to limit it to 5 or less (average crystal grain size = 0.021 mm). The method for determining the γ grain size number is JIS G
According to 0551. The steel sheet obtained by the above-mentioned structure has a structure mainly of lower bainite, which is sufficiently hardened in the central portion of the plate thickness, and satisfies T _NDT ≦ −40 ° C.

[0030]

EXAMPLES The inventors of the present invention performed quenching and tempering on steel plates 1 to 16 having a chemical composition shown in Table 1 and having a plate thickness of 40 to 80 mm under the heat treatment conditions shown in Table 2, and thereafter, at the center of the plate thickness. Mechanical properties (tensile strength TS), Charpy impact characteristics (fracture transition temperature vTs), NRL drop weight characteristics (T
_NDT ) and weldability (y _STOP : crack prevention preheating temperature) were investigated.

1 to 10 in Table 1 are steel types of the present invention,
11 to 16 indicate comparative steels which are out of the scope of the present invention in terms of composition. The investigation results shown in Table 2 will be described below in order of characteristics.

1) Mechanical Properties The yield strength and tensile strength of each steel type were measured using JIS No. 4 test pieces, and those having a tensile strength of 610 N / mm ² or more were accepted. The steel type 11 of the comparative example has a quenching heating temperature of 85.
Steel grade 13 has a low Hp of 2.31
4 has a low C of 0.070%, and steel type 15 has a Ceq of 0.
It was as low as 40%, and poor strength was recognized in all cases due to insufficient hardenability.

2) Fracture surface transition temperature The fracture surface transition temperature (vTs) is JIS No. 4 test piece (JIS
Z2202) and a 2 mm V notch Charpy impact test (weight 98N) were used. Fracture transition temperature of base metal is -4
Those having a temperature of 0 ° C. or less were accepted. As to the steel types 11 and 13 to 15 of the comparative example, as described above, all of them had insufficient hardenability, and the steel type 12 had a high S of 0.01%, and thus poor toughness was recognized.

3) NRL drop weight characteristic The NRL drop weight characteristic was carried out using a P-3 test piece by the test method defined by ASTM E208, and the NDT temperature (Nil-Ductility-Transition Temperature: T _NDT ) was -40 ° C. The following items were accepted. Comparative example steel type 1
1 and 13 to 15 for the lack hardenability and steel grade 12 of the comparative example has high and 0.01% S, both T _NDT
Does not satisfy ≦ −40 ° C.

4) Weldability Weldability was evaluated by the JIS Z3158 y-type weld cracking test, and those in which y _STOP was 100 ° C. or less were accepted. The steel type of the comparative example has a high Ceq of 0.47%, and y _STOP
= 125 ° C, the weldability deteriorates.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

As described above, according to the present invention, the plate thickness of 4
Even with a steel plate having a thickness of 0 mm or more, it is possible to form a lower bainite structure in the center portion of the plate thickness where it is difficult to quench, and the NRL drop weight characteristics in this center portion of the plate thickness satisfy T _NDT ≦ −40 ° C.

Claims (4)

[Claims] 1. C: 0.08 to 0.16% by weight,
Si: 0.15 to 0.55%, Mn: 0.9 to 1.5
%, P: 0.01% or less, S: 0.005% or less, N
i: 0.15 to 0.6%, Mo: 0.01 to 0.5%,
Nb: 0.005-0.03%, V: 0.01-0.1
%, B: 0.0005 to 0.002%,
(1) in Hp defined by the Hp ≧ 66.04 (mm), thick material 610N / mm ² class high strength steel sheet balance excellent following NRL drop weight characteristic thickness 40mm or 80mm consisting of iron and unavoidable impurities . Hp = 0.32 x (% solid solution C) ^0.5 x (1-8.90% undissolved Nb) x (1 + 0.64% Si) x (1 + 4.10% Mn) x (1 + 0.27% Cu) × (1 + 0.52% Ni) × (1 + 2.33% Cr) × (1 + 3.14% Mo) × (1 + 1.75% V) × 33.02 (mm) (1) 2. C: 0.08 to 0.16% by weight,
Si: 0.15 to 0.55%, Mn: 0.9 to 1.5
%, P: 0.01% or less, S: 0.005% or less, N
i: 0.15 to 0.6%, Mo: 0.01 to 0.5%,
Nb: 0.005-0.03%, V: 0.01-0.1
%, B: 0.0005 to 0.002%, and Cu:
0.05 to 0.3%, Cr: 0.01 to 0.3%, containing 1 or 2 types, and Hp ≧ defined by the following formula (1):
A method for producing a thick 610 N / mm ² class high-strength steel sheet with a balance of 66.04 (mm) and a balance of iron and inevitable impurities with a sheet thickness of 40 mm or more and 80 mm or less and having excellent NRL drop weight characteristics. Hp is Hp = 0.32x (% solid solution C) ^0.5x (1-8.90% undissolved Nb) x (1 + 0.64% Si) x (1 + 4.10% Mn) x (1 + 0.27% Cu) × (1 + 0.52% Ni) × (1 + 2.33% Cr) × (1 + 3.14% Mo) × (1 + 1.75% V) × 33.02 (mm) (1) 3. C: 0.08 to 0.16% by weight,
Si: 0.15 to 0.55%, Mn: 0.9 to 1.5
%, P: 0.01% or less, S: 0.005% or less, N
i: 0.15 to 0.6%, Mo: 0.01 to 0.5%,
Nb: 0.005-0.03%, V: 0.01-0.1
%, B: 0.0005 to 0.002%,
Hp defined in (1) is Hp ≧ 66.04 (mm), and the balance is 1000 to 120 for steel consisting of iron and unavoidable impurities.
After heating to a temperature range of 0 ° C, rolling is completed within a range of 800 to 1050 ° C to obtain a steel plate having a plate thickness of 40 mm or more and 80 mm or less, and the heating temperature is 900 to 950 ° C and the soaking time is 3
A method for manufacturing a thick 610 N / mm ² class high-strength steel sheet having excellent NRL drop weight characteristics, which is quenched for 0 minutes or more and then tempered at a temperature of 550 to 680 ° C. Hp = 0.32 x (% solid solution C) ^0.5 x (1-8.90% undissolved Nb) x (1 + 0.64% Si) x (1 + 4.10% Mn) x (1 + 0.27% Cu) × (1 + 0.52% Ni) × (1 + 2.33% Cr) × (1 + 3.14% Mo) × (1 + 1.75% V) × 33.02 (mm) (1) 4. C: 0.08 to 0.16% by weight,
Si: 0.15 to 0.55%, Mn: 0.9 to 1.5
%, P: 0.01% or less, S: 0.005% or less, N
i: 0.15 to 0.6%, Mo: 0.01 to 0.5%,
Nb: 0.005-0.03%, V: 0.01-0.1
%, B: 0.0005 to 0.002%, and Cu:
0.05 to 0.3%, Cr: 0.01 to 0.3%, containing 1 or 2 types, and Hp ≧ defined by the following formula (1):
After heating the steel with 66.04 (mm) and the balance of iron and inevitable impurities to a temperature range of 1000 to 1200 ° C., 800
Rolling is completed within the range of 1050 ° C to a steel plate with a plate thickness of 40 mm or more and 80 mm or less.
Quench at 950 ° C, soaking time of 30 minutes or more, then 55
A method for producing a thick 610 N / mm ² class high-strength steel sheet which is tempered at a temperature of 0 to 680 ° C. and has excellent NRL drop weight characteristics. Hp is Hp = 0.32x (% solid solution C) ^0.5x (1-8.90% undissolved Nb) x (1 + 0.64% Si) x (1 + 4.10% Mn) x (1 + 0.27% Cu) × (1 + 0.52% Ni) × (1 + 2.33% Cr) × (1 + 3.14% Mo) × (1 + 1.75% V) × 33.02 (mm) (1)