JP7398970B2 - Thick steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a thick steel plate and a method for manufacturing the same. In particular, the present invention relates to a high-strength thick steel plate with excellent base material toughness and a method for manufacturing the thick steel plate.

For example, as LPG tanks and the like become larger, demand is increasing for thick steel plates that have high strength, low-temperature toughness of the base material, and low-temperature toughness of the HAZ.

As a high-strength steel with excellent HAZ toughness, for example, Patent Document 1 states that the C concentration in the average chemical analysis value of the central segregation part of the steel sheet is 1.2 times or less the average C concentration of the steel material, The cleanliness of inclusions measured according to JIS standards is 0.03% or less, and the number of oxide inclusions with an average diameter of 10 μm or more observed in the cross section of the steel plate is 1 or less/1 mm ² , 0.05 to 5 μm A steel in which the number of oxide and nitride precipitates satisfies 100 or more/1 mm ² is shown.

Patent Document 2 discloses a material that satisfies predetermined chemical components, has an acicular ferrite structure fraction of 50% or more, and further has an island-shaped martensite (MA) structure fraction of 1 to 5 μm in average circular equivalent diameter of 3 to 10%. By meeting the requirements, a low yield ratio, high tensile strength steel plate with excellent base material low temperature toughness and HAZ low temperature toughness can be obtained.

Patent Document 3 discloses a steel material that satisfies a predetermined composition, has a bainitic microstructure, and has a yield strength of 500 N/mm ² or more and a tensile strength of 610 N/mm ² or more. Furthermore, this steel material does not require annealing heat treatment to remove residual stress after welding, and has been shown to be suitable for manufacturing tanks for LPG/ammonia carrier ships.

In Patent Document 4, the parameters 9×Ceq+4×P≧4.8 and [C]/([Mo]+[Ti]+[Nb]+[V]) satisfy a predetermined component composition. 0.6 to 1.7 steel is heated to a temperature of 1100 to 1300°C, hot rolled at a rolling end temperature of 750°C or higher, and then cooled to a temperature of less than 400°C at a cooling rate of 20°C/s or higher. A method for producing high-strength steel sheets with excellent SR (Stress Relief) properties is provided, which is characterized by performing accelerated cooling and then immediately reheating to 550 to 700°C at a temperature increase rate of 0.5°C/s or more. It is shown.

Japanese Patent Application Publication No. 8-158006 Japanese Patent Application Publication No. 2009-127065 Japanese Patent Application Publication No. 2008-025014 Japanese Patent Application Publication No. 2007-270194

Although a good balance between strength and toughness is obtained in Patent Document 1, the examples only consider plate thicknesses of 40 mm or less, and no technology has been proposed that takes thicker steel plates into consideration. Patent Document 2 attempts to achieve both low-temperature toughness and strength of the base material and HAZ, but low-temperature toughness is evaluated at -60°C, and further study is required to achieve excellent toughness at lower temperatures. I think that it is necessary. In Patent Document 3, the mechanical properties are evaluated only at the plate thickness t/4 position, and furthermore, the mechanical properties inside the steel plate are not taken into consideration. Patent Document 4 discloses a method for producing a thick steel plate that has good mechanical properties even after SR, but the toughness evaluation temperature is only -10°C, and toughness at lower temperatures has not been investigated. . The present invention has been made in view of the above circumstances, and its purpose is to exhibit an excellent strength-toughness balance throughout the interior of the steel plate even when the plate thickness is large, and in particular, to achieve high strength and a lower temperature than before. An object of the present invention is to provide a thick steel plate exhibiting excellent toughness, and a method for manufacturing the thick steel plate.

Aspect 1 of the present invention is
The ingredient composition is
C: 0.02% by mass to 0.070% by mass,
Si: more than 0 mass%, 0.40 mass% or less,
Mn: 1.30% by mass to 1.95% by mass,
P: more than 0 mass%, 0.015 mass% or less,
S: more than 0% by mass, 0.005% by mass or less,
Al: 0.005% by mass to 0.070% by mass,
Nb: 0.015% by mass to 0.048% by mass,
Ti: 0.005% by mass to 0.024% by mass,
N: 0.0030% by mass to 0.0080% by mass, and Ca: more than 0% by mass and 0.0040% by mass or less, the remainder consisting of Fe and inevitable impurities,
Satisfies Di+10Nb: 1.20 to 2.50 obtained from the following formula (1),
Among the crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15° or more, the total area fraction SA of crystal grains with an equivalent circle diameter of 7.5 μm or less is 34% or more at the 1/4 position of the plate thickness, and It is a thick steel plate with a thickness of 27% or more at the 1/2 position.
Di=1.16×([C]/10) ^0.5 ×(0.7×[Si]+1)×(5.1×([Mn]-1.2)+5)×(0.35× [Cu]+1)×(0.36×[Ni]+1)×(2.16×[Cr]+1)×(3×[Mo]+1)×(1.75×[V]+1)×(200 ×[B]+1)...(1)
In formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are each expressed in mass%. The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are shown, and elements not included are set to zero.

Aspect 2 of the present invention further includes:
The thick steel plate according to aspect 1, containing one or more elements selected from the group consisting of Cu: more than 0 mass% and 0.75 mass% or less, and Ni: more than 0 mass% and 1.4 mass% or less. be.

Aspect 3 of the present invention further includes:
Mo: more than 0 mass%, 0.50 mass% or less,
V: more than 0 mass%, 0.060 mass% or less,
The thickness according to aspect 1 or 2, containing one or more elements selected from the group consisting of Cr: more than 0 mass% and 0.8 mass% or less, and B: more than 0 mass% and 0.0007 mass% or less. It is a steel plate.

Aspect 4 of the present invention further comprises:
Any one of embodiments 1 to 3 containing one or more elements selected from the group consisting of REM: more than 0 mass% and 0.0060 mass% or less, and Zr: more than 0 mass% and 0.0050 mass% or less. This is the thick steel plate described.

Aspect 5 of the present invention is
A method for manufacturing a thick steel plate according to any one of aspects 1 to 4, comprising:
A step of heating a steel billet having a composition according to any one of aspects 1 to 4 to a temperature higher than 1020° C. and lower than 1200° C., and a hot rolling step after the heating,
In the hot rolling process, the number of rolling passes is 3 or more, and manufacturing of a thick steel plate is performed by hot rolling and cooling after the hot rolling so as to satisfy all of the following conditions (a) to (d). Method.
(a) Cumulative rolling reduction of 40% or more in the temperature range of 850°C or less (b) Average rolling reduction of 5.5% or more in the final three passes of rolling (c) Finish rolling temperature of 720 to 830°C
(d) After hot rolling, it is cooled from a cooling start temperature of the finish rolling temperature to 690°C to a cooling stop temperature of 320 to 550°C at an average cooling rate of 0.5 to 20°C/s.

According to the present invention, a thick steel plate exhibits an excellent strength-toughness balance throughout the interior of the steel plate even if the plate is thick, and in particular, exhibits high strength and superior toughness at lower temperatures than conventional steel plates, and a method for manufacturing the same. can be provided.

Figure 1 is a graph showing the relationship between the Y value and the total area fraction SA of crystal grains with an equivalent circle diameter of 7.5 μm or less among crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15° or more. be.

In the present invention, intensive research has been conducted to obtain a thick steel plate with an improved balance between strength and toughness at low temperatures than before in the state before SR, that is, in the as-hot-rolled state. In particular, even if the plate thickness is thick, the strength-toughness balance is excellent at the 1/4 position and 1/2 position of the plate thickness. Specifically, the parameter regarding the strength-toughness balance defined by the present invention Y=20×vTrs- We conducted extensive research to obtain a steel plate with a sufficiently low 7×YP.

As a result, while controlling the component composition, we ensured a predetermined amount of fine acicular ferrite structure at the 1/4 and 1/2 positions of the plate thickness, and more specifically, the equivalent of a circle measured by the method described below. It has been found that the above-mentioned good characteristics can be obtained by securing a predetermined amount of acicular ferrite having a diameter of 7.5 μm or less. Hereinafter, the 1/4 position of the plate thickness may be referred to as the "t/4 position", and the 1/2 position of the plate thickness may be referred to as the "t/2 position".

Furthermore, we have found that it is effective to carry out all of the following (A) to (C) in order to form a predetermined amount of the above-mentioned fine acicular ferrite structure. The above-mentioned acicular ferrite may be referred to as "AF" below.
(A) Before the transformation from the austenite phase to the ferrite phase, sufficient processing strain is applied to the austenite phase by hot rolling. A microstructure is realized by generating AF crystal grains with the dislocation structures and deformation bands introduced by this processing as nuclei.
(B) Solid solution Nb is secured before hot rolling in the non-recrystallized region. By doing so, it becomes easier to obtain processing strain prior to transformation, and as described above, it becomes easier to generate fine AF crystal grains. As described later, it is effective to ensure the solid solution Nb by setting the pre-rolling heating temperature to over 1020°C and lowering the rolling reduction during rolling at 850°C or higher.
(C) Appropriately control the transformation temperature to ferrite phase. If the transformation temperature to the ferrite phase is high, a grain boundary ferrite structure is formed prior to AF generation, and the amount of AF decreases. On the other hand, if the transformation temperature to the ferrite phase is low, a martensitic structure is generated without forming an AF structure. In order to appropriately control the transformation temperature to the ferrite phase, if the component composition contains at least one of C, Mn, Cu and Ni, the content of these and the range of Di + 10Nb should be controlled, as will be described later. Generally, the average cooling rate in the predetermined temperature range after hot rolling is preferably 0.5° C./s or more.

Below, the steel structure, component composition, characteristics, and manufacturing method of the thick steel plate of the present invention will be explained in order.

1. Steel Structure The steel structure of the thick steel plate of the present invention will be explained in detail below. In the description of the steel structure below, the mechanism by which various properties can be improved by having such a structure may be explained. Although these are mechanisms considered by the present inventors based on the knowledge obtained at the present time, it should be noted that they do not limit the technical scope of the present invention.

In the present invention, among crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15 degrees or more, crystal grains with an equivalent circle diameter of 7.5 μm or less are defined as a fine acicular ferrite (AF) structure. In the present invention, "forming a predetermined amount of fine acicular ferrite (AF) structure" refers to crystal grains surrounded by large-angle grain boundaries with a crystal misorientation of 15 degrees or more and an equivalent circle diameter of 7.5 μm or less. This means securing 34% or more of the crystal grains at the t/4 position and 27% or more at the t/2 position, as shown below.

In the present invention, in order to obtain a thick steel plate that exhibits an excellent strength-toughness balance even when the plate thickness is thick, among the crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15° or more, those with an equivalent circle diameter of 7.5 μm or less The total area fraction SA of crystal grains, that is, the area fraction of the fine acicular ferrite (AF) structure, is defined at both the t/4 position and the t/2 position. Specifically, the total area fraction SA is made to satisfy 34% or more at the t/4 position and 27% or more at the t/2 position. The total area fraction SA at the t/4 position is preferably 35% or more, more preferably 36% or more. Further, the total area fraction SA at the t/2 position is preferably 28% or more, more preferably 30% or more. Note that, from the viewpoint of obtaining an excellent strength-toughness balance, the upper limit of the above-mentioned total area fraction SA at the t/4 position and the t/2 position is not particularly limited. Considering the manufacturing conditions of the thick steel plate of the present invention, the upper limit of the total area fraction SA at the t/4 position is about 80%, and the upper limit of the total area fraction SA at the t/2 position is about 70%.

Examples of structures other than the fine acicular ferrite include bainite, ferrite, cementite, retained austenite, and martensite. As long as the total area fraction SA is within the range defined by the present invention, the acicular ferrite having an equivalent circle diameter of more than 7.5 μm may be present.

2. Composition The composition of the thick steel plate according to the present invention will be explained below.

C: 0.02% by mass to 0.070% by mass
C has the effect of appropriately controlling the ferrite transformation temperature and suppressing the formation of grain boundary ferrite, which acts as a brittle fracture starting point and causes deterioration of the strength-toughness balance, before AF formation. From the viewpoint of exhibiting this effect, the amount of C is 0.02% by mass or more, preferably 0.023% by mass or more, and more preferably 0.030% by mass or more. On the other hand, if the amount of C is excessive, a hard martensitic structure is generated and acts as a starting point for brittle fracture, thereby deteriorating the strength-toughness balance. Therefore, the amount of C is set to 0.070% by mass or less. The amount of C is preferably 0.065% by mass or less, more preferably 0.060% by mass or less.

Si: More than 0% by mass, 0.40% by mass or less Si is a deoxidizing element, and its content is more than 0% by mass. The amount of Si may be 0.05% by mass or more, and further may be 0.10% by mass or more. On the other hand, if the amount of Si is excessive, a hard martensitic structure is generated and acts as a starting point for brittle fracture, thereby deteriorating the strength-toughness balance. Therefore, the amount of Si is 0.40% by mass or less, preferably 0.38% by mass or less, more preferably 0.35% by mass or less.

Mn: 1.30% by mass to 1.95% by mass
Mn has the effect of appropriately controlling the ferrite transformation temperature and suppressing the formation of grain boundary ferrite, which acts as a brittle fracture starting point and causes deterioration of the strength-toughness balance, before AF formation. From the viewpoint of exhibiting this effect, the amount of Mn is 1.30% by mass or more, preferably 1.40% by mass or more, and more preferably 1.45% by mass or more. On the other hand, if the amount of Mn is excessive, a hard martensitic structure is generated and acts as a brittle fracture starting point, thereby deteriorating the strength-toughness balance. Therefore, the amount of Mn is 1.95% by mass or less, preferably 1.90% by mass or less, more preferably 1.80% by mass or less.

P: More than 0% by mass, 0.015% by mass or less P is an impurity element, and if it is included in excess, the grain boundaries will become brittle and the strength-toughness balance will deteriorate. Therefore, the amount of P is set to 0.015% by mass or less. The amount of P is preferably 0.008% by mass or less, more preferably 0.007% by mass or less. On the other hand, since it is industrially difficult to reduce the amount of P to 0% by mass, the lower limit of the amount of P is more than 0% by mass.

S: more than 0% by mass and not more than 0.005% by mass S is an impurity element, and if it is included in excess, grain boundaries become brittle and the strength-toughness balance deteriorates. Therefore, the amount of S is set to 0.005% by mass or less. The amount of S is preferably 0.004% by mass or less, more preferably 0.003% by mass or less. On the other hand, since it is industrially difficult to reduce the amount of S to 0% by mass, the lower limit of the amount of S is more than 0% by mass.

Al: 0.005% by mass to 0.070% by mass
Al is a deoxidizing element. In order to perform sufficient deoxidation to reduce oxygen in the steel and suppress deterioration of the strength-toughness balance due to oxides, the amount of Al is set to 0.005% by mass or more. The amount of Al is preferably 0.010% by mass or more, more preferably 0.015% by mass or more. On the other hand, if the amount of Al is excessive, coarse oxides are formed and the strength-toughness balance deteriorates. Therefore, the amount of Al is 0.070% by mass or less, preferably 0.050% by mass or less, more preferably 0.045% by mass or less.

Nb: 0.015% by mass to 0.048% by mass
Nb is an element that promotes the generation of AF. In order to sufficiently generate a fine AF structure and obtain a good strength-toughness balance, the Nb content is set to 0.015% by mass or more, preferably 0.016% by mass or more, and more preferably 0.018% by mass or more. do. On the other hand, if the amount of Nb is excessive, a hard martensitic structure is generated, and this structure acts as a brittle fracture starting point, thereby deteriorating the strength-toughness balance. Therefore, the amount of Nb is 0.048% by mass or less, preferably 0.045% by mass or less, and more preferably 0.040% by mass or less.

Ti: 0.005% by mass to 0.024% by mass
Ti is an element that contributes to improving HAZ toughness by forming TiN. From the viewpoint of exhibiting this effect, the amount of Ti is 0.005% by mass or more, preferably 0.007% by mass or more, and more preferably 0.009% by mass or more. On the other hand, if the amount of Ti is excessive, coarse crystallized TiN will be produced and the strength-toughness balance will deteriorate. Therefore, the amount of Ti is 0.024% by mass or less, preferably 0.022% by mass or less, more preferably 0.020% by mass or less.

N: 0.0030% by mass to 0.0080% by mass
N is an element that contributes to improving HAZ toughness by forming TiN. From the viewpoint of exhibiting this effect, the amount of N is 0.0030% by mass or more, preferably 0.0032% by mass or more, and more preferably 0.0035% by mass or more. On the other hand, if the amount of N is excessive, solid solution N increases and the strength-toughness balance deteriorates. Therefore, the amount of N is 0.0080% by mass or less, preferably 0.0075% by mass or less, and more preferably 0.0070% by mass or less.

Ca: more than 0% by mass, not more than 0.0040% by mass Ca is a deoxidizing element, and its content is more than 0% by mass. Furthermore, when the amount of Mn in the steel is large, coarse MnS is likely to be generated at the t/2 position due to Mn concentration during casting, and it is considered that the toughness at the t/2 position is likely to decrease. In order to suppress the formation of MnS, the amount of Ca is preferably greater than 0% by mass, more preferably 0.0008% by mass or more, and still more preferably 0.0010% by mass or more. On the other hand, if the amount of Ca is excessive, coarse oxides are formed and the strength-toughness balance deteriorates. Therefore, the amount of Ca is 0.0040% by mass or less, preferably 0.0028% by mass or less, more preferably 0.0025% by mass or less.

The remainder is Fe and inevitable impurities. As unavoidable impurities, for example, trace elements such as As, Sb, and Sn that are introduced depending on the conditions of raw materials, materials, manufacturing equipment, etc. are allowed. Note that, for example, there are elements such as P and S, which are preferably contained in smaller amounts and are therefore unavoidable impurities, but whose composition ranges are separately specified as described above. Therefore, in this specification, the term "inevitable impurities" constituting the remainder is a concept excluding elements whose composition range is separately defined.

The thick steel plate of the present invention only needs to contain the above elements in its composition. Although the selected elements described below do not need to be included, high strength etc. can be more easily achieved by including them together with the above elements as necessary. Further, the desired structure can be more easily secured, and the strength-toughness balance required by the present invention can be more easily achieved. The selected elements will be described below.

One or more elements selected from the group consisting of Cu: more than 0 mass% and 0.75 mass% or less, and Ni: more than 0 mass% and 1.4 mass% or less. This has the effect of suppressing the formation of grain boundary ferrite, which acts as a brittle fracture starting point and causes deterioration of the strength-toughness balance, before AF formation. From the viewpoint of exhibiting this effect, when Cu is contained, it is preferably more than 0% by mass, more preferably 0.05% by mass or more, still more preferably 0.10% by mass or more, even more preferably It is 0.15% by mass or more. When Ni is contained, it is preferably more than 0% by mass, more preferably 0.10% by mass or more, still more preferably 0.15% by mass or more, even more preferably 0.20% by mass or more. . On the other hand, if these elements are in excess, a hard martensitic structure is generated, which acts as a brittle fracture starting point, leading to a deterioration of the strength-toughness balance. Therefore, the amount of Cu is preferably 0.75% by mass or less, more preferably 0.70% by mass or less, still more preferably 0.68% by mass or less. The amount of Ni is preferably 1.4% by mass or less, more preferably 1.2% by mass or less, still more preferably 1.0% by mass or less.

Mo: more than 0 mass%, 0.50 mass% or less, V: more than 0 mass%, 0.060 mass% or less, Cr: more than 0 mass%, 0.8 mass% or less, and B: more than 0 mass%, One or more elements selected from the group consisting of 0.0030% by mass or less These elements are effective for improving strength. From the viewpoint of exhibiting this effect, when Mo is contained, it is preferably more than 0% by mass, more preferably 0.05% by mass or more, and still more preferably 0.10% by mass or more. When V is contained, it is preferably more than 0% by mass, more preferably 0.01% by mass or more, still more preferably 0.02% by mass or more. When Cr is contained, it is preferably more than 0% by mass, more preferably 0.10% by mass or more, still more preferably 0.20% by mass or more. When B is contained, it is preferably more than 0% by mass, more preferably 0.0003% by mass or more.

On the other hand, if the content of these elements is excessive, a hard martensitic structure is generated, which acts as a brittle fracture starting point, leading to a deterioration of the strength-toughness balance. Therefore, the amount of Mo is preferably 0.50% by mass or less, more preferably 0.45% by mass or less, still more preferably 0.40% by mass or less. The amount of V is preferably 0.060% by mass or less, more preferably 0.050% by mass or less, still more preferably 0.045% by mass or less. The amount of Cr is preferably 0.8% by mass or less, more preferably 0.70% by mass or less, still more preferably 0.60% by mass or less. The amount of B is preferably 0.0007% by mass or less, more preferably 0.0006% by mass or less.

One or more elements selected from the group consisting of REM: more than 0 mass% and 0.0060 mass% or less, and Zr: more than 0 mass% and 0.0050 mass% or less These elements are deoxidizing elements. In order to exhibit this effect, when REM is contained, it is preferably more than 0% by mass, more preferably 0.0010% by mass or more, and still more preferably 0.0015% by mass or more. When Zr is contained, it is preferably more than 0% by mass, more preferably 0.0010% by mass or more, and still more preferably 0.0012% by mass or more. On the other hand, if these elements are in excess, coarse oxides will be formed and the strength-toughness balance will deteriorate. Therefore, the amount of REM is preferably 0.0060% by mass or less, more preferably 0.0050% by mass or less, still more preferably 0.0045% by mass or less. The amount of Zr is preferably 0.0050% by mass or less, more preferably 0.0045% by mass or less, still more preferably 0.0040% by mass or less. The above-mentioned REM includes lanthanide elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

Di+10Nb: 1.20 to 2.50 (Di is obtained from the following formula (1))
Di=1.16×([C]/10) ^0.5 ×(0.7×[Si]+1)×(5.1×([Mn]-1.2)+5)×(0.35× [Cu]+1)×(0.36×[Ni]+1)×(2.16×[Cr]+1)×(3×[Mo]+1)×(1.75×[V]+1)×(200 ×[B]+1)...(1)
In formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are each expressed in mass%. The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are shown, and elements not included are set to zero.

Di+10Nb is a parameter that affects the ferrite transformation temperature. By appropriately controlling the ferrite transformation temperature, it is possible to suppress the formation of grain boundary ferrite before AF formation, suppress the excessive formation of pearlite structures and martensitic structures other than AF, and promote AF formation. . From these viewpoints, in the present invention, Di+10Nb is set within the range of 1.20 to 2.50. If the value of Di+10Nb is too small, a pearlite structure is generated together with grain boundary ferrite, and the strength-toughness balance deteriorates. Therefore, Di+10Nb is set to 1.20 or more. Di+10Nb is preferably 1.25 or more, more preferably 1.30 or more. On the other hand, if the value of Di+10Nb is too large, a hard martensitic structure is generated and acts as a brittle fracture starting point, thereby degrading the strength-toughness balance. Therefore, Di+10Nb is set to 2.50 or less. Di+10Nb is preferably 2.20 or less, more preferably 2.00 or less.

The thickness of the thick steel plate of the present invention is preferably 16 mm or more, more preferably 30 mm or more, still more preferably 40 mm or more, particularly preferably more than 40 mm. Note that the upper limit of the plate thickness is not particularly limited, and is preferably 80 mm or less, for example.

The thick steel plate of the present invention has the following characteristics and is suitable for manufacturing large LPG tanks, ships, etc., for example.

3. Characteristics In the present invention, the Y value, which is a parameter expressed by the following formula (2), is used to evaluate the balance between strength and toughness at a temperature lower than conventional ones. The Y value includes the yield strength YP and the brittle-ductile transition temperature vTrs, as shown in the following formula (2). In addition, as the yield strength YP in the following formula (2), if the SS curve (also referred to as "stress-strain diagram") is a round shape with no clear yield point, use 0.2% yield strength (0.2YS). However, if it has a yield point, use YP.

The thick steel plate of the present invention is as hot-rolled and satisfies that Y at the t/4 position is less than -5200 and Y at the t/2 position is less than -4700 in the C direction perpendicular to the rolling direction. The case is evaluated as having an excellent strength-toughness balance.
Y=20×vTrs-7×YP...(2)

The Y value at the t/4 position is preferably -5300 or less, more preferably -5400 or less, and the Y value at the t/2 position is preferably -4800 or less, more preferably -5000 or less. be. The lower these values, the better the strength-toughness balance.

The present invention only needs to achieve the above parameters as characteristics. Regarding each of the yield strength YP and the brittle-ductile transition temperature vTrs, on the premise that the above parameters are achieved, for example, the yield strength YP is preferably in the range of 350 to 550 MPa, and the brittle-ductile transition temperature vTrs is preferably in the range of less than -70°C. range, more preferably less than -80°C. Also, depending on the measurement position, at the t/4 position, on the premise that the above parameters are achieved, for example, the yield strength YP is preferably 410 MPa or more, more preferably 430 MPa or more, still more preferably 460 MPa or more, and even more preferably is 480 MPa or higher, particularly 500 MPa or higher, and the brittle-ductile transition temperature vTrs is preferably -90°C or lower, more preferably -95°C or lower, even more preferably -100°C or lower, Even more preferably it is -110°C or lower. Further, at the t/2 position, on the premise that the above parameters are achieved, for example, the yield strength YP is preferably 400 MPa or more, more preferably 430 MPa or more, still more preferably 460 MPa or more, even more preferably 480 MPa or more, In particular, it is 500 MPa or higher, and the brittle-ductile transition temperature vTrs is preferably -80°C or lower, more preferably -90°C or lower, even more preferably -95°C or lower, and even more preferably -100°C or lower. Even more preferably.

4. Manufacturing method The method for manufacturing a thick steel plate according to the present invention includes a step of heating a steel piece having the above-mentioned composition to a temperature above 1020°C and below 1200°C, and a hot rolling step after the heating. In the rolling process, the number of rolling passes is 3 or more, and a method for producing a thick steel plate in which hot rolling and cooling after the hot rolling are performed so as to satisfy all of the following conditions (a) to (d).
(a) Cumulative rolling reduction of 40% or more in the temperature range of 850°C or less (b) Average rolling reduction of 5.5% or more in the final three passes of rolling (c) Finish rolling temperature of 720 to 830°C
(d) After hot rolling, it is cooled from a cooling start temperature of the finish rolling temperature to 690°C to a cooling stop temperature of 320 to 550°C at an average cooling rate of 0.5 to 20°C/s.
Each manufacturing condition will be explained in detail below.

[Step of heating a steel piece having the above-mentioned composition above 1020°C and below 1200°C]
In heating for hot rolling, if the heating temperature is 1020° C. or lower, NbC generated during casting will not be sufficiently dissolved in solid solution, and the effect of promoting AF generation by solid solution Nb will not be obtained. Therefore, the heating temperature is set to higher than 1020°C, preferably higher than 1040°C, more preferably higher than 1050°C, still more preferably higher than 1060°C. On the other hand, if the heating temperature is 1200° C. or higher, the austenite grains become coarse and the overall structure becomes coarse. Therefore, the heating temperature is less than 1200°C, preferably 1180°C or less, more preferably 1150°C or less.

[Hot rolling process after heating]
As will be described later, the present invention is characterized in that the average rolling reduction rate of the final three passes is controlled, and after the heating, hot rolling is performed with three or more rolling passes, but the total number of passes (rolling The number of passes) does not affect the organization and characteristics and is not limited. The number of rolling passes may be 7 or more, more preferably 10 or more, and 60 or less from the viewpoint of productivity.

In the present invention, the hot rolling is performed so as to satisfy all of the following conditions (a) to (d), and cooling is performed after the hot rolling.

(a) Cumulative rolling reduction rate at temperatures below 850°C: 40% or more In order to introduce sufficient working strain into the austenite phase with the aim of obtaining a sufficient AF structure, it is necessary to The cumulative reduction rate (also referred to as "total reduction rate") in the temperature range needs to be 40% or more. By the way, it is thought that when the rolling reduction rate during rolling at 850° C. or higher increases, NbC precipitates during rolling and solid solution Nb decreases. Also from the viewpoint of suppressing the cumulative reduction rate in the rolling at 850°C or higher, the cumulative reduction rate in the temperature range of 850°C or lower is set to 40% or more. The cumulative reduction rate is preferably 50% or more, more preferably 55% or more. The upper limit of the cumulative reduction rate is about 80% from the viewpoint of productivity.

(b) Average rolling reduction ratio of the final 3 passes: 5.5% or more The dislocation structure introduced during the rolling of the final 3 passes moves to the cooling process with relatively little recovery, so the AF promotion effect is large. . In the present invention, in order to introduce sufficient working strain into the austenite phase to introduce the above-mentioned dislocation structure, the average reduction ratio in the final three passes of rolling is set to 5.5% or more. In the present invention, by controlling the average rolling reduction rate of the final three passes, a desired structure with an AF structure of a certain level or higher can be obtained even inside the steel sheet, especially at the t/2 position. This is different from the conventional method, which does not control the average rolling reduction rate. The average rolling reduction ratio is preferably 5.8% or more, more preferably 6.0% or more. On the other hand, from the viewpoint of rolling mill load, the upper limit of the average rolling reduction is about 20%.

(c) Finishing Rolling Temperature (FRT): 720-830°C
When the temperature of the steel material exceeds 850° C., even if the final three passes of rolling are performed at the above-mentioned average reduction ratio, sufficient working strain is not introduced into the austenite phase, resulting in an insufficient amount of AF structure. In the present invention, in order to ensure a sufficient amount of AF structure, the finish rolling temperature is set to 830° C. or lower. The finish rolling temperature is preferably 820°C or lower, more preferably 810°C or lower. On the other hand, if the finish rolling temperature is lower than 720° C., coarse ferrite is formed during rolling and the toughness deteriorates. Therefore, the finish rolling temperature is set to 720°C or higher, preferably 750°C or higher, and more preferably 760°C or higher.

(d) After hot rolling, it is cooled from a cooling start temperature of the finish rolling temperature to 690°C to a cooling stop temperature of 320 to 550°C at an average cooling rate of 0.5 to 20°C/s.
By cooling a certain temperature range at an average cooling rate of 0.5 to 20° C./s, a sufficient AF structure can be secured. If the average cooling rate exceeds 20° C./s, martensitic transformation occurs without sufficient formation of the AF structure, which is not preferable. The average cooling rate is preferably 15°C/s or less, more preferably 12°C/s or less. As mentioned above, in order to appropriately control the transformation temperature to the ferrite phase and sufficiently secure the AF structure, if the component composition contains at least one of C, Mn, Cu and Ni, the content of these, and Di+10Nb, and the average cooling rate is set to 0.5° C./s or more. If the average cooling rate is less than 0.5° C./s, coarse grain boundary ferrite is generated during cooling, resulting in insufficient AF structure. The average cooling rate is preferably 2.0°C/s or more, more preferably 3.0°C/s or more. An example of a cooling method having the above-mentioned average cooling rate is water cooling.

The starting temperature of the cooling mentioned above is an arbitrary temperature between the finish rolling temperature and 690°C. If the temperature of the steel material falls below 710° C., problems such as grain boundary ferrite being generated before the start of cooling and processing strain introduced into the austenite phase being recovered will occur. As a result, the amount of AF tissue becomes insufficient. In the present invention, in order to ensure a sufficient amount of AF tissue, the cooling start temperature (Start Cooling Temperature, SCT) at the above average cooling rate is set to 690° C. or higher. The cooling start temperature is preferably 710°C or higher, more preferably 720°C or higher.

The finishing temperature of the cooling (Finish Cooling Temperature, FCT) is an arbitrary temperature between 320 and 550°C. If cooling at the above average cooling rate, for example, water cooling, is stopped in a temperature range above 550° C., grain boundary ferrite is generated during slow cooling after water cooling is stopped, making it difficult to secure a sufficient AF structure. Therefore, the end temperature is set to 550°C or less. The finishing temperature is preferably 500°C or lower, more preferably 480°C or lower. On the other hand, if cooling is performed at the above average cooling rate to a temperature range below, for example, 320° C., martensitic transformation occurs without sufficient formation of the AF structure. Therefore, the end temperature is set to 320°C or higher. The end temperature is preferably 340°C or higher, more preferably 360°C or higher.

The manufacturing method of the present invention is not particularly limited except for the hot rolling step, and may be carried out under commonly used conditions.

Those skilled in the art who have come into contact with the manufacturing method of the high-strength steel sheet according to the embodiment of the present invention described above will be able to obtain the high-strength steel sheet according to the present invention by a manufacturing method different from the above-mentioned manufacturing method through trial and error. There is a possibility that it can be done.

Hereinafter, the present invention will be explained in more detail with reference to Examples. The present invention is not limited by the following examples, and can be implemented with appropriate changes within the scope that can meet the spirit described above and below, and all of these are within the technical scope of the present invention. included in

1. Sample Preparation 150 kg of steel with the composition shown in Table 1 was melted in a VIF (Vacuum Induction Furnace) or an actual converter, and the slab obtained by casting was hot rolled under various conditions shown in Table 2. A steel plate having the thickness shown in Table 2 was obtained. In the hot rolling, the total number of passes (number of rolling passes) was more than 20. In Table 1, "-" indicates that it was not added intentionally. In Table 2, FRT indicates finish rolling temperature, SCT indicates cooling start temperature, and FCT indicates cooling end temperature. The cooling stop temperature FCT was measured by a radiation thermometer at 1 to 3 points on the surface of the steel plate in the longitudinal direction, and the average value was calculated. Further, FRT and SCT were determined by measuring one point on the surface of the steel material using a radiation thermometer.

2. Steel structure EBSD (Electron Back Scatter Diffraction) was measured at t/4 (t: plate thickness) and t/2 positions of the cross section perpendicular to the rolling width direction of the hot rolled material. The measurement conditions are as follows.
EBSD measurement conditions/equipment: JEOL-5410 or JSM-IT100
・Observation magnification: 400 times ・Measurement area: 200 x 200 μm
・Step (pixel) size: 0.4μm
・Phases to consider: ferrite, austenite

The obtained EBSD data was analyzed using analysis software OIM Analysis manufactured by TSL Solutions Co., Ltd. In the obtained data, points with a Confidence Index of 0.100 or less were removed, and grain boundaries with a crystal orientation of 15° or more with respect to adjacent pixels were defined as large-angle grain boundaries. Among the units surrounded by this large-angle grain boundary, units with a pixel size of 10 or more were regarded as large-angle grains. Furthermore, large square grains located at the edges of the measurement field of view were excluded from the analysis. The average equivalent circular diameter of the large square grains was determined, and the total area fraction SA of the large square grains having an average equivalent circular diameter of 7.5 μm or less in the total area of the large square grains was calculated.

3. Mechanical properties (yield strength YS)
ASTM round bar tensile test pieces were taken parallel to the plate width direction (C direction) at the t/4 and t/2 positions of the as-hot rolled steel plate, and a tensile test was performed according to the ASTM procedure to determine yield. The strength YS was measured.
(Low temperature toughness of base material)
V-notch Charpy test pieces were taken parallel to the sheet width direction (C direction) at the t/4 and t/2 positions of the as-hot-rolled steel sheet, and a Charpy impact test was conducted in accordance with ASTM. The temperature vTrs at which the brittle fracture ratio was 50% was then evaluated.

The yield strength YP and brittle-ductile transition temperature vTrs obtained by the above measurement were substituted into the following equation (2) to determine the Y value at each of the t/4 position and the t/2 position. The results are also listed in Table 2. In Table 2, when vTrs>-30°C, the Y value was calculated as vTrs=-30°C. In addition, when vTrs<-130°C, the Y value was calculated using vTrs=-130°C.
Y=20×vTrs-7×YP...(2)

The following can be seen from Tables 1 and 2. Experiment No. In Nos. 1 to 11, thick steel plates were produced that satisfied the chemical composition specified in the present invention and under the specified conditions, so that the obtained thick steel plates had an excellent strength-toughness balance throughout the inside of the steel plate, even if the plate thickness was large. There is. In particular, it exhibits high strength and superior toughness at low temperatures than conventional products. On the other hand, experiment no. In Nos. 12 to 17, at least one of the component composition and manufacturing conditions was not within the range specified by the present invention, so the obtained thick steel plates had poor strength-toughness balance.

Experiment No. Although the composition of No. 12 is within the scope of the present invention, the AF structure was not present at both the t/4 and t/2 positions due to insufficient cumulative rolling reduction in the temperature range of 850°C or lower in the manufacturing conditions. This resulted in an inferior strength-toughness balance.

Experiment no. Although the component compositions of Nos. 13 and 14 were within the scope of the present invention, the AF structure was insufficient at both t/4 and t/2 positions due to the high finish rolling temperature in the manufacturing conditions, resulting in poor strength-toughness. The result was an imbalance.

Experiment No. In No. 15, Di+10Nb was below the specified range, resulting in poor strength-toughness balance. This experiment no. In No. 15, it is considered that the above Di+10Nb was below the specified range, and thus a pearlite structure was generated together with grain boundary ferrite. As a result, it is thought that the AF structure was insufficient and the strength-toughness balance was poor.

Experiment No. In Nos. 16 and 17, Di+10Nb exceeded the specified range, resulting in poor strength-toughness balance. This experiment no. In Nos. 16 and 17, it is considered that hard martensitic structures were generated because the above Di+10Nb exceeded the specified range. As a result, it is thought that this hard martensitic structure acted as a brittle fracture origin, resulting in poor strength-toughness balance.

Experiment No. Although the composition of No. 18 is within the scope of the present invention, in the manufacturing conditions, the heating temperature before hot rolling is low and the finish rolling temperature is high, so it was not possible to secure a sufficient AF structure, and the strength was - The result was poor toughness balance.

Experiment No. Although the composition of No. 19 is within the scope of the present invention, in the manufacturing conditions, the heating temperature before hot rolling is high, so it is not possible to secure a fine AF structure above a certain level, and the strength-toughness balance is poor. This was the result.

Experiment No. In No. 20, the amount of B is 0.0008%, which exceeds the upper limit of 0.0007%, so hard martensite is generated and SA at the t/4 position and t/2 position is insufficient, resulting in poor characteristics. became worse.

Experiment No. In No. 21, although the component composition was within the scope of the present invention, the average rolling reduction of the final three passes during hot rolling was too low under the manufacturing conditions, making it impossible to secure a certain level of fine AF structure. However, the strength-toughness balance was poor.

Experiment No. In No. 22, the amount of Nb is insufficient, and under the manufacturing conditions, the cumulative reduction rate in the temperature range of 850 ° C or less and the average reduction rate of the final three passes of rolling are also low, so a fine AF structure must be secured above a certain level. This resulted in an inferior strength-toughness balance.

Figure 1 shows the total area fraction SA of crystal grains with an equivalent circle diameter of 7.5 μm or less among crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15° or more, created based on the above example. , is a graph showing the relationship between Y values. Note that the downward arrow in Figure 1 means that the measured vTrs was lower than -130°C, so the Y value is estimated to be lower than the plotted value, and the upward arrow indicates that the Y value is estimated to be lower than the plotted value. , the measured vTrs was higher than −30° C., meaning that the Y value is assumed to be higher than the plotted value.

From FIG. 1, there is a correlation between the total area fraction SA and the Y value at both the t/4 position and the t/2 position on the steel plate, and at the t/4 position, the Y value is -5200. In order to make the Y value less than -4700 at the t/2 position, the total area fraction SA must be 27% or more. It turns out that it is necessary to do this.

Claims (5)

The ingredient composition is
C: 0.02% by mass to 0.070% by mass,
Si: more than 0 mass%, 0.40 mass% or less,
Mn: 1.30% by mass to 1.80 % by mass,
P: more than 0 mass%, 0.015 mass% or less,
S: more than 0% by mass, 0.005% by mass or less,
Al: 0.005% by mass to 0.070% by mass,
Nb: 0.015% by mass to 0.048% by mass,
Ti: 0.005% by mass to 0.024% by mass,
N: 0.0030% by mass to 0.0080% by mass, and Ca: 0.0008% by mass or more and 0.0040% by mass or less,
Furthermore, it contains one or more elements selected from the group consisting of Cu: more than 0 mass% and 0.75 mass% or less, and Ni: more than 0 mass% and 1.4 mass% or less,
The remainder consists of Fe and unavoidable impurities,
Satisfies Di+10Nb: 1.20 to 2.50 obtained from the following formula (1),
Among the crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15° or more, the total area fraction SA of crystal grains with an equivalent circle diameter of 7.5 μm or less is 34% or more at the 1/4 position of the plate thickness, and A thick steel plate with a thickness of 27% or more at the 1/2 position.
Di=1.16×([C]/10) ^0.5 ×(0.7×[Si]+1)×(5.1×([Mn]-1.2)+5)×(0.35× [Cu]+1)×(0.36×[Ni]+1)×(2.16×[Cr]+1)×(3×[Mo]+1)×(1.75×[V]+1)×(200 ×[B]+1)...(1)
In formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are each expressed in mass%. The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are shown, and elements not included are set to zero. Furthermore,
Mo: more than 0 mass%, 0.50 mass% or less,
V: more than 0 mass%, 0.060 mass% or less,
The thick steel plate according to claim 1, containing one or more elements selected from the group consisting of Cr: more than 0 mass% and 0.8 mass% or less, and B: more than 0 mass% and 0.0007 mass% or less. . Furthermore,
REM: more than 0 mass%, 0.0060 mass% or less, and Zr: more than 0 mass%, 0.0050 mass% or less, containing one or more elements selected from the group consisting of Thick steel plate. The thick steel plate according to any one of claims 1 to 3, having a plate thickness of more than 40 mm. A method for manufacturing a thick steel plate according to any one of claims 1 to 4, comprising:
A step of heating a steel piece having the composition according to any one of claims 1 to 4 to a temperature higher than 1020°C and lower than 1200°C, and a hot rolling step after the heating,
In the hot rolling process, the number of rolling passes is 3 or more, and manufacturing of a thick steel plate is performed by hot rolling and cooling after the hot rolling so as to satisfy all of the following conditions (a) to (d). Method.
(a) Cumulative rolling reduction of 40% or more in the temperature range of 850°C or less (b) Average rolling reduction of 5.5% or more in the final three passes of rolling (c) Finish rolling temperature of 720 to 830°C
(d) After hot rolling, it is cooled from a cooling start temperature of the finish rolling temperature to 690°C to a cooling stop temperature of 320 to 550°C at an average cooling rate of 0.5 to 20°C/s.

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