JP2020176329A - Thick steel plate and method for manufacturing the same - Google Patents

Abstract

To provide a thick steel plate exhibiting excellent strength-toughness balance over the inside of the steel plate even when a plate thickness is thick, especially, showing high strength and more excellent toughness at low temperature than before, and a method for manufacturing the same.SOLUTION: The thick steel plate satisfies a predetermined component composition and satisfies Di calculated from the following formula (1)+10 Nb:1.20-2.50. In crystal grains surrounded by a large angle grain boundary having a crystal orientation difference of 15° or more, the total area fraction SA of crystal grains having a circle equivalent diameter of 7.5 μm or less is 34% or more at a t/4 position and is 27% or more at a t/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).SELECTED DRAWING: Figure 1

Description

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

For example, with the increase in size of LPG tanks and the like, there is an increasing demand for thick steel sheets having high strength, low temperature toughness of base material, and low temperature toughness of HAZ.

As a high-strength steel having excellent HAZ toughness, for example, Patent Document 1 states that the C concentration of the average chemical analysis value of the central segregated portion of the steel sheet satisfies a predetermined component and is 1.2 times or less the average C concentration of the steel material. The cleanliness of inclusions measured by JIS standard is 0.03% or less, and the number of oxide-based inclusions with an average diameter of 10 μm or more observed on the cross section of the steel sheet is 1 or less / 1 mm ² , 0.05 to 5 μm. Steels in which the number of oxide and nitride precipitates is 100 or more / 1 mm ² are shown.

In Patent Document 2, a predetermined chemical component is satisfied, an acicular ferrite structure fraction is 50% or more, and an island-shaped martensite (MA) structure fraction having an average circle equivalent diameter of 1 to 5 μm is 3 to 10%. By satisfying the requirements, a low yield ratio high-strength steel plate having excellent low-temperature toughness of the base material and low-temperature toughness of HAZ is obtained.

In Patent Document 3, satisfies a predetermined component composition, microstructure is bainite, the steel is disclosed is the yield strength 500 N / mm ² or more and a tensile strength of 610N / mm ² or more. Further, it has been shown that the steel material does not require annealing heat treatment for removing residual stress after welding and is suitable for manufacturing a tank for an LPG / ammonia carrier.

In Patent Document 4, 9 × Ceq + 4 × P ≧ 4.8 and [C] / ([Mo] + [Ti] + [Nb] + [V]), which satisfy a predetermined component composition and are parameters, are Steel of 0.6 to 1.7 is heated to a temperature of 1100 to 1300 ° C., hot-rolled at a rolling end temperature of 750 ° C. or higher, and then to a temperature of less than 400 ° C. at a cooling rate of 20 ° C./s or higher. A method for producing a high-strength steel plate having excellent SR (Stress Relief) characteristics, which comprises accelerating cooling and then immediately reheating to 550 to 700 ° C. at a heating rate of 0.5 ° C./s or higher. It is shown.

Japanese Unexamined Patent Publication No. 8-158006 JP-A-2009-127065 Japanese Unexamined Patent Publication No. 2008-025014 Japanese Unexamined Patent Publication No. 2007-270194

In Patent Document 1, although a good balance between strength and toughness is obtained, only a plate thickness of 40 mm or less is considered in the examples, and a technique considering a thicker steel plate has not been proposed. Patent Document 2 attempts to achieve both low-temperature toughness and strength of the base material and HAZ, but the low-temperature toughness is evaluated at -60 ° C, and further studies are made to realize excellent toughness at lower temperatures. I think it is necessary. In Patent Document 3, the mechanical properties are evaluated only at the plate thickness t / 4 position, and the mechanical properties inside the steel sheet are not taken into consideration. Patent Document 4 discloses a method for producing a thick steel sheet having good mechanical properties even after SR, but the evaluation temperature of toughness is only -10 ° C, and toughness at a lower temperature has not been examined. .. The present invention has been made in view of the above circumstances, and an object of the present invention is to exhibit an excellent strength-toughness balance over the inside of a steel sheet even if the sheet thickness is thick, especially at high strength and lower temperature than before. It is an object of the present invention to provide a thick steel sheet exhibiting excellent toughness, and a method for producing the thick steel sheet.

Aspect 1 of the present invention is
Ingredient composition
C: 0.02% by mass to 0.070% by mass,
Si: More than 0% by mass, 0.40% by mass or less,
Mn: 1.30% by mass to 1.95% by mass,
P: More than 0% by mass, 0.015% by 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, 0.0040% by mass or less, and the balance is composed of Fe and unavoidable impurities.
Di + 10Nb: 1.20 to 2.50 obtained from the following formula (1) is satisfied.
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 the crystal grains having a circle-equivalent 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 that is 27% or more at the position of 1/2 of the plate thickness.
Di = 1.16 x ([C] / 10) ^0.5 x (0.7 x [Si] + 1) x (5.1 x ([Mn] -1.2) + 5) x (0.35 x) [Cu] +1) x (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] + 1) x (1.75 x [V] +1) x (200 × [B] +1) ・ ・ ・ (1)
In the formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are represented by mass%, respectively. The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B are shown, and the elements not contained are set to zero.

Aspect 2 of the present invention further comprises.
The thick steel sheet according to embodiment 1, which contains one or more elements selected from the group consisting of Cu: more than 0% by mass, 0.75% by mass or less, and Ni: more than 0% by mass, 1.4% by mass or less. is there.

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

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

Aspect 5 of the present invention
A method for producing a thick steel sheet according to any one of aspects 1 to 4.
A step of heating a steel piece having the component composition according to any one of aspects 1 to 4 to above 1020 ° C. and below 1200 ° C. and a hot rolling step after the heating are included.
In the hot rolling step, a thick steel sheet is manufactured by hot rolling and cooling after the hot rolling so that the number of rolling passes is 3 or more and all of the following conditions (a) to (d) are satisfied. Method.
(A) Cumulative rolling reduction rate in the temperature range of 850 ° C. or lower is 40% or more (b) Average rolling reduction rate of rolling in the final 3 passes is 5.5% or higher (c) Finishing rolling temperature is 720 to 830 ° C.
(D) After hot rolling, the temperature from the finishing rolling temperature to 690 ° C. to the cooling stop temperature of 320 to 550 ° C. is cooled at an average cooling rate of 0.5 to 20 ° C./s.

According to the present invention, a thick steel sheet exhibiting an excellent strength-toughness balance throughout the inside of the steel sheet even if the sheet thickness is thick, particularly a thick steel sheet exhibiting high strength and excellent toughness at a lower temperature than before, and a method for producing the same. Can be provided.

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

In the present invention, intensive research has been carried out in order to obtain a thick steel sheet having an improved balance between strength and toughness at a lower temperature than before in the state before SR, that is, in the state of hot rolling. In particular, even if the plate thickness is thick, the strength-toughness balance is excellent at the 1/4 position and the 1/2 position of the plate thickness. Specifically, the parameter Y = 20 × vTrs- regarding the strength-toughness balance defined in the present invention. 7 × YP conducted diligent research to obtain a sufficiently low steel sheet.

As a result, the composition of the components is controlled, and a predetermined amount of a fine acicular ferrite structure is secured at the 1/4 and 1/2 positions of the plate thickness. More specifically, the equivalent of a circle measured by the method described later. 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. In the following, the 1/4 position of the plate thickness may be referred to as “t / 4 position”, and the 1/2 position of the plate thickness may be referred to as “t / 2 position”.

Further, it has been found that it is effective to carry out all of the following (A) to (C) in order to form a predetermined amount of the 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 fine structure is realized by forming AF crystal grains around the dislocation structure and deformation zone introduced by this processing.
(B) A solid solution Nb is secured before rolling in the unrecrystallized region while hot. By doing so, it becomes easy to obtain processing strain prior to transformation, and as described above, fine AF crystal grains are likely to be generated. As will be described later, it is effective to set the pre-rolling heating temperature to more than 1020 ° C. and reduce the rolling reduction rate in rolling at 850 ° C. or higher to secure the solid solution Nb.
(C) The transformation temperature to the ferrite phase is appropriately controlled. When the transformation temperature to the ferrite phase is high, a grain boundary ferrite structure is formed prior to AF formation, and the amount of AF decreases. On the contrary, when the transformation temperature to the ferrite phase is low, a martensite structure is formed without forming an AF structure. In order to appropriately control the transformation temperature to the ferrite phase, when at least one of C, Mn, Cu and Ni in the component composition is contained, the content thereof and each range of Di + 10Nb are controlled, and will be described later. As you can see, the average cooling rate in the predetermined temperature range after hot rolling is preferably 0.5 ° C./s or more.

Hereinafter, the steel structure, composition, characteristics, and manufacturing method of the thick steel sheet of the present invention will be described in order.

1. 1. Steel structure The steel structure of the thick steel sheet of the present invention will be described in detail below. In the following description of the steel structure, a mechanism capable of improving various properties by having such a structure may be described. It should be noted that these are the mechanisms considered by the present inventors based on the findings obtained at present, but do not limit the technical scope of the present invention.

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

In the present invention, in order to obtain a thick steel sheet showing an excellent strength-toughness balance even if the plate thickness is thick, among the crystal grains surrounded by large grain boundaries with a crystal orientation difference of 15 ° or more, the equivalent circle diameter is 7.5 μm or less. The total area division SA of the crystal grains, that is, the area division of the fine acicular ferrite (AF) structure is defined at both the t / 4 position and the t / 2 position. Specifically, the total surface integral SA is set to satisfy 34% or more at the t / 4 position and 27% or more at the t / 2 position. The total surface integral SA at the t / 4 position is preferably 35% or more, more preferably 36% or more. The total surface integral SA at the t / 2 position is preferably 28% or more, more preferably 30% or more. From the viewpoint of obtaining an excellent strength-toughness balance, the upper limit of the total surface integral SA at each of the t / 4 position and the t / 2 position is not particularly limited. Considering the manufacturing conditions of the thick steel sheet 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 the structure other than the fine acicular ferrite include bainite, ferrite, cementite, retained austenite, martensite and the like. As long as the total surface integral SA is within the range specified in the present invention, acicular ferrite having a diameter equivalent to a circle of more than 7.5 μm may be present.

2. Composition The composition of the thick steel sheet according to the present invention will be described 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 exerting the 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 martensite structure is formed and acts as a brittle fracture starting point, so that the strength-toughness balance deteriorates. 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 martensite structure is formed and acts as a brittle fracture starting point, so that the strength-toughness balance deteriorates. Therefore, the amount of Si is 0.40% by mass or less, preferably 0.38% by mass or less, and 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 exerting the 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 martensite structure is formed and acts as a brittle fracture starting point, so that the strength-toughness balance deteriorates. Therefore, the amount of Mn is 1.95% by mass or less, preferably 1.90% by mass or less, and 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 contained in excess, the grain boundaries become brittle and the strength-toughness balance deteriorates. 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 set the P amount to 0% by mass, the lower limit of the P amount is more than 0% by mass.

S: More than 0% by mass, 0.005% by mass or less S is an impurity element, and if it is contained in excess, the 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 set the S amount to 0% by mass, the lower limit of the S amount is more than 0% by mass.

Al: 0.005% by mass to 0.070% by mass
Al is a deoxidizing element. The amount of Al is set to 0.005% by mass or more in order to reduce oxygen in the steel by performing sufficient deoxidation and suppress deterioration of the strength-toughness balance due to oxides. 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, a coarse oxide is 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, and 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 formation of AF. In order to sufficiently generate a fine AF structure and obtain a good strength-toughness balance, the amount of Nb is 0.015% by mass or more, preferably 0.016% by mass or more, and more preferably 0.018% by mass or more. To do. On the other hand, if the amount of Nb is excessive, a hard martensite structure is formed, and this structure acts as a brittle fracture starting point, so that the strength-toughness balance deteriorates. 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 the improvement of HAZ toughness by forming TiN. From the viewpoint of exerting the 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 is generated and the strength-toughness balance deteriorates. Therefore, the amount of Ti is 0.024% by mass or less, preferably 0.022% by mass or less, and 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 the improvement of HAZ toughness by forming TiN. From the viewpoint of exerting the 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, the 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, 0.0040% by mass or less Ca is a deoxidizing element, and its content is more than 0% by mass. Further, when the amount of Mn in the steel is large, it is considered that coarse MnS is likely to be generated at the t / 2 position due to Mn enrichment during casting, and 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 more than 0% by mass, more preferably 0.0008% by mass or more, and further 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, and more preferably 0.0025% by mass or less.

The balance is Fe and unavoidable impurities. As unavoidable impurities, for example, trace elements such as As, Sb, and Sn that are brought in depending on the conditions of raw materials, materials, manufacturing equipment, etc. are allowed to be mixed. It should be noted that, for example, there are elements such as P and S, which are usually preferable as the content is smaller, and therefore are unavoidable impurities, but the composition range thereof is separately specified as described above. Therefore, in the present specification, the "unavoidable impurities" constituting the balance is a concept excluding the elements whose composition range is separately defined.

The thick steel sheet of the present invention may contain the above elements in the component composition. The selective elements described below may not be contained, but high strength and the like can be more easily achieved by containing them together with the above elements as needed. In addition, the desired structure can be more easily secured, and the strength-toughness balance required in 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% by mass, 0.75% by mass or less, and Ni: more than 0% by mass, 1.4% by mass or less These elements have appropriate ferrite transformation temperatures. It 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 exerting the 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, still 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, still more preferably 0.20% by mass or more. .. On the other hand, when these elements are excessive, a hard martensite structure is formed, and the structure acts as a brittle fracture starting point, resulting in 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, and further 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, and further preferably 1.0% by mass or less.

Mo: more than 0% by mass, 0.50% by mass or less, V: more than 0% by mass, 0.060% by mass or less, Cr: more than 0% by mass, 0.8% by mass or less, and B: more than 0% by mass, One or more elements selected from the group consisting of 0.0030% by mass or less These elements are effective elements for improving the strength. From the viewpoint of exerting the effect, when Mo is contained, it is preferably more than 0% by mass, more preferably 0.05% by mass or more, and further 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, and further 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, and further 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 martensite structure is formed, and the structure acts as a brittle fracture starting point, resulting in 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, and further 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, and further 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% by mass, 0.0060% by mass or less, and Zr: more than 0% by mass, 0.0050% by mass or less These elements are deoxidizing elements. In order to exert the effect, when REM is contained, it is preferably more than 0% by mass, more preferably 0.0010% by mass or more, and further 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 further preferably 0.0012% by mass or more. On the other hand, if these elements are excessive, coarse oxides are formed and the strength-toughness balance deteriorates. Therefore, the amount of REM is preferably 0.0060% by mass or less, more preferably 0.0050% by mass or less, and further 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, and further preferably 0.0040% by mass or less. The REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium).

Di + 10Nb: 1.20 to 2.50 (Di is calculated from the following formula (1))
Di = 1.16 x ([C] / 10) ^0.5 x (0.7 x [Si] + 1) x (5.1 x ([Mn] -1.2) + 5) x (0.35 x) [Cu] +1) x (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] + 1) x (1.75 x [V] +1) x (200 × [B] +1) ・ ・ ・ (1)
In the formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are represented by mass%, respectively. The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B are shown, and the elements not contained 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 structure and martensite structure other than AF, and promote AF formation. .. From these viewpoints, in the present invention, Di + 10Nb was set within the range of 1.20 to 2.50. If the value of Di + 10Nb is too small, a pearlite structure is formed 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 martensite structure is formed and acts as a brittle fracture starting point, so that the strength-toughness balance deteriorates. 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, and particularly preferably more than 40 mm. 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, for example, a large LPG tank, a ship, or the like.

3. 3. Characteristics In the present invention, the Y value, which is a parameter represented by the following equation (2), is used to evaluate the balance between strength and toughness at a lower temperature than before. The Y value includes the yield strength YP and the brittle ductile transition temperature vTrs as shown in the following formula (2). As the yield strength YP in the following formula (2), 0.2% proof stress (0.2YS) is used when the SS curve (also referred to as “stress-strain diagram”) is a round type whose yield point is not clear. However, if it has a yield point, YP is used.

The thick steel sheet of the present invention remains hot-rolled and satisfies Y at the t / 4 position of less than -5200 and Y at the t / 2 position of 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. is there. The lower these values are, the better the strength-toughness balance is.

The present invention may achieve the above parameters as characteristics. For each of the yield strength YP and the brittle ductility 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 ductility transition temperature vTrs is preferably less than −70 ° C. The range, more preferably the range of less than -80 ° C. Further, for each measurement position, on the premise that the above parameters are achieved at the t / 4 position, 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, still more preferably. Is 480 MPa or more, particularly 500 MPa or more, and the brittle ductile transition temperature vTrs is preferably −90 ° C. or lower, more preferably −95 ° C. or lower, and further 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, further preferably 460 MPa or more, still more preferably 480 MPa or more. In particular, it is 500 MPa or more, and the brittle ductile transition temperature vTrs is preferably −80 ° C. or lower, more preferably −90 ° C. or lower, further preferably −95 ° C. or lower, and more preferably −100 ° C. or lower. Is even more preferable.

4. Manufacturing Method The method for manufacturing a thick steel sheet according to the present invention includes a step of heating a steel piece having the above-mentioned composition to a temperature of more than 1020 ° C. and a temperature of less than 1200 ° C., and a hot rolling step after the heating. The rolling step is a method for manufacturing a thick steel sheet in which hot rolling and cooling after the hot rolling are performed so that the number of rolling passes is 3 or more and all of the following conditions (a) to (d) are satisfied.
(A) Cumulative rolling reduction rate in the temperature range of 850 ° C. or lower is 40% or more (b) Average rolling reduction rate of rolling in the final 3 passes is 5.5% or higher (c) Finishing rolling temperature is 720 to 830 ° C.
(D) After hot rolling, the temperature from the finishing rolling temperature to 690 ° C. to the cooling stop temperature of 320 to 550 ° C. is cooled at an average cooling rate of 0.5 to 20 ° C./s.
Hereinafter, each manufacturing condition will be described in detail.

[Step of heating a steel piece having the above component composition to more than 1020 ° C and less than 1200 ° C]
In the heating of hot rolling, if the heating temperature is 1020 ° C. or lower, the NbC generated during casting is not sufficiently solid-dissolved, and the effect of promoting AF generation by the solid-dissolved Nb cannot be obtained. Therefore, the heating temperature is more than 1020 ° C., preferably 1040 ° C. or higher, more preferably 1050 ° C. or higher, still more preferably 1060 ° C. or higher. On the other hand, when the heating temperature is 1200 ° C. or higher, the austenite grains are coarsened and the structure is generally coarsened. Therefore, the heating temperature is less than 1200 ° C., preferably 1180 ° C. or lower, and more preferably 1150 ° C. or lower.

[Hot rolling process after heating]
As will be described later, the present invention is characterized in that the average rolling reduction rate of rolling in the final three passes is controlled. After the heating, hot rolling with three or more rolling passes is performed, but the total number of passes (rolling). The number of passes) does not affect or limit the organization and characteristics. The rolling pass may be further 7 passes or more, further 10 passes or more, and 60 passes 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 reduction rate in the temperature range of 850 ° C or lower: 40% or more In order to introduce sufficient machining strain into the austenite phase for the purpose of obtaining a sufficient AF structure, it is 850 ° C or lower in hot rolling. The cumulative rolling reduction rate in the temperature range (also referred to as "total rolling reduction rate") needs to be 40% or more. By the way, when the rolling reduction rate in rolling at 850 ° C. or higher increases, it is considered that NbC is precipitated during rolling and the solid solution Nb decreases. From the viewpoint of suppressing the cumulative rolling reduction rate in rolling at 850 ° C. or higher, the cumulative rolling reduction rate in the temperature range of 850 ° C. or lower is set to 40% or higher. 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 reduction rate of rolling in the final 3 passes: 5.5% or more The dislocation structure introduced under the reduction in the final 3 passes shifts to the cooling process with relatively little progress in recovery, and thus has a large AF promoting effect. .. In the present invention, the average rolling reduction in the final 3-pass rolling is set to 5.5% or more in order to introduce sufficient processing strain into the austenite phase for the introduction of the dislocation structure. In the present invention, by controlling the average rolling reduction of rolling in the final 3 passes, a desired structure having an AF structure of a certain level or more can be obtained inside the steel sheet, particularly at the t / 2 position, and rolling in the final 3 passes. It is different from the conventional method in which the average rolling reduction rate of is not controlled. The average reduction rate 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 rate is about 20%.

(C) Finishing Rolling Temperature (FRT): 720 to 830 ° C.
When the temperature of the steel material exceeds 850 ° C., sufficient processing strain is not introduced into the austenite phase even if the final three-pass rolling is performed at the above average rolling ratio, and the AF structure amount is insufficient. In the present invention, the finish rolling temperature is set to 830 ° C. or lower in order to secure a sufficient amount of AF structure. The finish rolling temperature is preferably 820 ° C. or lower, more preferably 810 ° C. or lower. On the other hand, when 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 720 ° C. or higher, preferably 750 ° C. or higher, and more preferably 760 ° C. or higher.

(D) After hot rolling, the temperature from the finishing rolling temperature to 690 ° C. to the cooling stop temperature of 320 to 550 ° C. is cooled 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, the AF structure can be sufficiently secured. If the average cooling rate exceeds 20 ° C./s, martensitic transformation occurs without sufficient AF structure being formed, which is not preferable. The average cooling rate is preferably 15 ° C./s or less, more preferably 12 ° C./s or less. As described above, in order to appropriately control the transformation temperature to the ferrite phase and sufficiently secure the AF structure, when at least one of C, Mn, Cu and Ni in the component composition is contained, these contents are used. And each range of Di + 10Nb is controlled, and the average cooling rate is set to 0.5 ° C./s or more. When the average cooling rate is less than 0.5 ° C./s, coarse grain boundary ferrite is generated during cooling, and the amount of AF structure is insufficient. The average cooling rate is preferably 2.0 ° C./s or higher, more preferably 3.0 ° C./s or higher. As a cooling method of the average cooling rate, for example, water cooling can be mentioned.

The cooling start temperature is an arbitrary temperature in the finishing rolling temperature to 690 ° C. If the temperature of the steel material is lower than 710 ° C., problems such as grain boundary ferrite being generated before the start of cooling and processing strain introduced into the austenite phase are recovered. As a result, the amount of AF tissue is insufficient. In the present invention, in order to secure a sufficient amount of AF structure, the starting cooling temperature (SCT) at the 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 finish cooling temperature (FCT) is an arbitrary temperature of 320 to 550 ° C. When cooling at the above average cooling rate, for example, water cooling is stopped in a temperature range of more than 550 ° C., grain boundary ferrite is generated during slow cooling after the water cooling is stopped, and it becomes difficult to sufficiently secure the AF structure. Therefore, the end temperature is set to 550 ° C. or lower. The end temperature is preferably 500 ° C. or lower, more preferably 480 ° C. or lower. On the other hand, when cooling at the above average cooling rate is performed up to a temperature range below, for example, 320 ° C., martensitic transformation occurs without sufficiently forming 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 described above, and may be carried out under normally performed conditions.

A person skilled in the art who has come into contact with the method for producing a high-strength steel sheet according to the embodiment of the present invention described above can obtain the high-strength steel sheet according to the present invention by a production method different from the above-mentioned production method by trial and error. There is a possibility that it can be done.

Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited by the following examples, and can be carried out with appropriate modifications within a range that can meet the above-mentioned and later-described intent, and all of them are the technical scope of the present invention. Included in.

1. 1. Sample preparation The steel having the composition shown in Table 1 was melted in a 150 kg VIF (Vacuum Induction Furnace) or an actual furnace, 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 (the number of rolling passes) was set to more than 20 times. In Table 1, "-" indicates that it was not intentionally added. In Table 2, FRT indicates the finishing rolling temperature, SCT indicates the cooling start temperature, and FCT indicates the cooling end temperature. The cooling stop temperature FCT was measured at 1 to 3 points on the surface of the steel sheet in the longitudinal direction with a radiation thermometer, and the average value was calculated. The FRT and SCT were obtained by measuring one point on the surface of the steel material with a radiation thermometer.

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

The obtained EBSD data was analyzed by analysis software OIM Analysis manufactured by TSL Solutions Co., Ltd. In the obtained data, points having a Confidence Index of 0.100 or less were removed, and grain boundaries having a crystal orientation of 15 ° or more with adjacent pixels were defined as large-angle grain boundaries. Among the units surrounded by the large-angle grain boundaries, the units having a pixel size of 10 or more were regarded as large-angle grains. In addition, large-angle grains on the edge of the measurement field of view were excluded from the analysis. The average circle equivalent diameter of the large square grains was obtained, and the total surface integral SA of the large square grains having the average circle equivalent diameter of 7.5 μm or less in the total area of the large square grains was calculated.

3. 3. Mechanical properties (yield strength YS)
At the t / 4 and t / 2 positions of the hot-rolled steel sheet, an ASTM round bar tensile test piece is taken in parallel with the plate width direction (C direction), and a tensile test is performed in the manner of ASTM to yield. The intensity YS was measured.
(Low temperature toughness of base material)
At the t / 4 and t / 2 positions of the hot-rolled steel sheet, V-notch Charpy test pieces were collected in parallel with the plate width direction (C direction), and a Charpy impact test was carried out in the manner of ASTM. Then, the temperature vTrs at which the brittle fracture surface ratio was 50% was evaluated.

The yield strength YP and the brittle ductility transition temperature vTrs obtained by the above measurement were substituted into the following equation (2), and the Y values at the t / 4 position and the t / 2 position were obtained. The results are also shown in Table 2. In Table 2, when vTrs> −30 ° C., vTrs = −30 ° C. was calculated when calculating the Y value. When vTrs <−130 ° C., vTrs = −130 ° C. was calculated when calculating the Y value.
Y = 20 × vTrs-7 × YP ... (2)

The following can be seen from Tables 1 and 2. Experiment No. Since 1 to 11 satisfy the component composition specified in the present invention and the thick steel sheet is manufactured under the specified conditions, the obtained thick steel sheet has an excellent strength-toughness balance over the inside of the steel sheet even if the sheet thickness is thick. There is. In particular, it exhibits high strength and is superior in toughness at low temperatures than before. On the other hand, Experiment No. Since at least one of the component composition and the production conditions of 12 to 17 is not within the range specified in the present invention, the obtained thick steel sheet has a poor strength-toughness balance.

Experiment No. In No. 12, although the component composition was within the range of the present invention, the cumulative reduction rate in the temperature range of 850 ° C. or lower was insufficient under the manufacturing conditions, so that the AF structure was formed at both the t / 4 and t / 2 positions. Insufficient, resulting in poor strength-toughness balance.

Experiment No. Although the composition of components 13 and 14 was within the range of the present invention, the AF structure was insufficient at both t / 4 and t / 2 positions due to the high finishing rolling temperature under the manufacturing conditions, and the strength-toughness was increased. The result was inferior in balance.

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

Experiment No. In 16 and 17, Di + 10Nb exceeded the specified range, resulting in poor strength-toughness balance. This experiment No. In 16 and 17, it is considered that a hard martensite structure was formed because the above Di + 10Nb exceeded the specified range. As a result, it is considered that the strength-toughness balance is inferior because this hard martensite structure acts as a brittle fracture starting point.

Experiment No. No. 18 has a component composition within the range of the present invention, but under the manufacturing conditions, the heating temperature before hot rolling is low and the finish rolling temperature is high, so that a sufficient AF structure cannot be secured and the strength is increased. -The result was poor toughness balance.

Experiment No. No. 19 has a component composition within the range of the present invention, but under the manufacturing conditions, the heating temperature before hot rolling is high, so that a fine AF structure cannot be secured above a certain level, and the strength-toughness balance is inferior. The result was.

Experiment No. In No. 20, the amount of B is 0.0008%, which exceeds the upper limit of 0.0007%. Therefore, hard martensite is generated, and SA at the t / 4 position and the t / 2 position is insufficient, which is a characteristic. Has gotten worse.

Experiment No. In No. 21, although the component composition was within the range of the present invention, the average rolling reduction of the final three-pass rolling during hot rolling was too low under the manufacturing conditions, so that a fine AF structure could be secured above a certain level. However, the result was that the strength-toughness balance was inferior.

Experiment No. In No. 22, the amount of Nb is insufficient, and the cumulative reduction rate in the temperature range of 850 ° C. or lower and the average reduction rate of rolling in the final 3 passes are also low under the manufacturing conditions, so that a fine AF structure is secured above a certain level. The result was that the strength-toughness balance was inferior.

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

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 sheet, and the Y value is set to −5200 at the t / 4 position. To make it less than, the total area fraction SA must be 34% or more, and 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.

Claims (5)

Ingredient composition
C: 0.02% by mass to 0.070% by mass,
Si: More than 0% by mass, 0.40% by mass or less,
Mn: 1.30% by mass to 1.95% by mass,
P: More than 0% by mass, 0.015% by 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, 0.0040% by mass or less, and the balance is composed of Fe and unavoidable impurities.
Di + 10Nb: 1.20 to 2.50 obtained from the following formula (1) is satisfied.
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 the crystal grains having a circle-equivalent 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 that is 27% or more at 1/2 of the plate thickness.
Di = 1.16 x ([C] / 10) ^0.5 x (0.7 x [Si] + 1) x (5.1 x ([Mn] -1.2) + 5) x (0.35 x) [Cu] +1) x (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] + 1) x (1.75 x [V] +1) x (200 × [B] +1) ・ ・ ・ (1)
In the formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are represented by mass%, respectively. The contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B are shown, and the elements not contained are set to zero. In addition
The thick steel plate according to claim 1, which contains one or more elements selected from the group consisting of Cu: more than 0% by mass and 0.75% by mass or less, and Ni: more than 0% by mass and 1.4% by mass or less. .. In addition
Mo: More than 0% by mass, 0.50% by mass or less,
V: More than 0% by mass, 0.060% by mass or less,
The invention according to claim 1 or 2, which comprises one or more elements selected from the group consisting of Cr: more than 0% by mass and 0.8% by mass or less, and B: more than 0% by mass and 0.0007% by mass or less. Thick steel plate. In addition
Any of claims 1 to 3 containing one or more elements selected from the group consisting of REM: more than 0% by mass, 0.0060% by mass or less, and Zr: more than 0% by mass, 0.0050% by mass or less. The thick steel plate described in. The method for producing a thick steel sheet according to any one of claims 1 to 4.
A step of heating a steel piece having the component composition according to any one of claims 1 to 4 to more than 1020 ° C. and lower than 1200 ° C. and a hot rolling step after the heating are included.
In the hot rolling step, a thick steel sheet is manufactured by hot rolling and cooling after the hot rolling so that the number of rolling passes is 3 or more and all of the following conditions (a) to (d) are satisfied. Method.
(A) Cumulative rolling reduction rate in the temperature range of 850 ° C. or lower is 40% or more (b) Average rolling reduction rate of rolling in the final 3 passes is 5.5% or higher (c) Finishing rolling temperature is 720 to 830 ° C.
(D) After hot rolling, the temperature from the finishing rolling temperature to 690 ° C. to the cooling stop temperature of 320 to 550 ° C. is cooled at an average cooling rate of 0.5 to 20 ° C./s.

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