KR102684370B1 - Heavy steel plate and manufacturing method thereof - Google Patents

Abstract

The component composition satisfies the predetermined C, Si, Mn, P, S, Al, Nb, Ti, N, and Ca, and the remainder consists of Fe and inevitable impurities, and Di+ is obtained from the formula (1) below. 10Nb: Satisfies 1.20 to 2.50, and among crystal grains surrounded by diagonal grain boundaries with a crystal orientation difference of 15° or more, the total area fraction SA of grains with an equivalent circular diameter of 7.5 μm or less is 34% or more at 1/4 of the plate thickness, Also, thick steel plate with more than 27% thickness at 1/2 of the plate thickness. 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 equation (1), [element] represents the content of each element expressed in mass%, and elements not included are set to zero.

Description

Heavy steel plate and manufacturing method thereof

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

For example, with the increase in the size of LPG tanks, etc., the demand for thick steel plates that has high strength and has both the low-temperature toughness of the base material and the low-temperature toughness of the HAZ is increasing.

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

In Patent Document 2, the predetermined chemical composition is satisfied, the acular ferrite structure fraction is 50% or more, and the island-like martensite (MA) structure fraction of 1 to 5 μm in average equivalent circle diameter is 3 to 10%. By satisfying this, a resistance compound ratio high-strength steel sheet with excellent base material low-temperature toughness and HAZ low-temperature toughness is obtained.

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

In Patent Document 4, the predetermined component composition is satisfied, and the parameters 9 Steel with -1.7 is heated to a temperature of 1100-1300°C, hot-rolled at a rolling end temperature of 750°C or higher, then accelerated cooling is performed at a cooling rate of 20°C/s or higher to a temperature below 400°C, and immediately thereafter. A method of manufacturing a high-strength steel sheet with excellent SR (Stress Relief) properties is shown, which involves reheating to 550-700°C at a temperature increase rate of 0.5°C/s or more.

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

In Patent Document 1, a good balance of strength and toughness is obtained, but in the examples, only a plate thickness of 40 mm or less is considered, and no technology considering thicker steel plates is proposed. In Patent Document 2, coexistence of low-temperature toughness and strength of the base material and HAZ is attempted, but low-temperature toughness is evaluated at -60°C, and it is believed that additional examination is necessary to realize excellent toughness at lower temperatures. In Patent Document 3, the mechanical properties are evaluated only at the plate thickness t/4 position, and the mechanical properties inside the steel plate are not further considered. Patent Document 4 discloses a method for manufacturing a thick steel plate that has good mechanical properties even after SR, but the evaluation temperature for toughness is only -10°C, and toughness at lower temperatures has not been examined. The present disclosure has been made in consideration of the above-mentioned circumstances, and its purpose is to demonstrate excellent strength-toughness balance throughout the interior of the steel plate even when the plate thickness is thick, and in particular, to exhibit high strength and excellent toughness at lower temperatures than before. The object is to provide a steel plate and a manufacturing method for the thick steel plate.

Embodiment 1 of the present invention is,

Ingredient composition,

C: 0.020 mass% to 0.070 mass%,

Si: more than 0 mass% and less than or equal to 0.40 mass%,

Mn: 1.30 mass% to 1.95 mass%,

P: More than 0 mass% and less than or equal to 0.015 mass%,

S: More than 0 mass% and less than or equal to 0.005 mass%,

Al: 0.005 mass% to 0.070 mass%,

Nb: 0.015 mass% to 0.048 mass%,

Ti: 0.005 mass% to 0.024 mass%,

N: 0.0030 mass% to 0.0080 mass%, and

Ca: More than 0 mass% but less than 0.0040 mass%

satisfies, and the balance consists of Fe and inevitable impurities,

Di+10Nb: 1.20 to 2.50 determined from the following equation (1) is satisfied,

Among crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15° or more, the total area fraction SA of grains with an equivalent circular diameter of 7.5 μm or less is 34% or more at 1/4 of the plate thickness, and is 1/4 of the plate thickness. It is a thick steel plate with more than 27% at position 2.

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)···( One)

In equation (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:

Cu: greater than 0% by mass and less than or equal to 0.75% by mass, and

Ni: More than 0 mass% and less than or equal to 1.4 mass%

It is a thick steel plate according to aspect 1 containing at least one element selected from the group consisting of.

Aspect 3 of the present invention further,

Mo: greater than 0 mass% and less than or equal to 0.50 mass%,

V: greater than 0 mass% and less than or equal to 0.060 mass%,

Cr: greater than 0% by mass and less than or equal to 0.8% by mass, and

B: More than 0 mass% but less than 0.0007 mass%

It is a thick steel plate according to aspect 1 or 2 containing one or more elements selected from the group consisting of.

Aspect 4 of the present invention further includes:

REM: greater than 0% by mass and less than or equal to 0.0060% by mass, and

Zr: More than 0 mass% but less than 0.0050 mass%

A thick steel plate according to any one of aspects 1 to 3 containing one or more elements selected from the group consisting of.

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 process of heating a steel piece having the composition described in any one of aspects 1 to 4 to more than 1020°C and less than 1200°C, and a hot rolling process after the heating,

The hot rolling process is a method of manufacturing a thick steel plate in which the number of rolling passes is 3 or more, and hot rolling and cooling after the hot rolling are performed so as to satisfy all of the conditions (a) to (d) below.

(a) The cumulative reduction rate in the temperature range of 850℃ or lower is 40% or more.

(b) The average reduction rate of the final three passes of rolling is 5.5% or more.

(c) Finish rolling temperature is 720∼830℃

(d) After hot rolling, it is cooled from the cooling start temperature of the finish rolling temperature to 690°C to the cooling stop temperature of 320 to 550°C at an average cooling rate of 0.5 to 20°C/s.

According to the present disclosure, a thick steel plate that exhibits excellent strength-toughness balance throughout the interior of the steel plate even when the plate thickness is large, and in particular, exhibits high strength and excellent toughness at lower temperatures than before, and a method for manufacturing the same can be provided. .

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

In the present disclosure, intensive research was conducted to obtain a thick steel plate with an improved balance of strength and toughness at lower temperatures than before in the state before SR, that is, in the state as it is hot rolled. In particular, even if the plate thickness is thick, the strength-toughness balance is excellent at the 1/4 and 1/2 positions of the plate thickness, specifically, the parameter Y = 20 Extensive research was conducted to obtain a steel plate with sufficiently low 7×YP.

As a result, in addition to controlling the component composition, a predetermined amount of fine axial ferrite structure was secured at the 1/4 and 1/2 positions of the plate thickness. More specifically, the equivalent circle diameter measured by the method described later was 7.5. It was discovered that the above-mentioned good properties can be obtained by securing a predetermined amount of axial ferrite of μm or less. Hereinafter, the position of 1/4 of the plate thickness may be referred to as the “t/4 position” and the position of 1/2 of the plate thickness may be referred to as the “t/2 position.”

In addition, it was found that in order to form the fine axial ferrite structure in a predetermined amount, it is effective to carry out all of the following (A) to (C). The axial ferrite may hereinafter be referred to as “AF”.

(A) Before 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 generating AF grains using the dislocation structure or deformation zone introduced through this processing as the nucleus.

(B) Before rolling in hot unrecrystallized area, solid solution Nb is secured. By doing so, processing deformation preceding transformation is likely to be obtained, and fine AF crystal grains are likely to be generated as described above. As will be described later, it is effective to secure the solid solution Nb by setting the heating temperature before rolling to exceed 1020°C and lowering the reduction ratio during rolling above 850°C.

(C) The transformation temperature into the ferrite phase is appropriately controlled. 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 is reduced. Conversely, when the transformation temperature to the ferrite phase is low, a martensite structure is formed without an AF structure being formed. In order to appropriately control the transformation temperature to the ferrite phase, the C content, Mn content, and if at least one of Cu and Ni are included in the component composition, and the respective ranges of Di+10Nb are controlled. In addition, as will be described later, it is recommended that the average cooling rate in the predetermined temperature range after hot rolling be 0.5°C/s or more.

Hereinafter, the steel structure, component composition, characteristics, and manufacturing method of the thick steel plate of the present disclosure will be described in order.

1. River organization

Below, the steel structure of the thick steel plate of the present disclosure will be described in detail. In the description of the steel structure below, mechanisms that can improve various characteristics by having such a structure may be explained. Please note that although these are mechanisms conceived by the present inventors based on knowledge obtained at the present time, they do not limit the technical scope of the present disclosure.

In the present disclosure, among crystal grains surrounded by diagonal grain boundaries with a crystal orientation difference of 15° or more, grains with an equivalent circle diameter of 7.5 μm or less are defined as a fine axial ferrite (AF) structure. In the present disclosure, “forming a predetermined amount of a fine axial ferrite (AF) structure” refers to crystal grains surrounded by diagonal grain boundaries with a crystal orientation difference of 15° or more and having an equivalent circle diameter of 7.5 μm or less, as indicated below. Likewise, it means securing more than 34% at the t/4 position and more than 27% at the t/2 position.

In the present disclosure, in order to obtain a thick steel plate showing excellent strength-toughness balance even when the plate thickness is large, the total area fraction SA of grains with an equivalent circle diameter of 7.5 μm or less among crystal grains surrounded by diagonal grain boundaries with a crystal orientation difference of 15° or more, SA, That is, the area fraction of the fine axial ferrite (AF) structure is defined at both the t/4 position and the t/2 position. In detail, the total area fraction SA is set 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. Moreover, the total area fraction SA at the t/2 position is preferably 28% or more, and more preferably 30% or more. On the other hand, from the viewpoint of obtaining an excellent strength-toughness balance, the upper limit of the total area fraction 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 plate in the embodiment 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%. It becomes.

Structures other than the above fine axial ferrite include bainite, ferrite, cementite, retained austenite, and martensite. As long as the total area fraction SA is within the range specified in the embodiment of the present invention, ocular ferrite having an equivalent circle diameter of more than 7.5 μm may be present.

2. Composition

Below, the composition of the thick steel plate according to the present disclosure will be described.

C: 0.020 mass% to 0.070 mass%

C has the effect of appropriately controlling the ferrite transformation temperature and suppressing the generation of grain boundary ferrite, which acts as a brittle fracture origin and causes strength-toughness balance deterioration, before AF generation. From the viewpoint of exhibiting the effect, the amount of C is 0.020 mass% or more, preferably 0.023 mass% or more, and more preferably 0.030 mass% or more. On the other hand, if the amount of C is excessive, a hard martensitic structure is generated and acts as a brittle fracture origin, 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 mass% or less, more preferably 0.060 mass% or less.

Si: More than 0 mass% but less than 0.40 mass%

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

Mn: 1.30 mass% to 1.95 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 origin and causes strength-toughness balance deterioration, before AF generation. From the viewpoint of exhibiting this effect, the amount of Mn is 1.30 mass% or more, preferably 1.40 mass% or more, and more preferably 1.45 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 origin, thereby deteriorating the strength-toughness balance. Therefore, the amount of Mn is 1.95 mass% or less, preferably 1.90 mass% or less, and more preferably 1.80 mass% or less.

P: More than 0 mass% but less than 0.015 mass%

P is an impurity element, and if contained in excess, grain boundaries become embrittled 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 mass% or less, more preferably 0.007 mass% or less. On the other hand, industrially, it is difficult to set the P amount to 0 mass%, so the lower limit of the P amount is greater than 0 mass%.

S: More than 0 mass% but less than 0.005 mass%

S is an impurity element, and if contained in excess, grain boundaries become embrittled 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 mass% or less, more preferably 0.003 mass% or less. On the other hand, industrially, it is difficult to set the S amount to 0 mass%, so the lower limit of the S amount is greater than 0 mass%.

Al: 0.005 mass% to 0.070 mass%

Al is a deoxidizing element. 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 set to 0.005% by mass or more. The amount of Al is preferably 0.010 mass% or more, more preferably 0.015 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 Al amount is 0.070 mass% or less, preferably 0.050 mass% or less, and more preferably 0.045 mass% or less.

Nb: 0.015 mass% to 0.048 mass%

Nb is an element that promotes the production of AF. In order to sufficiently create a fine AF structure and obtain a good strength-toughness balance, the amount of Nb is set to 0.015 mass% or more, preferably 0.016 mass% or more, and more preferably 0.018 mass% or more. 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 origin, thereby deteriorating the strength-toughness balance. Therefore, the amount of Nb is 0.048 mass% or less, preferably 0.045 mass% or less, and more preferably 0.040 mass% or less.

Ti: 0.005 mass% to 0.024 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 mass% or more, preferably 0.007 mass% or more, and more preferably 0.009 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 mass% or less, preferably 0.022 mass% or less, and more preferably 0.020 mass% or less.

N: 0.0030 mass% to 0.0080 mass%

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

Ca: More than 0 mass% but less than 0.0040 mass%

Ca is a deoxidizing element, and its content exceeds 0% by mass. Additionally, when the amount of Mn in the steel is large, it is thought that coarse MnS is likely to be generated at the t/2 position due to Mn concentration during casting, and the toughness at the t/2 position is likely to decrease. In order to suppress the formation of MnS, the Ca amount is preferably greater than 0 mass%, more preferably 0.0008 mass% or more, and even more preferably 0.0010 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 Ca amount is 0.0040 mass% or less, preferably 0.0028 mass% or less, and more preferably 0.0025 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 brought in depending on the circumstances of raw materials, materials, and manufacturing facilities are allowed. On the other hand, for example, as with P and S, a lower content is generally preferable, and therefore they are unavoidable impurities, but there are elements whose composition range is separately specified as above. For this reason, in this specification, the term “inevitable impurities” constituting the remainder is a concept excluding elements whose composition ranges are separately specified.

The thick steel plate in the embodiment of the present invention may contain the above elements in its component composition. The optional elements described below do not need to be included, but high strength, etc. can be achieved more easily by including 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 embodiment of the present invention can be more easily achieved. Hereinafter, selected elements will be described.

Cu: more than 0 mass% but not more than 0.75 mass%, and Ni: one or more elements selected from the group consisting of more than 0 mass% and not more than 1.4 mass%.

These elements have the effect of appropriately controlling the ferrite transformation temperature and suppressing the formation of grain boundary ferrite, which acts as a brittle fracture origin and causes strength-toughness balance deterioration, before AF formation. From the viewpoint of demonstrating the effect, when Cu is contained, it is preferable to exceed 0 mass%, more preferably 0.05 mass% or more, further preferably 0.10 mass% or more, and even more preferably 0.15 mass%. It is more than %. When containing Ni, it is preferable to exceed 0 mass%, more preferably 0.10 mass% or more, further preferably 0.15 mass% or more, and even more preferably 0.20 mass% or more. On the other hand, if these elements are excessive, a hard martensitic structure is generated, and this structure acts as a brittle fracture origin, resulting in deterioration of the strength-toughness balance. Therefore, the amount of Cu is preferably 0.75 mass% or less, more preferably 0.70 mass% or less, and even more preferably 0.68 mass% or less. The amount of Ni is preferably 1.4 mass% or less, more preferably 1.2 mass% or less, and still more preferably 1.0 mass% or less.

Mo: more than 0 mass% but not more than 0.50 mass%, V: more than 0 mass% but not more than 0.060 mass%, Cr: more than 0 mass% but not more than 0.8 mass%, and B: more than 0 mass% and not more than 0.0007 mass%. one or more elements

These elements are effective elements for improving 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 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, and even 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, and even 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, and this structure acts as a brittle fracture origin, resulting in deterioration of the strength-toughness balance. Therefore, the Mo amount is preferably 0.50 mass% or less, more preferably 0.45 mass% or less, and still more preferably 0.40 mass% or less. The amount of V is preferably 0.060 mass% or less, more preferably 0.050 mass% or less, and still more preferably 0.045 mass% or less. The Cr amount is preferably 0.8 mass% or less, more preferably 0.70 mass% or less, and even more preferably 0.60 mass% or less. The amount of B is preferably 0.0007 mass% or less, and more preferably 0.0006 mass% or less.

REM: more than 0 mass% but not more than 0.0060 mass%, and Zr: one or more elements selected from the group consisting of more than 0 mass% and not more than 0.0050 mass%.

These elements are deoxidizing elements. In order to exert 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 containing Zr, 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 excessive, coarse oxides are formed and the strength-toughness balance deteriorates. Therefore, the amount of REM is preferably 0.0060 mass% or less, more preferably 0.0050 mass% or less, and even more preferably 0.0045 mass% or less. The amount of Zr is preferably 0.0050 mass% or less, more preferably 0.0045 mass% or less, and still more preferably 0.0040 mass% or less. The REM means including lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

Di+10Nb: 1.20 to 2.50 (Di is obtained from equation (1) below)

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)···( One)

In equation (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, the formation of grain boundary ferrite before AF formation can be suppressed, the excessive formation of pearlite structures and martensite structures other than AF can be suppressed, and AF formation can be promoted. From these viewpoints, Di+10Nb was set within the range of 1.20 to 2.50 in the embodiment of the present invention. If the value of Di+10Nb is too small, a pearlite structure is generated along 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 generated and acts as a brittle fracture origin, thereby deteriorating 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 in the embodiment of the present invention is preferably 16 mm or more, more preferably 30 mm or more, further preferably 40 mm or more, and particularly preferably more than 40 mm. On the other hand, 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 disclosure has the following characteristics and is suitable for manufacturing large-sized LPG tanks, ships, etc., for example.

3. Characteristics

In the present disclosure, the Y value, which is a parameter expressed in the following equation (2), is used to evaluate the balance between strength and toughness at a temperature lower than before. The Y value includes the yield strength YP and the brittle-ductility transition temperature vTrs, as shown in the following equation (2). On the other hand, as the yield strength YP in the following equation (2), when the SS curve (also referred to as “stress-strain diagram”) is round with an unclear yield point, 0.2% proof strength (0.2YS) is used, and the yield point is If you have , use YP.

The thick steel plate according to the embodiment of the present invention is as hot rolled, and in the C direction perpendicular to the rolling direction, Y at the t/4 position is less than -5200, and Y at the t/2 position is less than -4700. When it satisfies the requirements, the strength-toughness balance is evaluated as excellent.

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 It is -5000 or less. The lower these values are, the better the strength-toughness balance is.

In the embodiment of the present invention, the above parameters can be achieved as characteristics. For each of the yield strength YP and the brittle-ductility transition temperature vTrs, provided 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 -70. Examples include a range of less than ℃, more preferably a range of less than -80℃. In addition, for each measurement position, at the t/4 position, on the premise of achieving the above parameters, for example, the yield strength YP is preferably 410 MPa or more, more preferably 430 MPa or more, more preferably 460 MPa or more, and even more. Preferably it is 480MPa or more, especially 500MPa or more, and the brittle-ductility transition temperature vTrs is preferably -90°C or less, more preferably -95°C or less, more preferably -100°C or less, and even more preferably -110℃ or lower. Also, at the t/2 position, provided 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, and even more preferably 480 MPa. or more, especially 500 MPa or more, and the brittle-ductility transition temperature vTrs is preferably -80°C or less, more preferably -90°C or less, more preferably -95°C or less, and still more preferably -100°C or less. .

4. Manufacturing method

The method for producing a thick steel plate of the present disclosure includes a step of heating a steel piece having the above-mentioned component composition to more than 1020°C and less than 1200°C, and a hot rolling step after the heating, wherein the number of rolling passes is 3. This is a method of manufacturing a thick steel plate in which hot rolling and cooling after the hot rolling are performed so as to pass more than one pass and satisfy all of the conditions (a) to (d) below.

(a) The cumulative reduction rate in the temperature range of 850℃ or lower is 40% or more.

(b) The average reduction rate of the final three passes of rolling is 5.5% or more.

(c) Finish rolling temperature is 720∼830℃

(d) After hot rolling, it is cooled from the cooling start temperature of the finish rolling temperature to 690°C to the cooling stop temperature of 320 to 550°C at an average cooling rate of 0.5 to 20°C/s.

Hereinafter, each manufacturing condition will be described in detail.

[Process of heating a steel piece having the above chemical 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 dissolved in solid solution, and the effect of promoting AF production by dissolved Nb is not obtained. Therefore, the heating temperature is set to exceed 1020°C, preferably 1040°C or higher, more preferably 1050°C or higher, and even more preferably 1060°C or higher. On the other hand, when the heating temperature is 1200°C or higher, the austenite grains become coarse and the overall structure becomes coarse. Therefore, the heating temperature is set to be less than 1200°C, preferably 1180°C or less, and more preferably 1150°C or less.

[Hot rolling process after the above heating]

As will be described later, the present disclosure is characterized by controlling the average reduction ratio of the final three passes of rolling, and after the heating, hot rolling is performed with three or more rolling passes, but the total number of passes (rolling passes) is The number of times) does not affect the organization and characteristics and is not limited. The rolling passes can be 7 passes or more, and even 10 passes or more, and can be 60 passes or less from the viewpoint of productivity.

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

(a) Cumulative reduction ratio in the temperature range of 850℃ or less: 40% or more

For the purpose of obtaining a sufficient AF structure, in order to introduce sufficient processing strain into the austenite phase, the cumulative reduction ratio (also referred to as “total reduction ratio”) in the temperature range of 850°C or lower during hot rolling must be 40% or more. It is necessary to do so. However, it is thought that when the reduction ratio in rolling above 850°C increases, NbC precipitates during rolling and the dissolved Nb decreases. From the viewpoint of suppressing the cumulative reduction in rolling at 850°C or higher, the cumulative reduction in the temperature range below 850°C is set to 40% or more. The cumulative reduction ratio is preferably 50% or more, more preferably 55% or more. The upper limit of the cumulative reduction ratio is about 80% from the viewpoint of productivity.

(b) Average reduction rate of the final 3 passes of rolling: 5.5% or more

The dislocation tissue introduced in the final three passes of reduction transfers to the cooling process with relatively no recovery, so the effect of promoting AF is significant. In an embodiment of the present invention, the average reduction ratio in the final three passes of rolling is set to 5.5% or more to introduce sufficient processing strain into the austenite phase for introducing the dislocation structure. In an embodiment of the present invention, by controlling the average reduction ratio of the final three passes of rolling, a desired structure with an AF structure of a certain level or more can be obtained inside the steel sheet, especially at the t/2 position, so that the final three passes of rolling are obtained. It is different from the conventional method in which the average reduction rate is not controlled. The average reduction ratio is preferably 5.8% or more, and more preferably 6.0% or more. Meanwhile, from the viewpoint of rolling mill load, the upper limit of the average reduction ratio is about 20%.

(c) Finishing Rolling Temperature (FRT): 720∼830℃

If the temperature of the steel exceeds 850°C, even if the final three passes of rolling are performed at the average reduction ratio, sufficient processing strain is not introduced into the austenite phase, and the amount of AF structure is insufficient. In the embodiment of the present invention, the finish rolling temperature is set to 830° C. or lower in order to sufficiently secure the amount of AF tissue. 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 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, it is cooled from the cooling start temperature of the finish rolling temperature to 690°C to the 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, AF tissue can be sufficiently secured. If the average cooling rate exceeds 20°C/s, it is undesirable because martensite transformation occurs without the AF structure being sufficiently formed. 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 the component composition contains C content, Mn content, and at least one of Cu and Ni, The content and each range of Di+10Nb are controlled, 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, and the amount of AF tissue is insufficient. The average cooling rate is preferably 2.0°C/s or more, more preferably 3.0°C/s or more. As a cooling method at the above-mentioned average cooling rate, water cooling can be used, for example.

The starting temperature of the cooling is set to an arbitrary temperature within the range of finish rolling temperature to 690°C. If the temperature of the steel is lower than 710°C, problems such as grain boundary ferrite being formed before cooling begins or processing strain introduced into the austenite phase being recovered are likely to occur. As a result, AF tissue volume is likely to be insufficient. In an embodiment of the present invention, in order to secure 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 any temperature in the range of 320 to 550°C. If cooling at the above-mentioned average cooling rate, for example, water cooling, is stopped in a temperature range exceeding 550°C, grain boundary ferrite is generated during slow cooling after water cooling is stopped, making it 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, if cooling at the above average cooling rate is performed to a temperature range below 320°C, for example, martensite transformation occurs without the AF structure being sufficiently formed. 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 disclosure is not particularly limited other than the hot rolling process, and may be carried out under normal conditions.

Anyone skilled in the art who is familiar with the manufacturing method of the high-strength steel sheet according to the embodiment of the present invention described above may be able to obtain the high-strength steel sheet according to the embodiment of the present invention through trial and error by a manufacturing method different from the above-described manufacturing method. there is.

Example

Hereinafter, embodiments of the present invention will be described in more detail through examples. The present disclosure is not limited by the following examples, and may be implemented with appropriate changes within the scope consistent with the spirit described above and below, and all of them are included in the technical scope of the present disclosure.

1. Sample production

The steel with the composition shown in Table 1 is melted in a 150 kg VIF (Vacuum Induction Furnace) or an actual converter, and the obtained slab is cast and hot rolled under various conditions shown in Table 2, and the plate thickness shown in Table 2 is obtained. steel plate was obtained. In the above hot rolling, the total number of passes (number of rolling passes) was set to exceed 20. In Table 1, “-” indicates that it was not added intentionally. In Table 2, FRT represents the finish rolling temperature, SCT represents the cooling start temperature, and FCT represents the cooling end temperature. The cooling stop temperature FCT was measured with a radiation thermometer at 1 to 3 points along the steel sheet surface in the longitudinal direction, and the average value was calculated. Additionally, FRT and SCT were determined by measuring one point on the surface of the steel material with a radiation thermometer.

2. River organization

EBSD (Electron Back Scatter Diffraction) measurements were performed at positions t/4 (t: sheet thickness) and t/2 of the cross section perpendicular to the rolling width direction of the hot rolled material. The measurement conditions are as follows.

EBSD measurement conditions

· Device: Nippon Electronics JEOL-5410 or JSM-IT100

· Observation magnification: 400 times

· Measuring area: 200μm×200μm

· Step (pixel) size: 0.4μm

Phases considered: Ferrite, austenite

The obtained EBSD data was analyzed using analysis software OIM Analysis manufactured by TSL Solutions Co., Ltd. From 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 adjacent pixels were defined as diagonal grain boundaries. Among the units surrounded by these diagonal grain boundaries, units with a pixel size of 10 or more were considered diagonal. In addition, diagonal ribs at the ends of the measurement field of view were excluded from the analysis. The average equivalent circular diameter of the diagonal grains was determined, and the total area fraction SA of diagonal grains with an average equivalent circular diameter of 7.5 μm or less, which occupies the total area of the diagonal grains, was calculated.

3. Mechanical properties

(yield strength YS)

At the t/4 position and the t/2 position of the as-hot-rolled steel sheet, an ASTM round bar tensile test piece is taken parallel to the sheet width direction (C direction), a tensile test is performed according to ASTM, and the yield strength YS is determined. Measured.

(Low-temperature toughness of base metal)

V-notch Charpy test pieces were taken parallel to the sheet width direction (C direction) at the t/4 position and t/2 position of the as-hot-rolled steel sheet, and a Charpy impact test was performed according to ASTM. And the temperature vTrs at which the brittle fracture ratio was 50% was evaluated.

The yield strength YP and the brittleness-ductility transition temperature vTrs obtained through the above measurements were substituted into the following equation (2) to obtain the Y values at each of the t/4 position and the t/2 position. The results are listed in Table 2. Meanwhile, in Table 2, in the case of vTrs > -30°C, the Y value was calculated as vTrs = -30°C. Additionally, in the case of vTrs < -130°C, the Y value was calculated as vTrs = -130°C.

Y = 20×vTrs-7×YP···(2)

From Tables 1 and 2, the following can be seen. Experiment No. 1 to 11 satisfy the component composition specified in the embodiment of the present invention, and since the thick steel plate was manufactured under the specified conditions, the obtained thick steel plate maintains a strength-toughness balance throughout the interior of the steel plate even if the plate thickness is large. is excellent. In particular, it exhibits high strength and has superior toughness at low temperatures compared to conventional products. In contrast, experiment no. In 12 to 17, at least one of the component composition and manufacturing conditions was not within the range specified in the embodiment of the present invention, so the obtained thick steel plate had a poor strength-toughness balance.

Experiment No. 12, the component composition is within the scope of the embodiment of the present invention, but in the manufacturing conditions, the cumulative reduction ratio in the temperature range of 850 ° C. or lower was insufficient, so AF at any position of t / 4 and t / 2 Due to the lack of structure, the strength-toughness balance was poor.

Experiment No. 13 and 14, the component composition is within the range of the embodiment of the present invention, but in the manufacturing conditions, the finish rolling temperature was high, so the AF structure was insufficient at any position of t/4 and t/2, resulting in low strength. -The result was a lack of personality balance.

Experiment No. In case 15, Di+10Nb was below the specified range, resulting in poor strength-toughness balance. This experiment no. In Fig. 15, it is believed that a pearlite structure was created along with grain boundary ferrite because the Di+10Nb was below the specified range. As a result, it is thought that the strength-toughness balance was poor due to the lack of AF tissue.

Experiment No. In cases 16 and 17, Di+10Nb exceeded the specified range, resulting in poor strength-toughness balance. This experiment no. In cases 16 and 17, it is believed that a hard martensitic structure was generated because Di+10Nb exceeded the specified range. As a result, it is believed that this hard martensite structure acts as a brittle fracture origin, deteriorating the strength-toughness balance.

Experiment No. 18, the component composition is within the scope of the embodiment of the present invention, but in the manufacturing conditions, the heating temperature before hot rolling is low and the finish rolling temperature is high, so a sufficient AF structure cannot be secured, and the strength-toughness balance is poor. resulted in lagging behind.

Experiment No. 19, the component composition is within the range of the embodiment of the present invention, but in the manufacturing conditions, the heating temperature before hot rolling is high, so the fine AF structure cannot be secured beyond a certain level, resulting in poor strength-toughness balance. It has been done.

Experiment No. In 20, the amount of B was 0.0008%, exceeding the upper limit of 0.0007%, so hard martensite was generated, SA at the t/4 position and t/2 position was insufficient, and the characteristics deteriorated.

Experiment No. 21, although the component composition is within the range of the embodiment of the present invention, in the manufacturing conditions, the average rolling reduction rate of the final three passes during hot rolling was too low, and a fine AF structure could not be secured above a certain level. , resulting in poor strength-toughness balance.

Experiment No. 22, the amount of Nb is insufficient, and in the manufacturing conditions, the cumulative reduction rate in the temperature range of 850°C or lower and the average reduction rate of the final 3 passes of rolling are low, so a fine AF structure cannot be secured above a certain level, and the strength -The result was a lack of personality balance.

Figure 1 is a graph showing the relationship between the total area fraction SA of grains with an equivalent circle diameter of 7.5 μm or less among grains surrounded by diagonal grain boundaries with a crystal orientation difference of 15° or more and the Y value, prepared based on the above examples. Meanwhile, the downward arrow in FIG. 1 means that the Y value is assumed to be lower than the plotted value because the measured vTrs was lower than -130°C, and the upward arrow means that the measured vTrs is lower than -30°C. Since it was higher than ℃, it means that the Y value is estimated to be a higher value than the plotted value.

From this Figure 1, there is a correlation between the total area fraction SA and the Y value at either the t/4 position or the t/2 position in the steel plate, and the Y value is less than -5200 at the t/4 position. In order to set the total area fraction SA to 34% or more, and to make the Y value less than -4700 at the t/2 position, the total area fraction SA needs to be 27% or more. You can see that it exists.

This application claims priority based on Japanese Patent Application No. 2019-081271, with a filing date of April 22, 2019, and Japanese Patent Application No. 2020-007626, with a filing date of January 21, 2020. It is accompanied by a priority claim as a basic application. Japanese Patent Application No. 2019-081271 and Japanese Patent Application No. 2020-007626 are incorporated herein by reference.

Claims (3)

Ingredient composition,
C: 0.020 mass% to 0.070 mass%,
Si: more than 0 mass% and less than or equal to 0.40 mass%,
Mn: 1.30 mass% to 1.80 mass%,
P: More than 0 mass% and less than or equal to 0.015 mass%,
S: More than 0 mass% and less than or equal to 0.005 mass%,
Al: 0.005 mass% to 0.070 mass%,
Nb: 0.015 mass% to 0.048 mass%,
Ti: 0.005 mass% to 0.024 mass%,
N: 0.0030 mass% to 0.0080 mass%, and
Ca: More than 0 mass% but less than 0.0040 mass%
satisfy,
Contains one or more elements selected from the group consisting of Cu: more than 0 mass% and not more than 0.75 mass%, and Ni: more than 0 mass% and not more than 1.4 mass%,
The balance consists of Fe and inevitable impurities,
Di+10Nb: 1.20 to 2.50 determined from the following equation (1) is satisfied,
Among the grains surrounded by diagonal grain boundaries with a crystal orientation difference of 15° or more, the total area fraction SA of grains with an equivalent circular diameter of 7.5 μm or less is 34% or more at 1/4 of the sheet thickness, and also at 1/2 of the sheet thickness. It is more than 27%,
Y calculated from the following equation (2) at 1/4 of the plate thickness is less than -5200, and Y at 1/2 of the plate thickness is less than -4700,
Heavy steel plate with a plate thickness of 40 mm or more.
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)···( One)
In equation (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.
Y = 20×vTrs-7×YP···(2)
In equation (2), vTrs is the brittle-ductile transition temperature (°C), and YP is the yield strength (MPa). According to claim 1,
A thick steel plate that satisfies any one or more of the following (i) and (ii).
(i) Additionally:
Mo: greater than 0 mass% and less than or equal to 0.50 mass%,
V: greater than 0 mass% and less than or equal to 0.060 mass%,
Cr: greater than 0% by mass and less than or equal to 0.8% by mass, and
B: More than 0 mass% but less than 0.0007 mass%
Contains one or more elements selected from the group consisting of
(ii) Additionally:
REM: greater than 0% by mass and less than or equal to 0.0060% by mass, and
Zr: More than 0 mass% but less than 0.0050 mass%
Contains one or more elements selected from the group consisting of A method for manufacturing a thick steel plate having a plate thickness of 40 mm or more according to claim 1 or 2,
A process of heating a steel piece having the component composition according to claim 1 or 2 to more than 1020°C and less than 1200°C, and a hot rolling process after the heating,
In the hot rolling process, the number of rolling passes is 3 or more, and hot rolling and cooling after the hot rolling are performed so as to satisfy all the conditions (a) to (d) below.
(a) The cumulative reduction rate in the temperature range of 850℃ or lower is 40% or more.
(b) The average reduction rate of the final three passes of rolling is 5.8% or more.
(c) Finish rolling temperature is 720∼830℃
(d) After hot rolling, it is cooled from the cooling start temperature of the finish rolling temperature to 690°C to the cooling stop temperature of 320 to 550°C at an average cooling rate of 0.5 to 20°C/s.