JP2008045195A - High tensile strength thick steel plate and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick steel plate having a high strength level in a 490 MPa class particularly suitable for a vessel, and having excellent uniform elongation and weldability (HAZ toughness). <P>SOLUTION: The high tensile strength thick steel plate is composed of a steel having chemical components comprising, by mass, 0.02 to 0.20% C, 0.2 to 0.5% Si, 1 to 1.8% Mn, Cu and/or Ni by 0.2 to 1% in total, 0.005 to 0.025% Ti, 0.0015 to 0.010% N and 0.0005 to 0.003% B, and the balance Fe with inevitable impurities, and in which the contents of the Ti, N and B satisfy inequality (1), a ferrite fraction is ≥80 vol.%, a retained austenite fraction is ≥1 vol.% and the C content in the retained austenite is 0.80 to 1.10 mass%; wherein, 0≤[N]-0.292×[Ti]≤(14/10.8)×([B]-0.0003): (1). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a thick steel plate used for large structures such as ships, and a method for producing the same, and particularly has a tensile strength of 490 MPa or more and less than 590 MPa (hereinafter sometimes referred to as a 490 MPa class) and a thickness of about 5 mm or more. It relates to steel plates.

  When ships collide with each other or the ships are stranded, the hull breaks and opens a hole, floods and sinks, and cargo and fuel flow out of the ship, causing marine pollution and becoming a social problem. Yes. Accordingly, various steel plates for hulls have been proposed that can absorb the impact energy received by the hull when the vessels collide with each other or the ground of the vessel is stranded, thereby preventing adverse effects due to the hull destruction.

  As a steel plate for a hull having excellent impact energy absorption capability, Patent Document 1 discloses that a surface austenite (residual γ) of 1.0 to 20% in area ratio is formed on the front and back layers of at least 1/8 of the thickness of the steel plate. Including steel plates have been proposed. A steel sheet in which an austenite (γ) structure remains is known as a TRIP (Transformation Induced Plasticity) steel sheet, and when the TRIP steel sheet is deformed by processing, the residual γ is induced and transformed into martensite by stress, and is excellent due to γ. Elongation and high strength due to martensite.

  In Patent Document 1, as the relationship between the impact energy absorption capacity (EA) and the mechanical properties of the steel material, the product of the elongation property [total elongation (El)] and the strength property (YP, TS) of the steel material increases. It is described that the impact energy absorption capacity (EA) is improved.

  However, the evaluation of absorbed energy based on the total elongation (El) does not necessarily lead to an evaluation of the safety of the hull structure, and is supported by a stiffener with a long span that cannot be compared with the distance between gauge points in a tensile test. In order to evaluate the elongation deformation of a hull skin plate, it is considered that it is necessary to evaluate with a uniform elongation (uniform elongation) considered to have a high correlation with the elongation characteristics of the hull skin plate (see Patent Document 2). ).

  By the way, the present applicant has previously proposed a technique in which uniform elongation of a high-tensile steel plate of 590 MPa class is increased as a TRIP steel plate suitably used for a building structure or the like (see Patent Document 3 and Patent Document 4). . In Patent Document 3, high steel strength is ensured by making the steel structure mainly bainite, and uniform elongation is improved by generating residual γ in the steel. In Patent Document 4, the strength of the base material is ensured by making the steel structure a low-temperature transformation bainite, and the uniform elongation is enhanced by generating island-like martensite and residual γ.

However, in the steel plate for the hull, the required yield stress is determined from the design requirements, and the strength grade of the steel plate is changed according to the site to be used. Unnecessary strength is not required. On the other hand, according to the methods described in Patent Document 3 and Patent Document 4 above, uniform elongation is only about 15%, and further improvement of uniform elongation is required to be suitable for ships. . Further, as for the steel plate for ship hulls, as in the 590 MPa class high-tensile steel plate described in Patent Document 3 and Patent Document 4, the toughness in the heat-affected zone (hereinafter sometimes referred to as HAZ) when welded. Is required to be excellent.
Japanese Patent Laid-Open No. 11-246934 JP 2001-262272 A JP 2002-266048 A JP 2003-160835 A

  The present invention has been made in view of such a situation, and an object of the present invention is a thick steel plate having a high strength level of 490 MPa particularly suitable for ships, and has uniform elongation (uniform elongation) and weldability. The object is to provide a thick steel plate excellent in (HAZ toughness). Moreover, the other objective of this invention is to provide the manufacturing method of such a thick steel plate.

The high-strength thick steel plate according to the present invention that has solved the above problems is C: 0.02 to 0.20% (meaning mass%, hereinafter the same for chemical components), Si: 0.2 to 0.00. 5%, Mn: 1 to 1.8%, Cu and / or Ni: 0.2 to 1% in total, Ti: 0.005 to 0.025%, N: 0.0015 to 0.010%, B : 0.0005-0.003% steel, the balance being Fe and inevitable impurities, the content of Ti, N, B satisfies the following formula (1), ferrite fraction is 80 It has a gist in that it is at least volume%, the retained austenite fraction is at least 1 volume%, and the C content in the retained austenite is 0.80 to 1.10 mass%. However, in the following formula (1), [] indicates the element content.
0 ≦ [N] −0.292 × [Ti] ≦ (14 / 10.8) × ([B] −0.0003)
... (1)

The C is preferably 0.02 to 0.10%. The thick steel plate of the present invention may further contain at least one of the following (a) to (c) as another element.
(A) Cr: 0.5% or less (excluding 0%)
(B) Mo: 0.2% or less (excluding 0%)
(C) one or more selected from the group consisting of V, Nb, Zr, Hf and Ta: 0.05% or less in total (excluding 0%)

  Moreover, the thick steel plate of this invention may contain Al: 1% or less (0% is not included) as another element. Further, the thick steel plate of the present invention further contains, as another element, at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ce and La in a total of 0.01% or less (not including 0%). ) May be contained.

  The high-tensile thick steel plate of the present invention has a first step of producing 80% by volume or more of ferrite and 1% by volume or more of retained austenite, and the amount of C in the retained austenite is 0.80 to 1.10% by mass. A second step of controlling within a range, and as the second step, is any temperature range between 500 and 300 ° C. gradually cooled at an average cooling rate of 1 ° C./second or less for 50 to 3600 seconds? Or it can manufacture by employ | adopting the manufacturing method hold | maintained for 50 to 3600 second at arbitrary temperatures between 500-300 degreeC.

  According to the present invention, since the amount of C (CγR) in the residual γ is appropriately controlled, the TRIP effect due to the residual γ is reliably exhibited, and the uniform elongation of the thick steel plate can be significantly increased. Moreover, according to this invention, since the component composition of a steel plate is adjusted appropriately, HAZ toughness can be improved even if it welds. Since the steel plate of the present invention has a high strength level of 490 MPa particularly suitable for ships, for example, by using it as a material for a hull, even if a collision accident or a grounding accident occurs, the hull breaks. Thus, it is possible to prevent the hole from being opened, or to significantly reduce the fracture area as compared with the conventional case.

  In order to provide a 490 MPa class TRIP thick steel plate excellent in uniform elongation, the present inventors have conducted intensive studies based on the 590 MPa class TRIP thick steel plate described in Patent Document 3 and Patent Document 4 described above. As a result, even if the residual γ generating means disclosed in these patent documents is simply applied as it is and the matrix is made of a soft ferrite structure in order to reduce it to a strength level suitable for ships, the desired uniform structure is obtained. Many basic experiments by the present inventors revealed that no elongation (specifically, 18% or more) can be obtained.

  Therefore, as a result of further studies by the present inventors, (a) by appropriately controlling the amount of C in the residual γ, the uniform elongation of the 490 MPa class TRIP thick steel plate can be remarkably increased, and (b) the residual In order to appropriately control the amount of C in γ, among the components in steel, in particular, Si is reduced to 0.5% or less, and Cu and / or Ni are added as essential components, and after hot rolling, 500% The inventors have found that it is necessary to strictly adjust the cooling conditions in the temperature range of ˜300 ° C. in relation to the holding time, and completed the present invention.

  As described above, the present invention is characterized in that the uniform elongation is increased to 18% or more by controlling the amount of C in the residual γ in the range of 0.80 to 1.10% by mass. By appropriately controlling the amount of C in the residual γ, the TRIP effect due to the residual γ can be reliably exhibited even in a thick steel plate having a strength level of 490 MPa. This will be described with reference to the drawings.

  FIG. 2 is a graph showing the relationship between the amount of C in the residual γ and the uniform elongation based on the results of examples described later. In the figure, ◯ indicates No. in Tables 4-7. 1-5, No. 1 7, no. 13, no. 16-23, no. 29, no. 30, no. 40, no. 42-46, no. 48-51, no. 53, No. in Table 4 to Table 7 10, no. 11, no. 14, no. 37-39, no. 41 results. In FIG. 2, only an example in which the ferrite fraction and the residual γ fraction satisfy the range defined in the present invention is plotted.

  As is apparent from FIG. 2, the uniform elongation of the thick steel plate can be remarkably increased by controlling the amount of C in the residual γ within the above range.

  Further, as taught in Patent Document 3 and Patent Document 4, even if a predetermined amount of residual γ (specifically, 1% by volume or more) is generated by controlling the cooling conditions after rolling, the desired uniform elongation is I can't get it. This is also the case in the examples described later. 5 and No. 11 is apparent (see Tables 4 and 6 below). These are examples in which the amount of residual γ is 2.0 area%, and the amount of C in residual γ (CγR) is changed by controlling the components in the steel. Even if the range satisfying the above condition is satisfied, the CγR falls outside the range defined by the present invention. No. 11 showed a decrease in uniform elongation (see Table 6 below). Hereinafter, the present invention will be described in detail.

  First, chemical components of the thick steel plate of the present invention will be described. The thick steel plate of the present invention contains C, Si, Mn, Ti, N, B and Cu and / or Ni as basic elements. The contents of these elements are as follows.

C: 0.02 to 0.20%
C is an element necessary for ensuring strength, and is contained particularly for generating ferrite and residual γ. However, in order to increase the HAZ toughness when welding, it is preferable that C is small. However, if the amount of C is less than 0.02%, a large amount of ferrite is generated and a predetermined amount of residual γ is not generated, so that uniform elongation cannot be increased. Therefore, C is 0.02% or more, preferably 0.03% or more. On the other hand, if the amount of C exceeds 0.20%, the generation of ferrite decreases and the uniform elongation decreases. Further, the ferrite fraction is lowered and the toughness of the base material is deteriorated. Therefore, C is 0.20% or less, preferably 0.15% or less, more preferably 0.12% or less. In particular, in order to further increase the HAZ toughness when welding, C is preferably 0.10% or less, more preferably 0.08% or less.

Si: 0.2 to 0.5%
Si is an important element for suppressing the decomposition of γ to cementite and maintaining a predetermined amount of residual γ and the amount of C in the residual γ when held in a temperature range of 500 to 300 ° C. Si is an element that increases the tensile strength of the base material by solid solution strengthening. However, if the Si content is less than 0.2%, the decomposition of γ cannot be sufficiently suppressed, and the residual γ useful for enhancing the uniform elongation cannot be sufficiently generated. Therefore, Si is 0.2% or more, preferably 0.25% or more, more preferably 0.30% or more. On the other hand, even if Si exceeds 0.5%, residual γ is generated. However, if Si is excessively contained, the amount of C in the generated residual γ is excessively increased, so that the uniform elongation is lowered. Also, the ferrite becomes brittle and the HAZ toughness after welding deteriorates. Therefore, Si is 0.5% or less.

Mn: 1 to 1.8%
Mn is an element that suppresses the decomposition of residual γ and maintains the concentration of C in the residual γ when held in a temperature range of 500 to 300 ° C. Mn is an element that enhances hardenability and is an important element for securing strength. However, if the amount of Mn is less than 1%, the strength becomes insufficient and the amount of C concentrated in the residual γ decreases, so that the uniform elongation cannot be sufficiently increased. Therefore, Mn is 1% or more, preferably 1.2% or more, more preferably 1.3% or more, and further preferably 1.4% or more. On the other hand, if the amount of Mn exceeds 1.8%, the hardenability becomes too good, and it is difficult to produce ferrite, and the strength becomes too high. Further, when the ferrite fraction is lowered, the uniform elongation is also deteriorated. Therefore, Mn is 1.8% or less, preferably 1.6% or less.

Cu and / or Ni: 0.2 to 1% in total
Cu and Ni, like Si and Mn, are elements that act to suppress the decomposition of γ to form cementite and generate residual γ when held in the temperature range of 500 to 300 ° C. is there. Cu and Ni are elements that concentrate C in the residual γ to increase uniform elongation. In the present invention, Cu and Ni also have an effect as a substitute element for Si. As described above, the addition of a large amount of Si causes a decrease in base metal toughness and adversely affects weldability. However, according to the present invention, by adding Cu and / or Ni having the same action as Si, the amount of Si is increased. Even if it reduces, the desired residual (gamma) can be ensured and uniform elongation can fully be raised. Even if Cu or Ni is contained, the base material toughness is not impaired.

  Cu and Ni can be used alone or in combination. However, if the total amount of these elements is less than 0.2%, a predetermined amount of residual γ can be secured as shown in the examples described later. However, the amount of C in the residual γ decreases, and the uniform elongation cannot be sufficiently increased. Therefore, the total amount of these elements is 0.2% or more, preferably 0.3% or more, more preferably 0.4% or more. On the other hand, if the total amount of Cu and Ni exceeds 1%, the hardenability becomes too good, and it becomes difficult to generate ferrite and the uniform elongation cannot be increased. Therefore, the total amount of Cu and Ni is 1% or less, preferably 0.9% or less, more preferably 0.8% or less.

Ti: 0.005-0.025%
Ti is an element that forms TiN by combining with N in the thick steel plate, and this TiN exhibits an effect (pinning effect) that prevents crystal grains from coarsening when subjected to heat during welding. Increases HAZ toughness. However, if Ti is less than 0.005%, the amount of TiN produced is small and the HAZ toughness cannot be increased. Therefore, Ti is 0.005% or more, preferably 0.007% or more, and more preferably 0.008% or more. On the other hand, when Ti is contained excessively, TiN becomes coarse and the HAZ toughness improving effect is not exhibited. Therefore, Ti is 0.025% or less, preferably 0.02% or less, more preferably 0.018% or less.

N: 0.0015 to 0.010%
N is an element that forms TiN by combining with Ti, and is an element that acts to increase the HAZ toughness, similar to Ti. However, if N is less than 0.0015%, the amount of TiN produced is small and the HAZ toughness cannot be increased. Therefore, N is 0.0015% or more, preferably 0.002% or more, more preferably 0.003% or more. On the other hand, when N is contained excessively, TiN becomes coarse and the HAZ toughness improving effect is not exhibited. Therefore, N is 0.010% or less, preferably 0.08% or less, more preferably 0.05% or less.

B: 0.0005 to 0.003%
B is an element that suppresses the formation of intergranular ferrite [sometimes referred to as allotriomorphic ferrite or pro-eutectoid ferrite], and can improve HAZ toughness. However, if B is less than 0.0005%, the formation of grain boundary ferrite cannot be suppressed, and the HAZ toughness cannot be increased. Therefore, B is 0.0005% or more, preferably 0.0008% or more, more preferably 0.0010% or more. On the other hand, even if B is contained excessively, the effect of suppressing the formation of grain boundary ferrite is saturated, and excessive B combines with N or the like to form a boride, and on the contrary, deteriorates the HAZ toughness. Therefore, B is 0.003% or less, preferably 0.0028% or less, more preferably 0.0025% or less.

The content of Ti, N, and B needs to satisfy the following formula (1). In addition, in the following (1) Formula, [] has shown element content.
0 ≦ [N] −0.292 × [Ti] ≦ (14 / 10.8) × ([B] −0.0003)
... (1)

The atomic weight ratio of N and Ti is 0 ≦ [N] −0.292 × [Ti]
If this relationship is satisfied, N is bonded to Ti to bond TiN, or it exists in the steel as free N. At this time, since N is present in excess of the stoichiometric ratio of Ti, TiN can be refined and HAZ toughness can be improved.

Also, the contents of B, N and Ti are
[N] −0.292 × [Ti] ≦ (14 / 10.8) × ([B] −0.0003)
If the relationship shown by (3) is satisfied, 0.0003 mass% (3 ppm) or more of B required as free B exists in the thick steel plate, and generation of grain boundary ferrite can be suppressed. Note that 14 is the atomic weight of N (nitrogen), and 10.8 is the atomic weight of B (boron).

  The thick steel plate of the present invention contains the above elements, with the balance being Fe and inevitable impurities. Examples of inevitable impurities include P and S.

  P and S are elements that embrittle ferrite, and are preferably as small as possible. P is preferably 0.05% or less, more preferably 0.04% or less, and still more preferably 0.03% or less. S is preferably 0.05% or less, more preferably 0.03% or less, and still more preferably 0.02% or less.

  The thick steel plate of the present invention further includes at least one selected from the group consisting of Al, Mg, Ca, Sr, Ba, Ce, and La as other elements. An element may be contained.

(A) Cr: 0.5% or less (not including 0%), (b) Mo: 0.2% or less (not including 0%), (c) V, Nb, Zr, Hf and Ta One or more selected from the group: 0.05% or less in total (excluding 0%), at least one of Cr, Mo, V, Nb, Zr, Hf and Ta are all carbides in the steel It acts effectively to increase the strength by precipitation strengthening. However, if too much carbide is generated, the effect of the present invention is adversely affected because it inhibits the formation of residual γ.

  Therefore, Cr is 0.5% or less, preferably 0.4% or less, more preferably 0.3% or less. In order to exhibit the said effect | action, it is preferable to make it contain 0.02% or more, More preferably, it is 0.05% or more.

  Mo is 0.2% or less, preferably 0.15% or less, more preferably 0.1% or less. In order to exhibit the said effect | action, it is preferable to make it contain 0.02% or more, More preferably, it is 0.04% or more.

  V, Nb, Zr, Hf, and Ta are 0.05% or less in total, and preferably 0.04% or less. In order to exhibit the said effect | action, it is preferable to make it contain 0.01% or more, More preferably, it is 0.02% or more.

  The elements shown in the above (a) to (c) may be contained alone (for example, Cr only, Mo only, V only), or a combination of arbitrarily selected elements (for example, Cr and Mo). , Cr and V) may be contained.

Al: 1% or less (excluding 0%)
Al is an element that suppresses the decomposition of γ to form cementite and acts to generate a predetermined amount of residual γ. However, if the Al content exceeds 1%, the solid solution Al increases, the ferrite becomes brittle, and the uniform elongation decreases. Therefore, Al is 1% or less, preferably 0.5% or less, more preferably 0.3% or less. In order to exhibit the said effect | action, it is preferable to make it contain 0.02% or more, More preferably, it is 0.025% or more.

One or more elements selected from the group consisting of Mg, Ca, Sr, Ba, Ce and La in total 0.01% or less (excluding 0%)
Mg, Ca, Sr, Ba, Ce and La are elements that form fine oxides in the thick steel plate, and when this oxide is heated during welding, the structure of the HAZ part becomes coarse. This contributes to preventing HAZ and improving HAZ toughness. However, even if these elements are contained in excess of 0.01% in total, the effect of improving the HAZ toughness is saturated, and the amount of oxide becomes too large, which causes the tensile strength and toughness of the base material to deteriorate. Therefore, the total amount of the above elements is preferably 0.01% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. In order to exhibit the said effect | action, it is preferable to make it contain 0.0005% or more.

  Mg, Ca, Sr, Ba, Ce, and La may be contained alone or in combination of two or more elements that are arbitrarily selected.

  Next, the structure of the thick steel plate of the present invention will be described. The thick steel plate of the present invention contains a ferrite fraction of 80% by volume or more as a matrix structure and 1% by volume or more of residual γ as a second phase structure.

  In the thick steel plate of the present invention, the structure is mainly composed of ferrite in order to ensure a tensile strength of 490 MPa class. Ferrite is a soft and highly plastic structure and contributes to improvement of uniform elongation. In order to exert such an effect, the ferrite fraction is 80% by volume or more. The ferrite fraction is preferably 83% by volume or more, and more preferably 86% by volume or more. However, if ferrite is formed too much and the structure becomes a ferrite single phase, residual γ is not generated and uniform elongation is lowered, so the upper limit is 99% by volume. A preferred upper limit is 97% by volume, and a more preferred upper limit is 95% by volume.

  In the structure of the thick steel plate of the present invention, residual γ is generated as the second phase. Residual γ undergoes stress to transform into martensite and exhibits good uniform elongation due to the TRIP effect. In order to exert such an effect, the residual γ fraction must be 1% by volume or more. The residual γ fraction is preferably 2% by volume or more, and more preferably 2.5% by volume or more. However, if the residual γ fraction becomes too large, the amount of C in the residual γ decreases, and as will be described later, the residual γ is not stabilized and the TRIP effect cannot be obtained. Therefore, the residual γ fraction is preferably 20% by volume or less, more preferably 15% by volume or less, and still more preferably 10% by volume or less.

  Furthermore, in the present invention, it is important that the amount of C in the residual γ is 0.80 to 1.10% by mass. As demonstrated in the examples described later, the uniform elongation cannot be sufficiently increased only by using ferrite as a main component and generating a predetermined amount of residual γ as the second phase. The amount of C in the residual γ is closely related to the stability of the residual γ (the TRIP effect due to the generation of the residual γ). In the 490 MPa class thick steel plate targeted by the present invention, the amount of C in the residual γ By being controlled within the range, the best uniform elongation is exhibited when the residual γ undergoes a processing-induced transformation before the maximum load point. That is, if the amount of C in the residual γ is less than 0.80% by mass, the residual γ becomes unstable, the residual γ easily undergoes processing-induced transformation, and the uniform elongation cannot be increased. Therefore, the amount of C in the residual γ is 0.80% by mass or more, preferably 0.85% by mass or more, and more preferably 0.90% by mass or more. On the other hand, if the amount of C in the residual γ exceeds 1.10% by mass, the residual γ is overstabilized, so that the residual γ is difficult to undergo processing-induced transformation and the TRIP effect cannot be obtained. Therefore, the upper limit of the amount of C in the residual γ is 1.10% by mass, the preferable upper limit is 1.05% by mass, and the more preferable upper limit is 1.00% by mass.

  The thick steel plate of the present invention only needs to have a ferrite fraction of at least 80% by volume and a residual γ fraction of 1% by volume or more, and may contain a structure other than these. For example, bainite, martensite, pearlite, or the like may be generated. The remaining tissue ratio may be, for example, 19% by volume or less, and particularly preferably 15% by volume or less.

  What is necessary is just to measure the ratio of the structure of a thick steel plate, and the amount of C in residual (gamma) with the method described in the term of the Example mentioned later. In addition, about the ratio of the structure of a thick steel plate, it may be considered that the area ratio measured by microscopic observation is equal to the volume ratio.

  The steel plate of the present invention can be suitably used, for example, as a material for large structures such as ships that have a strength of 490 MPa and require high uniform elongation and weldability (HAZ toughness).

  The manufacturing method of the thick steel plate which concerns on this invention can be manufactured, for example using the method shown below. That is, a first step of generating 80% by volume or more of ferrite and 1% by volume or more of retained austenite, and a second step of controlling the amount of C in the retained austenite within a range of 0.80 to 1.10% by mass. In the second step, (a) an arbitrary temperature range between 500 and 300 ° C. is 50 to 3600 at an average cooling rate of 1 ° C./second or less (not including 0 ° C./second). It may be gradually cooled for 2 seconds, or (b) held at an arbitrary temperature between 500 and 300 ° C. for 50 to 3600 seconds.

  Specifically, after heating the steel having the above chemical components to a γ single phase temperature range, hot rolling and cooling the steel, an arbitrary temperature range between 850 ° C. and 620 ° C. is 2 ° C./second. Ferrite is generated by slow cooling at the following average cooling rate. After slow cooling, residual γ is generated by quenching at an average cooling rate of 10 ° C./second or higher to any temperature between 500 and 300 ° C. And (a) slowly cooling an arbitrary temperature range between 500 and 300 ° C. for a predetermined time at an average cooling rate of 1 ° C./second or less, or (b) any temperature between 500 and 300 ° C. By maintaining for a predetermined time, C can be concentrated in the residual γ, and the uniform elongation of the thick steel plate can be increased. These steps will be described with reference to the drawings.

  FIG. 1 is a diagram schematically showing the thermal history when manufacturing the thick steel plate of the present invention. In FIG. 1, the rolled material was processed under the conditions shown in Table 3 below using a water cooling facility and a heat treatment furnace. That is, the obtained rolled material was gradually cooled from the slow cooling start temperature T1 (° C.) to the slow cooling end temperature T2 (° C.) at an average cooling rate R1 (° C./second), and from T2, the slow cooling start temperature T3 ( C.) is rapidly cooled at an average cooling rate R2 (° C./second), and is gradually cooled from T3 to the annealing end temperature T4 (° C.) at an average cooling rate R3 (° C./second) (maintained at a constant temperature T3). Included). The time taken from the slow cooling start temperature T1 to the slow cooling end temperature T2 is defined as t1 (seconds), and the time taken from the slow cooling start temperature T3 to the slow cooling end temperature T4 is defined as t2 (seconds).

  First, the steel having the above chemical components is heated to a γ single-phase temperature range to be austenitized. The γ single phase temperature range is about 910 to 1300 ° C.

  After heating to the γ single phase temperature range, hot rolling is performed according to a conventional method. The finishing temperature of hot rolling may be about 900 to 800 ° C., and the finished plate thickness may be about 5 to 50 mm.

  After the hot rolling, an arbitrary temperature range T1 to T2 (° C.) between 850 to 620 ° C. is gradually cooled over 20 seconds (t1) at an average cooling rate of 2 ° C./second or less. This step is particularly important for securing the ferrite structure of the parent phase. When the annealing start temperature exceeds 850 ° C., the amount of ferrite produced decreases, and when the annealing end temperature falls below 620 ° C., austenite Is transformed into pearlite or bainite, the amount of ferrite produced is reduced, and residual γ is not produced.

  Moreover, when the residence time t1 in the said temperature range is less than 20 second, the production amount of a ferrite will decrease.

  The average cooling rate R1 (° C./second) in the temperature range T1 to T2 (° C.) may be 2 ° C./second or less, preferably 1 ° C./second or less. The average cooling rate R1 may be 0 ° C./second. For example, in the condition b of Table 3 of the example described later, the temperature is kept at 750 ° C. for 30 seconds (T1 = T2).

  After slow cooling or holding at constant temperature, rapid cooling is performed at an average cooling rate (R2) of 10 ° C./second or more while avoiding pearlite transformation or bainite transformation from temperature T2 to any temperature T3 (° C.) between 500 and 300 ° C. As a result, austenite that is not transformed into ferrite can be generated as residual γ, and the generated residual γ can be prevented from being transformed into pearlite or bainite. The average cooling rate R2 is preferably 15 ° C./second or more, more preferably 20 ° C./second or more.

  After the rapid cooling to the temperature T3, an arbitrary temperature range T3 to T4 (° C) between 500 to 300 ° C is gradually increased over 50 to 3600 seconds (t2) at an average cooling rate (R3) of 1 ° C / second or less. Cool or hold at an arbitrary temperature (T3) between 500 and 300 ° C. for 50 to 3600 seconds (t2) (At this time, R3 is 0 ° C./second and T3 = T4). When the slow cooling start temperature T3 or the constant temperature holding start temperature T3 exceeds 500 ° C., the residual γ undergoes pearlite transformation, and the residual γ cannot be generated. Further, even if the annealing end temperature is below 300 ° C., residual γ cannot be generated.

  This process is extremely important for ensuring the amount of C in the residual γ. In particular, the above-mentioned amount of C can be obtained for the first time only by gradually cooling or keeping the temperature range over a predetermined time (t2) (see Examples described later). In addition, since the method described in Patent Document 3 and Patent Document 4 described above does not include this step, it is considered that a desired CγR is not obtained.

  If the average cooling rate R3 in the temperature range T3 to T4 exceeds 1 ° C./second, or if the residence time t2 in this temperature range is less than 50 seconds, the concentration of C in the residual γ becomes insufficient, and the thickness The uniform elongation of the steel sheet cannot be increased. Therefore, the average cooling rate R3 is 1 ° C./second or less, preferably 0.5 ° C./second or less. The average cooling rate R3 may be 0 ° C./second. When the cooling rate is 0 ° C./second, the temperature is kept constant at T3, and T3 = T4. For example, in the condition d of Table 3 of the example described later, the temperature is maintained at 400 ° C. for 1800 seconds.

  The residence time t2 is 50 seconds or longer, preferably 100 seconds or longer, more preferably 300 seconds or longer. However, when the residence time t2 exceeds 3600 seconds, C is excessively concentrated in the residual γ, the residual γ is stabilized, and the uniform elongation is reduced. Therefore, the residence time t2 is 3600 seconds or less, preferably 3000 seconds or less, more preferably 2000 seconds or less.

  After slow cooling or holding at a constant temperature in the temperature range of 500 to 300 ° C., air cooling may be performed according to a conventional method.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

  After the steel composition (the balance is Fe and inevitable impurities) shown in the following Table 1 or Table 2 obtained by melting in a small vacuum melting furnace is heated at 1100 ° C. for 30 minutes to form an austenite single phase, Rolled for a while. The finish rolling temperature was 800 ° C., and the finished plate thickness was 12.5 mm. In the following Table 1 or Table 2, the value of “[N] −0.292 × [Ti]” and “(14 / 10.8) × The value of ([B] −0.0003) ”was calculated and the value was also shown.

  The obtained hot rolled material was cooled under the conditions shown in Table 3 below using a water cooling facility and a heat treatment furnace. In Table 3, T1 to T4, t1 to t2, and R1 to R3 correspond to the symbols given in FIG.

  After cooling to the annealing end temperature T4, air cooling to room temperature was performed to produce a thick steel plate (plate thickness 12.5 mm).

  About the obtained thick steel sheet, the structure [ferrite fraction, residual γ fraction, C amount in residual γ] and base material characteristics [yield point (YP), tensile strength (TS), uniform elongation in the following manner] (UE), toughness] was measured. The measurement results are shown in Tables 4 to 7 below.

[Organization]
In this example, (a) the ferrite fraction, (b) the residual γ fraction, and (c) the C content in the residual γ were measured at a thickness 1/4 portion of the thick steel plate. However, the observation site | part of a structure | tissue is not limited to this, For example, it has confirmed that the same structure | tissue as the thickness 1/4 site | part is produced | generated also at the board thickness 1/2 site | part.

(A) Ferrite fraction A test piece obtained by mirror-polishing a 1/4 thickness portion of a thick steel plate was etched (corroded) with a 3% nital solution, and then 10 pictures were taken at 400 times using an optical microscope. . For each photo, the white equiaxed area was traced as ferrite and the other areas as the second phase, and this was analyzed with image analysis software ("Image-pro" from micromedia) to measure the area ratio of ferrite. The average value of 10 fields of view was calculated. This ferrite area ratio was defined as the ferrite fraction.

In order to eliminate an error due to the area of the line when tracing, the ferrite area ratio was calculated using the following formula.
Ferrite area ratio = [area of ferrite surrounded by traced lines / (area of observation field-total area of traced lines)] × 100

(B) Residual γ fraction (VγR)
X-ray diffraction is performed on a test piece obtained by mirror-polishing a 1/4 thickness part of a thick steel plate, and the theoretical strength ratio is calculated from the peak strength ratio of the α-Fe (200) surface and γ-Fe (200) surface by the Liberty method. To determine the residual γ fraction. As the X-ray diffractometer, RAD-RU300 manufactured by Rigaku Denki was used, the target was Co, and the target output was 40 kV, 200 mA.

(C) C content in residual γ (CγR)
Si is applied as a standard material to a test piece obtained by mirror-polishing a 1/4 thickness part of a thick steel plate, and peak positions of Si and residual γ (γR) are determined. J. et al. Dyson et al. , Journal of The Iron and Steel Institute, (1970), p469-474, the lattice constant a0 of γR was measured. The peaks used are (111), (200), (220), and (311).

From the lattice constant a0, the amount of C in residual γ (CγR) was determined using the following equation.
CγR = (a0−3.578−0.00095 × [Mn] + 0.0002 × [Ni] −0.0006 × [Cr] −0.022 × [N] −0.0056 × [Al] +0.0004 × [Co] −0.0015 × [Cu] −0.0031 × [Mo] −0.0051 × [Nb] −0.0039 × [Ti] −0.0018 × [V] −0.0018 × [ W]) / 0.033

[Base material properties]
(A) Tensile test “Metal material tensile test method” defined in JIS Z2241 using a No. 14 test piece (parallel part diameter: 10 mm) defined in JIS Z2201 from a 1/4 thickness portion of a thick steel plate. The yield point (YP), tensile strength (TS), and uniform elongation (UE) were measured based on the above. The test speed during the tensile test was 0.5 mm / second. The case where TS was 490 MPa or more and less than 590 MPa was regarded as acceptable, and the case where UE was 18.0% or more was regarded as acceptable. UE means elongation at the highest load point. FIG. 2 shows the relationship between the amount of C in the residual γ and the uniform elongation.

(B) Impact test Using a V-notch test piece stipulated in JIS Z2202 from a 1/4 thickness part of a thick steel plate, a Charpy impact test was conducted based on the “metal material impact test method” stipulated in JIS Z2242. The absorption energy (vE- ₄₀ ) at _-40 degreeC was measured by performing. A case where vE- ₄₀ was 100 J or more was regarded as acceptable.

(C) HAZ toughness In order to simulate a heat-affected zone (HAZ zone) during welding, the thick steel plate was heated at 1350 ° C. for 5 seconds and then continuously cooled. At this time, the temperature range from 800 ° C. to 500 ° C. was cooled over 40 seconds, and then air cooled. After air cooling, a V-notch test piece specified in JIS Z2202 is cut out, and a Charpy impact test is performed based on the “metal material impact test method” specified in JIS Z2242, so that the absorbed energy at −20 ° C. (vE _-20 ) was measured. A case where vE- ₂₀ was 100 J or more was regarded as acceptable.

  Table 4 to Table 7 can be considered as follows. No. 1-5, No. 1 7, no. 13, no. 16-23, no. 29-30, no. 40, no. 42-46, no. 48-51, no. No. 53 is an example that satisfies the requirements specified in the present invention. Since the composition and structure of the thick steel plate are appropriately controlled, the uniform elongation is high, and the HAZ toughness is excellent. A thick steel plate was obtained.

  On the other hand, no. No. 6 is an example in which C is low, and ferrite is generated, but the amount of residual γ generated is small and uniform elongation is low. No. 8 is an example in which there is a large amount of C, and since the generation of ferrite is small, the strength is too high. Further, the uniform elongation is low, and the base metal toughness and the HAZ toughness are also low. No. No. 9 is an example with little Si, and no residual γ is generated. Therefore, the uniform elongation is low. No. 10 is an example with much Si, and uniform elongation is low. Also, the base metal toughness and the HAZ toughness are poor. No. 11 is an example in which Mn is small, the amount of C in the residual γ is small, and the uniform elongation is low. No. No. 12 is an example with much Mn, and since the ferrite fraction is small, the strength becomes too high and the uniform elongation is also low. No. No. 14 is an example not containing Cu and Ni, the amount of C in the residual γ is small, and the uniform elongation is low. No. 15 is an example in which the total content of Cu and Ni is large, the ferrite fraction is small, the strength becomes too high, and the uniform elongation is low. No. Nos. 24-28 are examples in which the contents of Ti, B, N deviate from the range defined in the present invention, and the HAZ toughness is low. No. No. 41 is an example in which the contents of Cu and Ni are small, the amount of C in the residual γ is small, and the uniform elongation is low. No. 47 is an example in which the content of Ti, N, and B does not satisfy the above formula (1), and the HAZ toughness is deteriorated.

  No. 31-39 and No.3. No. 52 is an example in which the production conditions deviate from the range recommended in the present invention, and whether the ferrite fraction is low (No. 31 and 32, No. 52) or whether residual γ is generated (No. .31, 33 to 36), and the amount of C in the residual γ cannot be controlled within the range defined by the present invention (No. 37 to 39, No. 52), and thus the uniform elongation is low.

FIG. 1 is a diagram schematically showing the thermal history when manufacturing the thick steel plate of the present invention. FIG. 2 is a graph showing the relationship between the amount of C in the residual γ and the uniform elongation.

Claims (6)

C: 0.02 to 0.20% (meaning mass%; hereinafter the same for chemical components),
Si: 0.2 to 0.5%
Mn: 1 to 1.8%,
Cu and / or Ni: 0.2 to 1% in total,
Ti: 0.005 to 0.025%,
N: 0.0015 to 0.010%,
B: 0.0005 to 0.003% is contained,
The balance is steel composed of Fe and inevitable impurities,
The content of Ti, N, B satisfies the following formula (1),
Ferrite fraction: 80% by volume or more,
Residual austenite fraction: 1% by volume or more, and the C content in the retained austenite is 0.80 to 1.10% by mass.
0 ≦ [N] −0.292 × [Ti] ≦ (14 / 10.8) × ([B] −0.0003)
... (1)
[In the above formula (1), [] indicates the element content. ]   The high-tensile thick steel plate according to claim 1, wherein the C is 0.02 to 0.10%. Furthermore, the high-tensile steel plate of Claim 1 or 2 which contains at least 1 sort (s) among following (a)-(c) as another element.
(A) Cr: 0.5% or less (excluding 0%)
(B) Mo: 0.2% or less (excluding 0%)
(C) one or more selected from the group consisting of V, Nb, Zr, Hf and Ta: 0.05% or less in total (excluding 0%)   Furthermore, Al: 1% or less (0% is not included) as another element, The high-tensile steel plate in any one of Claims 1-3.   Furthermore, as another element, 1 or more types chosen from the group which consists of Mg, Ca, Sr, Ba, Ce, and La are contained 0.01% or less (excluding 0%) in total. The high-tensile thick steel plate according to any one of the above. In producing the high-tensile thick steel plate according to any one of claims 1 to 5,
A first step of generating 80% by volume or more of ferrite and 1% by volume or more of retained austenite, and a second step of controlling the amount of C in the retained austenite within a range of 0.80 to 1.10% by mass. And the second step includes
Slowly cool any temperature range between 500-300 ° C at an average cooling rate of 1 ° C / second or less for 50-3600 seconds, or hold at any temperature between 500-300 ° C for 50-3600 seconds A method for producing a high-tensile thick steel plate.

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