JP2006241556A - High tensile strength steel sheet having reduced acoustic anisotropy and excellent weldability and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength steel sheet having reduced acoustic anisotropy and excellent weldability, in which the amount of Mn to be added is increased, and further, the amounts of Ni and Cu to be added can be reduced, and which also has high tensile strength satisfying the yield strength of ≥450 MPa, and to provide its production method. <P>SOLUTION: The high tensile strength steel sheet has a composition comprising, by mass, 0.03 to 0.07% C, 0.10 to 0.60% Si, 2.0 to 3.0% Mn, ≤0.06% Al, ≥0.025% Nb, ≥0.005% Ti and 0.002 to 0.008% N, and in which the Nb and Ti satisfy 0.045%≤Nb+2×Ti≤0.105%, and comprising P: 0.020-0.04(C+Mn/20)(%) or below and S: 0.015-0.04(C+Mn/20)(%) or below, and in which the value of A=(Nb+2×Ti)×(C+N×12/14) is 0.0022 to 0.0055, and the balance iron with inevitable impurities. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a high-tensile steel plate having small acoustic anisotropy and excellent weldability, and a method for producing the same.

High-strength steel sheets with a yield strength of 450 MPa or more used as welded structural members such as bridges, ships, building structures, offshore structures, pressure vessels, penstocks, and line pipes require strength and toughness as well as weldability. In recent years, weldability with particularly high heat input is often required, and many studies have been made on improving the properties. Many proposals have been made for the composition and production conditions of this high-strength steel sheet. Yes. For example, in the manufacturing method, after rolling a steel plate, the method of carrying out reheating quenching offline, and also the reheating tempering process has already been proposed (for example, refer patent documents 1 and 2).
Moreover, the technique regarding what is called direct hardening which performs quenching online after rolling a steel plate is proposed (for example, refer patent documents 3-5).
Thus, in any case of reheating quenching and direct quenching, an offline tempering process is required.

By the way, in the manufacturing method of the conventional high-tensile steel sheet, since the tempering process step is performed offline, there is a disadvantage that the productivity is lowered. Therefore, in order to increase the productivity, the tempering process is omitted. So-called non-tempered manufacturing methods that do not require off-line heat treatment have been proposed (see, for example, Patent Documents 6 to 9).
As a non-tempered manufacturing method, an accelerated cooling-intermediate stop process in which accelerated cooling after rolling a steel sheet is stopped in the middle has already been proposed.

This accelerated cooling-intermediate stop process is a rapid cooling process by stopping water cooling at a relatively high temperature after transformation while rapidly quenching to below the transformation temperature by accelerated cooling to obtain a quenched structure. In this method, a tempering effect is obtained in the slow cooling process, and reheating and tempering are omitted.
In this accelerated cooling-intermediate stopping process, a high-tensile steel sheet having a tensile strength of 570 MPa or higher is obtained (Patent Document 9).
Further, as a technique for improving the non-tempered manufacturing method, a non-tempered process in which water cooling is not performed after rolling has been proposed (see, for example, Patent Document 10). This non-tempering process has an advantage that acoustic anisotropy does not increase because controlled rolling at low temperature is not performed.
JP-A-53-119219 JP-A-01-149923 JP 52-081014 A JP-A-63-033521 Japanese Patent Laid-Open No. 02-205627 JP 54-021917 A Japanese Patent Laid-Open No. 54-071714 JP 2001-064723 A JP 2001-064728 A JP 2002-053912 A

  By the way, since the conventional non-tempered manufacturing methods all require controlled rolling at a relatively low temperature in order to obtain toughness and strength, the temperature at which the rolling is finished is around 800 ° C., and the temperature waiting time Therefore, it cannot be said that productivity is high. On the other hand, especially in applications to bridges, buildings, etc., it is required to have low acoustic anisotropy in order to affect the accuracy of the ultrasonic oblique flaw detection test of welds, but rolling at a temperature of about 800 ° C. In the controlled rolling that terminates, the texture is formed and the steel sheet has a large acoustic anisotropy, and is not necessarily suitable for use in bridges, buildings, and the like.

Further, in the conventional accelerated cooling-intermediate stop process, a high-tensile steel sheet having a tensile strength of 570 MPa or higher is obtained, but V contained in this steel sheet contributes to precipitation strengthening even in the slow cooling stage after the accelerated cooling stop. In reality, however, V has a slower precipitation rate in the slow cooling stage after stopping the accelerated cooling on the way compared to Nb and Ti, and it is difficult to think that it is positively contributing to precipitation strengthening. In this component composition, a stable strength is not always obtained.
In addition, in the non-tempering process in which water cooling is not performed after rolling, in order to obtain strength, the content of expensive metal components such as Cu and Ni is large, and there is a problem in the stability and productivity of characteristics in the manufacturing process, In addition, there will be a problem with economy.

  The present invention has been made in view of the above circumstances, and can increase the addition amount of Mn and reduce the addition amount of expensive Ni and Cu, and the yield strength is 450 MPa or more. It is an object of the present invention to provide a high-tensile steel sheet having high tensile strength, small acoustic anisotropy and excellent weldability, and a method for producing the same.

There are several means for strengthening high-strength steel, but methods using precipitation strengthening such as carbides or nitrides of Nb, V, Ti, Mo, and Cr can be strengthened with relatively few alloy components. At that time, in order to obtain a large precipitation strengthening amount, it is important to form precipitates that are consistent with the substrate.
In particular, in the accelerated cooling-intermediate stop process, the steel structure is austenite at the stage of rolling, and is transformed by accelerated cooling to become a structure of a ferrite base such as bainite or ferrite. Precipitates precipitated in austenite before rolling and accelerated cooling lose consistency with the substrate after transformation, and the strengthening effect is reduced. In addition, precipitates deposited at an early stage of rolling become coarse and cause toughness to decrease. Therefore, it is important to suppress precipitation of precipitates during rolling and before accelerated cooling, and to precipitate as much as possible in the bainite or ferrite structure at the stage of slow cooling after the stop of accelerated cooling.

If it is a process in which tempering heat treatment is carried out by reheating after water cooling, sufficient temperature and time for precipitation can be taken, so that large precipitation strengthening can be easily obtained.
On the other hand, in the case of the accelerated cooling-interruption process without reheating and tempering, precipitation is expected during the slow cooling after the accelerated cooling stop, but the accelerated cooling stop temperature must be lowered to some extent to obtain a quenched structure. Therefore, both the temperature and time for precipitation are limited, which is generally disadvantageous for precipitation strengthening.
As described above, while the conventional non-tempering process is high in productivity, on the other hand, in order to obtain the same strength as the tempering process, either a large amount of alloying elements or controlled rolling at a low temperature is performed. That is why we had to take a method.

Accordingly, the present inventors have made maximum precipitation strengthening especially in order to obtain high strength without using a large amount of alloying elements or controlled rolling at a low temperature while using a highly productive accelerated cooling-intermediate stop process as a basic process. We studied earnestly on how to make the best use.
First, in order to clarify the precipitation behavior in the slow cooling process after stopping accelerated cooling, the precipitation rate and precipitation strengthening amount of carbide, nitride, carbonitride of each alloy element in bainite or ferrite structure or mixed structure thereof The relationship between temperature and holding time was examined in detail. As a result, in the bainite or ferrite structure or a mixed structure thereof, the precipitation rate of Nb carbonitride and Ti carbide is faster than the precipitation rate of other elements such as V, and these are consistent with the substrate. Therefore, it was found that the amount of strengthening is large, and in particular, the precipitation rate is high in the temperature range of 550 to 700 ° C., and the amount of strengthening is large.
In addition, when Nb and Ti or Nb, Ti and Mo are used in combination, precipitates that are consistent with the substrate can be finely dispersed and a large precipitation strengthening can be obtained even in a short time due to a synergistic effect. I understood that I could do it.

Further, when the content of Nb and Ti is too large, the generated precipitates tend to be coarse, and the number of precipitates is rather reduced, so that the precipitation strengthening amount is reduced, and the carbides of Nb and Ti, The precipitation rate of nitride and / or carbonitride in austenite and / or ferrite and the form of the precipitate are greatly influenced by the contents of Nb and Ti and the contents of C and N, respectively.
Through various experiments and analyses, the present inventors have determined that the precipitation rate and the precipitation form of Nb, Ti carbide, nitride and / or carbonitride are parameters A = (Nb + 2 × Ti) × (C + N × 12/14). ) Is well organized, and by suppressing this A value within a certain range, it has been found that fine precipitation during slow cooling after stopping during water cooling can be sufficiently obtained while suppressing precipitation during rolling. It was.

  That is, as the content of Nb and Ti increases, the content of C and N needs to be reduced. Here, if the value of A is too small, the precipitation rate in ferrite is slowed, and sufficient precipitation strengthening cannot be obtained. On the other hand, if the value of A is too large, carbides, nitrides and / or carbons in austenite. Since the precipitation rate of nitride becomes too fast, the precipitate becomes coarse, and the amount of consistent precipitation during slow cooling after the stop of accelerated cooling is also insufficient, so the precipitation strengthening amount is also lowered. Further, since Si also affects the rate of carbide formation, it is necessary to set the content within a certain range.

  These precipitation strengthening effects are also greatly affected by the structure. The bainite structure is easier to maintain a processed structure such as dislocation density than ferrite. In order to promote fine alignment precipitation, it is very effective that there are sufficient precipitation sites such as dislocations and deformation bands included in the processed structure. According to the study by the present inventors, in order to obtain sufficient strengthening, it is necessary to use a bainite single phase or a mixed structure of bainite and ferrite having a bainite volume fraction of 40% or more. In addition, pearlite and island martensite are precipitated at the phase interface and the strengthening effect is reduced, and the toughness is reduced. Therefore, the sum of the volume fractions of pearlite and island martensite must be 8% or less. It is.

Subsequently, the present inventors examined specific production conditions for obtaining the maximum precipitation strengthening effect, and obtained the following knowledge.
The present invention obtains strength by making the best use of precipitation strengthening of Nb, Ti, etc. in the accelerated cooling-intermediate stop process subsequent to rolling, and sufficient Nb, Ti when heating the steel slab or slab prior to rolling. It is necessary to make it solid solution. However, when Nb and Ti coexist, they tend to be harder to dissolve than in the case where they exist alone, and these are not always sufficient when heated to the solid solution temperature expected from their respective solubility products. It was found that it did not dissolve.

The present inventors investigated the heating temperature and the solid solution state of Nb and Ti in the high-strength steel of the present invention, and particularly analyzed in detail the relationship between the A value and the solid solution state of Nb and Ti.
As a result, the temperature T (° C.) calculated by the conditional expression including the A value as shown below for the heating temperature of the steel slab or slab.
T = 6300 / (1.9-Log (A))-273
However, A = (Nb + 2 × Ti) × (C + N × 12/14)
As a result, it was concluded that Nb and Ti can be sufficiently dissolved.

  Since precipitation of Nb and Ti in this rolling stage is promoted by rolling strain, rolling conditions in a high temperature range of austenite, so-called rough rolling conditions, greatly influence the final precipitation strengthening effect. Specifically, rough rolling is completed in a temperature range of 1020 ° C. or higher, and not rolling as much as possible in a temperature range of 920 to 1020 ° C. is a requirement for suppressing precipitation during rolling. However, if all rolling is completed in a temperature range of 1020 ° C. or higher, almost no work structure remains after accelerated cooling-interruption due to recovery and recrystallization, so there are sufficient precipitation sites such as dislocations and deformation bands. It does not exist and sufficient precipitation strengthening cannot be obtained. Therefore, it is an essential condition to perform necessary and sufficient rolling in the non-recrystallization temperature range and to perform accelerated cooling immediately after rolling. Specifically, relatively mild rolling with a cumulative reduction of 20 to 50% is performed in a limited range of 860 to 920 ° C. Under these conditions, the rolling strain is not excessively large, so that unnecessary precipitation of Nb and Ti is suppressed and a strong texture is not formed, so that acoustic anisotropy does not increase. In addition, a necessary amount of rolling strain can be ensured in order to leave an appropriate precipitation site even after the accelerated cooling is stopped.

  The accelerated cooling stop temperature of the accelerated cooling-intermediate stopping process is 550 to 700 ° C. so that each of Nb and Ti easily precipitates, but even at such a high stopping temperature, the volume fraction of bainite is 40% or more, And in order to obtain the steel structure whose sum of the volume ratio of a pearlite and an island-like martensite is 8% or less, while limiting the component composition of steel to the specific range mentioned later, in accelerated cooling, it is 2-30 degrees C / sec. A cooling rate is required.

The knowledge obtained here is a new way of controlling the precipitation of carbides or carbonitrides of Nb and Ti on-line during rolling, including high temperature ranges, during accelerated cooling, and during the slow cooling process after cooling stop. Further, precipitation strengthening over the conventional tempering process can be realized by an accelerated cooling-intermediate stop process that does not require an off-line heat treatment.
Further, according to this manufacturing process, the weld cracking susceptibility composition P _CM (= C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B) of the steel material composition can be kept low, and P _CM ≦ 0.22% Thus, it is possible to obtain a high-tensile steel sheet having high weld heat-affected zone toughness and excellent weldability even at high heat input and having a yield strength of 450 MPa or more.

As a result of intensive studies as described above, the present inventors have completed the present invention, and the gist of the present invention is as follows.
(1) By mass%, C: 0.03 to 0.07%, Si: 0.10 to 0.60%, Mn: 2.0 to 3.0%, Al: 0.06% or less, Nb: Containing 0.025% or more, Ti: 0.005% or more, N: 0.002 to 0.008%,
And Nb and Ti satisfy 0.045% ≦ Nb + 2 × Ti ≦ 0.105%,
Further, P: 0.020-0.04 (C + Mn / 20) (%) or less, S: 0.015-0.04 (C + Mn / 20) (%) or less,
Furthermore, the value of A represented by A = (Nb + 2 × Ti) × (C + N × 12/14) is 0.0022 to 0.0055,
A high-tensile steel sheet having a small acoustic anisotropy and excellent weldability, wherein the balance is iron and inevitable impurities.

(2) The high-tensile steel sheet having a small acoustic anisotropy and excellent weldability according to (1), further comprising, by mass%, Mo: 0.05 to 0.30%.

(3) Further, Cu and Ni satisfying Ni: 1.0% or less, Cu: 1.0% or less, Cu / 2 ≦ Ni ≦ Mn / 1.5, Ni + Cu ≦ 1.5% by mass% The high-tensile steel sheet having a small acoustic anisotropy and excellent weldability according to (1) or (2),

(4) Furthermore, Cr: 0.05-0.50%, V: 0.01-0.05%, W: 0.1-2.0%, B: 0.0002-0. (1), (2) or (3) characterized in that it has a small acoustic anisotropy and excellent weldability, comprising one or more selected from the group of 003% Tensile steel plate.

(5) Further, it is characterized by containing one or two of Mg: 0.0002 to 0.005% and Ca: 0.0005 to 0.006% by mass% (1 A high-tensile steel plate having small acoustic anisotropy and excellent weldability according to any one of (1) to (4).

(6) The fraction of bainite at a quarter thickness of the steel sheet is 40% or more, and the sum of the fractions of pearlite and island martensite is 8% or less (1) to ( 5) A high-tensile steel sheet having small acoustic anisotropy and excellent weldability.

(7) P _{CM of the} steel sheet = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
High-tensile steel sheet acoustic anisotropy and excellent small weldability according to (4) that in represented by weld crack susceptibility composition P _CM is equal to or less than 0.22%.

(8) The high strength steel sheet having low acoustic anisotropy and excellent weldability according to any one of (1) to (7), wherein the yield strength of the steel sheet is 450 MPa or more.

(9) A method for producing a high-tensile steel sheet having low acoustic anisotropy and excellent weldability according to any one of (1) to (8),
A steel piece or slab having the steel composition according to any one of (1) to (5),
T = 6300 / (1.9-Log (A))-273
A = (Nb + 2 × Ti) × (C + N × 12/14)
Is heated to a temperature of not less than T and not more than 1300 ° C., then is roughly rolled at a temperature of not less than 1020 ° C., and then a cumulative rolling reduction in a temperature range of 920 to 1020 ° C. is 15% or less and a temperature of 860 to 920 ° C. Rolled so that the cumulative reduction ratio in the range is 20 to 50%, then cooled at a cooling rate of 2 to 30 ° C./second from a temperature range of 800 ° C. or higher, and then from a temperature range of 550 to 700 ° C. A method for producing a high-tensile steel sheet having low acoustic anisotropy and excellent weldability, characterized by cooling at a cooling rate of 0.4 ° C./second or less.

According to the high-tensile steel sheet having small acoustic anisotropy and excellent weldability according to the present invention, Nb and Ti satisfy 0.045% ≦ Nb + 2 × Ti ≦ 0.105%, and P: 0.020− 0.04 (C + Mn / 20) (%) or less, S: 0.015-0.04 (C + Mn / 20) (%) or less, and further A = (Nb + 2 × Ti) × (C + N × 12 / 14) Since the value of A shown in FIG. 14 is 0.0022 to 0.0055, the acoustic anisotropy can be reduced, the weldability can be improved, and the yield strength is maintained at 450 MPa or more. Can do.
In addition, the relatively inexpensive Mn content is set to 2.0 to 3.0%, and as a result, the content of expensive metals such as Cu and Ni is reduced, so that the acoustic anisotropy is small and the weldability is reduced. It is possible to provide an inexpensive high-tensile steel sheet having a yield strength of 450 MPa or more.

According to the method for producing a high-tensile steel sheet having low acoustic anisotropy and excellent weldability according to the present invention, a steel slab or slab having the steel composition of the present invention,
T = 6300 / (1.9-Log (A))-273
A = (Nb + 2 × Ti) × (C + N × 12/14)
Is heated to a temperature of not less than T and not more than 1300 ° C., then is roughly rolled at a temperature of not less than 1020 ° C., and then a cumulative rolling reduction in a temperature range of 920 to 1020 ° C. is 15% or less, Rolled so that the cumulative reduction ratio in the range is 20 to 50%, then cooled at a cooling rate of 2 to 30 ° C./second from a temperature range of 800 ° C. or higher, and then from a temperature range of 550 to 700 ° C. Since cooling is performed at a cooling rate of 0.4 ° C / second or less, an economical steel composition and production in which a large amount of relatively inexpensive Mn is added and, as a result, the content of expensive metals such as Cu and Ni is reduced. By virtue of the synergistic effect of the accelerated cooling / intermediate stop process, a high-tensile steel sheet having small acoustic anisotropy and excellent weldability and yield strength of 450 MPa or more can be produced with good productivity and at low cost.

An embodiment of a high-tensile steel plate having a small acoustic anisotropy and excellent weldability according to the present invention and a method for producing the same will be described.
Note that this embodiment is described in detail for better understanding of the gist of the invention, and thus does not limit the present invention unless otherwise specified.

The high-tensile steel plate of the present invention is
In mass%, C: 0.03 to 0.07%, Si: 0.10 to 0.60%, Mn: 2.0 to 3.0%, Al: 0.06% or less, Nb: 0.025 % Or more, Ti: 0.005% or more, N: 0.002 to 0.008%,
And Nb and Ti satisfy 0.045% ≦ Nb + 2 × Ti ≦ 0.105%,
Further, P: 0.020-0.04 (C + Mn / 20) (%) or less, S: 0.015-0.04 (C + Mn / 20) (%) or less,
Furthermore, the value of A represented by A = (Nb + 2 × Ti) × (C + N × 12/14) is 0.0022 to 0.0055,
The balance consists of iron and inevitable impurities.

Here, the reason why the composition of the high-tensile steel plate is limited as described above will be described.
C is an important element that is a main element of the steel strengthening mechanism, and increases the strength of the steel by forming carbides and carbonitrides with Nb and Ti in the steel sheet. It is limited to a range of 0.07%. Here, the reason for limiting the content of C as described above is that if it is less than 0.03%, the amount of precipitation during slow cooling after accelerating cooling stop is insufficient, and a desired strength cannot be obtained. On the other hand, if it exceeds 0.07%, the precipitation rate in the γ austenite region during rolling is increased, and as a result, the amount of consistent precipitation during slow cooling after stopping accelerated cooling is insufficient, and the desired strength cannot be obtained. .

  Si is an element necessary as a deoxidizing element on steelmaking, and is an element that controls the precipitation rate of carbides. By adding an appropriate amount of Si, it suppresses the precipitation of carbides in the austenite region during rolling. The amount is limited to a range of 0.10 to 0.60%. Here, the reason for limiting the Si content as described above is that if it is less than 0.10%, precipitation of carbides in the austenite region cannot be suppressed, whereas if it exceeds 0.60%, This is because the precipitation rate of carbides in the austenite region during rolling becomes too slow, and the toughness of the weld heat affected zone may be reduced. A more preferable range of Si is 0.25 to 0.60%.

Mn is an element necessary for improving hardenability and stably obtaining a bainite single phase or a mixed structure of bainite and ferrite having a bainite fraction of 40% or more, and is in a range of 2.0 to 3.0%. It is limited.
Here, the reason why the Mn content is limited as described above is that when the content is less than 2.0%, a tensile strength of 400 MPa or more can be stably obtained without adding an expensive metal such as Cu or Ni. On the other hand, if it exceeds 3.0%, the base material toughness may be lowered.

Al is most commonly used as a deoxidizing element, but Si or Ti is sufficient for deoxidation, and the lower limit is not particularly limited, and includes 0%. Therefore, it can be selectively added as a deoxidizing element supplementing Si or Ti.
In particular, in the case of Ti-added steel, in some cases, it is desirable not to contain Al in order to use intragranular transformation. When the content is increased, not only the cleanliness of the steel but also the toughness of the weld metal is deteriorated, so the upper limit of the content was set to 0.060%.

Nb and Ti are Cb and N together with NbC, Nb (CN), TiC, TiN, Ti (CN), or a composite precipitate thereof, and a composite precipitate of Mo and N in an accelerated cooling-intermediate stop process. It is an important element that forms an object and becomes the main element of the strengthening mechanism of the steel of the present invention.
In order to obtain sufficient composite precipitates in this accelerated cooling-intermediate stop process, the Nb and Ti contents need to be in an appropriate range in consideration of the precipitation rate.

  Therefore, the Nb content is 0.025% or more, preferably 0.035% or more, the Ti content is 0.005% or more, and Nb and Ti are 0.045% ≦ Nb + 2 × Ti ≦ 0.105%, preferably 0.055% ≦ Nb + 2 × Ti ≦ 0.105%, and the value of A represented by A = (Nb + 2 × Ti) × (C + N × 12/14) Was set to 0.0022 to 0.0055.

P and S are impurities in the steel of the present invention, and as the content increases, the toughness of the steel sheet and the like are deteriorated. However, reducing the content more than necessary increases the load on the steelmaking process and is not a good measure in terms of productivity and cost.
Therefore, the upper limits of the contents of P and S are limited according to the contents of C and Mn, respectively.

Here, as the contents of C and Mn are large (as a result, segregation is also promoted), the upper limit values of P and S are suppressed, so the contents of P and S are as follows. Limited to the street.
P ≦ 0.020−0.04 (C + Mn / 20) (%)
S ≦ 0.015-0.04 (C + Mn / 20) (%)
In addition to P, S, it is more preferable that the total amount of P and S is 0.018% or less.
Here, the reason why the total amount of P and S is limited to 0.018% or less is that the toughness of the base material and the welded portion is not greatly deteriorated.

  N combines with Ti to form TiN. When TiN is finely dispersed, it suppresses the coarsening of the weld heat affected zone structure by the pinning effect and improves the toughness of the weld heat affected zone, but if N is insufficient, TiN becomes coarse and the pinning effect is obtained. It becomes impossible. In order to finely disperse TiN, N must be contained in an amount of 0.002% or more, preferably 0.003% or more. On the other hand, if N is contained excessively, the toughness of the base metal may be lowered, so the upper limit was made 0.008%.

In addition to the above composition, the high-tensile steel plate of the present invention has
Furthermore, it is preferable to contain Mo: 0.05 to 0.30% by mass%.
Mo greatly contributes to precipitation strengthening by improving hardenability and forming a composite precipitate with Nb and Ti. In order to obtain this effect, the lower limit of the Mo content is set to 0.05%. On the other hand, if Mo is added excessively, there is a possibility that the weld heat-affected zone toughness may be impaired, so the upper limit is made 0.30%.

In addition to the above composition, the high-tensile steel plate of the present invention has
Furthermore, in mass%, Ni: 1.0% or less, Cu: 1.0% or less, Cu / 2 ≦ Ni ≦ Mn / 1.5, Ni + Cu ≦ 1.5%
It is preferable to contain Cu and Ni that satisfy the requirements.

Here, the reason why Ni and Cu are limited as described above will be described.
Both Ni and Cu improve the strength and toughness of the base metal without adversely affecting the weldability and weld zone toughness, but the steel sheet of the present invention increases the Mn content and is expensive. Since the purpose is to reduce Ni and Cu as much as possible, considering this point, the upper limits of Ni and Cu are not limited in terms of metallurgy and technology. Is not particularly stipulated.

  In the steel sheet of the present invention, Ni is allowed to be up to about 3.0% and Cu up to about 2.0% due to the characteristics of the steel, but in the present invention, the required characteristics are satisfied depending on the required strength and thickness. However, even when it is unavoidably added, it is necessary to keep these elements as low as possible. Considering these points, the maximum value of the addition amount of Ni and Cu is 1.0%, so the upper limit of the content of Ni and Cu is 1.0%.

Further, if Cu is added in a large amount by itself, a crack with Cu as a nucleus is generated during hot rolling, so that the hot rolling process itself becomes difficult to perform. Therefore, in order to prevent this, Cu is always allowed to coexist with Ni, and the Ni content in this case is set to 1/2 or more of the Cu content.
When the Cu content is 0.60% or more, a remarkable precipitation hardening phenomenon is exhibited by applying appropriate manufacturing conditions. Therefore, the Cu content is preferably 0.60% or more.

Furthermore, in order to clarify the characteristics of the present invention, the Ni content was limited to 1 / 1.5 or less of the Mn content, and the total amount of Ni and Cu was limited to 1.5% or less.
Here, except that the Ni content is limited to 1/2 or more of the Cu content, all are not for metallurgical reasons, but only for clarifying the characteristics of the present invention.

In addition to the above composition, the high-tensile steel plate of the present invention has
Further, in terms of mass%, Cr: 0.05 to 0.50%, V: 0.01 to 0.05%, W: 0.1 to 2.0%, B: 0.0002 to 0.003% It is preferable to contain 1 type (s) or 2 or more types selected from the group.

  These elements, like Mn, have the effect of enhancing the hardenability of steel and making it easier to obtain a bainite structure. Therefore, the lower limit is a minimum value for stably expressing the effect. On the other hand, the upper limit value is not a limit / critical value that can provide the properties that the steel material should originally have in order to supplement Mn to be added in a relatively large amount, but is suppressed to clarify the characteristics of the present invention. It was. Keeping the upper limit low is advantageous not only in alloy costs but also in weld toughness.

The specific ranges of the contents of these elements are as follows: Cr is 0.05 to 0.50%, V is 0.01 to 0.05%, W is 0.1 to 2.0%, and B is 0.00. 0002 to 0.003%.
Note that V has a small strengthening effect compared to Nb and Ti, but also has an effect of increasing precipitation strengthening to some extent. Further, B is often used as a tank steel or the like in cases where stress corrosion cracking is a concern, and a reduction in the hardness of the base material and the weld heat affected zone is often the point. For example, in order to prevent sulfide stress corrosion cracking (SSC), HRC ≦ 22 (HV ≦ 248) is essential, but in such a case, addition of B that increases hardenability is not preferable.

In addition to the above composition, the high-tensile steel plate of the present invention has
Furthermore, it is preferable to contain any one or two of Mg: 0.0002 to 0.005% and Ca: 0.0005 to 0.006% by mass%.

By containing any one or two of Mg and Ca, sulfides and oxides can be formed to improve the base metal toughness and weld heat affected zone toughness. Therefore, it is preferable to add it positively.
The lower limit of the Mg content capable of obtaining the above effect is 0.0002%, and the lower limit of the Ca content is 0.0005%.
On the other hand, if the content is too large, coarse sulfides and oxides are produced, and there is a possibility that the toughness is lowered. Therefore, the upper limit of the Mg content is 0.005%, and the upper limit of the Ca content is 0.006%.

The high-tensile steel sheet of the present invention has the following characteristics due to the above composition.
(1) The volume fraction of bainite at a 1/4 thickness position of the steel sheet is 40% or more, and the sum of the volume fractions of pearlite and island martensite is 8% or less.
Here, the volume fraction of bainite was set to 40% or more because the bainite structure is easier to maintain the work structure such as the dislocation density than the ferrite, and the dislocation and deformation included in the work structure are promoted to promote fine alignment precipitation. In order to obtain sufficient strengthening, the presence of sufficient precipitation sites such as bands, etc. is very effective. To obtain sufficient strengthening, a bainite / ferrite mixed structure with a bainite volume fraction of 40% or more is required. Because it is necessary to.
Moreover, the sum of the volume fractions of pearlite and island martensite is set to 8% or less. When the sum of volume fractions exceeds 8%, pearlite, island martensite, etc. are precipitated at the phase interface and strengthened. This is because the effect is reduced and the toughness is also reduced.

(2) P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B The weld crack susceptibility composition P _CM represented by the following formula is 0.22% or less.
(3) The yield strength is 450 MPa or more.

Next, the manufacturing method of the high-tensile steel sheet having small acoustic anisotropy and excellent weldability according to the present invention will be described.
This manufacturing method comprises a steel piece or slab having the steel composition of the present invention,
T = 6300 / (1.9-Log (A))-273
A = (Nb + 2 × Ti) × (C + N × 12/14)
Is heated to a temperature of not less than T and not more than 1300 ° C., then is roughly rolled at a temperature of not less than 1020 ° C., and then a cumulative rolling reduction in a temperature range of 920 to 1020 ° C. is 15% or less and a temperature of 860 to 920 ° C. Rolled so that the cumulative reduction ratio in the range is 20 to 50%, then cooled at a cooling rate of 2 to 30 ° C./second from a temperature range of 800 ° C. or higher, and then from a temperature range of 550 to 700 ° C. This is a method of cooling at a cooling rate of 0.4 ° C./second or less.

Here, the heating temperature is T = 6300 / (1.9−Log (A)) − 273.
A = (Nb + 2 × Ti) × (C + N × 12/14)
When the heating temperature is lower than the above temperature T, Nb and Ti cannot be sufficiently dissolved, and the heating temperature is 1300 ° C. This is because the austenite grain size becomes coarse and the toughness is reduced.

In order to suppress the precipitation of Nb and Ti during rolling as much as possible, the rolling is roughly rolled at an appropriate reduction rate in a temperature range of 1020 ° C. or higher, and then the cumulative reduction rate in a temperature range of 920 to 1020 ° C. is 15 In order to obtain a necessary and sufficient processed structure as a precipitation site, rolling is performed so that the cumulative reduction ratio in the temperature range of 860 to 920 ° C. is 20 to 50%.
With this rolling condition, the formation of texture is suppressed, so that the acoustic anisotropy does not increase.

Subsequently, in order to suppress recovery of the processed structure and precipitation after the processing, accelerated cooling is performed immediately after the completion of the rolling process.
In this accelerated cooling, water cooling is performed under the condition that the cooling rate is 2 to 30 ° C./second from a temperature range of 800 ° C. or higher.
Here, the reason for setting the cooling rate of water cooling as described above is that a cooling rate of 2 ° C./second or more is necessary in order to make the volume fraction of bainite 40% or more, and pearlite and island-like This is because a cooling rate of 30 ° C./second or less is required to make the sum of the volume fractions of martensite 8% or less.

This accelerated cooling is stopped when the steel plate temperature reaches 550 to 700 ° C., and then gradually cooled (cooled) at a cooling rate of 0.4 ° C./second or less by cooling.
The purpose of this slow cooling is to ensure sufficient temperature and time for Nb, Ti and their composite precipitation, and further for the composite precipitation with Mo.
Here, the reason why the accelerated cooling is temporarily stopped in the temperature range of 550 to 700 ° C. and then gradually cooled is that if the accelerated cooling stop temperature is too higher than 550 to 700 ° C., it is difficult to obtain a bainite structure. Moreover, it is because precipitation will become late | slow and sufficient reinforcement | strengthening will not be obtained when an accelerated cooling stop temperature is too lower than 550-700 degreeC.

  Note that immediately after the accelerated cooling is stopped, the temperature at the center of the steel sheet is higher than the surface temperature, so that the temperature of the steel sheet surface once rises due to subsequent recuperation from the inside, and then turns to cooling. The accelerated cooling stop temperature is the highest temperature reached on the steel sheet surface after reheating.

  The high-tensile steel plate of the present invention is used in the form of a thick steel plate as a structural member of a welded structure such as a bridge, a ship, a building structure, an offshore structure, a pressure vessel, a penstock, and a line pipe.

Next, Examples 1 to 15 and Comparative Examples 16 to 21 will describe high-tensile steel sheets having small acoustic anisotropy and excellent weldability according to the present invention.
First, steel slabs having various compositions shown in Table 1 are melted in a converter and then rolled and cooled under the conditions shown in Table 2 to produce steel sheets having a thickness (25 to 100 mm) shown in Table 2. did.

Subsequently, the mechanical properties, weld heat affected zone toughness, and acoustic anisotropy shown in Table 2 were evaluated for each of the steel plates of Examples 1 to 15 and Comparative Examples 16 to 21.
Here, as mechanical properties, two points of yield strength and tensile strength were measured and evaluated.
For the yield strength and tensile strength, a No. 4 round bar tensile test piece defined in Japanese Industrial Standard JIS Z 2201 “Metal Material Tensile Test Specimen” is collected from the center of the thickness in the direction perpendicular to the rolling direction. Then, it measured and evaluated based on Japanese Industrial Standard JISZ2241 "Metallic material tensile test method".
As for the base material toughness, a 2 mm V notch impact test piece defined in the Japanese Industrial Standard JIS Z 2202 “Metal Material Impact Test Specimen” is collected from the center of the thickness in the direction perpendicular to the rolling direction, and then the Japanese Industrial Standard JIS. The fracture surface transition temperature (vTrs) of the impact specimen was measured and evaluated based on Z 2242 “Metallic material impact test method”.

With regard to the weld heat affected zone toughness, a thermal cycle corresponding to submerged arc welding with a heat input of 20 kJ / mm was given to a 2 mm V notch impact test piece defined in Japanese Industrial Standard JIS Z 2202 “Metal Material Impact Test Piece”. The impact energy (vE- ₂₀ ) at −20 ° C. of the impact test piece was measured and evaluated.
Here, the steel plate having a plate thickness of 32 mm or less is kept at the original thickness, and the steel plate having a plate thickness of more than 32 mm is ground to reduce the thickness to 32 mm. Was subjected to a large heat input submerged arc welding with a heat input of 20 kJ / mm, and the impact test piece defined above was taken so that the notch bottom was along the fusion line, and the absorbed energy at −20 ° C. (vE _{− 20} ) was measured and evaluated.

The acoustic anisotropy was determined by measuring the speeds of sound in two directions orthogonal to each other using a transverse wave vertical probe and determining the ratio of these speeds of sound. Here, it was evaluated that the acoustic anisotropy was small if the sound speed ratio was 1.02 or less.
The target values for each characteristic are: yield strength is 450 MPa or higher, fracture surface transition temperature (vTrs) is −30 ° C. or lower, absorbed energy (vE ₋₂₀ ) at −20 ° C. is 47 J or higher, and sound velocity ratio is 1.02 or lower. It was.

Furthermore, the bainite volume fraction and the sum of the pearlite and island martensite volume fractions were obtained and evaluated for the base metal structures of the steel plates of Examples 1 to 15 and Comparative Examples 16 to 21, respectively.
This volume fraction was calculated by observing 10 fields of 100 mm × 100 mm in a microscopic microstructure photograph at a magnification of 500 times.

  Table 1 shows the steel composition, and Table 2 shows the manufacturing method and various properties of the steel sheet.

According to these evaluation results, Examples 1 to 15 all show good characteristics. Further, by increasing the amount of Mn, the mechanical properties are extremely good even when expensive Ni or Cu is not added, and the cost is reduced.
On the other hand, Comparative Examples 16 to 21 deviating from the composition range of the present invention are inferior to Examples 1 to 15 in properties such as mechanical properties.

That is, in Comparative Example 16, both the C content and the A value are low, and the accelerated cooling stop temperature is also low. Therefore, precipitation strengthening is insufficient, and as a result, the yield strength is reduced.
In Comparative Example 17, both the content of Nb and Ti and the A value are both low, and the cumulative rolling reduction at 860 to 920 ° C. is high, so that precipitation strengthening is still insufficient, and as a result, the yield strength decreases. Yes. In addition, it is inferior in acoustic anisotropy because of the high cumulative rolling reduction at 860 to 920 ° C.
In Comparative Example 18, since the Si content is low and the heating temperature and the accelerated cooling stop temperature are also low, in addition to insufficient precipitation strengthening, the Mn content is also low, so the strength is low. Furthermore, since the Ni content is lower than the Cu content, so-called Cu cracking occurs, which is not suitable as a structural steel plate.

In Comparative Example 19, since the total content of Nb and Ti was high, the acoustic anisotropy was inferior and the weld heat affected zone toughness was also inferior. In addition, the heating temperature is low, the cumulative rolling reduction at 920 to 1020 ° C. is high, and the individual values of Nb and Ti are high, but the total content and the A value are high, so the strength is Nb The total content of Ti is the same as or lower than those of Examples 1 to 15.
Comparative Example 20 has a high content and P _CM and C, content of N be added to lower the order low accelerated cooling stop temperature, the sum of the volume fraction of pearlite and island martensite has high Not only is the base metal toughness inferior, but the weld heat affected zone toughness is also low.
In Comparative Example 21, in addition to the high contents of P and S, the cooling rate is low and the base material structure is not appropriate. Therefore, both the base material toughness and the weld heat affected zone toughness are inferior and the strength is low.

  By increasing the amount of Mn added and reducing the amount of expensive Ni or Cu added, the present invention has higher strength and lower cost while having high strength equal to or higher than that of existing steel materials. Because it is a high-tensile steel plate with low acoustic anisotropy and excellent weldability, it is possible to achieve welded structures such as bridges, ships, building structures, marine structures, pressure vessels, penstocks, line pipes, etc. It can be widely applied as a structural member, and its industrial utility value is extremely large.

Claims (9)

In mass%, C: 0.03 to 0.07%, Si: 0.10 to 0.60%, Mn: 2.0 to 3.0%, Al: 0.06% or less, Nb: 0.025 % Or more, Ti: 0.005% or more, N: 0.002 to 0.008%,
And Nb and Ti satisfy 0.045% ≦ Nb + 2 × Ti ≦ 0.105%,
Further, P: 0.020-0.04 (C + Mn / 20) (%) or less, S: 0.015-0.04 (C + Mn / 20) (%) or less,
Furthermore, the value of A represented by A = (Nb + 2 × Ti) × (C + N × 12/14) is 0.0022 to 0.0055,
A high-tensile steel sheet having a small acoustic anisotropy and excellent weldability, wherein the balance is iron and inevitable impurities.   The high-tensile steel sheet having a low acoustic anisotropy and excellent weldability according to claim 1, further comprising, by mass%, Mo: 0.05 to 0.30%. Furthermore, in mass%, Ni: 1.0% or less, Cu: 1.0% or less, Cu / 2 ≦ Ni ≦ Mn / 1.5, Ni + Cu ≦ 1.5%
The high-tensile steel sheet having a small acoustic anisotropy and excellent weldability according to claim 1 or 2, characterized by containing Cu and Ni satisfying the requirements.   Further, in terms of mass%, Cr: 0.05 to 0.50%, V: 0.01 to 0.05%, W: 0.1 to 2.0%, B: 0.0002 to 0.003% The high-strength steel sheet having a small acoustic anisotropy and excellent weldability according to claim 1, 2 or 3, comprising one or more selected from the group.   Furthermore, it contains any one or two of Mg: 0.0002 to 0.005% and Ca: 0.0005 to 0.006% by mass%. A high-tensile steel sheet having a small acoustic anisotropy and excellent weldability.   6. The volume fraction of bainite at a position of 1/4 thickness of the steel sheet is 40% or more, and the sum of the volume fractions of pearlite and island martensite is 8% or less. A high-tensile steel sheet having a small acoustic anisotropy and excellent weldability. P _{CM of the} steel sheet = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
High-tensile steel sheet acoustic anisotropy according to claim 4, wherein in weld crack susceptibility composition P _CM represented is equal to or less than 0.22% and excellent small weldability.   The high strength steel sheet having low acoustic anisotropy and excellent weldability according to any one of claims 1 to 7, wherein the yield strength of the steel sheet is 450 MPa or more. A method for producing a high-strength steel sheet having low acoustic anisotropy and excellent weldability according to any one of claims 1 to 8,
A steel slab or slab having the steel composition according to any one of claims 1 to 5,
T = 6300 / (1.9-Log (A))-273
However, A = (Nb + 2 × Ti) × (C + N × 12/14)
Is heated to a temperature of not less than T and not more than 1300 ° C., then is roughly rolled at a temperature of not less than 1020 ° C., and then a cumulative rolling reduction in a temperature range of 920 to 1020 ° C. is 15% or less and a temperature of 860 to 920 ° C. Rolled so that the cumulative reduction ratio in the range is 20 to 50%,
Next, cooling is performed at a cooling rate of 2 to 30 ° C./second from a temperature range of 800 ° C. or higher, and then cooling is performed at a cooling rate of 0.4 ° C./second or lower from a temperature range of 550 to 700 ° C. A method for producing a high-tensile steel sheet having low acoustic anisotropy and excellent weldability.