JP2662409B2 - Manufacturing method of ultra-thick tempered high strength steel sheet with excellent low temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing a high-quality tempered high-strength steel sheet having excellent low-temperature toughness and a tensile strength of 80 kgf / mm ² or more produced by quenching and tempering. Is involved.

[Related Art] In recent years, development of offshore petroleum resources has been actively promoted, but particularly recently, installed marine structures tend to be large. For this reason, the steel materials used are becoming thicker and more intensive. Further, since the recently developed sea area is shifting to a cold area, it is required to secure a low temperature toughness while achieving a thick high strength.

In tensile strength of 50 to 60 kgf / mm ² as high tensile steel, recently fine ferrite-upper bainite by developed thermomechanical treatment method (TMCP), Nb, by utilizing the precipitation strengthening elements such as V An extra-thick material having excellent low-temperature toughness is being developed while ensuring strength.

Further strength is high, in the tensile strength of 80 kgf / mm ² or more high tensile steel by direct quenching after reheating quenching or rolling after hot rolling in order to ensure the strength, upper bainite a hardened structure It is necessary to form a lower bainite or martensite-based structure in which the formation of is suppressed.

On the other hand, from the viewpoint of toughness, it is conventionally said that lower bainite or a mixed structure of lower bainite and martensite is superior to upper bainite-based structure and 100% martensite structure.

However, almost no quantitative knowledge has been obtained on how the mixing ratio of these structures should be desirable with respect to toughness, and under actual manufacturing conditions, how At present, there is no clear guideline as to whether or not a tough structure can be obtained.

When the sheet thickness is small, the cooling rate during quenching can be increased to the center of the sheet thickness, so that the structure control of the center part of the sheet thickness is easy. Rather, since the cooling rate of the surface layer is high, the structure of the surface layer becomes 100% martensitic and the toughness deteriorates greatly. Recently, however, the toughness of the surface layer has been improved by using a working heat treatment method such as direct quenching. Several methods have been proposed for securing toughness over the entire surface in the thickness direction.

However, for extremely thick materials with a thickness of about 75 to 200 mm,
Even if quenching is performed as hard as possible, the cooling rate at the center of the sheet thickness is at most several ° C / sec, so it is difficult to greatly change the cooling rate. In some cases, the influence of the processing during hot rolling is unlikely to reach the center of the sheet thickness, and therefore, measures for improving the toughness of thin materials are not very effective.

Except for the outermost layer, the cooling rate is not as high as that of thin materials except for the outermost layer, and if necessary, measures to improve toughness using the effects of thermomechanical treatment can be taken. Not so difficult. Therefore, it is difficult to control the structure at the center of the sheet thickness of an extremely thick material. Therefore, the most important issue is to improve the toughness of this part.

As a prior art of a method for manufacturing an ultra-thick tempered high-strength steel, there is a technique disclosed in Japanese Patent Publication No. 56-52970, etc., but recently the use environment has become severer and diversified. At the same time as the demands have become increasingly severe, various other properties have also been required, and there is a manufacturing method that can select higher toughness at lower cost and within a wide range of component composition. Is needed.

[Problems to be Solved by the Invention] The present invention relates to a central part of the thickness of a tempered steel sheet having a tensile strength of at least 80 kgf / mm ² and a thickness of about 75 to 200 mm produced by quenching and tempering. It is an object of the present invention to provide a production method that achieves excellent low-temperature toughness by controlling the structure of the steel.

[Means for Solving the Problems] As a result of a detailed study of the relationship between the structure and toughness of a high-strength steel, the present inventors found that the most optimal structure for toughness was substantially inhibited from forming upper bainite (upper bainite). Was determined to be a mixed structure of lower bainite and martensite, and that the formation of martensite should be as small as possible.

On the other hand, it is known that the addition of a small amount of B is effective for improving hardenability. If B is used effectively, the content of expensive alloy elements can be reduced.

B works effectively on hardenability in thin materials with a high cooling rate during quenching, and the amount of alloying elements can be reduced by that amount. However, it is difficult to use effectively with extremely thick materials with a low cooling rate. there were.

Therefore, the present inventors have clarified the composition of the steel material to obtain an optimal structure that achieves excellent low-temperature toughness, and as a means for minimizing the content of expensive alloy elements, Also, by combining a new heat treatment method that can effectively utilize B, the inventors succeeded in inventing a method for manufacturing a very thick tempered high-strength steel sheet having excellent low-temperature toughness.

 Hereinafter, the gist of the present invention will be described in detail.

In the present invention, after heating to a temperature range of 900 to 1050 ° C., air-cooling to 800 to 850 ° C. and then forcibly cooling to 500 ° C. or less at a cooling rate of 1 ° C./sec or more are necessary for improving the hardenability of B. This is for effective use.

That is, as described later, in the present invention, the amount of N is 0.0040.
% Or less. Also, to fix N during heating,
Contains appropriate amounts of Al and Ti. Under such conditions, B is not completely inactivated by being linked to N during heating or during cooling.

However, if the content is small, the amount of B that segregates at the austenite grain boundary in the heating stage and contributes to hardenability is also small, and if direct forced cooling is performed as it is, sufficient hardenability cannot be obtained. B
Increasing the content also increases the amount of grain boundary segregation B, but increasing the B content is not preferred because the toughness of the B precipitates is greatly reduced and the hardening of the heat affected zone is increased.

Therefore, it is necessary to reduce the B content as much as possible as long as the hardenability is maintained, and to effectively use the B content for the maximum hardenability.

In the present invention, the purpose of air cooling from the heating temperature to 800 to 850 ° C. is to improve the hardenability by segregating B in the grains in the cooling step at the grain boundaries.

If the temperature at which the forced cooling is started exceeds 850 ° C., segregation of B at the grain boundary during cooling is not sufficient, and if it is lower than 800 ° C., the segregation amount becomes too large, and the compound of B precipitates, Since the transformation is started and any of them is not suitable for improving the hardenability, the forced cooling start temperature needs to be in a temperature range of 800 to 850 ° C.

Regarding the heating temperature, if the heating temperature is lower than 900 ° C., the austenite grains are extremely fine and hardenability is low, and it is difficult to suppress the formation of upper bainite. Solid precipitates are present and the toughness deteriorates. If the temperature exceeds 1050 ° C, austenite coarsens and the toughness deteriorates.
The heating temperature was in the range of 900 to 1,050 ° C. because the effectiveness of the method was lost.

The reason why the end temperature of forced cooling is set to 500 ° C or lower is that if cooling is stopped at a temperature higher than this temperature, untransformed austenite may remain, and cooling or slow cooling after that may adversely affect toughness. This is because bainite may be generated.

By the above method, B can be most effectively utilized for improving hardenability in quenching thick materials. Accordingly, it is most advantageous from the viewpoint of saving alloys to design an alloy for obtaining an optimal structure on the premise of this manufacturing method.

Therefore, the present inventors used a test steel having a component range shown in Table 1 and set a heating temperature of 900 ° C. and a cooling rate before forced cooling of a thick steel sheet having a thickness of about 100 mm as conditions suitable for the above manufacturing conditions. After applying a heat treatment at 0.2 ° C / sec corresponding to the cooling condition at the center of the plate thickness and 3 ° C / sec in the forced cooling stage according to the cooling rate under the actual manufacturing conditions, the Charpy test was performed. The relationship between the 50% fracture surface transition temperature (vTrs) and the amount of alloy components was investigated, and the composition of the components for obtaining the optimal structure for toughness was examined.

The results are shown in FIG. 1, and it was found that the tissue morphology was substantially organized by the parameter x represented by the following equation.

x = [C (%)] ^1/2 x [1 + 0.64 x Si (%)] x [1 + 4.10 x Mn (%)] x [1 + 0.27 x Cu (%)] x [1 + 0. 52 × Ni (%)] × [1 + 2.33 × Cr (%)] × [1+
3.14 × Mo (%)] That is, when the value of x is in the range of 26 to 42, the formation of the upper bainite is almost suppressed, and the structure does not become a complete martensite structure. It becomes a mixed structure of martensite. When compared with a constant Ni content, the toughness is the best in this parameter range.

The region where the parameter x is large as the Ni content is large (x ≧
In 26), higher toughness can be achieved, but parameter x is 42
When it exceeds 100% and becomes a 100% martensite structure, the susceptibility of embrittlement due to subsequent tempering or stress relief annealing becomes extremely large even if the as-quenched toughness is good, so x = 42 was set as the upper limit.

In addition, setting this parameter range is to achieve the highest toughness under the condition of constant Ni content, in other words, achieve the highest toughness by minimizing the expensive Ni content. This is very effective from an economic point of view.

When the Ni content is small, the range of x having good toughness is narrowed, and there is a problem because the change in toughness due to a change in x is also rapid. If the Ni content is 1% or more, as shown in FIG.
Since the change in toughness in the region 26 is small and the level is good, the lower limit of the Ni content is set to 1%.

The steel produced by such composition and quenching is tempered at an appropriate temperature equal to or lower than the Ac ₁ transformation point, and the strength is adjusted as necessary before use. At this time, if the tempering temperature exceeds the Ac ₁ transformation point, there is a possibility that a structure that re-transforms and adversely affects the toughness may appear.
It should be ₁ transformation point or less. Under these conditions, if the tempering temperature is appropriately selected, the toughness is further improved as it is in the quenched state.

The method for maximizing the use of B for hardenability and the limited range of parameter x as an index for obtaining an optimal structure are based on the reasons described above. It is limited for the reasons listed below.

C is required to be 0.03% or more in order to secure the strength with extremely thick material, but if it exceeds 0.20%, the toughness is deteriorated regardless of the structure.
Even if the value of the parameter x is within the limited range, it is difficult to ensure sufficient toughness, so the range was 0.03 to 0.20%.

Si is an element that facilitates the formation of island martensite,
%, There is a problem in toughness because even if there is a small amount of upper bainite, the toughness deteriorates due to the island-like martensite in the upper bainite structure. On the other hand, if it is less than 0.05%, deoxidation becomes insufficient and In order to increase internal defects, the content is set in the range of 0.05 to 0.50%.

If Mn is less than 0.50%, there is a problem in securing strength and toughness.
If it exceeds 0%, tempering embrittlement becomes remarkable, so the range is 0.50 to 3.0%.

P is set to 0.010% or less in the high-strength steel targeted by the present invention because of the need to suppress temper embrittlement.

S is an element that forms MnS and reduces ductility,
Especially in high strength steel, the effect is large, so 0.010%
It was as follows.

Al fixes N, which inhibits the hardenability improvement effect of B, as AlN, and has the effect of increasing the hardenability improvement effect of B.
If the content is less than 03%, the effect is insufficient, while if it exceeds 0.10%, coarse precipitates are formed and the toughness is adversely affected. Therefore, the content is set in the range of 0.03 to 0.10%.

The lower limit of Ni is set to 1.0% for the reason described above. Also Ni
Has the effect of improving the toughness of the matrix itself, in addition to the effect of improving the toughness via parameter x, so that the higher the content, the more the toughness is improved. In the present invention, the upper limit is set to 10.0% because the sensitivity of toughness deterioration due to the steel is increased.

B is a trace amount and is an element effective for improving hardenability. According to the manufacturing method of the present invention, the hardenability can be improved to the maximum. The hardenability is not sufficiently improved because it cannot be secured.
%, The effect on hardenability saturates or slightly decreases, and the precipitation of the compound of B causes a large deterioration in toughness. Therefore, the range is 0.0005 to 0.0030%.

N is an element that inhibits the hardenability effect of B, and should be 0.0040% or less in the component composition and production method of the present invention.

In the present invention, Cu, Cr, and Mo are all parameters x.
In order to obtain an optimal tissue, it is necessary to contain a certain amount. However, since the influence on the tissue control is the same in any case, any one or two, and even all three, are contained. It is possible to do. 0.01% for all
If it is less than 1, the effect of improving the structure is not clear even if it is contained, and if it exceeds 1.50%, precipitation embrittlement becomes remarkable, so that it is 0.01 to 1.
The range was 50%.

The above are the basic components of the present invention. In the present invention, Ti is added in the range of 0.005 to 0.020%, and Nb and V are added to Nb +.
V can be contained in the range of 0.01 to 0.10%.

That is, Ti is effective in fixing N as in Al, and is effective in preventing austenite from becoming coarse during heating.
Ti is contained according to the component composition and the plate thickness. However, 0.00
If it is less than 5%, the effect is not sufficient, and if it exceeds 0.020%, coarse precipitates are easily formed, and on the contrary, the toughness is deteriorated.
Limited to the range of 0.020%.

In addition, Nb and V can be precipitated in a small amount and are effective in increasing the strength.

Since both elements have almost the same effect on precipitation strengthening, it is possible to include either one or two of them, but if the total content of Nb and V is less than 0.01%, the strength The rise is not clear, and if it exceeds 0.10%, the toughness deteriorates more than the strength increase.
Must be in the 10% range.

[Examples] Table 2 shows the Charpy characteristics (vTrs) at the center of the thickness of the steel manufactured according to the present invention in comparison with the results of comparative steel manufactured by a composition and manufacturing method outside the limited range of the present invention. Show.

The results of the Charpy test are shown together with the results after as-quenching and after tempering.

No. 1 to No. 10 are steels manufactured according to the present invention. The plate thicknesses are 100 mm and 150 mm, and the average cooling rate during quenching is approximately in the range of 1 to 3 ° C./sec. Even if the plate thickness is further increased, there is not much difference in the cooling rate at such a plate thickness.

If the parameter x is in the range of 26 to 42 in accordance with the method of the present invention and the heating temperature and the forced cooling start temperature are within the appropriate ranges, the Charpy characteristics are good even as-quenched. After tempering at 630 ° C., the toughness is further improved.

Steel No. 7 with high Ni content naturally shows extremely high toughness,
Even in steels with a low Ni content, for example, in steel Nos. 1, 9, and 10, the composition of other alloying elements is adjusted to adjust the value of parameter x.
In the range of 26 to 42, in order to effectively use B for hardenability,
If the forced cooling start temperature is in the range of 800 to 850 ° C., the as-quenched Charpy characteristics are secured at −60 ° C. or less in vTrs.

On the other hand, comparative steels Nos. 11 to 20 do not satisfy the limited range of the present invention, and all have inferior toughness as compared with steels manufactured according to the present invention.

For example, in Nos. 11 and 12, since the value of the parameter x was lower than the limited range, the structure at the center of the plate thickness was a coarse upper bainite-based structure, and the toughness was poor. No. 12 has a higher Ni content, so the toughness is better among the comparative steels,
For example, as can be seen in comparison with steel No. 8 according to the present invention, the toughness is not so good for the Ni content.

Steel Nos. 13 and 14 satisfy the conditions of the present invention for the value of parameter x, but No. 13 has a low Ni content, and
Has a high N content, so that the toughness is not so good. Steel No.1
Conditions 5 and 16 are conditions in which the value of the parameter x is too high than the limited range. Although the as-quenched or the toughness after tempering is better than high Nos. 11 and 12 where x is too low,
If the parameter x is such a high value, a 100% martensite structure will be formed even in the central portion of the sheet thickness, so that the temper embrittlement and the susceptibility to embrittlement due to stress relief annealing increase, which is not preferable.

Further, even if the Ni content is high as in steel No. 16, the toughness is not so improved, and it cannot be said that the composition and production method are appropriate as in steel No. 12 having a low x value. Although the value of parameter x of steel Nos. 17 to 20 satisfies the conditions of the present invention, the heating temperature or the forced cooling start temperature does not satisfy the limited range of the present invention. Does not achieve the expected toughness value.

It is apparent from the above examples that when the range of the present invention is not satisfied, excellent toughness cannot be obtained at the center of the extremely thick plate.

Low temperature toughness of high tensile strength [Effect of the invention] is, in order to produce 80 kgf / mm ² or more extra-thick tempered high tensile steel sheet, high tenacity achieved in the slow thickness center portion of the cooling rate during quenching In order to achieve this, it is necessary that the structure at this site be a mixed structure of lower bainite and martensite with a ratio of upper bainite of 10% or less, which is optimal for toughness.

The present invention clarifies an appropriate component composition range and a manufacturing method for the purpose, and the present invention makes the most effective use of alloying elements and B, and appropriately reduces the content of expensive alloying elements such as Ni. As is clear from the above examples, very excellent low-temperature toughness can be achieved in the center of the sheet thickness, and the industrial effect is extremely remarkable.

[Brief description of the drawings]

FIG. 1 shows the relationship between the parameter x and the 50% fracture surface temperature (vTrs) and microstructure in the Charpy test of a test material subjected to a heat treatment simulating the cooling condition of the center portion of a very thick plate according to the present invention. It is a chart.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Koichi Yamamoto 5-10-1 Fuchinobe, Sagamihara City, Kanagawa Prefecture New Nippon Steel Corporation Second Technical Research Institute (56) References JP-A-57-126955 (JP, A JP-A-62-177120 (JP, A) JP-A-49-63617 (JP, A)

Claims (4)

(57) [Claims] [Claim 1] In terms of% by weight, C: 0.03 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.50 to 3.0%, P: 0.010% or less, S: 0.010% or less, Al: 0.03 to 0.10%, Ni : 1.0 to 10.0%, B: 0.0005 to 0.0030%, N: 0.0040% or less, and Cu, Cr, Mo for Cu: 0.01 to 1.50%, Cr: 0.01 to 1.50%, Mo: 0.01 to 1.50%, And one or more kinds, the balance being iron and unavoidable impurities, and a parameter x represented by the following formula:
The thickness of steel slab with a value of 26 to 42 is 75 to 200 m by hot rolling.
After heating to a temperature range of 900 to 1050 ° C., air cooling to 800 to 850 ° C., followed by forced cooling to 500 ° C. or less at a cooling rate of 1 ° C./sec or more, and then Ac ₁ A method for producing a high-quality tempered high-strength steel sheet having a tensile strength of 80 kgf / mm ² or more, which is excellent in low-temperature toughness and characterized by tempering at a temperature below the transformation point. x = [C (%)] ^1/2 x [1 + 0.64 x Si (%)] x [1 + 4.10 x Mn (%)] x [1 + 0.27 x Cu (%)] x [1 + 0. 52 x Ni (%)] x [1 + 2.33 x Cr (%)] x [1 + 3.14 x Mo (%)] 2. The ultrathickness having an excellent low-temperature toughness having a tensile strength of 80 kgf / mm ² or more according to claim 1, wherein one or two of Nb and V are contained in the range of Nb + V = 0.01 to 0.10%. Manufacturing method of tempered high strength steel sheet. 3. A tensile strength of 80 kgf / mm ^{2 with} excellent low-temperature toughness according to claim 1, wherein Ti: 0.005 to 0.020% is contained.
The method for producing the above-mentioned extremely thick tempered high-tensile steel sheet. 4. The excellent low-temperature toughness according to claim 1, wherein one or two of Nb and V are contained in the range of Nb + V = 0.01 to 0.10% and Ti is contained in the range of 0.005 to 0.020%. Tensile strength
A method for producing ultra-thick tempered high-strength steel sheets of 80 kgf / mm ² or more.