JPH04293719A - Production of high strength steel plate for structural use excellent in toughness at low temperature and having high young's modulus - Google Patents

Abstract

PURPOSE:To economically and efficiently produce a high strength steel plate for structural use excellent in toughness at low temp. and remarkably improved in the Young's modulus in a direction perpendicular to a rolling direction. CONSTITUTION:A steel slab having a temp. between the Ar3 point and 1250 deg.C is hot-rolled at a temp. between the recrystallization finishing temp. and the Ar3 point at >=20% reduction of area, cooled without delay at a rate of >=5 deg.C/sec, subjected to rolling in two phase region at >=50% reduction of area at a temp. lower than the Ar3 point, hardened, and then subjected, within 1hr, to tempering treatment at a temp. in the region of <=700 deg.C for 5-30min. By this method, a Young's modulus of <=15% can be obtained.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is an efficient and economical way to produce high-strength structural steel plates that have excellent low-temperature toughness and dramatically improved Young's modulus in the direction perpendicular to the rolling direction (hereinafter referred to as the C direction). The present invention relates to a method for manufacturing the same.

0002

2. Description of the Related Art Generally, the rigidity of a steel plate is proportional to Young's modulus if the shape is constant.

[0003] In conventional steel, the Young's modulus is approximately 21,000 kgf, excluding special cases such as single crystal and electrical steel sheets.
/mm2, and was not considered to be a particularly noteworthy material property.

However, in recent years, there has been a demand for improved rigidity in a specific direction during use, and consideration has been given to applying the Young's modulus in the C direction to this purpose.

According to this method, it is possible to increase the rigidity of the structure without increasing the plate thickness or changing the shape.

On the other hand, there are various proposals regarding high Young's modulus steel, all of which involve developing a rolling texture through rolling in a two-phase region or a ferrite region to improve the Young's modulus in a specific direction of the steel sheet.

For example, in Japanese Patent Publication No. 58-14849,
A method for producing high Young's modulus steel is disclosed. The manufacturing method of the high Young's modulus steel material disclosed herein involves rolling steel with defined chemical composition in a two-phase region, controlling the cooling rate to 300°C after finishing rolling, and then tempering at a temperature of 700°C or less. It is said that the Young's modulus in the C direction can be increased by about 10%.

[0008] Furthermore, in Japanese Patent Publication No. 62-4448,
Steel containing less than 0.03% by weight of Ar3 or less 60
A method is described in which the Young's modulus in the C direction is increased to a maximum of 24,300 kgf/mm 2 by specifying the rolling reduction in a temperature range of 0° C. or higher and winding at a temperature of 450° C. or higher and 720° C. or lower.

[0009] Conventionally, there is a method of quenching and tempering as a method for producing structural steel plates having excellent strength and toughness.

[0010] To summarize the techniques for improving the strength and toughness of steel materials, 1) Japanese Patent Publication No. 1973 as a toughening method by heat treatment;
-36100 Publication, 2) N effective for significantly improving toughness
Special Publication No. 53-416 that combines i increase and tempering treatment
14, 3) Japanese Patent Publication No. 46-27139 which added pretreatment including reheating treatment prior to tempering treatment, and 4) combined pretreatment including heat treatment prior to tempering treatment and addition of Ni. Special Publication No. 46-13498, Publication No. 13498, Special Publication No. 51-
Publication No. 19409, Japanese Patent Publication No. 14971/1971, 5
) Special Publication No. 1694 (1974) as a rapid and short-time heat treatment method
It is roughly divided into Japanese Patent Application Laid-Open No. 55-2761.

[0011]

However, all of the above-mentioned methods have the following problems in practical use, and improvements are awaited for each of them.

First of all, regarding Young's modulus,
In the method of Publication No. 4849, a rolling method (large reduction rolling in α-γ two-phase region) is applied that significantly promotes the formation of texture in order to improve Young's modulus, but it is difficult to ensure base material toughness. Therefore, the guaranteed temperature is 0°C, which does not meet the low-temperature toughness of -60°C or lower, which has been required for important components of structures in recent years from the viewpoint of ensuring even stricter safety. Furthermore,
In recent years, shipbuilding manufacturers have been promoting reductions in operating fuel consumption, which has led to lighter ship hulls, and even if the thickness of the plate is reduced, it does not meet the requirements for a steel plate that can provide the necessary strength and rigidity.

[0013] In addition, the method disclosed in Japanese Patent Publication No. 62-4448 requires the component to be limited to C≦0.03%, and T
This relates to a manufacturing method for steel plates of S30 kgf/mm2 or less, and does not satisfy the strength of structural steel TS40 kgf/mm2 or more, which is the target of the present invention.

Regarding strength and toughness, 1) There is a limit to the level of strength and toughness that can be obtained by conventional toughening methods using heat treatment. 2) Increased amount of Ni and heat treatment are effective for significantly improving toughness. 3) There is a method that adds pretreatment including heat treatment prior to tempering treatment. 4) A method that combines pretreatment including heat treatment prior to tempering treatment and Ni addition is expensive. This is economically disadvantageous due to the unavoidable increase in manufacturing costs due to the use of a large amount of Ni element and the addition of a heat treatment process. However, the present invention solves the problems of the prior art, such as requiring a long time until the tempering process and inevitably reducing productivity, so it cannot be said to be an efficient manufacturing method. To establish an efficient and economical method for manufacturing structural steel plates with high strength, excellent low-temperature toughness, and a high Young's modulus.

[0015]

[Means for Solving the Problems] The gist of the present invention is to hot-roll a structural steel billet whose temperature is above Ar3 point and below 1250°C at a reduction rate of ≧20% at a temperature above Ar3 point below the recrystallization finish temperature. , Immediately cooled at a rate of 5°C/sec or more, rolled in a two-phase region with a rolling reduction of ≥50% at less than the Ar3 point, and then tempered within 1 hour at a temperature range of 700°C or less for 5 minutes or more and 30 minutes or less. This is a method for producing high-strength structural steel sheets with excellent low-temperature toughness and high Young's modulus.

[0016]

[Operation] As described in Japanese Patent Publication No. 58-14849, for example, the present invention combines the types of additive elements conventionally used in the field in order to obtain the required material properties of ordinary welded structural steel. Similar amounts can be used with equivalent action and effect. Therefore, steels including these are targeted.

[0017] The reason for addition and the amount of each component element are shown below.

C is added as an effective component to improve the strength of steel, but if the content exceeds 0.20%, it increases the deformation resistance during rolling in the two-phase region and makes rolling difficult. Otherwise, it precipitates island-shaped martensite in the welded area, significantly deteriorating the toughness of the steel, so it is restricted to 0.20% or less.

[0019]Si is a deoxidizing element for molten steel and is useful as an element for increasing strength, but when added in excess of more than 1.0%, it reduces the workability of the steel and deteriorates the toughness of the welded part. Moreover, since the deoxidizing effect is insufficient if it is less than 0.01%, the amount added is regulated to 0.01 to 1.0%.

Mn is also a deoxidizing component element, and if it is less than 0.3%, it reduces the cleanliness of the steel and impairs workability. In addition, 0.3% or more is added as a component that improves the strength of steel materials. However, since Mn lowers the transformation temperature, it is better to lower the temperature in order to roll in the two-phase region by adding Mn in excess, which causes an increase in deformation resistance, so the upper limit is set at 2.0%.

[0021] Al and N are added in order to have an effective function in matching the crystal orientation and recrystallization of the steel through solid solution and precipitation during the rolling process, in addition to refining the steel by Al nitride. However, if the amount added is small, it has no effect, and if it is excessive, it deteriorates the toughness of the steel, so Al: 0.0
01 to 0.20%, N: limited to 0.020% or less.

The above are the basic components of the steel targeted by the present invention; however, alloying elements are added depending on the required properties for the purpose of increasing base material strength or joint toughness. In this case, if the transformation temperature is lowered too much, the deformation resistance in the two-phase region increases and rolling becomes difficult, so Ni, Cr, Mo
, Cu, W, P, Co, V, Nb, Ti, Zr, Ta,
One or more of Hf, rare earth elements, Y, Ca, Mg, Te, Se, and B may be added, but it is preferable to limit the amount of the alloy added to within 4.5% in total.

The heating temperature of the steel billet produced in this manner is set at a lower limit of 900° C. and an upper limit of 900° C. in consideration of the usual heating conditions for this type of steel billet, that is, the workability of rolling due to temperature drop during rolling. The upper limit is set at 1250°C to prevent coarsening of austenite.

[0024] Also, if the reduction rate below the recrystallization finish temperature and above the Ar3 point is less than 20%, good low-temperature toughness cannot be obtained. did.

[0025] Furthermore, if the rolling reduction below the Ar3 point is less than 50%, the improvement in Young's modulus is less than 10%.
The rolling reduction ratio of less than 3 points was set to 50%.

In order to solve the problems of the prior art described above, the inventors of the present invention repeatedly conducted various experimental studies using general structural steel having the following chemical components.

C: 0.05-0.15% Si: 0.
15-0.25% Mn: 0.8-1.6% Al: 0
．． 01~0.05% N: 0.0020~0.0050%

As a result, FIGS. 1 to 4 were obtained. FIG. 1 shows the relationship between the intermediately controlled cooling rate and the low temperature toughness expressed in vTrs when the reduction amount in the non-recrystallized region is 25% and the reduction amount in the two-phase region is 50%.

As shown in the figure, the midway control cooling rate is 5°C.
It has been found that when the temperature is higher than /second, the toughness is improved to a level that is possible at temperatures below -60°C.

[0030] This is considered to be because the residence time at high temperatures is shortened, and grain growth of ferrite grains or austenite grains is suppressed, thereby improving toughness.

FIG. 2 shows the tempering temperature and C This figure shows the relationship between the improvement in Young's modulus in the direction (compared to the 21,000 kgf/mm2 level of conventional steel).

From this figure, the inventors found that the Young's modulus was improved and that there was an appropriate tempering temperature range.

This is because during rolling in the α+γ region or the α region, crystal grains with {211}<111> as the main orientation are formed in a direction perpendicular to the rolling direction, and furthermore, crystal grains that do not have this orientation as the main orientation form ferrite. This seems to be the result of substitution with crystal grains having {211}<111> as the main orientation during the tempering process at a temperature higher than the recrystallization temperature.

[0034] Furthermore, we have learned that if tempering treatment is performed at 700°C or higher, it is no longer possible to expect an improvement in Young's modulus.

This is because a specific and small number of crystal grains combine with other crystal grains and grow in the process of {211}
This seems to be because the crystal grains whose main orientation is 〉 disappear, and the overall crystal orientation becomes random.

Furthermore, FIG. 3 shows the tempering temperature and holding time when steel manufactured under the rolling conditions shown in FIG. Improvement in Young's modulus in the C direction (21,000 kgf/mm2 of conventional steel)
This shows the relationship between

The present inventors found from this figure that the holding time at the tempering temperature should be 5 minutes or more and 30 minutes or less, since the {211} <111> direction is perpendicular to the rolling direction. It was discovered that the Young's modulus can be increased by forming a texture that increases the Young's modulus.

FIG. 4 also shows that the steel manufactured under the rolling conditions shown in FIG.
This figure shows the relationship between the elapsed time from immediately after quenching to tempering, yield strength, and toughness when tempering was performed with a holding time of 10 minutes.

From this figure, the inventors found that the elapsed time from immediately after quenching to tempering should be within 1 hour.
It has been found that this improves strength and toughness.

(1) Test steel The steel composition of the present invention may be any combination of elements and addition amounts of the general structural steels mentioned above, but the chemical composition used in the examples is shown below. Both are shown in 1.

[0041] This is also a typical chemical composition of different strength levels in the field of structural steel.

(2) Manufacturing conditions and material results The manufacturing conditions and materials obtained are shown in Tables 2 and 3.

[0043] Regarding the test steels shown in Table 1, steel numbers 1 and 2 are 40 kg class steel, steel numbers 3 to 6 are 50 kg class steel, and steel number 7 is 60 kg class steel. In addition, the test steel may contain V, Nb, Ni, if necessary.
, Ti, Cu, Cr, Mo, and other alloying elements are added.

[0044]No. In all of the invention examples A1 to A14, structural steel plates having excellent properties such as low temperature toughness and Young's modulus were obtained.

In other words, the low-temperature toughness is excellent at -80°C to -130°C, and the Young's modulus in the C direction is compared to the Young's modulus of B9 to B15, which are comparative examples using the same steel type as A1 to A7 but not tempered. As is clear, the improvement margin is 5.0 to 8.
A structural steel plate was obtained which showed an improvement of 15% or more over the conventional level of 21,000 kgf/mm2 and fully achieved the objectives of the present invention.

In addition, regarding the influence of the elapsed time from quenching to tempering on the strength and toughness, inventive examples A1, A3, and A
Compared to B9, B11, and B12 of the same composition, the strength of No. 4 was increased by about 2.0 to 3.3 kgf/mm2, and the toughness was improved by -9 to -20°C.

In contrast, comparative example No. B1~B2
No. 2 had its own problems, and a structural steel plate that met the above requirements could not be obtained.

That is, the reduction rate in the two-phase region below the Ar3 point is 5
Comparative example No. less than 0%. In B1, B5, and B16, the improvement in Young's modulus did not reach the required level.

[0049] Comparative example No. where the heating temperature is as high as 1300°C.
．． B2, B17, Comparative Example No. where the rolling reduction in the recrystallization zone was less than 20%. B4, B19, and B22 all had poor toughness and could not be used for the intended purpose.

Comparative Example No. 1 in which no intermediate cooling was performed and in which the cooling rate during intermediate cooling was less than 5° C./sec. B18 toughness did not reach -60°C level.

B3 having a retention time of less than 5 minutes at the tempering temperature;
B7, B8 with retention time of 30 minutes or more, tempering temperature of 800
℃ and high B6, the improvement in Young's modulus did not reach the required range.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

[0055]

[Effects of the Invention] The present invention enables smooth and stable production of structural steel plates with excellent low-temperature toughness, high strength, and high stiffness (Young's modulus) in a specific direction of approximately 15% with extremely high productivity. The economic impact it brings to industry, especially in this field, is extremely large.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between intermediately controlled cooling rate and low-temperature toughness expressed in vTrs.

FIG. 2 is a diagram showing the relationship between tempering temperature and improvement in Young's modulus in the C direction.

FIG. 3 is a diagram showing the relationship between the retention time at the tempering temperature and the improvement in Young's modulus in the C direction.

FIG. 4 is a diagram showing the relationship between the elapsed time from quenching to tempering, strength, and toughness.

Claims (1)

[Claims] Claim 1: A structural steel billet whose temperature is between Ar3 and 1250°C is hot-rolled at a reduction rate of 20% or more at Ar3 or above the recrystallization end temperature, and immediately cooled at a rate of 5°C/sec or more. , After rolling and quenching in a two-phase region with a reduction rate of 50% or less at less than the Ar3 point, the
A method for producing a high-strength structural steel plate having excellent low-temperature toughness and a high Young's modulus, the method comprising performing a tempering treatment for at least 30 minutes.