JPH02166227A - Manufacture of high strength and high toughness thick steel plate having excellent weldability - Google Patents

Abstract

PURPOSE:To manufacture the steel plate by heating a steel slab contg. small amounts of specific alloy components to a specific temp., thereafter subjecting it to rolling under limited conditions and immediately to hardening and tempering treatment. CONSTITUTION:A steel slab contg., by weight, 0.01 to 0.10% C, 0.10 to 0.50% Si, 0.5 to 2.0% Mn, 0.01 to 0.10% Al, 0.8 to 1.5% Cu, 0.01 to 0.06% Nb, 0.005 to 0.020% Ti and <0.015% N or furthermore contg. one or more kinds among <3.0% Ni, <1.0% Cr, <0.5% Mo, <0.1% V, <0.0030% B and <0.0050% Ca is heated to a temp. of >=1100 deg.C at which Nb enters a solid solution sufficiently, is immediately rolled down not until the temp. range where the temp. of the surface of the slab comes to 850 to 1000 deg.C and is cooled at >=20 deg.C/sec cooling speed. Then, the slab is rolled at 60% draft and finish rolling is ended at the temp. of >=Ar3-30 deg.C; the slab is immediately subjected to direct hardening and is thereafter tempered, by which the high strength and high toughness thick steel plate of >=35mm thickness having excellent weldability can be manufactured.

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a technology for manufacturing high-strength, high-toughness thick-walled steel plates that undergo welding. (Prior art and problems to be solved) Thick steel plates with high strength, high toughness, and good weldability are used as materials for large structures. A, ST MA710 is known as a steel that utilizes age precipitation hardening of Cu in an equivalent component system. This steel contains 1.0 to 1.3% of Cu, and high strength is obtained by precipitating Cu through aging treatment. However, in the case of the above-mentioned steel, when it is made into a thick steel plate with a thickness of 35 mm or more, it is difficult to ensure excellent low-temperature toughness (fracture surface transition temperature vTrs of -80"C or less) using conventional manufacturing methods. On the other hand, when manufacturing this type of steel sheet that requires high strength and high toughness, a method of controlled rolling followed by direct quenching has been proposed.However, when such a method is applied to the above steel, , it is necessary to apply a large cumulative reduction in the austenite low temperature range.As a result, the finishing rolling temperature is extremely low, below 800°C, and therefore, even if quenching is performed directly after rolling, the quenching effect is not sufficient. It is difficult to obtain a high tensile strength of 70 kgf/mm'' or more even if the aging precipitation hardening of Cu is used. Moreover, as the thickness of the steel plate increases, large amounts of alloying elements such as Mn, Cr, and Mo must be added to ensure strength, and as a result, the carbon equivalent of the steel increases, making it difficult to weld. There is a problem in that it is significantly impaired. For example, JP-A-60-59018 discloses only cases where the plate thickness is 30 mm or less. For the above reasons, it has been extremely difficult to manufacture a high-strength, high-toughness steel plate with a thickness of 35 mm or more and excellent weldability by utilizing Cu precipitation hardening. However, there is a method in which the finishing temperature of the rolling is set relatively high, such as s o o 'c or higher, and quenching is performed directly after the rolling is completed (
JP-A-62-256915, JP-A-61-149845) or reheating and quenching method (JP-A-61-1-4)
No. 94.30, No. 62-149845), but in the former case, there is a problem in whether a good balance of strength and low-temperature toughness can be obtained when the plate thickness is 35 mm or more, and in the latter case, it is difficult to obtain a good balance of strength and low-temperature toughness. There is a problem with varnish hair bubbling due to heating. The present invention was made under such circumstances, and
Utilizing Cu precipitation hardening with a low carbon equivalent component system, and applying controlled rolling to further improve low-temperature toughness, the plate thickness was reduced to 35 mm.
mm or more, has high strength with a tensile strength of 70 kgf/mm” or more, has high toughness with a fracture surface transition temperature of 180°C or less at the center of the plate thickness, and has high strength with excellent weldability. The object of the present invention is to provide a method for obtaining a high toughness thick copper plate. (Means for solving the problem) In order to achieve the above object, the present inventors have improved the effect of controlled rolling and directly Focusing on Nb, which has the effect of promoting transformation strengthening during quenching and precipitation hardening during tempering, we conducted extensive research on how to effectively utilize this.As a result, we found that while appropriately adjusting the chemical components, In particular, by appropriately controlling the rolling conditions at the rolling stage, we have discovered a method that can achieve the above objectives and produce high-strength, high-toughness thick steel plates with excellent weldability.Specifically, Mainly: (1) In order to improve weldability, the amount of C is 0.01 to 0.0.
(2) Contains 0.8 to 1.5% of Cu to increase strength, enhances the effect of controlled rolling, promotes transformation strengthening, and also contains Nb, which has a precipitation hardening effect. 0.010-0.060
(3) After sufficiently solid-dissolving Nb in the heating step, in the rolling step, the steel is first cooled at a cooling rate or higher without applying any rolling reduction, and then rolled. Based on these findings, we have conducted further experimental studies under various conditions, and have now completed the present invention. That is, in the present invention, C: 0.01 to 0.10%, Si
: 0.10-0.50%, Mn2 0.5-2.0%, 8Ω: 0.01-0.10%, Cu: 0, 8-1.5
%, Nb: 0.01~0.06%, Ti: 0.005~
0.020% and N: O, O' 15% or less, and if necessary, Ni: 3.0% or less, Cr: 1.0%
Below, Mo2 0.5% or less, V: 0.1% or less, B: O
, 0O30% or less and Ca: 0.0050% or less, and the balance is iron and unavoidable impurities, the temperature range in which Nb is sufficiently dissolved at 1100°C or higher. After heating, the slab is immediately cooled at a cooling rate of 20°C/min or more without applying any reduction until the slab surface temperature reaches a temperature range of 850 to 1000°C, and in subsequent rolling, a reduction of 60% or more is applied ( Manufacture of high-strength, high-toughness thick-walled steel plates with a thickness of 5 mm or more and excellent weldability, characterized by completing rolling at Ar3 transformation point temperature -30°C) or higher, immediately directly quenching, and further tempering. The gist is the method. The present invention will be explained in more detail below. (Function) First, the reasons for limiting the chemical components in the present invention are as follows. C: C is an effective element for increasing strength, and for that purpose 0.0
1% or more is required. However, from the viewpoint of ensuring toughness and preventing deterioration of weld cracking resistance, it is necessary to limit the upper limit to 0.10%. Therefore, the amount of C is 0.01 to 0.1
The range is 0%. Sl: Si is a deoxidizing element and needs to be added in an amount of 0.10% or more, but excessive addition deteriorates toughness, so the upper limit should be set at 0.10%.
It shall be 50%. Therefore, the amount of Si is 0°10-0.50
% range. Mn: Mn is effective in increasing strength, and for this purpose it is necessary to add 0.5% or more, but if it is added in excess of 2.0%, toughness deteriorates. Therefore, the amount of Mn is set in the range of 0.5% to 2.0%. Al: 80 is a deoxidizing element, and 0.01% or more is required, but excessive addition forms inclusions and deteriorates toughness, so the upper limit is set to 0.10%. Therefore, the amount of ΔΩ is 0.
The range is 0.01 to 0.10%. Nb: Nb is an important element that is the key point of the present invention. In other words, Nb is an element that suppresses recrystallization of austenite in a solid solution state, enhances the effect of controlled rolling, improves transformation strengthening by direct quenching, and precipitates during tempering, contributing to improvement of strength and toughness. be. Such an effect is 0.0
It is effective when the content is 1% or more, but if it exceeds 0.06%, the toughness of the weld zone deteriorates, which is not preferable. Therefore, N
The amount of b is in the range of 0.01 to 0.06%. Tj: T1 is an element that forms hardly soluble carbonitrides and suppresses the growth of austenite grains during slab heating or welding, and thus has the effect of improving the toughness of the base metal and weld zone. Also, c. By fixing N, there is also the effect of suppressing solid solution Nb from precipitating as carbon/nitride. 0 for that. 0.005% or more is required, but if it is added in excess of 0.020%, coarse inclusions will be formed and the toughness will deteriorate. Therefore, the amount of Ti is 0.005 to 0. The range is 0.020%. N: If N is added in excess of 0.015%, it will significantly deteriorate the toughness of the base metal and weld zone, so the amount of N should be 0.015%.
5% or less. Cu: Cu is an element effective for solid solution strengthening, precipitation strengthening, or transformation strengthening by improving hardenability. In order to exhibit these effects, it is necessary to add 0.8% or more. However, since excessive addition deteriorates toughness, the upper limit is set at 1.5%. Therefore, the amount of Cu is in the range of 0.8 to 1.5%. Each of the above elements is an essential component, but if necessary, Ni. Cr. Mo. V. One or more of B and Ca can be added in appropriate amounts. Although Ni has the effect of improving low-temperature toughness, it is expensive;
From the viewpoint of economy, it should be 3.0% or less. In addition, mainly for the purpose of increasing strength, Cr is added to 1.0% or less.
Mo 0.5% or less, ■ 0.1% or less, B 0, 0
It can be added at 0.30% or less. Further, Ca can be added in an amount of 0.0050% or less for the purpose of improving toughness. It should be noted that excessive addition of these elements exceeding the upper limit is undesirable because it deteriorates toughness and further deteriorates weldability. Next, the reasons for limiting the ingot heating and hot rolling conditions in the present invention will be described. When heating the slab, it is necessary to secure solid solution Nb, which effectively works to improve strength and toughness. For this purpose, it is necessary to heat it to at least 1100°C or higher. Next, hot rolling is performed, and the reason for limiting the rolling conditions is as follows. In general, Nb suppresses recrystallization of austenite in a solid solution state, so that rolling can refine crystal grains and is effective in improving toughness. However, the product thickness is 35mm
In the center of the thickness of a plate thicker than
It is difficult to improve toughness even by adding it. Therefore, in the present invention, rolling conditions are limited in order to provide a rolling effect to the center of the thickness of a thick material. That is, immediately after the above heating, the slab surface temperature reaches 850℃.
2 without applying pressure up to a temperature range of ~10oO℃.
It is cooled at a cooling rate of 0°C/min or more, and then rolled. This is the most important point of the invention. As described above, if the heated slab is immediately cooled, a large temperature difference will occur between the slab surface and the center, resulting in a difference in deformation resistance between the slab surface and the center. If rolling is applied in this state, the rolling effect on the center of the slab will be greater than in normal rolling where the temperature difference between the surface and center of the slab is small. Therefore, especially in steel containing Nb as a solid solution, it is possible to refine the grains by rolling and improve toughness. Furthermore, according to this method, since the heated slab is immediately cooled at a faster cooling rate than usual, it is possible to suppress the precipitation of Nb during cooling, which improves the effect of controlled rolling and improves toughness. At the same time, the strength can be significantly improved by promoting transformation strengthening during direct quenching and increasing the amount of Nb precipitation strengthening through tempering. For example, FIG. 1 shows an example of the results of a basic experiment.
〇, 8%Cu-0.4%Ni-0, 2%Mo-0,0
45%Nb-0.010%Ti-0,0045%N-0
, 035%AD, is a diagram showing the influence of the cooling stop temperature of the heated slab on strength and toughness when cooling is performed immediately after heating at a cooling rate of 20 ° C / min. be. The rolling completion temperature is approximately 750°C, the total reduction rate is 7o%, and after quenching, the rolling temperature is approximately 750°C.
Tempered with C. In the same figure, it can be seen that the strength tends to increase as the cooling stop temperature decreases, and the toughness tends to improve as the cooling stop temperature decreases. Through the above basic experiments, the bow tension strength 7, which is the objective of the present invention, was
It has been found that in order to achieve both 0 kgf/mm2 or higher and a fracture surface transition temperature of -80°C or lower, the cooling stop temperature needs to be 1000°C or lower. On the other hand, if the cooling stop temperature becomes too low, the deformation resistance of the slab surface becomes too large, making it difficult to secure a predetermined steel sheet shape in the subsequent rolling step. In order to prevent this, it is necessary to set the cooling stop temperature to 850° C. or higher. Further, the reduction in the rolling stage after stopping cooling imparts processing strain to the austenite, promotes refinement of the structure after transformation, and improves strength and toughness. Therefore, at least 6
0% reduction is required. In addition, the finish rolling finishing temperature is set at (Ar3 transformation point temperature - 30
℃) or higher. After rolling, the steel plate is immediately quenched directly. This is solid solution N
This is to maximize the effect of transformation strengthening by improving hardenability in b. Furthermore, this steel plate is subjected to a tempering treatment in order to obtain high strength by precipitating Nb present in a solid solution state after transformation. Next, examples of the present invention will be shown. (Example) A test steel having the chemical composition shown in Table 1 was melted and cast by a conventional method, and the obtained ingot was heated to the temperature shown in Table 2, and then under the conditions shown in the table. , cooling, rough rolling and finish rolling were performed, immediately quenched, and further tempered at 60°C. The mechanical properties (tensile strength TS, fracture surface transition temperature v T rs) of the obtained steel plate at the center of the plate thickness were investigated. The results are also listed in Table 2. As is clear from Table 2, Nα1(1
i1A), Nn 6 (Steel B), Nα'', 0 (Steel F),
Both Nα11 ([G) and Nα12 (Steel H) have a tensile strength of 70 kg f / mm2 or more, and the fracture surface transition temperature vTrs at the center of the plate thickness is 180 °C or less, providing an excellent strength-toughness balance. It can be seen that it has. Of course, since the chemical components, especially the amount of C, are low, the weldability is excellent. On the other hand, even if the steel has the same chemical composition as the example of the present invention,
At least one of the process conditions (heating, cooling, rolling conditions)
Comparative example where No. 2 is outside the scope of the present invention, i.e., No. 2
(Steel A)-Nn5 (Steel A) has a strength of 70 kgf/m
m2, and the fracture surface transition temperature at the center of the plate thickness is higher than 180°C. Also, comparative example NO,'y (steel c), Nα8 regarding steel C-E having a composition outside the chemical composition range of the present invention.
(Steel D) and No. 9 (Steel E) have a tensile strength of 70 kg even if the process conditions are within the range defined by the present invention.
f/mm2 or less, or even more so, the fracture surface transition temperature is 180°C or more, and the target characteristics are not obtained.

[Left below]

(Effects of the Invention) As described in detail above, according to the present invention, a slab heated to a predetermined temperature is cooled to a predetermined temperature range at a high rate without applying any reduction to steel with appropriate chemical composition. It is cooled, then rolled under specific conditions, immediately quenched, and then tempered, so it has a high tensile strength of 70 kgf/mm2 or more at a plate thickness of 35 mm or more, and a fracture surface transition temperature of -80 It is possible to produce a high-strength, high-toughness, thick-walled steel plate that has high toughness of 0.degree. C. or lower, has an excellent strength-ductility balance, and has excellent weldability.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the influence of the cooling stop temperature of a slab after heating on strength and toughness. Patent applicant Hisashi Nakamura, patent attorney representing Kobe Steel, Ltd.

Claims (2)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.01 to 0.1
0%, Si: 0.10-0.50%, Mn: 0.5-2
．． 0%, Al: 0.01~0.10%, Cu: 0.8~
1.5%, Nb: 0.01-0.06%, Ti: 0.0
05 to 0.020% and N: 0.015% or less, with the balance consisting of iron and inevitable impurities, 11
After heating to a temperature range of 00°C or higher where Nb is sufficiently dissolved, the slab is immediately cooled at a cooling rate of 20°C/min or higher without applying pressure until the slab surface temperature reaches a temperature range of 850 to 1000°C, and then Welding with a thickness of 35 mm or more, characterized by applying a reduction of 60% or more in rolling, completing the rolling at a temperature of (Ar_3 transformation point temperature -30°C) or more, directly quenching immediately, and further tempering. A method for producing high-strength, high-toughness thick-walled steel plates with excellent properties. (2) The steel has Ni: 3.0% or less and Cr: 1.0%
Below, Mo: 0.5% or less, V: 0.1% or less, B: 0
．． Claim 1: Contains one or more of Ca: 0.0030% or less and Ca: 0.0050% or less.
The method described in.