JPH0313524A - Production of thick high-toughness high tensile steel plate having excellent toughness on steel plate surface and in central part of thickness - Google Patents

Abstract

PURPOSE:To produce the thick high-toughness high tensile steel plate having the excellent low-temp. toughness in the central part of the thickness and the surface part of the steel plate by subjecting a steel stock having a specific compsn. to a water cooling at a specific cooling rate between a rough rolling and finish rolling at the time of producing the thick steel plate by subjecting the above-mentioned steel stock to the hot rough rolling, finish rolling and cooling. CONSTITUTION:The slab of a low-carbon steel contg., by weight, 0.01 to 0.10% C, 0.10 to 0.50% Si, 0.5 to 2.0% Mn, <0.015% P, <0.003% S, 0.010 to 0.10% Al, 0.005 to 0.020% Ti, 20 to 70ppm N, or further >=1 kind of <1.5% Cu, <3.0% Ni, <0.03% Nb, and <0.0050% Ca, is heated to the Ac3 point to 1150 deg.C and is roughly rolled at <15% rolling reduction in the temp. range of Ar3 to (Ar3+100 deg.C). The steel plate is then subjected to the finish rolling to >=50mm thickness at >30% rolling reduction in the temp. range of (Ar3 to 50 deg.C) to (Ar3 to 10 deg.C) steel plate surface temp. to >=(Ar3 to 50 deg.C) steel plate surface temp. and is then cooled down to <=500 deg.C at 1 to 20 deg.C/sec cooling rate immediately thereafter. The steel plate is subjected to >=1 times of the water cooling treatment at the cooling rate CR( deg.C/ sec) expressed by inequation between the rough rolling and the finish rolling.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing a high-toughness, high-strength steel plate having a thickness of 50 mm or more, which has excellent toughness in the central part of the plate thickness and also has excellent toughness on the surface of the steel plate. (Prior art and the *problem to be solved) Steel plates used for offshore structures in ice-covered areas include high-strength steel plates with a thickness of 50III11 or more that do not require preheating during welding and have excellent low-temperature toughness. requested. It is already known that it is effective to apply low-temperature rolling to improve the toughness of the central part of the sheet thickness and accelerated cooling methods that can reduce carbon equivalent in the production of such steel sheets. In other words, regarding low-temperature rolling to improve the toughness of the central part of the slab, it is necessary to ensure a considerable reduction in the austenite non-recrystallization temperature range at the central part of the slab, but the temperature at the central part of the slab during rolling is is higher than the surface temperature,
Generally, controlled rolling including a considerable amount of reduction is performed in a low temperature range where the slab surface temperature is lower than Ar). As a result, the Charpy fracture surface transition temperature is 160°C at the center of the plate thickness.
It is now possible to manufacture thick steel plates with extremely excellent toughness as shown below. However, when the rolling temperature is Ar on the slab surface. 5, pro-eutectoid ferrite is generated in the surface area during rolling, and a phenomenon of coarsening is observed, and the toughness near the surface of the final manufactured steel plate is inferior to that at the center of the plate thickness. It has the disadvantage of being The present invention eliminates the drawbacks of the prior art and provides a method for manufacturing a thick high-toughness steel plate with a thickness of 50++v+ or more, which has excellent toughness in the central part of the plate thickness and also has excellent toughness on the surface of the steel plate. The purpose is to provide the following. (Means for Solving the Problems) As a result of intensive research to solve the problems of the prior art, the present inventors found that pro-eutectoid ferrite during rolling, which causes deterioration of surface toughness, They found a method to suppress the formation and coarsening as much as possible, and developed a method for manufacturing thick-walled, high-toughness, high-strength steel plates with excellent toughness on the steel plate surface and in the center of the plate thickness. That is, in the present invention, C: 0.01 to 0.10%, Si
:0.10~0.50%, Mn:0.5~2.0%, P
: 0.015% or less, S: O, 003% or less, AQ: 0
．． 010-0.100%, Ti: 0.005-0.02
0% and N: 20 to 70 ppm, and if necessary, further contains Cu: 1, 5% or less, Ni: 3, 0% or less, Nb: 0.03% or less, and Ca: O, OO 50% or less. After heating a steel containing one or more of these and the remainder consisting of Fe and unavoidable impurities to a temperature of Ac) or higher and 1150°C or lower, the steel plate surface temperature is Ar3 or higher, (Ar2+
Rough rolling including a reduction of 15% or more is performed in a temperature range of 100℃ or less, and then the steel plate surface temperature is (Ar, -10℃
) and below, the reduction amount is 3 in the temperature range of (Ar, -50°C) or higher.
After finish rolling of 0% or more, the steel plate surface temperature reaches (Ar, -
50 ° C.) or higher to a predetermined thickness, immediately bring the temperature to a temperature range of 550 ° C. or lower in steps 1 to b, and perform intermittent water cooling one or more times between the rough rolling end temperature and the finish rolling start temperature, and. During this period, the average cooling rate CR(”
The gist of this invention is a method for producing a thick-walled, high-toughness, high-strength steel plate having a thickness of 50+s+n or more, which is characterized by cooling at a temperature of 50+s+n or more and exhibits excellent toughness on the steel plate surface and in the center of the thickness. CR>1.5x (0,166-6,14X10-'t)
...(1) Here, t: Slab thickness at completion of rough rolling (■■) 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 o or io%. therefore. The amount of C is in the range of 0.01 to 0.10%. Si: Si is a deoxidizing element and needs to be added in an amount of 0.10% or more, but excessive addition will reduce the upper limit to 0.
．． It shall be 50%. Therefore, the amount of Si is 0.10 to 0.5
The range is 0%. Mn: Mn is effective in increasing strength, and for this purpose, 0.65% or more is required, but if added in excess of 2.0%, toughness deteriorates. Therefore, the amount of Mn is set in the range of 0.5 to 2.0%. P: P is an element with a strong tendency to segregate, and deteriorates the toughness of the base metal and weld zone. Therefore, the amount of P needs to be kept low, and is suppressed to 0.015% or less. S: S forms A-based inclusions and deteriorates drawing properties in the thickness direction, lamellar tear resistance, and further toughness. Therefore, the amount of N needs to be as low as possible, and is regulated to 0.003% or less. Ti: Ti is an element that forms hardly soluble carbonitrides and suppresses the growth of austenite grains during slab heating or welding, so it has the effect of improving the toughness of the base metal and during welding. For that purpose, 0.005% or more is required, but 0.0
When added in an amount exceeding 20%, coarse inclusions are formed and the toughness is deteriorated. Therefore, the amount of Ti is in the range of 0.005 to 0.020%. AQ: AQ is a deoxidizing element and requires 0.01% or more, but excessive addition forms inclusions and deteriorates toughness, so the upper limit is set to 0.10%. Therefore, the AQ amount is 0.
The range is 0.01 to 0.10%. N: N forms a hardly soluble carbonitride with Ti, which has the effect of improving the toughness of the base metal and weld zone.
Although 0 ppm or more is required, excessive content exceeding 70 ppm will conversely significantly deteriorate the toughness of the base metal and weld zone. Therefore, the amount of N is set in the range of 20 to 70 ppp. Each of the above elements is an essential component, but depending on the necessity, one or two of the following elements: Cu, Ni, Nb, and Ca.
Seeds or more can be added in appropriate amounts. Cu: Cu is an effective element for solid solution strengthening, precipitation strengthening, or transformation strengthening by improving hardenability, but excessive addition deteriorates toughness, so it is limited to 1.5% or less. Nj: Ni has the effect of improving low-temperature toughness, but is expensive, so it is set to 3.0% or less in consideration of economic efficiency. Nb: Nb is an element that suppresses recrystallization of austenite in a solid solution state, enhances the effect of controlled rolling, and promotes transformation strengthening by accelerated cooling after rolling, contributing to improvement of strength and toughness. Since the addition of 0 deteriorates the toughness of the weld,
．． 03% or less. Ca: Further, Ca can be added for the purpose of improving toughness, but excessive addition significantly impairs toughness, so the amount of Ca is set to 0.0050% or less. Next, the reasons for limiting the heating, rolling, and cooling conditions for steel having the above chemical components in the present invention will be explained. It is necessary to completely austenite the entire slab in the heating process prior to rolling, and for this purpose it is necessary to heat the slab to a temperature higher than 1150°C, but heating at a temperature higher than 1150°C will cause the austenite grains to coarsen and deteriorate the toughness. This is undesirable because it causes
Defined as 0℃ or below. When hot rolling a slab heated to the above temperature range, it is necessary to control the cooling rate of the slab during rough rolling, finishing rolling, and adjusting the temperature of the slab from the rough rolling temperature to the finishing rolling temperature. There is. First, in order to improve the toughness of the central part of a thick steel plate, reduction in the austenite recrystallization temperature range and reduction in the austenite non-recrystallization temperature range in the central part of the thickness are essential. In this respect, the rough rolling temperature (Ar, or higher, (
(Ar, +100℃) or less) and finish rolling temperature (A
r, −10° C.) or lower, and (Ar, −50° C. or higher) respectively correspond to the austenite recrystallization temperature at the center of the plate thickness and the austenite non-recrystallization temperature. In order to ensure a Charpy fracture surface transition temperature of -60°C or less at the center of the plate thickness, rough rolling is required. It is necessary to ensure a rolling reduction of 15% or more and 30% or more in finish rolling, respectively. Further, the finishing temperature is set at (Ar, -50° C.) or higher to ensure the effect of increasing strength due to subsequent accelerated cooling. Next, a description will be given of means for improving the toughness of the steel plate surface, which is the most important point of the present invention. The present inventors discovered that the cause of the deterioration of toughness on the surface of cold-rolled steel sheets was due to the coarsening of pro-eutectoid ferrite generated during rolling. It was found that the formation and coarsening of pro-eutectoid ferrite on the back surface mainly occurred during cooling in the atmosphere from the completion of rough rolling to the starting temperature of finish rolling. Therefore, as a result of investigating the relationship between the average cooling rate and back surface toughness during this period, we found that, as shown in Figure 1, intermittent water cooling was performed one or more times at the slab thickness at the completion of rough rolling, and )
It has been discovered that the toughness of one surface area can be significantly improved by cooling at an average cooling rate CR that satisfies the formula. In other words, in order to improve the toughness of one surface area, between the completion of rough rolling and the start temperature of finish rolling, intermittent water cooling is performed once or more at the slab thickness at the time of completion of rough rolling, and the atmospheric pressure is reduced by the following formula: Cooling rate of natural cooling (empirical formula) CRAC ('C/
5ee) CRAC= 0.166-6.14 X 10
-' t, where t: slab thickness at completion of rough rolling (mu)
It is necessary to cool the temperature at 1.5 times or more. Intermittent water cooling during this period plays an important role in significantly suppressing the growth of ferrite grains formed on the surface of the steel sheet by lowering the surface temperature of the copper sheet below the temperature at which -H ferrite grains are likely to grow. This is an essential factor of the present invention. This is to obtain a strength of 1 to b up to a temperature range of 550° C. or lower after finish rolling. If the cooling stop temperature is higher than 550°C, this strength-increasing effect cannot be obtained, and if the cooling rate is less than 1°C/sec, the strength-increasing effect is small;
If the speed is faster than sec, the strength will increase too much and the toughness will deteriorate, so it is necessary to set the speed within this range. Next, examples of the present invention will be shown. (Example) A steel having a chemical composition shown in Table 1 was heated, rolled, and cooled under the conditions shown in Table 2 to produce a steel plate having a predetermined thickness. The tensile strength, yield strength, and Charpy fracture surface transition temperature of the steel plate surface (1 inch below the steel plate surface) and the center of the plate thickness were investigated for the obtained steel plate. The results are also listed in Table 2. The following considerations can be made from Table 2. In Tests &3 to Ha5, the heating and rolling conditions were outside the range of the present invention, so the Charpy fracture surface transition temperature (
vTrs) is -60°C or higher. It does not have suitable performance as a steel plate for low temperature use. Further, in both tests NQIO and Mail, the toughness at the center of the plate thickness was poor because the chemical composition of the steel was outside the scope of the present invention. In addition, test Na2. &7, Na9 has sufficient toughness in the center of the plate thickness as a steel plate for low temperature use, but because it was cooled in the atmosphere from the end of rough rolling to the start of finish rolling,
The toughness of the surface of the steel sheet is significantly inferior to that of the center of the sheet thickness. Compared to these comparative examples, the tests of the present invention example &1°&6,
All Na8 has excellent low-temperature toughness at the center of the sheet thickness, and because it is cooled at the appropriate cooling rate specified in the present invention from the end of rough rolling to the start of finish rolling, it has excellent toughness at the surface of the steel sheet. It also has excellent low-temperature toughness that far exceeds that at the center of the plate thickness.

[Left below]

(Effects of the Invention) As detailed above, according to the present invention, the chemical components are appropriately adjusted and the slab is processed under appropriate heating, rolling, and cooling conditions, especially between the completion of rough rolling and the start of finish rolling. Since the structure is configured to cool at an appropriate cooling rate, it is possible to produce a thick-walled, high-toughness, high-strength steel plate having a thickness of 50 Iam or more and having excellent low-temperature toughness in both the central part of the plate thickness and the surface part of the steel plate.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the cooling rate of a slab from the completion of rough rolling to the start of finish rolling and the Charpy fracture surface transition temperature (vTrs) of the surface portion of the steel sheet and the center portion of the sheet thickness.

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%, P: 0.015% or less, S: 0.003% or less, Al: 0.010 to 0.100%, Ti: 0.005
After heating a steel containing ~0.020% and N: 20 to 70 ppm, with the balance consisting of Fe and unavoidable impurities to a temperature of Ac_3 or higher and 1150°C or lower, the steel plate surface temperature is Ar_3 or higher and (Ar_3+100°C) or lower. Perform rough rolling with a reduction of 15% or more in the range, and then perform finish rolling with a reduction of 30% or more in a temperature range where the steel plate surface temperature is below (Ar_3-10℃) and above (Ar_3-50℃). Finish the steel plate to the specified thickness when the surface temperature is (Ar_3-50℃) or higher, and immediately heat the plate at 1 to 20℃/s to a temperature range of 550℃ or lower.
When manufacturing a steel plate by cooling at a cooling rate of EC, intermittent water cooling is performed one or more times between the rough rolling end temperature and the finish rolling start temperature, and during this period, the following formula (1) is satisfied. A method for producing a thick-walled, high-toughness, high-strength steel plate having a thickness of 50 mm or more and having excellent toughness on the steel plate surface and in the center of the plate thickness, characterized by cooling at an average cooling rate CR (° C./sec). CR＞1.5×(0.166-6.14×10^-^4
t)...(1) Here, t: Slab thickness at completion of rough rolling (mm) (2) The steel further contains Cu: 1.5% or less, Ni: 3.
0% or less, Nb: 0.03% or less and Ca: 0.005
The method according to claim 1, wherein the method contains one or more of 0% or less.