JPS6320414A - Production of high-toughness high-tensile steel plate - Google Patents

Abstract

PURPOSE:To improve the uniformity of the hardness in the thickness direction of a steel plate by heating a specifically composed steel contg. Nb, V or Ti to a prescribed temp. and rough-rolling the steel, then subjecting the rolled steel to primary force cooling and recuperation treatment of the surface layer part under specific conditions. CONSTITUTION:The steel contg., by weight %, 0.02-0.18 C, <=1.0 Si, 0.5-2.0 Mn, 0.005-0.080 solAl, and 0.005-0.150 respectively >=1 kinds among Nb, Ti and V, if necessary, contg. >=1 kinds of 0.05-1.00 Cr, Mo and Cu respectively and 0.1-3.0 Ni, and consisting of the balance Fe is prepd. Such steel is heated to 1,050-1,250 deg.C at which the Nb, Ti and V are thoroughly solutionized. The heated steel is roughly rolled and the rolled steel is force-cooled until the surface layer part of the steel plate falls down to the ferrite transformation initiation temp. or below at the time when a suitable plate thickness is attained in the mid-way of rolling. The steel plate is in succession subjected to the recuperation treatment for 30sec-10min to restore the temp. of the above-mentioned surface layer part to the austenite transformation end temp. or above and further, the steel plate is force-cooled down to <=650 deg.C at >=4 deg.C/sec cooling rate in the central part of the thickness.

Description

[Detailed Description of the Invention] <Industrial Field of Application> The present invention provides a high-toughness material with no non-uniformity in hardness in the thickness direction, and which is suitable for use in membrane structures and offshore structures. This invention relates to a method for stably and reliably manufacturing tensile steel plates.

In recent years, with the increase in the size of welded barrel structures such as ships, pressure vessels, line pipes, and buildings, there has been a desire for steel materials with even higher tensile strength from the standpoint of safety and economy. At the same time, steel materials are required to have high toughness and excellent weldability from the standpoint of safety as well as welding workability.

Of course, in response to these demands, the "controlled rolling-accelerated cooling method" in which forced cooling is performed after hot rolling, and the "direct quenching (Di
A manufacturing method for high-toughness, high-strength steel, such as the rect-quench method 1, has been developed, and the resulting steel has attracted attention for its good weldability and toughness, as well as its high strength, which can be achieved even with a reduction in added alloying elements. It has come to be commonly used for manufacturing line pipes, etc.

However, on the other hand, this method of manufacturing steel materials using accelerated cooling has the following problem: ``As the thickness increases, the hardness variation in the thickness direction increases (the surface layer becomes harder than the center).
'' problems were also pointed out. because,
Steel materials with large hardness fluctuations in the thickness direction and higher hardness near the surface than in the center are more susceptible to environmental cracking and do not exhibit sufficient cold workability when making line pipes, for example. be.

<Prior art and its problems> From the above, regarding the production of steel materials that have high strength and toughness and have little variation in the thickness direction,
Various studies have been made in the past, regardless of whether the steel is heat treated or controlled rolled, and some promising proposals have been made.

For example, in JP-A No. 60-169517, Kowa G has developed an rNb-added steel that is accelerated after hot rolling based on the knowledge that an acicular ferrite m-woven network has excellent hardness uniformity in the plate thickness direction. A method for obtaining a steel material with uniform hardness in the thickness direction by cooling and subsequently quenching and tempering to change the structure to an acicular ferrite structure is disclosed.

However, this method does not take into account that the hardenability of the steel to be treated remains uniform in the thickness direction, and when strong water cooling is performed for hardening, Since there is an unavoidable upper limit on the surface hardness due to the speed difference, it has been extremely difficult to achieve a sufficiently uniform hardness in the thickness direction.

On the other hand, Japanese Patent Publication No. 51-15804 and Japanese Patent Application Laid-Open No. 59-182916 proposed by the present applicant state that ``Preliminary cooling makes a difference in the U-weave between the surface part of the tuning plate and the center part before accelerated cooling. A method is disclosed in which the hardness is finally made uniform in the thickness direction by giving the same thickness and accelerating cooling. In other words, these methods reduce the hardenability of the steel plate by making the center part of the steel plate an austenite-phase structure and the surface part a mixed phase of [austenite + ferrite], thereby increasing the hardness in the thickness direction. This was an attempt to achieve a uniform steel plate, and although it was sufficiently effective in this respect, subsequent studies revealed that the surface layer of the product mesh plate obtained in this way was “two-phase”. It was pointed out that there was a problem that there was a concern that the elongation and toughness of the part might tend to deteriorate because the structure was quenched rapidly.

Means for Solving the Problems> Based on the above problems observed in conventional high-strength steel sheets, the present inventors have developed a steel sheet that has sufficiently uniform hardness in the thickness direction, Moreover, in order to provide a method for stably and reliably manufacturing high-toughness, high-strength steel materials that do not have any particular drawbacks in terms of properties such as elongation and toughness, we are conducting research from a comprehensive perspective of steel material composition, steel manufacturing conditions, etc. They found that ``a steel with a specific composition containing Nb, V, or Ti in a predetermined proportion is first heated to a temperature range where these elements are sufficiently dissolved in solid solution, hot-rolled, and then heated at an appropriate temperature during rolling. At the point when the plate thickness is reached, the surface layer is first forcedly cooled to below the ferrite transformation start temperature, followed by reheating treatment (from the center of the mouth plate to above the austenite transformation finish temperature within a specified time). (natural recuperation, rapid heating near the surface, etc.) will significantly reduce the hardenability of the surface layer of the mouth material, and even if accelerated cooling is performed afterwards, only the hardness of the surface layer will be prevented from increasing. This led to the discovery that a steel plate with no uniformity in strength in the transverse direction can be obtained.

This invention was made based on the above findings, and C: 0.02 to 0.18% (hereinafter referred to as % representing the component ratio).
is weight%), Si: 1.0% or less, Mn: 0.5 to 2.0
%, so It, A j! : 0.005~0.0
80%, Nb: 0.005 to 0.150%, Ti: 0.
005~0.150%, v: o, oos~0.15
0%, or further contains Cr: 0
．． 05-1.00%, Mo: 0.05-1.00%, Cu: 0.05-1.00%, Ni: 0.1-3.0%, and the remainder Steel consisting of Fe and unavoidable impurities is heated to a temperature of 1050 to 1250°C and rough rolled, then forcedly cooled until the surface layer temperature becomes below the ferrite transformation starting temperature, followed by reheating treatment for 30 seconds to 10 minutes. The surface layer temperature is reheated to the austenite transformation end temperature or higher, and then finish rolling is performed, and then the thickness center is forcibly cooled to 650°C or less at a cooling rate of 4°C/sec or more. The present invention is characterized in that it is possible to stably manufacture a high-toughness, high-strength steel plate with uniform directional hardness.

Next, the reason why the component ratio of the symmetrical tone and the heating and rolling conditions are limited as described above in this invention will be explained.

A) Component ratio of steel (Al C The C component has the effect of ensuring the strength of steel materials, but if its content is less than 0.02%, the desired effect cannot be obtained, and on the other hand, 0.18% If the C content exceeds 0.0%, the toughness of the base metal and welded part will deteriorate. Therefore, the C content was set at 0.02 to 0.18%.

Cb) 5i St is an effective component as a deoxidizing agent for steel, and is also effective in increasing strength through solid solution strengthening, but if it is contained in excess of 1.0%, toughness and weldability are adversely affected. (Thus, the Si content was determined to be 1.0% or less.

(c) The Mn 9o component has the effect of improving the strength and toughness of the base metal and welded part, but if it is less than 0.5%, the desired effect cannot be obtained; If the Mn content exceeds 0.00, the Mn content will deteriorate the toughness due to the enrichment of low-temperature transformation products such as bainite structure.
It was set at 5 to 2.0%.

(d) Sol, A1 11 is a component added as a deoxidizing agent for steel and for grain refinement, but if its content is less than o, oos% in Sol,^l, the purpose of these additions is On the other hand, soj and Ajl were 0.080%
If the content exceeds sol, 11', the amount of non-metallic inclusions will rapidly increase, leading to deterioration of toughness.
The content was determined to be 0.005% to 0.080%.

(e) Nbs Ti, and ■ These elements are steel components that play a particularly important role in this invention, and are precipitated as carbonitrides in austenite or ferrite, improving the strength and toughness of the steel. It is a type 1 type because it has the effect of reducing the hardenability of the steel material (particularly the surface layer of the steel material to which the above-mentioned heat history is applied) by providing a predetermined thermal history, and making the hardness of the accelerated cooling agent uniform in the thickness direction. Or, two or more types can be contained, but if each is less than o and oos%, the desired effect cannot be obtained in the above-mentioned action.On the other hand, if each is contained in excess of 0.15 (1%), the toughness of the welded part will deteriorate. From this, Nb, Ti
The content of one or more of (1) and (2) is 0.005 to 0.0, respectively.
It was set at 150%.

ff) Cr-MO% C11% and Ni These elements all have the effect of increasing the strength of tears, so if even higher strength is required, one or more of these elements are added. The reason for limiting the content of each component will be explained below along with the accompanying effects.

Q Cr and -〇The strength-improving effect of these components is due to their content being 0.
．． If the content of Cr and 40 is less than 0.05%, the desired effect will not be exhibited, while if the content exceeds 1.00%, the toughness of the base metal and welded part will deteriorate. It was set at 0.05-1.00%.

○ Cu The Cu component not only improves the strength of steel but also improves corrosion resistance, but if its content is less than 0.05%, the desired effect cannot be obtained; Since hot cracking is likely to occur in the slab if the content exceeds 0.05% to 1.00%.

○ Ni The Ni component not only improves the strength of steel, but also improves toughness and corrosion resistance, but if its content is 0.
If it is less than 1%, the desired effect cannot be obtained in the above action; on the other hand,
If the Ni content exceeds 3.0%, the toughness of the base metal and weld zone will deteriorate, so the Ni content should be 0.1 to 3.
．． It was set as 0%.

B) Heating/Rolling Conditions (a) Heating Temperature If the heating temperature prior to hot rolling is less than 1050°C, the added Nb, Ti, V, Cr, MOSCu, Ni, etc. will not be sufficiently solid-dissolved in the steel and will not form in the steel. Not only will it not be possible to impart the desired strength and toughness, but it will also no longer be possible to differentiate the hardenability between the surface layer portion and the center portion due to the thermal history during preliminary cooling, which will be described later. On the other hand, if the heating temperature exceeds 1250°C, the austenite grains will become too coarse, making it impossible to ensure sufficient toughness even with the subsequent "rolling-accelerated cooling" process. 1050~12
The temperature was set at 50°C.

Pre-cooling temperature Steel materials after rough rolling are once forcedly cooled (e.g. water-cooled) to reduce the hardenability of the surface layer.
The reason why the water-cooling stop temperature of the surface layer due to preliminary cooling is set as "below the ferrite transformation start temperature" is because if cooling is stopped at the ferrite transformation start temperature or higher (i.e., in the austenite region), reheating treatment will be performed afterwards. This is because even if this is done, the hardenability of the surface layer portion will not deteriorate and a steel material having uniform hardness in the thickness direction will not be obtained.

The means of pre-cooling is not particularly limited, but for the purpose of ensuring that the effect of thermal history is sufficiently distributed in the thickness direction, the pre-cooling means should be 300 to 30 mm for a plate thickness of 30 to 150 mm.
It is desirable to perform strong water cooling at about 00 ff/m''·min for about 2 to 60 seconds.

(C1 reheating treatment time and recuperation temperature In this invention, as mentioned above, in order to reduce the hardenability of the surface layer of the steel material, the steel material after preliminary cooling is processed by "natural reheating from the center of the steel material" or "near the surface area." The temperature of the surface layer reaches the end temperature of austenite transformation (Ac 3 points)
It is necessary to reheat so that the temperature reaches the above level. If the recuperation temperature is less than the Ac3 point, a steel material having uniform hardness in the thickness direction cannot be obtained, and coarse bainite may be mixed in, resulting in deterioration of the toughness and ductility of the surface layer.

In other words, by performing such "forced cooling-recuperation treatment", the effects of Nb5Ti and V are utilized to reduce the hardenability of the surface layer of the steel material, and fine-grained austenite is formed in the surface layer, improving its ductility. This improves toughness, but in order to ensure this effect more stably, the recuperating hot water temperature must be adjusted to Ac.

point +100°C] is preferable.

Furthermore, in order to ensure the effect of reducing the hardenability of the surface layer by Nb, Ti or
It is also important to adjust the time to between 30 seconds and 10 minutes. This is because short-time reheating treatment of less than 30 seconds does not sufficiently reduce the hardenability of the surface layer portion, while reheating treatment time of more than 10 minutes significantly reduces rolling efficiency and economic efficiency.

In addition, when adopting external heating means near the surface for recuperation, it is possible to use a gas burner, infrared heating device, induction heating device, etc., but from the viewpoint of temperature control and workability, induction heating is preferable. is desirable. From the viewpoint of ease of natural recuperation after preliminary cooling or recuperation treatment by local (surface) heating, the thickness of the rolled material is preferably about 30 to 150 mm.

(dl Accelerated cooling conditions after finish rolling This accelerated cooling is performed to improve the strength and toughness of the steel material.
This is an essential step for producing high-toughness, high-strength steel sheets, which is the objective of the method of this invention. However, if the average cooling rate at the center of the plate thickness is less than 4°C/sec or if the accelerated cooling (water cooling) stop temperature exceeds 650°C, sufficient strength and toughness improvement effects may not be achieved. can't get it.

Therefore, accelerated cooling is performed at a cooling rate of 4°C/sec at the center of the plate thickness.
It was specified that the test should be carried out at temperatures above 630°C and below 630°C.

Now, Figure 1 is a graph showing the relationship between the thermal history conditions due to preliminary cooling and the hardness of the obtained steel material when hot working steel having the composition shown in Table 1. ! After heating the small test piece to r 1200°C using the device,
After 25% processing at 00°C, 50% processing at 850°C, cooling rate: 30°C/se.
When cooled to room temperature at c (0 in the figure) and "Same <1"
After heating to 200°C, processing at 1000°C for 25%, cooling to 400°C, then reheating to 900°C by heat treatment for 1 minute, and then heating to 850°C for 5%.
After 0% processing, cooling rate: 30℃/s
It is possible to confirm the difference in hardness when cooled to room temperature using EC ([F] in the figure).

From these comparisons, it is found that steel containing Nb or Ti-V can achieve a significant decrease in hardness, that is, a decrease in hardenability, by cooling to 400℃ and then reheating to 900℃. It can be seen that in the comparison steel that does not contain Ti, Nb, and ■, even when subjected to such a heat history, the effect of reducing hardenability is extremely small. Furthermore, from Figure 1, even if the sample has been given a thermal history due to pre-cooling, ○ those that do not fall below the ferrite transformation temperature during forced cooling (O in the figure), ○ those whose reheating time is shorter than 30 seconds ( ■) in the figure shows that the hardness reduction is extremely small and the effect is not sufficient.Furthermore, in the cases where the recuperation temperature is low and does not exceed the austenite transformation temperature, a mixed structure of coarse bainite and ferrite is exhibited (Fig. It can be seen that sufficient effects cannot be obtained, such as medium [F]) and no decrease in hardness ([F] in the figure).

On the other hand, Figure 2 shows the change in hardness in the thickness direction of the steel material when rolling is performed under the thermal history conditions determined from the above findings, between the steel subject to the present invention (Steel A in Table 1) and the comparative steel (Steel A in Table 1). (The results for A steel are shown in Figure 2<a), and the results for Cfl are shown in Figure 2). In addition, in this investigation, each steel was heated to 1200 degrees and first hot rolled to a thickness of 6011, and then the flow density: 1
000 Il/m"・+min water cooling for 15 seconds to bring the surface temperature to 450°C), then natural reheating from the center of the plate thickness for 60 seconds to bring the surface temperature to 900°C, and then The test was carried out on a plate material that was rolled to 800°C to a plate thickness of 30 mm, immediately thereafter acceleratedly cooled to 550°C at a cooling rate (average cooling rate at the center of the plate thickness) of 10°C/sec, and then allowed to cool.

For comparison, FIG. 2 also shows the results obtained by allowing the material to cool as it is without pre-cooling between each rolling, and by accelerating cooling from 800.degree.

From the results shown in Figure 2, it is clear that the results do not give any thermal history (
O mark in the figure) and steel that does not contain either Nb or Tis V as a constituent component (Figure 2 (b)), the surface hardness of the obtained steel plate increases, and the strength in the thickness direction increases. On the other hand, in a steel plate whose composition satisfies the conditions of the present invention and has been given a thermal history according to the conditions defined by the present invention (marked with * in Fig. 2 (a)), the surface layer is non-uniform. It can be seen that the increase in hardness is suppressed, and extremely excellent uniformity of hardness in the thickness direction is ensured.

Next, the present invention will be explained using more specific examples and in comparison with comparative examples.

<Example> First, steel pieces having the chemical compositions shown in Table 2 were prepared, and these were hot-rolled under the heat history conditions shown in Table 3 and made into steel plates with a thickness of 3011 under accelerated cooling conditions.

Preliminary cooling (water cooling) during rolling is performed when the plate thickness reaches 70 mm, and finish rolling after reheating treatment is performed at [recuperation temperature ~
800°C], and immediately after finishing the rolling at 800°C, accelerated cooling (water cooling) was performed. Here, the recuperation temperature is 8
For those whose temperature was below 00°C, finish rolling was completed at a temperature as high as possible after reheating (the recuperation temperature), and accelerated cooling was immediately performed.

The mechanical properties of each of the thus obtained plates were measured, and the hardness distribution in the thickness direction was also investigated, and the results are also shown in Table 3.

The hardness distribution was evaluated by creating a distribution map as shown in Figure 2. Items with no hardness are marked with a ``○'' mark, items with slight variations in hardness are marked with a △ mark, and items with noticeable variations in hardness are marked with an x mark j.

As is clear from the results shown in Table 3, the steel plate manufactured according to the conditions specified in the present invention not only has excellent strength and toughness, but also has excellent hardness uniformity in the thickness direction. On the other hand, it can be seen that a steel sheet whose manufacturing conditions deviate from the conditions specified in the present invention does not fully satisfy the above characteristics (in addition, if the chemical composition of the steel sheet deviates from the conditions specified in the present invention, As shown in Figure 2 (bl
(as shown in ).

As explained above, according to the present invention, there is no non-uniformity in hardness in the thickness direction, and the high tensile F material has excellent toughness.
It is possible to stably and efficiently produce four types of steel structures, and it is possible to further improve the reliability of various steel structures such as ships, pressure vessels, line pipes, and building structures, and to fully cope with the increase in size. This brings about extremely useful effects industrially.

[Brief explanation of the drawing]

Figure 1 is a graph comparing the hardness of steels of various compositions hot rolled and accelerated cooled with different precooling conditions. Figure 2 is a graph comparing the hardness of steel sheets with different precooling conditions. This is a graph comparing the hardness distribution in the thickness direction when precooling is applied and when it is not applied.
1 is for the steel subject to the present invention, and 1) is for the comparative steel.

Claims (2)

[Claims] (1) Weight percentage: C: 0.02 to 0.18%, Si: 1.0% or less, Mn
:0.5-2.0%, sol. Al: 0.005-0.
080%, and also contains one or more of Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, V: 0.005 to 0.150%, and the remainder Steel consisting of Fe and unavoidable impurities is heated to a temperature of 1050 to 1250°C and rough rolled, then forcedly cooled until the surface layer temperature becomes below the ferrite transformation starting temperature, followed by reheating treatment for 30 seconds to 10 minutes. Finish rolling is performed after the surface layer temperature is reheated to a temperature equal to or higher than the austenite transformation end temperature, and then forced cooling is performed to 650°C or less at a cooling rate of the central part of the plate thickness of 4°C/sec or more. , a method for manufacturing high-toughness, high-strength steel plates with uniform hardness in the thickness direction. (2) Weight percentage: C: 0.02 to 0.18%, Si: 1.0% or less, Mn
:0.5-2.0%, sol. Al: 0.005-0.
080%, and one or more of Nb: 0.005-0.150%, Ti: 0.005-0.150%, V: 0.005-0.150%, and Cr: 0 .05 to 1.00%, Mo: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Ni: 0.1 to 3.0%. Steel, the remainder of which consists of Fe and unavoidable impurities, is heated to a temperature of 1050 to 1250°C and rough rolled, then forcedly cooled until the surface layer temperature falls below the ferrite transformation initiation temperature, followed by 30 seconds to 10 minutes of recovery. Finish rolling is performed after the surface layer temperature is reheated to a temperature equal to or higher than the austenite transformation end temperature by heat treatment, and then forced cooling is performed to 650°C or less at a cooling rate of 4°C/sec or more at the center of the plate thickness. A method for manufacturing high-toughness, high-strength steel plates with uniform hardness in the thickness direction.