JPS6156268A - High toughness and high tensile steel and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a high tensile steel sheet superior in toughness in sheet thickness direction, by combining solution treatment and working heat treatment in hot working steel slab having a specified compsn. to thick plate material. CONSTITUTION:Low alloy steel slab contg. 0.07-0.20% C, <0.5% Si, 0.6-2.0% Mn, 0.2-1.0% Cr, 0.05-1.0% Mo, <=0.02% both P and S, 0.01-0.10% SolAl, 0.0005-0.0020% B, <=0.0060% N, further specified quantities of >=one kind among Ni, Cr, V, Nb, Ca is solution treated. Next, said slab is heated to 900- 1,150 deg.C and hot rolled at 800-930 deg.C finish biting temp., >=40% cummulative draft against finished thickness, and at >=750 deg.C finishing temp. Next, said plate is cooled rapidly so that austenite grain is elongated and quenched structure is martensite under the surface layer, and austenite is granular and quenched structure is martensite + lower bainite at center part in plate thickness. This is tempered at <=AC1 point temp. to obtain high tensile steel plate of 70-100kg/mm.<2> class superior toughness.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Applicability) The present invention has a tensile strength of 70 to 100 kg/
This invention relates to VN2 class high tensile strength steel and its manufacturing method.

In recent years, energy demand has been on the rise, and energy-related steel structures such as the construction of offshore structures leading to the development of seabed resources and the construction of high-head penstocks for pumped storage power generation to adjust nighttime surplus power from thermal power generation have become increasingly popular. Construction is picking up steam. As the steel materials used in these products become larger and thicker, there is a desire to develop steel with superior strength and toughness to ensure greater safety.

(Prior art) Conventionally, reheating, quenching and tempering type materials have been the mainstream for high-strength materials, and it is difficult to satisfy strength and toughness over the entire thickness of the material, especially for thick materials.
Addition of hardenability-enhancing elements such as Cr*Mo has been widely used. However, more than reassuring Nl, Cr
, Mo, and other alloying elements are not favorable as they increase the carbon content, which is one of the indicators of weldability. Therefore, to date, many methods have been published to maximize the hardenability of B. For example, during reheating and quenching, keep B in a solid solution state as much as possible to 1000°C to 1050°C.
There is a method of fixing N as ktN during hot rolling at .degree. Also, as in Japanese Patent Publication No. 56-52970, low N
There is a method that uses the addition of Ti to suppress the deterioration of toughness caused by coarse TIN precipitates, the increase in strength due to Ti, and the effect of improving hardenability due to the addition of B. Also, special public service in Showa 5
5-40091, there is also a method of performing fine AtN'i precipitation on thick-walled materials as a pre-temperature treatment to homogeneously disperse solid solution B to improve hardenability. All of these relate to methods for producing high-strength steel of reheating, quenching, and tempering type, and can stably ensure hardenability at the center of the plate thickness even when the thickness is increased.

(Problems to be solved by the invention) However, the reheating, quenching and tempering molds produced by these methods,
Although strength and toughness were improved by improving the hardness in the center of the plate thickness, there was a problem in that sufficient toughness could not be obtained below the surface layer of the plate thickness. This is because as hardenability improves at the center of the plate thickness, the subsurface layer of the plate inevitably becomes a completely martensitic structure during quenching, and this tendency is particularly strong for thick-walled materials. Therefore, as one way to improve the toughness below the surface layer of the plate thickness, the toughness has been improved by repeating the reheating and quenching process two or more times to make the austenite grains finer. However, this method is also unfavorable in terms of manufacturing costs.

(Means for Solving the Problems) The present inventors focused on the deterioration of toughness under the surface layer of the plate thickness observed in reheating, quenching and tempering molds for thick-walled materials, and aimed to improve the toughness and improve the toughness in the thickness direction. As a result of experiments on various manufacturing methods, with the aim of further improving high-strength steel with brittle fracture propagation arresting properties and weldability,
It has been discovered that by combining solution treatment and processing heat treatment, the toughness below the plate thickness surface layer can be significantly improved and the desired steel can be manufactured. The present invention was constructed based on this knowledge, and The abstract is C0.07-0.20%, 810.5% or less,
Mh O16~2.0%, P≦0.02 excellent, S<0.0
2chi, Cr 0.2-1.0%, Mo 0.05-1.
0%, 5otAtO101~0.10%, B Q, O
Including OO5~0.0020chi, N<0.0060won,
Furthermore, NI is 0.01 to 3.0, CuO is 505 to 1.
0%, Vo, 01-0.2%, Nb0.005-0.1
Steel containing one or more of the following: 0%, Ca0.0010~0.0080%, with the balance consisting of Fe, and has elongated austenite grains below the plate thickness surface layer and a tempered martensitic structure. It is a high-toughness, high-strength steel with granular austenite grains in the center of the plate thickness and a tempered martensite + lower bainite structure.Such steel is made by heating the slab to 900-1150℃ after solution treatment. , in hot rolling, finish biting temperature 8oo~930
It is produced by applying a cumulative reduction of 40% or more with respect to the finished thickness at a temperature of 750°C or more, and immediately after the completion of rolling, quenching is performed, followed by tempering at a temperature of 1 Ac or less.

The present invention will be explained in detail below.

First, the reason why the steel of the present invention is limited to the above-mentioned steel components will be described.

C: If C is less than 0.07%, the strength is insufficient;
．． If it exceeds 20%, weldability and toughness will deteriorate.

81: St is effective in improving strength, but if it exceeds 0.5%i, it reduces toughness.

Mn: If it is less than 0.6%, the strength and toughness will be low, and if it exceeds 2.0%, it will deteriorate weldability and toughness. It is desirable to reduce Op, s; p, s as much as possible.
The reason why P is less than 0.02% is that P deteriorates tempering brittleness and toughness.

In addition, S produces non-metallic inclusions Mn5k, and MnS is formed by rolling.
0.0 to elongate and demonstrate the anisotropy of ductility and toughness.
6Cr: Less than 2%; 0.2 or more Cr is added to ensure hardenability, but if it exceeds 1.0%, weld hardenability increases.

Mo: Like Cr, Mo also increases hardenability, so if it is less than 0.05% it has no effect, and if it exceeds 1.0%, weldability and toughness deteriorate.

5otkt; At should be added in an amount of 0.01% or more to ensure that it is effective for hardenability, but 0.10%
When gold is exceeded, inclusions increase.

BIB is the most effective element for improving hardenability, obtaining a hardened structure in the present invention, and ensuring strength and toughness, but when it exceeds 0.0020 or more, its effect is saturated and the toughness is reduced.

This is because the hardenability improvement effect of B is stabilized by setting NUN to 0.0060% or less, and NUN is 0.0060% or less.
This is because if it exceeds 60%, weldability deteriorates.

The above component composition provides strength and toughness.

Furthermore, in the present invention, the following components are selectively added to improve the properties.

Nl: NI is added to ensure total strength and toughness, and if it is less than 0.01%, the effect is not sufficient;
If it exceeds 0%, the effect of improving toughness is small considering the strong rebound.
This is undesirable because it increases costs.

Cu; Cu1t0 to ensure hardenability and toughness
．． 0.05% is required, but if it exceeds 1.0%, the toughness will deteriorate.

v; v is required to be 0.01 inch or more to ensure strength. However, if it exceeds 0.2%, the toughness will be significantly deteriorated.

Nb: While Nb increases the strength, it is used especially in this process to make the austenite grains finer and improve toughness. It is effective when OQ is 5% or more, but when it exceeds 0.10%, toughness deteriorates.

Ca; Ca is extremely effective in spheroidizing non-metallic inclusions, and 0.0010 mm is necessary to reduce anisotropy, but if it exceeds o, 00 mm, inclusions increase and toughness decreases. lower.

Furthermore, the reason for specifying the crystal structure in the present invention will be described. In other words, we will explain why the main quenched structure must be elongated austenite grains and martensite grains below the surface layer of the sheet thickness, granular (spherical) austenite grains at the center of the sheet thickness, and a martensite + lower bainite structure. . Generally, the quenched structure becomes a martensite structure, a martensite+lower bainite structure, an upper bainite structure, etc. in descending order of cooling rate. Here, the upper bainite structure is a structure obtained due to insufficient quenching and is undesirable because it reduces strength and toughness. On the other hand, if the martensitic structure is coarse austenite grains, the toughness is also deteriorated. Therefore, the martensite + lower bainite structure is the structure that provides the highest toughness. That is,
This is because when the lower bainite structure is included, the effective grain size becomes even finer. On the other hand, in the case of elongated austenite grains and martensite structure, the austenite grain size becomes finer and at the same time, the effective grain size becomes finer due to the formation of deformation bands, etc., so the toughness is superior to that of granular grains.

On the other hand, when the central part of the plate thickness becomes elongated grains, the austenite grain size becomes finer, which increases the transformation point and reduces the hardenability due to the slow quenching cooling rate at the center of the plate thickness, forming upper bainite and decreasing toughness. A problem arises. As described above, only when the structure is limited to the above, uniform toughness in the thickness direction and high strength can be obtained. Furthermore, a manufacturing method for obtaining stable quality on a factory scale using the present invention will be described.

First, molten low-alloy steel having the above-mentioned composition is made into steel slabs by continuous casting or ingot blooming. Furthermore, the copper piece is subjected to solution treatment before hot rolling. This disease treatment is a treatment for dissolving coarse carbonitrides generated during slow cooling during billet production and dissolving them into austenite.The solution temperature is heated to a temperature higher than the austenitization temperature, preferably 1200℃ or higher. Then, after holding and completely solutionizing the precipitate, it is immediately quenched. In this case, the average cooling rate is a process of cooling at a rate of I O'C/min or more. This solution treatment may be performed using high-temperature heat after continuous casting or after agglomeration, instead of reheating as described above. Next, this solution-treated steel piece is
It is heated to 0 to 1150°C and hot rolled. The reason why the heating temperature is limited to 900 to 1150°C is that many fine carbonitrides are formed during heating, the heated austenite becomes finer, and since N is fixed at kt, solid solution B increases. This is to improve hardenability during rapid cooling after hot rolling. In other words, the temperature was set at 1150° C. as a temperature that would not dissolve finely precipitated carbonitrides. Moreover, at a low temperature of less than 900° C., the deformation resistance during hot rolling is large and the rolled shape becomes poor. Furthermore, the preferred temperature is 950-1
The temperature is 050°C. Figure 1 shows a low-alloy steel billet (C: 0.11%, s+: 0.25%r Mn) produced by continuous casting.
; 0.91%, P: 0.003%, S: 0.001
%#NI: 1.04% sCr: 0.56%,
Mo; 0.45%, 5otAt; 0.056%, B
;0.00116%, N: 0. (1031%
, V: 0.047%.

Ca; 0.0019%) 'r--Solution treatment, hot rolling after heating to heating temperatures of 950°C and 1250°C, and quenching hardness of a steel plate immediately quenched from a finishing temperature of 850°C. However, the material rolled at a heating temperature of 950°C has a quenching hardness of Hv350 or more up to the center of the plate thickness (7,
Heating temperature: 125 (1°C rolled material has insufficient hardenability at Hv of about 280.

In Figure 1, (1) shows general ratio treatment → 1250°C → 850°C finishing 9J1 (cumulative downstream rate 62 inches) → water cooling, ■ shows solution treatment → 950°C heating → 850°C finishing rolling (cumulative ITg reduction rate) 62%) → indicates water cooling.

Next is the hot rolling process, which includes the above-mentioned solution treatment and 900
The need for a heating temperature of ~1150°C is primarily a necessary means of obtaining grain refinement and improved hardenability at the center of the plate thickness. However, the toughness of rolled steel materials that have been refined and hardened is not highly evaluated. The cause is variation in the grain structure in the thickness direction. Therefore, in order to improve the rolling properties under the surface layer of the sheet and to impart toughness at the center of the sheet thickness, the present inventors have determined that in this hot rolling process, the layer below the surface layer of the sheet becomes elongated austenite grains and the center of the sheet thickness becomes granular austenite grains. It was discovered that rolling was necessary.

That is, in hot rolling, the finish biting temperature is 800
A cumulative reduction of 40% or more with respect to the finished thickness is carried out at ~930°C, the finishing temperature is set at 750°C or more, and immediately after this rolling is completed, it is rapidly cooled. Here, the finishing temperature is limited to 800 to 930°C; the reason for this is that the rolling temperature range below the surface layer of the sheet thickness is where elongated austenite grains are obtained, and if it exceeds 930°C, elongated austenite grains cannot be obtained; At temperatures below
The austenite grains extend to the center of the plate thickness, and the grain refinement impedes hardenability in the center of the plate thickness. On the other hand, the reason why the cumulative reduction ratio is limited to 40 inches or more with respect to the finished thickness is that the reduction ratio is sufficient to obtain elongated grains below the surface layer of the plate thickness. Figure 2 shows the effects of cumulative reduction on strength, toughness, and austenite grain size.
It can be seen that the results are significantly improved. Next, the finishing temperature is 750℃
The reason for the above limitation is that it is necessary to perform a rapid cooling treatment after the completion of hot rolling to obtain a quenched structure with a martensite structure below the surface layer of the plate thickness and a martensite-lower paynite structure in the center of the plate thickness. Therefore, at a temperature of 750° C. or lower, a sufficient hardened structure cannot be obtained particularly at the center of the plate thickness. FIG. 3(a) shows the thus obtained inventive material (steel F: t=50
This is a photograph of the austenite grain structure during quenching of gourd, finishing temperature 870°C, rolling reduction 67%).
A comparison with b) is shown. The steel materials (,) related to the present invention are:
The lower part of the plate thickness has elongated austenite grains, the quenched structure is a martensitic structure, and the center of the plate thickness has granular austenite grains, which is superior to steel materials.

Moreover, the quenched structure consists of martensite + lower bainite structure. However, while high strength can be obtained in this state, the toughness is insufficient, and it is necessary to perform tempering treatment at a temperature of A (11 points or lower). This results in significantly improved strength and toughness that is uniform in the thickness direction of the plate.

(Examples and effects of the invention) Next, examples of the present invention will be described. Based on the manufacturing conditions of the inventive method and the comparative method shown in Table 2, steel slabs with the respective compositions shown in Table 1, which were melted in a converter and manufactured by the continuous casting method, were cast into plates with a thickness of 50 to 100 m. Manufactured from steel plate. The test results at that time are shown in Table 3. As is clear from the results shown in Table 3 above, the mechanical properties of the steel plate obtained by the method of the present invention are as follows:
It has higher strength and toughness than steel plates obtained using comparative methods.
In particular, the toughness under the surface layer of the plate is significantly improved, and the austenite grain size is elongated and finer than that of steel made by the comparative method. In addition, the Kea value of the brittle fracture propagation arrest characteristic by the temperature gradient type ESSO test is also determined by the comparative method.

[Brief explanation of drawings]

Figure 1 is a diagram showing the effect of slazo heating temperature on quenching hardness in the thickness direction when a low alloy steel billet is solution-treated, heated, hot-rolled, and immediately water-quenched. 2
The figure shows the influence of the cumulative reduction during hot rolling on the strength, toughness, and austenite grain size of the material of the present invention. Figure 3 (,) is a photograph of the austenite grain structure showing the characteristics of the material of the present invention. (X200), Figure 3(b)
is a photograph (X200) of the austenite grain structure of a normal reheated and quenched material. Figure 1←--JOs port

Claims (2)

[Claims] (1) C 0.07-0.20% Si 0.5% or less Mn 0.6-2.0% Cr 0.2-1.0% Mo 0.05-1.0% P≦0.02 % S≦0.02% solAl 0.01-0.10% B 0.0005-0.0020% N≦0.0060%, further including Ni 0.01-3.0% Cu 0.05-1 .0% V 0.01-0.2% Nb 0.005-0.10% Ca 0.0010-0.0080% A steel containing one or more of the following, the balance being Fe, and A high-toughness, high-strength steel characterized by having elongated austenite grains and a tempered martensite structure in the lower surface layer of the plate thickness, and granular austenite grains in the center of the plate thickness, and consisting of tempered martensite + lower bainite structure. (2) C 0.07-0.20% Si 0.5% or less Mn 0.6-2.0% Cr 0.2 1.0% Mo 0.05-1.0% P≦0.02% S≦0.02% solAl 0.01-0.10% B 0.0005-0.0020% N≦0.0060%, and further contains Ni 0.01-3.0% Cu 0.05-1. Steel slab containing one or more of the following: 0% V 0.01-0.2%, Nb 0.005-0.10%, Ca 0.0010-0.0080%, with the remainder being Fe. After solution treatment, it is heated to 900-1150°C, and in hot rolling, the finishing biting temperature is 800-93°C.
A cumulative reduction of 40% or more of the finished thickness is carried out at 0°C, the finishing temperature is set to 750°C or more, and immediately after the completion of rolling, the plate is rapidly cooled to produce elongated austenite grains below the surface layer and a martensitic structure in the center of the plate thickness. is a granular austenite grain with a martensite + lower bainite structure, followed by A
A method for producing high toughness and high tensile strength steel, characterized by tempering at a temperature of c_1 point or lower.