JP2913426B2 - Manufacturing method of thick high strength steel sheet with excellent low temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a superior thickness direction of the low temperature toughness of the steel plate, the present invention relates method of manufacturing a tensile strength 70~100kgf / mm ² class high strength steel having a brittle fracture propagation stopping performance is there.

[0002]

2. Description of the Related Art In recent years, the demand for energy has been increasing more and more, and the construction of marine structures leading to the development of submarine resources or the construction of high-fall penstocks for pumped-storage power generation for nighttime surplus power adjustment of thermal power generation, etc. Construction of steel structures is active. The steel materials used for these are also becoming larger and thicker, and ensuring safety is an important issue. Therefore, development of a steel having high structural strength and excellent low-temperature toughness is desired as a material used for these. To meet the demand for safer and more reliable materials,
Development and quality improvement of various thick-walled high-strength steels have been performed.

Conventionally, reheating, quenching and tempering methods have been the mainstream heat treatment method for high-strength materials. Particularly, for thick-walled materials, good strength and toughness can be obtained by sintering them to the center of the sheet thickness. It was difficult. Therefore, in order to improve the hardenability with a small number of alloying elements, a method that utilizes the hardenability improving effect of B is often used. For example, as disclosed in Japanese Examined Patent Publication No. 60-20461, sol. A
make an l-total B mass diagram, limit the effective B range, and
Tensile strength of 70 to 80 kgf with limited carbon equivalent (Ceq) and weld crack susceptibility index (Pcm) from the viewpoint of weldability
/ Mm ² class thick high strength steel.

On the other hand, recently, a direct quenching and tempering method in which quenching is immediately performed without cooling after rolling and tempering is performed has attracted attention because the strength can be increased without lowering the economic efficiency. Was. For example, as disclosed in JP-B-63-66368, a small amount of T
If i is added and the steel containing Nb is directly quenched and tempered, Ti prevents the austenite crystal grains from being coarsened, and the austenite crystal grains are refined by hot rolling. Is completed, the recrystallization of austenite is promoted, and the deterioration of hardenability due to processing strain is prevented, and Nb (C, N) during tempering is reduced.
Precipitation improves the strength and provides a high-tensile steel with excellent strength and toughness balance.

[0005]

However, in the reheating quenching and tempering method, although the strength and toughness are improved by improving the hardenability at the center of the sheet thickness, there is a problem that sufficient toughness cannot be obtained below the sheet thickness surface layer. there were. This is because with the improvement in hardenability at the center of the sheet thickness, the cooling rate is high at the time of quenching inevitably under the sheet thickness layer, resulting in a complete martensitic structure, especially for thick materials. Appear strongly. The direct quenching and tempering method also requires quenching from the recrystallization temperature range in order to secure hardenability at the center of the sheet thickness in the case of thick-walled materials. A site structure is formed, and sufficient toughness cannot be obtained in the lower part of the surface layer as in the reheating quenching and tempering method.

[0006] In the above-mentioned direct quenching and tempering method, the strength and toughness of a relatively thin material having a thickness of 30 mm or less are improved. It cannot be said that sufficient strength and toughness are secured over the entire area.

Further, in order to ensure safety under severe conditions of use environment, the structure must not be deformed or broken, and especially for steel materials, the ability to stop brittle fracture propagation at low temperatures must be considered. Such considerations are not sufficiently given to the steel sheet obtained by the above-mentioned manufacturing method. Therefore, it cannot be said that thick materials are sufficiently safe for use.

[0008]

Means for Solving the Problems The gist of the present invention is that the weight%
, C: 0.03 to 0.15%, Si: 0.02 to 0.
5%, Mn: 0.4-2.0%, Ni: 0.05-3.
0%, Cr: 0.2 to 1.0%, Mo: 0.1 to 1.0
%, V: 0.01 to 0.1%, Al: 0.03 to 0.1
0%, B: 0.0005 to 0.0020%, N: 0.0
060% or less, or further Cu: 0.1 to
1.5%, Nb: 0.005 to 0.05%, Ti: 0.
005-0.02% strength-improving element group or Ca having an inclusion morphology controlling action: 0.0005-0.00
A steel slab containing 5% of one or more kinds and the remainder consisting of Fe and unavoidable impurities is heated to 950 to 1150 ° C., and is then cooled to 900 ° C. or higher after the steel slab or rough rolling. 1/10 or more 2/10 of the thickness from both surfaces
Cool the surface layer up to the following thickness ratio to Ar ₃ points or less,
Subsequently, after the surface layer of the slab is reheated to a temperature of not less than Ac ₁ point and not more than Ac ₃ points, finish rolling is started, rolling is performed at a cumulative reduction ratio of 40% or more with respect to the finished thickness, and rolling or rolling is completed. After the surface part, Ac ₃ points -50 ° C or higher Ac ₃ points +30
After being reheated to a temperature of not more than ₃ ° C., a quenching treatment is performed by cooling with water from a temperature of not less than ₃ points of Ar, followed by a tempering treatment at a temperature of not more than ₁ point of Ac. This is a method for manufacturing tensile steel.

[0009]

The present inventors have focused on the reduction in toughness under the surface layer of the sheet thickness observed in the reheating quenching and tempering type and the direct quenching and tempering type of thick-walled material. In order to develop a high-strength steel with high strength, homogeneous low-temperature toughness and brittle fracture propagation stopping performance, we conducted experiments on various manufacturing methods. In, after rough rolling, the front and back surfaces of the steel slab are water-cooled, followed by finish rolling while reheating the surface layer, and then directly quenched, the quenched structure is the surface layer
Part martensite + lower bainite structure consisting austenite grains fine, becomes a martensite + lower bainite tissue thickness center portion is made of relatively coarse austenite grains, toughness of the surface layer portion by tempering it Has been remarkably improved, uniform low-temperature toughness can be obtained at all positions in the thickness direction, and it is found that a target steel having brittle fracture arrestability can be produced.

Hereinafter, the operation of the present invention will be described in detail.

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

C: C is an element effective for improving hardenability and easily increasing strength. However, if it is less than 0.03%, the strength is insufficient, and if it exceeds 0.15%, the low-temperature toughness and the weldability decrease. Therefore, the range of the C content is set to 0.03 to 0.15%.

Si: Si is an unavoidable element in steel making,
0.02% is contained in the steel, but if it exceeds 0.5%, the low-temperature toughness decreases. Therefore, in order to secure a certain level of strength and not lower the low-temperature toughness, 0.02
-0.5%.

Mn: Mn improves quenching properties and is effective for securing strength and toughness. However, if it is less than 0.4%, strength and toughness decrease, and if it exceeds 2.0%, tempering brittleness increases. And lower the low temperature toughness. Therefore, the content of Mn is set to 0.4 to 2.0%.

Ni: Ni is the most effective element for improving strength and toughness. If it is less than 0.05%, the effect is not obtained, and if it exceeds 3.0%, the effect of improving the toughness is small for the strength, which is disadvantageous in economy. Therefore,
The content of Ni was set to 0.05 to 3.0%.

Cr: Cr is effective for improving hardenability and ensuring strength. However, if it is less than 0.2%, it has no effect, and if it exceeds 1.0%, toughness and weldability decrease.
Therefore, the content of Cr is set to 0.2 to 1.0%.

Mo: Mo secures strength by improving hardenability,
It is an element effective for preventing temper embrittlement. However, at less than 0.1%, the effect is not obtained.
%, The low-temperature toughness and weldability decrease. Therefore, the content of Mo is set to 0.1 to 1.0%.

V: V forms carbonitrides during tempering and is effective for securing strength by precipitation strengthening. In particular, in the direct quenching and tempering method, carbonitride is sufficiently dissolved before the quenching treatment, so that effective precipitation strengthening can be achieved. However, if it is less than 0.01%, the desired strength cannot be obtained, and if it exceeds 0.1%, the low-temperature toughness decreases.
Therefore, the content of V is set to 0.01 to 0.1%.

Al: Al is combined with N in the steel when heating the slab, and B is effective for hardenability in the subsequent quenching treatment.
Is required to be added in an amount of 0.03% or more. Further, the fine precipitates of AlN are also effective in reducing austenite grains. However, if it exceeds 0.10%, Al
Inclusions such as ₂ O ₃ increase and hinder low-temperature toughness. Therefore, the content of Al is set to 0.03 to 0.10%.

B: B is an element effective for improving the hardenability, and in combination with the hardenable elements of C, Mn, Cr, Mo and Ni, B is hardened by adding a smaller amount of these elements. It can demonstrate the nature. Furthermore, in the present invention, since the front and back surfaces of the slab are cooled by water cooling after slab or rough rolling, and then rolled while recuperating, the austenite crystal grains are refined in the surface layer , and processing strain is formed. Therefore, hardenability decreases. However, the addition of B can suppress the formation of ferrite from austenite grain boundaries, and can also form a lower bainite or a mixed structure of lower bainite and martensite in the surface layer . Further, in the case of the present invention, the plate thickness center portion is quenched from a sufficiently high recrystallization temperature, so that the effect of improving the hardenability by B can be sufficiently exhibited. Less than 0.0005% has no effect.
If it exceeds 0.0020%, the effect is saturated and the toughness is reduced. Therefore, the content of B is 0.00
05 to 0.0020%.

N: By making N less than 0.0060%, the effect of improving the hardenability by B is stabilized, and 0.0060%.
%, The weldability decreases. Therefore, the content of N is set to 0.0060% or less.

In the present invention, in addition to the above basic components, Cu,
One or more of Nb, Ti and Ca are added.

The Cu, Nb, and Ti components have a uniform effect of improving the strength of the steel. In order to ensure the desired effects, the lower limit contents are Cu: 0.1% and Nb:
Must be 0.005% and Ti: 0.005%.
However, Cu: 1.5%, Nb: 0.05%,
If the content of Ti exceeds 0.02%, the low-temperature toughness and the weldability are reduced, so that the content is limited as described above.

Ca: Ca is effective for spheroidizing nonmetallic inclusions and has the effect of reducing the anisotropy of toughness. Also,
Effective for preventing cracking due to residual stress removal annealing after welding. For this purpose, 0.0005% or more is required. However, if it exceeds 0.0050%, toughness is reduced due to an increase in inclusions. Therefore, the content of Ca is 0.000
5 to 0.0050%.

In addition to the above components, P and S as unavoidable impurities are harmful elements that lower the low-temperature toughness. Preferably, P ≦ 0.0
10%, S ≦ 0.005%.

Next, a manufacturing method which is another gist of the present invention will be described. That is, even with the steel component composition as described above, in order to increase the toughness of each position in the thickness direction of the thick material, and to improve the brittle fracture propagation stopping performance, a manufacturing method is appropriate. There must be. Here, the reasons for limiting the heating, rough rolling cooling, recuperation / rolling, cooling and tempering conditions of the billet will be described.

First, a slab having the above-mentioned component composition was prepared for 950 to 11
It is heated to 50 ° C. and hot rolled. This heating is a process of fixing N with Al, achieving fine precipitation of AlN, increasing solid solution B before quenching, and improving hardenability. In addition, the heating austenite grains are refined and tempered. Sometimes M
This is a process for sufficiently dissolving fine carbonitrides such as o and V. Therefore, at low temperatures below 950 ° C., Mo,
The solution effect of V and the like becomes insufficient. On the other hand, at a temperature exceeding 1150 ° C., although carbonitrides such as Mo and V are sufficiently dissolved, fine precipitates of AlN are re-decomposed, and the dissolved N is combined with B during rolling and re-precipitated as BN. In addition, the amount of solid solution B during quenching decreases, and the hardenability decreases. Further, the heated austenite grains are coarsened, and in the subsequent rolling, particularly, the austenite grains in the lower part of the surface layer are less likely to become finer, which causes a decrease in toughness in the lower part of the surface layer. Therefore, in consideration of these problems, the heating temperature of the steel slab was set to 950 to 1150 ° C.

Next, the steel slab thus heated is subjected to blanking or rough rolling, and then water-cooled from a temperature of 900 ° C. or more, and from both surfaces of the steel slab to 1/10 to 2/10 of the thickness. Less than
The surface portion up to the lower thickness ratio is cooled to the Ar ₃ point or lower (FIG. 1 (a) shows a temperature curve in the thickness direction immediately after water cooling of the slab of the present invention). Here, the reason for limiting the water cooling start temperature of the billet or the billet after the rough rolling to 900 ° C or more is as follows.
This is to ensure the hardenability of the central part of the sheet thickness after finish rolling. FIG. 2 shows the steel M shown in Tables 1 and 2, heated at 1050 ° C., the slab water cooling start surface temperature was changed to 800 to 1100 ° C., and then rolled, water cooled, and tempered under the production conditions of the present invention. It shows the toughness of the central part of a 50 mm thick material. When the water cooling start temperature is 900 ° C., high toughness is clearly obtained. However, if the temperature is lower than 900 ° C., the quenching temperature at the center of the sheet thickness also becomes low, the hardenability decreases, an upper bainite structure is generated, and the toughness decreases. The reason for cooling the surface portion from the both surfaces of the slab to a thickness ratio of 1/10 or more and 2/10 or less to Ar ₃ points or less is that the surface portion is γ → α → recuperation rolling. → It is because refinement of austenite grains can be efficiently achieved by passing through γ transformation. However, if the thickness ratio of the surface layer portion to the Ar ₃ point or less is less than 1/10, the surface layer portion fine-grained layer becomes thin, and the low-temperature toughness and the brittle fracture propagation stopping performance decrease. The thickness ratio is 2 /
If it exceeds 10, the surface portion is Ac ₃ points-5 in finish rolling.
It cannot be reheated to a temperature of 0 ° C. or higher, and grain refinement becomes insufficient. In addition, the temperature at the center of the sheet thickness also decreases at the same time, and the hardenability decreases and the toughness also decreases.

Next, after the surface slab of the slab having been cooled to the Ar ₃ point or lower is reheated to the point of the Ac ₁ point or more and the Ac ₃ point or less, the finish rolling is started. The reason for this is that when the surface layer is rolled from the α + γ two-phase region temperature, the austenite grains are most refined due to α → γ reverse transformation due to reheating during or after rolling. But A
c When rolling is started from less than _one point, the generation of fine austenite grains after reheating is small, and the mixed grains with elongated coarse ferrite grains are formed, and strength and toughness are reduced. Also, Ac
If the rolling is started after reheating to more than _three points, the austenite grains are insufficiently refined, and the toughness and the ability to stop brittle fracture propagation are reduced.

The slab whose surface layer has been reheated to such a temperature range is rolled at a cumulative draft of 40% or more with respect to the finished thickness.
Also, during rolling or after rolling, the surface portion is Ac ₃ points-50.
It is necessary to reheat to a temperature not lower than ₃ ° C. and not higher than Ac ₃ points + 30 ° C. Reason cumulative rolling reduction is required more than 40%, the table
This is because a work strain is introduced into the layer at a temperature in the two-phase region, thereby increasing the number of nucleus sites for generating fine austenite grains. However, when the cumulative rolling reduction is less than 40%, the introduction of working strain is small, and the austenite grains are insufficiently refined. Furthermore, even in the center of the sheet thickness, the rolling reduction is required to be 40% or more in order to cause rolling recrystallization in the recrystallization temperature range and to reduce the grain size to some extent. Further, during or after rolling, the surface portion is set to an Ac ₃ point of −50 ° C. or more and an Ac ₃ point of + 30 ° C.
It is necessary to reheat to the following temperature (FIG. 1 (b) shows a reheating temperature curve during or after the rolling of the present invention). The reason for this is that fine austenite grains are generated in the surface layer by α → γ reverse transformation. But Ac
_At a temperature lower than ₃ points −50 ° C., the formation of fine austenite grains is small, and at a temperature higher than Ac ₃ points + 30 ° C.,
The generated fine austenite grains grow, resulting in a decrease in toughness. 3 was investigated the effect of toughness obtained while recuperation changing tables layer portion during and after rolling or rolling end steel slabs before rolling produced by the present invention range finishing the steel M as shown in Tables 1 and 2 Things. The steel sheet which has been subjected to recuperation rolling at a temperature within the range of the present invention has high toughness in both the surface layer portion and the central portion. However, it can be seen that the reheat-rolled finished material that deviates from the scope of the present invention has reduced toughness in the surface layer portion or the central portion.

The hot rolling - recuperator steel is then Ar ₃
It is necessary to perform a quenching process of water cooling from a temperature higher than the temperature. The reason for this is that the steel layer rolled as described above is subjected to quenching treatment to form a martensite + lower bainite structure from fine austenite grains having a work strain in the surface layer portion and a work center portion having less work strain than the surface layer portion. This is because a lower bainite + martensite structure is formed from relatively coarse austenite grains. However, at a temperature lower than the Ar ₃ point, ferrite is formed and the hardenability of the surface layer portion is significantly reduced,
Even in the center of the thickness, the hardenability tends to decrease,
An upper bainite structure is formed, and strength and toughness cannot be obtained.

The steel that has been hot-rolled and water-cooled is then
c It is necessary to perform tempering at a temperature of _one point or less. This tempering treatment is for sufficiently strengthening the precipitation and strengthening of carbonitrides such as Mo and V to obtain strength and toughness. But Ac
_At temperatures above _one point, the strength drops significantly.

The steel obtained by such a manufacturing process has high strength and high toughness at all positions in the sheet thickness direction, and further has a remarkable improvement in brittle fracture propagation stopping performance.

[0034]

EXAMPLE From a steel slab obtained by smelting steel having the composition shown in Tables 1 and 2, a plate thickness of 30 to 150 mm was obtained based on the production conditions of the present invention method and comparative method shown in Tables 3 and 4. Was manufactured. The mechanical properties of the base material and the brittle fracture arrestability by a temperature gradient type ESSO test were investigated.

The mechanical properties of each part in the thickness direction obtained by the steel having the chemical composition shown in Tables 1 and 2 and the manufacturing conditions shown in Tables 3 and 4 and the Kca of the brittle fracture propagation stopping performance were obtained.
The test results are shown in Tables 5 to 7.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

[Table 5]

[0041]

[Table 6]

[0042]

[Table 7]

As shown in Tables 5 to 7, the present invention example 1
In A to 13-M, the strength and toughness of the base material are small enough in the thickness direction difference and sufficiently high. Also, ESSO
The Kca value of the test is also a sufficiently high value.

On the other hand, in the case where the steel composition satisfies the requirements but the manufacturing conditions do not satisfy the requirements, the comparative example 14-
In the case of M, the slab heating temperature is high, AlN is dissolved, solute B is reduced, and the hardenability is reduced, so that the strength and toughness of the central part are reduced, and the reheating peak temperature after rolling is also increased, The surface layer is coarse and the toughness is reduced. In Comparative Example 15-M, since the water cooling start temperature of the slab was low, the recuperation peak temperature during the subsequent finish rolling was also low, the hardenability at the center was reduced, and both the strength and toughness were reduced. In Comparative Examples 16-M and 21-B, since the surface layer portion having a thickness ratio of up to 3/10 from both surfaces of the slab was cooled to the Ar ₃ point or less, the center portion was not recrystallized and the hardenability was reduced, and the strength was reduced. In addition, the toughness is reduced, and the reheat temperature of the surface layer during rolling is also reduced, and ferrite is formed, and the strength and toughness are reduced. Comparative Example 17
In -M, the quenching property of the central part is improved because there is no slab water cooling and the finish rolling start temperature is high, but the quenched structure of the surface layer becomes a coarse-grained martensite structure, and the toughness is reduced. The rupture propagation stopping performance is also reduced. Comparative Example 18-
In I and 20-B, since the temperature of the steel strip after cooling the steel strip surface layer portion starts to finish rolling after being recuperation to a higher temperature than ₃ points Ac, toughness is not fine of the surface layer portion is reduced I have. In Comparative Example 19-I, the quenching temperature of the steel sheet after the finish rolling was lower than the Ar ₃ point, ferrite was generated, the hardenability was reduced over the entire area in the thickness direction, and both the strength and the toughness were reduced. I have. In Comparative Example 22-E, a steel sheet was manufactured by ordinary rolling without performing controlled rolling, and reheating and quenching was performed, so that the surface layer became coarse and the toughness was reduced.

[0045]

According to the present invention, it is possible to produce a high-strength, high-strength steel sheet having high strength and high toughness from the surface layer to the center. Particularly in the case of thick materials, in the conventional manufacturing method, in order to obtain high toughness from the surface layer part to the center part of the sheet thickness, the non-recrystallization zone rolling was performed on the surface layer part to reduce γ grains, but on the other hand, In addition, the hardenability of the central part is reduced, and the central part needs to be hardened from a high temperature by rolling in the recrystallization zone. On the other hand, there is no fear that the surface layer becomes coarse. As a result, the brittle fracture propagation stopping performance is significantly improved,
Sufficient safety is ensured.

[Brief description of the drawings]

FIG. 1 is a diagram showing a temperature curve (a) in the thickness direction immediately after water cooling of a slab of the present invention and a recuperation temperature curve (b) in the thickness direction of a steel sheet during or after rolling.

FIG. 2 is a diagram showing the relationship between the toughness of a steel sheet M at the center in the plate thickness and the steel slab water cooling start temperature.

FIG. 3 is a diagram showing the relationship between the toughness of each position in the thickness direction of steel M and the surface temperature of the steel sheet during or after rolling.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C21D 8/02

Claims (2)

(57) [Claims] 1. C .: 0.03-0.15%, Si: 0.02-0.5%, Mn: 0.4-2.0%, Ni: 0.05-3. 0%, Cr: 0.2 to 1.0%, Mo: 0.1 to 1.0%, V: 0.01 to 0.1%, Al: 0.03 to 0.10%, B: 0 0.0005% to 0.0020%, N: 0.0060% or less, the balance being Fe and unavoidable impurities. , And the surface layer from the both surfaces of the slab to the thickness ratio of 1/10 or more and 2/10 or less of the thickness is cooled to ₃ points or less of Ar.
c After reheating to a temperature of not less than ₁ point and not more than Ac ₃ points, finish rolling is started, rolling is performed at a cumulative reduction ratio of 40% or more with respect to the finished thickness, and the surface layer is subjected to Ac ₃ rolling during or after rolling.
After reheating to a temperature of −50 ° C. or more and Ac ₃ points + 30 ° C. or less, a quenching treatment is performed by water cooling from a temperature of Ar ₃ points or more, followed by a tempering treatment at a temperature of Ac ₁ point or less. Of high-strength high-strength steel with excellent low-temperature toughness. 2. In% by weight, C: 0.03-0.15%, Si: 0.02-0.5%, Mn: 0.4-2.0%, Ni: 0.05-3. 0%, Cr: 0.2 to 1.0%, Mo: 0.1 to 1.0%, V: 0.01 to 0.1%, Al: 0.03 to 0.10%, B: 0 0.0005 to 0.0020%, N: 0.0060% or less, Cu: 0.1 to 1.5%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.5%. A steel slab containing one or two or more of a strength-improving element group consisting of 02% or Ca: 0.0005% to 0.005% having an inclusion morphology controlling action, and the balance being Fe and unavoidable impurities is 950. After heating to ~ 1150 ° C, the steel slab or rough rolling is water-cooled from a temperature of 900 ° C or higher, and 1/10 or more of the thickness 2 / 10 or less
The surface layer up to the thickness ratio of ₃ is cooled to Ar ₃ points or less, and then the billet surface layer is reheated to a temperature of Ac ₁ point or more and Ac ₃ points or less, and then finish rolling is started. It rolled at a reduction ratio of 40% or more, and, after recuperation through the rolling or rolling after the end surface portion to a temperature below Ac ₃ point + 30 ° C. Ac ₃ point -50 ° C. or higher, water cooling from Ar ₃ point or more temperature A high-strength high-strength steel excellent in low-temperature toughness, characterized by performing a quenching treatment followed by a tempering treatment at a temperature of ₁ point or less of Ac.