JPH10306315A - Production of non-heat treated high tensile-strength steel excellent in low temperature toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel having high tensile strength and excellent in low temp. toughness without adding a large amt. of expensive elements by subjecting a stock having a specified compsn. to hot working under specified conditions to form into an approximately prescribed shape, thereafter subjecting it to air cooling to a specified temp., executing hot working, holding it at a specified temp. range and subsequently executing air cooling. SOLUTION: A stock having a compsn. contg. C, Si, Mn, P, S, Al, V and N, also satisfying 4.0 to 12.0 V/N, and the balance Fe or the like is heated at 1050 to 1250 deg.C and is subjected to hot working at a cumulative draft of >=30% in the temp. range of 1050 to 950 deg.C to form into an approximately prescribed shape. Next, It is air-cooled to a temp. T ( deg.C) satisfying the inequality and is hot-worked at a draft of <=5%. In the formula, Tps is calculated from the relation among the contents of V, N and Ti. Then, it is held or retained in the temp. range of the temp. T ( deg.C) or below to the Ar3 point or above for >=160 sec, the nitrides having 0.02 to 0.20 μm particle size are dispersed into austenite at the density of 10<5> to 10<10> pieces/mm<3> , and after that, it is air-cooled to a room temp.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a non-heat-treated high-strength steel material suitable for use in building structures, marine structures, line pipes, ships, storage tanks, civil engineering construction machines, and the like.
It should be noted that the non-heat-treated high-strength steel material of the present invention includes a thick steel plate, a steel strip, a shaped steel bar or a steel bar.

[0002]

2. Description of the Related Art As a method for ensuring the strength and toughness of a steel material in a well-balanced manner, TMCP (Thermo Mechanical Control PTM) is used.
There is known a method for producing a steel material by rocess). For example, JP-A-3-223419, then subjected to reduction of 30% or more in the steel material containing Nb (Ar ₃ +150 ℃) above the recrystallization temperature region, (Ar ₃ +150 ℃) ~Ar 50% in ₃ temperature range
There has been proposed a method of manufacturing a thick steel plate by applying the above reduction. In this method, a deformation zone is introduced by applying a strong pressure in a non-recrystallized region to make the structure finer.

[0003] Also, Japanese Patent Application Laid-Open No. 2-25968 discloses Ca, Ti
And a steel slab containing Nb or V are heated to 900-1100 ° C.
The rolling reduction below 00 ° C is 30% or more, and the rolling finish temperature is 6%.
There has been proposed a method of producing a thick-walled high-tension steel plate by performing hot rolling at 80 to 860 ° C and then cooling to 500 ° C or lower at a cooling rate of 3 to 10 ° C / sec. However, the effect of the rolling in the non-recrystallization temperature range as described above is sufficiently exhibited. Also,
Since a large load is applied to the rolling mill, a large amount of energy is consumed, and in the case of a thick material, a sufficient rolling reduction cannot be secured, and the waiting time for temperature adjustment increases, thereby reducing the rolling efficiency. The problem remained. Further, when a high-pressure condition at a low temperature cannot be ensured as in the case of an extremely thick steel plate, the introduction of a deformation zone becomes insufficient, so that ferrite nuclei are reduced and the structure cannot be refined. On the other hand, in the case of a thin steel plate, there are problems such as acoustic anisotropy due to the formation of a texture, and large residual stress and residual strain due to cooling to a low temperature of 500 ° C. or less.

On the other hand, unlike the above-mentioned method, a high-strength steel using VN to refine the structure and improve the strength and toughness of the as-rolled steel has been conventionally known (for example, iron and iron). Steel, vol. 77 (1991) No. 1, p171). In addition, Japanese Unexamined Patent Publication
No. 186848, V, N and Ti are added, and TiN-MnS-
There is disclosed a technique for dispersing VN composite precipitates to effectively act on ferrite-forming nuclei and improve the toughness of a heat affected zone (HAZ). However, with these technologies, VN
Has not been exhibited efficiently, and the properties of the as-rolled base metal have been insufficient.

[0005]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and has a tensile strength (TS) of 490 MPa or more and a low-temperature toughness without adding a large amount of expensive elements. An object of the present invention is to provide a method for producing an excellent non-heat treated high-tensile steel material.

[0006]

Means for Solving the Problems The present inventors have conducted intensive studies in order to achieve the above object, and have obtained the following findings. By controlling the amounts of V and N to precipitate and disperse VN in austenite, these precipitates act as precipitate nuclei for ferrite, and a fine ferrite + pearlite structure is formed. VN particles having a size of 0.01 to 0.10 μm act as precipitation nuclei of ferrite.

The amount of VN precipitated and the size of the particles can be controlled by adjusting the processing and cooling rate in a specific temperature range related to the V and N contents, and by adjusting the residence time in a specific temperature range by maintaining the isothermal temperature. There is a temperature range in which VN effectively acting as precipitation nuclei increases. VN precipitates in ferrite in large quantities after ferrite transformation, and thus greatly contributes to an increase in strength. Also, VN
Since a large amount of fine precipitates even with relatively slow cooling, it is possible to suppress variations in strength and toughness in the cross section of the steel sheet and occurrence of residual stress and residual strain.

If the temperature range in which grain boundary ferrite is formed is accelerated and cooled, the formation of fine intragranular ferrite is promoted and the structure is further refined. The present invention has been completed based on the above findings. That is, according to the present invention, C: 0.05 to 0.18
%, Si: 0.10 to 0.60%, Mn: 0.80 to 1.80%, P: 0.030
%, S: 0.015% or less, Al: 0.005 to 0.050%,
A material containing V: 0.04 to 0.15%, N: 0.0050 to 0.0150%, and satisfying V / N: 4.0 to 12.0, the balance being composed of Fe and unavoidable impurities is heated to 1050 to 1250 ° C.
After performing hot working with a cumulative rolling reduction of 30% or more in a temperature range of 50 ° C. or less and 950 ° C. or more to obtain a substantially predetermined shape, the following equation (1) Tps ≦ T ≦ Tps−100 (1) , T: temperature (° C.), Tps (° C.) = ｛V (N−0.
292 Ti) × 10 ⁵ +425 mm / 0.480, V, N, Ti: Content (wt%)) is air-cooled to a temperature T (° C.), and the draft at the temperature T (° C.): 5% or less And then kept or retained for at least 160 seconds in a temperature range of not more than the temperature T (° C.) and not less than ₃ points of Ar, and having a particle size of 0.02 in austenite.
After the V nitride ~0.20μm was dispersed at a density of 10 ⁵ to 10 ¹⁰ cells / mm ^3, it is a method for producing a non-tempered high tensile steel having excellent low temperature toughness characterized by air cooling to room temperature .

In the present invention, instead of the air cooling up to the temperature T (° C.), accelerated cooling may be performed at a cooling speed exceeding the air cooling. Further, in the present invention, instead of the air cooling to the room temperature, (Ar ₃ −60) ° C.
It is preferable to cool to an arbitrary temperature of 600 ° C. or higher. Further, in the present invention, the material is C: 0.
05 ~ 0.18%, Si: 0.10 ~ 0.60%, Mn: 0.80 ~ 1.80%,
P: 0.030% or less, S: 0.015% or less, Al: 0.005-0.
050%, V: 0.04-0.15%, N: 0.0050-0.0150%, Nb: 0.003-0.030%, Ti: 0.005-0.030
% Or more, and (V + Ti) / N: 4.0 to 12.0 is satisfied, and the material is preferably composed of the balance of Fe and unavoidable impurities. , Further Cu: 0.05-0.50%, Ni: 0.05-0.50
%, Cr: 0.05 to 0.50%, Mo: 0.02 to 0.20%, one or more kinds selected from among them, and / or B: 0.0003
~ 0.0020%, REM: 0.0010 ~ 0.010%, Ca: 0.0010 ~ 0.
One or more selected from 010% may be contained.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The chemical composition of a material suitable for the present invention will be described. C: 0.05 to 0.18% C is an element that increases the altitude of steel, and it is necessary to add 0.05% or more to ensure strength. However, if added in excess of 0.18%, the base material toughness and the HAZ toughness decrease, so C was limited to the range of 0.05 to 0.18%. In addition, it is preferably 0.08 to 0.16%.

Si: 0.10 to 0.60% Si is an element that acts as a deoxidizing material and further increases the strength of steel by solid solution strengthening. To get this effect,
It is necessary to add 0.10% or more, but if it exceeds 0.60%,
Significantly degrades HAZ toughness. For this reason, Si is 0.10 ~
The range was 0.60%. Preferably, 0.20 to 0.45%
It is.

Mn: 0.80-1.80% Mn is an element that increases the strength of steel.
0.80% or more must be added. However, if it exceeds 1.80%, the structure becomes a structure mainly composed of low-temperature products such as ferrite + pearlite and bainite, and the toughness of the base material decreases. For this reason, Mn was limited to 0.80 to 1.80%. In addition, it is preferably 1.00 to 1.70%.

P: not more than 0.030% P segregates at the grain boundaries and lowers toughness. For this reason, it is desirable to reduce as much as possible, but up to 0.030% is acceptable. Incidentally, the content is preferably 0.020% or less. S: 0.015% or less Since S forms nonmetallic inclusions and deteriorates ductility and toughness, it is limited to 0.015% or less. Incidentally, preferably 0.010
% Or less.

Al: 0.005 to 0.050% Al acts as a deoxidizing agent, but when added in a large amount, non-metallic inclusions increase, thereby lowering cleanliness and deteriorating toughness. In addition, Al bonds with N to easily form AlN, and inhibits stable precipitation of VN. Therefore, the content of Al is set in the range of 0.005 to 0.050%. V: 0.04 to 0.15% V is an important element in the present invention and combines with N to form V nitride (VN), which precipitates in austenite during hot working or subsequent cooling. This V nitride (VN) acts as a ferrite precipitation nucleus, refines ferrite crystal grains, and improves toughness. V nitride (VN) also precipitates finely in ferrite after ferrite transformation,
The strength of the base material can be increased without performing strong cooling, and the uniformity of the properties within the thickness of the steel sheet, the residual stress and the residual strain can be reduced. In order to obtain these effects, it is necessary to add 0.04% or more, but if added over 0.15%, the base material toughness,
Deterioration of weldability. For this reason, V is limited to the range of 0.04 to 0.15%. In addition, it is preferably 0.04 to 0.12%.

N: 0.0050 to 0.0150% N combines with V and / or Ti to form a nitride, suppresses the growth of austenite grains during heating, acts as a ferrite precipitation nucleus, refines the ferrite grains, and reduces the toughness. Improve. 0.0050 to get these effects
%, But if it exceeds 0.0150%, the amount of solute N increases, deteriorating the base metal toughness and the HAZ toughness. For this reason, N was limited to the range of 0.0050 to 0.0150%. In addition, it is preferably 0.0060 to 0.0120%.

V / N, (V + Ti) / N: 4.0 to 12.0 In the present invention, the calculation is made by V / N when Ti is not added, and by (V + Ti) / N when Ti is added. When Ti is not added, the amounts of V and N are set within the above ranges, and the V / N is adjusted to a range of 4.0 to 12.0. When Ti is added, the amounts of V, Ti, and N are set within the above ranges, and (V
+ Ti) / N is adjusted in the range of 4.0 to 12.0. If V / N or (V + Ti) / N is less than 4.0, the amount of solute N increases, causing strain aging and further deteriorating the toughness of the HAZ. When V / N or (V + Ti) / N exceeds 12.0,
V or V and Ti combine with C to form excessive carbides and reduce the base material. Therefore, V / N or (V + T
i) / N was in the range of 4.0 to 12.0. Preferably,
5.0 to 10.0.

Nb: 0.003 to 0.030%, Ti: 0.005 to 0.03
One or two of Ti and Nb selected from 0% both have an effect of promoting the precipitation of V nitride. Ti combines with N to form TiN, which suppresses the growth of austenite grains during heating, and has the effect of remaining or precipitating in austenite to promote the precipitation of VN in austenite. To obtain this effect, 0.005% or more must be added, but 0.030%
If it exceeds, the cleanliness of the steel is reduced, the precipitation of V nitride is suppressed, and the toughness of the base material is deteriorated. Therefore, Ti is set in the range of 0.005 to 0.030%. Preferably,
0.010 to 0.025%.

Nb improves the strength and toughness by grain refinement and precipitation hardening, and promotes the precipitation of V nitride, similar to Ti. To obtain these effects, it is necessary to add 0.003% or more, but if it exceeds 0.030%, the weldability and the toughness of the HAZ part are deteriorated. Therefore, Nb is limited to the range of 0.003 to 0.030%. Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50%, Cr: 0.05 to 0.50
%, Mo: one or two selected from 0.02 to 0.20%
Any of Cu, Ni, Cr and Mo are effective elements for improving hardenability and increasing strength, and one or more of them can be added. In order to exhibit such effects, Cu,
Ni and Cr each require addition of 0.05% or more, and Mo requires 0.02% or more. However, even if Cu and Ni are added in excess of 0.50%, the effect is saturated and the cost becomes high economically. For this reason, C
Both u and Ni are limited to 0.05 to 0.50%. Cr and Mo are each
If it exceeds 0.50% or 0.20%, weldability and toughness will deteriorate. For this reason, Cr was limited to 0.05 to 0.50%, and Mo was limited to 0.02 to 0.20%.

B: 0.0003-0.0020%, REM: 0.0010-0.
010%, Ca: one or more kinds selected from 0.0010 to 0.010% B, REM, and Ca all have an effect of contributing to the refinement of ferrite grains. More than one species can be added. B segregates at the grain boundaries, suppresses the precipitation of coarse grain boundary ferrite, and contributes to the refinement of ferrite grains. To obtain this effect, 0.0003% or more must be added, but if it exceeds 0.0020%, the toughness is reduced. For this reason, B was limited to 0.0003 to 0.0020%.

REM and Ca form stable oxides even at high temperatures and are finely dispersed in the steel, suppress the growth of austenite grains, and refine the ferrite grains after rolling. Also HA
It also has the effect of refining the structure of the Z part and improving the toughness of the HAZ part. Less than 0.0010% has little effect, 0.01
If it is added in excess of 0%, the amount of oxides increases, the cleanliness decreases and the toughness is impaired. For this reason, both REM Ca
Limited to 0.010%.

Others are Fe and inevitable impurities. For elements other than the elements specified above, O: 0.010%
Below, the content of Zr: 0.02% or less and Mg: 0.02% or less is allowable. Next, the reasons for limiting the manufacturing method will be described.
For the smelting of the steel having the above-mentioned composition, any commonly known smelting method such as a converter and an electric furnace can be applied, and there is no particular limitation. The smelted molten steel is solidified by a continuous casting method or an ingot-making method to be a working material.

The material is heated to 1050-1250 ° C. If the heating temperature is lower than 1050 ° C., the precipitated elements such as V and Nb do not sufficiently form a solid solution, and it is difficult to sufficiently exert the effects of these elements. It becomes difficult to secure. On the other hand, when the temperature exceeds 1250 ° C., the crystal grains become coarse, and the scale loss and the frequency of furnace repair increase. Therefore, the heating temperature of the material is 1050-1250 ° C
Limited to the range.

The heated material is subjected to hot working with a cumulative rolling reduction of 30% or more in a temperature range of 1050 ° C. or less and 950 ° C. or more,
It has a substantially predetermined shape. Austenite is recrystallized and refined by hot working with a cumulative rolling reduction of 30% or more in a temperature range of 1050 ° C or lower and 950 ° C or higher. If the cumulative rolling reduction is less than 30%, austenite grain refinement is insufficient, and if the processing temperature exceeds 1050 ° C. or less than 950 ° C., austenite grain refinement becomes insufficient.

The material which has been subjected to hot working at a temperature of 950 ° C. or more to have a substantially predetermined shape is then subjected to the following equation (1): Tps ≦ T ≦ Tps−100 (1) (where T: temperature (° C.) ), Tps (° C.) = ｛V (N−0.
292 Ti) × 10 ⁵ +425 mm / 0.480, V, N, Ti: hot working at a temperature T (° C.) satisfying the content (wt%)) and a draft of 5% or more; A V nitride precipitation treatment is performed to hold or stay at a temperature range of not more than T (° C.) and Ar not less than ₃ points for not less than 160 seconds, preferably not more than 3000 seconds.

Thus, the austenite has a particle size of 0.1
V-nitride of 10 ~ 02 ~ 0.20μm^Five~Ten ^TenPieces / mm^ThreeAt the density of
Spread. After hot working at 950 ° C or more,
The cooling condition up to T is air cooling or air cooling
The cooling may be accelerated at the cooling speed. V effective for microstructural refinement
The nitride is a V nitride having a particle size of 0.01 to 0.10 μm. particle
V nitrides with a diameter of less than 0.01 μm are too fine
V nitrides that are unlikely to become precipitation nuclei and that exceed 0.10 μm
Becomes the starting point of fracture and deteriorates toughness. But this
Even if V nitride with such a particle size is precipitated,
Degree 10^FivePieces / mm ^ThreeIf less, the effect of refining the structure is small.
10^TenPieces / mm^ThreeIf it exceeds, the presence of excessive precipitates
The property is deteriorated.

If the processing temperature T (° C.) is lower than Tps or higher than Tps-100, then the isothermal holding or Ar ₃
The precipitation of V nitride, which can be a ferrite precipitation nucleus even when staying in a temperature range above the point, is small, and the micronization of the structure becomes insufficient. Also, if the rolling reduction at the temperature T is less than 5%,
The precipitation density of V nitride, which can be a ferrite precipitation nucleus, is low. The amount of reduction at the temperature T is preferably 10% or more from the viewpoint that the precipitation is effectively promoted and uniform processing is performed in the thickness direction.

After such processing for accelerating the precipitation of V-nitride, the steel material is kept isothermally or retained in a temperature range of not less than its processing temperature T (° C.) and not less than _three points of Ar. Isothermal holding refers to a process of holding at a constant temperature within the above-mentioned temperature range, and stagnation refers to elapse of a predetermined time within the above-mentioned temperature range. If the isothermal holding time or residence time is less than 160 seconds, the precipitation of V nitride, which can be a ferrite precipitation nucleus, is small. In addition, the toughness deteriorates due to the presence of coarse precipitates.

In the present invention, the Ar ₃ point uses a value defined by the following equation (2). Ar ₃ (° C) = 910-230C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb ... (2) The steel material is kept at a temperature T (° C) or less for ₃ hours or more in the isothermal state for a predetermined time. After being retained, it is air-cooled to room temperature. Slow cooling such as air cooling reduces variations in strength and toughness, and reduces residual stress and residual strain.

In the present invention, instead of air cooling to room temperature, it is preferable to accelerate cooling to an arbitrary temperature of (Ar ₃ −60) ° C. or less and 600 ° C. or more at a cooling rate of 30 ° C./s or less. . As a result, generation of grain boundary ferrite is suppressed, austenite is supercooled, and the effect of grain refinement due to generation of intragranular ferrite having V nitride as a nucleus becomes remarkable. However, if the cooling rate exceeds 30 ° C./s, the temperature difference in the thickness direction becomes remarkable, and the strength and toughness vary, and the occurrence of residual stress and residual strain becomes remarkable.

When accelerated cooling is stopped at a temperature exceeding (Ar ₃ -60) ° C., the effect of accelerated cooling is not recognized. On the other hand, when accelerated cooling is performed to a temperature lower than 600 ° C., a large amount of low-temperature products such as bainite are generated, and toughness is deteriorated. The temperature and cooling rate specified in the present invention are values at the center of the thickness of the steel material.

[0031]

EXAMPLES Steel having the composition shown in Table 1 was melted in a converter and slabs having a thickness of 240 to 310 mm were produced by continuous casting. Next, these slabs were heated to the temperature shown in Table 2, and after hot rolling to obtain a thick steel plate having a substantially predetermined shape, a precipitation treatment of V nitride was performed under the conditions shown in Table 2 and cooled to room temperature.

From these thick steel plates, test specimens were taken from the center of the plate thickness, and the tensile properties and toughness of the base metal were examined. Table 2 shows the results.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

From Table 2, it can be seen that the tensile strength (T
S) is 500 MPa or more, and vE- ₂₀ is 210 J or more, which is excellent in both strength and toughness. On the other hand, the comparative examples deviating from the present invention have insufficient strength, or have insufficient microstructure to deteriorate toughness.

[0037]

According to the present invention, a non-refined high-strength steel material having excellent tensile strength of 490 MPa or more in both strength and toughness can be subjected to strong pressure reduction at a low temperature without adding a large amount of expensive elements. It can be easily manufactured industrially and has great industrial effects.

Continuing on the front page (72) Inventor Kenji Oi 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. No address) Inside Mizushima Steel Works of Kawasaki Steel Corporation (72) Inventor Kinichi Amano 1-chome, Kawasaki-dori Mizushima, Kurashiki City, Okayama Prefecture (No address) Inside Mizushima Steel Works of Kawasaki Steel Corporation

Claims (6)

[Claims] C .: 0.05 to 0.18%, Si: 0.10 to 0.60%, Mn: 0.80 to 1.80%, P: 0.030% or less, S: 0.015% or less, Al: 0.005 to 0.050%, V : A material containing 0.04 to 0.15%, N: 0.0050 to 0.0150%, and satisfying V / N: 4.0 to 12.0, the balance being composed of Fe and inevitable impurities is heated to 1050 to 1250 ° C, and 1050 ° C to 1050 ° C. After performing hot working with a cumulative rolling reduction of 30% or more in a temperature range of 950 ° C. or more to obtain a substantially predetermined shape, a temperature T satisfying the following equation (1) is obtained.
(° C), subjected to hot working at a reduction rate of 5% or more at the temperature T (° C), and then held or retained for 160 seconds or more in a temperature range of the temperature T (° C) or less and _three or more points of Ar. the V nitride particle size 0.02~0.20μm in austenite ¹⁰⁵
A method for producing a non-heat-treated, high-strength steel material having excellent low-temperature toughness, which is dispersed at a density of about 10 ¹⁰ pieces / mm ³ and then air-cooled to room temperature. Note Tps ≦ T ≦ Tps−100 (1) where T: temperature (° C.) Tps (° C.) = {V (N−0.292 Ti) × 10 ⁵ +425} / 0.
480 V, N, Ti: Content (wt%) 2. Instead of air cooling to the temperature T (° C.),
The method for producing a non-heat treated high-strength steel material according to claim 1, wherein accelerated cooling is performed at a cooling rate exceeding air cooling. 3. An air-cooled super-cooler, instead of said air-cooling up to room temperature.
° C. / s or less cooling rate (Ar ₃ -60) ℃ claim 1 or 2 non-tempered high tensile method for producing steel wherein cooling to any temperature above the following 600 ° C.. 4. The material is, by weight%, C: 0.05 to 0.18%, Si: 0.10 to 0.60%, Mn: 0.80 to 1.80%, P: 0.030% or less, S: 0.015% or less, Al: 0.005 to 0.005%. 0.050%, V: 0.04 to 0.15%, N: 0.0050 to 0.0150%, Nb: 0.003 to 0.030%, Ti: 0.005 to 0.030%, and (1) 4. The method for producing a non-heat treated high-strength steel material according to claim 1, wherein the material satisfies V + Ti) / N: 4.0 to 12.0, and has a composition comprising the balance of Fe and unavoidable impurities. . 5. The material further comprises, by weight: Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50%, Cr: 0.05 to 0.50%.
%, Mo: one or two selected from 0.02 to 0.20%
The method for producing a non-heat-treated high-strength steel material according to any one of claims 1 to 4, comprising at least one kind. 6. The material further comprises, by weight: B: 0.0003-0.0020%, REM: 0.0010-0.010%, Ca:
The method for producing a non-heat treated high-strength steel material according to any one of claims 1 to 5, further comprising one or more selected from 0.0010 to 0.010%.