JP3290595B2 - Method for manufacturing high-tensile steel plate with excellent toughness and weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for manufacturing a high-tensile steel sheet, and more particularly to a thick steel sheet in which strength, toughness, and weldability are balanced.

[0002]

2. Description of the Related Art As a method for ensuring strength, toughness, and weldability of steel plates in a well-balanced manner, TMCP (ThermoMech) is used.
2. Description of the Related Art A method of manufacturing a thick steel plate by an anical control process is known. For example, Japanese Patent Application Laid-Open No. Sho 64-73019 discloses that a steel material containing Nb is made of (Ae ₃ -200 ° C) to (Ae ₃ -250 ° C).
A method has been proposed in which two-phase rolling is performed in the temperature range described above, ferrite is precipitated during processing, and a fine ferrite structure is formed after processing. 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
_A method of manufacturing a thick steel plate in which a reduction of 50% or more is applied in the temperature range of ₃ has been proposed. In this method, a deformation zone is introduced by applying a strong pressure in a non-recrystallized region to make the structure finer. Further, JP fairness 2-25968, Ca, Ti and the billet containing Nb or V is heated to 900 C. to 1100 ° C., at 900 ° C. The following reduction rate of 30% or more, and the rolling finishing temperature After hot rolling at 680 to 860 ° C, 500 to 500 at a cooling rate of 3 to 10 ° C / sec.
There has been proposed a method for producing a thick-walled high-strength steel cooled to a temperature of not more than ° C.

However, in order to sufficiently exert the effect of rolling in the non-recrystallization temperature range as described above, it is necessary to apply a higher pressure at a lower temperature, which imposes a large load on a rolling mill and a large load. In addition to consuming energy, in the case of thick materials, there have been problems such as an increase in waiting time for temperature control and a reduction in rolling efficiency. In addition, when it is difficult to ensure high pressure at low temperatures as in the case of extremely thick steel sheets, the introduction of deformation bands is insufficient, ferrite nuclei are reduced, and the microstructure cannot be achieved. In such a case, there are problems such as anisotropy of sound due to formation of a texture and increase in 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 in which the structure is refined by utilizing the precipitation of VN to improve the strength toughness as rolled has been known (for example, iron). And Steel, Vol.77 (1991) No.1, p171.). In Japanese Patent Application Laid-Open No. 5-186848, Ti is added in addition to V and N to form TiN-
There is disclosed a technique of dispersing a complex precipitate of MnS-VN, effectively acting a ferrite generation function, and improving the toughness of a HAZ portion. However, in these techniques, particularly in the case of an extremely thick steel plate, the effect of VN is not always efficiently exhibited, and there is a problem that the properties of the base material as-rolled are insufficient.

[0005]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, provides a well-balanced strength, toughness and weldability, and provides a high yield strength steel plate having a low yield ratio and excellent toughness and weldability. The purpose is to propose a manufacturing method.

[0006]

Means for Solving the Problems The present inventors have conducted intensive studies on a method of manufacturing a high-tensile steel plate capable of ensuring a good balance of strength, toughness and weldability, and have obtained the following findings. By controlling V, N, and Ti amount, limiting (V + Ti) / N value,
By dispersing an appropriate amount of VN, TiN precipitates or composite precipitates of TiN and VN in steel, these precipitates act as nuclei for ferrite, and a fine ferrite + pearlite structure is formed.

[0007] After the ferrite transformation, VN is finely precipitated in a large amount in ferrite, and thus greatly contributes to an increase in strength. VN is finely precipitated in a large amount even with relatively slow cooling, so that the variation in strength and toughness in the cross section of the steel sheet is small, and the occurrence of residual stress and residual strain can be suppressed. In addition to controlling the amount of V, N, Ti,
After heating to 1050-1350 ° C, it is the first time to combine with recrystallization refinement rolling in which a reduction of 5% or more at a reduction rate / pass is applied at a cumulative reduction rate of 20% or more in a temperature range of 1100-950 ° C. Sufficient grain refinement is obtained.

The present invention has been completed based on the above findings. That is, in the present invention, C: 0.
05-0.18%, Si: 0.10-0.60%, Mn: 0.60-2.00%,
P: 0.030% or less, S: 0.015% or less, Al: 0.005-0.
050%, V: 0.04-0.15%, Ti: 0.005-0.05%, N:
0.0050 to 0.0150%, and (V + Ti) / N: 4.0
~ 12.0, Cu: 0.05-0.50%, Ni: 0.05
~ 0.60%, Cr: 0.05 ~ 0.50%, Mo: 0.02 ~ 0.10%, Nb:
Contains one or more selected from 0.003 to 0.020%, and is defined by the following equation (1): Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) The rolled material consisting of the balance Fe and unavoidable impurities with a Ceq of 0.36 to 0.45% is heated to 1050 to 1350 ° C, and the rolling reduction per pass is 5% or more in the temperature range of 1100 to 950 ° C. After applying the rolling reduction at a cumulative rolling reduction of 20% or more,
This is a method for producing a high-strength steel plate having excellent toughness and weldability, characterized by air cooling to room temperature.

In the present invention, the rolled material is, by weight%, C: 0.05 to 0.18%, Si: 0.10 to 0.60%, Mn: 0.60%.
2.00%, P: 0.030% or less, S: 0.015% or less, Al:
0.005 to 0.050%, V: 0.04 to 0.15%, Ti: 0.005 to 0.
05%, N: 0.0050 to 0.0150%, and (V + Ti)
/ N: 4.0 to 12.0, and Cu: 0.05 to 0.50
%, Ni: 0.05 ~ 0.60%, Cr: 0.05 ~ 0.50%, Mo: 0.02 ~
0.10%, Nb: One or more selected from 0.003 to 0.020%, and B: 0.0003 to 0.0020%
%, REM: 0.0010-0.0200%, Ca: 0.0010-0.0100%, and Ceq defined by the above formula (1) is 0.36-0.45%.
A rolled material composed of Fe and inevitable impurities may be used.

In the present invention, instead of air cooling to the room temperature, cooling to a room temperature at a cooling rate of less than 3.0 ° C./sec may be performed. In the present invention, instead of air cooling to the room temperature, the following formula (2) is used at a cooling rate of less than 3.0 ° C./sec. Ar ₃ = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb ... After cooling to the temperature range of Ar ₃ or less (Ar ₃ −100 ° C.) or more defined in 2), the temperature may be cooled to room temperature at a cooling rate of 3.0 ° C./sec or more.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is applicable to hot rolled steel sheets, steel pipes, section steels, and steel bars in addition to thick steel sheets. First, a chemical composition of a rolled material suitable for use in the present invention will be described. C: 0.05 to 0.18%, C is an element that increases the strength of steel, and it is necessary to add 0.05% or more to secure the strength. However, if added in excess of 0.18%, the base metal toughness and weldability deteriorate, so C was limited to the range of 0.05 to 0.18%. In addition, it is preferably in the range of 0.08 to 0.16%.

Si: 0.10 to 0.60% Si is an element that acts as a deoxidizing agent 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%. In addition, it is preferably 0.20 to 0.45%.

Mn: 0.60 to 2.00% Mn is an element that increases the strength of steel, and it is necessary to add 0.60% or more to secure the strength. However, if it exceeds 2.00%, the structure becomes ferrite + pearlite or ferrite + bainite, and the base material toughness deteriorates. Therefore, Mn is set in the range of 0.60 to 2.00%. Preferably,
1.00-1.70%.

P: not more than 0.030% P segregates at the grain boundaries and lowers toughness. For this reason, it is reduced as much as possible, but up to 0.030% is acceptable. In addition,
The content is preferably set to 0.015% or less. S: 0.015% or less S combines with Mn to form MnS, and has an effect of refining the structure after rolling and cooling. However, if it exceeds 0.015%, the ductility and toughness in the thickness direction deteriorate. For this reason,
S was limited to 0.015% or less. In order to obtain the effect of MnS for grain refinement, S is preferably in the range of 0.004 to 0.010%.

Al: 0.005 to 0.050% Al acts as a deoxidizing agent, but when added in a large amount, nonmetallic inclusions increase, the cleanliness decreases, and the toughness deteriorates.
In addition, Al easily bonds with N to form AlN, and inhibits stable deposition of VN. Therefore, the content of Al is set in the range of 0.005 to 0.050%. Incidentally, the content is preferably 0.010 to 0.040%.

V: 0.04 to 0.15% V combines with N to form VN, which precipitates in austenite during rolling after cooling, acts as ferrite precipitation nuclei, refines crystal grains, and improves toughness. Further, after the ferrite transformation, fine precipitates are also formed in the ferrite, whereby the strength of the base material can be increased without strengthening the 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, 0.04% or more must be added, but if added in excess of 0.15%, the base material toughness and weldability deteriorate. 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%.

Ti: 0.005 to 0.050% Ti combines with N to form TiN, suppresses the growth of austenite grains during heating, and further retains or precipitates in austenite to promote precipitation of VN in austenite. Has the effect of causing. To get this effect,
Addition of 0.005% or more is necessary, but if it exceeds 0.050%, the cleanliness of the steel is reduced and the toughness of the base material is deteriorated.
Therefore, Ti is set in the range of 0.005 to 0.050%. In addition,
Preferably it is 0.010 to 0.025%.

N: 0.0050 to 0.0150% N combines with V and Ti to form a nitride, suppresses the growth of austenite grains during heating, acts as ferrite precipitation nuclei, refines crystal grains and improves toughness. It has the effect of causing. If it is less than 0.0050%, the required amount of nitride is insufficient. On the other hand, if it exceeds 0.0150%, the amount of solute N increases, and the toughness and weldability of the base material deteriorate. Therefore, N is
Limited to the range of 0.0050-0.0150%. The preferred range is 0.0060 to 0.0120%.

(V + Ti) / N: 4.0 to 12.0 In the present invention, V, Ti, and N are set in the above ranges.
V, T so that (V + Ti) / N is in the range of 4.0 to 12.0.
Adjust i, N amount. If (V + Ti) / N is less than 4.0,
The amount of solute N increases, causing strain aging and further reducing weldability. When (V + Ti) / N exceeds 12.0, T
i and V combine with C and lower the base material toughness. For this reason,
(V + Ti) / N was in the range of 4.0 to 12.0. Preferably, it is 5.0 to 10.0.

Cu: 0.05-0.50%, Ni: 0.05-0.60%, C
r: 0.05 to 0.50%, Mo: 0.02 to 0.10%, Nb: 0.003 to 0.0
One or more selected from 20% Cu, Ni, Cr, Mo, and Nb all have an effect of improving hardenability, and one or more of them can be added. Also, Cu, Ni, Cr, Mo, Nb all reduce the Ar ₃ point,
This makes the ferrite grains finer and increases precipitation hardening due to VN. To achieve this effect, C
u, Ni, Cr, Mo, Nb are 0.05%, 0.05%, 0.05 respectively
%, 0.02%, 0.003% or more is required. However, when a large amount of Cu is added, hot workability deteriorates.
It is preferable to add about the same amount of Ni at the same time, but if it exceeds 0.50%, the effect is saturated and the cost becomes high. For this reason, Cu was limited to the range of 0.05 to 0.50%. Ni
Is saturated even if added in excess of 0.60%, and is economically expensive. Therefore, Ni is set in the range of 0.05 to 0.60%. Cr, Mo, and Nb are 0.50%, 0.10%, and 0.020%, respectively.
If it exceeds 300, the weldability and the base material toughness deteriorate. For this reason, Cr
Is 0.05-0.50%, Mo is 0.02-0.10%, Nb is 0.003-0.02%
Limited to the 0% range.

Ceq: 0.36 to 0.45% Ceq is defined by the following equation (1). Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) Ceq adjusts the chemical composition to ensure good weldability and high strength, and ranges from 0.36 to 0.45% Limited to. Ceq is
If it is less than 0.36%, it is difficult to secure the strength of the base metal and the HAZ softened part, and if it exceeds 0.45%, the susceptibility to weld cracking increases,
HAZ toughness deteriorates.

B: 0.0003-0.0020%, REM: 0.0010-0.
0200%, Ca: one or two or more selected from 0.0010 to 0.0100% B, REM, and Ca each have an effect of contributing to the refinement of ferrite grains. Two or more 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. In order to obtain such effects, 0.0002% or more is required. However, if it exceeds 0.0020%, toughness is reduced. For this reason, B was limited to the range of 0.0002 to 0.0020%.

REM and Ca form stable oxides even at high temperatures and are finely dispersed in the steel to suppress the growth of austenite grains and refine the ferrite grains after rolling. REM is
If the content is less than 0.0010%, the effect is small. On the other hand, if the content exceeds 0.0200%, the amount of precipitates increases and coarsens, so that the cleanliness decreases and the toughness deteriorates. Therefore, RE
M was limited to the range of 0.0010-0.0100%.

If the content of Ca is less than 0.0010%, the effect is small,
If it is added in excess of 0.0100%, the amount of precipitates increases and coarsens, so that cleanliness decreases and toughness deteriorates. For this reason, Ca was limited to the range of 0.0010 to 0.0100%. Others are Fe and inevitable impurities. For elements other than the elements specified above, 0: less than 0.0025%, Zr: 0.020
% Or less, Mg: 0.020% or less is acceptable.

Next, 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 rolled material. The steel of the present invention is formed into a thick steel plate by hot rolling.

The heating temperature of the hot rolling is 1050 to 1350 ° C. If the heating temperature is less than 1050 ° C, the deformation resistance increases,
It is difficult to secure a predetermined rolling reduction / pass and a cumulative rolling reduction.
On the other hand, when the temperature exceeds 1350 ° C., the heating furnace basic unit is deteriorated, and the scale loss and the number of furnace repairs are increased. For this reason, the heating temperature of the rolled material was set in the range of 1050 to 1350 ° C.

After the rolled material is heated, it is then heated at 1100-950 ° C.
After a reduction of 5% or more per pass in a temperature range of 20% or more in terms of a cumulative reduction rate, air cooling is performed to room temperature.
In the present invention, hot rolling is repeated at a rolling reduction of 5% or more per pass in the austenite partial recrystallization region at 1100 to 950 ° C., and the cumulative rolling reduction is 20% or more. Thereby, recrystallization refinement of austenite grains can be achieved.

If the rolling temperature exceeds 1100 ° C., the effect of recrystallization refinement of austenite is small, and if it is lower than 950 ° C., a texture is easily formed, and the acoustic anisotropy becomes remarkable.
Residual stress and residual strain occur. Therefore, the rolling temperature is 11
The range was limited to the range of 00 to 950 ° C. Further, if the rolling reduction per pass is less than 5%, a sufficient grain refining effect cannot be obtained, so the rolling reduction per pass is limited to 5% or more. From the viewpoint of recrystallization refinement, the larger the rolling reduction per pass, the better, but the upper limit is limited due to the limitation of the rolling mill capacity.
It is about 30%.

If the cumulative rolling reduction in the austenite partial recrystallization region is less than 20%, a sufficient recrystallization refinement effect cannot be obtained, and the final structure is coarse even when the structure refinement effect by TiN or VN is exerted. And the base material toughness decreases. For this reason, 1100
The cumulative rolling reduction in the temperature range of 950950 ° C. was limited to 20% or more. After the end of the rolling, air-cool to room temperature.

By performing slow cooling such as air cooling, variation in strength and toughness, and residual stress and residual strain can be reduced.
In the present invention, instead of air cooling to the room temperature, 3.
It may be cooled to room temperature at a cooling rate of less than 0 ° C./sec. If the cooling rate is less than 3.0 ° C./sec, the precipitation of VN is not suppressed, so that the cooling of the steel material can be accelerated and the production efficiency can be improved. VN when the cooling rate exceeds 3.0 ° C / sec
Precipitation is suppressed, the structure becomes coarse, and the structure tends to be mainly bainite, and the toughness is deteriorated.

In the present invention, instead of air cooling to the room temperature, the following formula (2) is preferably used at a cooling rate of not less than air cooling and less than 3.0 ° C./sec: Ar ₃ = 910-273C + 25Si-74Mn-56Ni-16Cr- 9Mo -5Cu -1620Nb ... (2) After cooling to Ar ₃ or less (Ar ₃ -100 ° C.) or more temperature range defined by, may be cooled to room temperature at 3.0 ° C. / sec or more cooling rate.

When the temperature range for cooling at a cooling rate of less than 3.0 ° C./sec exceeds the Ar ₃ point, the micronization of the structure becomes insufficient,
If the temperature is lower than (Ar ₃ -100 ° C.), the generation of residual stress and residual strain becomes remarkable. Therefore, the cooling temperature at less than 3.0 ° C / sec is
The temperature range was set to a temperature range of Ar ₃ or less (Ar ₃ −100 ° C.) or more.
Thereafter, by cooling to room temperature at a cooling rate of 3.0 ° C./sec or more, growth of ferrite grains is suppressed, the structure is further refined, and the base material toughness is improved.

The temperature and cooling rate specified in the present invention are values at the center of the thickness of the steel sheet.

[0034]

EXAMPLES Steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. Next, these slabs were heated to the temperatures shown in Table 2, and were made into thick steel plates under the hot rolling conditions shown in Table 2. Immediately after the completion of the rolling, cooling was performed at a cooling rate shown in Table 2. Using these product plates, the tensile properties, toughness,
And weld HAZ toughness were investigated. Table 2 shows the results.

[0035]

[Table 1]

[0036]

[Table 2]

The methods for testing the tensile properties, toughness, and toughness of the heat affected zone of the base metal are as follows. (1) Tensile test of base material From the 1/4 T section of the above product sheet, JI was applied in the direction perpendicular to the rolling direction.
A No. 4 test piece specified in SZ2201 was sampled, and the yield point (YP) and tensile strength (TS) were determined in accordance with JIS Z2241. (2) Toughness test of base material From the 1/4 T section of the above product sheet, JI was applied in the direction perpendicular to the rolling direction.
Sample No. 4 specified in SZ 2202 was collected, and the Charpy absorbed energy at 0 ° C (vE
₀ ). (3) Toughness test of heat-affected zone of welding 12
A test specimen of t × 75 × 80mm was collected and the HAZ coarse grain area of submerged arc welding with a heat input of 100kJ / cm using a high frequency heating device.
After applying the heat cycle (maximum heating temperature 1400 ° C) to which the part is subjected, a No. 4 test piece specified in JIS Z 2202 is collected and JI
The Charpy absorbed energy at 0 ° C. (vE ₀ ) was determined according to SZ 2242.

As can be seen from Table 2, in the examples of the present invention, a high strength of 500 MPa or more in TS and a high toughness of 150 J or more in vE ₀ are obtained, the yield ratio is 80% or less, and the toughness of the HAZ portion is also vE _0. With high toughness of 150 J or more.
It can be seen that both the toughness is excellent. Also, Ar ₃ ~
Example No. 3 of the present invention in which the cooling rate was changed at (Ar ₃ −100 ° C.)
At vE ₀ , high toughness of 250 J or more is obtained.

Comparative Example N whose chemical composition is outside the scope of the present invention
vE ₀ is low in o.16 to No.21. Comparative Example No. 22 in which Ceq is out of the range of the present invention has low YS and TS. In addition, rolling
Comparative Example No.5 cooling conditions outside the range of the present invention, strength and toughness balance in No.6 is poor, a large base material characteristic change of the thickness in the cross section in Comparative Example No.5.

[0040]

According to the present invention, variations in the properties of the base material within the cross section of the sheet thickness are small, the balance between strength, toughness and weldability is good, and a low yield ratio and excellent toughness and weldability are obtained. A high-strength steel plate having high strength can be easily manufactured industrially, and its use as a steel for welded structures can be further expanded.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tatsumi Kimura 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. 1-chome (without address) Kawasaki Steel Corporation Mizushima Works (56) References JP-A-10-68016 (JP, A) JP-A-10-88232 (JP, A) JP-A-7-242944 (JP, A) JP-A-56-142826 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/02 C22C 38/00-38/60

Claims (4)

(57) [Claims] C. 0.05 to 0.18%, Si: 0.10 to 0.60%, Mn: 0.60 to 2.00%, P: 0.030% or less, S: 0.015% or less, Al: 0.005 to 0.050%, V : 0.04 to 0.15%, Ti: 0.005 to 0.05%, N: 0.0050 to 0.0150%, and satisfy (V + Ti) / N: 4.0 to 12.0,
Furthermore, one or more selected from among Cu: 0.05 to 0.50%, Ni: 0.05 to 0.60%, Cr: 0.05 to 0.50%, Mo: 0.02 to 0.10%, Nb: 0.003 to 0.020% And Ceq defined by the following equation (1) is set to 0.36 to 0.45%,
Rolled steel material consisting of Fe and unavoidable impurities
Heating to 1350 ° C, applying a reduction of 5% or more per pass in a temperature range of 1100 to 950 ° C at a cumulative reduction of 20% or more, and then air cooling to room temperature. Manufacturing method of high tensile strength steel sheet with excellent heat resistance Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) 2. The rolled material is, by weight%, C: 0.05 to 0.18%, Si: 0.10 to 0.60%, Mn: 0.60 to 2.00%, P: 0.030% or less, S: 0.015% or less, Al: 0.005% 0.050%, V: 0.04 to 0.15%, Ti: 0.005 to 0.05%, N: 0.0050 to 0.0150%, and satisfy (V + Ti) / N: 4.0 to 12.0,
Furthermore, one or more selected from among Cu: 0.05 to 0.50%, Ni: 0.05 to 0.60%, Cr: 0.05 to 0.50%, Mo: 0.02 to 0.10%, Nb: 0.003 to 0.020% B: 0.0003-0.0020%, REM: 0.0010-0.0200%, Ca: 0.0010-0.0100%, and one or more selected from the group consisting of Ceq defined by the following formula (1). 0.36 to 0.45%
2. The method for producing a high-tensile steel plate according to claim 1, comprising Fe and unavoidable impurities. Note Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) 3. Instead of air cooling to room temperature, 3.0 ° C.
The method for producing a high-tensile steel plate according to claim 1 or 2, wherein the steel plate is cooled to room temperature at a cooling rate of less than / sec. 4. In place of air cooling to room temperature, 3.0 ° C.
After cooling to a temperature range of Ar ₃ or less (Ar ₃ −100 ° C.) or more defined by the following equation (2) at a cooling rate of less than / sec,
Claim 1, or the method for manufacturing a high-tensile steel plate of 2, wherein the cooling at 3.0 ° C. / sec or more cooling rate to room temperature. Ar ₃ = 910-273 C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb ... (2)