JPH0610304B2 - Method of manufacturing low yield ratio non-heat treated steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a low yield ratio non-heat treated steel.

(Prior Art) In recent years, in various fields such as shipbuilding and industrial machinery, we have reduced welding work to improve competitiveness, pursued the ultimate limit of steel material characteristics such as bending workability, improved weldability and reduced steel cost. Demand is growing. In order to improve the bending workability of thick steel plates, it is necessary to develop thick steel plates having a low yield ratio of less than 70%. In the fields of architecture and bridges, it is desired to reduce the yield ratio to improve the stability of structures.

Recently, development of steel materials with strength of 50 kgf / mm ² or more using accelerated cooling technology after steel plate rolling aimed at improving low temperature toughness and weldability of base materials mainly for shipbuilding, line pipes, etc. The production of a low yield ratio steel sheet with good workability has not been examined.

In the conventional controlled rolling-controlled cooling process, in order to improve the low temperature toughness, hot rolling is used to make the grains as fine as possible and accelerated cooling from the austenite one-phase region. However, even with this method, there is a problem that the yield point rises due to the fine graining and hardening of ferrite and partly bainitization of pearlite, which increases the yield ratio and lowers the bending workability.

Some of the inventors of the present invention have examined a method of lowering the yield point by using a controlled rolling-controlled cooling process in Japanese Patent Laid-Open No. 59-215528, and as a result, while also obtaining good low temperature toughness with fine-grained ferrite. In addition, a method for producing a steel sheet having a low yield point and a low yield ratio and a strength of 50 kgf / mm ² or more was developed.

That is, after heating at 900 to 1200 ° C., a cumulative reduction of 30% or more with Ar ₃ or more is performed to achieve fine graining, and then Ar
Properly precipitate the soft pro-eutectoid ferrite by air cooling to ₃ or less, and when forced cooling is performed, due to the structure obtained by proper mixing of the soft pro-eutectoid ferrite and the remaining austenite, ferrite-pearlite-bainite, It was found that only the yield point is lowered without lowering the tensile strength and the low temperature toughness.

30 (invention Problems to be solved) However, in a result of various studies, the temperature range of Ar ₃ from 900 ° C. to requests for subsequent further low yield ratio of
By the cumulative reduction of not less than%, the ferrite and the carbide of the second phase are made finer and finer than necessary. In order to obtain a low yield ratio, it is necessary to have a low yield point and high tensile strength. The yield point is determined by the ferrite part, and the tensile strength is determined by the second phase carbide (especially high carbon island martensite).

Therefore, if the ferrite is made finer than necessary, it will have a high yield point, and at the same time, the second phase carbide will be made finer, and the decomposition of the carbide by tempering will be promoted, resulting in a low tensile strength. The result is a high yield ratio.

(Means for Solving Problems) In order to meet such demands, the present invention makes it possible to manufacture a steel plate having a low yield ratio and a strength of 50 kgf / mm ² or more. The gist thereof is , By weight, C:
0.03 to 0.30%, Si: 0.05 to 0.60%, M
n: 0.50 to 2.5%, including Al: 0.005 to 0.1%,
Steel containing the balance Fe and unavoidable impurities is added to 900-12
00 was heated at ° C. If the rolling end between either or 900 ° C. to Ar ₃ rolling ends at a temperature exceeding 900 ° C. In hot rolling, to a plate thickness finish is between 900 ℃ ~Ar ₃ 30%
Less than the cumulative rolling reduction, and then air cooling the steel plate surface temperature
From between the _{Ar 3 -20 ℃ ~Ar 3 -80 ℃} , water density 0.3 m ³
/ M ² · min or more, cooling is started until the steel plate temperature reaches 250 ° C. or less, and then tempering heat treatment is performed, which is a method for producing a low yield ratio non-heat treated steel.

The second invention further comprises, in addition to the above components of the first invention, Cu: 2.0% or less, Cr: 1.0% or less, Mo: 0.50 or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.15% or less. One or more of the improving element groups are contained.

In the third invention, the components of the first invention further contain one or two toughness improving element groups consisting of Ni: 4.0% or less and Ca: 0.01% or less.

The fourth invention further comprises the components of the first invention with Cu: 2.0% or less, Cr: 1.0% or less, Mo: 0.50% or less, Nb: 0.1%.
Hereinafter, one or more of the strength improving element group consisting of V: 0.1% or less and Ti: 0.15% or less, and Ni: 4.0 or less, C
a: A method for producing a low yield ratio non-heat treated steel characterized by containing one or two toughness improving element groups consisting of 0.01 or less.

(Operation) The present invention mainly performs rolling at a temperature of over 900 ° C., and when rolling at a temperature of 900 ° C. or less, by controlling the reduction amount to a low level, the grain size of ferrite and the second phase are increased more than necessary. By suppressing the refining of the carbide, the cooling stop temperature is lowered, and then the tempering treatment is performed, the low temperature toughness can be improved while maintaining the low yield ratio.

Next, the reasons for limiting the components in the present invention will be described.

Although 0.03% or more of C is necessary to secure the strength, if it increases, it impairs the toughness and weldability of steel, so the upper limit of the content is 0.30%.

Si is deoxidized, so 0.05% or more is necessary and added,
If it increases, the weldability will be impaired, so the content should be 0.6% or less.

Mn is useful as an element for inexpensively increasing the strength, and 0.5% or more is necessary to secure the strength, but if it increases, the weldability is impaired, so the content is made 2.5% or less.

Al needs to be 0.005% or more for deoxidation, but if it increases, the amount of inclusions in the steel becomes too large and the properties of the steel deteriorate, so the upper limit is 0.5%.

The present invention limits the steel containing the above elements as basic components in the present invention by heating-rolling-heat treatment with a low yield ratio,
Although it ensures good low temperature toughness, one or more of the following elements may be added depending on the properties required of the steel.

Cu, Cr, Mo, Nb, V, and Ti have an equal effect of improving the strength and are added in one kind or two or more kinds, but the addition amount of each element is limited as follows.

Even if Cu is added in excess of 2.0%, there is almost no increase in strength, so the upper limit of the content is 2.0%. If the Cr content increases, it impairs the low temperature toughness and weldability, so the content is 1.
The upper limit is 0%. When Mo increases, the weldability is impaired, so the upper limit is 0.5%. When Nb, V, and Ti increase, the weldability is impaired, so the upper limits are 0.1%,
0.1% and 0.15%.

Ni and Ca have an equal effect of improving toughness and are added in one or two kinds, but the addition amount of each element is limited as follows.

Since Ni is an expensive element, the upper limit of its content is 4.0%. When Ca increases, it forms inclusions in the steel and deteriorates the properties of the steel, so the upper limit of the content is 0.01%.

Next, heating, rolling and cooling conditions which are important requirements of the present invention will be described.

The lower limit of the heating temperature is 900 ° C., which is a temperature at which heating can be sufficiently performed in the austenite region. If the temperature is too high, the austenite grains will become too large, which will deteriorate the properties of the steel.
The upper limit of the heating temperature is 200 ° C.

Rolling can be divided into rolling at over 900 ° C and rolling at 900 ° C or less, but in applications where a low yield ratio steel sheet is used, it is sufficient to improve toughness by controlling rolling at a temperature over 900 ° C. It is desirable to complete rolling above 900 ° C. Rather, the cumulative rolling reduction is 30 by controlled rolling below 900 ° C.
%, The ferrite becomes finer than necessary and the second
High yield ratio is achieved by refining the carbide of the phase. There 90
The cumulative rolling reduction between 0 ° C and Ar ₃ is 30% of the finished plate thickness.
Below.

Next, after the rolling, air cooling is performed prior to the accelerated cooling. The air cooling is preferably performed immediately after rolling to any temperature between Ar ₃ -20 ° C. and Ar ₃ -80 ° C., which makes it soft. It allows the precipitation of an appropriate amount of proeutectoid ferrite.

The upper limit of the accelerated air cooling start temperature is set to Ar ₃ −20 ° C. in order to lower the yield point, and the lower limit is set to Ar ₃ −80 ° C., and the effect of accelerated cooling is obtained by cooling from a temperature lower than this. This is because it is difficult to secure the strength because the thin tensile strength decreases. The water amount density is set to 0.3 m ³ / m ² · min or more because the strength increase is small below this.

The reason why the cooling stop temperature for accelerated cooling is 250 ° C. or lower is that when cooling is stopped in a high temperature range exceeding 250 ° C. and then tempering heat treatment is carried out, the strength is slightly lowered and at the same time, the low temperature toughness is deteriorated. Here, the water cooling stop temperature is set to 35
The reason for adopting the method of performing water cooling to 250 ° C. or less and then performing tempering heat treatment instead of the method of setting the temperature to 0 to 600 ° C. is that the latter improves the low temperature toughness more than the former.

The tempering temperature is set to Ac ₁ or lower.

(Example) Next, the Example of this invention is given with a comparative example.

Table 1 shows the chemical composition of the test material, and Table 2 shows the heating, rolling and cooling conditions and the mechanical properties of the obtained steel sheet.

Steel A, G, H, I, J, K, L, M, N, O, P is 50
kgf / mm ² grade, steel B, C, D, E, F, Q, R, S, T,
U is a component system aiming at strength of 60 kgf / mm ² grade, and as shown in Table 2, steel plate Nos. A1, A2, B1, C1, D1, E
1, F1, G1, I1, J1, K1, L1, M1, N
1, O1, P1, Q1, R1, S1, T1, U1 is the present invention embodiment, each ^{50kgf / mm 2, 60kgf / mm} 2
It has sufficient strength and good low temperature toughness as a grade steel, and achieves the low yield ratio aimed at by the present invention.

On the other hand, Steel Plate No. A3 has a low temperature toughness because the heating temperature is too high. A4 has a high yield ratio for high yield point too fine of ferrite cumulative rolling is too high between 900~Ar _3. B2 is an example in which the forced cooling start temperature is too high, and the yield ratio is high. B3 is an example in which the forced cooling end temperature was too high, and the strength was slightly low and the low temperature toughness was low. C2 is an example in which the forced cooling start temperature is too low, and the strength is low and the yield ratio is high. C3 is an example having a low water density, and therefore has a low strength and a high yield ratio.

(Effects of the Invention) As described in detail above, the present invention provides a low yield ratio thick steel sheet having a high strength of 50 kgf / mm ² or more and good bending workability without using a particularly expensive alloying element, The invention can be manufactured at low cost by the forced rolling-controlled cooling method, and is an invention that has a great effect industrially.

Claims (4)

[Claims] 1. A steel containing C: 0.03 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.5%, Al: 0.005 to 0.1% and a balance Fe and inevitable impurities in a weight ratio. ,
Was heated at 900 to 1200 ° C., or rolling end at a temperature above 900 ° C. In the hot rolling, or to exit rolling between 900 ° C. to Ar _3, with respect to the plate thickness finish is between 900 ° C. to Ar ₃ 3
The cumulative rolling reduction is less than 0%, and after air cooling, the steel plate surface temperature is between Ar ₃ −20 ° C. and Ar ₃ −80 ° C.
^A method for producing a low yield ratio non-heat treated steel, which comprises cooling at a rate of ³ / m ² · min or more, cooling until the steel sheet temperature reaches 250 ° C or less, and then performing tempering heat treatment. 2. By weight ratio, C: 0.03 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.5%, Al: 0.005 to 0.1% and further Cu: 2.0% or less, Cr: 1.0% Hereinafter, steel containing one or more of the strength improving element group consisting of Mo: 0.50% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.15% or less, and the balance Fe and unavoidable impurities, 900-1200 ℃
In heating, or rolling end at a temperature above 900 ° C. In the hot rolling, or 900 ° C. to Ar to exit rolling between ₃ relative to the plate thickness finish is between 900 ° C. to Ar _3, the accumulation of less than 30% The reduction ratio is set, and then air cooling is performed to make the steel plate surface temperature Ar
_{From 3} -20 ℃ to Ar ₃ -80 ℃, water density 0.3m ³ / m ² ·
A method for producing a low-yield ratio non-heat treated steel, which comprises cooling for at least minutes, cooling until the steel plate temperature reaches 250 ° C. or lower, and then performing tempering heat treatment. 3. By weight ratio, C: 0.03 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.5%, Al: 0.005 to 0.1% are further included, and Ni: 4.0% or less, Ca: 0.01% or less. % Or less of one or two elements of a toughness improving element group, and the balance F
Steel consisting of e and unavoidable impurities is heated at 900 to 1200 ° C. to finish rolling at a temperature exceeding 900 ° C. in hot rolling, or 900 ° C. to Ar ₃ when rolling is finished between 900 ° C. and Ar _3. Between the finished plate thickness and the cumulative reduction ratio of less than 30%, and then air-cooling the steel plate surface temperature to Ar ₃
The feature is that cooling is started from −20 ° C. to Ar ₃ −80 ° C. at a water amount density of 0.3 m ³ / m ² · min or more, cooling is performed until the steel plate temperature reaches 250 ° C. or less, and then tempering heat treatment is performed. Method for producing low yield ratio non-heat treated steel. 4. By weight ratio, C: 0.03 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.5%, Al: 0.005 to 0.1% are further contained, and Cu: 2.0% or less, Cr: 1.0 % Or less, Mo: 0.50% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.15% or less, and one or more elements of a strength improving element group, Ni: 4.0% or less, Ca: 0.01% Contains one or two of the following toughness improving element groups, with the balance F
Steel consisting of e and unavoidable impurities is heated at 900 to 1200 ° C. to finish rolling at a temperature exceeding 900 ° C. in hot rolling, or 900 ° C. to Ar ₃ when rolling is finished between 900 ° C. and Ar _3. Between the finished plate thickness and the cumulative reduction ratio of less than 30%, and then air-cooling the steel plate surface temperature to Ar ₃
The feature is that cooling is started from −20 ° C. to Ar ₃ −80 ° C. at a water amount density of 0.3 m ³ / m ² · min or more, cooling is performed until the steel plate temperature reaches 250 ° C. or less, and then tempering heat treatment is performed. Method for producing low yield ratio non-heat treated steel.