JP2708540B2 - Method for producing high-strength steel sheet mainly composed of ferrite structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing a steel plate by direct quenching and tempering, and in particular, a high strength steel plate for general use having a tensile strength of 60 kgf / mm ² or more. And a method for producing the same.

<Conventional technology> Steel materials manufactured by direct quenching-tempering treatment are:
Compared to the steel material manufactured by the normal reheating quenching-tempering process, the added element is more fully dissolved in austenite,
It is known that quenching properties are improved due to homogenization and large austenite grains, and high strength is easily obtained.

The rolling finish temperature in the direct quenching-tempering process is generally selected to be as high as _three or more Ar points as disclosed in JP-A-61-23715 and JP-A-61-48517. That is, it is generally considered unfavorable when the rolling finish temperature is lower than or equal to the Ar ₃ point temperature because soft ferrite is precipitated and the steel sheet strength is reduced.

However, when the rolling finishing temperature is set to a high temperature here, the austenite grains of the steel sheet are coarse, and the toughness is disadvantageous as compared with the reheat-quenched-tempered material. Therefore, for the purpose of this improvement, various attempts have been made in, for example, rolling in a component system, a recrystallized region or a non-recrystallized region as disclosed in Japanese Patent Publication No. 61-60891.

However, the above-mentioned technique strictly controls the heating temperature and the rolling conditions in a relatively high-component system, and requires a high-level control technique and also impairs economic efficiency. In addition, in the production of relatively thin steel plates,
There is a drop in temperature during rolling, and a great deal of capital investment and consideration of rolling is necessary to secure a high-temperature finishing temperature.Even if the temperature can be secured, the strength of the steel sheet changes significantly due to changes in the quenching cooling rate. It was difficult to stably produce a large number of steel sheets.

On the other hand, a technique focusing on ferrite precipitation is disclosed in
It is disclosed in JP-A-52-59017 and JP-A-55-41927. Wherein the former are those quenching the steel sheet, thereby improving the fracture propagation arrestability by mixing 5-20% of trace ferrite martensite structure, thin steel sheet latter was the rolling finishing temperature Ar ₃ point or more Air cooling and ferrite is 5
It is quenched at the time of precipitation of 析出 60% to produce a thin steel plate with good workability. Both techniques are intended for the quenching of thin steel plates with a very fast cooling rate, and the main strengthening mechanism is by martensitic transformation strengthening, and therefore has an upper limit on the amount of ferrite precipitation.

However, even with these techniques, as described above, it is expected that the tensile properties of the steel sheet will fluctuate greatly with a change in the cooling rate, and there will still be difficulties in mass production and stable production.

<Problems to be Solved by the Invention> The present invention is directed to the production of a high-strength steel sheet having a tensile strength of 60 kgf / mm ² or more, in which the production conditions are relatively easy, the cost is low, and the strength does not change much due to the quenching cooling rate. It is intended to provide a method.

<Means for Solving the Problems> Hints and requirements leading to the present invention are as follows.

For a 60 kgf / mm ² grade steel sheet, the desired strength can be obtained by mixing the ferrite and bainite (or martensite) systematically, and the ferrite precipitation is expected to refine the structure and improve the low-temperature toughness. it can.

The strength of the ferrite + bainite (or martensite) mixed structure steel sheet can be explained by a mixing rule of σ _TS = σ _F · f _F + σ _B · (1−f _F ). Here, σ _TS indicates the tensile strength of the steel sheet, σ _F indicates the tensile strength of ferrite, f _F indicates the area ratio of the ferrite phase, and σ _B indicates the tensile strength of the second phase other than ferrite. Therefore, although it is generally assumed that σ _TS approaches σ _F and the tensile strength of the steel sheet decreases due to a large amount of soft ferrite phase precipitation, if σ _F and σ _B are increased, f _F
It is expected that a predetermined strength can be ensured even if is large.

The increase in σ _F can be achieved by accumulation of rolling strain, and the increase in σ _B can be achieved by enrichment of C.

The tensile strength of the ferrite + bainite (or martensite) mixed structure steel sheet is not easily affected by the quenching cooling rate.

Based on the above hints, the present inventors studied in detail a low-temperature rolling experiment focusing on the control of the work hardening and the amount of precipitation of ferrite, and studied in detail the optimum amount of C for increasing σ _{B of} the second phase and the like. Invented the invention.

That is, the present invention provides, by weight%, C: 0.06 to 0.25%, Si:
0.05 ~ 0.50%, Mn: 0.60 ~ 2.50%, Mo: 0.03 ~ 0.30%, Al: 0.
010 to 0.100%, P: 0.015% or less, and if necessary, V: 0.080% or less, Ni: 1.5% or less, Cu: 1.5% or less, Cr: 1.0%
The following, Nb: 0.10% or less, Ca: 0.01% or less, REM: 0.01% or less, and one or more of Ti: 0.05% or less, and hot rolling is performed on steel consisting of the balance of Fe and unavoidable impurities. , (Ar ₃
-30) 20% to 40% reduction in the temperature range from ℃ to the rolling finish temperature
%, And after the ferrite precipitation amount is increased to 70% or more, immediately cooled at a cooling rate of 1 to 15 ° C / s to a temperature of 150 ° C or less, and then tempered at a temperature of ₁ Ac or less. This is a method for producing a high-strength steel sheet mainly containing a ferrite structure.

<Operation> First, the reasons for limiting the chemical components will be described.

C is 0.06% from securing the strength of the second phase other than ferrite
(Same as the weight% or less) is required, but if it exceeds 0.25%, the toughness and weldability of the base material will decrease, so 0.06 to 0.25%
Range.

Mn is required to be at least 0.60% to form a solid solution in ferrite, increase the ferrite hardness, lower the Ar ₂ transformation temperature and Ms point, and increase the quenchability, but if it exceeds 2.50%, the steel sheet workability and weldability will increase. Since it decreases, it is set in the range of 0.60 to 2.50%.

Si is required as a deoxidizing agent at the time of steelmaking and 0.05% or more to secure strength by solid solution strengthening of ferrite.
If it exceeds 50%, the toughness of the steel sheet and welded parts will deteriorate.
The range is 0.05 to 0.50%.

Mo has an effect on refinement of austenite grains in the high-temperature rolling zone, suppresses pearlite transformation, and
_{(1) Since} the element has an effect of lowering the transformation point, it is an essential element in the present invention.
%, The effect saturates, so the content is set in the range of 0.03 to 0.30%.

Al has a deoxidizing effect and requires 0.010% or more.
If it exceeds 0.100%, the toughness of the steel sheet and the welded portion deteriorates, so the content is made 0.010 to 0.100%.

P is set to 0.015% or less because it deteriorates the toughness of the steel sheet and the welded portion and promotes tempering embrittlement.

Further, in addition to the above components, one or more of the following components can be added to steel for the purpose of improving the strength and toughness of the welded portion.

V has the effect of improving hardenability, increasing tempering softening resistance, and hardening ferrite.
If it exceeds 0.08%, the toughness of the weld will deteriorate, so the content is made 0.08% or less.

Ni is effective in improving the strength and toughness of the steel sheet and the welded portion, but is set to 1.5% or less from the viewpoint of economy.

Although Cu is effective in increasing the strength of the steel sheet, if it is too much, the hot workability and the weldability decrease, so the upper limit is made 1.5%.

Cr is effective in increasing the strength of steel sheets and welds,
If the content is too large, the weldability is reduced and the sensitivity to SR cracking is increased, so the upper limit is made 1.0%.

Nb forms carbonitrides, suppresses the growth of austenite grains, facilitates grain refinement, expands the non-recrystallization temperature range, facilitates accumulation of rolling strain, and reduces temper softening resistance. Therefore, it is effective to secure the strength of the steel sheet, but if too much, the toughness of the welded portion is significantly deteriorated. Therefore, the upper limit is set to 0.100%.

Ca has a sulfide morphological control effect and reduces anisotropy, but if too much, the cleanliness is reduced, so the upper limit is made 0.01%.

REM is effective in improving weld toughness, but 0.01%
If the value exceeds, the effect is saturated, so the upper limit is made 0.01%.

Ti has the effect of increasing the strength and improving the toughness of the welded portion. However, if the content is too large, the effect is saturated and the possibility of surface cracking may occur, so the upper limit is made 0.05%.

 Next, the reasons for limiting the rolling conditions will be described.

The basic feature of the present invention is to precipitate a large amount of ferrite,
Another object of the present invention is to harden the ferrite by rolling strain and to harden a second phase other than the ferrite.
Therefore, in the latter half of the rolling, ferrite precipitates,
In addition, it is important to select a temperature range in which the rolling strain is not released and rolling conditions, and it is necessary to contain a sufficient amount of C to sufficiently cure the second phase.

Roll at a rolling reduction of 20 to 40% in the temperature range from (Ar ₃ −30) ° C. to the rolling finish temperature, and make the ferrite precipitation amount 70% or more.

Here, when the rolling finish temperature exceeds (Ar ₃ −30) ° C., the amount of ferrite precipitated is small, and the accumulation of processing strain is small, so that the desired performance cannot be satisfied. Further, the rolling finish temperature is preferably set to 650 ° C. or higher. Rolling finish temperature is 6
If the temperature is lower than 50 ° C., the load on the rolling mill and the rolling efficiency are significantly impaired, which is not preferable.

If the cumulative draft in this temperature range is less than 20%, precipitation of ferrite and accumulation of rolling strain are insufficient, and
If it exceeds, the load on the rolling mill increases significantly, and the anisotropy of the material becomes remarkable. Therefore, the range of the rolling reduction is set to 20 to 40%.

If the amount of ferrite deposited is less than 70%, the curing of the second phase is insufficient within the range of the present invention, and the desired strength cannot be obtained. Therefore, the amount of ferrite to be precipitated is set to 70% or more.

Next, as an example, 0.10% C-0.25% Si-1.30% Mn-0.
When a steel containing 012% P-0.12% Mo-0.032% Al was manufactured by the method of the present invention, the amount of ferrite precipitated and its hardness, the hardness of the second phase and the strength of the steel sheet at various quenching cooling rates were 900.
Fig. 1 shows a comparison with those of the high-temperature finished steel sheets at ℃.

From the figure, when the method of the present invention is used, the amount of precipitated ferrite is increased as compared with the comparative example at each cooling rate, and both the hardness of the ferrite and the hardness of the second phase are hardened.
It can be seen that the tensile strength of the steel sheet is substantially constant at each cooling rate and has a value of 60 kgf / mm ² or more in the method of the present invention. Here, in a rapid cooling rate region of more than 15 ° C./s, since the amount of ferrite decreases, the toughness is likely to deteriorate due to the increase in the second phase, and the balance between strength and toughness is lost. On the other hand, in the slow cooling rate range of 15 ° C./s or less, the second phase hardening is remarkable, and excellent toughness can be obtained while maintaining the high tensile strength of the steel sheet.

Further, when the cooling rate is slow at less than 1 ° C./s, the strength is significantly reduced. Therefore, it has excellent toughness in the cooling rate range of 1 to 15 ° C./s, and can stably secure a steel sheet tensile strength of 60 kgf / mm ² or more. The cooling is performed to 150 ° C. or less, in which the second phase is completely quenched, and then tempering is performed at a temperature of ₁ point or less of Ac in order to recover the toughness of the second phase and adjust the strength of the steel sheet.

In the hot rolling, the slab heating temperature is not particularly specified, but according to a conventional method, the added element is completely dissolved, and austenite grains are not significantly coarsened at 1050 to 1200 ° C.
Is preferred.

Rolling in a high temperature recrystallization temperature region is not particularly specified, but reduction of 50% or more is desirable because it is more advantageous to refine austenite grains by sufficient rolling and recrystallization to improve toughness.

As described above, the present invention is completely different from the conventional technical idea of increasing the area ratio of bainite or martensite as a high-temperature rolling finish as much as possible to achieve high tensile strength. This is a completely new technique for achieving high tensile strength by mixing the processed ferrite and the C-enriched second phase.

<Examples> Ingots of the respective components shown in Table 1 were smelted by a vacuum melting method, and predetermined quenching was performed by hot rolling as steel plates having a thickness of 15 mm, 50 mm, and 100 mm. Table 2 shows the quenching conditions. These various steel plates
After tempering at 600 ° C, impact test pieces and round bar tensile test pieces were sampled and investigated for material properties.

The obtained results are shown in Table 3-1 and Table 3-2. From the table, it can be seen that Steel A is a low C type steel, has low hardenability, and cannot satisfy a predetermined strength. Component steels B, C and E of the present invention have a tensile strength of 60 kgf / mm ² or more by rolling according to the method of the present invention, and v
Trs also has good toughness of -80 ° C or less. further,
It can be seen that the same mechanical properties are obtained even when the cooling rate changes by applying the method of the present invention. Comparative steels D and F are a system in which Mo was not added and C was excessively added, and the method of the present invention was not sufficiently exhibited.
It can be seen that the toughness of both components is deteriorated.

Invention steels G to J are intended to increase strength and improve toughness.
Although V, Cu, Ni, Cr and Nb were added, it is understood that the combination with the method of the present invention has the best balance of strength and toughness.

<Effect of the Invention> The present invention is directed to direct quenching which is an economical manufacturing process.
Relates the preparation of tensile strength 60 kgf / mm ² or more grade steel in tempering process, yet not only enabled the cost manufacture of saving component system, quenching cooling rate also vary considerably It is possible to manufacture steel sheets with the same mechanical properties, expanding the way to stable mass production,
The industrial effects are significant.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the cooling rate and various mechanical properties when a steel sheet is manufactured by the method of the present invention and the comparative method.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-145717 (JP, A) JP-A-59-211529 (JP, A) JP-A-62-196326 (JP, A)

Claims (2)

(57) [Claims] (1) C: 0.06 to 0.25%, Si: 0.05 to 0.50% by weight
%, Mn: 0.60 ~ 2.50%, Mo: 0.03 ~ 0.30%, Al: 0.010 ~ 0.100
%, P: 0.015% or less, hot rolling is performed on steel consisting of the balance of Fe and unavoidable impurities, and rolling at a rolling reduction of 20 to 40% in the temperature range of (Ar ₃ −30) ° C to the rolling finishing temperature. Immediately after the ferrite precipitation amount is increased to 70% or more, the ferrite structure is cooled at a cooling rate of 1 to 15 ° C / s to a temperature of 150 ° C or less, and then tempered at a temperature of ₁ point or less of Ac. Manufacturing method of high strength steel sheet mainly. 2. C .: 0.06 to 0.25% by weight, Si: 0.05 to 0.50% by weight.
%, Mn: 0.60 ~ 2.50%, Mo: 0.03 ~ 0.30%, Al: 0.010 ~ 0.100
%, P: 0.015% or less, V: 0.080% or less, Ni: 1.5
%, Cu: 1.5% or less, Cr: 1.0% or less, Nb: 0.100% or less, C
a: Hot rolling is performed on steel containing one or more of 0.01% or less, REM: 0.01% or less and Ti: 0.05% or less, the balance being Fe and unavoidable impurities, and (Ar ₃ −30) ° C. After rolling at a rolling reduction temperature of 20 to 40% in the temperature range of the rolling finish temperature and making the amount of ferrite precipitation 70% or more, immediately cool at a cooling rate of 1 to 15 ° C / s to a temperature of 150 ° C or less, and then Ac A method for producing a high-strength steel sheet mainly comprising a ferrite structure, which is tempered at a temperature of ₁ point or less.