JP4528184B2 - Method for producing alloyed hot-dip galvanized high-strength steel sheet with good workability - Google Patents

Description

  The present invention relates to a method for producing an alloyed hot-dip galvanized high strength steel sheet for processing.

  For high-strength steel sheets used in automobiles, etc., the main background is the reduction of vehicle body weight, which is an effective means of improving fuel efficiency that originates from global environmental problems, and the securing of collision safety for the purpose of protecting passengers during collisions. The demand is growing. However, even though it is a high-strength steel plate, excellent workability is required, and a steel plate that balances strength and workability is required. Of the workability, it is important to lower the yield ratio as well as improve ductility. The yield ratio is the ratio of the yield strength (YP) to the tensile strength (TS) of the steel sheet, YP / TS, and by lowering this, the shape freezing property deteriorated by the increase in strength, the reduction in press load, Wrinkle generation can be suppressed.

  Conventionally, as a high-strength steel sheet used for applications requiring good elongation, there is a dual phase steel (hereinafter referred to as DP steel) composed of ferrite and martensite, and is widely used for automobiles and the like. . This DP steel is characterized by a strength-ductility balance superior to that of a solid solution strengthened steel sheet and a precipitation strengthened steel sheet, and a low yield ratio.

  In automobiles, high corrosion resistance is required depending on the application site. For such applications, galvannealed steel sheets are preferred. Therefore, an alloyed hot-dip galvanized high-strength steel sheet with excellent corrosion resistance, excellent ductility, and low yield ratio has become an indispensable material for further promoting the reduction of automobile weight and the improvement of collision safety. Yes.

  Alloyed hot-dip galvanized steel sheets are usually manufactured by the Sendzimer method or non-oxidizing furnace method, but the annealing equipment and plating equipment are continuous, cooling after annealing is interrupted at the plating temperature, and through the process The cooling rate is necessarily reduced. Therefore, it is difficult to contain martensite generated under cooling conditions with a high cooling rate in the steel sheet after plating. Further, in the DP steel, Si is added to improve ductility. However, when the Si content is high, Si is concentrated on the surface of the steel sheet and oxidized, so that non-plating is likely to occur during hot dipping.

On the other hand, in Patent Documents 1 and 2, there is a method for producing an alloyed hot-dip galvanized high-strength steel sheet in which a Si-added high-strength steel sheet is rapidly heated to 430 to 500 ° C. after Ni pre-plating and alloyed after galvanization. Are listed. In the case of this method, it is possible to use a cold-rolled steel plate manufactured by a cold-rolling-continuous annealing process in which the material has already been made as the original plate, and the maximum temperature reached is about 550 ° C. It is considered that an alloyed hot-dip galvanized steel sheet can be produced without significantly impairing the properties. Further, non-plating is less likely to occur even when the Si content is high due to a treatment such as Ni pre-plating. However, when a cold-rolled steel sheet of DP steel actually manufactured by a cold-rolling-continuous annealing process was used and an alloyed hot-dip galvanized steel sheet was manufactured using the methods of Patent Documents 1 and 2, the yield ratio relative to the original sheet There was a significant increase in the production and a decrease in ductility.
Japanese Patent No. 2526320 Japanese Patent No. 2526322

  An object of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet by Ni pre-plating using a cold-rolled steel sheet of DP steel as an original sheet without significantly impairing the workability of the original sheet.

As described above, the present inventors, as a result of investigating the cause of greatly damaging the workability of the original plate when producing an alloyed hot-dip galvanized steel plate by Ni pre-plating method using a cold rolled steel plate of DP steel as the original plate, It has been found that the temper rolling applied to the cold-rolled steel sheet is the cause. Normally, DP steel is subjected to temper rolling with an elongation of about 0.2 to 0.6% in order to reduce the yield ratio and correct the shape. However, when the alloyed hot-dip galvanizing process by the Ni pre-plating method is performed while this temper rolling is applied, the workability of the original plate is greatly impaired by the heat treatment. Furthermore, the present invention has been completed by finding that it can be produced without damaging the workability of the original plate unless temper rolling is applied. The present invention has been made to solve the above problems, and the gist thereof is as follows.
(1) In mass%,
C: 0.05-0.20%,
Mn: 1.0 to 3.0%
Si: 0.3-1.8%
P: 0.05% or less,
S: 0.03% or less,
sol. Al: 0.005 to 1.0%,
N: Including 0.01% or less, the balance is a steel slab composed of Fe and inevitable impurities, hot-rolled, pickled, cold-rolled, and then annealed at 750-900 ° C, from 600-700 ° C to 350 ° C or lower 5 is cooled at 0 to 100 ° C. / sec, pickling, without applying temper rolling in the middle, and Puremekki Ni or Ni-Fe, 5 ° C. / after heating sec to from 430 to 500 ° C. or more galvanizing bath An alloy with good workability, characterized by being galvanized therein, subjected to alloying heat treatment at 460 to 550 ° C. for 5 to 40 seconds, and subjected to final temper rolling at an elongation of 0.2 to 1% producing how the galvanized high strength steel sheet.

  According to the production method of the present invention, it is possible to provide an alloyed hot-dip galvanized high-strength steel sheet having good ductility and a low yield ratio, which is widely used for automobiles and has great industrial merit.

  First, the reason why the components and the component ranges of the steel sheet targeted by the present invention are limited will be described. Basically, it is a component of common cold rolled steel of DP steel. Hereinafter, mass% in the composition is simply referred to as%.

  C is a hardening element and is effective in generating martensite. However, if it is less than 0.05%, the desired high strength cannot be obtained, and if it exceeds 0.20%, the weldability is deteriorated. Therefore, the C content is set to 0.05 to 0.20%.

  Mn strengthens the steel by solid solution strengthening and enhances hardenability to promote the formation of martensite. In order to exert such an effect, 1.0% or more is necessary. Moreover, even if it exceeds 3.0%, an effect will be saturated and the effect corresponding to content cannot be expected. Therefore, the amount of Mn is set to 1.0 to 3.0%.

  Si is a ferrite stabilizing element that inhibits the precipitation of cementite and promotes the formation of retained austenite by concentrating C in the austenite. Residual austenite improves ductility by the effect of strain-induced accelerated transformation. Therefore, Si is an element effective for improving the ductility of DP steel. Such an effect is recognized at 0.3% or more. On the other hand, if it exceeds 1.8%, the formation of martensite is inhibited and the plating property is remarkably deteriorated. Therefore, the Si amount is set to 0.3 to 1.8%.

  Since P is inevitably contained as an impurity and adversely affects elongation, the upper limit was made 0.05%.

  If S increases, it causes hot brittleness and deteriorates workability, so the upper limit was made 0.03%.

  Al is added as a steel deoxidizer and contained in the steel. 0.005% or more is necessary for Al. Further, it is a ferrite stabilizing element like Si, and can be used in place of Si by increasing the amount. However, sol. If it exceeds 1% with Al, the inclusions in the steel sheet will increase and ductility will deteriorate. Therefore, sol. It was made into 0.005-1.0% with Al.

  N is contained as an inevitable impurity, but if the amount of N is large, workability is deteriorated, so the upper limit is made 0.01%.

Next, the manufacturing method of the galvannealed steel plate by this invention is demonstrated in detail.
The molten steel may be one produced by a normal blast furnace method or one using a large amount of scrap as in the electric furnace method. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting. The slab may be once cooled and then heated in a heating furnace before hot rolling, or may be a so-called HCR or DR that is placed in a heating furnace while being cooled.

  Hot rolling is performed under normal manufacturing conditions for the above-described cold-rolled steel sheets. A coil box that winds and holds the coarse bar after rough rolling may be used. Further, a so-called hot rolling continuous process in which the rolled coarse bar is joined and rolled with the preceding coarse bar when unwinding may be used.

  The pickling and cold rolling are also carried out under normal manufacturing conditions for the above-described cold rolled steel sheet. In the continuous annealing process after cold rolling, first, annealing is performed at 750 to 900 ° C. to recrystallize ferrite and to generate austenite. If it is less than 750 ° C., recrystallization does not occur sufficiently, resulting in deterioration of workability. On the other hand, when the temperature exceeds 900 ° C., the surface properties deteriorate due to abnormal grain growth. In this case, the holding time is preferably about 30 to 200 seconds. Next, it is rapidly cooled to 350 ° C. or less at 50 ° C./second or more. Thereby, martensite is generated. Cooling to over 350 ° C. or cooling at less than 50 ° C./second does not sufficiently generate martensite, leading to strength reduction and workability deterioration. Desirably, it should be cooled to below 300 ° C. Moreover, about rapid cooling start temperature, about 600-700 degreeC is desirable. After the rapid cooling, it may be cooled to room temperature as it is, or may be cooled to room temperature after being kept near the cooling end temperature for several minutes. In order to remove the scale generated during the continuous annealing, it is necessary to pickle again at this stage.

  The temper rolling after the continuous annealing is the most important point in the present invention. As shown in FIG. 1, the elongation ratio of temper rolling is 0, that is, the yield ratio increases little if not applied at all. However, if the elongation is 0.1% or more, that is, if temper rolling is applied even a little, the yield ratio increases significantly. There is a similar tendency for the decrease in elongation. Therefore, temper rolling at this intermediate stage should not be applied. The above phenomenon is presumed to be due to a strain aging phenomenon in which solid solution C or solid solution N adheres to the movable dislocations generated in the steel sheet by temper rolling and the dislocations are difficult to move during the plating process. In the case of DP steel, since hard martensite is scattered in the soft ferrite phase, mobile dislocations are likely to occur at the interface between ferrite and martensite, and the above phenomenon is considered to occur remarkably.

In the galvanizing process, first, Ni or a Ni—Fe alloy is pre-plated to ensure plating adhesion. The plating amount is preferably about 0.2 to ² g / m ² . The pre-plating method may be any of electroplating, immersion plating, and spray plating. Then, it heats to 430-500 degreeC at 5 degree-C / second or more in order to plate. At a temperature increase rate of less than 5 ° C./second, solute C and solute N easily move, resulting in deterioration of workability. Desirably, the deterioration is further suppressed by raising the temperature at 30 ° C./second or more. Moreover, if it is less than 430 degreeC, it will be easy to produce non-plating at the time of plating, and if it exceeds 500 degreeC, the red rust resistance of a process part will deteriorate. Next, galvanization is performed in a galvanizing bath, and alloying heat treatment is performed at 460 ° C. to 550 ° C. for 5 to 40 seconds. If it is less than 460 ° C. or less than 5 seconds, alloying does not occur sufficiently. Moreover, when it heats exceeding 550 degreeC or it heats exceeding 40 seconds, deterioration of workability will become large.

  After the galvanization process, final temper rolling is performed for final shape correction and loss of yield point elongation. If the elongation is less than 0.2%, the effect is not sufficient, and if the elongation exceeds 1%, the allowance for increasing the yield ratio is large. Therefore, the elongation rate was set to 0.2 to 1%.

  Each process after hot rolling as described above, pickling, cold rolling, continuous annealing, temper rolling (intermediate), pre-plating, galvanizing process (including alloying treatment), temper rolling (final) are independent of each other. It may be a process, or may be a partially continuous process. From the viewpoint of production efficiency, it is ideal that everything is continuous.

A 250 mm thick continuous cast slab having the composition shown in Table 1 is reheated to 1200 ° C in an actual continuous hot rolling line, roughly rolled, finish-finished at 850 ° C, and finished with a plate thickness of 3.0 mm. The coil was taken up at 600 ° C. This hot-rolled coil was made into a cold-rolled steel sheet in an actual machine line that continued from pickling to cold rolling to continuous annealing to temper rolling. Cold-rolled to a thickness of 1.6 mm, annealed at 820 ° C. for 90 seconds, then cooled to 650 ° C. at 2 ° C./second, cooled from 650 ° C. to 250 ° C. at 100 ° C./second, and held at 250-200 ° C. for 360 seconds. Then, after cooling to room temperature, pickling was performed, and a sample was collected without applying temper rolling. This sample was subsequently processed in the laboratory. The temper rolling was not applied or it was applied at an elongation of 1% or less. Thereafter, Ni pre-plating of 0.5 g / m ² is performed per one side of the steel sheet, heated to 470 ° C. at 30 ° C./second, galvanized in a galvanizing bath, and subjected to alloying heat treatment at 500 ° C. for 10 seconds. The final temper rolling was applied at an elongation of 0.4%. The material of the steel plate was investigated by a tensile test using a JIS No. 5 tensile test piece. The results are shown in Table 2. For comparison, Table 2 also shows the material of the cold-rolled steel sheet in the intermediate stage. There were no problems with plating quality such as plating adhesion and powdering properties.

  As shown in Table 2, in the example of the present invention, it is possible to minimize an increase in yield ratio and a decrease in strength × elongation (TS × EL) relative to the cold-rolled steel sheet. On the other hand, in the comparative example, the increase in yield ratio and the decrease in strength × elongation are large.

With various plated steel sheets manufactured within the scope of the present invention except the intermediate temper rolling elongation and cold-rolled steel sheets in the middle stage, the yield ratio increases (yield ratio of plated steel sheets-elongation of cold-rolled steel sheets) ) And the average value plotted against the elongation of intermediate temper rolling.

Claims (1)

% By mass
C: 0.05-0.20%,
Mn: 1.0 to 3.0%
Si: 0.3-1.8%
P: 0.05% or less,
S: 0.03% or less,
sol. Al: 0.005 to 1.0%,
N: Including 0.01% or less, the balance is a steel slab composed of Fe and inevitable impurities, hot-rolled, pickled, cold-rolled, and then annealed at 750-900 ° C, from 600-700 ° C to 350 ° C or lower 5 is cooled at 0 to 100 ° C. / sec, pickling, without applying temper rolling in the middle, and Puremekki Ni or Ni-Fe, 5 ° C. / after heating sec to from 430 to 500 ° C. or more galvanizing bath An alloy with good workability, characterized by being galvanized therein, subjected to alloying heat treatment at 460 to 550 ° C. for 5 to 40 seconds, and subjected to final temper rolling at an elongation of 0.2 to 1% For producing high-strength hot-dip galvanized steel sheets.

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