CodeIgniter form_open_multipart, how to use

I'm using CodeIgniter in a project but I'm having a hard time sending information, I have a view that contains a form with the registration fields:                             

            <?php

             echo form_open_multipart('Controller_site/cadastro_form');

             ?>

            <input type="text" class="input-text" name="pais" placeholder="País">

        </div>
    </div>
</div>

In this view I have several other fields that are being "wrapped" with divs (Bootstrap) to assemble the layout. I'm closing the form in the last div:

 <div class="row">
<div class="col-md-12" align="center">

    <button type="button" class="btn btn-success">Cadastrar</button>

    <?php
     echo form_close();

     ?>

</div>

In the Controller_site I have the following function:

 public function cadastro_form(){


    $data = 'null';


        $data = array(
            'pais' => $this->input->post('pais'),
            'name_users' => $this->input->post('name_users'),
            'last_name' => $this->input->post('last_name'),
            'date_nasc' => $this->input->post('date_nasc'),
            'sexo' => $this->input->post('sexo'),
            'cpf' => $this->input->post('cpf'),
            'mail' => $this->input->post('mail'),
            'phone' => $this->input->post('phone'),
            'cep' => $this->input->post('cep'),
            'logradouro' => $this->input->post('logradouro'),
            'bairro' => $this->input->post('bairro'),
            'num' => $this->input->post('num'),
            'city' => $this->input->post('city'),
            'state' => $this->input->post('state'),
            'payment' => $this->input->post('payment'),
            'num_cart' => $this->input->post('num_cart'),
            'validade' => $this->input->post('validade'),
            'agencia' => $this->input->post('agencia'),
            'conta' => $this->input->post('conta'),
            'cvv' => $this->input->post('cvv'),
            'login' => $this->input->post('login'),
            'senha' => $this->input->post('senha'),
            'confirmar_senha' => $this->input->post('confirmar_senha'),





        );


        $this->Model_site->cadastro($data);

        redirect("index", 'redirect');


    }
}

No model:      public function register ($ data) {

    return $this->db->insert('tb_users',$data);
}
} 

I fill in the form data and nothing happens the page stays the same way the "looks" button does not have a button effect.

Note: The database file is already configured.